Atoll Global RF Planning Solution Version 2.6.2
AFP Reference Guide
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AFP Reference Guide
Contact Information Forsk (Head Office) 7 rue des Briquetiers 31700 Blagnac France
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Atoll 2.6.2 AFP Reference Guide Release AT262_ARG_E1
© Copyright 1997 - 2007 by Forsk The software described in this document is provided under a licence agreement. The software may only be used/copied under the terms and conditions of the licence agreement. No part of this document may be copied, reproduced or distributed in any form without prior authorisation from Forsk. The product or brand names mentioned in this document are trademarks or registered trademarks of their respective registering parties.
About AFP Reference Guide This document is aimed at frequency planning engineers using Atoll AFP module to perform automatic frequency planning of their networks. This document introduces the AFP with a high level description of the frequency planning process in Atoll. Then descending lower to the practical level, this document describes in detail every aspect of frequency planning in Atoll. Main topics covered in this document include AFP pre-requisites, AFP usage, AFP minimization target and some possible problems that may come up during training. This document begins with a basic user guide containing a short operational introduction to the AFP process in Atoll. Then it goes on to summarize most aspects of the practical planning process and provides detailed discussions on certain topics. It also explains the means to evaluate a frequency plan. Furthermore, a chapter is dedicated to advanced topics and troubleshooting in the end. The appendices describe the technical aspects of the cost function, the BSIC allocation algorithm, the IM calculation, and the dimensioning process.
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Table of Contents
Table of Contents
1 1.1 1.1.1 1.1.2 1.2
2 2.1 2.2 2.3 2.4 2.5 2.6 2.6.1 2.6.2 2.7 2.8 2.8.1 2.8.2
3 3.1 3.1.1 3.1.2 3.1.3 3.1.3.1 3.1.3.2 3.1.3.3 3.1.3.3.1 3.1.3.4 3.1.3.5 3.1.4 3.1.5 3.1.6 3.1.7 3.1.8 3.1.9 3.1.10 3.1.11 3.1.12 3.2 3.2.1 3.2.1.1 3.2.2
4 4.1 4.1.1 4.1.2 4.1.3 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.3 4.3.1 4.4
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Overview ......................................................................................... 13 Introduction to AFP ................................................................................................................................ 13 Frequency Assignment as a Cost Minimization Problem ................................................................. 13 Abbreviations.................................................................................................................................... 13 Architecture ............................................................................................................................................ 14
Basic AFP Tutorial .......................................................................... 19 AFP Process in Atoll .............................................................................................................................. 19 Loading and Validating the Network ...................................................................................................... 20 Definition of the AFP Scope ................................................................................................................... 22 Preparing to Launch the AFP................................................................................................................. 23 Launching the AFP and Monitoring its Progress.................................................................................... 25 AFP Outputs........................................................................................................................................... 27 Partial Commit Functionality............................................................................................................. 29 Automatic Constraint Violation Resolution ....................................................................................... 30 Visualising and Manipulating Results..................................................................................................... 31 Manual Frequency Allocation................................................................................................................. 31 Manual Frequency Allocation for SFH Case .................................................................................... 31 Manual Frequency Allocation for NH Case ...................................................................................... 31
Frequency Planning Prerequisites .................................................. 35 Atoll Data Model..................................................................................................................................... 35 Reliability and Propagation............................................................................................................... 35 HCS Layers ...................................................................................................................................... 35 Subcells............................................................................................................................................ 36 Key Roles of Subcells................................................................................................................. 36 Concentric Cells and Dual-band Cells ........................................................................................ 36 Minimum C/I................................................................................................................................ 36 Quality Targets...................................................................................................................... 36 Traffic Loads ............................................................................................................................... 36 Local Domain Restrictions .......................................................................................................... 36 TRXs ................................................................................................................................................ 37 Freezing Flags.................................................................................................................................. 37 AFP Weights .................................................................................................................................... 37 Spectrum Administration .................................................................................................................. 37 Redundancy and Subcell Audit ........................................................................................................ 37 Neighbour Importance ...................................................................................................................... 38 SeparationConstraints Table ............................................................................................................ 38 SeparationRules Table and Rule Priority ......................................................................................... 38 Adjacency Suppression .................................................................................................................... 38 AFP Performance Indicators .................................................................................................................. 38 AFP TRX Rank ................................................................................................................................. 38 TRX Rank Usage........................................................................................................................ 39 Total Cost and Separation Violation Cost Component ..................................................................... 39
Frequency Plan Optimisation .......................................................... 43 Step 1 (Optional): Traffic Model Usage.................................................................................................. 43 Creating a Traffic Map Based only on Clutter Weighting ................................................................. 43 Performing a Traffic Capture ............................................................................................................ 43 Creating IMs Based on Traffic .......................................................................................................... 44 Step 2 (Optional): Neighbour Relations and Relative Weighting ........................................................... 44 Automatic Neighbour Allocation ....................................................................................................... 44 Importing Neighbour Importance ...................................................................................................... 45 Extending Existing Neighbour Relations .......................................................................................... 45 Importing Partial Sources of Neighbour Importance ........................................................................ 46 Step 3 (Optional): Using Dimensioning .................................................................................................. 47 Optimal Dimensioning of an Existing Network ................................................................................. 47 Step 4: Optimal Usage of the Atoll AFP ................................................................................................. 48 AT262_ARG_E1
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4.4.1 4.4.1.1 4.4.1.2 4.4.1.3 4.4.1.4 4.4.1.5 4.4.1.6 4.4.1.7 4.4.1.8 4.4.1.9 4.4.1.10 4.4.1.11 4.4.2 4.4.2.1 4.4.2.2 4.4.2.3 4.4.2.3.1 4.4.2.3.2 4.4.2.4
5 5.1 5.1.1 5.1.1.1 5.1.1.1.1 5.1.1.1.2 5.1.1.2 5.1.1.3 5.1.2 5.1.2.1 5.1.2.1.1 5.1.2.1.2 5.2 5.2.1 5.2.2 5.3 5.3.1 5.3.1.1
6 6.1 6.1.1 6.1.2 6.1.3 6.1.4 6.1.5 6.1.6 6.1.7 6.1.8 6.1.9 6.1.9.1 6.1.10 6.1.11 6.1.11.1 6.2 6.2.1 6.2.1.1 6.2.1.2 6.3 6.4 6.5 6.6 6.6.1 6.6.2 6.6.3 6.7
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Introduction to the AFP Cost Function ..............................................................................................48 Combination of Separation Violation and Interference Probabilities ...........................................48 Counting TRXs (Nodes) Instead of Relations (Edges) ................................................................48 Each TRX Cost............................................................................................................................49 Separation Violation Cost ............................................................................................................49 Interference Cost.........................................................................................................................50 Probabilistic Cost Combination ...................................................................................................50 Missing TRX Cost........................................................................................................................50 Corrupted TRX Cost ....................................................................................................................51 Out-of-domain Frequency Assignment Cost ...............................................................................51 Quality Target..............................................................................................................................51 Modifiable and Non-Modifiable Costs..........................................................................................51 Most Important Cost Function Parameters and Tuning ....................................................................52 Interference Weight vs. Separation Weight .................................................................................52 Cost of Changing a TRX .............................................................................................................52 Quality Target and C/I Weighting ................................................................................................53 Quality Target ........................................................................................................................53 C/I Weighting .........................................................................................................................53 Separation Weights Settings .......................................................................................................54
Means to Evaluate Frequency Plans ...............................................57 Estimating Frequency Plan Quality.........................................................................................................57 Using Interference Studies................................................................................................................57 Various Interference Studies .......................................................................................................57 TRX Based Interference Study ..............................................................................................57 Worst Case Interference Study..............................................................................................58 Visualising TRX Ranks with a TRX Based Interference Study....................................................58 Visualising C/I Distributions with a TRX Based Interference Study.............................................58 Using Audit........................................................................................................................................59 Global Separation Fitness Expression ........................................................................................59 Forsk Independent Separation Fitness Expression (FISFE) .................................................59 Main Separation Violation Item Summary .............................................................................59 Using Point Analysis ...............................................................................................................................60 Example 1: Combination of Interference Effects...............................................................................61 Example 2: Counting Strong Interference Only Once .......................................................................61 Uniform Frequency Usage Distribution...................................................................................................61 When Uniform Distribution and Quality do not Coincide ...................................................................62 Domain Range Effect and Adjacent Constraints .........................................................................62
Advanced Topics and Troubleshooting ...........................................65 Various AFP Related Features ...............................................................................................................65 SFH (HSN, MAL, MAIO) ...................................................................................................................65 Definition of Atom..............................................................................................................................65 Synchronous Networks .....................................................................................................................65 Optimising Hopping Gains ................................................................................................................65 Fractional Load .................................................................................................................................65 Domain Use Ratio .............................................................................................................................66 User Defined MAL Length.................................................................................................................66 HSN Allocation ..................................................................................................................................66 MAIO Allocation ................................................................................................................................66 Staggered MAIO Allocation .........................................................................................................66 BSIC Allocation .................................................................................................................................66 Robustness of Atoll AFP ...................................................................................................................67 Value Ranges and Limitations at Validation ................................................................................67 Managing Consistency in Atoll and the AFP...........................................................................................68 Service Zone of a Subcell .................................................................................................................68 Specifying Correct Interference Study Coverage Criteria ...........................................................68 Selecting “All servers” or “Best Server” Service Zone .................................................................69 Event Viewer...........................................................................................................................................69 Interference Study Quality Criteria..........................................................................................................69 Calculation Zone Border Effect...............................................................................................................69 Frequency Planning Techniques ............................................................................................................70 Basics................................................................................................................................................70 Post-processing of Hot Spots............................................................................................................70 Learning the Network and Solving for Hot Spots ..............................................................................70 Performance and Memory Issues in Large GSM Projects......................................................................70
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7 7.1 7.2 7.3 7.4 7.4.1 7.4.1.1 7.4.1.2 7.4.1.3 7.4.2 7.5
8 8.1 8.1.1 8.1.2 8.1.3 8.1.3.1 8.1.3.2 8.1.4 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.4.1 8.2.4.2 8.2.4.3 8.2.5 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.4 8.4.1 8.4.2 8.4.3 8.4.3.1 8.4.3.2 8.4.3.2.1 8.4.3.2.2 8.4.3.2.3 8.4.3.3 8.4.4 8.4.4.1 8.4.4.2 8.4.4.3
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Interference Matrices ...................................................................... 75 Types of Supported Interference Matrices ............................................................................................. 76 Interference Matrices Storage................................................................................................................ 76 Multiple File Import................................................................................................................................. 77 Maximum Likelihood Combination ......................................................................................................... 77 Scope and Context of Interference Matrices .................................................................................... 77 Interference Matrix Context ........................................................................................................ 77 Interference Matrix Scope........................................................................................................... 79 Keeping the Scope and Context Up to Date............................................................................... 80 Interference Matrix Combination in Atoll AFP Module...................................................................... 80 Interference Matrix Calculation .............................................................................................................. 81
Appendices ..................................................................................... 85 Appendix 1: Description of the AFP Cost Function ................................................................................ 85 Notations .......................................................................................................................................... 85 Cost Function ................................................................................................................................... 85 Cost Components............................................................................................................................. 87 Separation Violation Cost Component........................................................................................ 87 Interference Cost Component..................................................................................................... 88 I_DIV, F_DIV and Other Advanced Cost Parameters ...................................................................... 90 Appendix 2: Interferences ...................................................................................................................... 91 Using Interferences .......................................................................................................................... 91 Cumulative Density Function of C/I Levels....................................................................................... 91 Precise Definition ............................................................................................................................. 91 Precise Interference Distribution Strategy ........................................................................................ 92 Direct Availability of Precise Interference Distribution to the AFP .............................................. 92 Efficient Calculation and Storage of Interference Distribution .................................................... 92 Robustness of the IM.................................................................................................................. 92 Traffic Load and Interference Information Discrimination................................................................. 92 Appendix 3: BSIC Allocation .................................................................................................................. 94 Definitions......................................................................................................................................... 94 Hard Criterion ................................................................................................................................... 94 Soft Criterion .................................................................................................................................... 94 Behaviour ......................................................................................................................................... 94 Appendix 4: Traffic Capture and Dimensioning...................................................................................... 95 Introduction....................................................................................................................................... 95 Traffic Map Generation..................................................................................................................... 95 Traffic Capture Process.................................................................................................................... 95 Inputs .......................................................................................................................................... 95 The Engine ................................................................................................................................. 96 Traffic Distribution ................................................................................................................. 96 Average Timeslot Capacity ................................................................................................... 97 Integration ............................................................................................................................. 97 Outputs ....................................................................................................................................... 98 Network Dimensioning Process ....................................................................................................... 99 Inputs .......................................................................................................................................... 99 Dimensioning .............................................................................................................................. 99 Outputs ....................................................................................................................................... 99
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List of Figures
List of Figures
Figure 2.1: Figure 2.2: Figure 2.3: Figure 2.4: Figure 2.5: Figure 2.6: Figure 2.7: Figure 2.8: Figure 2.9: Figure 2.10: Figure 2.11: Figure 2.12: Figure 2.13: Figure 2.14: Figure 2.15: Figure 2.16: Figure 2.17: Figure 2.18: Figure 2.19: Figure 2.20: Figure 2.21: Figure 2.22: Figure 2.23: Figure 2.24: Figure 3.1: Figure 4.1: Figure 4.2: Figure 4.3: Figure 4.4: Figure 4.5: Figure 4.6: Figure 4.7: Figure 4.8: Figure 5.1: Figure 5.2: Figure 5.3: Figure 5.4: Figure 5.5: Figure 5.6: Figure 5.7: Figure 5.8: Figure 6.1: Figure 7.1: Figure 7.2: Figure 7.3: Figure 7.4: Figure 8.1: Figure 8.2: Figure 8.3: Figure 8.4: Figure 8.5: Figure 8.6: Figure 8.7:
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AFP Process in Atoll .................................................................................................................................. Interaction of the AFP with Other Elements............................................................................................... AFP Outputs .............................................................................................................................................. AFP Launch Wizard - AFP Session Tab.................................................................................................... AFP Launch Wizard - Separations Tab ..................................................................................................... AFP Launch Wizard - Global Parameters Tab .......................................................................................... Event Viewer - Sample Messages............................................................................................................. Message 1 ................................................................................................................................................. Message 2 ................................................................................................................................................. AFP Launch Window ................................................................................................................................. Partial Interference Matrices - Report........................................................................................................ Complete Interference Matrices - Report................................................................................................... AFP Progress Window............................................................................................................................... Event Viewer Message - Solution Kept ..................................................................................................... AFP Progress Window............................................................................................................................... Cost Distributions on Frequencies............................................................................................................. Frequency Usage Distributions.................................................................................................................. AFP Results Window ................................................................................................................................. Separation Constraint Violation Details Message...................................................................................... AFP Results Window - Partial Commit Feature......................................................................................... AFP Results Window - Partial Commit Feature......................................................................................... Constraint Violation Resolution Tool.......................................................................................................... Scanning for Frequencies.......................................................................................................................... Scanning for Frequencies.......................................................................................................................... Model Standard Deviation - Default Value................................................................................................. Automatic Neighbour Allocation................................................................................................................. Automatic Neighbour Allocation Results.................................................................................................... Neighbours Table ...................................................................................................................................... Dimensioning Process ............................................................................................................................... Atoll AFP Module Properties - Separation Weights Tab............................................................................ Atoll AFP Module Properties - Cost Tab.................................................................................................... C/I Weighting ............................................................................................................................................. Atoll AFP Module Properties - Separation Weights Tab............................................................................ Interference Study Report.......................................................................................................................... TRX Based Interference Studies ............................................................................................................... TRX Based Interference Study - C/I Distributions ..................................................................................... Event Viewer Messages ............................................................................................................................ Event Viewer Message 1 ........................................................................................................................... Event Viewer Message 2 ........................................................................................................................... Combinatin of Interference Effects ............................................................................................................ Counting Strong Interference Only Once................................................................................................... Hopping Sequence Numbers..................................................................................................................... Interference Matrix Properties Dialog - General Tab ................................................................................. Interference Matrix Properties Dialog - Advanced Tab.............................................................................. Interference Matrix Scope.......................................................................................................................... AFP Interference Matrices Parameters ..................................................................................................... Atoll AFP Module Properties - Advanced Tab ........................................................................................... The cumulative density of C/I levels between [TX1, BCCH] and [TX2, BCCH] ......................................... Traffic Maps Overlay.................................................................................................................................. Traffiic Overflow......................................................................................................................................... Intra-Layer Distribution .............................................................................................................................. Traffic Distribution in Atoll .......................................................................................................................... Network Dimensioning Process.................................................................................................................
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Chapter 1 Overview
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Chapter 1: Overview
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Overview This document describes every aspect of frequency planning in Atoll, from high level description of the frequency planning process to the practical level detail. Main topics covered in this document include AFP prerequisites, AFP usage, AFP minimisation target and some possible problems that may come up during training. This document begins with a basic user guide, a short operational introduction to the AFP process in Atoll, and goes on to summarize most aspects of the practical planning process with detailed discussions on certain topics. It also explains the means to evaluate a frequency plan available in Atoll. A chapter is dedicated to advanced topics and troubleshooting in the end. Four appendices contain in-depth information on technical aspects of the cost function, the BSIC allocation algorithm, the IM calculation and the dimensioning process respectively. All in all, this document is almost self sufficient with respect to the use of Atoll AFP.
1.1
Introduction to AFP The main role of an Automatic Frequency Planner (AFP) is to assign frequencies (channels) to the network such that the overall network quality is optimised. With the evolution of GSM over the years to integrate many improvements, additional requirements have emerged in the process of radio network planning. The implementation of baseband and synthesised frequency hopping, discontinous transmission and network synchronisation, for example, has led to higher sophistication in the process of frequency planning. These enhancements require that an AFP also be intelligent and advanced enough to help the frequency planner through out his tedious task. The Atoll AFP considers a large number of constraints and directives; for example, ARFCN separation requirements between transmitters, interference relations, HSN assignment methods, frequency domain constraints, a certain fractional load to maintain etc. Hence, the AFP depends on a variety of input data, such as the interference matrix, neighbourhood relations, traffic information and so on. This document not only explains how to use the Atoll AFP, by describing the AFP GUI, but also includes detailed descriptions of the various constraints, directives, and data sources. The primary target of this document is to explain the technical background of the AFP.
1.1.1
Frequency Assignment as a Cost Minimization Problem From the technical point of view, the Frequency Assignment Problem (FAP) is considered as a minimization problem. This means that the AFP will generate a set of Frequency Plans (FPs), and propose the one that has the lowest cost as the “Best Solution”. Therefore, the AFP cost is the equivalent of AFP quality estimation: the lower the cost, the better should be the quality from the AFP point of view. The approach of cost minimization is not only the most common approach to the FAP but probably also the easiest to understand and control. It provides the user with means of guiding the AFP in its task. For example, by setting the cost of interference violation low, the AFP will concentrate its efforts on resolving the separation violations. There are AFP tools in which certain types of objectives are presented as “hard constraints”. If a hard constraint is not satisfied, the AFP does not offer any solution or offers a partial solution (with fewer frequencies and satisfying hard constraints). The philosophy of hard constraints vs. soft constraints has nothing to do with the quality of an AFP engine, it is merely a behaviour convention. In Atoll, we prefer always offering a solution to offering partial assignments or violating domain limitations. This ensures that you will always get a result when you launch the Atoll AFP. This result will very well depict the difficulty of the FAP. The cost of this solution will clearly indicate if unacceptable violations have occurred or if this plan has improved the current frequency plan. The cost function definition permits you to place as much emphasis as required on certain elements of the cost function. This manipulation will make the AFP behave as if it were guided by hard constraints, from the optimisation viewpoint, while retaining its property of being a quality monitor and a hardness-of-assignment monitor both.
1.1.2
Abbreviations Some abbreviations and terminologies used in the document are listed below:
© Forsk 2007
GSM
Global System for Mobile Communications (Groupe Speciale Mobile)
GPRS
General Packet Radio Service
EDGE
Enhanced Data rates for GSM (or Global) Evolution
EGPRS
EDGE based GPRS
TSL
Timeslot
TX
Transmitter or sector
TRX
Transceiver
BCCH
Broadcast Control CHannel. A term usually employed in Atoll to refer to the TRX carrying this channel.
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1.2
TCH
Traffic CHannel. A term usually employed in Atoll to refer to a TRX carrying traffic with usually the same coverage area as the BCCH.
TCH_INNER
Inner Traffic CHannel. A term usually employed in Atoll to refer to a TRX carrying traffic but usually having a coverage area less than that of a TCH.
HR/FR
Half Rate/Full Rate
CS
Circuit-switched
PS
Packet-switched
HCS
Hierarchical Cell Structure
Subcell
An entity defined by the pair [TX, TRX Type]
HO
Handover
kbps
Kilobits per second
GoS
Grade of Service
QoS
Quality of Service
KPI
Key Performance Indicators
TL
Traffic Load
P
Probability
C
Carrier power (Signal strength)
C/I
Carrier to Interference ratio
AFP
Automatic Frequency Planner/Planning
DTX
Discontinuous transmission
GUI
Graphical User Interface
FP
Frequency Plan
BBH
Baseband Hopping
SFH
Synthesized Hopping
NH
No Hopping
MAL
Mobile Allocation List. In the context of SFH, MAL is the group of frequencies used by the frequency hopping TRX.
AMR
Adaptive Multi-Rate
CC
Concentric Cells
Transmitter
Atoll synonym for cell or sector in conventional GSM jargon
FER
Frame Erasure Rate
FH
Frequency Hopping
DLPC
Down Link Power Control
RRM
Radio Resource Management
Synchronised transmitters
Transmitters that are synchronised and can, therefore, share the same HSN.
Data Model
A project can be saved in a filename.ATL file or as a database. In both cases, most of the project’s information is saved in database tables. We refer to these tables as the data model.
IM, IMco, IMadj
Interference Matrix, Co-channel / Adjacent-channel Interference Matrix
FN
Frame Number
CDF
Cumulative Density Function
TSC
Training Sequence Code
FAP
Frequency Assignment Problem
#
Number of
Architecture The Atoll Automatic Frequency Planning (AFP) module is an optional module that enables you to generate frequency plans for GSM and TDMA networks automatically. The Atoll AFP module can allocate the following parameters: • • • •
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TRX rank (can be used to prioritise the use of good frequencies) Performance Indicators at Site/Cell/TRX levels
Atoll works with an open AFP interface. Any AFP built using this interface can be able to allocate the following additional parameters. Future versions of the Atoll AFP module are planned to assign the following parameters as well: • • •
Group ID (better administration of the frequency resources) TN offsets FN offsets
Atoll AFP implements simulated annealing, taboo search, graph heuristics and machine learning. It manages its time resources to match the users time directive. If allowed enough time, the AFP will employ a major part of this time in “learning” the network. During the learning phase, the AFP tunes up its internal parameters. Towards the end of the user-defined time, the AFP switches to a randomised combinatorial search phase.
Remark: The role of this learning phase is extremely important in order to get good results. You should often let the AFP run over a night or a weekend by specifying corresponding target time. If you never run the AFP specifying a long time period, it will never be able to calibrate itself and will always perform from 10 to 70 solutions and stop. Network learning is performed by executing numerous fast and deterministic instances of the AFP. The one that obtains the best performance is memorized in the document and is, therefore, the most suitable for the specific network. The next time an AFP is executed it will start where the learning process ended and it will use the parameter profile of the best solution stored in the document. Note: •
The following scenario will demonstrate the usefulness of AFP learning capabilities: - Create a GSM GPRS EGPRS project and import its network elements and maps. - Create a copy of “Atoll AFP module” and name it “Atoll AFP module 2”. - If the network has X transmitters, run “Atoll AFP module 2” for X / 10 minutes to obtain a cost Y. (Short execution) - Now run “Atoll AFP module 2” for a longer time (for example, X / 5 hours). - Another cost, Z, is obtained, which is better than Y (i.e. Z < Y). The network dependent information is memorized in the “Atoll AFP module 2” instance whereas the “Atoll AFP module” instance remains unchanged. - Now if you perform a short execution with “Atoll AFP module 2”, you can get the improved result (Z) right away. While a short execution of the “Atoll AFP module” instance will give the initial cost (Y). - If X / 5 hours is too long, you can perform the “learning” on a small (representative) part of the network.
The Atoll AFP is built based on a specified COM interface designed as a part of Atoll’s open platform strategy. The interface is designed in such a way that puts aside elements that are not inherent to the AFP process. At the same time, through the modelling capabilities of the planning tool, the AFP can support complete list of features expected from an AFP.
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Chapter 2 Basic AFP Tutorial
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Chapter 2: Basic AFP Tutorial
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Basic AFP Tutorial Atoll AFP framework complies with its global open architecture strategy. Any AFP module, Atoll AFP or 3rd party AFP, can be interfaced and made available to RF planning engineers through Atoll. Furthermore, different AFP modules are activated, accept their main inputs and generate their main outputs in the same manner. This section teaches the basics of activating an AFP in Atoll.
2.1
AFP Process in Atoll The AFP process is a cycle in which the AFP is only one of its many steps:
Figure 2.1: AFP Process in Atoll The figure below gives a better view of interaction of the AFP with other elements in Atoll:
Figure 2.2: Interaction of the AFP with Other Elements The following figure depicts the outputs of the AFP:
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Figure 2.3: AFP Outputs
2.2
Loading and Validating the Network To launch the AFP, choose the Automatic Allocation… command from the Frequency Plan menu of the Transmitters folder context menu. This initiates a series of dialogs called the AFP wizard.
Figure 2.4: AFP Launch Wizard - AFP Session Tab
Figure 2.5: AFP Launch Wizard - Separations Tab
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Chapter 2: Basic AFP Tutorial Here you can, • • • • •
Specify the AFP module you would like to use and set its parameters, Choose the network parameters and AFP performance indicators you want the AFP to allocate, Specify the network’s default separation requirements, Consult the network’s “Exceptional Pairs” and define other separation constraints for them, and Indicate whether interferences are to be included in calculations or not.
For explanations of AFP performance indicators, refer to section 3.2 "AFP Performance Indicators". The last wizard dialog contains some global parameters that often vary from one AFP instance to another:
Figure 2.6: AFP Launch Wizard - Global Parameters Tab The most important option here is the one proposing the two sources of the traffic load information. Traffic load can be read directly from the subcells table, which could have been filled manually, by the dimensioning process or by a KPI calculation. You can also specify that the traffic load should be read from the default traffic capture (explained later). Notes: •
In case the traffic load is taken from the Subcells table, committed after a KPI calculation, you must be aware of a certain difference: in the KPI calculation, Atoll divides the captured traffic by the timeslot capacity of the existing number of TRXs, while the AFP requires it to be divided by the timeslot capacity of the required number of TRXs.
•
The traffic load is artificially increased to 0.1, if it is too low (less than 0.1), in order to maintain the AFP robust against partial data conditions. Hence, the AFP cannot completely ignore the existence of a frequency in a TRX.
Clicking Validate will start the data verification and storage optimisation aimed at providing fast access to data needed by the AFP. This stage may generate many warnings for real-life networks (for example, values out of range). These are displayed in the Event viewer. It is recommended to revise the network data according to these messages and continue once all the data are clean and coherent. If a certain message is not clear or self evident, you can always contact Forsk’s technical support. The figure below depicts the Event viewer with some sample messages:
Figure 2.7: Event Viewer - Sample Messages Let us look at two of these messages:
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Figure 2.8: Message 1 This means that the value entered in the AFP weight column of the Transmitters table for the transmitter 19941 is invalid. In the database, this field’s name is “COST_FACTOR”. A value of –2 for the cost factor implies that the AFP should generate the worst assignment possible for the transmitter. It would be interesting to investigate the origin of this erroneous value as it may avoid possible errors in the future. Atoll automatically resets this value to 1 in order to avoid such calculation errors.
Figure 2.9: Message 2 This message informs that 3678 subcells were loaded successfully. The next section explains the significance of the term ‘effectively selected’ and why 3678 subcells were loaded and only 6 selected for the AFP process.
2.3
Definition of the AFP Scope In the example above, the 6 subcells effectively selected for the AFP process had many potential interferers, neighbours, neighbours of neighbours, and/or transmitters with exceptional separation constraints with them. No AFP can perform a good allocation for these 6 subcells without “dragging in” a large part of the network. The AFP considers the part that is “dragged in” to be “frozen”. On the other hand, there are many other ways to freeze network elements in Atoll. Some precise definitions are provided in order to avoid misconceptions. Let us define 4 groups of transmitters (ALL, NET, SEL, RING): • • • •
ALL = All the transmitters in the project. NET = Active transmitters that pass the filters on the main Transmitters folder and on the main Sites folder. SEL = Transmitters belonging to the (sub)folder for which the AFP was launched and that are located inside the focus zone. RING = Transmitters belonging to NET, not belonging to SEL and having some relationship with the transmitters in SEL: - If interferences are to be taken into account (see the dialog above), all transmitters whose calculation radii intersect the calculation radius of any transmitter in SEL will be included in RING. For large calculation radii (20 km for example), a single site can have a very large RING loaded. - Neighbours are always included in RING. - If one transmitter of an Exceptional Pair is included in SEL and the other is not, then the other will be included in RING as well. - If BSIC assignment is required, then all the second order neighbours (neighbours of a neighbour) will be included in RING as well.
Both the RING and the SEL parts of the network are loaded. It is important to know which subcells are loaded as the cost is calculated for all loaded subcells. The RING part is frozen for all assignments (BSIC, HSN, MAL, MAIO and channels). The SEL part may be assigned some parameters but only the ones specified in the dialog above. For example, if the user did not select BSIC, it will not be assigned. In addition to the generic freezing options above, there are some finer freezing options available in the data structure: 1. 2.
Individual transmitters can be frozen for channel (and MAL), HSN and/or BSIC assignment. Individual TRX’s can be frozen for channel (and MAL) assignment.
In an Atoll project, it is strongly recommended to avoid TRX’s without channels. For this reason, never create transmitters automatically if there are no channels to assign to them. Therefore, if the user does not ask for MAL/MAIO assignment, all SFH subcells are considered frozen and no TRX will be created for them. The same occurs when only a MAL/MAIO assignment is requested. In this case, all NH and BBH subcells will be considered frozen and no TRXs will be created. Note: •
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See Developer Reference Guide for details on the TO_ASSIGN and FROZEN assignment states available in the AFP API.
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2.4
Preparing to Launch the AFP Once the network is loaded and all warnings resolved, the AFP launch dialog will appear. This dialog contains a short summary of the state of the loaded network, SEL + RING.
Figure 2.10: AFP Launch Window Interference matrices can be managed through the Interference Matrices folder. You can have more than one interference matrices in your document. The top most active interference matrices set is used by the AFP. You can either embed the interference matrices in the document or store them in external files. Atoll compresses the interference matrices if stored in the .atl document itself. It is not necessary to load IMs or look for them each time AFP is launched. You can view the reports on different interference matrices available in the Interference Matrices folder. This report has a summary section which indicates the current state of the IMs.
Example 1: When partial IM info exists, we can see that 9 transmitters out of 24 do not have any interferers.
Figure 2.11: Partial Interference Matrices - Report
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Example 2: When complete IM info exists, observe that the IM topology is more or less normal.
Figure 2.12: Complete Interference Matrices - Report The AFP launch dialog also lets you define a generator initilialisation number. This number serves as a directive of randomness for the AFP process being launched. If the generator initialisation is set to 0, the AFP will be fully random. An integer other than 0 will define a given deterministic sequence for the AFP process. Each generator initialisation number (other than 0) corresponds to a deterministic sequence. Therefore, each AFP instance launched with the same generator initialisation number will yeild the same results. You can use this option if you want to have the same set of solutions every time you launch the AFP for the same part of the same network. The Atoll AFP has a single algorithm with a number of steps. The AFP ignores some of these steps if the alloted target calculation time is too short. One of these steps is deterministic, i.e. independent of the generator initialisation number, while the other steps are initialized by this number. • •
Generator Initialisation = 0 (default value) signifies that this intialisation number will be calculated randomly. Generator Initialisation ≠ 0 means that the number will be the one set by the user. Every time you define the same number, the AFP algorithm will be initialised in the same way, and hence the set of solutions will be the same.
It is advised to set Generator Initialisation = 0, and let the AFP reach the end of the Target Computation Time defined. However, you must keep in mind that all the AFP computations are deterministic in the start, independent of the generator initialisation. The AFP must be allowed to compute during the target time to observe the effects of randomness. Notes:
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•
Since the method chosen by the AFP depends on the target time provided, you might not get the same results using the same generator initialisation number if the defined target times are different. Therefore, to actually get the exact same results from the AFP process, you must define a certain target time and a certain generator initialisation.
•
The AFP may be perfectly deterministic during a portion of the target computation time (5 15%). During this period, the randomness seed will have no effect on the solutions. If you want to see the effect of randomness, let the AFP calculate until the end of the target time, or set a shorter target time.
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2.5
Launching the AFP and Monitoring its Progress Provide a time quota and a generator initialisation number in the above dialog and launch the AFP by clicking Run. It is important to set a long time quota from time to time to allow the AFP to calibrate itself. If not stopped, the AFP will usually continue for a while before stopping by itself. Important: •
If only a short time is specified, the full optimisation potential of the AFP will not be utilised.
The window below opens when the AFP is started, and displays information about the AFP process:
Figure 2.13: AFP Progress Window The Progress section of this window in the top left displays the target time allocated to the AFP, the time elapsed and the number of AFP solutions that have been evaluated so far. The general information and interference matrices report section in the top right gives some general information about the current solution in real time. This display depends on the selected AFP module. This section lists the status of the current solution, the initial cost, the cost of the current best solution, the cost of the previous solution and whether the previous solution was kept or rejected. You can use the >> button to switch to the report on the currently used interference matrices. The Event viewer has been made accessible through the AFP progress dialog in order to help the user keep track of all the important warnings and messages generated before and during the AFP process. This also enables you to export these messages as an AFP log file. If a solution is kept, a corresponding message appears in the Event viewer. Double-clicking the message in the Event viewer will open a dialog with the full details of this message, which will look something like the following figure.
Figure 2.14: Event Viewer Message - Solution Kept After the AFP is allowed to compute solutions and try to optimise the network for a while, the AFP progress dialog would look somewhat like this:
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Figure 2.15: AFP Progress Window The Best Frequency Plan Costs section displays the current values of modifiable and total costs, and their respective separation components. This section also displays the total weighted Erlangs of the network concerned in the AFP process, i.e. the total cost of a 100% interfered frequency plan). It gives a general idea of how good the cost of a certain frequency plan is. The cost of any solution remains between 0 and the Network Weighted Erlangs. The cost is as better as it is closer to 0. Apart from the above information, this section also contains a table listing the initial frequency plan and all the AFP solutions kept so far sorted in ascending order of cost. This table can display: • • • • • •
Modifiable costs Total costs Frozen costs Summed components Main components (separation violation cost component, interference component and modified TRX component) Additional taxes (corrupted, missing or out of domain TRXs)
For detailed description of modifiable and non-modifiable parts of the total cost, please refer to section 4.4.1.11 "Modifiable and Non-Modifiable Costs". Using the buttons available in the Plan comparison section in the bottom right, it is possible to visually compare the initial frequency plan and the current best solution (with the Best Plan column in the AFP cost details table checked). Clicking these buttons opens dialogs containing graphs corresponding to ’Cost Distribution on Frequencies’ and ’Usage Distribution on Frequencies’. The cost of a frequency f is given as: Cost ( f ) =
∑
FL ( i ) × Cost ( i )
i ∈ TRXs using f
Where, FL(i) is the fractional load of frequency f in the MAL of i, and cost(i) is the AFP cost of TRX i in Erlangs.
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Figure 2.16: Cost Distributions on Frequencies
Figure 2.17: Frequency Usage Distributions You can pause or stop the AFP process any time to check the current best solution, and resume optimising the network after you have checked it. Pausing the AFP process opens the AFP results window with the current best solution results listed.
2.6
AFP Outputs When calculations stop (completed or paused to view the current situation), Atoll displays the frequency plan proposed by the AFP. All results/violations are listed in a dialog window. This window contains a table listing all the assigned resources.These resources and related items (transmitters, subcells) are coloured differently to indicate different reasons: • • • • • • •
Arctic blue: frozen resource Red: resource modified compared to the previous allocation but with separation violation Green: resource modified compared to the previous allocation respecting the separation constraints Black: resource not modified Blue: resource assigned with no separation violation Purple: resource assigned but with separation violation Grey: items and resources involved in computation but not available for allocation
Positioning the cursor over a resource in the table displays the reason for its colour in a tool tip. The AFP result dialog is a non-blocking dialog. It enables the user to access other Atoll windows while the AFP is still pending. Thus, it is possible to view other data or warning/error messages in the Event viewer (for example, the history of AFP solutions). From this stage, it is possible to commit, to resume or to quit the AFP. It is good practice to keep a report through the export option before resuming the AFP. The user can also partially commit some of the results as explained in the next section. The results window displays all the results of the AFP session. It is possible to only display some of the results by checking/ un-checking the relevant choices in the Display options menu. You can choose to display the results related to: • • •
Cells (BSICs) Subcells (HSNs) TRXs (Channels/MAL, MAIO) and related separation violations
Selected AFP performance indicators (AFP TRX ranks, and total and separation costs at TRX, subcell, transmitter and site levels) will also be available in the results window. These AFP performance indicators are also available to export. You can choose whether to display the AFP indicators in the results as separate columns. The Show AFP Indicators command in the Display options menu controls the display of AFP TRX ranks, and total costs and separation cost components at TRX, subcell, transmitter, and site levels.
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Figure 2.18: AFP Results Window As the network had been loaded according to both the items to assign and the ones they relate to, it is possible to display the frequency plan of either: • •
Items belonging to the selected transmitters (see the definition of SEL), or Items belonging to the loaded transmitters (see the definition of SEL + RING). In the preceding example, there were no transmitters in the RING set, so the option is not available.
It is also possible to display detailed information about separation constraint violations, i.e. the co-channel and adjacent channel collision probabilities for relevant TRXs. You can choose to display these separation constraint violations through the Display options menu. The Separation violations column lists each each type of separation constraint violation realted to a given TRX, i.e. exceptional pair, co-transmitter, co-site, or neighbour. Another column titled ’With the TRX’ contains a button for each type of separation constraint violation. This caption of this button shows the TRX with which the separation constraint violation occurs. Clicking this button takes you to the corresponding TRX row in the table. Right-clicking a row with a separation constraint violation opens a Separation Constraint Violations context menu, which opens a dialog mentioning the reason of violation when clicked. For example:
Figure 2.19: Separation Constraint Violation Details Message Use the Commit button to assign the allocated resources and AFP performance indicators. The resume button permits resuming the AFP optimisation from where it stopped the last time. Note: •
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At the bottom of the AFP results window, messages related to the last solution are displayed, which may list problems as well.
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2.6.1
Partial Commit Functionality It is often required to commit only a part of the automatically generated frequency plan rather than committing it entirely. The purpose is to avoid committing TRXs that violate separation constraints (sometimes referred to as “not closing the frequency plan”). Future Atoll versions will incorporate advanced automatic filters for partial commit. The dialog examples below depict a case where removing a TRX eliminates a separation constraint violation on neighbours. Once a TRX is manually removed from the resulting plan, separation violations are recalculated (may take a few seconds). If the TCH TRX of transmitter Site36_3, causing neighbour separation constraint violations, is removed from the sample frequency plan below, the resulting frequency plan has no neighbour separation constraint violations on the TCH TRX of transmitter Site36_1.
Figure 2.20: AFP Results Window - Partial Commit Feature It is possible to specify the action to be taken with each TRX individually, or globally delete all TRXs with separation violations. It is also possible to mix the old plan and the new plan. Though this is not recommended, since it can cause interferences of which the user might be unaware. The dialog examples below depict how this operation can be carried out. The Delete the TRX option implies that the resulting frequency plan will not respect the number of required TRXs. In the above example, note than the neighbour separation constraint violations at transmitter Site36_1 vanished once the TCH TRX at Site36_31 was deleted.
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Figure 2.21: AFP Results Window - Partial Commit Feature
2.6.2
Automatic Constraint Violation Resolution Different types of constraint violations, i.e. co-transmitter, co-site, neighbour, and exceptional pair, can automatically be eliminated from the propsed frequency plan using the Automatic Constraint Violation Resolution tool. This tool is accessible from the Actions button menu. The aim of this tool is to find the TRXs in the currently proposed frequency plan that cause constraint violations of any of the four following types: 1. 2. 3. 4.
Co-transmitter Co-site Neighbour Exceptional pair
Once it finds the TRXs that satisfy the criteria, it sets their corresponding values to Delete the TRX in the Channel Assignment column of the AFP results window. This tool lets you resolve any type of constraint violations for different types of TRXs, control or traffic. You can also define a threshold of co-channel and adjacent channel collision probabilities. This restriction will only set those TRXs to Delete the TRX, which have a co-channel or adjacent channel collision probability higher than the threshold you defined.
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Figure 2.22: Constraint Violation Resolution Tool
2.7
Visualising and Manipulating Results The Commit button copies the frequency plan to the data structure. It is not necessary to save the document or commit the changes to the database right away as the AFP cycle has not yet ended. At this stage, various generic and specific tools are available in Atoll, and can be used to inspect the candidate frequency plan. Interference and C/I prediction studies and various consistency checks are described in the following chapters of document. In addition to these, a useful tool is also available in Atoll, called the Search tool. Its function is to facilitate visualising co-channel and adjacent-channel transmitters. This tool is explained in detail in the User Manual. Other means of inspection include the common grouping, filtering, advanced filtering, display and tool tip management features.
2.8
Manual Frequency Allocation This section describes quick and useful techniques for performing manual frequency allocations in Atoll.
2.8.1
Manual Frequency Allocation for SFH Case It is possible to perform frequency allocations for irrgular pattern networks, i.e. patten allocation of type 1/N. The following set of operations will results in a frequency allocation even if the network is not a 100% regular pattern network. 1. 2. 3. 4. 5.
2.8.2
Run the AFP so that it creates the required number of TRXs. Group the transmitters by azimuth and manually assign the MALs to the most important azimuth groups. Filter out these azimuth groups and delete the TRXs of all transmitters that were not assigned a MAL manually. Run the AFP again selecting MAIO assignment only. This will assign proper MAIOs to the TRXs to which MAL was manually assigned. Remove the filter and freeze the existing TRXs. Now use the AFP to complete the assignment (assigning all resources).
Manual Frequency Allocation for NH Case To carry out manual frequency assignment: 1. 2. 3. 4.
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Create a Best Server map and display it, Display neighbours of the transmitter for which you want to find a frequency manually, Open the Search tool, By scanning the spectrum a good frequency can easily be found and can be allocated to the transmitter.
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Figure 2.23: Scanning for Frequencies In this example, frequency 11 is not a good choice since it is used as a neighbour co channel. Frequencies 10 and 12 present similar characteristics.
Figure 2.24: Scanning for Frequencies On the other hand, frequency 14 is a good one and can be possibly allocated. None of the frequencies {13, 14, 15} are allocated at the selected transmitter of at its neighbours.
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Chapter 3 Frequency Planning Prerequisites
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3
Frequency Planning Prerequisites The principal difference between AFP and other planning activities is that the impacts of poor frequency planning are more widespread in a network. For example, poor planning of a site or a cell will have somewhat local influences, while implementing a poor frequency plan will affect a much larger part of the network. Moreover, creating a poor frequency plan is rather relatively easy, the presence of a single faulty parameter in the process can be sufficient for the damage the entire plan. Therefore, it is mandatory that the AFP user acquires a minimum level of knowledge regarding Atoll data model. This chapter familiarises the user with the essentials of the data model and depicts their relations with the AFP.
3.1
Atoll Data Model
3.1.1
Reliability and Propagation Often the user senses that the AFP does not have enough constraints: • •
The unfrozen part of the AFP cost is 0 and the AFP stops due to this fact. There appear to be close frequency reuses in the resulting frequency plan.
This means that the problem is too “easy” for the AFP and the user would like to create a more difficult IM in order for the AFP to have a more difficult problem to solve. The best method to accomplish this is to increase the cell edge reliability and recalculate the IMs. When the reliability requirement is elevated, a larger part of the standard deviation is reduced from “C” when calculating the C/I for each IM entry. The user should also verify that the standard deviation is properly defined in all clutter classes and its default value. This verification is more important in the case of Atoll documents converted from older versions or connected to a database.
Figure 3.1: Model Standard Deviation - Default Value
3.1.2
HCS Layers HCS layers have several roles in Atoll. Their most important role is related to the way Atoll manages traffic maps. Different layers have different priorities and mobility limitations. There is also the possibility to manage traffic overflow from one layer to another. The objective of all these options is to model the behaviour of a real network, where two potential servers that do not belong to the same layer usually do not compete for best server. When calculating an IM, or when generating an interference study, HCS layers are used in generating service zone maps, the basis of these calculations. If two transmitters belong to different layers, they can both serve the same pixel even if received signal from one is much stronger than the other’s. For equal HO margins, more HSC layers mean higher overlapping levels in the network. As the overlapping level increases, the constraint level in the IM and the amount of interference in an interference study also increase. Note: •
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Be sure to study the priority mechanism in your network, both in the re-selection process and in the handover process. Define the corresponding HCS layers once you know its working. When using a traffic model, make sure that there are a few levels of mobility in order to model high speed / low speed mobility behaviours.
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3.1.3
Subcells Subcells are defined as a group of TRXs in the same transmitter. Two subcells of the same transmitter can request frequencies from different domains, require different C/I qualities, have different downlink power offsets and even have different Radio Resource Managements (RRM). Different RRMs can lead to different service zones under the same cell. Subcells are crucial for modelling concentric and dual band transmitters. In these cases, the TRXs belonging to the “inner” subcell serve traffic within a limited zone. Note: •
3.1.3.1
Key Roles of Subcells • • • • •
3.1.3.2
All TRXs in a subcell share the same TRX type.
Associating TRX groups with required quality definitions Associating TRX groups with weak / strong constraints (interference , separation) Associating TRX groups with different domain limitations Visualising and filtering by TRX Type The following additional parameters are also defined in the Subcells table: - HSN (since the inner zone HSN may be different from the outer zone HSN) - Power offset - Reception threshold (can limit the zone of the inner subcell) - Hopping mode - Assignment mode (in SFH, “group constrained mode” limits the choice of MAL to one of the groups in the domain) - Support of DTX - Traffic load and supplementary AFP weight - Some other parameters influencing the AFP indirectly (for example, the overflow rate)
Concentric Cells and Dual-band Cells Concentric cells were created in order to exploit downlink power control (DLPC) and radio resource management (RRM) in frequency planning. This is accomplished by associating channels with subcells. Subcells may have different service zones with respect to the transmitter’s geographic coverage. For example, a subcell TCH_INNER covers a zone requiring minimum reception level of –75 dBm and TCH_OUTER covers a zone with minimum reception level of –94 dBm. In this case, the inner zone has a higher resistance to increasing interference. The AFP has the possibility ot assign a relatively interfered frequency to the TCH_INNER zone to give more choice to the outer zone. The other important property of concentric cells is the fact that a downlink power offset is associated with each subcell. The inner subcells can have higher DLPC implying that the frequencies assigned to the inner zones will interfere less with other transmitters. Concentric cells permit a higher reuse pattern between inner zones, providing up to 40% increase in capacity. Atoll can fully exploit this increase in capacity since it calculates interferences between subcells. It uses the power offset and the C/I threshold that defines the subcell boundaries. Furthermore, it is also possible to define separation constraints at subcell level.
3.1.3.3
Minimum C/I The required quality thresholds for BCCH and TCH are usually 12 and 9 dB respectively. But, since the GSM standard tests this behaviour under the comfortable reception conditions of 20 dB above thermal noise, it does not reflect the behaviour for, for example, received signals being only 15 dB above thermal noise. Atoll provides the possibility to define these thresholds at subcell level allowing maximum flexibility and possibility to support a mixture of old and new equipment. Moreover, the safety margins corresponding to these values can be defined in the AFP cost definition. Refer to section 4.4.2.3 "Quality Target and C/I Weighting" for more information.
3.1.3.3.1
Quality Targets Various quality targets can be set in Atoll by defining a C/I threshold value “min C/I”, with a probability threshold “% max interference”. These two values combined together define a quality target which implies that in order to have acceptable quality, the probability of having C/I lower than the “min C/I” value must be less than “% max interference”. This method enables Atoll to exploit the fact that a larger number of TCH channels can be assigned with quality requirements lower than the BCCH quality. This results in less constraining interferences and an easier and faster assignment. Refer to section 4.4.2.3 "Quality Target and C/I Weighting" for more information.
3.1.3.4
Traffic Loads Traffic loads of all the subcells are used as input to the AFP. These traffic loads can be calculated by Atoll or read from the Subcells table. Traffic loads are discussed in detail afterwards.
3.1.3.5
Local Domain Restrictions Low level domain restrictions can be introduced at subcell level through the excluded channels column in the Subcells table.
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3.1.4
TRXs Atoll’s TRX table enables the following: • • • •
Support of an external ID space of the TRXs of a transmitter (important for import and export utilities). MAL / channel at TRX level. MAIO at TRX level. Fine freezing: The user can freeze specific TRXs in an unfrozen transmitter.
The TRX table does not contain an “active” field. Therefore, all TRXs in it should contain a valid frequency or MAL and are all considered to be on air. It is better to remove an entire TRX record than removing only the frequency or MAL from its channels list.
3.1.5
Freezing Flags A multilevel freezing mechanism enables freezing resources at TRX level as well as at transmitter level. This, in turn, enables the user to use an existing plan while assigning only newly added demand for channels. These options are in addition to the working zone limitations. Note: •
3.1.6
When freezing channels, keep in mind that the MAIOs are not frozen.
AFP Weights The AFP weight field in the Transmitters table enables the user to assign high or low weightings to certain transmitters. It can be used to improve quality at a problematic location or to boost quality in a particular covered region of the network. An additional AFP weight field exists at the subcell level. It enables the user to assign weighting to subcells. A conventional idea could be to assign a higher weight to the BCCH. The AFP uses the multiplicative product of transmitter level AFP weight and subcell level AFP weight.
3.1.7
Spectrum Administration Many levels of administration exist relative to frequency planning. In order to avoid confusion, here is a comprehensive list: •
ARFCNs ARFCN is the method employed by the GSM/DCS standards to enumerate 200 kHz frequency carriers.
•
Frequency Bands Frequency Bands are subgroups of ARFCNs. Different equipment may be limited to different frequency bands (BTS, MS, …). In addition, propagation models use the central frequency of the band for calculating propagation.
•
Frequency Domains Domains are used for managing the usage of the Frequency Bands. For example, an operator may use frequencies 1 to 50 while the other uses 52 to 100. Splitting the band on channel usage basis is of great importance as well (BCCH frequencies, TCH frequencies, Hopping layer).
•
Domain Groups Domain groups are used for further managing the use of the frequencies in a domain. For example, f1 and f2 can be assigned at the same transmitter if and only if they belong to the same group. Another frequent use for groups is in the MAL assignment.
In Atoll, a domain is defined as a union of groups. It points to a frequency band and must be included therein. The AFP respects domain limitations at subcell level.
3.1.8
Redundancy and Subcell Audit Atoll incorporates some deliberate redundancies between the subcells and TRX levels, and the Transmitters table: • • •
The channel list in the Transmitters table is the intersection of all channels appearing in the TRXs of a transmitter. The hopping mode of a transmitter is the hopping mode of it’s default traffic carrier (the TCH TRX Type) The frequency band of the transmitter (the one used by the propagation model to deduce the central frequency), is read from the domain of the BCCH subcell of the transmitter.
Atoll considers the low level to be the accurate source of information. For example: • •
Atoll will automatically update the TRX table if the channel list of a transmitter in the transmitter table is changed. The frequency band of a transmitter cannot be edited.
These redundancies provide some additional features (for example, grouping transmitters according to the frequency bands). On the other hand, there is a chance of mistakes and bugs which may damage a redundancy in the ATL file. Therefore, it is recommended that the audit tool be used from time to time in order to fix these problems (right click the Transmitters folder, choose Audit from the Subcells menu).
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3.1.9
Neighbour Importance Neighbour importance field exists in the neighbour relation tables. It is also available in the AFP and can assist in resolving congestion situations. This is discussed in detail in subsequent chapters.
3.1.10
SeparationConstraints Table It is a separation exceptional-pair table containing pairs of subcells with associated separation requirements. Special separations have a higher priority with respect to all other separations and can be used to relax separation constraints as well.
3.1.11
SeparationRules Table and Rule Priority The SeparationRules table is simple to understand once the order of priority that exists between various separation rules is kept in mind: 1. Highest priority: exceptional pairs 2. Second higher: co-transmitter 3. Third priority: co-site 4. Last priority: Neighbour. For example, if two subcells are neighbours and at the same site, their associated separation requirement will be according to the co-site separation rules. And, if this separation requirement is not fulfilled, their separation violation costs will be weighted by the co-site weight. Separation rules depend on equipment, and refer to the non-hopping configuration. Separation rules are "administration rules" that are set once according to the equipment and are not meant to be modified during routine operations. Separation rules do not depend on whether SFH is available in the network or not. Atoll and the AFP consider SFH independent of the separation rules. If you relax the separation constraints, and have SFH TRXs, this means that you are asking the AFP and Atoll to take into account the effect of SFH twice.
3.1.12
Adjacency Suppression Adjacency suppression is defined as the difference between the required C/I and the required C/A (C/A being the “Carrier to Adjacent Intensity ratio”). By default this is set to 18 dB following the standard. It is available in the Predictions folder properties dialog window under the name “Adjacent channel protection level”. The GSM standard requires this desired behaviour but does not specify any amplification level. It is recommended to be sure that the physical equipment in the network support this value. The value of this parameter is used in the AFP when extracting the interference caused by an adjacent channel, and in Atoll in interference and C/I studies. It might be a good idea to use a safety margin for this parameter and set it to 16 dB, for example.
3.2
AFP Performance Indicators The AFP can be used to generate different AFP performance indicators and listing them in the AFP results window. These performance indicators describe the states of different network entities, such as TRXs, subcells, transmitters and sites.
3.2.1
AFP TRX Rank AFP TRX Rank provides a ranking of the TRXs in a subcell. If a TRX rank is high, it implies that the frequency (channel) corresponding to this TRX has bad usage conditions. TRX ranks indicate the best and worst quality TRXs in each subcell, which maybe candidate GPRS TRXs or potentially removable TRXs to improve overall network quality. The OMC might use rank (or preference) information for better RRM. Notes: •
Rank = 1 is the best rank.
•
TRX Rank is the corresponding field in the TRX table.
As it is during an AFP process that frequencies and MALs/MAIOs for different TRXs of a subcell are chosen, the AFP tool stores and manipulates the information about TRXs in good and in bad conditions. If you choose AFP Rank indicator to be allocated when starting an AFP session, each cost improving solution will go through a TRX rank assignment. If no improving plan is found, TRX rank will be assigned for the initial plan (like BSIC). TRX ranking within a subcell is performed on the basis of TRX costs. In many cases of MAL/MAIO assignment, only one or two of a TRX’s MAIOs violate separation constraints. Therefore, a higher ranking will be assigned to the MAIO violating the separation constraints.
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3.2.1.1
TRX Rank Usage This information can help increase performance in certain cases where a cell and its neighbour are not loaded with traffic at the same time (for example, a stadium and its parking lot). In such cases, it is possible to decrease call blocking by adding the TRXs in bad conditions to the concerned cells. If the BTSs do not recognize TRXs in bad conditions, the overall network behaviour will either be very poor or difficult to predict even if the BTS knows how to track ranking in real time. TRX ranks may be required by the OMC in order to optimise the spectral efficiency. In some networks, a part of the decision-making process at the OMC may be transferred to the BTSs when this information is available. Even if such a “smart” system exists, it might be better to know the TRX ranks in advance to improve predictability and consistent behaviour. Apart from these uses, AFP TRX ranks can be used in post-AFP optimisation. For example, once you perform AFP, you can freeze all TRXs with ranks less than or equal to X. So that a new AFP instance will concentrate on a smaller subset of the most interfered TRXs in the most loaded subcells. A TRX will not be considered frozen for TRX Rank assignment if and only if it is selected for AFP allocation and has not been frozen at Transmitter level or by the AFP launch Wizard.
3.2.2
Total Cost and Separation Violation Cost Component Total cost and separation violation cost component at the TRX, subcell, transmitter and site levels can be computed and displayed as AFP performance indicators. These are the cumulated total costs and the cumulated separation violation costs of each TRX, subcell, transmitter and site. In order to be able to compute and display these results, you must add AFP_COST and AFP_SEP_COST fields (of type SINGLE) to the TRX, Subcells, Transmitters and Sites tables. AFP_COST field and AFP_SEP_COST field correspond to the total cost and separation cost component respectively. These AFP performance indicators are available in the list of AFP performance indicators to be computed available when launching the AFP tool. The AFP cost assignment to the TRXs, subcells, transmitters and sites is carried out at the same time as the TRX rank assignment. Once a frequency plan is committed, the next instance of the AFP can concentrate more on the problematic TRX/subcell/transmitter/site to improve results. Another use of this feature can be to automatically limit the modification scope to the problematic cells/sites. This feature can deliver a significant quality gain.
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Frequency Plan Optimisation
4.1
Step 1 (Optional): Traffic Model Usage It is not possible to solve a difficult optimisation problem without having a traffic model. Following are the 4 principal roles of a traffic model: 1. To reduce the required number of TRXs where they are least needed and spectrum not available. 2. To indicate the least loaded TRXs, since they are less important and interfere less with others TRXs. 3. To reduce the constraint level of the IM (for example where interferences are limited to low density surface). 4. To provide an accurate quality estimation of the resulting frequency plan. The first point is currently implemented through a dimensioning model, which is explained in this chapter. Moreover, it will be available as an AFP option in one of the future versions (i.e. the AFP will optimise the decisions so as not to respect the required number of TRXs. In other words, it will perform a spectrum oriented dimensioning). The second role is carried out by the traffic loads. In order to understand traffic loads better, traffic capture is also described subsequently. The third point is explained alongwith the description of the IM and the last point is detailed in chapter 5. The Atoll traffic model is quite advanced. To gain familiarity with the concepts of user profiles, environments, services, mobile types, terminal types etc. a new user should refer to the Atoll User Manual. These traffic model entities can be used to benefit from all possible capacity gaps in a network. The simplest application here, would be to use a clutter weight oriented model. The more advanced models and techniques of creating traffic maps, based on traffic-by-transmitter etc., are also explained in detail in the Atoll User Manual. This chapter provides with the basic know-how on creating the simplest model using clutter information and clutter weights.
4.1.1
Creating a Traffic Map Based only on Clutter Weighting There are two simple methods: •
Using a raster map -
•
Define a simple user profile for an active user with voice service, speaking 3600s per hour (i.e. consuming 1 Erlang). Create a traffic environment of this kind of user profile with a density of 1 and pedestrian mobility. Any mobility can be used (e.g. 1), as it will be used for calculating IM where only the relative weight matters. Assign appropriate clutter weighting to this traffic environment. In the Geo tab, create a new traffic map based on environments through the GSM GPRS EGPRS Traffic folder context menu. On the drawing toolbar, select the traffic environment created earlier, click the polygon button and draw a polygon surrounding the computation zone. This raster map will appear in the Traffic folder.
Using a vector map -
-
Define a simple user profile for an active user with voice service, speaking 3600s per hour (i.e. consuming 1 Erlang). In the Geo tab, create a new traffic map based on user profiles through the GSM GPRS EGPRS Traffic folder context menu. Select the user profile just created with pedestrian mobility and assign density to the Density field. Assign appropriate clutter weighting. Click the polygon button on the drawing toolbar and draw a polygon surrounding the computation zone. Double-click it and assign a density of 1. This vector map will appear in the Traffic folder.
Both traffic maps are stored in the document and can be exported. An exported vector map is smaller than a raster one.
4.1.2
Performing a Traffic Capture Traffic capture is a means to cumulate one or more traffic maps, for voice and/or data services, for different terminals and provide a spreading of traffic per sector respecting layer priorities, frequency bands and other rules that can be defined by the user. The details of this process are described in 8.4 "Appendix 4: Traffic Capture and Dimensioning". Once traffic analysis is performed, a traffic capture object is available in the Explorer window Data tab. This traffic capture object contains traffic demand per [service, subcell] pair in terms of Erlangs for CS traffic and kbps for PS traffic. This traffic demand provides Atoll with an estimate of average demand in terms of # TSL used. The AFP combines this traffic capture with the number of required TRXs and their timeslot configurations to generate traffic loads (assuming the AFP will create the required number of TRXs indicated in the subcell table). The dimensioning process reads the basic information contained in the traffic capture to find out the number of TRXs needed to support a user defined blocking rate, HR ratio etc. See 8.4 "Appendix 4: Traffic Capture and Dimensioning" for details. The KPI calculations combine traffic capture with the current number of TRXs in the network and their timeslot configurations to generate current traffic loads.
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4.1.3
Keep in mind that the required number of TRXs is the number of TRXs required to carry a given traffic. This is the number of TRXs (usually) calculated through the dimensioning process. The number of existing TRXs is the current actual number to TRXs at a transmitter.
Creating IMs Based on Traffic IM calculation is either based on uniform distribution or on the maps used to perform the default traffic capture. In order to calculate IMs based on a traffic clutter weighting, • • • •
Create the traffic map as described earlier, Perform a traffic capture using only this traffic map, Make this traffic capture the default one, and Select the option "Traffic spreading based on the maps used in the default traffic capture" in the IM calculation dialog.
This option has the following advantages: •
Interference over “hot spots” will have more weight -
•
Example: Sites covering an important highway will interfere over the highway but the interfered surface will be less compared to the coverage. Therefore, not significant if no traffic is used.
Interference over dead spots will not create overhead constraints -
Example: A large hilly park in the middle of a city is often not covered by a dedicated site since it has low traffic. The slopes of this hill are covered by many overlapping cells and tend to create many undesirable IM entries. If the weight of these slopes is reduced due to very little traffic, this can simplify an over-constrained problem.
See 8.3 "Appendix 3: BSIC Allocation" to understand further why traffic loads and interference information are not combined together in Atoll.
4.2
Step 2 (Optional): Neighbour Relations and Relative Weighting In many cases, neighbour relations are the most constraining elements for the AFP. Neighbour importance field of the neighbours table permits the AFP to partially ignore weak / far-away neighbours and concentrate more on the more important neighbours. This section details the use of this new feature in various scenarios.
Tip: Check neighbour allocation before running the AFP. Often a bad neighbour relation definition causes poor frequency plan performance.
4.2.1
Automatic Neighbour Allocation Neighbour importance has two major roles in Atoll: 1. Weighting the neighbour relation in the AFP. 2. Ranking the neighbours so that Atoll can select the n most important neighbours. The configuration presented below is recommended in order to use the resulting neighbour importance in the AFP. • • •
Coverage Factor: Adjacency Factor: Co-site Factor:
1% to 81% 20% to 90% 70% to 100%
Notes: •
The default values for computing importance values are: - Coverage Factor: 1% to 30% - Adjacency Factor: 30% to 60% - Co-site Factor: 60% to 100%
•
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The neighbour allocation algorithm works as in earlier versions with these default values. Changing these values changes the priority definitions of the neighbour allocation algorithm. Refer to the Technical Reference Guide for more details.
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Figure 4.1: Automatic Neighbour Allocation AFP can be launched once the results of the neighbour allocation have been generated and committed. Note: •
4.2.2
In the results generated by Atoll after neighbour allocation, the sum of importance values of all neighbour relationships of a sector is not 1.
Importing Neighbour Importance Various sources of neighbour importance exist: • • •
OMC HO statistics Test mobile data measurements (which ignore interferences between non-neighbours) Other
As with any other source of information, it is the user’s task to prepare and import this external data. The units of the neighbour importance are probabilities and are expected to reamin between 0 and 1.
4.2.3
Extending Existing Neighbour Relations Extending an existing neighbour relation should be performed often either to solve some HO problems or because of addition of new sites. Such operations usually imply that a fresh frequency allocation be carried out. The AFP would be required to use the original neighbour relations as well as the new additional neighbours, yet in a different way (with a different weight). In addition, the AFP would require access to the former (complete or partial) source of neighbour importance as well as to the new values of neighbour importance calculated for the recently added relations. The neighbour importance of the original neighbour assignment is probably more reliable than the one calculated using path loss calculations. This section explains how this can be done: 1. Export the current neighbour relation into a file called AllCurrentNei.txt using the generic export feature available through the context menu of the table, 2. Export all the relations for AllCurrentNei_Importance.txt,
which
there
are
reliable
neighbour
importance
into
a
file
named
3. Import the file AllCurrentNei.txt into the neighbour exceptional pairs so that the existing neighbour allocation is forced (usual operation for extending an existing allocation), 4. Run automatic neighbour allocation in order to extend your neighbour relations and/or assign importance where it was not already assigned. To keep important values lower than X%, all Max% values in the importance part of the dialog should be kept less than X. For example, if X is 50%, the configuration shown below can be used,
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Figure 4.2: Automatic Neighbour Allocation Results As can be observed in the figure above, all new neighbour relations have weak importance values. 5. Commit the allocation, 6. Import the file, and answer “no” if asked to remove neighbours of modified transmitters. The screenshot below shows that the neighbour relations now comprise old neighbours with a higher importance and new neighbours with a lower importance automatically calculated by Atoll.
Figure 4.3: Neighbours Table
4.2.4
Importing Partial Sources of Neighbour Importance Atoll’s generic import feature can be used to import the data easily. In order to import, the user should know the location to place this imported data (Importance column of the Neighbours table) and the data units (probabilities between 0 and 1). If your network statistics do not provide you directly with the importance of neighbours, you can calculate neighbour importance from other statistics. Then, this calculated importance can be imported to Atoll and provided to the AFP as input. For example, if you have statistics about the number of handovers between two sectors, you can calculate the importance of the different neighbours of each cell from these statistics. Consider two sectors, A and B. Let X be the "Average Activity of a Relationship" in the network, i.e., the sum of all handovers of all the sectors divided by the number of neighbour rela-
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Chapter 4: Frequency Plan Optimisation tionships. If the number of handovers from sector B (neighbour of sector A) is Y, the importance of sector B for sector A can be given by: ⎧ 1 If Y ≥ X ⎪ Impor tan ce = ⎨ Y ⎪ ---- If Y < X ⎩ X In this way, when a relationship has more than the average number of handovers, its importance will be the highest it can be in Atoll, i.e., 100%. Otherwise, the importance will be less than the average.
4.3
Step 3 (Optional): Using Dimensioning The Atoll dimensioning model, combined with the traffic capture, is a strong tool for frequency plan optimisation. In most cases, where a spectrum problem exists and the problem does not originate from the neighbour relation, the second most important task is to reduce the number of required TRXs in a selective and careful way. This optimisation can currently be carried out with the help of the dimensioning model. In future versions, it may be available directly through the AFP.
4.3.1
Optimal Dimensioning of an Existing Network 1. Run the AFP and commit the resulting frequency plan. Proceed to the next step if this frequency plan is not satisfactory and the TRX demands have to be reduced. 2. Increase the service blocking rates (from 2% to 4% for example). The screenshot below shows: -
Where this can be done (Services table). That the dimensioning model is based on blocking. The effect this change has on the required number of TRXs (the number of existing TRXs being the previous number of required TRXs for 2% blocking rate committed in Step 1).
Figure 4.4: Dimensioning Process 3. Recalculate the traffic capture since service definitions have changed and then launch dimensioning. Some transmitters will have less required TRXs while others, which were more loaded, have the same number of required TRXs as before.
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AFP Reference Guide Running the AFP once more can return an improved frequency plan, as the following example shows:
Action performed
AFP cost for the empty network, indicating the number of weighted Erlangs
Sum of Number of Required TRXs
Original network
603.6
100
106
9.7
0
After increasing the Blocking Probability to 4% and dimensioning
601.4
95
96.6
9.4
0
Separation Interference Violation Cost Cost
Missing TRX Erlangs
The above experiment proves that the capacity difference between the two networks is very low (first column, around 2 Erlangs). This means that the reduction of 5 TRXs leads to a very minor decrease in capacity. This is due to the fact that this was done by dimensioning considerations rather than other possible considerations. The AFP generates a better plan after this decrease in the number of TRXs. The AFP cost units are Erlangs, therefore, we can compare the 2 Erlangs lost because of capacity to the 10 Erlangs gained because of better AFP cost. Note: •
It is possible to set a “Maximum number of TRXs” in the Transmitters table. You can copy and paste the current demand to this column, thus forcing the dimensioning process to respect the current state of the network as an upper bound. This possibility is a handy in all possible cases of difficult frequency allocation.
4.4
Step 4: Optimal Usage of the Atoll AFP
4.4.1
Introduction to the AFP Cost Function
4.4.1.1
Combination of Separation Violation and Interference Probabilities The cost function of Atoll AFP has two main components. The first component is the cost for violations of separation constraints and the second component is the cost for creating interference. Atoll AFP gives each separation violation a cost equivalent to a certain amount of interference, making it possible to sum both costs and minimize their sum. For example, the user can define that a separation violation of 1 costs the same as x% of interfered traffic. This is weighted by the type of violation (co-transmitter separation violations have higher impact than neighbour separation violations). Through this equivalence, It is possible to sum separation violation and interference costs that share a common unit, i.e. percentage of interfered traffic. Following this principle, all other cost elements are also calculated in the same manner, the cost of Missing TRXs, the cost of corrupted TRXs, the cost of a TRX assigned out-of-domain frequencies and the cost of changing a TRX’s assignment.
4.4.1.2
Counting TRXs (Nodes) Instead of Relations (Edges) In the following example, each separation violation represents an edge and each TRX a node. All the 3 frequency plans proposed in this example do not respect all separation requirements for all TRXs, meaning that they all have bad nodes and bad edges. Now the question is that whether the AFP minimization target should try to minimize the number of bad edges or the number of bad nodes. Example: • •
Imagine a network with 6 TRXs, all having a separation constraint of 1 with each other (i.e. 6 nodes, 15 Edges). The following 3 cases demonstrate the way the AFP calculates the cost of an allocation.
Case 1
Case 2
Case 3
F1 is used 4 times, F2 and F3 are used one time each.
F1 is used 3 times, F2 twice, and F3 is only used one time.
F1, F2, and F3 are used two times each.
Number of separation violations is 6 (6 bad edges)
Number of separation violation is 4 (4 bad edges)
Number of separation violations is 3 (3 bad edges)
Two TRXs have good assignments
Only one TRX has a good assignment
No TRX has a good assignment
The spectrum is not equally used
•
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The spectrum is equally used
Atoll AFP prefers Case 1 by default. Nevertheless, it can be configured to opt for Case 3.
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Chapter 4: Frequency Plan Optimisation The parameters that control the capability of Atoll AFP to be more Edge-oriented than Node-oriented are explained next. But, before this explanation, following are the three main advantages of the Node-oriented approach: a. The cost function has meaningful units, i.e. Interfered Erlangs. b. The ability to focus problems on a TRX that is already 100% interfered and to improve the others instead of spreading interference on several TRXs. c. The capability to respect a TRX based quality target, i.e. dismissing interference at a TRX that does not sum up to a certain considerable value (explained below). The Node-oriented approach is an important feature of the Atoll AFP and provides a tighter correspondence between the AFP cost and the network quality.
4.4.1.3
Each TRX Cost The AFP cost function is summed up for each TRX according to the following logic: • • • • •
If TRX α is corrupted, the tax of being corrupted is added to the cost, and multiplied by T(α). If TRX α is missing (the required number of TRXs and the actual number of TRXs being different), the tax of missing TRX is added to the cost, and multiplied by T(α). If TRX α has out-of-domain frequencies assigned to it, the tax of out-of-domain frequency assignment is added to the cost, and multiplied by T(α). Otherwise, the separation cost, the interference cost and the changing load of this TRX are summed up (probabilistically) and added to the cost, and multiplied by T(α). If this sum is very small, it is discarded (see section 4.4.1.10 "Quality Target")
Here, T(α) is an estimation of the traffic Erlangs using TRX α weighted by the AFP weight for this TRX. The user can fully control the AFP cost target by determining the value of the cost function parameters. Some of these parameters belong to the data model, e.g. “Maximum MAL Length” and “Minimum C/I”, while others are present in the specifc AFP GUI. Appendix 2 explains how to find each of these parameters. Note: •
4.4.1.4
The AFP cost is the cost of the entire loaded network, not only the cost of the selected or non-frozen TRXs. In many cases, the AFP is authorized to change only a part of the network. Therefore, the part of the cost corresponding to the non-frozen part of the network and the part of the cost corresponding to the frozen part of the network are indicated.
Separation Violation Cost In this section, interference cost is ignored in order to understand the separation violation cost. A TRX having only one separation violation is considered. Let Sij denote the required separation between two transmitters. If f1 is assigned at i and f2 at j such that ( f 1 – f 2 ) < S ij , this means that the separation constraint is not satisfied. Separation constraints can be violated strongly or weakly: •
For example, the pair of frequencies (1, 2) violates a separation requirement of 3. The pair of frequencies 1 and 3 violate this requirement as well but is still a better solution than (1, 2) and, therefore, should have a lower cost.
Frequencies that are part of a MAL with a low fractional load and that disobey a separation constraint, should not be weighted the same as in non-hopping separation violation. In fact, the separation component is weighted by the burst collision probability, which is the multiplication of the victim’s fractional load and the interferer’s fractional load.
Figure 4.5: Atoll AFP Module Properties - Separation Weights Tab
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AFP Reference Guide Example: Let us consider the following simple case of a network comprising two TRXs in the same cell. The first, TRXi, has a MAL denoted as MALi. It is interfered by TRXk having MALk. TRXi and TRXk have a separation requirement of 2. Their MAL lengths are respectively 5 and 4. Unfortunately, one of their frequencies is the same (i.e. separation = 0), while all other frequencies are correct. For the case of a co-channel violation when the required separation is 2, the cost of the separation violation is 90%, as shown in the dialog above. Since only one channel of each TRX causes interference, with length(MALi)=5 and length(MALk)=4, the collision probability is 1/20. Hence, the cost to consider is divided by 20, i.e. 90/20 = 4.5% for each TRX. In addition, since the two TRXs have different MAL lengths, they have different interferer diversity gains: a gain of 1.4 for MAL length of 5 and a gain of 1.2 for MAL length of 4 (see 8.2 "Appendix 2: Interferences" for details). Applying interference diversity gain of 1.4 dB means that the cost will be divided by the value: 10 this will give 4.5% / 1.38 = 3.25%.
( 1.4 ⁄ 10 )
≈ 1.38 . For TRXi,
1 90 For TRXk, the cost to consider will be ------ -----------------------= 3.41% . 20 10 ( 1.2 ⁄ 10 ) Now, in order to get the exact contribution to the separation cost component, these values are multipilies by the traffic load (Erlangs / timeslot) and by the number of traffic carrier timeslots in each TRX. Assuming the traffic load to be 1 and that each TRX has 8 traffic carrier timeslots, we will get (8 x 3.25 + 8 x 3.41), i.e. about 0.5 Erlangs for the two TRXs together. Note: •
4.4.1.5
In this example, the AFP weight was assumed to be 1, the traffic loads were assumed to be 1, no DTX was involved, no other interference or violation was combined with the above, the global separation cost was set to be 1, and the co-transmitter separation weight was set to 1 as well.
Interference Cost Traffic on a TRX will be interfered if and only if co/adjacent-channel reuse exists within interfering transmitters. Each such reuse will reduce the amount of good traffic and increase the interference cost. It will be weighted by the global interference weighting factor, and will take into account the burst collision probability in the same way as in the example above. For more information, see section 8.1.3.2 "Interference Cost Component".
4.4.1.6
Probabilistic Cost Combination Assume that TRX α is subject to a separation violation causing a cost of 30% of T(α) (T(α) = Traffic of TRX α) and in addition, a co/adjacent-channel reuse causing this TRX to be 40% interfered. This section explains how these two events are summed together. The solution to this problem is provided by a probabilistic approach: all different costs are considered as bad events and are combined as if these events were independent. The probabilities of events in this example are p(Violation) = 0.3 and p(Interference) = 0.4. The cost of the two together is given by: 1 – ( 1 – p ( Violation ) ) × ( 1 – p ( Interference ) ) = 0.58 or 58% Let P1, P2, ….Pn be the violation probability costs of the given TRX (one for each of its n violations). Let Pn+1, Pn+2, ….Pm be the interference probability costs of the given TRX (one for each of its (m-n) interferences). Let Pm+1 be the “changing TRX cost” described below. n
⎛ ⎞ The separation cost of this TRX will therefore be: ⎜ 1 – ( 1 – P i )⎟ ⎜ ⎟ ⎝ ⎠ i=1
∏
⎛ ⎜ The additional cost of this TRX will be ⎜ 1 – ⎝
m+1
n
∏
∏
⎞ ⎛ ⎞ ( 1 – P i )⎟ – ⎜ 1 – ( 1 – P i )⎟ ⎟ ⎜ ⎟ ⎠ ⎝ ⎠ i=1 i=1
The interference cost uses the “min C/I” value, defined at subcell level, for which it may have precise pair-wise interference information. It may apply various gains to this C/I quality target due to frequency hopping and/or DTX.
4.4.1.7
Missing TRX Cost It is easy to have a 0-cost solution if the required number of TRXs criterion is not fulfilled (by removing all TRXs, for example). This is the main purpose of missing TRX cost. By default, the exact traffic that a missing TRX was supposed to carry will be counted by the cost function. However, the user can increase this tax (to 200% for example) if needed. In certain cases, creating more TRXs will not only generate interferences for the newly assigned TRXs but also for other TRXs that, otherwise, would have correct assignments. This parameter enables the user to tune the AFP in its tradeoffs between respecting the number of required TRXs and optimising quality.
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4.4.1.8
The Atoll AFP always assigns as many TRXs as the “Required TRXs” field indicates. The user can only decide regarding the better plan, i.e. the previous plan having a better quality but some missing TRXs or the new plan having lower quality but having all required TRXs assigned. In future versions, the AFP will be capable of optimising these decisions as well.
Corrupted TRX Cost It is easy to generate a 0-cost solution if frequency domain constraints are not satisfied (by putting an arbitrary channel, for example). This cost component exists in order to avoid this behaviour. By default, 1000% of the traffic that a corrupted TRX is supposed to carry will be considered impaired. In some cases, fixing the assignment on a group of corrupted TRXs will not only result in an interfered assignment for these TRXs but also for many other TRXs that, otherwise, would have correct assignments. It is for this reason that this tax is higher than 100%. Note: •
A TRX is corrupted not only if it contains frequencies that are out of domain but also when: - An NH TRX has a MAL with more than one frequency - A TRX has no channel at all - A group constrained SFH TRX is assigned a MAL that is not strictly a group of its domain - An SFH TRX has no MAIO, or no HSN, or has an out of domain HSN / MAIO value.
In future versions, different corruption reasons will have different tax levels and the assignment of a corrupted TRX will not always be ignored.
4.4.1.9
Out-of-domain Frequency Assignment Cost TRXs may have proper ARFCN assigned yet may not comply with the frequency domain definition. In many cases, the frequency domain limitations are not fully respected in the surroundings of the zone considered for frequency planning. This cost component can not be ignored or be modelled by the corrupted TRX cost because such a corrupted TRX can interfere and be interfered, which will be ignored in both the cases. If a TRX is assigned out-of-domain frequencies (channels) but has correct ARFCNs, it will have dual influence on the cost: 1. The normal cost of interference, separation and/or modification. 2. An additional cost of having out-of-domain channels, multiplied by the number of frequencies out of domain and divided by the MAL length. Note: •
4.4.1.10
If SFH and group constrained subcells have out-of-domain channels, and are frozen, the frozen TRX will be ignored altogether.
Quality Target It is often required to handle small and large amounts of interference in different manners. For example, an operator might prefer to have 10 transmitters with 2% interfered traffic in each, rather than to have 2 transmitters with 10% interfered traffic in each. The Global Cost section of the Atoll AFP properties dialog window’s Cost tab provides an option to dismiss interference and separation costs that do not sum up to the value of the parameter "% Max Interference" defined in the Subcells table for each subcell. TRXs having less percentage of interference than that defined in the "% Max Interference" are considered to have 0 interference and are excluded from the cost. This feature can be used to distribute the interferences equally among some transmitters in stead of having a few with very low interferences and others with high interferences.
4.4.1.11
Modifiable and Non-Modifiable Costs Interfered Erlangs or separation constraint violations between frozen TRXs can not be resolved by the AFP. The AFP can only reduce the non-frozen cost, which is the modifiable cost. Modifiable and non-modifiable parts of the total netwok cost are linked in concept with the definition of the AFP scope. See section 2.3 "Definition of the AFP Scope" for more information. Four groups of transmitters can be defined with respect to AFP: • • • •
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ALL = All the transmitters in the project. NET = Active transmitters that pass the filters on the main Transmitters folder and on the main Sites folder. SEL = Transmitters belonging to the (sub)folder for which the AFP was launched and that are located inside the focus zone. RING = Transmitters belonging to NET, not belonging to SEL and having some relationship with the transmitters in SEL: - If interferences are to be taken into account, all transmitters whose calculation radii intersect the calculation radius of any transmitter in SEL will be included in RING. For large calculation radii (20 km for example), a single site can have a very large RING loaded. AT262_ARG_E1
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Neighbours are always included in RING. If one transmitter of an Exceptional Pair is included in SEL and the other is not, then the other will be included in RING. If BSIC assignment is required, then all the second order neighbours (neighbours of neighbours) will be included in RING.
The total cost of the network corresponds to SEL+RING. It contains the modifiable as well as the non-modifiable parts of the network costs. The modifiable part of the total cost of the network corresponds to SEL. However, this cost does not include costs corresponding to frozen entities of transmitters in SEL. The non-modifiable part of the total cost of the network corresponds to RING. It includes the costs corresponding to frozen entities of transmitters in SEL. In each instance of an AFP process, there might be entities frozen by the user. In addition to the generic freezing options, there are finer freezing options available in the data structure: 1. Individual transmitters can be frozen for channel (and MAL), HSN and/or BSIC assignment. 2. Individual TRX’s can be frozen for channel (and MAL) assignment.
4.4.2
Most Important Cost Function Parameters and Tuning It is strongly advised that the user should fully understand the different cost function parameters before manipulating them. In order to understand the parameters that are not explained in this section, please refer to sections 8.1 "Appendix 1: Description of the AFP Cost Function" and 8.3 "Appendix 3: BSIC Allocation". The figure below depicts the Cost tab of the AFP properties dialog:
Figure 4.6: Atoll AFP Module Properties - Cost Tab
4.4.2.1
Interference Weight vs. Separation Weight The most important parameters are the ones outlined by rectangles in the figure above. Interference and separation weights are used as multiplicative factors before each interference or violation event. Therefore, these parameters have the ability to reduce one type of event cost compared to the other. If these two parameters are set to low values (for example, 0.1 and 0.035 respectively), AFP will be forced to work according to the edge-oriented strategy, which is probably not the recommended approach. By default, interference events are less important than separation violation events.
4.4.2.2
Cost of Changing a TRX The second most important parameter (also outlined with a rectangle in the figure above), is the cost of modifying a TRX. This is recommended to be used if the non-frozen part of the network should be changed as little as possible. The following experiment shows the effects using this parameter can have:
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Test case: A network with 90 transmitters in total, 15 frozen transmitters and sum of required TRXs = 257. Only 193 good TRXs were already allocated. 64 TRXs should be created / newly allocated with as little influence as possible on the other 193 TRXs. For a cost of changing a TRX = 1
AFP changed only 98 TRXS
For a cost of changing a TRX = 0.3
AFP changed only 129 TRXS
For a cost of changing a TRX = 0.1
AFP changed only 139 TRXS
For a cost of changing a TRX = 0
AFP changed 162 TRXS
4.4.2.3
Quality Target and C/I Weighting
4.4.2.3.1
Quality Target The Global Cost section (outlined with a rectangle in the figure above) lets you define the quality target mechanism for the AFP cost. 1. If you select "Do not include the cost of TRXs having reached their quality targets (% Max Interference)", Atoll will not take into account the cost of those TRXs which have less % of interference than that defined in their corresponding "% Max Interference" in the Subcells table. The total cost will only include the costs of TRXs whose interference is still higher than this threshold value. 2. If you select "Take into account the cost of all TRXs", Atoll will consider the cost of all the TRXs ignoring whether they have reached their quality targets, defined in the "% Max Interference" in the Subcells table, or not. For example, consider a frequency plan with only one interfered TRX with 10% interfered traffic, and another frequency plan with 10 interfered TRXs with 1.5% interfered traffic in each. Assume that you have set the "% Max Interference" for all the TRXs in both the cases to 2%. If you choose the 1st option in the AFP global cost settings, Atoll will prefer the solution with 10 interfered TRXs with 1.5% interfered traffic rather than having 1 interfered TRX with 10% interfered traffic.
4.4.2.3.2
C/I Weighting When the C/I weighting option (the bottom rectangle), related to the quality threshold, is used, the AFP takes into account that the traffic having close-to-threshold C/I conditions is neither 100% satisfactory nor 100% corrupted.
Figure 4.7: C/I Weighting In this way, safety margins on the threshold C/I conditions can be avoided. Therefore, the user must specify a margin around which a “slope” is created, as illustrated in the figure above. This figure corresponds to an interference relation between two TRXs. It describes the distribution of traffic according to C/I conditions. It depicts the effect of 3 different quality requirements on the interference cost of a co-channel frequency reuse. It can be observed that, when a low quality (C/I > 8 dB) is required, less traffic is considered as interfered than for a high quality (C/I > 11 dB). The option ‘C/I >10 dB + 2dB margin’ has the advantage of not being too strict on one hand, and yet trying to achieve high quality if possible. It is visible from the above figure how it integrates the different traffic classes into the interference cost.
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4.4.2.4
Separation Weights Settings The Separation Weights tab enables the user to define relative weights of different separation types with respect to each other and of one co-channel violation with respect to an adjacent channel violation and so on. The figure below shows the Separation Weights tab of the AFP properties dialog.
Figure 4.8: Atoll AFP Module Properties - Separation Weights Tab Other tabs of the AFP module properties dialog are more advanced. Please refer to the Atoll User Manual for more information on AFP module properties tabs.
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Chapter 5 Means to Evaluate Frequency Plans
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Chapter 5: Means to Evaluate Frequency Plans
5
Means to Evaluate Frequency Plans
5.1
Estimating Frequency Plan Quality
5.1.1
Using Interference Studies The full description of the interference study features is outside the scope of this document. Nonetheless, it is important to understand that a geographically based interference study has an important advantage over any simulation based on pair-wise interference matrices (or histograms). Atoll interference studies are geographic studies that analyse each point on the map. For each point, an interference study estimates the carrier power and then sums up the interference powers taking burst collision probabilities, DTX, and traffic load into account. As the AFP works with interference matrices, it is limited to coarse estimations of interference combination and loses knowledge of the geographical location of interference events. It is due to this reason that the interference study output is much more accurate than the AFP cost.
Figure 5.1: Interference Study Report The column “Erlangs (based on traffic load)” is available in the interference study report as seen above. This column uses a traffic model similar to the one used by the AFP: • •
It spreads the traffic of each subcell (#TRX x traffic load x # timeslots) on the service zone of the subcell. Then, it sums up the interfered traffic (in Erlangs) of each interfered TRX.
This means that it is a TRX based estimation of interference and is much more accurate than any other tool available in Atoll. In order to be able to use this option, you must check the “Detailed results” option when specifying conditions for an interference study.
5.1.1.1
Various Interference Studies Following are the main differences between a TRX based interference study and a transmitter level worst case interference study.
5.1.1.1.1
TRX Based Interference Study If there are more than one TRXs per cell, a “TRX based” frequency plan analysis is usually required. This can be performed in Atoll using the “Detailed results” option of an “Interfered zones” study. The following figures depict this function:
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Figure 5.2: TRX Based Interference Studies For each TRX, Atoll retains a map of all the pixels that do not comply with the quality threshold (one map for each TRX). The threshold can be global or dedicated to each subcell (from the Subcells table). The maps can be visualised by the user and contribute to the overall statistics. The column mentioned earlier weighs the “bad” surface of each TRX map by the traffic carried by the TRX as seen by the AFP. Therefore, it is the most appropriate tool for frequency plan interference evaluation.
5.1.1.1.2
Worst Case Interference Study A Worst Case Interference study allows generating the Worst C/I (I being the worst interferer) per pixel or per serving transmitter at pixel. Statistics on these maps are available with or without traffic weighting. The principal drawback is that pixels are coloured in both cases: when 1 out of n TRXs is interfered and when n out of n TRXs are interfered. Moreover, there is no information about TRXs (channels or frequency) responsible of these worst C/I values.
5.1.1.2
Visualising TRX Ranks with a TRX Based Interference Study You can also visualise and compare the AFP TRX ranks with the results of a detailed interference study. In the interference study report table, as shown in the earlier figure, the “Erlangs (based on traffic load)” column should rank the TRXs in the same order as the AFP TRX ranks. Make sure that the AFP cost is based on interferences and not separation constraints. In case of large networks, where it might be easier to compare these results using MS-Excel, you might not get a 100% match between these results and small variations in the order may exist.
5.1.1.3
Visualising C/I Distributions with a TRX Based Interference Study The figure above and the example below show how to get very detailed information about the various C/I conditions at TRX level. The different information seen in this screen-shot is part of the report obtained by creating several instances of an interfered zone study. The interfered zone study whose properties dialog is shown, is the closest to the default configuration. Its interference definition references the subcell quality threshold. In the two other interfered zones studies, the global threshold for the minimum and maximum TRX C/I to be included in each TRX’s map were fixed at the reference values. These studies show that there are more “weak interferences” (11.3 Erlangs) than “strong interferences” (6.3 Erlangs). The “weak interferences” being 8 < C/I <= 12 and the strong interference being C/I <= 8.
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Figure 5.3: TRX Based Interference Study - C/I Distributions
5.1.2
Using Audit The consistency checking tool gives precise information about separation constraint violations: • • • •
Number of constraints violated Types of constraints violated (co-transmitter, co-site, neighbour, special) Transmitters and TRXs affected Synchronisation, HSN, MAL and MAIO are taken in to account.
The consistency check tool is described in detail in the Atoll User Manual. A quantified summary of separation constraint violations is available. It makes the evaluation of a frequency plan much easier than before. The section below demonstrates how this evaluation can be carried out.
5.1.2.1
Global Separation Fitness Expression It is often interesting to quantify the “amount” of separation violation in a frequency plan. The difficulty in this is that many different cases and subcases exist. It is not possible to simply count the violating pairs since the SFH violation with low fractional reuse will affect the count. Another complexity is the difference between co-channel and adjacent-channel violations. Therefore, a Separation Fitness Expression that tries to assign relative weights to different types of separation violations is defined. It is composed of 7 separate summations (N1 through N7), each assigned its respective weight. It is called Forsk Independent Separation Fitness Expression (FISFE) and it is described below. The FISFE expression is part of our open architecture AFP strategy. There are more than one AFP integrated with the Atoll platform and, therefore, it is interesting to know the frequency plan FISFE values. The answer must be independent of the AFP used to generate the plan.
5.1.2.1.1
Forsk Independent Separation Fitness Expression (FISFE) FISFE ( FP ) = 7.5 × { 0.8 ×N1 + 0.3 × N2 + 0.2 × N3 + 0.1 × N4 + 0.07 × ( N5 – N1 – N2 – N3 – N4 ) + 0.3 × N6 + 0.15 × N7 } The value “7.5” is arbitrary. It is used to upscale the FISFE(FP) value to “close to Erlang” units. On the other hand, the weights specified for each component reflect the idea of their relative importance. N1 through N7 are now available in the summary of the consistency checking tool.
5.1.2.1.2
Main Separation Violation Item Summary •
© Forsk 2007
N1 is the number of TRXs subject to a co-channel reuse violation where the separation violation concerns two non SFH TRXs.
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N2 is the additional number of TRXs subject to an adjacent-channel reuse violation where the separation violation concerns two non SFH TRXs. N3 is the additional number of TRXs subject to a co-channel reuse violation where the separation violation concerns two TRX where at least one is SFH. N4 is the additional number of TRXs subject to an adjacent-channel reuse violation where the separation violation concerns two TRX where at least one is SFH. Notes:
• •
•
If a TRX is counted in N1 it will not be counted in N2.
•
If a TRX is counted in N1 or N2 it will not be counted in N3.
•
If a TRX is counted in N1, N2 or N3 it will not be counted in N4.
N5 is the number of TRXs subject to whatever separation violation that exists. N6 and N7 correspond to a different counting system, the pair-wise counting system. N6 and N7 are only used in cases where the fractional load is less than 1, which means that at least one of the TRXs in the pair is SFH. The pair-wise violation counting is less important than TRX counting and therefore has a low weight coefficient. On the other hand, it cannot be ignored, since in 1/1 SFH plans, all TCH TRXs will have violations with neighbours and yet it would be required to minimize the volume of these violations. N6 and N7 are defined as following:
• •
N6 is summed over all pairs subject to a co-channel reuse violation, where at least one TRX in the pair is SFH and where the probability of a burst having a co-channel violation is summed. N7 is summed over all pairs subject to an adjacent-channel reuse violation, where at least one TRX in the pair is SFH and where the probability of a burst having an adjacent-channel violation is summed.
In the following example, the values of N1 through N7 are {0, 23, 62, 35, 123, 7.695, 2}
Figure 5.4: Event Viewer Messages
Figure 5.5: Event Viewer Message 1
Figure 5.6: Event Viewer Message 2
5.2
Using Point Analysis It is often useful to know what exactly causes interference conditions at a point. This is one of the important roles of the point analysis tool. Yet because of it’s complexity, some users are afraid to use it, which is a pity. The point analysis is complicated only because it is a very rich tool. It provides the user with the information of how are the interferers of a TRX at a point, what are the different gains (power offsets, burst collision probability, DTX, adjacency suppression), and how do the different components combine to a “total interference”.
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5.2.1
Example 1: Combination of Interference Effects This figure depicts the case where two adjacent interferences are combined to create total interference (the gain value (the empty part – 18 dB) shows that they are adjacent). For each of the two adjacent interferers, C/I > 11 dB, while for their combination, the total interference, C/I < 11 dB. This example demonstrates the fact that geographic interference combination is more accurate than the interference cost of the AFP. Assuming the required quality to be 11 dB, this specific point would not contribute to the AFP cost, while it would be considered as interfered in the interference study.
Figure 5.7: Combinatin of Interference Effects
5.2.2
Example 2: Counting Strong Interference Only Once In this case, two strong interferences are combined to create an extra strong total interference. C/I is very weak for both interferers. Therefore, the point under analysis contributes to both IM entries, which are considered in the AFP cost. This example demonstrates the fact that geographic interference combination is more accurate than the interference cost of the AFP because of counting this point only once as an interfered point (and not twice as in the AFP).
Figure 5.8: Counting Strong Interference Only Once
5.3
Uniform Frequency Usage Distribution A frequency distribution analysis tool is available. This feature is accessible through the Frequency Distribution command from the Frequency Plan menu in the Transmitters folder context menu. The output of this tool consists of a 3 column table listing, • • •
© Forsk 2007
The ARFCN, The number of TRXs in which the ARFCN appears Its relative load.
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5.3.1
When Uniform Distribution and Quality do not Coincide While it is clear that in some cases the frequency usage distribution can be a quality indicator, it is not always the case. The following two examples prove the same. Therefore, a cost has not been dedicated to non-uniformity of spectral use in Atoll and thus marking it an “objective” of the AFP. As a result Atoll AFP will create non-uniform frequency distributions in the following cases: 1. When the FAP is easy, the AFP reaches a 0-cost solution and stops immediately. If it was instructed to use the minimum possible spectrum, the AFP will use the smaller ARFCNs more than the larger ones (and will leave the largest ARFCNs untouched, for future use). Otherwise, the AFP will try to spread the spectrum usage. By default this directive is free for AFP tuning. Therefore, the AFP will not create a uniform frequency usage distribution when the FAP is easy. 2. If the FAP is somewhat difficult, the frequency usage distribution will be somewhat non-uniform. In order for it to be uniform, the corresponding directive must be set in the Spectrum tab of the AFP properties dialog. 3. If the FAP is difficult, all frequencies will be used and the allocation heuristics will result in a balanced allocation. Lastly, when the FAP is extremely difficult (many separation violations, for example), an unbalanced allocation may result because of the reason explained in chapter 7.1, concerning the fact that the AFP cost is a TRX (node) based cost. 4. AFP can also create non-uniform frequency distributions due to the domain range effects described below.
Advice: Do not expect a balanced distribution of frequencies in every case. Sometimes the best solution requires an unbalanced assignment.
5.3.1.1
Domain Range Effect and Adjacent Constraints Other sources of unbalanced frequency distribution are the domain range limits. The first and the last frequencies in a domain have lesser separation constraints related to them. Consequently, they are used more than other frequencies. The second and the second last are used less since frequencies adjacent to them are used more and so on. This effect is stronger for more regular networks. If the domain is small, if ((first – last) / 2) is an integer and if the network is regular, the AFP may hardly use a part of the spectrum. This will happen if and only if there are many adjacent constraints. The following example can be interesting: In a domain of 1 to 7 in a typical hexagon network with 6 neighbours for each transmitter, one solution can be a 1/4 pattern using only the frequencies 1, 3, 5 and 7, while 2, 4 and 6 will not be used at all. Another solution for the same can be a 1/ 7 pattern. Although, if the separation constraints between neighbours are 2, the 1/4 pattern (based on odd number frequencies only) will give a better result.
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6
Advanced Topics and Troubleshooting
6.1
Various AFP Related Features
6.1.1
SFH (HSN, MAL, MAIO) Atoll AFP is capable of performing free MAL assignment and predefined MAL assignment both. The instruction indicating the assignment mode to be used is at subcell level. In free assignment mode, the AFP is free to assign any MAL that satisfies the requirements of the TRXs. The length of MAL, the HSNs and the MAIOs are assigned in compliance with the user’s directives. If the assignment mode is group constrained, the AFP can assign one of the predefined groups in the domain. The success of the assignment depends on the definition of groups in the domain. Each MAL length, represented by one or more groups in the domain, is supposed to fulfil the conditions: • •
that there must be a number of other groups having the same length, and that on the whole they cover the domain as much as possible.
For example, a domain containing groups of lengths 3, 5 and 8 will be a badly planned domain if there are many groups of length 3, many of length 8 and only one of length 5. If restricted to such a domain, the AFP will not produce an optimum plan. On the other hand, by adding a few more groups of length 5, the quality can be much improved. Another solution could be to simply remove the single group of length 5. When many groups are defined, the quality is almost as good as with free assignment. Notes:
6.1.2
•
Currently, the AFP always assigns the same MAL to all TRXs within a subcell.
•
The “group constrained” assignment mode is applicable for SFH only. In NH and BBH, the assignment mode is always free.
Definition of Atom An atom is a set of synchronised subcells that share the same HSN, the same frequency domain and have the same length MAL. The MAIO assignment of an atom manages the frequency collisions between the MALs of the atom. If an atom contains more than one subcell, the AFP may assign to it partially different MALs (depending on a user-definable option) but it will always consider the fact that the subcells are synchronised. Atoms can be determined by the user or by the AFP via the HSN allocation. Some restrictions on this definition exist due to some extreme cases: 1. If two subcells have different domains, they cannot belong to the same atom. 2. If two subcells have different limitations on “Max MAL Length”, they cannot belong to the same atom. A warning is generated when HSN assignment directives contradict with these restrictions. An important feature is the possibility to force the AFP to always assign the same MAL among subcells of an atom. Furthermore, improved results can be obtained by post-relaxation of this constraint, performed on a carefully selected and small subset of transmitters.
6.1.3
Synchronous Networks Through working at atom level, and consulting a user defined synchronisation reference given in the subcell table, the AFP can fully exploit the benefits of synchronisation in a GSM network. It is capable of extending Atoms beyond the limit of a site and, by doing so, using the MAIO assignment to further resolve violations or interference.
6.1.4
Optimising Hopping Gains If the AFP has a certain degree of freedom when choosing MAL lengths, it may opt for longer MAL lengths. In this way, it can profit more from the hopping gains. On the other hand, it may be increasingly hard to find frequencies for these MALs.
6.1.5
Fractional Load Both HSN assignment and MAL length determination processes are tuned to obtain a user defined fractional load. A fractional load of δ is obtained if the number of TRXs using a certain MAL is only δ times the size of the MAL. Atoll’s notion of fractional load does not require the traffic load to be taken into account. Since fractional load cannot always be obtained, this parameter is considered as a guide rather than a constraint. When it can be obtained, AFP chooses either a MAL length 1/δ times longer than the number of TRXs in the biggest subcell of the atom or a MAL length 1/δ times longer than the sum of all TRXs in the atom. These are called “the short MAL strategy” and “the long MAL strategy” respectively. You can choose between the two in the MAL tab of the properties dialog. The value of the fractional load parameter can also be edited and, furthermore, it can even be automatically calibrated by the AFP.
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6.1.6
Fractional load is 1 for Baseband hopping.
Domain Use Ratio Both HSN assignment and MAL length determination processes are tuned to avoid exceeding a user defined Domain Use Ratio. Domain Use Ratio is the MAL length divided by the total number of frequencies in the domain. For example, a 1/1 reuse pattern has a frequency reuse ratio of 1. A 4/12 reuse pattern can have a reuse ratio between 1/4 and 1/12, depending on whether all TRXs in a site have the same MAL (and HSN) or not.
6.1.7
User Defined MAL Length The MAL length has an upper limit defined in the “Max MAL length” parameter of the subcell table. The user can instruct the AFP to strictly use this value.
6.1.8
HSN Allocation The AFP assigns HSNs at subcell level. It chooses different HSNs for interfering and non-synchronous subcells. For synchronous subcells (usually within a site), the AFP can opt to assign the same HSN and different MAIOs within the set of same-HSN subcells. According to the adapted convention on HSNs for BBH TRXs, the AFP allocates different HSNs to the BCCH TRX and TCH TRXs. The 1st HSN corresponds to timeslots 1 through 7 of the BCCH and TCH TRXs, and the second HSN corresponds to the timeslot 0 of the TCH TRXs only. The second HSN is used in studies.
Figure 6.1: Hopping Sequence Numbers The user can control the HSN allocation so that it performs one of the following: • • • •
6.1.9
Assigns the same HSN to all subcells of a site Assigns the same HSN to all subcells of a transmitter Assigns pair-wise different HSNs if a pair of subcells has mutual interference. Optimise HSN assignment so that the frequency assignment is better (free HSN).
MAIO Allocation The AFP assigns MAIOs to TRXs so that the same MAL can be reused within a subcell, within a transmitter or even within a site. The separation requirements must be satisfied for frequencies that are on air, at all frame numbers. The cost function averages the cost upon all frame numbers in the synchronised case and upon all collision probabilities in the nonsynchronised case. See section 8.1 "Appendix 1: Description of the AFP Cost Function" for details.
6.1.9.1
Staggered MAIO Allocation If this option is selected, the AFP will allocate a MAIO set of 3 numbers: {First, step, total Number}. For example: • • • •
6.1.10
{17, 85} {2, 3, 4, 5} {2, 3, 5} {2, 4, 6}
OK, Step is 68 OK. Step is 1 Not OK, 4 is missing OK. Step is 2
BSIC Allocation Atoll AFP allocates BSICs according to two criteria, a soft criterion and a hard criterion. Not respecting the hard criterion is considered an error, while not respecting the soft criterion provokes a warning. The soft constraints are logically stronger, meaning these have a higher probability of not being satisfied. The hard criterion is easier to satisfy but must not be broken as it will cause handover failures. The hard criterion is based on the second order neighbour relation and BCCH co-channel reuse. The soft criterion uses interference information as well and tries to induce a larger [BSIC, BCCH] reuse distance. Appendix 4 details the new algorithm and both criteria.
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Chapter 6: Advanced Topics and Troubleshooting The BSIC allocation is compliant with the BSIC domains of transmitters and the strategy indicated in the BSIC tab of the AFP module property dialog. Either the algorithm selects a minimum number of BSICs in the related BSIC domain (Minimal option), or it chooses as many BSICs as possible while keeping them evenly distributed in the related BSIC domain (Maximal and homogeneous option). Notes: •
In the Atoll AFP, the directions of neighbour relations are all considered equivalent, so that the BSIC allocation is subject to harder constraints. Let us consider the following two examples: A
A N
N
N
C
B st
1 case: B and C neighbours of A
B nd
2
N
C
case: A neighbour of B and C
From the operational point of view, the first case is much more critical in terms of (BSIC, BCCH) collision. However, both cases are considered as violations of hard constraints by the AFP and error messages are generated (if the domain limitation provokes violations).
6.1.11
•
The soft criteria add additional constraints due to interference and adjacent reuse between BCCH channels. Warning messages are generated when soft criteria are not fulfilled.
•
Atoll AFP also considers Training Sequence Code collisions for synchronous networks. The AFP tries to avoid having the same TSC when interference exists (i.e. TCH interference, not only BCCH reuse).
•
BSIC assignment cannot be performed if the BSIC domain is empty or not assigned. When no BSIC domain is assigned, Atoll dislays a warning message that says that the BSIC domain is either null or empty. When an empty BSIC domain is assigned, Atoll displays an error message telling the user that BSIC allocation is not possible, and the network loading is stopped.
Robustness of Atoll AFP There are three main checks to avoid corrupt data entries and illegal data manipulations in Atoll: •
• •
The first is available in the GUI and protects Atoll’s tables from NULL and out of range values, non-existing references and other inconsistencies. The deficiency of these checks is that these can be partially bypassed by automated import procedures and external database manipulations. The second comprises Network Validation processes as described in the User Manual. The setback of this check is that the user can ignore it altogether. The third check is available when launching the AFP. As the AFP puts together information collected from many different sources, it is quite possible that some elements be conflicting and/or missing. Before launching the AFP, a validation process is launched that controls the possibility to launch the AFP.
If possible, missing / incorrect data are changed and default values are used (with warnings for each in the Event viewer). If any serious problem is detected, an error message appears and access to the AFP is denied. The table below lists the most common data limitations enforced in order to protect the frequency planner.
6.1.11.1
Value Ranges and Limitations at Validation
Limitation
Value
Comments
Maximum number of subcells in the loaded part of 1,000,000 the network Highest possible HSN
63
Lowest possible HSN
0
Limitation on the number of different frequency domains
10,000
Each exclusion of frequencies at a transmitter may create a new domain
Highest possible BSIC
77
Lowest possible BSIC
0
Longest possible MAL length
62
Shortest possible MAL length
0
Default Max MAL length
62
Used if the parameter is out of range.
Interference calculations are performed for cochannel and adjacent interference only.
If separation <= 1
Performance
Highest frequency ARFCN
1024
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So that with the BCCH frequency there are not more than 63 frequencies in a list to avoid exceeding the 255 character limit in Access.
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62
Highest value of AFP weight
100
Lowest value of AFP weight
0
Default value of AFP weight
1
Highest value of Traffic Load
10
Lowest value of Traffic Load
0.1
Default value of Traffic Load
0.1 / or 1 if the field is NULL.
Used if the parameter is out of range.
Highest value of “% max interference”
100
Appears in subcell and cell type configuration tables.
Lowest value of “% max interference”
1
Appears in subcell and cell type configuration tables.
Default value of “% max interference”
2
Used if the parameter is out of range. (The parameter appears in subcell and cell type configuration tables).
Highest value of “min C/I”
25
Lowest value of “min C/I”
2
Used if the parameter is out of range.
Default value of “min C/I”
12
Maximum power offset
25
Highest value of “Reception threshold”
-50
Lowest value of “Reception threshold”
-116
Default value of “Reception threshold”
-94
Used if the parameter is out of range.
Limitation on separation requirements
Must be <= 7
Will be eliminated in the future.
Limitation on the actual number of TRXs in a transmitter
50
Limitation on the number of interfering and neighbour subcells
1000
6.2
Used if the parameter is out of range.
Managing Consistency in Atoll and the AFP Atoll can generate many prediction studies, some of which can be used to analyse the AFP results. In each case, it is the task of the user to ensure consistency between the prediction study settings and the AFP settings.
6.2.1
Service Zone of a Subcell To render flexibility, optimised storage, validity checking and backward compatibility, each of the following Atoll processes has its own definition of service zone: 1. Neighbour allocation 2. Network dimensioning 3. Prediction studies 4. Interference calculation To avoid confusion, it is recommended that the user be aware of service zone model being used. In fact, the user should decide a unique service zone definition to use throughout a project.
Advice: Before running the AFP, select the definition of the service zone to use for the AFP and all Atoll prediction studies intended for AFP results analysis.
6.2.1.1
Specifying Correct Interference Study Coverage Criteria The AFP reads required interference information from either calculated or imported interference histograms. For interferences calculated by the AFP, the service zone is determined by the minimum reception level found in the Subcells table, and by the option, “all servers” / “best server”. Atoll interference study has a wider range of options to determine the service zone. A small service zone in the AFP interference calculation and a large one in the Atoll interference study will imply great amount of interferences in the interference study, even if the AFP had declared the cost to be 0. Moreover, for imported interference matrices, there is no means to check whether the imported files are a complete source of information. The user could very well calculate interference matrices for a part of the network, export it, and later import it as interference data for the entire network. This would result in an interfered frequency plan.
Advice:
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1. settings.
Check that the options in the Atoll interference study are consistent with the AFP interference calculation
2.
Verify that the small IM report does not indicate the existance of many non-interfered transmitters.
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Advice: 3. If AFP interferences have been imported, check that they are consistent with the service zone of Atoll interference study.
6.2.1.2
Selecting “All servers” or “Best Server” Service Zone Except neighbour allocation, all other calculations and processes allow choosing their service zone models through the options “All” or “Best server” with a margin. It is important to understand the difference between these options and why these options exist in Atoll. The “All servers” option implies that a transmitter is assumed to serve the region where its signal is stronger than a minimum reception threshold. This means that more than one transmitter can be serving a given point. The service zone is based on the minimum reception level “C”. The “Best server” option implies that a transmitter is assumed to serve the region where its signal is the strongest among all signals stronger than a minimum reception threshold. In other words, where C/N > 0 (C being reception level and N being the strongest neighbour reception level). If a margin M is defined, then the condition translates to C/N > – M. There may be more than one serving transmitters at a point but only one best server. By tuning the minimum reception level and the margin value, any desired server overlapping can be achieved. Large overlaps will result in a denser interference relation, more neighbours and a rather pessimistic analysis of C/I levels or interferences. It might be interesting to manipulate the overlapping zones study and the coverage by transmitter study to determine the correct service zone model. Coming back to the question of choosing between the two models, since both “C” and “N” values contain errors, the better model would be the one with the least error. The choice depends on the correlation between the errors in “C” and in “N”. If these errors are geographically correlated, the “C/N > – M” model should be preferred. Moreover, the error in “C” and the one in “N” are not evenly distributed: the further you are from the site, the bigger the error is. Therefore, when “N” is much stronger than “C” it is probably more reliable as well. For these reasons, it is recommended to use the “C/N > – M” model. As a final argument, it should be kept in mind that the C/N > – M model is bound by the C model.
Advice: Use a "Best server with margin" coverage study as service zone for both AFP and Atoll studies dedicated to analyse AFP results. The fact that “C/N > – M” is bounded by the “C” model might incur misunderstandings. For example, increasing the margin will not essentially increase the service zone, but change nothing because service zones are limited by coverage and not by other best servers. Note: •
6.3
For simplicity, hierarchical layers have been ignored in this discussion.
Event Viewer The AFP outputs various messages in the Event viewer. For example, when it finds a zero cost solution, when it stops because of a data problem or when it finishes without improving. The events, warnings and errors may be helpful in understanding the AFP behaviour, especially when a problem occurs.
6.4
Interference Study Quality Criteria The AFP uses the quality specified in the data model (min C/I field in the subcell table). In Atoll interference study, there is a wider range of quality criteria determination. To assess a frequency plan quality in accordance with AFP calculations, ensure that the interference study quality threshold has been specified accordingly (use subcell C/I threshold as study setting).
Advice: Select "Subcell min C/I threshold" instead of "Specified min threshold" in the Atoll interference study Condition tab.
6.5
Calculation Zone Border Effect If a calculation zone is used to geographically select a set of transmitters for assignment. The following problem will occur without the user being aware of it: Since the calculation zone limits the influence area in the interference studies (and IM calculation), all subcell service zones are, therefore, limited to the calculation zone. Hence, the transmitters at the borders will have very small service zones. All of the traffic corresponding to these transmitters will be considered as if concentrated in these few pixels and their interference matrix entries will be calculated so.
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Advice: Use a focus zone inside the calculation zone in order to focus the study on properly modelled transmitters. Preferably, use calculation zones that are geographically isolated. See the example below:
6.6
Frequency Planning Techniques
6.6.1
Basics For any network and any amount of available time, fill in the target computation time field to instruct the AFP to behave according to its default and basic allocation strategy.
6.6.2
Post-processing of Hot Spots Assuming X hours of available computation time, the strategy is as following: • • • • • •
Launch the AFP during X * 30 minutes (half of the time), then stop it and commit the results (if good). Freeze all TRXs in the network. Find the areas that generate problems. For example, some sites with separation violations. Unfreeze the worst 10 sites. For each such site, unfreeze 2 – 4 neighbouring transmitters. Run the AFP for an additional X * 30 minutes (the remaining half of the time).
AFP is also capable of committing cell level and TRX level quality indicators into the data structure. This makes the selection of hot spots a much easier task.
6.6.3
Learning the Network and Solving for Hot Spots • • • • • • • •
6.7
Apply this technique to networks having 4000 to 40000 Erlangs (500 to 5000 TRXs). Run the AFP for at least 10 solutions, specifying a short time period. Find the areas that generate problems. For example, some sites with separation violations. Create a calculation zone around these areas (referred to as the network core). Make sure that this network core is not too big or too small. For example: 100 to 150 transmitters, in one chunk, if possible. The network core should not be too small because it is representative of the entire network. Specify a long execution time (1000 to 5000 minutes) and let the AFP work on the core over a weekend. Commit the plan and run the AFP on entire network. Specify a short time (between 5 and 40 minutes) but let the AFP run for a long time (a night if possible, to get at least 50 solutions). Optional: You can freeze the network core before this last AFP execution.
Performance and Memory Issues in Large GSM Projects Memory problems might be experienced in the C/I coverage prediction studies, interference matrices calculations, and the AFP, while working on large GSM networks. Large network projects are more susceptible to these problems. Although, if the network is large though homogeneous, these problems may only appear if the number of transmitters is more than 15,000 or so. But, if there are large city centers involved, with each pixel having many overlapping path loss matrices, then this size limit might decrease to around 5,000 transmitters or so.
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Chapter 6: Advanced Topics and Troubleshooting Also, if the Atoll session has been open for a long time, memory problems may even appear while working on smaller networks. This is because the process memory space (memory space allocated to Atoll by the operating system) becomes fragmented. Following is a list of advices, which you can follow in order to avoid such problems: •
Use regionalisation or site lists: If you load a large network, Atoll will be required to load a lot of data that might not be useful all of the time. For example, in a typical large GSM network, you might have around 10,000 transmitter records, 20,000 subcell records, 50,000 TRX records, and up to 150,000 neighbor records.
•
Externalise embedded interference matrices: In Atoll 2.4.1 and 2.5.0, embedded interference matrices can be exported to external files and then removed from the .atl file to free up some memory. This might be interesting to do before launching a large C/I coverage prediction study. In Atoll 2.5.1 and above, you can externalise or embed each separate interference matrix in the IM folder. Atoll manages the loading of interference matrices from the disk to RAM so that it occupies memory only when needed. You will also reduce the .atl file size by externalising the interference matrices.
•
Try to adapt calculation radii to the cell type and the EIRP: Before calculating path loss matrices, take care in correctly associating calculation radii and resolutions to different types of cells. If you calculate path loss matrices for all types of cells over a large calculation radius, it will unnecessarily burden the C/I and interference matrices computations.
•
Properly configure the interference thresholds: These thresholds indicate the level from which onwards an interferer can be ignored. The default value for this threshold (-130 dBm), defined in the Predictions tab of the Predictions folder’s Properties dialog, implies that the computations will take into account all the intererers. However, if you set it too high, you might lose important interference information. The proper value for this threshold depends on the Reception Thresholds and the C/I Thresholds defined in the Subcells table. The optimum value would be Min AllSubcells ( RT i – CIT i – M ) . Which means the minimum value of the factor RT i – CIT i – M computed for all subcells, i. Where, RTi is the reception threshold of the subcell i, CITi the C/I threshold of this subcell, and M is a safety margin. Since this interference threshold is used both in interference matrices calculation and in interference predictions, it is important to take at least 3 dB margin for the interference energy aggregation in C/I studies. We recommend a safety margin of 5 dB, which can be reduced any problem is encountered.
•
Do not define very high C/I quality thresholds (Default values: 12 dB for BCCH and 9 dB for TCH). If you want a certain TRX type to carry GPRS/EDGE traffic, you can add 1 or 2 dB to this value for that TRX type, and use the option of safety margin in the AFP module’s Cost tab. The 12 dB and 9 dB default values already include safety margins. If you increment these values too much, it will unnecessarily load the interference matrix generation and the AFP.
•
Do not start an AFP session if the interference matrices report indicates problems: All the transmitters should have interferers and very few of them (not more than 20%) should have more than 70 interferers. If there are too many or too few entries in your interference matrices, the AFP plan will not be optimum.
•
If the memory-critical task is interference matrices generation: You can generate interference matrices in a piecewise manner. This means that you can generate nation-wide interference matrices with low resolutions based on the % of interfered area (to improve computation time), with a cell edge coverage probability of 50% (which implies no access to clutter forreading standard deviation values), and an interference threshold of -112 dBm. This will provide rough global interference matrices which can be locally improved. These interference matrices will be less memoryconsuming. Then, use polygon or site list filters to focus on each important location, and calculate local interference matrices with higher resolutions and reliabilities. Make sure that the computation zone in your project completely encompasses the filtering zones that you define. If you are working with Atoll 2.3.1 upto 2.5.0, you must first import the high resolution interference matrices, and then the low resolution matrices. This is automatically performed by Atoll 2.5.1 and above.
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•
If the memory-critical task is the AFP session: Try to make the document lighter, e.g. remove coverage prediction studies, exit and restart Atoll, and try to generate interference matrices having less number of entries.
•
If the memory-critical task is the traffic capture: You can use traffic load field of the Subcells table to provide traffic loads directly to the AFP, and possibly skip this step.
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Interference Matrices Atoll can simultaneously use more than one interference matrices. There is no size limit to the interference information that you can use. And, it is possible to keep the interference matrices updated when transmitters are renamed or deleted, or the radio network data is reloaded from a database. You can create different scenarios in Atoll, such as: •
Working with many small (regional) interference matrices instead of one country wide matrix -
•
Working with interference matrices of different resolutions -
•
If recalculation is required, you can recalculate just the concerned matrices instead of recalculating an entire country-wide matrix. The calculation of smaller matrices is quicker as well. Using the AFP for a certain region only requires to load the interference information for that region.
You can have, for example, a country-wide matrix of lower resolution completed by high resolution regional matrices. The AFP can use the country-wide matrix where the high-resolution matrices are not available.
Working with interference matrices of different types -
Interference matrices based on drive tests can be updated from the measurements as they are carried out from time to time in each region. Interference matrices based on RXLEV statistics from the OMC can be updated as the statistics are made available from the OMC. Each time the information will be collected with a different list of (dummy) neighbours. The actual neighbours remain the same; therefore, these interference matrices can be combined based on a "Worst case combination". If they are combined by averaging, a neighbour interferer will be n times stronger than an equivalent non neighbour interferer.
You can combine interference matrices of the same type and import for use in Atoll, or you can import these interference matrices in Atoll as lower bound interference matrices. In both cases, the AFP uses the interference information correctly. Interference matrices of the different types, resolutions, and sizes are listed under the Interference Matrices folder. The following sections describe how interference matrices can be stored, imported, combined, and calculated.
Features in Atoll The following features are available concerning interference matrices in Atoll. •
Support of different types of interference matrices. You can import and work with interference matrices from different sources: -
•
Based on path losses (propagation) Based on data from the OMC Based on drive test data
Support of different sizes of interference matrices. You can work with either a lot of of small (local) or fewer large interference matrices, with different resolutions.
•
Support of more than one interference matrix with the possiblity to activate and deactivate the matrices. You can import and work with more than one interference matrix. All the interference matrices, available in the Interference Matrices folder, correspond to a network state. -
•
You can work with interference matrices with different interference information about the same pixel. Duplicate information is managed by Atoll. The information in the interference matrices is compressed. Interference matrices only require memory (RAM) when in use. Interference matrices can be saved to external files, without any loss in performance, so that the .atl file size is not impacted. Each interference matrix has its own scope and context.
Support of maximum likelihood combination. The Atoll AFP module can combine interference matrices, using their scopes and contexts and a maximum likelihood combination method. -
The Atoll AFP module has access to the the scopes and contexts of interference matrices, and can intelligently combine different interference matrices. The combination process takes into account no-interference and interference information from interference matrices. The Atoll AFP only loads the active and relevant interference matrices in memory during calculations. Inactive interference matrices and interference matrices that do not belong to the studied area are not loaded.
General API Features Atoll’s general API can be used to manage interference matrices. You can read and write interference data, and the scope and context of interference matrices via the API. It is also possible to run interference matrix calculations (in batch mode) using the API, which allows you to automatically prepare the interference matrices for the AFP in the background.
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AFP API Features Automatic frequency planning tools can access active interference matrices through the AFP API. The AFP can select the interference matrices to load, and has access to the scope and context parameters of each IM. The AFP can also read the different interference values predicted by each of the loaded interference matrices.
7.1
Types of Supported Interference Matrices Interference matrices can be generated based on different sources of information and propriety software. The following list includes 9 typical information sources that can be used to create interference matrices supported by Atoll. 1. Based on path loss (propagation data) matrices Reliability depends on the accurate and correct network and geo data. 2. Based on reselection statistics from the OMC Reliability usually low due to difference between the locations where mobiles are switched on and where they are actually used to access the network. 3. Based on handover statistics from the OMC Reliability usually low due to the fact that interference is measured only among existing neighbours, which might not be correctly assigned. This type of interference matrices are highly correlated with the neighbour relations. It can be used as a measure to remove excess neighbour constraints. However, it can not be used to complete any missing neighbour information. Another reason for low reliability is that interference information is collected from the handover regions only, which is different from the service area. 4. Based on RXLEV statistics from the OMC Can be a very good source of interference information if it is statistically stable because it is not sensitive to data errors. On the other hand, it has many disadvantages, such as: -
Transmitters with the same BSIC and BCCH cannot be differentiated. Transmitters having the same BCCH will never have an interference entry. Information is lost when more than 6 interferers exist at a location. If many interferers share the same BCCH, they increase the interference levels of each other. HCS layers may cause problems: more servers at a point, macro layer servers are stronger, a correction margin might be introduced in some equipment, etc.
This type of interference matrices can be created using an extended neighbours list. 5. Based on test mobile data Reliability can be low because usually the test mobile data sampling zone and the traffic model are not related. Secondly, the measurements are carried out for existing neighbours. 6. Based on CW measurements Reliability can be low because usually the measurements do not reflect the traffic model. However, this source of information can be very reliable for a subset of transmitters that was properly scanned. Carrying out CW measurements is expensive, which implies that often the collected information is partial or out of date. 7. Based on scan data drive tests Highly reliable and an excellent source of information, but not usable in a radio planning tool because no information is available for mapping transmitters to the received signals at any pixel (x, y). 8. Upper bound interference matrix The source of this type of interference matrix is not defined. It can be based on user experience. The information contained in this interference matrix is used as an upper limit, i.e., if this interference matrix indicates a certain level of interference, it should not be exceeded because other interference matrices show higher interference. If an upper bound interference matrix does not contain information about an entry, it is ignored. 9. Lower bound interference matrix The source of this type of interference matrix is not defined. It can be based on user experience. The information contained in this interference matrix is used as a lower limit. This type of interference matrix can be very useful because you can edit entries in this interference matrix, and be certain that the interference will be at least as high as the value you entered. This approach can be used when user experience shows a certain level of interference which the radio network planning tool is unable to calculate.
7.2
Interference Matrices Storage The interference matrices listed in this folder can be stored in the .atl file by embedding, or in external files, by externalising: •
Embedded interference matrices: Advantages: -
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Interference matrices are stored as binary format data, and can be (re)loaded to the RAM when needed (by the AFP, for generating reports, or for exporting) Read/Write operations are quicker
Disadvantages: •
File size limit restricts the amount of information that can be stored in a .atl file.
Externalised interference matrices: Atoll can externalise interference matrices to one of the supported file formats. However, only the .clc file format can store complete information of an interference matrix. Please refer to the Technical Reference Guide for more information on the different file formats. Advantages: -
The size of the Atoll document is not affected by the size of the externalised interference matrices listed in the Interference Matrices folder.
Disadvantages: -
Read/Write operations take a bit longer with external files, therefore, once the interference matrix is loaded to the RAM, it is kept loaded. File sharing between users is not possible.
You should externalise interference matrices if the .atl file size is close to the file size limit. It is, otherwise, recommended to keep the interference matrices embedded.
7.3
Multiple File Import More than one interference matrix can be imported. If you have an interference matrix that covers a part of the network, and another that covers the entire network, and you want that the larger IM complete the smaller one, you must import the smaller one before the larger one. So, when Atoll imports the smaller IM first, and the larger one afterwards, if an entry already exists in the smaller IM, Atoll will ignore that entry in the larger IM. It is also possible to select more than one file in the Open dialogue when importing interference matrices. In this case, files are imported in the order in which they are selected.
7.4
Maximum Likelihood Combination One of the more advanced features of an AFP is to intelligently combine different interference matrices, and, in the case of a frequency plan with interference, to determine which interference corresponds to which interference matrix. Different types of interference matrices have different weak points. The maximum likelihood approach detects and avoids these weak points. Another important aspect of maximum likelihood combination is the capability to differentiate between no-interference and unknown-interference. Moreover, the maximum likelihood method keeps the information about the type of the interference matrix, its quality indicators, and its scope. The next two sections explain the maximum likelihood combination performed by the Atoll AFP Module. Before describing the combination process, the scope and context of interference matrices is explained.
7.4.1
Scope and Context of Interference Matrices
7.4.1.1
Interference Matrix Context The context of an interference matrix refers to the following properties associated with each matrix: 1. Name (and comments, if any) 2. External file name (if the matrix in externalised) 3. Active or not 4. Type (one of the 9 types described in 7.1 "Types of Supported Interference Matrices") 5. Type-dependent quality indicators The context of an interference matrix is mainly used to indicate the statistical quality if the interference matrix so that the AFP can weight the information read from the interference matrix accordingly. Atoll can support a number of AFP tools. The interference matrix combination process, which is a part of the cost function, can be different in different AFP tools. The context of interference matrices allow a common representation and significance of the parameters influencing the combination process. These parameters are, therefore, described as a set of quality indicators, with comprehensive units, such as the number of measurement days, standard deviation, calculation resolution, and whether the interference matrix is based on traffic or surface area. The General tab of the Interference Matrix Properties dialog gives you access to the first three (see Figure 7.1: "Interference Matrix Properties Dialog - General Tab").
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Figure 7.1: Interference Matrix Properties Dialog - General Tab The Advanced tab lists the type and the quality indicators of the interference matrices (see Figure 7.2: "Interference Matrix Properties Dialog - Advanced Tab").
Figure 7.2: Interference Matrix Properties Dialog - Advanced Tab Depending on the matrix type, the quality indicators available in the Advanced tab include: 1. For matrices based on path loss (propagation data) matrices -
The standard deviation The resolution Whether the interference information (probabilities) correspond to traffic or surface area
2. For matrices based on reselection statistics from the OMC -
The statistic duration Whether the interference information (probabilities) correspond to traffic or surface area
3. For matrices based on handover statistics from the OMC -
The standard deviation, depending on the equipment quality and measurement post-processing The average number of points collected in each matrix calculation point The volume of information Whether the interference information (probabilities) correspond to traffic or surface area
4. For matrices based on RXLEV statistics from the OMC -
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5. For matrices based on test mobile data -
The standard deviation, depending on the equipment quality and measurement post-processing The average number of points collected in each matrix calculation point
6. For matrices based on CW measurements -
The standard deviation, depending on the equipment quality and measurement post-processing The average number of points collected in each matrix calculation point The volume of information Whether the interference information (probabilities) correspond to traffic or surface area
7. For matrices based on scan data drive tests -
The standard deviation, depending on the equipment quality and measurement post-processing The average number of points collected in each matrix calculation point The volume of information Whether the interference information (probabilities) correspond to traffic or surface area
The context of an interference matrix is not included in the interference matrix files. That is why Atoll asks the user to setup the type and quality indicators of the interference matrix manually.
7.4.1.2
Interference Matrix Scope The scope of an interference matrix is a mapping between a transmitter ID and the following information: 1. Transmitter name 2. BSIC (as was when IM statistics were gathered) 3. BCCH (as was when IM statistics were gathered) 4. % of victim coverage (an integer between 0 and 100) 5. % of interferer coverage (an integer between 0 and 100)
Figure 7.3: Interference Matrix Scope The most important information of the scope is contained in the columns "% of victim coverage" and "% of interferer coverage". In order to understand their significance as well as their use, the following should be kept in mind: •
Interference matrices must provide interference information between each pair of subcell in the network. A large amount of memory would be required for a simple sequential representation of the interference matrix, which would make it impossible to work with such interference matrices in large networks. Therefore, interference matrices are represented as a set of entries for which interference exists. If an entry (i, j) does not exist in the set: -
Either j does not interfere i (no-interference), Or the interference information is missing in the interference matrix because at least one of the two was out of the scope of the interference matrix (unknown-interference).
In other words, the lack of information can be interpreted as either no or unknown interference. •
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If there is only one interference matrix, it can be considered complete since it is the only source of interference information. In this case, there is no difference between no and unknown interference. If there is more than one
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Creation of the Interference Matrix Scope The scope of an interference matrix is created by the tool that creates the interference matrix. If the interference matrix is created by Atoll, the AFP scope is used as an initial set of victims, which It corresponds to SEL + RING (see 2.3 "Definition of the AFP Scope"). This means that even when only one transmitter is present inside the computation zone, many other transmitters may be taken into account in the set of victims. Atoll adds all potential interferers to this set, and calculates the interference matrix. The interference matrix is accurate only between transmitters that are in the victim set. Inaccurate entries are removed, and the scope of the interference matrix becomes the victim set with 100% at both victim and interferer coverage. Other software can be used to edit the interference matrix scope using the general API features, or by externalising the interference matrix in the .clc format and editing it. The .clc format can store all the interference matrix information (see the Technical Reference Guide for more information). Notes: •
The scopes of the interference matrices are automatically created when old .clc, .im0, .im1, or .im2 files are imported. The scope is created using the current BSIC and BCCH allocation, and finding the set of all victims and the set of all interferers.
•
The interference matrix scope internally manages the transmitter ID’s. When exchanging information with a .clc file, these ID’s are visible to the user. They are arbitrary numbers used to index the interference matrix entries. Even if an addin is used to create the interference matrix, the association of transmitter names to ID’s is carried out by Atoll. The addin will associate the interference information to pairs of transmitter ID’s.
•
The .clc and .dct files have the same mapping of transmitter names to transmitter ID’s. There are no restrictions on transmitter ID’s as long as they are unique integers under 231.
Two possibilities (examples) for editing the interference matrix information could be: •
•
An Addin that imports an interference matrix should should know its scope. For example, if it is an OMC addin, and the OMC covers 50 transmitters, the scope will contain 50 transmitters. Their indexes will be supplied by Atoll once added to the scope. The % of victim and interferer coverages should be 100%. When generating an interference matrix from CW measurements, there might be a few transmitters which were correctly scanned and others that were not. In this case, the correctly scanned transmtters would have good % of victim and interferer coverages, while the others would not.
Use of the BSIC and BCCH in the Scope The BSIC and BCCH fields in the scope are used for the cases where the BSIC and BCCH allocation, during the period when the interference matrix information was gathered, was different from the current BSIC and BCCH allocation.
7.4.1.3
Keeping the Scope and Context Up to Date When a .clc file (and its corresponding .dct) are imported, the transmitter indexes in the files can be arbitrary. In order to improve access time, Atoll changes these indexes to the ADO record ID as index. When you rename or delete a transmitter, or when the ADO index is changed, the interference matrix is automatically updated, and saved when the Atoll document is saved. Instead of updating the interference matrix every time a transmitter is renamed or deleted, Atoll stores the events in memory, and updates the interference matrix only when it is used. It checks the ADO record ID’s and, if they have been changed, the changes are taken into account.
7.4.2
Interference Matrix Combination in Atoll AFP Module Interference matrices are combined considering the following criteria: •
The cost function does not change Earlier, interference values were read from a single interference matrix. Now, they are read from more than one interference matrix.
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If the interference matrices are correctly managed in Atoll, no further parameterisation (weighting) is required.
The Interference Matrice tab (see Figure 7.4: "AFP Interference Matrices Parameters") available in the Atoll AFP Module properties dialog lets you set up the interference matrix combination by defining its three weighting components. The interference matrix combination is carried out as follows: 1. The Atoll AFP Module asks Atoll to load a subset of the active interference matrices of the document. This subset is determined by comparing each interference matrix scope with the AFP scope. Only the interference matrices whose scope intersects the AFP scope are loaded. 2. The Atoll AFP Module then reads the scope and context information of each loaded interference matrix. At a given pixel, the interference, p(i, v, x), of subcell i (interferer) on subcell v (victim) for a given C/I level x, can be read from more than one interference matrix. 3. The Atoll AFP Module combines all the values of p(i, v, x) by performing a weighted average. Therefore, itcalculates as many weights as the number of p(i, v, x) entries for a pixel. These "reliability weights" are calculated by multiplying the following three components, which are defined in the Interference Matrice tab of the Atoll AFP Module properties dialog: a. Component quantifying the membership to the AFP scope: VictimCoverage(Transmitter(v)) x InterfererCoverage(Transmitter(i)) For interference matrices based on OMC statistics, if the scope indicates that both i and v had the same BCCH, the component will be 0. b. Component depending on the interference matrix type. c. Component depending on the interference matrix quality indicators (see "Reliability Calculation") Note: •
In Atoll version 2.5.2, the AFP considered the first value of p(i, v, x). And in Atoll version 2.6.0, the AFP worked with the highest value of p(i, v, x).
Figure 7.4: AFP Interference Matrices Parameters
7.5
Interference Matrix Calculation Atoll can calculate two types of interference matrices, and their related information such as scope and context. 1. Interference matrices calculates from path loss data: The quality of the interference matrices depends upon the standard deviation of the propagation model for the corresponding clutter classes. The interference matrices are calculated using the default calculation resolution defined in the Predictions tab of the Predictions folder’s properties dialog. You can either base the calculation on the percentage of overlapped traffic or on the percentage of overlapped surface area.
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AFP Reference Guide 2. Interference matrices calculated from test mobile data: The victim and interferer scope is the transmitters included in the test mobile data path. The number of points per victim is calculated and associated with the quality indicator related with the volume of information (see "Reliability Calculation" for more details). A basic statistical analysis of the test mobile data measurements is carried out in order to estimate the values of "% of victim coverage" and "% of interferer coverage" for each transmitter.
Reliability Calculation Depending on the type of interference matrix, one of the following three equations is used to determine the third reliability component: 1. Interference matrix based on propagation: 75 7.5 --------------- × -------r + 25 σ Where σ is the standard deviation of the propagation model, and r is the calculation resolution. A resolution of 50 m and a standard deviation of 7.5 dB gives a weight of 1. 2. Interference matrix based on measurements from the OMC performed during n days: 1+n ----------------3 Which gives a weight of 1 for 8 days of measurements. 3. Interference matrix based on drive test analysis: 0.4
[1 + n × r + 1] ---------------------------------------------4 × (σ + 1) 3 parameters determine the weight: a. The standard deviation σ , which is assumed to be lower than the one of a propagation model. b. The number of measurements considered at each calculation point, r c. The number of calculation points per transmitter, n
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Chapter 8 Appendices
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Chapter 8: Appendices
8
Appendices
8.1
Appendix 1: Description of the AFP Cost Function
8.1.1
Notations The notations listed hereafter are used to describe the cost function: • • • •
TRG denotes a group of TRXs. # is used instead of “number of”. For example, # TRXi is the number of TRXs in TRGi. “⇔” denotes the relation “if and only if”. g denotes the size of any group g.
• • •
ARFCN denotes the set of all the frequencies, and 2 the set of all the subsets of frequencies. “TRGs” denote the set of all the TRGs. x denotes the largest integer ≤ x (x can be a real number).
•
A i, g denotes the number of times a group g ∈ 2
ARFCN
ARFCN
is assigned to TRGi in the assignment A.
For example: -
When i is NH, A i, g = 1 ⇔ g is a single member group containing one of the frequencies assigned at TRGi. If |g| is not 1 or if g does not contain a frequency assigned at i, then A i, g = 0 .
-
When i is BBH, A i, g can be either 0 or #TRXi. A i, g = #TRX i ⇔ g is the set of frequencies assigned to TRXs of TRGi. (|g| = # TRXi). When we talk about “TRXs of i using g”, and in the case of BBH, then there are |g| such virtual TRXs, each using the entire group g and having a virtual MAIO [0, |g| - 1].
-
When i is SFH, A i, g must be ≤ #TRX i . A i, g = n ⇔ g is the set of frequencies assigned to n TRXs of TRGi. We assume all the groups assigned to TRGi to have the same length.
• •
TSi denotes the number of timeslots available for each TRX in TRGi. TLi is the traffic load of TRGi. This parameter may either be calculated during dimensioning or be user specified. TL i = #Erlangs of a single TRX in TRGi divided by TSi
• • • • •
TSUi refers to the downlink timeslot use ratio (due to DTX) at TRGi. CFi is the cost factor of TRGi (AFP Weight field). QMINi is the minimum required quality (in C/I) at TRGi. PMAXi is the percentage permitted to have quality lower than QMINi at TRGi. REQi corresponds to the required number of TRXs at TRGi.
A communication uses the group g in TRGi if its mobile allocation is g. The probability to be interfered is denoted by P i, i', g ( A ) (i’ is the TRX index). Different TRX indexes may have different MAIOs. P i, i', g ( A ) is a function of the whole frequency assignment. The precise definition of the term “to be interfered” is provided afterwards. The probability penalty due to violating a separation constraint is P i, i', g ( A ) . It is a function of the whole frequency assignment as well. The term “Atom” will be used in the following context: For two TRGs, i and k, ATOM ( i ) ≡ ATOM ( k ) i and k are synchronised, have the same HSN, the same MAL length and the same hopping mode. NH TRGs or BBH TRGs are always in separate atoms. If two TRGs interfere but are not in the same atom, these can be taken as unsynchronised. The quality of unsynchronised TRGs is a function of all possible frequency combinations. For synchronised TRGs, pairs of frequencies emitted at the same time are known.
8.1.2
Cost Function The Atoll AFP cost function is a TRX based cost and not an interference matrix entry based cost. It counts the impaired traffic of the network TRXs in weighted Erlangs. The cost function Ψ is reported to the user during the AFP progress with the help of its 5 components: Ψ mis , Ψ sep , Ψ comp , Ψ corr and Ψ dom . Ψ = Ψ mis + Ψ sep + Ψ comp + Ψ corr + Ψ dom
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AFP Reference Guide where, Ψ mis represents the missing TRX cost component Ψ sep represents the separation component Ψ comp represents the additional cost component (interference, cost of changing a TRX) Ψ corr represents the corrupted TRX cost component Ψ dom represents the out-of-domain frequency assignment cost component Note: •
Ψ mis =
Do not let this form of representation mask the fact that the cost function is a TRX based cost function.
∑
( MIS_TRX i × λ ) × TL i × CF i × TS i
∑
( CORR_TRX i × Ω ) × TL i × CF i × TS i
∑
( DOM_TRX i × ω ) × TL i × CF i × TS i
i ∈ TRGs
Ψ corr =
i ∈ TRGs
Ψ dom =
i ∈ TRGs
Ψ sep
⎛ ⎜ ⎜ ⎜ = ⎜ i ∈ TRGs ⎜ ⎜ ⎝
Ψ comp
∑
∑ ARFCN
g∈2 i' ∈ TRXs of i using g
⎛ ⎜ ⎜ ⎜ = ⎜ i ∈ TRGs ⎜ ⎜ ⎝
∑
⎞ ⎟ ⎟ δ' i, i', g ( A )⎟ × TL i × CF i × TS i ⎟ ⎟ ⎟ ⎠
∑ ARFCN
g∈2 i' ∈ TRXs of i using g
⎞ ⎟ ⎟ δ'' i, i', g ( A )⎟ × TL i × CF i × TS i ⎟ ⎟ ⎟ ⎠
In the above equations, •
i’ is the TRX index belonging to { 0, 1, ..., A i, g – 1 } .
•
MIS_TRX i is the number of missing TRXs for the subcell i.
MIS_TRX i = MAX 0, REQ i –
∑ g∈2
• • • • •
A i, g
ARFCN
λ is the cost value for a missing TRX. This value can vary between 0 and 10. The default cost value is set to 1 and can be modified in the AFP module properties dialog. CORR_TRX i is the number of corrupted TRXs for the subcell i. Ω is the cost value of a corrupted TRX. This value can vary between 0 and 10. The default cost value is set to 10 and can be modified in the AFP module properties dialog. DOM_TRX i is the number of TRXs, for the subcell i, having out-of-domain frequencies assigned. ω is the cost value of a TRX with out-of-domain frequencies assigned. This value can vary between 0 and 1. The default cost value is set to 0.5 and can be modified in the AFP module properties dialog. And, as mentioned earlier, a virtual TRX is considered in case of BBH. If i’ is valid, the algorithm evaluates the cost of a valid TRX. This cost has two components, δ' i, i', g ( A ) and δ'' i, i', g ( A ) .
•
δ' i, i', g ( A ) is the separation violation probability penalty.
•
δ'' i, i', g ( A ) is complementary probability penalty due to interference and the cost of modifying a TRX. If the option “Take into account the cost of all the TRXs” available in the AFP module properties dialog is selected, then, δ' i, i', g ( A ) = P' i, i', g ( A ) and δ'' i, i', g ( A ) = P'' i, i', g ( A )
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Chapter 8: Appendices Or if the option “Do not include the cost of TRXs having reached their quality target” available in the AFP module properties dialog is selected, the algorithm compares P' i, i', g ( A ) + P'' i, i', g ( A ) with the quality target specified for i, P MAX : If P' i, i', g ( A ) + P'' i, i', g ( A ) > P MAX , Then δ' i, i', g ( A ) = P' i, i', g ( A ) and δ'' i, i', g ( A ) = P'' i, i', g ( A ) . Otherwise, Both δ' i, i', g ( A ) and δ'' i, i', g ( A ) will be equal 0. P' i, i', g ( A ) is the same as δ' i, i', g ( A ) (separation violation probability penalty) and P'' i, i', g ( A ) the same as δ'' i, i', g ( A ) (complementary probability penalty due to interference and the cost of modifying a TRX) in most cases. These are explained in detail in the next sections.
8.1.3
Cost Components Separation violation and interference cost components are described hereafter. Parameters considered in the cost function components can be fully controlled by the user. Some of these parameters are part of the general data model (quality requirements, percentage of interference allowed per subcell), while others (such as separation costs and diversity gains) can be managed through the properties dialog of the Atoll AFP module.
8.1.3.1
Separation Violation Cost Component The separation violation cost component is evaluated for each TRX. Estimation is based on costs specified for the required separations. Let SEP_CONSTR i, k denote the required separation constraint between TRGi and TRGk. Let Cost s, z denote the user defined separation penalty for a required separation “s” and actual separation “z”. SEP i, k, v is used instead of Cost SEP_CONSTR
i, k,
z
as abbreviation.
Note: •
The AFP module properties dialog takes probability percentages as inputs while this document deals in probability values.
ξ ii'kgg'k' is considered to be the effect of a separation violation on the i' th TRX of TRGi assigned the group g, caused by the k' th TRX of TRGk assigned the group g' . γ denotes the overall weight of the separation violation cost component. This value can be between 0 and 1, set to 1 by default. It can be modified in the AFP module properties dialog. γik represents the weight of the specific separation constraint between i and k. This specific weight depends on the type of separation violation and follows the following priority rule: 1. Exceptional pairs 2. Co-transmitters 3. Co-site 4. Neighbours For example, if a pair of subcells are co-site and neighbours at the same time, they will be considered as co-site because higher priority. Hence, γik of these subcells will be the weight of co-site relations. If only a neighbour relation exists between two subcells, then γik will be further weighted by the neighbour relation importance. The value of γik remains between 0 and 1. The default weights of each type of separation are available in the Separation cost tab. If ATOM ( i ) ≠ ATOM ( k )
∑
SEP i, k,
f – f'
f∈g f' ∈ g'
Then ξ ii'kgg'k' = γ × γik × ---------------------------------------------- , which is same for all values of k. g × g' If ATOM ( i ) = ATOM ( k )
∑
Then ξ ii'kgg'k' = γ × γik ×
SEP i, k,
g υ – g' τ
f_n ∈ { 0, 1, ..., F_N – 1 } -------------------------------------------------------------------------------------
F_N
In the above equations, F_N ( g ) is the number of frames in the MAL g. F_N ( g ) = g .
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AFP Reference Guide Note: •
Since F_N ( g ) = F_N ( g' ) , we shortly denote the two as F_N .
Let f_n denote the instantaneous frame number from 0 to F_N . While υ = ( f_n + MAIO A And τ = ( f_n + MAIO A
th
i, g, i'
k, g', k'
) modulo F_N and g υ is the υ frequency in g,
) modulo F_N and g' τ is the τ
th
frequency in g’.
In addition, frequencies belonging to a MAL with a low fractional load, and breaking a separation constraint, should not be weighted equally as in a non-hopping separation breaking case. Therefore, the cost is weighted by an interferer diversity gain. 1 ˆ G i, k, g, g' = ------------------------------------------------------------------( 0.1 × SEP_GAIN ( i, k, g , g' ) ) 10 The separation gain, denoted by SEP_GAIN ( i, k, g , g' ) is basically a function of the MAL length (and, of course, of the hopping mode). With frequency hopping, the effects of DTX and traffic load become more significant (due to the consideration of the average case instead of the worst case). For this reason, it is possible to consider these effects in SEP_GAIN ( i, k, g , g' ) through the relevant option available in the Advanced tab of the AFP module properties dialog. Without this option, the SEP_GAIN ( i, k, g , g' ) is: SEP_GAIN ( i, k, g , g' ) = I_DIV ( g ) I_DIV ( g ) is the user defined interferer diversity gain (dB) for a given MAL length. It is used in P i, i', g ( A ) definition as well. On the other hand, if this option is selected, the SEP_GAIN ( i, k, g , g' ) becomes, ( 2 + ASYN_GAIN ( i, k, g' ) ) SEP_GAIN ( i, k, g , g' ) = I_DIV ( g ) +⎛0.5 × TSU_GAIN ( k ) × min ⎛ 10, 4 + ⎛ ( 2 + I_DIV ( g ) ) × ------------------------------------------------------------------------⎞ ⎞ ⎞ ⎝ ⎠⎠⎠ ⎝ ⎝ 4 1 Where TSU_GAIN ( k ) = log 10 ⎛ -------------------------------⎞ , ⎝ TL k × TSU k⎠ And ASYN_GAIN ( i, k, g' ) =
0 I_DIV( g' )
if ATOM(i) = ATOM(k) Otherwise
ˆ More than one separation violations may exist for a TRX. Many “small” G i, k, g, g' and ξ' ii'kgg' have to be combined to form one cost element, the P' i, i', g ( A ) . This is done through iterating over all violating assignments and by summing up an equivalent to the probability of not being violated while considering each separation violation as an independent probability event. This sum is naturally limited to 100% of the TRX traffic, and is given by, ⎛ ⎜ ˆ ( 1 – ξ ii'kgg'k' × G i, k, g, g' ) ⎜1 – ⎜ P' i, i', g ( A ) = ⎜ k ∈ TRGs ⎜ ARFCN ⎜ g' ∈ 2 ⎜ k' ∈ TRXs of k using g' ⎝
∏
⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠
In the above formula, if ( k = i ) , then ( k' ≠ i' ) , so that interference with itself is not taken into account.
8.1.3.2
Interference Cost Component The interference cost component is evaluated for each TRX. Its estimation is based on interference histograms calculated for pairs of subcells. In addition, it takes into account frequency and interferer diversity gains and models frequency hopping and gain due to DTX. Note: •
Interference histograms are described in User Manual (GSM GPRS EGPRS project management, GSM GPRS EGPRS network optimisation, GSM GPRS EGPRS generic AFP management). Interference histograms can also be exported to files. For further description, refer to section 8.2 "Appendix 2: Interferences".
When estimating P'' i, i', g ( A ) , the following problems are encountered: • •
88
The QMINi C/I quality indicator corresponds to the accumulated interference level of all interferers while the C/I interference histograms correspond to pair-wise interferences. Both QMINi and the histograms correspond to a single frequency. In case of a MAL containing more than one frequencies, interferences on several different frequencies of a MAL must be combined.
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Chapter 8: Appendices This estimation, presented below, is the simplest possible as it solves the first problem by linear summation and truncation at the value of 1 and it solves the second problem by averaging and adding the two diversity gains: •
F_DIV ( g ) , the frequency diversity gain, and
•
I_DIV ( g ) , the interferer diversity gain.
Hereafter, α denotes the global weight of interference cost component. This value can vary between 0 and 1 and is set to 0.35 by default, which can be modified in the AFP module properties dialog. Let F_N ( g ) be the number of frames in the MAL g. F_N ( g ) = g . Let f_n denote the instantaneous frame number from 0 to F_N . Let MAIO A
k, g', j
be the j’th MAIO of A k, g' , where j is one of the { 0, 1, ..., A k, g' – 1 } TRXs.
The value of MAIO A
k, g', j
is one of { 0, 1, ..., g' }
If TRGk is NH, then MAIO A
k, g', j
If TRGk is BBH, then MAIO A
= 0.
k, g', j
= j.
As said earlier, in case of BBH, we consider g' virtual TRXs, the jth TRX has the MAIO j. Let g i be the ith frequency in the group g. Similar to the definition of ξ ii'kgg'k' , ξ' ii'kgg'k' is defined as an interference event. ξ' ii'kgg'k' is the effect interference on the i' th TRX of TRGi assigned the group g, caused by the k' th TRX of TRGk assigned the group g' . If ATOM ( i ) ≠ ATOM ( k )
ξ' ii'kgg'k' =
Then
∑
f ∈ g, f' ∈ g'
Where
C Probability ⎛ ----- < Q_UB i, k, f, f'⎞ ⎝ I ik ⎠ -------------------------------------------------------------------------------g × g'
Q_UB i, k, f, f' = QMIN i –
f – f' × ADJ_SUP + INTERF_GAIN ( i, k, g , g' )
If ATOM ( i ) = ATOM ( k ) Then, Since F_N ( g ) = F_N ( g' ) , these are both represented by F_N .
ξ ii'kgg'k'
C ⎧ Probability ⎛ ----- < Q_UB i, k, f, f'⎞ ⎫ ⎝ I ik ⎠⎪ ⎪ -⎬ = ⎨ ------------------------------------------------------------------------------F_N ⎪ ⎪ f_n ∈ { 0, 1, ..., F_N – 1 } ⎩ ⎭
∑
Where, f = gυ , f' = g' τ , υ = ( f_n + MAIO A τ = ( f_n + MAIO A
i, g, i'
) modulo F_N ,
k, g', k'
) modulo F_N ,
Q_UB i, k, f, f' = QMIN i –
f – f' × ADJ_SUP + INTERF_GAIN ( i, k, g , g' )
⎧ ⎪ ⎪ ⎪ Therefore, we have, P'' i, i', g ( A ) = 1 – ⎨ ( 1 – P' i, i', g ( A ) ) × ⎪ ⎪ ⎪ ⎩
∏ k ∈ TRGs ARFCN
⎫ ⎪ ⎪ ⎪ ( 1 – ξ ii'kgg'k' ) ⎬ – P' i, i', g ( A ) ⎪ ⎪ ⎪ ⎭
g' ∈ 2 k' ∈ TRXs of k using g'
In the above formula, if ( i = k ) , then ( k' ≠ i' ) , so that interference with itself is not taken into account. The sum is limited to 100% of the TRX traffic. INTERF_GAIN ( i, k, g , g' ) is quite similar to SEP_GAIN ( i, k, g , g' ) . The only difference is the frequency diversity gain, F_DIV ( g ) , added to SEP_GAIN ( i, k, g , g' ) .
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8.1.4
I_DIV, F_DIV and Other Advanced Cost Parameters When combining interference effects (or separation violation effects) on different frequencies belonging to a MAL, the following considerations should be taken into account: 1. Non-linearity of Frame Error Rate (FER) with respect to average C/I conditions and MAL length. 2. Interference Diversity Gain. This factor represents that the effect of average negative effects over user geographic location are directly proportional to the MAL length. 3. Frequency Diversity Gain. This factor models the gain due to diversity of multi-path effects and should be applied to the interference cost component only. 4. The fact that long MALs with synthesized hopping permit discarding the worst case estimation and include a gain due to DTX and low traffic load at the interferer end. The Advanced properties tab shown in the figure below facilitates modelling these effects. The Interference Diversity Gain table lists the values of I_DIV provided as a functions of MAL length. This gain is applied to the interference cost component and to the separation constraint violation cost component. Therefore, it provides a means to model the non-linear FER effects and interference diversity both. The default values in this table correspond to the curve y = 2 × log 10 ( x ) . This equation generates values somewhat lower than empirical best-found values (this is because we prefer a slightly pessimistic cost function to be on the safe side). The other table contains the F_DIV values, which are the same as the I_DIV values by default.
Figure 8.1: Atoll AFP Module Properties - Advanced Tab
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8.2
Appendix 2: Interferences In the Technical Reference Guide, Chapter 3.8 (File Formats: Interference Histogram Formats), there is a detailed description of the Atoll’s interference representation. The User Manual contains additional descriptions of interference usage: importing, calculating, keeping up-to-date etc. This appendix explains what is carried out at high level.
8.2.1
Using Interferences If interferences are to be taken into account by the AFP, they must be calculated or imported beforehand. In order to do this, the user should previously decide to take interferences into account (enabling the loading of all the potential interferers). Otherwise, Atoll does not allow performing their computation by disabling the histogram part in the corresponding dialog.
8.2.2
Cumulative Density Function of C/I Levels For each [interfered subcell, interfering subcell] pair, Atoll calculates a C/I value on each bin of the interfered subcell service area (as if the two subcells share the same channel). Then, Atoll integrates these C/I values to determine a C/I distribution and transforms this distribution function into a cumulative density function in the normal way. In Atoll, both the IMco and IMadj are represented by this Cumulative Density function This implies that each query for the probability to have C/I conditions worse than X dB requires a single memory access: the co-channel interference probability at X dB. In order to deduce the adjacent interference probability value, Atoll looks up the cumulative density function at the value corresponding to X - Y dB, Y dB being the adjacency suppression value. The following example may be helpful in further clarifying this concept: Example: Let [TX1, BCCH] and [TX2, BCCH] be the interfered and interfering subcells respectively. The service areas for both have been defined by Best Server with 0 dB margin. The interference probability is stated in percentage of interfered area.
Figure 8.2: The cumulative density of C/I levels between [TX1, BCCH] and [TX2, BCCH] In this case, we observe that the probability for C/I (BCCH of TX2 effecting the BCCH of TX1) being greater than 0 is 100% (which is normal because TX1 is the Best Server). The probability of having a C/I value at least equal to 31 dB is 31.1%. For a required C/I level of 12 dB on the BCCH of TX1, the interference probability is 6.5% (as this requirement is fulfilled with a probability of 93.5%). Note: •
8.2.3
The subcell power offset does not enter the calculation results in the .clc file. It is added later by the AFP interface. On the other hand, its influence on the subcell service zone is taken into account in the .clc file.
Precise Definition Pci ( v, n, C_I ) is defined to be the probability of a communication (call) occupying a timeslot in subcell v (victim) to have C/I conditions of C_I with respect to a co-channel interference from the BCCH TRX of cell n (neighbour). We assume C_I values to be discrete and in dB. CDF(Pci) is the cumulative density function of Pci: CDF ( Pci ( v, n, C_I ) ) =
∑
Pci ( v, n, x )
x ≥ C_I
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8.2.4
Precise Interference Distribution Strategy Why does Atoll calculate and maintain precise interference distributions, while the most common solution (used by most other tools) is rather to compress the information into two values: the co-channel and adjacent-channel interference probabilities? The reason is simply that it, • • •
8.2.4.1
improves the AFP result, introduces very little (or no) overhead, and creates more generic interference information.
Direct Availability of Precise Interference Distribution to the AFP In the presence of frequency hopping, and when one or more frequencies are common (or adjacent) in two interfering MAL sequences, the hopping gain depends on following factors: • • • •
the MAL length, the traffic load on the interferer TRX, DTX level, and the number of common (and adjacent) frequencies in the two MALs.
All these factors cannot be pre-calculated since it is the AFP that determines the MAL length and the MAL frequencies.
8.2.4.2
Efficient Calculation and Storage of Interference Distribution In the innermost loop of the calculation process Atoll increments a counter each time a C/I level has a certain value. In the case of a two-entry IM, there are only two counters for each [interfered, interferer] pair. In the case of precise distribution information, there are about 40 counters per pair. In both cases, the number of operations is the same: one increment of an integer value. Once Atoll finishes the counting for an [interfered, interferer] pair, it compresses the information from the counters to a Cumulative Density Function (CDF) representation. In this way, access to interference probability at a certain level is instantaneous. Thus, the only overheads are the read / write times to the files and the memory occupation at running time. These two overheads are negligible and do not affect the calculations, the heaviest part of the task.
8.2.4.3
Robustness of the IM By having precise C/I distributions calculated and exported, the user is free to change the following settings without the need for recalculating their interference distributions: 1. Quality requirements of network elements (required C/I, % Probability Max, …), 2. C/I weighting (the interference levels above and below the C/I target), 3. Separation requirements and/or neighbour relations, 4. Hopping gain values, DTX activities, traffic load levels, HSNs, synchronisation information, 5. Any frequency assignment setting (MAL length directives, frequency domains, assignment strategies, number of required TRXs, cost function parameters, …), or 6. Remove equipment By not mixing any of the elements above, the interference information keeps its original probability units and is easier to check and validate. Therefore, the user spends less time on interference recalculations than in the case of a two-entry matrix (where “everything” is included).
8.2.5
Traffic Load and Interference Information Discrimination Atoll maintains the traffic load separate from the interference information. The reasons for implementing this strategy are explained here. Let us look at the possible alternatives to this strategy: 1. The mixed option: The interference information contains the traffic information as well. In this way, each IM entry will contain the quantity of traffic interfered if a co-channel / adjacent channel reuse exists. 2. The separated option: The AFP has separate access to traffic load information and to interference probabilities (As in Atoll). Knowing the difference between the two alternative solutions explains why the second strategy has been opted for for Atoll. However, in detail, this has been done because: • • • • • •
92
Option 2 is a superset that contains option 1. But option 1, being a subset, does not contain option 2 (i.e. once the information are mixed they cannot be separated). It does not create any overhead (the size of the additional information is negligible compared to the size of the IM). It helps keeping the unit definitions simpler. It is facilitates merging IMs with different traffic units. The traffic information can be used for weighting the separation violation component. The traffic load can be used in deciding whether a TRX can be left uncreated.
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Chapter 8: Appendices For example, if there are too many TRXs at a site and the user wishes that the AFP remove one of them, in order to be able to not violate site constraints, the AFP must know the traffic loads in order to choose a low load TRX to be removed. •
The gain introduced by the traffic load of the interferer depends on the hopping mode and the MAL size. Incorporating this gain in the IM (as a result of the mixed option) means that the IMs become hopping-mode and MALsize dependent. This is a bad idea since the AFP should be able to change the MAL. And the user should be able to change the hopping mode without recalculating the IM. In addition, an IM calculated externally to Atoll, with a non-hopping BCCH can be used for the hopping TCH.
A third option also exists. Though, this option is so practically useless due to its inefficiency. It consists in mixing IM and traffic but still keeping the traffic in its isolated form. This is again a bad idea because of the unit definition and the variety of IM sources. It involves less benefits than the option chosen in Atoll.
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8.3
Appendix 3: BSIC Allocation The soft and hard criteria of BSIC Allocation are described here. As explained earlier, not respecting the hard criterion is considered an error while not respecting the soft criterion just provokes a warning. The hard criterion is easier to satisfy but must not be violated since it causes handover failures. It is based on the second order neighbour relation and BCCH co-channel reuse. The soft criterion uses interference information as well and expands the BSIC reuse prohibition over adjacent-channel BCCH use. It aims at inducing a larger (BSIC, BCCH) reuse distance. When unable to satisfy this criterion, Atoll BSIC allocation algorithm will use the interference and separation relations to choose the “least interfering” BSIC to assign.
8.3.1
Definitions Here, • • •
“iff” is used instead of “if and only if”. “Pred” is used instead of predict. Let Pred_Nei(X, Y) be True iff X is a neighbour of Y or Y is a neighbour of X. Note: •
• • • •
8.3.2
For all X, Y in the network, if Pred_Nei(X, Y) then Pred_Nei(Y, X).
Let Pred_Nei_Of_Nei(X, Y) be true iff there exist Z such that Pred_Nei(X, Z) and Pred_Nei(Y, Z). Let Pred_Co_BCCH(X, Y) be true iff X and Y both have the same BCCH. Let Pred_Adj_BCCH(X, Y) be true iff X and Y both have adjacent BCCH frequencies. Let Pred_Int(X, Y) be true iff X interferes with Y or Y interferes with X.
Hard Criterion Prohibit using the same BSIC in both X and Y iff, (Pred_Nei_Of_Nei(X, Y) OR Pred_Nei(X, Y)) AND Pred_Co_BCCH(X, Y)
8.3.3
Soft Criterion Prohibit using the same BSIC in both X and Y iff, (Pred_Nei_Of_Nei(X, Y) OR Pred_Int(X, Y) OR Pred_Nei(X, Y)) AND (Pred_Co_BCCH(X, Y) OR Pred_Adj_BCCH(X, Y))
8.3.4
Behaviour As obvious, the soft constraint is much stronger and has a higher probability of not being respected. For any criterion not respected, the AFP issues a warning message counting the number of times the AFP was blocked due to each of the two criteria. The Consistency checking tool considers the cases where the hard criterion is not respected as errors. The previous algorithm (version 2.2.1) had a weaker soft criterion and had a tendency of using fewer BSICs. For a realistic test-bench network (using only one NCC), following BSIC usage results were obtained:
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8.4
Appendix 4: Traffic Capture and Dimensioning
8.4.1
Introduction This appendix describes key GSM (and other TDMA based) network design processes in Atoll, namely Traffic Capture process and Network Dimensioning process. The purpose of this appendix is not to detail the algorithms and formulas on which these processes are based, but rather to describe their concepts and processing steps. A glossary of terms used throughout this document is provided in the beginning to familiarize the reader with terminology employed in Atoll and in this document as well. It should be noted that the sequence in which these processes are explained is also, usually, the sequence of designing a GSM network. Though modifications (optimisation or otherwise) in a mature GSM network may not necessarily follow this rule. The features described in this appendix are: • •
Traffic Capture Network Dimensioning (and Key Performance Indicator calculation)
Traffic Capture in Atoll is defined as the process of reading raw traffic data from the traffic maps and integrating them to generate traffic demand for each subcell. In order to perform a Traffic Capture, Atoll collects traffic information (in the form of Services, Mobility types, Terminal types) from traffic maps and network parameters/criteria (HCS priorities, service zone model, HR/FR activity percentages, other compatibility criteria, etc.) from the traffic model. Then, it distributes this traffic according to the criteria over the network’s subcells, and generates and assigns the calculated traffic demand to each subcell. Traffic Demand is defined as the number of Circuit Switched/Packet Switched traffic Erlangs/kbps in a subcell. The results from this process are ready to be utilized in the next step. Network Dimensioning is the process that determines the number of TRXs per subcell required to carry the traffic assigned to it while respecting the QoS/GoS criteria defined. The traffic captured by the network’s subcells in the previous step is the main input to this process along with the Quality of Service (or Grade of Service for Circuit Switched traffic) criteria defined by the user. Assigning a large number of TRXs to a subcell may guarantee the QoS required but will make the generation of a good frequency plan more difficult. This will also increase the infrastructure cost that depends directly upon the quantity of equipment required. Therefore, network dimensioning is a more sophisticated process than a mere estimation. Additional outputs of this process are the Traffic Load values (used by the Automatic Frequency Planners) and the Key Performance Indicators (KPIs).
8.4.2
Traffic Map Generation Traffic map generation is the process of creating a geographic distribution of offered traffic demand based on user defined traffic inputs. Atoll is capable of creating, editing, importing and exporting various types of traffic maps. Atoll is capable of working with raster and vector maps, and incorporates the function of spreading live traffic data (from the network) in the form of a traffic map.
8.4.3
Traffic Capture Process Traffic capture is the process of determining and accumulating served traffic onto specific network elements and bearers. To facilitate comprehension, we will divide the description of the entire process into three parts, the inputs, the calculation engine and the outputs.
8.4.3.1
Inputs The following inputs are required for performing a traffic capture: • • •
Defined traffic model data (Services, Mobility types, Terminal types) Traffic map(s) (either internally generated, imported or edited) Network elements (Transmitters, Subcells, HCS layers, Frequency domains, equipment, …)
The traffic model data contain the requirements for a certain class of traffic. Numerous classes of traffic can be created through combinations of [Services, Mobility types, Terminal types]. For example, a Packet Switched service requiring a certain kbps data throughput, allowing a certain maximum number of simultaneous timeslots to be used, with a minimum blocking probability, together define a minimum Quality of Service criterion. When this service is accessed by a user with certain mobility through a certain type of terminal device, this constitutes a particular class of traffic. Traffic maps can comprise many such traffic classes with user densities for each geographically distributed over the entire network or a part of it. Different traffic maps, raster, vector and live data, can be overlaid as shown in the figure below. During the traffic distribution, Atoll combines or integrates the traffic data read from these multiple layers of traffic maps to generate the traffic per bin.
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Traffic Vector Map
Traffic Raster Map
Figure 8.3: Traffic Maps Overlay The next section describes how these data, traffic information, compatibility criteria, and requirement criteria, are utilized in distributing and assigning this traffic to the network’s sectors (subcells, to be precise).
8.4.3.2
The Engine The Traffic Capture performs three principal tasks. First of all, it computes a Best Server per HCS layer prediction study (with the possibility to define an handover margin and a coverage reliability level at the boundaries) to define boundaries and overlaps between sectors corresponding to different layers and to generate an estimate timeslot capacity in kbps/ timeslot per sector. You can choose whether the later be based on C or C/I and C/(I+N). Secondly, it divides the traffic input from the maps between different HCS layers of the network and between sectors of the same layer complying with the criteria defined in the data model. In the end, it integrates the traffic distributed to each subcell to generate the end result: traffic demand per [service, subcell] pair in terms of Erlangs for CS traffic and in terms of kbps for PS traffic. This traffic demand provides Atoll with an estimate of average demand in terms of # TSL used. These three steps are described one by one hereafter.
8.4.3.2.1
Traffic Distribution The distribution of traffic can itself be divided into two parts, one that is performed between different HCS layers and one that is performed between the sectors of the same layer. Here we will describe both respectively.
Inter-Layer Distribution Once the Best Server per HCS layer prediction study has been performed in the background, Atoll proceeds to distributing the traffic over the different HCS layers and between the subcells of each layer. In Atoll, a user can define any number of HCS layers as required and assign priorities to them. A possible, rather probable, priority structure could be: • • •
Micro layer: Macro layer: Umbrella layer:
assigned priority 3 assigned priority 2 assigned priority 1
Note: •
In Atoll, the priority levels increase directly as the integer assigned. This means that priority 3 is higher than priority 2, which is higher than priority 1, and so on.
Atoll starts by assigning traffic to the HCS layer with the highest priority and then moves down the priority scale. There are two levels of control for filtering the traffic for each layer: • •
Global control Local control
•
Global Control A user can define each HCS layer to have a certain priority in order to control its importance at the time of traffic distribution. The priority parameter can hence be termed as one of the global control parameters that guide Atoll to start with a certain HCS layer. Each HCS layer is assigned a maximum speed limit. Any user with mobility higher than the maximum allowed on a certain layer cannot be allocated to it. Therefore, all traffic with speeds higher than that permissible on an HCS layer will not be assigned to it. Next, each layer can have a certain operating frequency. Although this parameter is managed at the TX level, a network can very well have, for example, a GSM 900 layer and an 1800 layer. Only the traffic compatible with the operating frequency band of a layer will be distributed over that layer. A network can also have, for example, a CS GSM layer and a PS EDGE layer. This technology
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Chapter 8: Appendices compatibility criterion is also taken into account when distributing traffic over layers. This parameter is also at the level of transmitters in Atoll. •
Local Control The local control parameters enable much finer tuning at the level of subcells. Each subcell in a network can be assigned a ‘Target Traffic Overflow’. This is the percentage of traffic that is allowed to overflow from this subcell to the lower priority layer beneath it. The figure below gives an idea of the effect of this parameter over traffic distribution.
Figure 8.4: Traffiic Overflow In the example above, when the Target Traffic Overflow for the microcell layer is defined to be 0, the entire traffic (compatible to the micro layer, of course) covered by this cell will be carried by it. On the other hand, if there is a percentage of traffic allowed to overflow from the micro layer to the layer underneath (10% in this example), Atoll will be permitted in this case to assign this overflowing traffic to the macro layer. This parameter enables fine tuning of the network’s traffic distribution policy. This may be useful in order to avoid over-dimensioning the micro layer, or to avoid reaching the upper limit on the maximum number of TRXs that a sector can accommodate. To be a bit more precise on the two compatibility criteria, i.e. the frequency band and technology, this compatibility comparison is in fact performed at the sector level. Atoll compares whether the traffic to be assigned is technologically compatible with the higher priority sector before handing it over to the lower priority one. For example, a user with a GSM terminal using the 900MHz band cannot be assigned to a sector that operates on the 1800Mhz band. Similarly, a terminal accessing a PS service using the EDGE technology cannot be allocated to a sector that can only perform CS functions.
Intra-Layer Distribution This function enables the distribution of traffic between sectors of the same layer when there are more than one servers providing coverage to that traffic. As, for example, in the figure below, there is an overlap between the coverage of A and B that corresponds to a certain HO margin (fixed by the user). The traffic under this overlapping region is covered by both servers. Therefore, if the traffic complies with the compatibility criteria for both sectors, it will be equally distributed between the two. Similarly, when there are more than two servers for traffic in a bin, it will be equally distributed among all the potential servers of that bin.
Figure 8.5: Intra-Layer Distribution
8.4.3.2.2
Average Timeslot Capacity Atoll computes average TSL capacity for all sectors in terms of kbps for estimating the average TSL consumption relative to the PS traffic. This capacity can either be based on the C level or interpolation between C/I and C/(I+N) for bins in the sector’s coverage area. Knowing the average kbps per TSL through this computation, and the kbps demand from the traffic assigned to a sector, Atoll is able to generate an estimate demand in terms of # TSL for PS traffic.
8.4.3.2.3
Integration Atoll integrates the demand in terms of # TSL from the above results to generate the resulting total demand per sector for each service. This means that the CS traffic demand in Erlangs and PS traffic demand in kbps are converted into a total demand for each sector. This traffic demand per sector is later utilized when performing the network dimensioning process.
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8.4.3.3
Outputs The outputs of this entire process are CS and PS traffic demands in Erlangs and kbps respectively, the average CS and PS traffic demand in terms of # TSL, and the average traffic demand in terms of # TSL. The Traffic Capture process generates an item called ‘Traffic Capture X’, that is required as input to the dimensioning process.
Figure 8.6: Traffic Distribution in Atoll
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Chapter 8: Appendices
8.4.4
Network Dimensioning Process The Network Dimensioning process is based on the output of a preceding traffic distribution. Once the traffic from the maps has been distributed over the network following and respecting the criteria described earlier, it is possible to perform the dimensioning process for the network. Again, we will break the description into three parts to facilitate explanation and understanding.
8.4.4.1
Inputs The input to the dimensioning process is the output of the traffic distribution process described earlier. Once the traffic has been distributed over the network’s layers and sectors, the dimensioning process can be launched. Apart from the traffic demand, the dimensioning process also takes into account the minimum required QoS and GoS criteria defined for each type of service, the upper and lower limits on the # TSL for each service, and the upper limits on the number of TRX supported by each sector.
8.4.4.2
Dimensioning The dimensioning engine in Atoll converts the traffic demand into the required # TSL, and eventually the required # TRX, following the QoS and GoS criteria. For example, for a CS service, let’s say voice, allowing a minimum GoS of 2%, the traffic demand in Erlangs can easily be converted into # of TSL required to carry that traffic demand while respecting the required GoS through the Erlang B and Erlang C formulas. The type of model, either Erlang B or Erlang C, can be selected by the user. The computation of # TSL required to carry a certain PS traffic demand is more complicated than the simple conversion process described above. PS traffic implies a more complex definition of Quality of Service than the simple Grade of Service of CS traffic. It implies, apart from a certain blocking probability, a certain data throughput (kbps) to be maintained with a maximum allowable delay. These parameters that indicate the PS traffic quality requirements are known as Key Performance Indicators or KPIs.
Figure 8.7: Network Dimensioning Process
8.4.4.3
Outputs To abridge the numerous results generated at the end of a dimensioning process, we can say that the principal result is always the # TRX required for each subcell, and eventually each sector. Another chief output of the dimensioning process is the Traffic Load. More specifically, the Atoll Network Dimensioning process provides the # TSL required for CS traffic, for PS traffic, and the KPIs and the Traffic Load for each subcell. The Traffic Load is defined as the ratio of traffic demand to the # TSL carrying that traffic. It is perhaps one of the most important outputs of the process as it is further utilized in the Automatic Frequency Planning process. The consumption for a subcell can be extracted through multiplying the # TRX required by the Traffic Load and by the TSL multiplexing factor (8 for the GSM standard). Atoll also incorporates an intelligent allocation methodology for assigning the CS and PS TSL inside a TRX. The user has the possibility to define the TSL allocation schemes for each type of TRX. This is known as timeslot Configuration.
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Index
Index
Symbols % max interference 36, 51, 53, 68
A Additional cost component 86 Adjacency suppression 38, 60 Adjacent channel protection level 38 Adjacent channel violation 54 Adjacent constraint 62 Adjacent-channel reuse violation 59 Advanced cost parameters 90 AFP 13 AFP performance indicators 38 AFP process in Atoll 19 AFP scope 22 AFP weight 22, 36, 37 AFP wizard 20 AMR 14 ARFCN 13, 37, 51, 61, 67 Assignment mode 36 Atoll AFP 67 Atoll data model 35 Atom 65, 85 Audit 59 Automatic constraint violation resolution 30 Automatic frequency planner 13 Automatic neighbour allocation 44 Average timeslot capacity 97
B Baseband hopping 66 BBH 14, 65, 85 BCCH 13, 36, 66, 76, 79, 93 BCCH co-channel reuse 94 Blocking rate 47 BSIC 13, 22, 38, 67, 76, 79, 94 BSIC allocation 66, 94 BSIC domain 67 Burst collision probability 49, 60
C C/I 14 C/I coverage prediction 70 C/I distribution 58 C/I threshold 36 C/I weighting 53 Calculation zone border effect 69 CDF 14, 91 Cell edge coverage reliability 35 Channel reuse 50 Close-to-threshold C/I conditions 53 Clutter weighting 43 Co-channel interference 91 Co-channel reuse violation 59 Co-channel violation 54 Collision probability 50, 66 Combination of separation violation and interference probabilities 48 Concentric Cells 36 Consistency check tool 59, 94 Constraints 35, 59 Corrupted TRX cost 51
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Corrupted TRX cost component 86 Co-site 28, 38, 87 Cost 13, 48, 58, 61 Cost components 87 Cost distribution on frequencies 26 Cost function 13, 48, 85 Cost function parameters 52 Cost minimization 13 Cost of changing a TRX 52 Co-transmitter 28, 38, 87 Creating IMs based on traffic 44 CS 14, 43, 96 Cumulative density function of C/I levels 91 CW measurements 76
D Dimensioning 13, 43, 47, 95, 99 DLPC 14, 36 Domain range effect 62 Domain use ratio 66 Downlink power control 36 Downlink power offset 36 DTX 14, 36, 57, 60, 88 Dual-band Cells 36
E EDGE 13 EGPRS 13 Embedded interference matrices 76 Erlang 26, 43, 48, 57, 70, 85, 96 Erlang B 99 Erlang C 99 Estimating frequency plan quality 57 Exceptional pairs 21, 28, 38, 87 Extending existing neighbour relations 45 Externalised interference matrices 77
F F_DIV 90 FAP 13 FER 14, 90 FH 14 FISFE 59 FN 14 FN offsets 15 FR 14 Fractional load 26, 49, 65, 88 Frame number 66, 88 Free MAL assignment 65 Freezing mechanism 37 Frequency assignment problem 13 Frequency band 37 Frequency distribution 61 Frequency diversity gain 89 Frequency domain 36, 37, 65 Frequency hopping 92 Frequency load 62 Frequency panning techniques 70 Frequency plan 39 Frequency plan optimisation 43 Frequency plan quality 57 Frequency planning 13, 36
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G Generator initilialisation 24 Global cost 51 Global separation fitness expression 59 GoS 14, 95 GPRS 13, 38 Group constrained mode 36, 65 Group constrained subcells 51 GSM 13
H Handover 46, 66 Hard criterion 66, 94 HCS 14, 96 HCS layers 35, 76 Histogram 57 HO 14 HO margin 35 HO statistics 45 Hopping gain 65 Hopping mode 36, 37 HR 14, 43 HSN 22, 36, 51, 59, 65, 67, 85 HSN assignment 65
I I_DIV 90 IM 13, 35, 43, 44, 61, 68, 92 Importing neighbour importance 45 Inner zone 36 Interfered Erlangs 49, 51 Interfered traffic 48, 53, 57 Interfered zones 57 Interference 31, 38, 51, 57, 68, 91 Interference calculation 68 Interference component 26 Interference cost 49, 61 Interference cost component 88 Interference distribution strategy 92 Interference histogram 88 Interference information discrimination 92 Interference matrices 23, 57, 75 Interference matrix combination 80 Interference matrix context 77 Interference matrix scope 79 Interference probability 48 Interference study quality criteria 69 Interference weight 52 Interferer diversity gain 88 Introduction to the AFP cost function 48
K kbps 14, 43, 96 Key performance indicators 95 Key roles of subcells 36 KPI 14, 21, 43
L Local domain restrictions 36 Lower bound interference matrix 76
102
M MAIO 22, 31, 37, 38, 51, 59, 65 MAIO allocation 66 MAL 14, 22, 36, 37, 38, 50, 51, 59, 65, 92 MAL length 67 MAL length determination 65 MAL size 62 Managing consistency in Atoll and the AFP 68 Manual frequency allocation for NH 31 Manual frequency allocation for SFH 31 Max MAL length 65, 66 Maximum likelihood combination 75, 77 Maximum MAL length 49 Maximum number of subcells 67 Maximum number of TRXs 48 Means to evaluate frequency plans 57 Minimum C/I 36, 49 Missing TRX cost 50 Missing TRX cost component 86 Mobility 35 Modifiable and non-modifiable costs 51 Modifiable cost 26 Modified TRX component 26
N Neighbour 28, 38, 59, 62, 87 Neighbour allocation 44, 68 Neighbour importance 38, 44 Neighbour relations 44 Neighbour separation violation 48 Network dimensioning 68 Network dimensioning process 99 NH 14, 65, 85 NH TRX 51 Non-frozen cost 51 Non-synchronous subcells 66 Number of different frequency domains 67
O OMC 38, 45, 76 Optimal dimensioning of an existing network 47 Optimising hopping gains 65 Out-of-domain frequency assignment cost 51 Out-of-domain frequency assignment cost component 86
P Pair-wise interference matrices 57 Pair-wise violation 60 Partial sources of neighbour importance 46 Performance and memory issues in large GSM projects 70 Point analysis 60 Power offset 36, 60, 68 Predefined MAL assignment 65 Prediction studies 68 Probabilistic cost combination 50 Probability threshold 36 Propagation 35 PS 14, 43, 96
Q QoS 14, 95 Quality indicator 62 Quality indicators 78 Quality target 36, 51, 53 Quality threshold 58
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Index
R Radio resource management 36 Rank 38 Raster traffic map 43 Reception threshold 36, 68 Required quality threshold 36 Required TRXs 51 RRM 14, 36, 38 RXLEV 76
TRX rank 15, 38, 58 TSC 14, 67 TSL 13, 43, 96 TX 13
U Uniform frequency usage distribution 61 Upper bound interference matrix 76 Usage distribution on frequencies 26 User profile 43
S Safety margin 36, 53 Scan data drive tests 76 Scope and context of interference matrices 77 Second order neighbour relation 66 Separation component 86 Separation constraint 38, 62 Separation constraint violation 28 Separation cost 49 Separation fitness 59 Separation rule priority 38 Separation violation 48, 59, 60 Separation violation cost component 26, 39, 87 Separation weight 52 SeparationConstraints table 38 SeparationRules table 38 Service zone 36 Service zone of a subcell 68 SFH 14, 59, 65 SFH TRX 51 Soft criterion 66, 94 Spectrum 62 Spectrum administration 37 Staggered MAIO allocation 66 Standard deviation 35 Subcell 14, 36 Subcell audit 37 Subcell quality threshold. 58 Synchronous networks 65 Synchronous subcells 66 Synthesised hopping 62
V Vector traffic map 43
W Worst case combination 75 Worst case interference study 58 Worst interferer 58
T Target computation time 24 TCH 14, 36, 66, 93 TCH_INNER 14, 36 TCH_OUTER 36 TDMA 14 Test mobile data 76 Thermal noise 36 Timeslot configuration 43 TL 14 TN offsets 15 Total cost 26, 39 Traffic capture 21, 43, 47, 95 Traffic capture process 95 Traffic distribution 96 Traffic environment 43 Traffic load 21, 36, 43, 57, 65, 68, 92, 99 Traffic map 43 Traffic model 43 Traffic overflow 35 Traffic overflow rate 36 Traffic weighting 58 Training sequence code 67 Transmitter 14 TRG 85 TRX 13, 15, 36, 85, 99 TRX based cost function 86 TRX based interference study 57 TRX cost 49
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