Fundamentals of RF Planning

D) Fundamentals of RF Planning Chapter 1

Introduction to RF Planning

Chapter 2

Propagation-1

Chapter 3

Propagation-2

Chapter 4

Frequency planning

Chapter 5

Antenna Fundamentals

Chapter 6

Advanced RF Planning

Chapter 7

Extending Cell Range

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Chapter 1

Introduction to RF Planning

Contents: Introduction to RF Planning Objectives The Planning Process Aims of the planning process Basic Planning Process Power Budget Preparation Power Budget Preparation Summary

Objectives:

The aims of this section are to enable the student to: • Describe basic GSM RF Planning and optimization • List the important characteristics of a good RF plan • List the basic steps involved in Cell/RF planning • Prepare a single Power Budget for both uplink and downlink

The Planning Process A well planned network not only gives reliable operation, it also provides a cost-effective network coupled with a high quality of service. There are certain issues that need to be examined to produce an effective network plan. These include: Sufficient capacity support Efficient use of the available frequency spectrum The minimum number of sites to provide the required service Flexibility for future expansion Adequate coverage in a given area with the minimum of interference. The first step in any plan will be to assess the requirements of the customer. This information will include: Business plan The number of subscribers and their distribution Grade of service Local constraints Available frequency spectrum Once the planned system is implemented, all assumptions need to be validated by following an optimisation process. The planning process can be simplified into 4 main headings: Capacity planning Coverage planning Parameter planning Optimisation Figure 1-1 illustrates a simplified diagram which itemises the planning process. Figure 1-2 illustrates a basic outline of the optimisation process. NOTE The information given in Figure 1-2 under ’Recommendations’ is not in any particular order of application.


Figure 1-2


Aims of the planning process Once the planning and optimisation processes have been carried out, the following aims should have been met: Coverage as per expectations and customer request. Co-channel and Adjacent channel interference levels as predicted and within limits. Minimum adjustments required to antennas during the optimisation process. Planning process should be well structured and optimisation restricted to the final stages of implementation and the commissioning of new sites. Expansion of the system should be easy and require minimum disruption and changes to the network. Basic Planning Process In order to provide the basic elements of a well planned network (good coverage and adequate capacity), the planner needs to know the customer’ s expectations. These may be specified as number of subscribers for a given coverage area or a set number of sites in an area for example. Armed with either of these, the planner can then begin to assess the number sites required (for a given number of subscribers), or the capacity capabilities (if given a number of sites). Certain assumptions are made for the planning process: 25 mE average traffic per subscriber Grade of Service (air interface) 2% Mobile to Mobile traffic 10% (Mobile originated & Mobile terminated) Mobile to PSTN traffic 70% (Mobile originated) Land to Mobile traffic 20% (Mobile terminated) Average Call duration 90 secs Traffic Capacity of 1 carrier with 7 TCHs: 2.94E (approximately 118 subscribers). A 1/1/1 site will have capacity of approximately 350 subscribers. Traffic capacity of 2 carriers with 14 TCHs: 8.2E (approximately 330 subscribers). A 2/2/2 site will have a capacity of about 990 subscribers. As an example, if the customer has given the maximum number of sites for a city as 20, then the capacity of those sites would be as follows: For a 1/1/1 site: 350 subscribers per site - 350 x 20 = 7000 subscribers For a 2/2/2 site: 990 subscribers per site - 990 x 20 = 19800 subscribers If the customer specified capacity required was for 10000 subscribers, then to support these subscribers using 1/1/1 sites, the planner would calculate 29 sites would easily support the subscribers. For a network utilising 2/2/2 sites, 11 sites would support the 10000 subscribers. The actual deployment of the sites and their configuration would depend on the subscriber distribution within the coverage area. Table 1-1 illustrates how subscribers/erlangs may be distributed over a given area. Subscriber Distribution Table 1-1 Subscriber Distribution Area Type

Traffic %

Erlangs

Subscribers

1/1/1 Sites

2/2/2 Sites

Urban - High Density

20

50

2000

6

2

Urban

30

75

3000

9

3

Industrial

15

37.5

1500

5

2


Suburban

25

62.5

2500

8

3

Highways

5

12.5

500

2

1

Quasi-open

5

12.5

500

2

1

Totals

100

250

10000

32

12

If it is decided that the sites whose traffic is _ 20% will be 2/2/2 configuration and the rest will be 1/1/1, then the total sites would be: 8 @ 2/2/2 + 9 @ 1/1/1 = 17 sites It is possible to calculate an approximate area covered for a given number of sites. For instance, a cell with a 1km radius can give coverage for about 3sq.km. Therefore, if we needed to provide coverage for a city with an area of 250sq. km, then we would require approximately 84 sites. The number of sites could be reduced if the less busy areas of the city were covered utilising larger cells with the more dense capacity areas using smaller cells. Any area can be divided into one or more of the following areas: Urban Suburban Quasi-Open Open Water Once sample site areas (areas selected which take into account all types of clutter) have been chosen several basic steps then need to be carried out: Site surveys - assessing building heights and construction obstructions, foliage, orientation of sectors, local legal requirements. Preparation of power budgets. Propagation tests - using drive tests to obtain data for the site to calculate coverage probabilities. Propagation model adjustment - using drive test results to modify propagation model for more accurate prediction. Power Budget Preparation One of the first steps in the planning process is the production of a power budget for both uplink and downlink. Table 1-2 shows a power budget spreadsheet using typical values. The following however should be noted: BTS receiver sensitivity is quoted at -107dBm, but it could be as good as 110dBm MS receiver sensitivity is quoted at -102dBm, but could be as good as 105dBm The actual peak power of a MS is typically 31-32dBm even though the peak power is stated at 33dBm, this means that the uplink power budget is normally 1-2dB worse than in Table 1-2. Diversity gain, although nominally quoted at 3dB, could be anything from 0-4dBm depending upon environment, MS location, diversity type, etc. The maximum permitted path loss (MPL) in the downlink is 2dB more than the uplink allowable loss. This may mean reduction of BTS output by 2dB to maintain system balance. If the difference was only 1dBm, then the BTS cannot be adjusted down to obtain system balance because it can only be adjusted in 2dB steps. In this case, the lower value of MPL would be taken as the design parameter. The majority of the system losses/gains are the same because they are equipment related. Fade margin is a function of the area coverage probability. If it is 4dB for 90% coverage, then the minimum isotropic receive Power that is required for 90% coverage probability outdoors is -92dBm. Fade Margin is discussed in a later section.


Power Budget Preparation The allowable path loss calculated in the power budget is the absolute maximum permissable and includes: Free Space Loss Clutter Factors Coverage Confidence Level (Probability of coverage of the area) If the actual loss value is better than that calculated, then the cell size is adequate to give the performance predicted. With the cell commissioned, drive test readings can be taken to give actual signal strengths. With these values, statistical tools can then be employed to produce coverage probabilities and the required Fade Margin. By using these values in the Power Budget, the processes can be repeated until the system is ’ fine tuned’ to produce the radius and performance required.


Power Budget will be discussed again in the Propagation Models section. Summary RF planning can be broken down into several basic steps and requires understanding of the following: Propagation Models Coverage requirements Link (Power) Budgets Antenna factors Frequency Planning and re-use


Chapter 2

Propagation-1

Contents: Propagation – 1 Objective Radio Signal Propagation Multipath Environment Knife-Edge Diffraction Cell Defination Urban Propagation Environment Building Penetration Propagation Models Okumura-Hata Model Hata’ s Propagation Formula Corrections to Okumura-Hata Model Cost 231-Hata Propagation Model Example 1 Cell Radius Example 2 Example 3 Walfisch-Ikegami Model Line-Of-Sight Propagation Non Line-Of-Sight Propagation Non Line-Of-Sight Propagation (cont’ d) Microcellular Environment Fresnel Zones Example 4 Ray Tracing Model Propagation Model Selection

Objectives: The aims of this section are to enable the student to show basic knowledge of, and calculate: • Radio propagation in fre space • Fresnel zones and their effect • Effects of the environment on radio propagation • Building losses and In-Building coverage • Path loss in different propagation environments and the use of relevant propagation models in prediction • Macro and Micro cellular principles



Multipath Environment In a mobile environment, there is seldom a direct line of sight between the mobile and the BTS. Hence the pure free space path loss calculated as per the formula given in the previous page is not directly applicable. The signal almost always arrives via multiple paths at the receiver end, be it a mobile or the BTS as shown in Figure 2-2. The multi path is due to reflection, diffraction and scattering of radio waves. The extent of these effects depends on the type and the total area of the obstruction. For instance, a plain surface will cause maximum reflection whilst a sharp edge like the corner of a building will cause scattering of signals known as ’ Knife-Edge Diffraction’ .

Knife-Edge Diffraction Propagation over rough terrain is dependant on the size of objects encountered over that terrain with respect to the frequency used.


If the wavelength of the signal is much less than the size of the object, then waves will be reflected. If the wavelength of the signal is much greater than the size of the object, the effect will be minimal. Values in between these extremes will have variable effect on propagation. The signals will tend to ’ curve’ around the objects. Figure 2-3 shows the effect on the signal of Knife-Edge Diffraction. Knife-Edge Diffraction

Cell Definition A cell is a geographical area, which is covered by radio signals. Conventionally, a practical cell is considered to have an irregular shape, with uniform Receive Signal Strength (RSS) all around. This is shown in Figure 2-4 (a). However, it is convenient to assume a regular shape for analytical and planning purposes. Ideally a cell should be circular in shape Figure 2-4 (b) with varying signal strengths all around. From a geometrical point of view this can be approximated by a hexagon, with different RSS values on the sides. As illustrated in Figure 2-4 (c).


Urban Propagation Environment Of all the types of propagation environment considered in a mobile communications network, the most common, and also most unpredictable is the Urban environment. The Urban environment consists of many types of ’ obstruction’ . The predominant features are the buildings within the area. As well as the effects of the environment (buildings, foliage etc.) on external propagation levels, account must be taken of the effect the construction of the buildings has on signal levels inside the buildings themselves. Building Penetration The attenuation given by a building is simply the difference between signal level outside the building and the level inside. The attenuation afforded by a building will depend on several factors: Construction materials Thickness of walls Size of the building Angle of arrival of the signals Typically, signal values can vary by as much as -40dB to 80dB depending on the various factors. Generally speaking, a building with a wall facing the signal origin will offer less penetration loss than one which is at an angle to the signal source. Also, Doors and windows will offer less resistance to RF signals than walls and will thus provide a better penetration of RF signals. Another factor which can contribute to the degradation of signals within a building is the amount of furnishing within it. A fully furnished building can give 2-3dB more attenuation than one which is empty. Figure 2-5 gives some typical values for varying building types and uses. NOTE The values given are for illustration purposes only and are not definitive. Building Attenuation Type of Building

Attenuation in dBs

Farms, Wooden Houses, Sport Halls

0-3

Small offices, Parking lots, Independent houses, Small apartment blocks

4-7


Row houses, Offices in containers, Offices, Apartment blocks

8-11 dB

Offices with large areas

12 -15 dB

Medium Factories, workshops without roof top windows

16 -19

Halls of metal, without windows

20 -23

Shopping malls, ware houses, buildings with metal/glass

24 -27

Figure 2-5 Propagation Models Propagation models are effectively a set way of applying an environment’ s characteristics to calculations to produce a prediction of signal levels within that environment. Models are created for different environments by carrying out tests at selected frequencies, over varying distances and times and with varying antenna heights. The data retrieved from these tests is then analyzed using mathematical tools and a curve is produced. Imperical formulae to match these curves are then generated and used as propagation models. Common propagation models are Log-Distance model Longley-Rice Model (Irregular terrain) Okumura Hata Cost 231 - Hata (Similar to Hata; used for 1500-2000MHz frequencies) Walfish-Ikegami Cost 231 Walfisch-Xia JTC XLOS (Motorola Proprietary) Deterministic Microcell Model (DMM) Bullington Du Path loss model Diffracting screens model Of all the models listed, the Hata and Walfisch-Ikegami models are agreed to be the most important. Motorola’ s NetPlan uses the proprietary XLOS model as well as the Deterministic Microcell Model (DMM). Any given model is only as accurate as the information provided to it and assuming it is used in the correct environment for which it is intended. For example, the Hata model is suitable for use in Urban/Suburban areas. While the Walfisch-Ikegami is more suited to the dense urban Microcell type areas. In the following pages, the common models will be examined in more detail. Common Propagation Models Log Distance model Longley-Rice Model (Irregular terrain) Okumura Hata Cost 231 - Hata (Similar to Hata; used for 1500-2000MHz frequencies) Walfish-Ikegami Cost 231 Walfisch-Xia JTC


XLOS (Motorola Proprietary) Deterministic Microcell Model (DMM) Bullington Du Path loss model Diffracting screens model Okumura-Hata Model In the early 1960’ s a Japanese engineer named Okumura carried out a series of detailed propagation tests for land-mobile radio services at various different frequencies. The frequencies were 200 MHz in the VHF band and 453, 922, 1310, 1430 and 1920 MHz in the UHF band. The results were statistically analyzed and described for distance and frequency dependencies of median field strength, location variabilities and antenna height gain factors for the base and mobile stations in urban, suburban and open areas over quasismooth terrain. The correction factors corresponding to various terrain parameters for irregular terrain, such as rolling hills, isolated mountain areas, general sloped terrain and mixed land-sea path were defined by Okumura. As a result of these tests carried out primarily in the Tokyo area, a method for predicting field strength and service area for a given terrain of a land mobile radio system was defined. The Okumura method is valid for: frequency range of 150 to 2000 MHz distances between the base station and the mobile stations of 1 to 100 km base station effective antenna heights of 30 to 100 m. MS antenna height assumed as 1.5m The results of the median field strength at the stated frequencies were displayed graphically. Different graphs were drawn for each of the test frequencies in each of the terrain environments (eg. urban, suburban, hilly terrain etc.) Also shown on these graphs were the various antenna heights used at the test transmitter base stations. The graphs show the median field strength in relation to the distance in km from the site. Figure 2-6 illustrates some of the resultant curves that were produced. As this is a graphical representation of results it does not transfer easily into a computer environment. However, the results provided by Okumura are the basis on which path loss prediction equations have been formulated. The most important work has been carried out by another Japanese engineer named Hata. Hata has taken Okumura’ s graphical results and derived an equation to calculate the path loss in various environments. These equations have been modified to take into account the differences between the Japanese terrain and the type of terrain experienced in Western Europe. Hata’s Propagation Formula Hata used the information contained in Okumura’ s propagation loss report of the early 1960’ s, which presented its results graphically, to define a series of empirical formulas to allow propagation prediction to be done on computers. The propagation loss in an urban area can be presented as a simple formula of: A + B log 10 R where: is: A frequency function B antenna height function R the distance from the transmitter.


Hata, using this basic formula which is applicable to radio systems is the UHF and VHF frequency ranges, added an error factor to the basic formula to produce a series of equations to predict path loss. To facilitate this action Hata has set a series of limitations which must be observed when using this empirical calculation method:







Street Canyon Effect


Non Line-Of-Sight Propagation Here, we assume that the BTS antenna is above roof level for any building within the cell and that there is no line of sight between the BTS and the mobile. We define the following parameters with reference to Figure 2-9: w the distance between street mobile and building hm mobile antenna height hB BTS antenna heights hr height of roof. hB difference between BTS height and roof top. hm difference between mobile height and rooftop. Under non line of sight propagation conditions, for the sake of simplicity, we assume that the environment has buildings of uniform height. For a mobile on the street, the signal undergoes diffraction from rooftops and also multiple diffraction due to the surrounding buildings. The total path loss is given by:





Fresnel Zones We know that radio signals get diffracted when they encounter an obstacle. We can imagine the signal to travel with spherical wave fronts. Looking at the cross section, Fresnel Zones are a set of concentric circles, which are loci of all points having the same signal strength. The Fresnel zones are apart from each other. Figure 2-12‘ illustrates the nature of Fresnel Zones. The radius of the Fresnel Zone is dependent on frequency and antenna height. For a given antenna height the signal will propagate further before the FIRST Fresnel Zone touches the ground. Also, the diffraction is maximum when the difference between the direct ray and the diffracted ray is Therefore we can write that

where d0 is the break point. The path loss slope is similar to LOS path loss within the break point. Diffractions and Multi path phenomena usually happen beyond this point.


/2.

Ray Tracing Model The propagation of radio waves could be studied by using either Statistical Prediction algorithms or Deterministic models. The latter are more accurate than the former, but require large computation time. Deterministic models relate the propagation parameters to the physical structure of the buildings, such as the wall orientation, materials used, their refraction and diffraction coefficients etc. The statistical models on the other hand only look at the path losses based on measurements made between buildings. The Ray tracing model of the NetPlan is one such Deterministic model. It treats the walls, roofs and floors as black mirrors. Losses in the path between transmitters and receivers are calculated using the mechanisms of direct transmission, reflection and diffraction. In small areas with “soft” walls (few metallic frames, unglazed surfaces), direct transmission and reflection are the most predominant mechanisms. Larger micro cell environments where buildings provide a canyon for propagation, diffraction is the major mechanism of propagation. When the beam strikes a wall, part of it gets reflected and the rest goes ‘through’ the wall. There are multiple reflections within the wall as shown in Figure 2-14. The ray tracing is performed by studying the wave’ s arrival time, intensity, phase and direction of impact. The intensity of each beam is a function of wall material, thickness and incident angle. NetPlan provides conductivity constants for various types of materials used in buildings. Diffraction is the predominant mode of propagation when the beam strikes the corner of a building. Further discussions on the ray-tracing model are beyond the scope of our


Propagation Model Selection Table 2-1 shows table giving general usage of the various propagation models. It is by no means exhaustive and is only intended as a guideline for model selection. The model chosen will be dictated by conditions specific to the area to be planned for. Propagation Model Selection Table 2-1 Environment Type

Model

Dense Urban Street Canyon propagation

Walfisch-Ikegami, LOS

Non LOS conditions, Microcells

Walfisch-Ikegami COST 231

Macrocells, antenna above rooftop

Okumura-Hata

Urban Urban areas

Walfisch-Ikegami

Mix of buildings of varying heights, vegetaion and open areas

Okumura-Hata

Suburban Business and residential areas, open areas

Okumura-Hata

Rural Large open areas, fields, difficult terrain with obstacles

Okumura-Hata


Chapter 3

Propagation-2

Contents: Propagation – 2 Objectives Radio Link Design Calculation of Mobile Receiver Sensivity Example 5 Signal Variations Probability Density Functions (PDFs) of Signals Variation of Gaussian Curve for Varying values of s Calculation of Standard Deviation Example 6 Example 6 Confidence Intervals The Concept of Normalized Standard Deviation Example 7 Calculation of Edge Probability and Fade Margin Example 8 Example 9 Example 10 In-Building Margins Example 11 Example 12 Fussy Logic Vs Fuzzy Logic Coverage Plots Cell Planning and C/I Example 13 Co-Channel interference C/I – Omni Cells Co-Channel interference C/I-Sectored Cells Adjacent Channel Interference

Objectives: The aims of this section are to enable the student to show basic knowledge of, and calculate: Explain the need for radio link design and calculate acceptable path loss for a given power budget Explain and calculate Fade Margin and signal variation Explain and calculate Probability Density Functions (PDF) of signals Calculate Standard Deviation Explain and calculate Normal Distribution and confidence intervals Explain and calculate Edge Probability Calculate In-Building coverage Explain coverage plots Explain and calculate C/I ratio for both Co-channel and Adjacent channel Interference


Radio Link Design The primary objective of Radio Link Design is to provide RF coverage over a desired area with a good deal of certainty. In practical terms it involves preparation of a Power budget, which takes into account the receiver’ s sensitivity, BTS transmit power, path losses etc. The link budget we studied in Chapter 1 could be described by a Level diagram shown in Figure 3-1. From the diagram we can write an expression for the maximum allowable path loss as: LPmax = PT – Pmob.rec + GT + GR – (LFT + LFR + FM) Where, LFT and LFR are feeder and connector losses at the transmitter and receiver respectively. FM is the fade margin. In RF design, the key factors are the relationship between Path Loss and Coverage area as well as how to arrive at the Fade Margin.

Calculation of Mobile Receiver Sensitivity The mobile should get a minimum signal which is above the thermal noise, with a specified Carrier to Noise ratio. It should also have adequate cushion to take care of any degradation in the performance of the RF circuitry due to aging, temperature variations etc. The noise level at the receiver is calculated as follows: NR = kTB Where: k is: the Boltzmann’ s constant = 1.38 x 10- 20 (mW/Hz/0Kelvin.) T the receiver noise temperature in degrees Kelvin. B the receiver bandwidth in Hz.




Variation of Gaussian Curve for varying values The normal or the Gaussian distribution depends upon the value of Standard Deviation. We get a different curve for each value of ó. The total area under the normal curve is unity.This is illustrated in Figure 3-5.



Figure 3-6


Area under Normal Curve


NOTE We added 0.5 to 0.4474 because, the condition that the RSS is better than –92dBm is true for the entire right hand side of the normal curve. Calculation of Edge Probability and Fade Margin: We define the following parameters: Propagation Index: This is the attenuation constant . This can be theoretically computed by using the formulae applicable for the specific propagation model chosen for the cell site. Or we can obtain RSS at various points at a desired distance from the BTS using drive tests and plot RSS vs distance. From the plot we can obtain the propagation constant. Area Probability: This is the fraction of the total area within which the RSS will be above a specified threshold.





Example 9 For the RSS calculated in Example 8, prepare a power budget for the uplink and down links. RSS required is – 92.425 dBm. This is taken as the sensitivity limit of the mobile. Example 9 For the RSS calculated in Example 8, prepare a power budget for the uplink and down links. RSS required is – 92.425 dBm.


Table 3-4



Table 3-5 Area

75% coverage

50% coverage

Central business area

20 dB

15 dB

Residential area

15 dB

12 dB

Industrial Area

12 dB

10 dB

In Car

6 to 8 dB


If the minimum RF signal strength for 90% coverage on the street is –92dBm, then, for 75% in building coverage in a central business area, we should have a signal level of –72dBm on the street. This will provide –92dBm inside the building. When we take in to account the building penetration loss as explained in the previous page, the cell radius will be reduced.


Fussy Logic Vs Fuzzy Logic Up to now, all the models that we have studied are purely empirical. The formulae we used do not at all take care of all possible environments. Hence only an iterative process could achieve the accuracy of planning based on these models. We also have computer tools to do the job of performing the elaborate and complex calculations, given the parameters and assumptions. Fuzzy Logic could be useful for experienced planners in making right guesses! Certain assumptions can be made: Divide the environment into 5 categories; Free Space Rural Suburban Urban Dense Urban. We assign specific attenuation constant values to each category, say, 0, 1 4.Fuzzy Logic helps us to guess the right value for , the attenuation constant for an environment which is neither rural nor suburban nor urban but a mixture, with a strong resemblance to one of the major categories. The following simple rules could be used:


Coverage Plots By using computers, we can calculate the coverage probabilities at various points from the BTS and plot Coverage Contour plots. Figure 3-9 shows a single coverage plot for a cell site and Figure 3-10 a composite coverage plot for a locality respectively.


Coverage Plots

Figure 3-9


Coverage Plots

Figure 3-10



Figure 3-12

.




Table 3-6 N

D/R = 3N

C/I = 10log [1/6(D/R)3.5]

3

3

8.917dB

4

3.46

11.08dB

7

4.58

15.35dB

9

5.19

17.25dB

12

6

19.45dB



Chapter 4

Frequency planning

Contents: Frequency Planning Objectives Frequency Planning and Re-use Patterns TCH re-use planning example Directional Re-use

Objectives: State the reasons for good frequency planning Explain the concept of manual frequency planning Explain the concept of automatic frequency planning Describe the concept of frequency reuse patterns Explain directional frequency reuse Frequency Planning and Re-use Patterns The ultimate goal of frequency planning in a GSM network is attaining and maintaining the highest possible C/I ratio everywhere within the network coverage area. A general requirement is at least 12 dB C/I, allowing tolerance in signal fading above the 9dB specification of GSM. The actual plan of a real network is a function of its operating environment (geography, RF, etc.) and there is no universal textbook plan that suits every network. Nevertheless, some practical guidelines gathered from experience can help to reduce the planning cycle time. Rules for synthesizer frequency hopping (SFH) As the BCCH carrier is not hopping, it is strongly recommended to separate bands for BCCH and TCH, as shown below

This has the benefits of: _ Making planning simpler _ Better control of interference If microcells are included in the frequency plan, the band usage shown below is suggested. Practical rules for TCH 1x3 re-use pattern BCCH re-use plan: 4x3 or 5x3, depending on the bandwidth available and operating environment. Divide the dedicated band for TCH into 3 groups with an equal number of frequencies (N). These frequencies will be the ARFCN equipped in the MA list of a hoUse an equal number of frequencies in all cells within the hopping area. The allocation of frequencies to each sector is recommended to be in a regular or continuous sequence (see planning example).pping system (FHI). The number of frequencies (N) in each group is determined by the design loading factor (or carrier-tofrequency ratio). A theoretical maximum of 50% is permitted in 1x3 SFH. Any value higher than 50% would practically result unacceptable quality.


Some commonly used loading factors (sometimes termed as fractional load factors) are 40%, 33%, 25%, etc.

No more than 48 frequencies in a cell with multiple carriers with GPRS timeslots Use the same HSN for sectors within the same site. Use different HSNs for different sites. This will help to randomize the co-channel interference level between the sites. Use different MAIOs to control adjacent channel interference between the sectors within a site TCH re-use planning example Bandwidth: 10 MHz S Site configuration: Mix of 2-2-2, 3-3-3 and 4-4-4 S Loading factor: 33% S Environment: Multi layer (micro and macro co-exist) The spectrum is split as shown below

A total of 49 channels are available and the first and last one are reserved as guard bands. Thus, there are 47 usable channels. 12 channels are used in the BCCH layer with a 4x3 re-use pattern.Based on 33% loading and a 4-4-4 configuration, N is calculated as N = 3 / 0.33 = 9 hopping frequencies per cell. Thus, a total of 27 channels are required for the hopping TCH layer. The remaining 8 channels are used in the micro layer as BCCH. One of the possible frequency and parameter setting plans is outlined in the table Table 4-1. Table 4-1 ARFCN

HSN

MAIO

Sector A

21, 24, 27, 30, 33, 36, 39, 42, 45

Any from {1, 2, ..... 63}

0, 2, 4

Sector B

22, 25, 28, 31, 34, 37, 40, 43, 46

Same as above

1, 3, 5

Sector C

23, 26, 29, 32, 35, 38, 41, 44, 47

Same as above

0, 2, 4

The above MAIO setting will avoid all possible adjacent channel interference among sectors within the same site. The interference (co or adjacent channel) between sites will still exist but it is reduced by the randomization effect of the different HSNs


.


Practical rules for TCH 1x1 re-use pattern 1x1 is usually practical in rural area of low traffic density, where the average occupancy of the hopping frequencies is low. With careful planning, it can be used in high traffic areas as well. BCCH re-use plan: 4X3 or 5X3, depending on the bandwidth available and operating environment. The allocation of TCH frequencies to each sector is recommended to be in a regular or continuous sequence. Use different HSNs to reduce interference (co and adjacent channel) between the sites. Use the same HSNs for all carriers within a site and use MAIOs to avoid adjacent and co--channel interference between the carriers. Repeated or adjacent MAIOs are not to be used within the same site to avoid co-channel and adjacent channel interference respectively. A maximum loading factor of 1/6 or 16.7% is inherent in a continuous sequence of frequency allocation. Since adjacent MAIOs are restricted, the maximum number of MAIOs permitted is:


In a 3 cell site configuration, the logical maximum loading factor is 1/6 or 16.7%. The following figure illustrates how co-channel and adjacent channel interference can be avoided: Rules for baseband hopping (BBH) All the rules outlined for SFH are generally applicable to BBH. As the BCCH is in the hopping frequency list, a dedicated band separated from TCH may not be essential. An example of frequency spectrum allocation is shown below Directional Re-use In a sectored site, a group of channels (ARFCNs) is transmitted in the direction of antenna orientation. This is based on a tricellular platform consisting of 3 identical cells as shown in Figure 4-1.Every cell is considered as an OMNI logically. The cells are excited from the corners, separated by 1200. The axes of the diagram represent the 3 directions of reuse. These are designated as {f(00) }, { f(1200) } and { f(2400) } Because we use directional antennae, the worst co channel interference will be from only one interfering station in the same direction.

We form a generic combination of the tri-cell pattern using 7 such patterns, as shown in Figure 4-2. From this, we can see that each of the three axes has three parallel layers. This results in a total of six or multiples of six frequency groups. While assigning frequencies to individual cells we have to take the directions of reuse in to account.



Chapter 5

Antenna Fundamentals

Contents: Antenna Fundamentals Objectives Antenna Considerations Antenna Location Combating Multipath Fading Space Diversity Antenna Spacings Polarisation Diversity Antenna Configurations Air Combining System Antenna Specifications Downtilt Example 16 Downtilt Notch

Objectives : The objectives of this section are to enable the student to: Explain the factors to be considered regarding antenna selection Describe the advantages and disadvantages of certain antenna locations Explain antenna diversity Describe different antenna configurations Explain the various specifications given for antennas Explain downtilt

Antenna Considerations The primary objective for a proper antenna location and choice of an appropriate diversity scheme is to provide a uniform coverage within the cell area and minimum interference to and from other BTS antennae. Choice of antenna location ( cell site ) is based on proper containment of coverage and alignment of the sites in to a specific hexagonal pattern. The choice may be limited due to availability of space, links to BSC etc. Containment of Coverage in Urban/Suburban areas: In Urban areas, the following conditions usually exist: Several Sites may be needed Frequency re use is unavoidable In-building penetration is a must Large coverage obtained by keeping an antenna at a height may not satisfy in-building coverage requirements. In fact, one can rely on the buildings to serve as radio path shields, limiting the coverage area. Also the reflections from the buildings provide coverage to areas which would not have been possible in the normal LOS mode. (Street Canyons). These additional paths consequently increase in-building penetration also. Antenna Considerations Uniform Coverage in the cell Alignment with hexagonal pattern Space availability Connectivity to BSC/MSC Urban areas may have the following conditions:


• • • • • • • •

Several Sites may be needed Frequency re use is unavoidable In-building penetration is a must. Buildings act as RF shield and contain coverage. Buildings reflect signals and provide coverage to areas where LOS would have failed. Such additional paths improve in-building penetration. Antenna at a very high point may not meet In-building coverage requirements.

Antenna Location The location of an antenna needs to be chosen not only for coverage needs, but also to ensure that the minimum interference to and from other sites is acheived. Choice of location is driven by proper containment of coverage and site alignment within the confines of the specified pattern. In urban areas, there are certain conditions which prevail: Several sites will be required The re-use of frequencies is common In-building penetration needs to be provided Merely placing an antenna at the highest point is not the answer to providing best coverage. As well as giving a source of interference ot other sites in the coverage area, in-building coverage will not be fully acheived as succesfully as more specific solutions. Figure 5-1 illustrates the possibilities of antenna location within a built-up area. In the first case, whilst a large area is covered by the high mounted antenna, interference control is difficult and in-building coverage is limited. In the second instance, the buildings act as a natural containment for the propagation, and can also give coverage in areas that would otherwise be considered ’ dead spots’ . In-building coverage is also improved in this scenario. In Figure 5-2, location of the antenna at a high point within a suburban environment will be more beneficial than in a city environment and cause less of an interference problem. Antenna Location Location of antennae at high points needs careful examination of site coverage, type of area etc.


Combating Multipath Fading We have the following techniques by which the effects of Multi path fading can be minimized: In the Time Domain: Interleaving In the Frequency Domain: Frequency Hopping In the Spatial Domain: Space Diversity In the Polarisation Domain: Polarisation Diversity. Of the 4 different schemes listed above, the last two techniques are related to antenna systems. In general, a diversity antenna system provides a number of receive paths ( normally 2). The diverse output from each path is combined by the receiver to give a signal of sufficient S/N. Thus a Diversity antenna System essentially has: Two or More antennae A combiner circuitry. Another major requirement of Diversity antenna systems is that the signals arriving at the different receive paths/ports should have very low correlation. This is because if a signal is fading at one port, the chances of it happening in the other port should be LOW. This is the basis of Diversity. Antenna Diversity A Diversity antenna System essentially has: Two or More antennae A combiner circuitry

Space Diversity There are 3 ways in which Space Diversity could be realized: Horizontal Separation Vertical Separation Composite Separation Figure 5-4 shows the three different configurations for spatial diversity


Antenna Spacings: The separation between antenna is a function of the correlation coefficient. To achieve a desired correlation coefficient, say <0.7, different configurations need different spacings. Figures given in Table 5-1 are for minimum required separation. If space is not a constraint, larger separation is always recommended. Horizontal separation is preferred because it provides low correlation values. However, horizontal separation suffers from angular dependence (as in Figure 5-4). Vertical separation does not suffer much from angular dependence. It also requires minimum supporting fixtures and does not occupy a lot of space. But, as the distance increases, the correlation between the RF signals at the antenna points increases rapidly, thereby negating the very advantage of space diversity.


View Angles

Figure 5-5 Space Diversity can be achieved using: 2 antennae systems 3 antennae systems The 2 antennae system is preferred where the space for the antenna structure is limited or where the operators want to use less number of antennae. The 3 antennae system provides very good spatial separation between the two receive antennae and avoids the use of Duplexers. This reduces the risk of generating inter modulation products. Figure 5-6 shows the use of two antennas for space diversity, and Figure 5-7 shows configuration for three antenna configurations.


Figure 5-7 Polarisation Diversity Operating Principles (see Figure 5-8): A plane polarised wave has two components namely Vertical and Horizontal components. The two fields exhibit a good degree of de correlation. This means that dual polarization can be used as a diversity system. A Dual- Polarisation antenna consists of 2 sets of radiating elements which radiate, or in reciprocal, receive, 2 orthogonal fields. The antenna has 2 input connectors which separately connect to each set of the elements. The antenna has therefore the capability to transmit and receive two orthogonally polarised fields simultaneously. Figure 5-9 shows the two types of Dual-Polarisation antennas.


Advantages of Dual Polarisation: Reduced support structure for the antenna Reduced weight Slim Towers and hence quicker construction and low cost. Cost of One dual polarised antenna is generally lower than the cost of two space diversity antennae. Choice of Dual Polarisation type: H/V type: As most mobiles are held at an angle of 450, H/V is more likely to cause balanced signals at the two branches. The diversity performance is less dependent on the mobile’ s location. Slant type: Correlation between the two elements is angular dependent. Unbalanced signals at the two arms of the receive antenna, since one of the signals could be at the same angle as the mobile.

Antenna Configurations Figure 5-10 shows both the single and double antenna systems. One Antenna System: Needs a Duplexer for the port Transmitting and Receiving. Needs a Duplexer or an External Filter for the receive only arm. The filter can be avoided if the isolation between the two ports Is better than 30 dB. Two Antenna System: Receiving antenna is the Dual Polarised antenna.


Transmitting antenna is the conventional vertical poalrised antenna. Both transmit and receive antennae should have identical characteristics such as beam width, gain etc. Transmit antenna is usually mounted below the receive antenna. As most of the systems have down tilts, keeping the transmit antenna below the receive antenna minimizes shadowing of the receive antenna by the transmitter.

Air Combining System This enables both the ports of the antenna to simultaneously transmit and receive. The air combining system requires a duplexer. The system has the following advantages: It reduces combining losses. Ideal for SFH as it minimizes the losses caused by hybrid combiners. Enhances isolation, because of the Duplexer. The use of the duplexer increases the risk of high inter-modulation products. It is essential to choose low inter-mod product duplexers for this configuration. Figure 5-11 shows the configuration using the Air Combining system.




Figure 5-13 Downtilt When the main radiation lobe of the antenna is intentionally adjusted above or below its plane of propagation, the result is known as a “beam tilt”. When tiled downward, we get the “Down Tilt”. Down tilt can be done in 2 ways: Electrical Down tilt Mechanical Down tilt. Mechanical Downtilt Mechanical down tilt is done by physically changing the antenna position. A common method is by the use of scissor-type mounting brackets. Figure 5-14 illustrates the effect of Mechanical downtilt on the pattern. As can be seen from the diagram, mechanical downtilt not only causes the main lobe of the antenna to be lowered, but it also causes the back


lobe to be raised by the same degree. This can cause interference problems and therefore is not always the best solution. Lowering the Site power may be a better answer for instance.

Electrical Downtilt Electrical down tilt is achieved by changing the phase value of the RF signal at the inputs of a phased array. It is normally factory set and can be field-adjusted by changing externally the phasing cables supplied by the manufacturer. Figure 5-15 shows an example of the effect of using different length phasing cables. Figure 5-16 shows an the resultant coverage changes for differing values of electrical downtilt.


The down tilt required at a given site depends on the coverage planned. With reference to Figure 5-17, it can be seen that the coverage diminishes rapidly outside the main lobe of the transmitted signal. Keeping the interference objective in mind, it is preferable to limit the outer edge of the main lobe to the cell radius. In general down tilt angles greater than 7-10 degrees are not recommended. Also not more than 2 degrees difference in down tilt angles between any two adjacent sectors in a given site is desirable. The down tilt angle for a cell may be calculated from geometry. It is given by the equation:


Typical vertical beam width of an antenna is 10-12 degrees. Example 16

Downtilt Notch When the antenna is physically down tilted, a notch at the centre of the horizontal beam pattern is produced. This notch becomes larger as the down tilt angle increases.This notch can be effectively used to control interference as shown in Figure 5-19


Downtilt Notch


Chapter 6

Advanced RF Planning

Contents: Objectives Introduction Planning Steps Other Considerations Customer Requirements Surveys In-Building Coverage RF Propagation Tests Planning Tool Preparation and Model Calibration Model Calibration Link Budget and Other Steps Coverage Esstimation Capacity Considerations Fine Tuning Site Selection

Objectives:

The aims of this section are to enable the student to: List the various steps involved in the RF Planning process Explain each of the steps of the planning process State other factors to be considered in the planning process State customer requirements Explain the use of Drive Tests Explain the use of Planning tools Describe the customisation of planning models Perform an RF Planning exercise

Introduction Chapter 1 introduced the planning and optimisation process in general terms. This chapter looks at each step of the process in detail. The planning processes will be referenced to an imaginary city known as ’ Utopia’ . Planning Steps The first step is to understand the customer requirements in terms of: Coverage levels In-building coverage expectations Proposed roll out plans Start-up number of sites which the operator may have in mind. The next step is to Survey the city of Utopia to understand Traffic pattern and distribution Probable growth areas High business areas etc. Conducting Propagation tests for in-building coverage and on street signal level estimates. From the survey data the planning tool is set up and run. Then a draft plan is prepared by dividing the city into a number of regions like Busy business area with Excellent outdoor coverage, medium in-building coverage, good in-building coverage etc. For each of these classifications, a simple link budget along with


an appropriate propagation model, the number of sites required per region. The draft plan is then reviewed with the customer and fine tuned. Planning Steps 1. Understand Customer requirements Coverage requirements In-building coverage expectations Initial Roll out plans Pre determined number of sites ? 2. Survey: Traffic Distribution and Pattern Growth areas High density business/residential areas Propagation tests for in-building coverage estimates and model calibrations. 3. Prepare Planning Tool Get Digitized maps Load maps in the Planning tool. Use survey data and run the programme. 4. Draft Plan: Divide city in to number of regionsBusy business areas Areas that need excellent in-building coverage Medium in-building coverage areas Use appropriate model and link budgets to calculate the number of sites required per region. 5. Fine Tune Plan: Perform more drive tests, confirm plan predictions. Review plan with customer and fine tune the plan. Other Considerations The planning exercise depends on the type of site under consideration- is it a macro site ( large area ) or a mini site? Is it a micro cell or a pico cell? This is to a large extent defined by the traffic growth projections, amount of spectrum available, coverage required at the time of launch and capacity planned for the first 3-5 years of operation. To start with Macro cells may be designed to give maximum coverage with limited number of sites. As the traffic and capacity increase, cells may be split, or new cells may be added to get more focussed coverage. Tall sites are modified by reducing antenna heights to get better coverage and minimize interferences. Other Considerations Site Selection- Cell Types: Macro ( Large Cells) Mini Micro Pico Depends on:


Traffic Growth Projections Site Density required Spectrum Availability Coverage needed at launch Macro Sites to start with: Maximum Coverage Fewer Sites. Hence lower initial investment. During Growth phase: Split Cells Add New Cells Modify “ tall” sites by reducing antenna height. All this gives increased capacity and better in- building coverage. On going activities: Optimization techniques Capacity enhancements Frequency Hopping techniques Customer Requirements The success of the planning process depends on how well we understand what the customer wants.It would be a good idea to have a questionnair answered by the customer. Some of the important questions should include precise boundary limits for the network (does the Govt. have any special conditions?), areas in which medium in-building coverage is considered adequate, areas where excellent coverage is needed, what are the potential growth areas, what are the implementation strategies and specific requirements if any. Is there any limit on the initial investment for the customer? What is the minimum the customer is willing to support with reference to the competition? It is important to understand these customer requirements so that the draft plan will be in tune with what the customer wants and fine tuning becomes easier. Customer Requirements What are the boundaries for the network? Are there any special pockets to be covered due to Govt. requirements? What are the areas in which medium to average in-building coverage is acceptable? What are the areas where excellent in building coverage is needed? Areas with high growth potential New colonies under development High revenue areas Shopping malls, office complexes, industrial estates etc. Initial implementation Strategy: High usage, high revenue users first? High end residential and business areas? Street Coverage first? Special areas like 5 Star hotels, Commercial buildings with fine in-building coverage? High way coverage critical? Total coverage on day one? Number of sites more than the competition? Etc etc. Any budget limitations? Give an ideal plan to start with. Let the customer cut corners. Not an easy job!!


Surveys This is basically a scouting exercise. It is better to spend at least a week in each city/locality. It helps in identifying the network requirements for the city/location. Keep an open eye for the major traffic routes, main roads, the general city layout, types of buildings, location of major hotels, shopping centres, airport, railway station, typical consumer behaviour, telephone density, number of restaurants, bars and clubs, parks and open areas, any historical buildings, old buildings, congested localities with narrow streets, any lakes, ‘ nullahs’ ( narrow canals/waterways ) and so on. In-Building Coverage Identify different types of buildings in the city, such as hotels, restaurants, commercial buildings, shopping malls residential and so on. Select a few buildings in each type and conduct propagation tests. Receive signal strength is noted at the entrance ( on the road) of the buildings and inside- more importantly in the basement and ground floor. This is because the in building coverage is usually less only in these areas. As we go higher up in the building, the coverage usually improves. With reference to the signal strength in the road, the building penetration loss can then be computed. Repeat the process for all buildings and for all categories of buildings. This will give an estimate of building loss for the locality under consideration. In-Building Coverage _ Classify Buildings – Hotels/Restaurants Commercial Industrial Residential Shopping malls/markets Propagation tests in a number of buildings in each variety. RF signal on the road Vs. inside building gives building penetration loss. Repeat tests in as many buildings as possible to get an estimate of building loss for the area. In-building coverage affected mostly in ground floor/basements. Typical Values: ( examples only ) Hotels/Restaurants 15 dB Commercial bldgs 20 dB Shopping malls 15 dB Industrial Estates 12-15 dB Residential buildings 15-20 dB Old/Historical buildings 25-30 dB. RF Propagation Tests RF survey is done by performing drive tests within the proposed cell site area. For a new network, we set up a test transmitter at a chosen point and measure RF signal strengths at various points. In an existing network, the drive test is used to collect data from the network itself. A typical RF propagation kit is listed in Table 6-1. Table 6-1 Battery powered Transmitter

10 or 20 watts output; frequency in GSM 900/1800 MHz bands.


Portable mast

Adjustable up to 5m. With 1m antenna on top, effective height above ground is 6m.

Transmit antenna

High gain Omni or directional antenna as required.

Receiver (TEMS mobile),

Hand held mobile phone with RS232 connection to a lap top.Or, an accurate portable RF sensitivity meter/ CW receiver if model calibration is required.

Positioning System

GPS system,with PCMCIA card

Computer

Lap top PC with TEMS software and GPS software.

Cables & accessories:

Calibrated cable lengths (10m) of low loss feeder with known attenuation values; 12 Volts battery with appropriate cable to connect to transmitter.

Power meter VSWR meter Planning Tool Preparation and Model Calibration The concepts of RF planning were covered in Section 3. It is possible to do this planning for a number of sites by manually handling the propagation test results. However, to do iterative operations and get a coverage map it is essential to use planning tools to automate the process. There are many planning tools available to day: NetPlan ( Motorola) PlaNet (MSI) CellCad (LCC) Odyssey (Aethos) Asset (Aircon) Planning tool Requirements: The planning tool must be easy to use, should be compatible with tools like the TEMs, should be economical and require minimum hardware. Obtain Maps: It is important to have the map of the city/area under consideration. Such map information could be obtained as paper copies from authorized sources or from a satellite image. The latter option is very expensive. The maps could be obtained from the local authorities. Where contour maps are required they could be obtained from the Survey of India department. The maps should be preferably on 1:50000 or 1:25000 scale. Normally the data would be in 50m resolution. Less than this is not suitable for macro cell designs as it calls for large memory requirements for the planning tool.For micro cells, less than 30 m resolutions can be considered, where a “ Ray Tracing Tool” is used.The geographical data of the area is digitized under 3 different categories:


_ Land Use _ Digital Terrain Map (DTM) _ Vectors ( Roads, Railway tracks etc). Planning tool preparation and Model Calibration: There are many planning tools available to day: NetPlan (Motorola) PlaNet (MSI) CellCad (LCC) Odyssey (Aethos) Asset (Aircon) A planning tool should be: Easy to Use Compatible with tools like TEMs Minimum Hardware requirements Economical. Maps collected from authorized sources. 1:50000 or 1:25000 scale 50 m resolution for macro cells. Less than 30 m resolution possible for Micro cell planning using “ Ray Tracing Tool” . Maps are digitized under 3 categories: Land Use Digital Terrain Map ( DTM ) Vectors ( Roads, Railways etc). Most Planning tools use corrections for the land use or clutter. The propagation model can be tuned by assigning values to: Clutter factor (gain or loss due to clutter) Clutter heights (for diffraction modelling) Different types of clutter are defined in these models/tools: Dense Urban Urban Suburban Suburban with Dense Vegetation. Rural Industrial Area Utilities (marshalling yards, docks, container depots etc ) Open area Quasi Open area Forest Water Too many clutter type definitions complicate the planning process. 10 to 15 is typical DTM. Provided by the map vendor Provides Contour information as a digital map. Vectors Highways Main Roads Railways Canals/Water ways Coast line Rivers Each category is digitized as separate layers Displayed separately if required. Map information is set up in the planning tool. Model Calibration carried out.


Model Calibration Typically RF propagation tests are conducted for 5-8 proposed BTS sites covering as much of the area as possible. Efforts should be made to include all types of clutters during the propagation test. Almost all the planning tools have provision for changing the clutter values to match the propagation test results. They all have different directory structures and means of handling the geographical data. As an example, the PlaNet has a procedure for tuning the Macro Cell models. NetPlan has a Custom Path Loss Model which enables the planer to play with the values of various types of clutter and fine tune the model. For the sake of illustration, an excerpt from the “ User Reference Guide” of PlaNet is placed as an Annexure. All tools have provision for manipulating clutter values. Different tools have different Directory structures and means of handling geographical data. Macro Cell Tuning procedure described in “ User Reference Guide” of PlaNet kept in Annexure for illustration purposes. The procedure mainly talks about ensuring correct data header files to include: BTS location EIRP of BTS Antenna Type BTS antenna height Description of surrounding area. Procedure uses a general core model equation: The equation has constants k1 to k6 and a constant for clutter, kclutter. Initial values for the constants are set as per the model chosen (say Okumara-Hata). PlaNet programme is run repeatedly to make RMS error values for all data files ZERO or a minimum. For each run of the programme, the values of k1 to k6 are manipulated. This completes Model calibration. Link Budget and Other Steps Once the geographical data is entered in to the planning tool, a coverage map is required to be generated for each site. The most important step is then to prepare a Link Budget for the site under consideration. Some of the key points to be considered are: What is the desired probability of the receive signal strength at the mobile, within the entire coverage area? ( usually 90 or 95% and is decided in consultation with the customer). What is the expected in-building coverage? What is the probability on the “ EDGE” of the coverage area that the receive signal strength is sufficient for the mobile to make and hold a call? What is the fade margin available? What is the maximum permissible path loss ( from the Link Budget) What is the radius of the cell? What are the areas of different types of coverage planned for? For example Main business area could be 100 sq. Km and the suburban area could be 200 sq. Km and so on. How many sites are required for each type of area? ( from coverage point of view) Is the number of sites calculated as above adequate for capacity? Decide on more sites for capacity. Link Budgets and Other Steps Key Points to be considered: Coverage Probability Expected in-building coverage? Edge Probability Fade Margin required Maximum permissible path loss (from the Link Budget) What is the radius of the cell? Number of Sites required (from coverage point of view)


Is the number of sites calculated as above adequate for capacity? Decide on more sites for capacity. Coverage Estimation Having calculated the Maximum permissible path loss, the cell radius can be determined as explained in Section 2. For a given cell radius, we can calculate the estimated coverage area based on the formula:

The site separation distance D = (3N) R. ( R is the radius of the cell, same as d) For a given area, we can then calculate the number of sites required. Number of Sites = Total Area / Cell area.

Capacity Considerations Having determined the number of sites, it is then required to check if the capacity requirements are also met. This depends on the spectrum available, which will decide the site configurations. The availability of other features like frequency hopping etc is also to be taken into account. If capacity requirements are not met, then more sites may have to be added. If the number of sites as calculated above is not acceptable to the customer, then a second round of calculations may be required, assuming 50% in building coverage in place of 75%. Fine Tuning


Having prepared the preliminary cell plan, the planning tool can be used for returning coverage predictions. These will be more accurate than the rough models used for estimating the site density.Typically this activity will go through a number of iterations in consultation with the customer. Search Areas: Based on the final plan, “ search areas” are issued for each site location. In addition to the rough location, information like latitude and longitude of the place, approximate antenna heights, specific areas if any and the size of the search areas also needs to be given. The search area size would depend upon the criticality of the site. As per the information given by the RF planner, site acquisition teams scout for suitable buildings. Site Selection The choice of a site location depends on factors like the antenna heights above ground as well as above clutter. A generic rule of thumb could be used depending upon the type of area under consideration.

Central business area: Here the search area size is very critical, usually within 100m the antenna should be at or slightly above the average surrounding clutter height All near field obstructions should be avoided Antenna orientation may be along major roads Try and mount antennas close to solid structures like a lift motor room, this minimizes back lobe radiation effects As far as possible avoid towers on building tops as this might cause interference to neighboring cells. Residential/Suburban areas: Here the site separations may be larger than the first case and to that extent the location of the site is also less critical. Search areas are typically 200 m Antenna height should be 3 to 5 metres above the average clutter height Try and locate sites close to major routes or at large junctions Back lobe radiation is less critical, though it is desirable to avoid it. Industrial areas: A suitable location as much at the centre of the area as possible should be selected. Quasi Open areas: Search areas can be bigger, say, about 500 m Propagation is more dependent on the terrain than on the clutter. Hence more care is needed in hilly areas Use of towers is common Antenna orientation is not a critical factor. Availability of infrastructure like land, power etc is important. High ways: Search areas are along the road and are close High gain antennas required Tall sites are suggested. More are needed in hilly areas.


Chapter 7

Extending Cell Range

Contents: Extending Cell Range Objectives Extending Cell Range Increasing BTS Transmit EIRP Air Combining Techniques Improving BTS Sensitivity

Objectives : Explain the reason for extending the range of cells Explain the various methods used for range extension Discuss the results of the various methods Extending Cell Range Most operators are nowadays looking for achieving maximum coverage with a minimum or optimum number of sites. This requirement becomes significant especially in the 1800 MHz band. For in these frequencies, the path loss in more and cell sizes are much smaller than GSM 900 frequencies. Hence to get more coverage, the number of sites increases. Extending the range of each cell helps in reducing the number of sites required. The range of a cell depends upon the BTS transmit power, BTS receive sensitivity and the mobile’ s receive sensitivity. If we can improve these parameters, the cell can be ‘extended’ . Increasing BTS Transmit EIRP To maximize BTS o/p power, single carrier cells can be used, this will avoid the combination losses of multiple carrier cells. The output power at the top of the cabinet could be set to 40Watts, giving an increase in signal strength of 3 dB. Another way to maximize Tx and Rx signals is to implement low loss feeder cables. A typical 7/8” Andrews coaxial cable has an attenuation of 3.92 dB/100mt. If a 1-5/8” Andrews cable with an attenuation of 2.16 dB/100mt is used, then an increase of 1.6 dB can be obtained per 100m.


Air Combining Techniques For sites with 2 or more carriers we can enhance the EIRP of the BTS by using air combining techniques. By using duplexers, this can be achieved with 2 or 3 antennae as shown in Figure 7-1. For sites with 2 carriers we can enhance the EIRP of the BTS by using air combining techniques. When applied to Polarization antennae, we can get air combining using a single antenna or with 2 antennae as shown in Figure 7-2.

Improving BTS Sensitivity

Figure 7-2

BTS sensitivity can be improved by: Better devices in the BTS receiver Using Mast-head amplifiers with very low Noise figures. Better RF cables. Figure 7-3, Figure 7-4 and Figure 7-5 show relationship between sensitivity vs range; Tx EIRP vs range; sensitivity and BTS EIRP vs range respectively.


Extending the receiver sensitivity of the BTS has certain advantages: • On the BTS receiver side, we can improve the receiver sensivitiy by using Mast Head Amplifiers. • Mast-head amplifiers (MHAs)/Tower Mounted Amplifiers (TMAs) stretch the BTS receiver sensitivity to – 110 dBm as against the standard –107 or –108 dBm.


This results in better coverage probability. Experiments with MHAs have shown that quality of coverage has improved• In areas with 50% probability to approximately 70% probability. • In areas with 70% probability to approximately 85% probability. • In areas with 85% probability to approximately 95% probability. • In areas with 95% probability to approximately 98% probability.


Fundamentals of RF Planning

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