GSM RF OPTIMIZATION_IMP

Company Confidential

17/7/2005

1

Troubleshooting

Blocked Calls Poor Quality and Drop calls Abnormal Handovers Interference Termination Failures


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Blocked Call

Troubleshooting


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Blocked Calls Blocked Calls can occur due to : •Access Failures •SDCCH Congestion •SDCCH Drop •TCH Congestion

Trouble shooting cause : •Use Layer 3 messages to analyze the cause •Decode System Information Type 3 messages. •Note the parameter , “max_retransmission” ; “CCCH CONF” and “BS_AG_BLKS_RES”


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The best way of analyzing blocked calls, to identify the cause, is from a Layer III protocol log. * Paging Failure A paging message always originates from the MSC and is sent to all the BSCs in the Location Area of the MS to be paged. The BSC will then calculate the Paging group of the MS and send a Paging Command to the BTSs controlling the Location Area of the MS. On the air interface there are two cases of Paging Failure, either the Mobile receives no Paging message or it receives a Paging message, but is not able to respond (not able to send a RACH) which could be due errors in the Paging message. * Access Failure Irrespective of the purpose, for any communication required with the network, a mobile sends a Channel Request (for SDCCH) on a RACH and waits for some time for a response which should come from the BTS on an AGCH. A mobile will do several retransmission of RACHs (predefined) and if it still does not get a response, it goes back to idle mode and preferably does a cell reselection. At this stage we call it an Access Failure. * SDCCH Blocked Once a mobile has sent a Channel Request on a RACH , it expects a response from the BTS on the AGCH. This should be an Immediate Assignment Command to an SDCCH. If an Immediate Assignment Reject comes instead , then this is SDCCH blocking. * TCH Blocked After the completion of call set-up signaling, a mobile expects an Assignment Command to a TCH so that speech can commence. If no Assignment occurs for a specific period and the Mobile has to return to idle mode, then it is due to TCH congestion.


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Blocked Call Analysis - L3 messages 1

3 Channel Request RACH . : RACH max_retrans NO RESPONSE FROM N/W ACCESS FAILURE !

Channel Request Imm Assignment Service Request Signalling : Signalling

2 NO TCH ASSIGNMENT Mobile Returns To Idle TCH BLOCKED !

Channel Request RACH Imm Assign Reject SDCCH BLOCKED !


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Blocked Call - Cause troubleshooting

Access Failures - CCCH Overload at the Base Station - Uplink Interference at the Base Station - Low Rxlev at the Base Station - Base Station TRX decoder malfunctioning - Downlink Low Rxlev ( Coverage Hole ) - Downlink Interference - Excess Cell Range


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Access Failure - Cause Troubleshooting No Downlink Message after Channel Request

RR RR RR RR RR RR RR RR RR RR RR RR

Channel Request Channel Request Channel Request Channel Request Channel Request Channel Request Channel Request Channel Request System Information 1 Paging Request Type 2 Paging Request Type 2 System Information

1st RACH Transmission Multiple Re- Transmissions ( number depends on max_retrans)

Phone back to Idle Mode Call Blocked !!! Access Failure !!!

Causes : Downlink problem ( Coverage Hole / Interference)


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Access Failure - Troubleshooting Access Failure - Uplink Problem

Causes: 1. 2. 3. 4. 5.

AGCH Overload at Base Station RACH Collisions MS out of Range Poor Uplink quality BTS Receiver Problem


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Access Failure - Troubleshooting Access Failure - Uplink Problem -- Identification RR RR RR RR RR RR RR RR RR RR RR RR RR RR RR RR

Channel Request Paging Request Type 1 Immediate Assigment Sytem Information Type 3 Channel Request Immediate Assignment Immediate Assignment Extended Paging Request Type 1 Channel Request Immediate Assignment Immediate Assignment Paging Request Type 1 System Information 1 Paging Request Type 2 Paging Request Type 2 System Information

Channel Request max retransmissions

Return to Idle State Blocked Call !!! Access Failure !!!

Cause : Downlink Messages are seen after Channel Request. Problem suspected in Uplink !!!!! Company Confidential

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Access Failure - Troubleshooting Uplink Problem - Cause : AGCH Overloading RR RR RR RR RR RR RR RR RR RR RR RR RR RR

Channel Request Immediate Assignment Extended Immediate Assignment Extended Channel Request Immediate Assignment Extended Immediate Assignment Extended Channel Request Immediate Assignment Immediate Assignment Extended Paging Request Type 1 System Information 1 Paging Request Type 2 Paging Request Type 2 System Information


Multiple Immediate Assignment’s seen after Channel Requests

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Access Failure - Troubleshooting AGCH Overloading - Root Cause Analysis •If Multiple Immediate Assignment Extended Messages are seen, problem could be AGCH overloading OR RACH Collisions/Non-detection •If max_retrans and Tx-Integer are set to a lower value, problem could be more towards RACH Collisions/ Non-detection •If set high, then possibility of overloading is high!! •Check for CCCH_CONF and BS_AG_BLKS_RES. •If less blocks are reserved , problem is overloading. •Analyze the OMC data for the same period for the following stats: - No of Deletions - No of Successful RACHs - RACH Busy counts - No of RACH’s with invalid establishment cause Company Confidential

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Access Failure - Troubleshooting RACH Collisions/ Max Range - Root Cause Analysis •Max_Retrans and Tx-Integer set low - RACH Collisions Possible

•Check for Distance from Base Station -- Plot a map for BCH ARFCN -- Export the Channel Request and CellID Label -- Import the Site ID’s Raster Images on the Map -- Calculate distance between “Channel Request” and BTS -- Compare this distance with the “Max_Distance_Allowed” set for thjs cell. -- Note: Max_Dist_Allowed is a BSC paramter ( not on Air ) -- If the MS distance is more than max_distance then problem with Max Range •If both the above conditions don’t meet, then problem is Non-Detection


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Access Failure - Troubleshooting RACH Non-Detection - Root Cause Analysis •Downlink is fine !!! Parameters are well set for RACH control !!! •Problem could be Uplink Quality / Base Station •Analyze the following OMC Data -- No of Invalid RACHs -- Interference on Idle Channel -- SDCCH RF Loss / TCH RF Loss •If Interference and RF Losses are above normal, problem is Uplink Interference. •If RF Losses are high, but interference is low , problem is Uplink level

•Uplink level poor indicates Link imbalance. •If all above conditions are satisfactory and still “No of Invalid RACH’s” high, problem could


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Access Failure - Troubleshooting RACH Non-Detection

Downlink Fine

Analyze OMC Data No of Invalid RACH’s No

High Yes

Interference on Idle Channels High

Yes

Uplink Intrfer

Yes

No

High No

Uplink Imbalance

BTS TRX BTS Testing Abis Monitoring

SDCCH/TCH RF Losses

UL-Interference Test No


Link Imbalance Test Problem Found

Yes

Identify Cause Troubleshoot

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Now let us go a step further in understanding the most probable causes behind call block problems. * Access Failures It could simply be caused by coverage holes . Interference could however play an important role. Uplink interference on a serving cell can result in RACH rejections and hence no AGCH assignments. Improper channel distribution between AGCH and PCH (paging channel) can result in RACH/AGCH overloading. Paging Failures can be impacted by BCH pollution (cochannel and adjacent channel interference). * SDCCH Blocked Heavy Traffic and excessive Location Updates can result in congestion of SDCCH resources. Interference can block the channels , so though resources are available they may not be able to be used. * TCH Blocked Heavy Traffic is the main cause of TCH congestion. The TCH can also be blocked due to continuous interference in the uplink. Solutions to access failures would be to ensure continuous coverage and optimization of CCCH configuration parameters . For TCH and SDCCH congestion, the hot spots need to be identified and load sharing techniques implemented. Some techniques that have been used successfully involve adjusting cell powers to vary the coverage and therefore the location where mobiles will handover from one cell to the next. Interference management is essential for optimum network performance. Location updates can be optimized by independent drive tests on the ALL BCH carriers. The delta is measured of each BCH with the current serving BCH and the Reselect Hysteresis parameters adjusted appropriately. Company Confidential

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Blocked Call Analysis SDCCH Congestion Cause Location Updates to be analysed with OMC statistics first. If high, determine the source to target cell ratio Drive around the suspected area in the Idle Mode Configure “Delta LAC < > Constant 0” alarms Optimize Location Updates Interference Analyze OMC statistics on “ Idle Channel Interference” Carry out Uplink Interference Measurements using Viper Heavy Traffic Verify from OMC statistics SDCCH Congestion Carry Call Setup Time measurements Optimize set up time if high, else modify channel configuration


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Blocked Call TCH Blocked - Causes •Interference -- Verify Idle Channel Interference reports from OMC -- If suspected, carry out uplink interference measurements •Heavy Traffic -- Verify the TCH Holding time and no of attempts statistics from OMC -- During low traffic hours, Activate Cell barring in the cell -- Carry out Time slot testing , by setting Ignore Cell Barring. Solutions To Blocked Calls Optimize coverage Optimize Cell loading Interference management Channel configurations Optimize neighbors Company Confidential

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Blocked Call - Interference •Base Station Measures Uplink Interference on Idle Timeslots •At regular intervals, categorizes Timeslots into Interference Bands. •There are Five Interference Bands. •Each Interference Band has a range of interference level. Example : Interference Band “1” = -110 to -100 dbm Interference Band “2” = - 99 to -90 dbm Interference Band “3” = - 89 to -75 dbm Interference Band “4” = - 75 to -60 dbm Interference Band “5” = -59 dbm and above

•Network will assign Timeslots starting from lower band •Interference Band “5” Timeslots are considered as “BLOCKED” •OMC reports Hourly average statistics for each timeslot. Company Confidential

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Timeslot - Testing •Activate Cell Barring from OMC. •Remove this cell from the neighbor list of other cells. •Get the cell configuration •ARFCN’s and Timeslots configured for TCH.

•For BCH carrier select the Timeslot and carry out the Testing •For TCH Carriers: Block the BCH Timeslots from OMC •Carry out Timeslot testing. •If more than 1 TCH Carrier is activated, block all others .


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Congestion Relief - Redirection •Most of the manufacturers now provide this feature Channel Request

Immd Assign MM/CC Signaling ALL TCH’s Busy

Assign Command

BSC Assignment Request

MSC

Allocates a Free TCH

*** Some of the systems may also do handover existing calls to strong neighbors and assign TCH for this call from the same cell Company Confidential

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Congestion Relief - Redirection Role of Drive Testing

•OMC statistics may not reveal the actual congestion in a cell. •Drive Testing may be required in the Peak traffic hours to estimate how many times this happens. •Drive Testing may also be required to optimize the neighbor list for effectiveness of this feature.


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Dropped Call Troubleshooting


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Call drops are identified through SACCH messages. A Radio Link Failure Counter value is broadcast on the BCH. The counter value may vary from network to network. At the establishment of a dedicated channel, the counter is set to the broadcast value (which will be the maximum allowable for the connection). The mobile decrements the counter by 1 for every FER (unrecoverable block of data) detected on the SACCH and increases the counter by 2 for every data block that is correctly received (up to the initial maximum value). If this counter reaches zero, a radio link failure is declared by the mobile and it returns back to the idle mode. If the counter reaches zero when the mobile is on a SDCCH then it is an SDCCH Drop. If it happens on a TCH, it is a TCH drop.

Sometimes an attempted handover, which may in itself have been an attempt to prevent a drop, can result in a dropped call. When the quality drops, a mobile is usually commanded to perform a handover. Sometimes however, when it attempts to handover, it finds that the target cell is not suitable. When this happens it jumps back to the old cell and sends a Handover Failure message to the old cell. At this stage, if the handover was attempted at the survival threshold, the call may get dropped anyway. If on the other hand the thresholds were somewhat higher, the network can attempt another handover.


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Drop Calls Analysis 1

2 Channel Request

Channel Request

Imm Assignment

Imm Assignment

Service Request

Service Request

Signalling : Signalling

SDCCH Signalling : Speech TCH RLT = 0 ; DROPS TCH DROP !

RLT = 0 ; DROPS SDCCH DROP !

3

SDCCH / TCH Handover Command Hand Access Handover Failure HANDOVER FAILURE DROP ! Company Confidential

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Dropped Call Analysis • SDCCH Drops - Causes – Coverage – Interference & Multipath – BTS performance • TCH Drops - Causes – Coverage – Interference & Multipath – BTS performance – Pre-emption

• Solutions to Dropped Calls – Optimize Coverage – Interference Management – Optimize neighbors – Optimize handover parameters – Effective Frequency Hopping – Use of DTX & Power control

• Handover Failure - Causes – Threshold parameters – Missing neighbors Company Confidential

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We will examine the potential causes behind call drops and some solutions to combat them. * Coverage Poor non-contiguous coverage will reduce C/N and hence will reduce the Ec/No and will result into call drops. * Interference This is one of the major causes of dropped calls. Interference could be co-channel, adjacent channel or external. Under certain severe cell interference conditions, the call will be dropped before a handover can be initiated.. Multipath interference can also add to the problems. Strong signal reflections result in time dispersion issues resulting in a large coherence bandwidth.

* Network initiated drops Certain network features, like preemption, can kill an ordinary call to provide connection to an emergency class subscriber. A handover is the key to survival from dropping calls. But if there are problems in the Handover process itself, then this will not avoid a drop. Dropped calls can be effectively reduced by improving coverage, detecting and reducing interference, setting appropriate Handover Margins , thresholds for handovers and the correct selection of neighbors. Use of DTX and dynamic downlink power control will also reduce average interference which should lead to some improvements. Company Confidential

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Drop Call - Troubleshooting DROP CALL

Troubleshoot Link Imbalance UL-Interference

Handover Attempted ? Handover Failed ?

YES

RLT=0

No

L3 Message Analysis

YES

Uplink Problem !!!

No

Troubleshoot Multipath/ Co-Channel/ BSIC decode

RxQual > 4 YES

YES

Coverage Hole !!!

Rx_Lev < -95

Area of concern

No TCH=BCH

YES

No **

Optimize Neighbors/ Coverage

C/IAb <= -9

Multipath

No

YES

No YES

No No

Pri/Int <= 9

C/IAa <= -9 No

Co-Channel / Multipath Interference!!

YES

Adjacent Channel Interference!!

TCH -Pri/Int <= 9

Troubleshoot Source Cell Using BCH Analyzer

YES ** If TCH is Hopping, then all causes needs to be verified further, before concluding on the root cause and optimizing.


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•SDCCH Drop - Coverage •SDCCH Drop - Co- Channel Interference •SDCCH Drop - Adjacent Channel Interference •SDCCH Drop - Uplink Problem •TCH Drop - Coverage •TCH Drop - Co-Channel Interference •TCH Drop - Adjacent Channel Interference •TCH Drop - Uplink Problem •Handover Failure


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Poor Quality •Poor Speech Quality could be due to •Patchy Coverage ( holes) •No Target cell for Handover •Echo , Audio holes, Voice Clipping •Interference ---: •Co-channel •Adjacent channel •External •Multipath •Noise


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Speech Quality Parameters RxQUAL : Measured on the midamble. Indicates poor speech quality due to radio interface impairments FER : Measured on the basis of BFI ( Ping -Pong effect on speech ) Preferred under Frequency Hopping situation

Echo and distortion : Generally caused by the Transmission and switching system. Audio holes : Blank period of speech, due to malfunctioning of Transcoder boards or PCM circuits. Voice Clipping : Occurs due to improper implementation of DTX. Mean Opinion Score (MOS) : ITU standard for estimating speech quality.


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Speech Quality Parameters Mean Opinion Score Criteria for Voice Quality : A set value “x” at which “y” percent of customers rate the voice quality at Circuit Merits(CM) 4 - 5.

MOS

Quality Scale

5

Excellent ( speech perfectly understandable)

4

Good ( speech easily understandable, some noise)

3

Fair ( speech understandable with a slight effort, occasional repetitions needed)

2

Poor ( speech understandable only with considerable effort, frequent repetitions needed)

1

Unsatisfactory ( speech not understandable)


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Speech Quality - Estimation Speech BAD BAD BAD BAD BAD GOOD CLIPPING CLIPPING ECHO

RxQual HIGH HIGH LOW LOW GOOD HIGH LOW LOW LOW

FER LOW HIGH HIGH HIGH HIGH LOW HIGH LOW LOW

FH NO NO NO YES YES YES YES YES Y/N

DTX NO NO NO NO NO NO YES YES Y/N

Reason Air Interface Impairments Severe Air Interface Impairments Transmission and Switching system , Transcoder Air Interface Impairments Transmission and Switching system , Transcoder Hopping Implementation Hopping Implementation and VAD VAD Transmission and Switching system , Transcoder

MOS up to a certain extent can easily estimated by configuring an algorithm using the Alarms in the HP E74XX systems for the following elements, an example of subset of which is illustrated above - RxQual Full and Sub - RxLev Full and Sub - FER and RLT - L3 Measurement Report - L3 Handover specfic messages Company Confidential

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Speech Quality - Troubleshooting Poor Quality

Troubleshoot Link Imbalance UL-Interference

Patchy Quality Multipath High Probability with Hopping ON

MOS < 4

YES

No

RxQual> 6

Hopping ON

YES

No DTX_DL ON

YES

No

Uplink Problem !!!

No

FER > 1%

YES

FER > 2%

No Troubleshoot Multipath/ Co-Channel/ BSIC decode

A

YES YES

Coverage Hole !!!

Rx_Lev < -95

Transmission/ Switching Systems Transcoder

A

Area of concern

No TCH=BCH

Pri/Int <= 9

No **

Optimize Neighbors/ Coverage

C/IAb <= -9

No

RLT going down

A

YES

Co-Channel Interference!!

No YES

C/IAa <= -9

TCH -Pri/Int <= 9

YES

Adjacent Channel Interference!!

No

B YES

No

YES

B Troubleshoot Source Cell Using BCH Analyzer

YES

** If TCH is in Hopping, then all interference causes needs to be verified further, before concluding on the root cause and optimizing.


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Troubleshooting Poor Speech Quality Method 1 : Post Processing

Create the Following PLAN Wizards in Export Quality-PHONEDATA

Quality-Cell Information

Interference

Rx_Lev Rx_Qual FER RLT current Value Timing Advance Tx Level ARFCN ( BCH and TCH) Serving Cell Information -- CellID

Serving Cell Information -- CellID -- BSIC Neighbor Cell Measurements Neighbor Meas Count() Timeslot ARFCN

Co-Channel Interference -- Channel -- Primary/Interferer -- Primary BSIC -- Secondary BSIC -- Textual Meas Status -- Numeric Meas Status -- Fading Adjacent Channel Interf -- AdjA Channel -- AdjA N/N-1 -- AdjA N/N+1 -- AdjB Channel -- AdB N/N-1 -- AdjB N/N+1 ARFCN


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Filtered Spots- RxQual > 4

Bad Spot 1


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Troubleshooting - Bad Spot 1


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Which is this CELL ??

•Select QualPhoneData Layer. •Check on Labels

•Select Labels •Select CellID. •Display it on the MAP


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Which is the cell ? 47450 !!


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Did the Call Drop ???

•Create RLT.tab Query for RLT < 8 •Import as Label Layer


RLT = 0 , DROP CALL !!

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Conclusion •BAD Spot 1 has poor quality and Call Drop •This spot is covered by Cell 47450 •Poor Coverage. Level below -97 dbm •But Call should not Drop •The other Problem is Interference. •Mobile is Hopping on 99 and 84 •99 is also the BCH. •Co-Channel on BCH is very high. •50% of the time quality will be poor. •But Poor Quality is consistent. •Channel 84 is also suffering from Interference. •No Adjacent Channel on 84 and 99 •This means there is Co-Channel on 84 also. •It could also be multipath issue on 84. •WHY NO HANDOVER Company Confidential

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Troubleshooting Handover Problems


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Troubleshooting - No Handover Weak Neighbors - Reported by Phone Create a Handover PLAN Total Attempted Calls Total Dropped Calls Total Blocked Calls RxQual Full RxLeve Full RLT Current Value ARFCN Neighbor Cell Measurements RR Message Phone State Sequency Number


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Analyzing - No Handover

No Neighbor( very weak)


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Missing Neighbors Real Time

Frequency Reuse

BCH Analyzer : TOP N = 7 Configure an Alarm for Missing Neighbor “Subset BCH TOP N not-subset Value ARFCN”

'B' NET

CH 35

BSIC 16

'B' NET

CH 35

BSIC 21

'A' NET

CH 88

BSIC 53

'B' NET

CH 40

'A' NET

BSIC 15

CH 98

'B' NET

BSIC 57

'B' NET

CH 29

PHONE REPORTS

'B' NET

CH 27 35 40 25

CH 27

RxQual 1

RxLev -80 -85 -83 -95

BSIC 23

'A' NET

BSIC 22

CH 25 BSIC 17

CH 72

BSIC 75

Channel 29 is not in the neighbour list ! Company Confidential

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Missing Neighbor is a very critical problem in optimizing GSM neighbors. Missing Neighbor as such doesn’t means that there is no neighbor. We define neighbors to a cell based on the geography, I.e which base stations are nearby. In certain cases we may also use the Planning Tools to predict neighbors. It is such not desirable to use excessive neighbors also, since this reduces the samples collected per neighbor. Since Mobile has to look for each neighbor in one TDMA frame, and we have 104 frames for measurement. This means if we define 32 neighbors , the number of samples per neighbor will be only 3-4, which means the authenticity of Handovers will not be there. So we need to define appropriate neighbors only. In certain cases the neighbors we define may not be able to cope up with traffic or at certain significant patches of the cell, these neighbors signals would be weak and may result into no handovers or handover failures. So in such kind of problem spots , we should be see the possibility that signals from any other cell is coming or not, so that we can define that as a neighbor. So how do we find such missing neighbors.


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Troubleshooting - Handover Parameter Settings •Decide the Target Cell for handover in the Trouble Spot •Filter the Handover.txt file on Handover Attempts ( before AND after numbers) •Filter again on Neighbor ARFCN = Target Cell ARFCN •Create another column as HO_Margin , with Delta for Neighbor_Level to RxLev •Plot this on the MAP and see wether Handover Margin can be reduced to improve quality OR increased to avoid Ping-Pong effects •If handover margin settings are proper, and still handover is not occurring then it could be a problem with Handover decision and processing parameters at the BSC.


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Interference Measurements and Troubleshooting


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Types of Interference We already know …. Co-Channel •BCH - BCH •TCH - BCH •BCH - TCH •TCH - TCH Adjacent Channel Interference •BCH - BCH •TCH - BCH •BCH - TCH •TCH - TCH

Multipath Company Confidential

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TroubleShooting


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Uplink Problems


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Locating Uplink quality Bad Spots

RX_Level is near P_Con Threshold Ms Tx Level is max ( 5)


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Locating Uplink Bad Quality Spots.

Trend shows: High receive level And High Ms Tx Levels in this area.


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Locating Uplink Bad Quality Spots •At some areas for low signal level ( < -88), MS at full power •In another area for high signal level ( -75 to -85 ) , MS full power •MS Tx Level is controlled by the network. •Network will ask MS to increase power if Uplink level is poor or Quality is poor.

•Link Imbalance will result into low uplink level even if DL Level is good. •Then MS will be at high power in all those spots where level is good. BUT THIS IS NOT THE CASE !!! ( as seen from two spots) •This means this is a problem of Uplink quality being poor at good level. •This can happen due to UPLINK INTERFERENCE. •This interference seems to be burst in nature !!!!


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Troubleshooting - Uplink Interference •Uplink Interference can be due to: • Mobiles in reuse and adjacent channel cells • Spurs and Intermods from base stations • External sources •Interference due to Mobiles will be bursty and intermittent. •The behavior and its effect on quality will be time dependent. •More interference during heavy traffic hours. •Interference from external sources can be continous or also time dependent if the source is not continously ON.

•Uplink Interference measurement needs long term monitoring , collection of data, processing the data and estimating the probability of interference , and also estimating the source.


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Termination Failures Troubleshooting


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Troubleshooting - Termination Failures

•Termination Failures could be due to: •Paging Problems with Network •No response to page from MS •Blocking of Resources •If Paging Success Rate = 100 %, but still the caller reports termination failures, then this Blocking. •If Paging Success Rate < 100 % ,then this could be : - Paging Problems with Network Or - No response to page from MS


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Troubleshooting-Termination Failures

•Verify and Isolate the problem as: •Interference •Excessive Cell Reselection •Excessive Location Updates •None of above indicates Network Problem ( no paging)

•Excessive Cell Reselection and Location Updates can occur due to •Interference ( Downlink signaling Counter reaches 0) •BCH Pollution •Overlapping of Location Areas. •Carry out protocol analysis on Abis and A Interface to isolate network problem further


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Cell Optimization


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Optimization Cell Loading

C 2

C 3

C 1

•Cells C1 and C2 are reporting Blocking •Cell C3 is added to cater for capacity •Still C1 and C2 report blocking. Why ????? Company Confidential

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Optimizing - Cell Loading More Traffic could be persuaded towards C3 by : •Increasing the Power Output of C3 BTS •Tilting Antenna’s towards Hot spots in C1 and/or C2 •Applying Reselect Offsets to certain less loaded neighbor cells







With this approach more MS's will camp on or do cell reselection to C3, thus reduce load in C1 & C2. These issues are very common to fast growing networks, because new cells when are planned , we don't know the HOT SPOTS. In most cases , new cells investment could also be minimized by understand the RF environment and optimizing cell loading.


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Optimizing - New Cell Site Location & Antenna Tilts •New cell sites in the earlier phase of the network are implemented to enhance coverage. •Coverage enhancements takes priority over areas of predicted traffic •Once coverage objectives are met, cell expansion takes place purely on the basis of traffic. •New BTS are sited near to the cells which are generating blocking conditions. •The objective behind new cell site investment is to take the load from the existing congested cells and hence increase revenue. •An existing congested cell would be covering an area of 1 sqKm. •Traffic in this area would be concentrated in small patches known as “HOT SPOTS”. •Network Planner always intends to optimize the cell site location or the antenna tilts to target on some of the Hot spots . •But the question is do we precisely know where are the Hot spots ???


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Optimizing - Cell Reselections •Mobile Monitors the Neighbor cells BCCH Carriers and decodes SCH of the neighbors in the idle mode. •The list of the BCCH ARFCN’s which the mobiles will monitor in the idle mode is broadcasted in the System Information Type 2 Message . •More the neighbors , less the samples taken by the mobile per neighbor to the calculate the value of C1. •More neighbors of good and near to equal power levels will result into excessive reselections. •Excessive reselections can result into missing of paging messages •Solution to this problem would be to adjust power levels of base stations. •Adjusting power levels most of the time either effects coverage or enhances interference, since the terrain is not flat. •To optimize cell reselections, the best possible solutions are: • Optimizing the BA list •Implement C2 Reselections Company Confidential

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Optimizing - Neighbor lists •Maximum 32 neighbor •Less samples, averaged reported value not authentic •Results in Ping-Pong effect of Handovers •Neighbor list must be optimized


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Mobile in the dedicated mode measures the BCH of the neighbor cells. The list of BCH to be measured is informed to the Mobile in the System Information Type 5 message which can have maximum 32 neighbors BCH’s Mobile has to report the measurements of the neighbors to the network , every 480ms which corresponds to 104 TDMA frames. Mobile will take several samples of the neighbors, average it and then send the report as one value of Rxlev of the neighbor, with a minimum requirement of measuring at least 1 neighbor in 1 TDMA Frame. So , more the BCH’s in the neighbor list, less will be the number of samples. If the Mobile just meets the minimum requirement, then for a neighbor list of 32, only 3 - 4 samples per neighbor will be available and 12-13 samples per neighbor with a list of 8 neighbors If the samples collected are less, then average value reported will not be authentic, and can result into Ping-Pong effect of handovers. So, the Neighbor list must be optimized with the minimum best possible neighbors


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Optimizing -Handover Traffic

D

C

A

B


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As shown above, Cell B is a cell which is reporting Blocking due to heavy traffic. Cell B on OMC analysis is found to have traffic generated due to call setups within the cell , as well as handovers, since it covers a major highway with heavy mobility traffic coming from Cell A. Instead of implementing an excess TRX or a new cell to resolve this issue, we can optimize this problem, we can optimize the neigbbor lists, so as to drag the Mobiles coming from A to handover to cells other than B, unless B is the only candidate for handover. We can drive through the area between Cell A and Cell B and mark the spots where handover occurs. Look for the continuity between BCH’s. This will indicate to us, which are the strong BCH’s other than Cell B which could be potential targets for handovers and with what handover margins, so in case we don’t get an alternative for a certain patch which is above the handover margin threshold, we can reduce the handover margin .


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Optimizing - Handover Margin • Handover Margin is value set for all cells, which will allow mobiles to handover to the cell, only when this cells power budget balance exceeds the source cells power budget balance by this value. • Handover Margins main objective is to avoid ping-pong effects of handovers on cell periphery, when the power levels of two cells are near to equal , so due to multipath and mobility, this will result into frequent handovers between these two cells. • Setting this value low, will result into fast handover to the target cell, which may result in improvement in quality. • Setting this to high value will delay the handover to the target cell, and ensure that when handover takes place, the probability of the mobile going back to the source cell is very low for some significant period. • Two examples of use of this parameter are illustrated ( next page)


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Optimizing - Handover Margin Adjacent Channel Interference Reduction 



Tight Re-use patterns can permit the use of Adjacent ARFCN in adjacent cell. This should not pose an problem if Handover margin is set appropriately low. C/Ia = -1 db

C/Ia = -6 db C/Ia = -9 db

C2

C1 A



B

C

If Handover Margin is set to 6 dB, Handover will occur from C1 to C2 at spot B itself Company Confidential

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Optimizing - Handover Margin Multipath solutions    

Antenna Tilts Using Directional Antenna's Shifting the BTS site

Handover Parameters

C3 Reduce HO_margin Increase HO_margin

C1

C2 Reduce HO_margin

H Company Confidential

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Optimizing - Handover Margin • Selecting an appropriate value of handover margin is essential to ensure good quality communication . • Handover margin set for a cell is applicable to all calls going on in the cells, to which this cell is a neighbor.


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Optimization for Interference After drive test - Co-Channel benchmarking, we know which

cells are generating severe co-channel problem We also know by decoding BSIC , the interefering source Following processes could be adopted to optimize interference ---- Power Control ---- Antenna Tilts ---- Frequency Reallocations ---- Transmitter Tests ---- Mobile dispatch inspection ---- Space Diversity ---- Frequency Hopping /DTX


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In order to optimize interference the cells which are generating high level of interference should be located from the OMC Performance Database. It is essential to locate the source of interference whether it is external or internal. If it is internal then it could be due to multipath, adjacent, co-channel, inter-modulation or spurious. This can be located by analyzing the spectrum. There are various methods involved in optimizing interference. Appropriate Power Control of the interfering entities could be done. For Co-channel Interference problems, Tilting of antenna could be done with proper coverage aspects, for adjacent channel analysis should be done to locate if any adjacent channel is allocated to the neighbors , If it is then frequency allocation could help with this. It is very essential to carry out Transmitter and Mobile tests for Power v/s time output spectrum and intermodulation products, generally a mobile should be tested for these basic tests before being dispatched. In the Uplink Space Diversity is the worldwide adopted method for reducing the effect of Multipath fading, where as in the downlink it is the mobiles equalizer which has to do this job. One very relevant solution to interference is frequency hopping where the mobile constantly changes frequency for every burst which reduces the effect of interference. Implementation of DTX.DRX also reduces average interference.


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Co-Channel Optimization 

Static Power Control



Antenna Tilting



Trade Off with EC/No



Frequency Reallocation



Implementing Features like Dynamic Power Control, DTX and Frequency Hopping


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Antenna Tilts M

N A

50m

x 6 km

   

 

1 km

Point x is having problem of C/I from Cell M Estimate the C/I improvement required at Point x. Refer to the Antenna Vertical Pattern, and calculate the tilting angle required Example : To get an improvement of 3 dB , a tilt of 10 degrees is required. Tilting of Antenna in certain cases may reduce coverage also. Tilting of Antenna should be done after proper study.


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GSM RF OPTIMIZATION_IMP

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