Drive Test Analysis - A Case Study

RADIO PLANNING AND OPTIMISATION

Drive Test Log file Analysis- A Case Study (Presentation/Discussion – 19/06/2009) By

Grant Chivandire

Grant Chivandire - Radio Planning and Optimisation Specialist

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Objectives This presentation will cover the following:  Key drive-test log-file aspects  Layer 3 Messages  Call Drops  Handover Issues  Case Analysis 1  Case Analysis 2

Grant Chivandire - Radio Planning and Optimisation Specialist

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Key Drive Test Log-file Aspects  BCCH of serving cell ad neighbours  CGI – MCC, MNC, LAC, CI  RxLev of serving cell and neighbours  BSIC of serving cell and neighbours  Time Slot  Channel type/mode  Hopping Information – HSN, frequencies  Timing Advance  Tx Power  Radio Link Timer  C/I, C/A  RxQual, BER, FER, SQI Grant Chivandire - Radio Planning and Optimisation Specialist

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Layer 3 Messages  Synchronization Channel Information  Synchronization of MS to BSS  Repeated many times during a call  Contains BCCH, BSIC, Frame number etc  System Information Type 5  Contains BCCH Allocation list  List used to determine the neighbour list  Measurement Report  Contains the created neighbour list  BCCH, BSIC, RxLev and RxQual  System Information Type 6  Contains Location Area Identification information  MCC, MNC, LAC, RLINKT, NCC PERM Grant Chivandire - Radio Planning and Optimisation Specialist

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Call Drops - Summary

Fishbone Diagram: Call Drops Grant Chivandire - Radio Planning and Optimisation Specialist

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Call Drops – Summary (cont…)

Fishbone Diagram : Call Drops Grant Chivandire - Radio Planning and Optimisation Specialist

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Handover Issues - Types HANDOVER TYPES  Power Budget Handover  Difference in RxLev > PBGT margin  Level Handover  DL/UL RxLev < HO Threshold DL/UL

 Quality Handover  Quality level < Quality thresholds  Interference Handover  High RxLev but poor quality

Grant Chivandire - Radio Planning and Optimisation Specialist

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Handover Issues – Layer 3 Messages Layer 3 Messages  Handover Command  Cell description  Handover reference number  Power command  Synchronization indicator  Channel mode/description  Frequency channel sequence  Mobile Allocation  Time stamp  Handover Access  Handover reference number  Handover Complete/Failure  When MS establishes signaling link successfully/unsuccessfully Grant Chivandire - Radio Planning and Optimisation Specialist

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Handover Issues - Problems  Short Neighbour List  Less than 6 neighbours in neighbour list  Confirm you do not need to define more neighbours  Late Handover  Check handover margins between neighbours  Hierarchical Cell Structure  Power control

 Ping pong Handovers  No dominant server  Adjust hysteresis, handover margins and power  May require new site Grant Chivandire - Radio Planning and Optimisation Specialist

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Handover Issues – Problems (cont…)  Unnecessary Handover  Increasing number of Handover increases quality problems

 Missing Neighbours  No handover to neighbour cell that seems like best server  Sudden appearance of strong cell in neighbour list after handover  Stepping stone effect  Fake Neighbours  Good candidate but handover does not take place leading to a call drop  Fake neighbour – same BCCH with defined neighbour, itself not defined  Missing BCCH in measurement list, NCC Perm, Co-BCCH Grant Chivandire - Radio Planning and Optimisation Specialist

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Handover Issues – Problems (cont…)  Overshooting  Island effect  No good candidate to handover to  Timing Advance much greater than half of your inter-site distance

 Handover to another BSC  Data configuration should be consistent across the 2 BSCs  CGI, BCCH, BSIC and Power parameters  Target Cell congested  HO signalling performed over FACCH by stealing bits from TCH  Penalties applied to that particular MS  High HO Failure rate and drop due to dragging Grant Chivandire - Radio Planning and Optimisation Specialist

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Handover Issues – Problems (cont…)  Interference  Causes difficulties in decoding corrupt MRs on BCCH  Signalling failures due to high interference on TCH (FACCH)  Too many neighbours  Increase BSC CP load and leads to signalling congestion  Small window period to measure every neighbor may lead to incorrect decisions

 Swapped sectors  Defined neighbours will show poor levels  May exhibit “stepping stone” effect  May lead to drag and drops Grant Chivandire - Radio Planning and Optimisation Specialist

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Handover Issues - Summary

Fishbone Diagram : Handover Failure Grant Chivandire - Radio Planning and Optimisation Specialist

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Case Analysis 1

Grant Chivandire - Radio Planning and Optimisation Specialist

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Case Analysis 1 (cont…)  Log file  Central_longCalls_20090514_CELLC_CENTRAL004.dlf  Analysis  High interference at call drop  Handover failure 8seconds before call drop  Serving Cell =>BCCH=1012, BSIC=57, CI=41776, RxLev=-78, RxQual=7  Confirm serving cell details with Signalling messages and other database – BSCplan, Asset etc  Channel => Hopping TCH  Handover Failure => look at Handover Command and Handover Failure messages  Check if neighbour is defined properly in database  Measurement Reports => observe target cell is 4th candidate in neighbour list Grant Chivandire - Radio Planning and Optimisation Specialist

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Case Analysis 1 (cont…)  Analysis  Confirm neighbour cell information in database  Notice the proximity of the MS to nearest neighbour cells  Formulate ideal behaviour – why did the MS try to handover to the fourth candidate leaving the better 3?  Suspect fake neighbour problem  In Actix map, show CI properties on cells  In database, look for the BCCH of the 3 candidates and identify defined cell and CI  Notice that the 2nd sector of serving site, a defined neighbour, has the same BCCH as the first candidate  Missing neighbor has been detected

Grant Chivandire - Radio Planning and Optimisation Specialist

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Case Analysis 1 (cont…)  Analysis  Notice the distance of serving cell compared to nearby cells  Check terrain profile using Asset, clutter using Google Earth and neighbour candidates levels to see if acceptable  Site is overshooting, consider down tilting

Grant Chivandire - Radio Planning and Optimisation Specialist

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Case Analysis 2

Grant Chivandire - Radio Planning and Optimisation Specialist

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Case Analysis 2 (cont…)  Log file  Central_longCalls_20090514_CELLC_CENTRAL004.dlf  Analysis  High interference at call drop  Handover failure 15seconds before call drop  Repeated inter-cell handover failures following a series of intra-cell handover  Neighbour defined correctly  Handover failure due to timer expired  Deduce that high interference corrupted signalling information  Notice intra-cell handover did not help interference problem  Review hopping parameters Grant Chivandire - Radio Planning and Optimisation Specialist

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Case Analysis 2  Analysis  From the map, notice that 21626 is facing the location but not in neighbour list  Check from database if it is defined as a neighbour  Perform site audit and do coverage check  Check for swapped sectors  Perform scans with drive-test tool and determine best server  Observe the cell reselection after call drop – CI=21962  Check if defined in database  Notice that the defined cell with same BCCH is CI=31622  This is a fake neighbour  Explain that after failing to HO to 31626, an attempt to 21962 was expected  Make corrections and propose new site to enhance coverage Grant Chivandire - Radio Planning and Optimisation Specialist

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THANK YOU !