Drive Test Analysis - A Case Study

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RADIO PLANNING AND OPTIMISATION

Drive Test Log file Analysis- A Case Study

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Presentation/Discussion – 19/06/2009

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By

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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

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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

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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

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Call Drops - Summary

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Call Drops – Summary (cont…)

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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

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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

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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

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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

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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

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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

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Handover Issues - Summary

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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

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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

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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

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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

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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

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