Drive Test Analysis - A Case Study
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Transcript of Drive Test Analysis - A Case Study
Grant Chivandire - Radio Planning and Optimisation Specialist
1
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
2
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 calldrop
Fake neighbour – same BCCH with defined neighbour, itself notdefined
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 incorrectdecisions
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
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Case Analysis 1
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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 otherdatabase – BSCplan, Asset etc
Channel => Hopping TCH
Handover Failure => look at Handover Command and Handover Failuremessages
Check if neighbour is defined properly in database
Measurement Reports => observe target cell is 4th candidate inneighbour 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 thefourth 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 identifydefined cell and CI
Notice that the 2nd sector of serving site, a defined neighbour, has thesame 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 andneighbour 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-cellhandover
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 inneighbour 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 wasexpected
Make corrections and propose new site to enhance coverage
THANK YOU !