wcdma call drop

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OWJ104102 WCDMA Call Drop Analysis

OWJ104102 WCDMA Call OWJ104102 WCDMA Call Drop AnalysisDrop Analysis

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Confidential Information of Huawei. No Spreading without Permission. Security Level: Internal

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OverviewOverview

Call drop rate monitors the UTRAN capability of keeping a call after the call setup is success. This KPI can be optimized by driving test and the consequent trouble-shooting based on call drop analysis.


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ObjectivesObjectives

! Be familiar with the common reasons for call drop in UMTS

! Do analysis on call drop problems independently

When finished this course,you will be able to:


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Course ContentsCourse Contents

Definition of call drop problems

Call drop problems analysis process

Case study on call drop problems


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Definition of call drop problemsDefinition of call drop problems

Definition of call drop problems :


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Call drop problems analysis processCall drop problems analysis process

!! General call drop analysis flowGeneral call drop analysis flow

! Categories for call drop reasons


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General call drop analysis flow:


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Call drop location analysis: based on the location of the call drop

event in the map window in the post processing tool

Neighbor list analysis: to find out any cell not defined in the neighbor

list

Analyze the pilot coverage to make sure whether following coverage

events take place or not: Coverage limited, System interference, Poor

uplink coverage, Poor downlink coverage, pilot pollution

Handover analysis to make sure whether following cause exist or not:

insufficient handover area, too late to start CM, too late to add target

cell, too early to delete serving cell

Signaling analysis is used for those call drops that are still confused

about the exact reason through analysis mentioned above


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! General call drop analysis flow

!! Categories for call drop reasonsCategories for call drop reasons


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Categories for call drop reasons:

1. RF problems

2. Equipment problems

RF problems usually can be find out by neighbor list analysis

coverage analysis handover analysis, while signaling analysis is

helpful to equipment problems.


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Case study on call drop problemsCase study on call drop problems

!! RF concerned problemsRF concerned problems1. Neighbor missing2. Coverage Limited 3. Poor Downlink Coverage 4. Pilot Pollution 5. Handover problems

6. Inter System Interference

! Equipment concerned problems1. UE problem

2. Data configuration problem


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RF concerned problemsRF concerned problems

Call drop location-1-ScannerRscp:


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Call drop location-2-ScannerEc/I0 :


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Call drop location-3-ScannerBestserver :


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Before call drop, SC489 goes poor, while SC504 is good the

moment before drop, according to the measurements by UE

shown below; but SC03 which is not measured by UE is

becoming good gradually :


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SC03 is becoming good gradually measured by Scanner:


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Further investigation shows different performance between UE

and Scanner. Before the drop, the Ec/Io degrades for UE ONLY

while scanner shows no degradation:


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Before the drop, the RSCP degrades for UE ONLY while scanner

shows no degradation:


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Before the drop, the best server for the UE is not the same as that

of the scanner :

Corresponding measurement control shows there’s

no neighbor relation between SC489 and SC03: C:\NeighborMissi


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!! RF concerned problemsRF concerned problems1. Neighbor missing2. Coverage Limited3. Poor Downlink Coverage 4. Pilot Pollution 5. Handover problems





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General definition of “coverage limited” :

The signal of strongest cell should be in ranges:

CPICH_EcNo_in_ActiveSet < Threshold1

And, CPICH_RSCP_in_ActiveSet <Threshold2

And, UeTransmittedPower > Threshold3.

Theshold1~3 is “-15 dB” “-95 dB” “10 dB” respectively in

our analysis


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Call drop location-1-ScannerEc/I0 :


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The strongest cell’s Cpich_Ec/I0 and Cpich_RSCP (shown in next

page) is poor, while UE TxPower reaches the threshold of 21dBm:


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The strongest cell’s Cpich_Ec/I0 and Cpich_RSCP is poor :


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!! RF concerned problemsRF concerned problems1. Neighbor missing2. Coverage Limited 3. Poor Downlink Coverage4. Pilot Pollution 5. Handover problems





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General definition of “Poor Downlink Coverage ” :

The signal of strongest cell should be in ranges:

CPICH_EcNo_in_ActiveSet < Threshold1

And, CPICH_RSCP_in_ActiveSet <Threshold2

And, UeTransmittedPower < Threshold3.

Theshold1~3 is “-15dB” “-95 dB” “-15 dB” respectively in

our analysis.


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UE TxPower is not strong, while downlink signal seems so poor

that UE can hardly demodulate it :


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Measurements from the scanner shows the same thing. Little

difference is because that the antenna of the scanner is mounted

on the top of the vehicle, while the UE’s is in the car, the car

penetration loss is nearly 10 dB.


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!! RF concerned problemsRF concerned problems1. Neighbor missing2. Coverage Limited 3. Poor Downlink Coverage 4. Pilot Pollution5. Handover problems





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General definition of “Pilot Pollution ” :

The pilot pollution event occurs when 4 or more pilots

with their Ec/No above -15 dB are in the active or monitored

set, more strict definition should consider a relative

threshold between the best and the fourth pilot such as

“8dB”.


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There are too many servers at the call drop place, and cells in

active set go bad severely in 5 seconds before drop :


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There are too many servers at the call drop place, and cells in

active set go bad severely in 5 seconds before drop :


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UE Txpower keep rising before drop :

That’s a typical pilot pollution point, although from the point of view

of signaling, it’s caused by handover not in time.


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Call drop caused by handover problems, is helpful for us to get

solutions to parameters’ optimization. Here’s an example. Call

drop place is shown below:


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Call drop location-ScanneEc/I0:

The UeTxPower is not high:


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The drop call occurred in the area of frequent change of best

server shown by the scanner :


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Compare Ec/Io from both scanner and UE at the time of the drop:

the UE Ec/Io drop to < -18 dB while the scanner remained above -

6 dB :


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RF concerned problemsRF concerned problemsComparing the best servers from the UE and the scanner at the time of

drop: for the scanner and UE SC506 is the best server prior to the drop.

However, before the drop, the scanner selected SC505 as the best server

while the UE continued to have only SC506 in its active set resulting in the

drop call. Immediately after the drop, the UE camps on SC505:


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RF concerned problemsRF concerned problemsIt seems that the best server changes from SC506 to SC505 too fast for the UE to

perform soft handover on time. As the handover threshold of 1a event is 4.5dB,

the delay is 0dB, the delay trigger time is 320ms, the filter coefficient is 3 (1

second is required), so the requirement for the handover area is: the lasting time

when (Ec/Io of 506 cell – Ec/Io of 505 cell) < 4.5dB is about 2 seconds :


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Uu_Interface messages that 1a event of SC505 was triggered at

54:04:


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But SC506 went bad too severe for the procedure of “Active Set

Update” to be finished:


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Inter system interference has great impact on performances of

our system, here’s an example . Call drop place is shown below :


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DownlinkCodePower during call drop:

UE Txpower before drop :


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The NodeB RTWP scan shows high load (nearly 96%) when the

cell is not loaded actually. On that condition, user’s access will

probably fail or in other words handover to this cell will lead to call

drop:

Further spectrum measurements show that periodic inter-system

interference exist indeed


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! RF concerned problems1. Neighbor missing2. Coverage Limited 3. Poor Downlink Coverage 4. Pilot Pollution 5. Handover problems


!! Equipment concerned problemsEquipment concerned problems1. UE problem



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Equipment concerned problemsEquipment concerned problems

During tests, problems caused by UE happen occasionally, we’ll

give an example that we met with when testing with Motorola

A835. Call drop place is shown below :


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Call drop location-ScannerEc/I0 :


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Call drop location-ScannerBestserver :


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Before call drop, SC507 went poor, and SC473 was becoming

good according to the measurements by UE :


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Further investigation shows that UE reports event 1b of SC500

before drop instead of reporting event 1a of SC473 :


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Ec/I0 of SC500 before drop measured by UE :


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Ec/I0 of SC500 before drop measured by scanner :

There must be something wrong with the performance of UE’s

measurement. Further reliability of UE is required .


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! RF concerned problems1. Neighbor missing2. Coverage Limited 3. Poor Downlink Coverage 4. Pilot Pollution 5. Handover problems


!! Equipment concerned problemsEquipment concerned problems1. UE problem



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Problems caused by data configuration are always indiscoverable,

here is an example. Call drop place is shown below :


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Call drop location-ScannerEc/I0 , The position of the drop call was

at the border of two RNCs :


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Before call drop, SC473 went poor, and SC31 SC16 were

becoming good according to the measurements by UE :


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Further investigation shows that UE reports event 1a of SC31

SC16 before drop:


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That active set updating about adding Sc31 was successful :


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While active set updating about adding Sc16 failed for the

neighbor RNC cell configuration mistake:


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SummarySummary

We’ve discussed of several kinds of call drop, so typical they are as mentioned above. Engineers may meet with call drops that seem more puzzling. Taking no account of equipment problems, RF problems can be solved by suggestions below mostly:

1. If coverage is poor, we can reduce the downtilt and adjust the

azimuth, or we can increase the pilot transmit power or maximum downlink transmit power for service. The negative effect: is other places may be interfered, and soft handover ratio maybe higher.


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SummarySummary

2. If the interference is strong, first make sure whether it is internal network interference or external network interference. If it is internal network interference, find the interference source cell, increase the downtilt and adjust the azimuth, or reduce the pilot transmit power or maximum downlink transmit power for the service. The negativeeffect is coverage in other places may become discontinuous, andsoft handover areas maybe shrink.

3. As for the handover area is insufficient, and if it’s too slow to add the target cell, we can modify corresponding 1a event parameters. If it’s too fast to delete the serving cell, we can modify corresponding 1b event parameters. The negative effect is other places may be interfered and soft handover ratio maybe increase.


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