Friday, 16 October 2015

FDD-FL-PingPong Issue Analysis

  Incident Description (Incident Phenomena)
   

1 Overview

In one project, customer found that the user experience is not so good. UE can often drop from 4G to 3G and also go back to 4G. It seems there is a 4G/3G Ping-Pong issue.
 
As discuss with customer, user experience 4G/3G Ping-Pong when the user keeps the smart phone in “idle mode”, so Engineer do the drive test with a smart phone in “idle mode”.
Note: here “idle mode” just means the user do not try to do any data service with his smart phone. It is not the idle_mode as defined in protocol.  

Problem Cause Analysis
 

2 Test Analysis

 

2.1 Coverage Overview

We found that UE shift from 4G to 3G several times during the drive test. This figure show the LTE serving cell RSRP, the legend Invalid means that UE lost 4G signal.
 
At the same time, we check the same log for UE 3G signal, we found that UE catch 3G signal exactly when it lost 4G signal. That means UE shift from 4G to 3G.
This figure show the WCDMA best RSCP, the legend Invalid means that UE do not catch 3G signal.
 

2.2 Lost 4G issue Analysis

We analyzed the log and found that UE shift from 4G to 3G after two message: Measurement Report and RRC Connection Release.
In Measurement Report, UE report LTE serving cell RSRP=-92 dBm and UMTS neighbor cell RSCP=-72 dBm.
 
UE received RRC Connection Release, this message command UE redirection from 4G to 3G at ARFCN 10663.

 

2.3 Parameters Analysis

In LTE radio parameters table UeRATMeasurement for ratMeasCfgIdx=1060, these two parametersrsrpSrvTrd and rscpSysNbrTrd
Ø  rsrpSrvTrd
Absolute Decision Threshold of E-UTRAN Serving Cell for RSRP Measurement (dBm)
Ø  rscpSysNbrTrd
Absolute Decision Threshold of UTRAN System for RSCP Measurement (dBm)
These two parameters means that when LTE serving cell RSRP lower than rsrpSrvTrd and UMTS neighbor cell RSCP is higher than rscpSysNbrTrd, UE will handover (redirection if UE or network does not support handover) from LTE to UMTS.
We check BASE LTE network parameters, the value for these two parameters are:
 
rsrpSrvTrd
rscpSysNbrTrd
Absolute Decision Threshold of E-UTRAN Serving Cell for RSRP Measurement (dBm)
Absolute Decision Threshold of UTRAN System for RSCP Measurement (dBm)
long:[-140~-43];default:-115
long:[-120~-24];default:-95
-90
-86
 
The thresholds make UE easily shift from 4G to 3G; after UE shift to 3G, it may reselect back to 4G after a while because 4G have a higher priority. It causes a 4G/3G Ping-Pong.
suggest change rsrpSrvTrd from -90 to -110 and rscpSysNbrTrd change from -86 to -90.
 Solution
 

3 Re-test After Parameters Modification

After changing parameters in the related sites, we re-test the same road and found that UE stay always in 4G.
 
UE never shift to 3G in the same log when we re-test the same road.
 
 Summary and Notes
 

4 Suggestion

In order to make UE stay in 4G longer and not so easily shift to 3G,suggest all sites implement the following parameters modification
In LTE radio parameters table UeRATMeasurement for ratMeasCfgIdx=1060, these two parametersrsrpSrvTrd and rscpSysNbrTrd
  suggest change rsrpSrvTrd from -90 to -110 and rscpSysNbrTrd change from -86 to -90

Budi Prasetyo

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