2G HOSR/IHSR sudden degradation...!

Hi Experts,
I am facing an issue regarding HOSR/IHSR sudden degradation on some sites which belongs to same servring area ( As shown in below picture) causing BSC level degradation in the network, I am working on ZTE vendor. Kindly help me out to resolve it. Also these sites belong to one BSC with no external boundary or MSC boundary.

BSC level HOSR/IHSR Trend.

Cell level trend

Degradaed cells geographical location.

Below are the action pionts which have been analyzed/checked till now by us.

  • No Hardware related issue observed
  • Dump/Radio export comparison is done, No parameter change is observed
  • No frequency changes has been done on these cells before and after degradation.
  • No other KPI degraded on cells which map the HOSR/IHSR degradation
  • No Packet loss observed.
  • Increase in PBGT/UL/DL quality has been observed.
  • Site Reset, Nbrs deletion/addition has been done, No impact
  • In relational level stats HO OUT/ HO IN is degraded majorly in between these impacted cells .
  • As all cell are serving same are Transmission end has also been checked, But no issue observed.
  • External interference testing has also been conducted for the entire band, Not thing observed.
  • NO obvious hardware alarms were found of the issue sites. But one alarm were found(Almost every issue site once had this alarm,this alarm will not always appear):1588 Clock source fault. Core End says that they haven’t observed any alram related to Clock issue.

Issue has also been also escalated to our experts below is there analysis

  • Also from the DT results,no obvious handover failures were found. From the RCT tracing results, we have found that the HO Fail cause: Channel Mode unacceptable. And we have checked that only Speech Version3 have big increasse HO attempts. Our Experts says that the Speech version also need to be configured by CN, pls also let them help check. For the illustration of speech version, pls see following:

Further they say that there may be a certain type of Mobiles which does not support preferSpeechVersion AMR 3. Kindly support for resolution.

Check power control windows and handover control windows

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Check if there is any traffic balancing between 900 and 1800 band.

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Power control and handover control windows are the same as before degradation

First step in solving issue to to determine scale. Usually HO failure is caused by issues on target cell…so better to work with IHOSR. check ihosr degradation, is it on one cell or one site or multiple sites?