Degradation in INHSR, OHSR and increase in TCH Drops after swat from BTS312 to BTS3900A

Publication Date:  2014-03-23 Views:  1803 Downloads:  61
Issue Description
BTS: BTS3900A
Configuration: S222_222

There were transmission alarms on site. Also radio link failure was reported on second and third sector.  This was causing degradation in all major KPI's for second and third sector of this site.
Alarm Information
Tranmission Alarms reporting on site:

1. 25803 - E1/T1 Loss of Frame Alignment (This alarm is reported when Loss of Frame Alignment (LFA) is detected on the E1/T1 RX port)

2. 25802 - E1/T1 Remote Alarm Indication Signal (The peer E1/T1 device sends Remote Alarm Indication (RAI) to the local E1/T1 device when the peer E1/T1 device cannot detect the frame synchronization signals correctly)

3. 25801 -  E1/T1 Alarm Indication Signal (This alarm is reported when the all 1s signal (Alarm Indication Signal, abbreviated as AIS) is detected on the E1/T1 RX port)

4. 25806 -  E1/T1 Excessive Bit Error Rate (This alarm is reported when the bit error rate (BER) on the E1/T1 link exceeds the preset threshold

5. Radio Link Failure due to abnormal release in second and third sector on DCS1800 TRX's
Handling Process
I checked the cell level parameters for second and third sectors and found the following parameters wrongly configured:
1. The parameter  Enhanced concentric Allowed was set to OFF and I set it to ON, Because both cells were concentric cells i.e GSM900_DCS1800.





2. The cell basic handover parameter i.e Concentric circles handovers was set to NO and I configured it to yes so that handovers can be performed between overlaid and underlaid cells.




3. FSE was sent on site to troubleshoot the hardware and It was found that Partial swap (Swap in DCS1800 feeder cables) was present in second and third sector. On site engineer removed the partial sector swap.
After performing the above steps KPI’s for the site were improved and TCH drop rate was decreased in second and third sector. Also INHSR and OHSR was normal. It was also observed the Assignment success rate was improved after troubleshooting. Given below are POST troubleshooting benchmarks.


Root Cause

High BER on E1’s was removed by customer transmission but no improvement in KPIs was observed. I made a detailed analysis of site KPI’s and checked the following measurements:
Radio Link Failures on second and third sector were observed  and trend showed increased radio link failures in both sectors after E1 troubleshooting:



I made a detailed analysis of Radio Link failures on TRX level and found that DCS1800 TRX in second and third sector were having increased radio link failures


Downlink Levels were observed during radio link failures and it was found that during failures the level was not OK.



 

I checked the Mean timing advance during radio link failures for both sectors and found out that downlink levels were not good as compared to timing advance and uplink level for both cells.

  


 


It was clearly stating some sort of hardware fault  for second and third sector. 

 

Suggestions

When radio Link Failure alarms are encountered due to Abnormal Releases first of all the transmission alarms (if any) on site should be cleared, Basic configuration parameters of cocentric cells and basic handover parameters of concentric cells should be checked to be correct. Site engineer should make sure that there is no complete or partial sector swap carefully.

END