I am working on Nokia systems and facing a problem in approx 20 cells having high SD Drop and TCH assignment low.Traffic is good.In few cells SD and TCH blocking is there.
In reports there is no issue of Path Imbalance.There are quality issue while looking at the report but no such possible interferer is found on MCOM.The quality is bad on TCH TRX.Hopping is implemented throughout the network.HOSr is good.
In overall network overshooting is prominent.
it’s either bad coverage, interference or TRX hardware problem.
you have to investigate more into details :
what is the ratio of quality/level handovers compared to better cell handovers ?
what is the sub-cause of SD drop ?
what is the sub-cause of tch asgn fail ?
can you see the split of drops & fails per trx ?
are those cells having a large coverage, or a resurgence of coverage, meaning that they are covering some subscribers with a very low rxlev ?
do you have access to radio mesurement statistics ?
I am not familiar with Nokia equip but for such a wide spread problem start by looking at the common elements (BSC hardware) among the 20 cells.
Look also at the hardware KPIs for the BSC (CPUs etc) and it would be good if you gave us more details such as: Is the problem limited to a certain time of the day? etc
Check the transcoder resources connected to the BSC controlling the 20 sites; i bet they are all in the same BSC. If yes, it’s likely there is congestion on the transcoders so the calls drop on the SDCCH and indirectly affects TCH assignment.