I think that you work with an Ericsson network so I would base my answer on that.
There are 2 known formulas for the SDCCH Drop:
The second formula corresponds to the TCH Congestion excluded. So, the first formula really goes off the limit when there is congestion.
The reason counters are the following:
CDISTA: Excessive TA, that is if the TA value at a dropped connection is higher than or equal to the cell parameter TALIM.
CDISSS: Low signal strength in downlink and/or uplink per cell (CDISSS or CDISSSSUB), that is if the signal strength is less than the BSC exchange properties LOWSSUL and LOWSSDL in uplink or downlink respectively, or in both the uplink and downlink.
CDISQA: Bad quality in downlink and/or uplink (CDISQA or CDISQASUB), that is if the radio link quality is less than the BSC exchange properties BADQUL and BADQDL in uplink or downlink respectively, or in both the uplink and downlink.
As you can see the Reasons also depend on the parameter configuration of the TALIM, LOWSSDL, LOWSSUL, BADQDL and BADQUL. Maybe you can check the values and consider to move a couple of them and after check the new distribution.
You are right. The Other reason is normally around 70% – 75% in the Ericsson networks that I have seen.
The SDCCH Drop can be improved by:
– Adjusting the LA border so the interaction is minimum.
– Using parameters such as the ACCMIN, CCHPWR and/or MAXTA. But be careful with the coverage and some complaints.
– Better antenna performance. Remember to use electrical tilt (if possible) instead of too much mechanical.
– In you have a MSC in Pool and High SDCCH Drop check if there is any issue with the synchrony.
– I turned off the SDCCH HO (SCHO) and it worked in some sectors. Maybe you can try that.