IF i’m getting you, you want to know the situation when the three cases mentioned by you occure and what can be the possible solutions for that. As per my knowledge:
1)Excessive TA : It happens when the distance between the MS and Serving BTS is more or you are at the cell boundary. To reduce the tch drop due excessive TA, best method is to use the downtilting of thr GSM antena (Both E & M). You can also limit the TA (I’m sure in NOKIA System) with the certain parameters settings.
2)Low Signal Strength : The possible reasons for low signal strength may be VSWR, any obstruction in front of GSM antena, Low BTS power, etc.
In general i have seen that if your parameter setting is OK this problem happens mainly due to Hardware issues (e.g. VSWR High, Antena Connection Lost etc.)
3)Bad Qality : It may occure due to both high interference or your Hardware.
Hope this will help you.