Hi Every Body,
SD drop are cosed either by:
1)hardware problems
2)coverage & interference problems
for the first case try to resolve your HW problem, check your alarm and try to restart your TRX. check quality of each TRX and try to have SD TS over the best ones. also we should have one SD TS over your BCCH at least.
2) for the second case try analyze your HO causes & TA & TRX Qual if interference is found try change frequenceies. otherwise check your parameter try reduce your cell edge (RxLevAccessMin high value), increase your BS Transmission power and finaly try increase your T200 timer to give your cell sufficient time to wait for the ack before retransmission check.