Hi Jerrys,
In easy way, if you have TCH congestion, than add TRX. Than you will have much more capacity. If you have SDCCH congestion, than add SDCCH channels (change channel configuration from Comb to NonCom) or add more than one NonComb timeslots. One Comb is 4 SDCCH channels and one NonComb is 8 SDCCH channels. But if you have SDCCH drop than you have probably problem with interference. Check Idle Band. And try to find better BCCH frequency.
BR Tomar