Hello A. Qadah,
At international borders, you must get a lot of interferences from other operators, since there is usually no agreement on which frequencies belong to whom.
Therefore, in order to reduce interferences without knowing interfering frequencies, you must use synthetized frequency hopping, with as many frequencies as possible in each FHS. This way, it should average as much as possible all possible interference without too much work.
Keep in mind that the BCCH TRX will use a non-hopping frequency, so place your SDCCH timeslots on hopping TRXs.
If you’re experiencing hardware failures, or if there is no interference in this area, you must perform further investigations.
Finally, you may experience SDCCH congestion rate (due to high number of imsi attach / imsi detach caused by roaming ?) but it is not the same as sdcch drop rate.