Normally, SD high drop rate problem is caused by :
– above 4%. Hardware failure, either BTS or transmission >> replace the most problematic module even no alarm appears
– Specific User, massive SMS usage >> above 8% drop rate. In the case, you can figure out the problem by applying CDR(Record on specific cell). It also accompanies with massive SDCCH load.
– Imperfect Frequency Assignment >> 1% to 4% drop rate. Of course, co- or adj- channel interference can cause the problem. But you also have to care about the “Too Close BCCH assignment”. The MS usually can not correctly detect the right BSIC if there are two neighbours with the same BCCH. It’s ocassionally error detected. Try to change a new BCCH !!