SD Assignment Failure is either:
– congestion (but that’s not considered as a failure, strictly speaking)
– radio problems
– hardware problems (abis, trx…)
If it’s not congestion or hardware, then it’s a radio issue:
1- Coverage ? does this cell has a huge coverage ? then you can’t do anything unless increasing the RX LEV ACCESS MIN from -104dBm to -100dBm for instance, or reducing the TIMING ADVANCE LIMIT.
2- Interference ? Try changing the frequency. Try putting the SDCCH timeslot on another TRX.
I noticed high sdcch failure rate (50%) for sites using abis over satellite. Can’t do anything about this, so far (need to go drive-tests there one day…)