what is the “standard” SDCCH drop rate ?
what is the SDCCH “LU” drop rate ?
please provide hourly values for one typical day.
Even though you are not in location area Border, the MS are still doing LU. Could you compare the amount of LU done in this cell, compared to the immediate neighbour ? Do you see a big difference ? Values should be comparable, assuming the number of idle MS is similar in both cells.
If you do see a difference, you may assume that the LU received by your “bad” cells are “fake”.
1/ changing the BCCH/BSIC
2/ reduce MAXTA
As you suggested, the problem might come from 3G. But the LU are due to MS leaving the 3G and falling back to the 2G. Not the opposite –> you must set your UMFI IDLE to proper values.
In your 3G cell, check out the number of reselection from 2G to 3G (such counter exist in E/ 3G and in ALU 3G). Are they high ?
If yes, then it looks like some ping pong is occuring between 2G and 3G.
Those are all the ideas I have so far. Waiting for some numerical examples….