I’ve listed down some Common reasons for 3G-2G handover failures
1). Improper neighbour definition-this can be verified from SIB 11 message
2). Wrong settings for event 2D thresholds & TTT (time to trigger values)
3). Wrong Settings for event 3A (thresholds & TTT)
4.) Inaccurate Timer settings (eg for T309 it is recommended to have more than 5s)
above are related to wrong configurations, remember that there could be other reasons as well
furthermore if we consider the different phases of the HO process, followings can be identified as some of the reasons for failures;
1. failures During Preparation phase (eg- due to no radio resources in target cell)-these are generally called as “CS-iRAT-HO_Failure_1”
2. failures in IuB & generally called as “CS-iRAT-HO_Failure_2”
3. Due to NBAP link failures
thanks