What is INTER-RAT Mobility and why Inter-RAT is done?

Question

Since both the events B1 and B2 are used for Inter RAT measurement reporting. Which one is chosen by eNodeB and on what basis ? Is there any dependency on UE capability for that ?

Answer 1

InterRAT mobility mean to be UE is moving from one cell to another cell by changing its Radio Access Technology..

consider that a UE is moving form Cell A to Cell B. Assume Cell A is LTE cell and in Cell B is an UMTS cell, if handover happens from cell A to Cell B then this kind of handover is called as InterRAT Handover.

Now coming to your second part Why is Inter-RAT is done
Reason is simple if word is uniform i.e. everywhere ALL LTE then it is not required so to provide the handover from one network technology to another one InterRAT mobility is required.

Answer 2

I just want to add few points to answer your question "why Inter-RAT is done ".

• There could be a primarily reason of doing handover from LTE to UMTS, GSM and CDMA2000 network just because of "Circuit Switch call". If an operator does not have support of VoLTE in its deployed LTE network then CS call back is the best option since operator may have their legacy system in place which will take care of mobile originating/terminating CS call.