In Contention Based Handover how the C-RNTI's would be exchanged in b/w S-eNodeB and T-eNodeB?

Question

In case of hub-and-spoke topology, X2 messaging can travel all the way back to an aggregation point, which may be co-located with EPC. In such scenario both X2 and S1 traverses same physical path (a.k.a X2 over S1). Please consider both Intra-MME and Inter-MME handover scenarios. Would X2 based handover in such a case yield advantage over S1 based handover. Thanks.

Answer 1

FYI there is no term like contention based Handover , The RACH access procedure may be contention based or contention free when UE selects any eNodeB.

This contention based RACH happens at initial cell selection or in RLF failure case where too late Handover occurs.

In case of contention free RACH which happens in case of successful handover ,Target eNB share the newUE-Identity C-RNTI and target PCI to Source eNB in handover Ack message and source eNB shares newUE-Identity C-RNTI and target PCI with the UE using mobility control info message.

But in case of contention based UE selects a random preamble and sends to ENB ,Then enB allocates Temp C-RNTI.As there is a chance of other UE may send same Preamble id so eNB sends contention resolution message and resolves contention.So in this case C-RNTI of target ENB can not be shared previously with UE so UE acquires it using contention based RACH procedure.

Answer 2

Contention Based Handover: When the Handover requires Random Access Procedure.

When the handover is Contention based, UE would be informed by the source eNodeB through rrcConnectionReconfiguration message.

Explanatory: In RRCConnectionReconfiguration message, It has mobilityControlInfo Information Element. Which has another field called new-UE identity, which contains the info of RandomAccessPreambleIdentifier.

This preamble will be selected by the UE and will be transmitted in the available sub-frame. The target eNodeB answers using the same preamble and will assign the temporary C-RNTI to UE. This is how the C-RNTI will be exchanged.

UE will get the information of his new C-RNTI and UE will use this information in MSG3 of handover.

I hope I have answered your query. Further doubts are welcome.