Intra-RAT (inter-eNB) handovers are handled by the X2 interface unless any of the following conditions are true:
The S1 interface handles inter-RAT handovers. It also handles handovers from and to HeNBs.
In what follows, the relevant signaling for the X2 and S1 interfaces are described.
Elementary procedures for mobility management in the X2 interface include: Handover Preparation; SN Status Transfer; UE Context Release; and Handover Cancel.
Handover Preparation involves the source eNB sending a Handover Request to the target eNB. When the target eNB receives the request, it performs an admission control request based on the resources indicated to be required for the UE's radio bearers. If sufficient resources are available, the target eNB acknowledges the request with a Handover Request Acknowledge. If not, the target eNB sends a Handover Preparation Failure message to the source eNB. This terminates that Handover Preparation procedure. If beyond a certain time the source eNB does not receive an indication of either ...