Mobility Management Signaling

Intra-RAT (inter-eNB) handovers are handled by the X2 interface unless any of the following conditions are true:

  • No X2 interface between the source and target eNBs.
  • The source eNB has been configured to initiate handover to the particular target eNB via the SI interface in order to enable the change of en an EPC node (MME and/or Serving GW).
  • The source eNB has attempts to start an inter-eNB handover via X2 but receives a negative reply from the target eNB with a specific cause value.
  • The serving PLMN changes during handover.

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.

X2 Mobility Management

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 ...

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