In LTE the handover is carried out over the X2 interface. In case the X2 link is unavailable the S1 interface comes into picture for the handover.
The handovers in the LTE have a new concept.
Here is how the handover occurs in LTE
- The involved eNBs starts by agreeing about the handover then the UE is moved to the new cell.
- When the UE is safely locked to the new cell, the MME is notified that a new S1-UP connection should be established.
- During the time the UE moves to the new cell until the new S1-UP is established the remaining data from source eNB and new data coming on old S1-UP are forwarded over X2-UP.
If X2 HO is not possible a S1 handover can be done instead. The S1 HO uses the MME to transfer HO messages. At S1-based handover, packet forwarding can take place either directly from the source eNodeB to the target eNodeb
In cases when the UE moves between eNBs that belong to different pooling areas the handover procedure necessarily has to be executed via the S1 interface. In such cases at least the MME function, holding the UE context has to be relocated from one MME node in the first pool to another MME node in the second pool. There is possibility to relocate also the S-GW node during the S1 handover procedure, if it is needed (e.g., if no IP connectivity exists between the target eNB and the current S-GW). However, it is assumed that in the typical case the S-GW will not have to be relocated during an S1 handover, since IP connectivity will be configured in the network such that all S-GW’s will have connectivity to all eNBs.
In case of handover failure , the user equipment must first loose the connection and then perform an idle mode cell re-selection in order to move from one cell to another. This involves the user equipment moving from connected mode to idle mode in order to perform cell re-selection with direction from the network.