Handover Algorithm and Procedure in LTE

A Handover will be initiated by a measurement report, which is sent via the Radio Resource Control (RRC) protocol. Upon the reception of this measurement report, the handover algorithm will decide whether a handover should take place.

In response to the handover decision, the handover execution will be carried out using the corresponding procedures. After the handover execution, the handover algorithm will be informed, whether the handover was successful or not.

The Handover procedure is composed of a number of single functions:

• Measurements
• Filtering of measurements
• Reporting of measurement results
• Hard handover algorithm
• Execution of handover

When the UE is in LTE_ACTIVE state, mobility handling takes place via network controlled handovers with UE assistance. UE assistance here simply means that the UE does measurements and reports them to the eNB to assist in the handover decision. Currently it is planed that neighbor cell measurements are based on the UE’s cell detection capabilities rather than on a network supplied neighbor cell list.

When the source (current serving) eNB decides to start a handover of an UE to a neighbor cell in a new (target) eNB it will contact this target eNB. This is done via the X2-AP message HANDOVER REQUEST. The message will contain the target cell for the UE, the current serving MME and SAE GW. It is task of the target eNB to allocate virtual capacity in the target cell via its admission control function.

If this is done the target eNB returns part of the handover message for the UE within the X2-AP message HANDOVER REQUEST ACKNOWLEDGE. In this message also a data forwarding tunnel (TEID from target eNB) is indicated. It allows the source eNB to forward still buffered or still arriving downlink packets to the target eNB.

The source eNB can now give the HANDOVER COMMAND (RRC) to the UE. The command contains the configuration for the UE in the new cell and possibly already an UL/DL resource allocation. The UE will detach from the old cell and synchronize itself to the new cell. In the mean time the source eNB can start downlink packet forwarding via X2 interface.

Once synchronization between UE and the new cell is achieved, the UE confirms the handover with RRC message HANDOVER CONFIRM. This will trigger a HANDOVER COMPLETE message of S1-AP to be sent to the MME. It simply informs the MME that now a new eNB is responsible for the UE. Thus this message will contain the IP addresses and TEIDs of the target eNB for the S1 tunnels.

The MME’s task is to send this information via GTP-C UPDATE BEARER REQUEST to the SAE GW. This will switch the traffic path now completely from SAE GW to target eNB.
When the path is switched, the old eNB will get the S1-AP message RELEASE RESOURCE which will clear down all allocated resources for the UE that is already
in the new eNB.