Handover Call Flow
Handover Call Flow
This diagram shows X2 interface-based handover call flow at a high level.
1. The handover procedure is used only when a UE moves from one cell to another in the RRC Connected state.
2. Once a UE is in RRC Connected, the eNB will send the RRC Connection Reconfiguration message, which will define the measurement configuration, to the UE.
3. One of the key measurement configuration parameters is s-Measure. S-Measure is the level of RSRP for the current cell below which the UE is required to start handover-related measurements. As long as the current cell RSRP is below s-Measure, the UE will continue its measurements.
4. When UE measurements meet event triggers defined by the eNB, the UE will send a Measurement Report message to the source (current) eNB.
5. Assuming that the reported neighbor cell has the X2 interface is defined and enabled, the source eNB will send a Handover Request message across the X2 interface to target eNB. It will send information about current bearers and related details to aid the target eNB in its admission control process.
6. If the target eNB can support the UE coming in, it will send Handover Request Ack message, including any resource allocations such as a new C-RNTI for the UE, PRACH resource information etc.
7. The source eNB will then capture the received information from target eNB and send it in an RRC Connection Reconfiguration message to the UE.
8. The UE moves to the target cell and initiates PRACH access procedures. On successful completion of the procedures, and getting timing adjustment done, the UE sends an RRC Connection Reconfiguration Complete message to the target eNB.
9. The target eNB then sends the UE context release message to the source eNB to conclude the radio link part of handover.
10. The data path switch will then be completed between the S-GW and source and target eNBs with the help of the MME.