Cell Searching

Page 1Inter-eNodeB Handover over the X2 Interface (I)

The UE has accessed to the cell and performs services. The source eNodeB delivers measurement control information and notifies the UE of neighboring-cell measurement. The UE returns the measurement report after detecting a cell that meets handover conditions. The source eNodeB decides a handover based on the handover algorithm and current state. Handover preparation: The source eNodeB sends a Handover Request message to the target eNodeB. The Handover Request message contains the service information and other access layer information (encryption, integrity, and measurement information) of the current UE. The target eNodeB returns the admission result and radio resource configuration information to the source eNodeB, completing the handover preparation. After receiving the Handover Request message from the source eNodeB, the target eNodeB performs admission control based on the contained service information and radio resource configuration. The source eNodeB sends the container delivered by the target eNodeB to the UE through the radio message to inform the UE to trigger a handover. Sequence number (SN) information is used to transfer the SN status and hyper frame number (HFN) information of the UE from the source eNodeB to the target eNodeB for the purpose of data retransmission and encryption integrity protection. Data forwarding. For details, see subsequent description in this document. 11. Prerequisites for inter-eNodeB handovers over the X2 interface: The source eNodeB and target eNodeB are configured with X2 interfaces and must belong to the same MME pool.

4.The source eNodeB sends a Handover Request message containing necessary information to the target eNodeB for the purpose of handover preparation on the target eNodeB side. The necessary information includes the UE X2 signaling context of the source eNodeB, UE S1 EPC signaling context, target cell ID, key, RRC context containing the C-RNTI of the source end, and AS configuration (excluding physical-layer configuration). UE X2 and S1 signaling context helps the target eNodeB locate the source eNodeB and EPC. EPS bear context includes necessary RNC and TNL address information and QoS configuration of the EPS bear.

5.If resources are available on the target eNodeB, the admission control on the target eNodeB increases the handover success rate based on the EPS bear QoS information received. The target eNodeB configures required resources and reserves a C-RNTI for the UE based on the EPS bear QoS information received, and may also provide an RACH preamble for the UE. The AS configuration on the target eNodeB can be set separately or be set as an optional part of AS configuration on the source eNodeB.

6. The target eNodeB prepares corresponding layer-2 and layer-3 resources for the handover and then sends a Handover Request Acknowledge message to the source eNodeB. The Handover Request ACK message contains a transparent container, which will be a part of the Handover Command message sent to the UE.

7.The source eNodeB generates and sends a Handover Command message (RRC message) to the UE. The Handover Command message contains the transparent container received from the target eNodeB. The handover takes priority over the HARQ/ARQ and DRX.

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