With development of mobile Internet, a terminal user has a higher requirement for a bandwidth. To increase a network throughput, multiple small sites may be introduced to a cell of a macro base station (Macro eNB) to construct small cells (small cells), and the small sites are distributed in service hotspot areas, or coverage hole areas. In this case, when user equipment UE moves to these areas, services of the UE may be handed over to these small cells, to implement service offloading or coverage compensation, thereby achieving an objective of increasing a system capacity, increasing a user throughout, or improving coverage.
The following describes, by using an LTE (long term evolution) network as an example, a process of handing over a downlink service of UE.
A communications system includes: UE, an SeNB (source base station), a TeNB (Target eNB, target base station), and an SGW (service gateway).
The first is a handover preparation process, and perform step 1: The SeNB performs measurement configuration on the UE, where a measurement result of the UE is used to assist the SeNB in making a handover decision.
Step 2: The UE sends a measurement report to the SeNB according to the measurement configuration.
Step 3: After receiving the measurement report, the SeNB makes a handover decision according to the measurement report.
Step 4: The SeNB sends a handover command message to the TeNB, and the TeNB prepares for handover.
Step 5: The TeNB executes admission control according to received QoS (Quality of service) information borne in an EPS (evolved packet system).
If the TeNB includes sufficient resources, the TeNB reserves resources for the UE according to the received QoS information borne by the EPS, generates radio resource configuration information, and reserves one C-RNTI (cell radio network temporary identifier) for the UE, to prepare for the handover
After the TeNB completes the preparation, go to step 6: The TeNB sends a handover request acknowledgement message to the SeNB, to notify the SeNB that the handover preparation is completed, where the handover request acknowledgement carries the generated radio resource configuration information and the C-RNTI.
Step 7: Start a handover execution process, and the SeNB sends a handover command message to the UE.
The handover command carries necessary information, such as a new C-RNTI, a security algorithm identifier of the TeNB, a dedicated random access preamble, an end time indication of a dedicated preamble, and a system information block of the TeNB.
Step 8: The SeNB sends an SN (Sequence number, data packet sequence number) status transmission message to the TeNB, to notify the TeNB of a receiving status of a downlink PDCP SN (packet data convergence protocol sequence number) of the UE, so that the TeNB knows a current data transmission status of the UE.
In the following steps 9 to 11, after the handover command is received, the UE executes a process of synchronizing with the target eNB and connects to a target cell.
Step 9: The UE executes a synchronization process, to synchronize with the TeNB, and a network responds to the synchronization.
Step 10: After the UE successfully connects to the TeNB, the UE sends a handover acknowledgement message to the target base station.
Step 11: The TeNB sends a path switch response (Path switch Request response) message to an MME (mobility management entity) of the UE, to notify the MME that a base station that serves the UE changes.
Step 12: After receiving the path switch response, the MME of the UE sends a modify bearer request message to the SGW of the UE, to notify the SGW that a connection of a user plane needs to be switched from the SeNB to the TeNB, that is, a downlink path of the user plane is switched to the TeNB.
Step 13: The SGW returns a modify bearer response message to the MME, to confirm a connection update of the user plane.
Step 14: The MME returns a path switch response acknowledgement character (path switch request ACK) to the TeNB.
Step 15: The TeNB sends a resource information release message of the UE to the SeNB, to notify the SeNB that the handover has been completed, and a resource and information that are related to the UE can be released.
Step 16: The SeNB releases information related to the UE, such as radio and control plane resources.
It can be seen that, a handover preparation process needs to take a long time. When ping-pong handover occurs on a cell edge or the UE rapidly moves between cells, the foregoing handover preparation process needs to be performed each time the UE is handed over, and rapid handover between cells cannot be implemented; therefore, handover efficiency of the UE is reduced, and a case of poor quality of service of a service or even a call drop occurs, which affects user experience for the service.