1. A source base station comprising:a controller and a transceiver;
wherein the controller is configured to control the transceiver to:
communicate with a target base station over an X2 interface;
transmit, to the target base station over the X2 interface, an X2 application protocol (X2AP) handover request message which includes a Mobility Management Entity User Equipment S1AP (MME UE S1AP) identity and a global MME identity, wherein the MME UE S1AP identity uniquely identifies a UE over an S1 interface within the MME; and
receive, from the target base station over the X2 interface, an X2AP handover request acknowledgement message.