(Currently Amended) A method, performed by a first base station, for controlling the operation mode of the first base station with multiple base stations (103-3, 103-5), comprising: the first base station measuring resource usage for UE associated with the first base station; when the resource usage at the first base station is below a predefined threshold: the first base station performing a sleep mode negotiation including the following steps a) to c): a) sending a sleep mode request to a second base station, the request including an identifier of the first base station, an identifier of the second base station, a proposed sleeping interval, UE identity information and associated UE resource usage information of the UE; b) receiving a sleep mode response from the second base station, the response including the identifier of the first base station, the identifier of the second base station, an acceptance indicator, the UE identity information and sleeping interval information corresponding to that in the sleep mode request; c) starting a handover process with the second base station in accordance with the UE identity information in the sleep mode response; and the first base station further entering a sleep mode in response to determining that no UE is left in the first base station after a completion of the handover process and in response to determining by the first base station that the sleep mode response does not include a decline indication to decline the handover process; and when there is still active UE left at the first base station after a completion of the handover process with the second base station, the first base station performing a new round of said sleep mode negotiation with a third base station to handover the still active UE at the first base station. the first base station measuring resource usage for UE associated with the first base station; when the resource usage at the first base station is below a predefined threshold: the first base station performing a sleep mode negotiation including the following steps a) to c): a) sending a sleep mode request to a second base station, the request including an identifier of the first base station, an identifier of the second base station, a proposed sleeping interval, UE identity information and associated UE resource usage information of the UE; b) receiving a sleep mode response from the second base station, the response including the identifier of the first base station, the identifier of the second base station, an acceptance indicator, the UE identity information and sleeping interval information corresponding to that in the sleep mode request; c) starting a handover process with the second base station in accordance with the UE identity information in the sleep mode response; and the first base station further entering a sleep mode in response to determining that no UE is left in the first base station after a completion of the handover process and in response to determining by the first base station that the sleep mode response does not include a decline indication to decline the handover process; and the first base station performing a sleep mode negotiation including the following steps a) to c): a) sending a sleep mode request to a second base station, the request including an identifier of the first base station, an identifier of the second base station, a proposed sleeping interval, UE identity information and associated UE resource usage information of the UE; b) receiving a sleep mode response from the second base station, the response including the identifier of the first base station, the identifier of the second base station, an acceptance indicator, the UE identity information and sleeping interval information corresponding to that in the sleep mode request; c) starting a handover process with the second base station in accordance with the UE identity information in the sleep mode response; and a) sending a sleep mode request to a second base station, the request including an identifier of the first base station, an identifier of the second base station, a proposed sleeping interval, UE identity information and associated UE resource usage information of the UE; b) receiving a sleep mode response from the second base station, the response including the identifier of the first base station, the identifier of the second base station, an acceptance indicator, the UE identity information and sleeping interval information corresponding to that in the sleep mode request; c) starting a handover process with the second base station in accordance with the UE identity information in the sleep mode response; and the first base station further entering a sleep mode in response to determining that no UE is left in the first base station after a completion of the handover process and in response to determining by the first base station that the sleep mode response does not include a decline indication to decline the handover process; and when there is still active UE left at the first base station after a completion of the handover process with the second base station, the first base station performing a new round of said sleep mode negotiation with a third base station to handover the still active UE at the first base station.