When it is determined that a source cell configuration is changed and this change causes a change in a pre-configured candidate target cell configuration, an S-SN transfers a T-SN modification required message to an MN (operation 13-05). The T-SN modification required message may include information which is the same as the message of operation 10-15 of FIG. 10 (or at least one piece of information among pieces of information included in the message transmitted in operation 10-15), and may be transferred to the MN. When the T-SN Modification Required message is received, the MN transfers a T-SN modification request message to a T-SN of pre-configured candidate cells (operation 13-10). The T-SN modification request message may include information which is the same as the message of operation 10-30 of FIG. 10 (or at least one of pieces of information included in the message transmitted in operation 10-30). The T-SN having received the T-SN modification request message generates configuration information of a new candidate cell. The T-SN may generate or update configuration information of a new candidate cell by using a delta signal based on “configuration information of a source cell” received from the T-SN modification request message. In another embodiment, the T-SN may generate or update candidate cell configuration information by using a delta signal based on configuration information of an existing candidate cell rather than configuration information of a source cell. The updated configuration information of the candidate cell may be loaded in a T-SN modification request ack message and transferred to the MN again (operation 13-15). The T-SN modification request ack message corresponds to the message transmitted in operation 10-40 of FIG. 10. In operation 13-20, the MN transmits a T-SN modification response message to the S-SN. The T-SN modification response message may correspond to the message transmitted in operation 10-45 of FIG. 10. The S-SN having received the T-SN modification response message may transmit a T-SN modification response ack message to the MN. The T-SN modification response ack message may correspond to the message transmitted in operation 10-55 of FIG. 10.
