Disclosed is a base station which transmits and retransmits to a terminal first and second downlink data in first and second component carriers, respectively, wherein a first configuration pattern of UL (uplink) and DL (downlink) subframes is set for the first component carrier and a second configuration pattern of UL and DL subframes is set for the second component carrier. The base station receives from the terminal in the first component carrier an ACK/NACK for the first and second downlink data received by the terminal, which stores retransmission data of the first and second downlink data in a soft buffer, wherein the soft buffer for the second downlink data is sized according to a maximum number of downlink HARQ retransmission processes executable in a reference configuration pattern of UL and DL subframes, and the reference configuration pattern is determined according to the first and second configuration patterns.