1. A first base station that receives signals transmitted from one terminal in cooperation with a second base station and performs synthesis processing of the received signals of the second base station and the received signals of the first base station, the first base station comprising:a determination circuit that determines that the synthesis processing is necessary in a case where there is an error in a result of decoding of the received signals of the first base station at the time of the first transmission of the terminal;
a communication circuit that transmits to the terminal a Negative Acknowledgement (NACK) indicating that there is the error in the result of decoding, and transmits to the second base station a message requesting forwarding of the received signals in a case where the synthesis processing is determined to be necessary, and receives the forwarded signals that are forwarded from the second base station at the time of retransmission of the terminal; and
a synthesis circuit that synthesizes the forwarded signals from the second base station and the received signals of the first base station in a case where the synthesis processing is determined to be necessary,
wherein the forwarded signals are obtained by synthesizing signals that have been received at the second base station from the first transmission until the current retransmission.