Buffer-Aided Cooperative Non-Orthogonal Multiple Access for Downlink Transmission

A downlink buffer-aided cooperative nonorthogonal multiple access (C-NOMA) system is investigated in this paper. Both the direct transmission from the base station to the users and the buffer-aided cooperative transmission between two users are considered. In particular, a novel buffer-aided C-NOMA scheme is proposed to adaptively select a direct or cooperative transmission mode, based on the instantaneous channel state information and the buffer state. Then, the system outage probability of the proposed scheme is theoretically derived with a closed-form expression. Furthermore, the full diversity order of three is demonstrated to be achieved if the buffer size is not less than three, which is larger than conventional non-buffer-aided C-NOMA schemes whose diversity order is only two in the considered C-NOMA system.