NOMA-Based Cooperative Opportunistic Multicast Transmission Scheme for Two Multicast Groups: Relay Selection and Performance Analysis

The orthogonal frequency division multiple access (OFDMA)-based cooperative multicast (CM) technology realizing the intra-cooperation of multicast group (MG) has improved the performance in throughput and coverage. In the MG scenario, users are uniformly distributed. Due to the small effective transmission range of relay user (RU), the cooperation is hindered, which limits the performance of CM. Therefore, non-orthogonal multiple access (NOMA)-based two MGs joint cooperative opportunistic multicast scheme (termed as COM-NOMA) is proposed. In COM-NOMA scheme, two MGs are served as one quasi-MG. First, on the same frequency band, base station (BS) opportunistically multicasts the superposition signal, which is formed by two MGs' signals, to the users with good channel conditions in the two MGs using NOMA way. Second, some successfully receiving users are selected as RUs. The superposition signal is forwarded to the other users by Device-to-Device multicast way. Then, according to the coverage ratio of BS and the decode condition of NOMA, the signal to interference and noise ratio threshold for RU selection is given. Finally, the spectral efficiency of COM-NOMA is analyzed and the approximate expression of system coverage ratio is derived. The simulation results show that, spectral efficiency at unit transmit power (spectral efficiency per watt) and system coverage ratio is improved in comparison with other OFDMA-based CM schemes.