Outage Performance Enhancing for Downlink Cooperative Non-Orthogonal Multiple Access Systems

Abstract

For non-orthogonal multiple access (NOMA) systems, the near user who has the better channel condition can act as the cooperative relay to enhance the diversity of the far user in general. To make good use of such a nature to fully exploit the advantages of NOMA, in this paper, we study the outage performance enhancing scheme for the downlink cooperative NOMA systems, where two users communicate with the base station with the assist of a single relay. Assuming Nakagami-m fading channels and the regenerative protocol is applied to the relay, system's ergodic sum rate and users' outage probability are derived in closed form. Numerical results verify the accuracy of the theoretical analyses and demonstrate that with the diversity enhancing strategy, the proposed NOMA scheme outperforms the conventional NOMA and time division multiple access (TDMA) schemes in terms of outage probability greatly. Moreover, it shows that the proposed NOMA scheme can almost achieve the same ergodic sum rate as the conventional NOMA scheme has in the high SNR region.