Relay Selection for Spatially Random Full-Duplex Cooperative Non-Orthogonal Multiple Access Networks

This paper investigates the relay selection problem and proposes a three-stage relay selection strategy with power allocation (TRSPA) for a spectrum-sensing-based full-duplex (FD) user relaying cooperative non-orthogonal multiple access (CNOMA) scheme. Uniformly-distributed strong user relays in the investigated scheme help a weak user communicates with the base station in an efficient and reliable way. The proposed TRSPA strategy maximizes the transmission data rate of the selected relay while ensuring successful transmissions for the weak user by precisely narrowing down relay candidates step-by-step and dynamically allocating optimal power coefficients. Exact and asymptotic outage probabilities and ergodic rates are worked out. Accordingly, diversity orders and spatial multiplexing gains are derived. We further exploit the impact of self-interference (SI) on TRSPA for FD-CNOMA and then compare its performance with TRSPA applied in other relaying modes, that is half-duplex and orthogonal multiple access. Finally, simulation results reveal that: (i) theoretical derivation results are correct; (ii) TRSPA always outperforms other relay selection strategies in terms of outage probability and ergodic rate; and (iii) TRSPA for FD-CNOMA in a real-world scenario achieves better performance than other relaying modes in spite of the adverse effect of SI in FD mode.