New look on relay selection strategies for full-duplex multiple-relay NOMA over Nakagami-m fading channels

By removing the orthogonal use of radio-resources, non-orthogonal multiple access (NOMA) has been introduced to improve the spectral efficiency of fifth generation (5G) and beyond networks. This paper studies the system performance in a dual-hop multi-relay NOMA using decode-and-forward (DF) scheme over Nakagami-m fading channels. A group of NOMA users is considered, i.e. the near and far users which are decided by how strong these related channels are. Specifically, we obtain a closed-form expression of the outage probability of the near/far NOMA users when the several relay selection schemes are adopted for selecting the best among M intermediate relays. As main finding, this paper introduces three strategies including two-stage relay selection, max-min and power allocation based relay selection schemes. As main benefit, the NOMA users are considered to employ selection combining technique in order to improve signal transmissions for an increased reliability in the context of massive connections in 5G wireless communications. By conducting numerical simulations, we evaluate the impact of the number of intermediate relays, the NOMA power allocation factor, and the Nakagami-m fading severity parameter on the outage performance of the NOMA users. Finally, the outage probability along with throughout in delay-limited transmission mode are provided via numerical results and the necessary comparisons are provided.