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In this letter, we investigate the outage performance of cooperative relaying transmissions in two-user non-orthogonal multiple access systems, wherein simultaneous wireless information and power transfer is employed at the near users to power their relaying operations. To this end, a best-near best-far (BNBF) user selection scheme is proposed. Considering three relaying protocols, i.e., decode-and-forward (DF), amplify-and-forward (AF), and hybrid DF/AF protocols, tight closed-form approximate expressions for the outage probability are derived to evaluate the system performance. Numerical results reveal that, for any relaying protocols used, the diversity order achieved by the BNBF scheme is M+1, where M is the number of far users, and does not depend on the number of near users.
Nhu et al. (2016) studied this question.