Abstract— In this letter, we investigate the outage performanceof cooperative relaying transmissions in two-user non-orthogonalmultiple access systems, wherein simultaneous wireless information and power transfer is employed at the near usersto power their relaying operations. To this end, a best-nearbest-far (BNBF) user selection scheme is proposed. Consideringthree relaying protocols, i.e., decode-and-forward (DF), amplifyand-forward (AF), and hybrid DF/AF protocols, tight closed-formapproximate expressions for the outage probability are derived toevaluate the system performance. Numerical results reveal that,for any relaying protocols used, the diversity order achieved bythe BNBF scheme is M +1, where M is the number of far users,and does not depend on the number of near users.Index Terms— Non-orthogonal multiple access (NOMA),outage performance, simultaneous wireless information andpower transfer (SWIPT), user selection.