Design of Relay Selection Schemes for Uplink Cooperative NOMA Over Nakagami-$m$ Fading

This paper investigates relay selection (RS) schemes for uplink cooperative non-orthogonal multiple access (NOMA) systems, where two users with different priorities would like to transmit their information to a base station by means of multiple relays. Based on the degree of the use for the channel state information (CSI), five RS schemes with different user ordering and power allocation (PA) strategies are proposed, in which different tradeoffs between the performance and system overhead or complexity are considered. To evaluate the performance, we derive the closed-form expressions of the outage probability along with the diversity orders for all these RS schemes. The analytical results show that non-zero diversity orders can be harvested if the instantaneous CSI is employed when designing the user ordering or PA strategies. Furthermore, the provided simulation results demonstrate that better outage performance gain can be obtained if the collected CSI is used more fully when designing the RS schemes. Also, the improvement due to the use of dynamic PA strategies is more prominent in contrast to the situation where only user ordering strategies are enhanced, since lower outage probability can be always observed and there is no extra constraint for the PA coefficients.