Performance analysis of cooperative cognitive radio networks based on hybrid NOMA/OMA and best relay selection

In non-orthogonal multiple access (NOMA) systems, it may not be advisable to send the superimposed signal of all users to individual users with retransmission requirements during the retransmission phase. This paper investigates a novel cognitive radio network based on hybrid NOMA/orthogonal multiple access (CR-H-NOMA/OMA) in underlay spectrum sharing, whereby a hybrid NOMA/OMA cooperative mechanism is proposed for the secondary network by considering that the access modes of secondary users (SUs) and relays can be switched between NOMA and OMA. In order to compensate the loss of throughput caused by successive interference cancellation (SIC) during the cooperative phase, one or two relays with the best channel quality between themselves and the destination users are selected according to the states of the initial signal decoded by SUs, using truncated automatic repeat request (T-ARQ) to retransmit only the desired signal of SU instead of the entire superimposed signal. Exact closed-form expressions for outage probabilities of SUs and system throughput of the secondary network are derived under the interference constraint of the primary network to evaluate the performance of the proposed cooperative scheme. Numerical and simulation results demonstrate the superiority of the proposed hybrid NOMA/OMA cooperative scheme when compared with conventional pure NOMA and pure OMA based on time division multiple access (TDMA) in terms of secondary throughput. Also, the outage and throughput performance depend critically on the choices of key paraments such as power allocation factor and target rate.