Underlay cognitive radio networks with cooperative non-orthogonal multiple access

In this study, a cooperative non-orthogonal multiple access (NOMA) scheme is investigated in an underlay cognitive radio network. With this aim, a number of secondary users are concerned in the cooperative NOMA, in which a user with strong channel gains is properly selected (to act as a relay) by a multi-antenna base station (BS) for assisting another user with poor channel gains in the presence of a primary network. Closed-form expressions for the outage probability of the secondary network are derived, based on which the asymptotic outage behaviours of each secondary user assuming various realistic cases are discussed. Our analytical results clearly reveal that an outage floor may exist in the outage probability of the secondary users, being determined by the interference constraint and the number of antennas at the BS. The impact of the multiple antennas and the number of cooperative NOMA users on the system performance is examined, and it shows that the cell-edge user under poor channel gains can benefit from both the cooperative NOMA and opportunistic relay transmission.