User Pairing and Power Allocation in Underlay Cognitive NOMA Networks

Non-Orthogonal Multiple Access has attracted extensive research attention as a candidate multiple access technique for fifth-generation mobile networks. In a multichannel NOMA system with a large number of users, the literature has shown that the best performance is achieved if no more than three users share a channel. Most research typically allocates two users to a channel. One of the key issues then is user pairing, which must be done in order to maximize the network capacity. A number of user schemes have been derived in the literature, but these have been done mostly for noncognitive radio networks. In this paper, we determine near-optimal user pairing schemes in an underlay cognitive radio non-orthogonal multiple access network. We propose an underlay random pairing algorithm and an underlay channel state sorting pairing algorithm, for use in a large-scale network. Then we employ an optimal power allocation method to maximize the system throughput. The algorithms are then compared in terms of throughput and performance.