Efficient user pairing for minimization of capacity loss and outage probability in downlink non-orthogonal multiple access systems

Non-orthogonal multiple access (NOMA) is a potential technology for the fifth-generation (5G) cellular systems. The main task of the user pairing (UP) in NOMA is the selection of the paired users according to their channel gains. In this paper, an efficient UP algorithm is proposed to enhance the performance metrics of NOMA systems such as capacity, fairness, and outage probability. Firstly, the impact of the paired users' channel gains on the performance of NOMA systems is investigated and discussed. The proposed UP is based on the minimization of the capacity loss to maximize the NOMA network capacity. Also, the proposed UP targets to reduce the divergence between paired users' channel gains to avoid capacity loss and the degradation of the user fairness and the user data rate. Performance evaluation results have revealed that the proposed UP significantly outperforms the existing UP algorithms in maximizing the capacity and improving the fairness and the outage probability. Also, it achieves very small values of the outage probability which range from 1% to 10% of the achieved values by the other UP algorithms.