Proportional fairness-based power allocation and user set selection for downlink NOMA systems

The non-orthogonal multiple access (NOMA) has been investigated recently as a candidate radio access technology for the next generation mobile networks. It realizes powerdomain user multiplexing by employing the successive interference cancellation (SIC). This paper focuses on the power allocation and user set selection problems in multi-user downlink NOMA systems with the aim of providing proportional fairness. The theoretical optimal power allocation solution for a candidate user set is derived in closed form. We propose a tree searching-based user set selection scheme in which the redundant user sets are removed by a delicately designed pruning method. The transmission performance of the proposed power allocation and user set selection schemes is evaluated by system-level simulations and verified to be close to the optimal value. The searching complexity for user set selection is significantly reduced compared to the exhaustive searching method. Finally, the performance gains brought by the multi-user NOMA are analyzed considering the practical application.