Efficient selection of user sets for downlink non-orthogonal multiple access

Non-orthogonal multiple access (NOMA), which uses superposition coding at the transmitter side and successive interference cancellation at the receiver side, is a promising new downlink multiple access scheme. The product of user average throughputs among all users within a cell can be maximized by optimal proportional fair (PF) scheduling, but this is computationally complex because it requires an exhaustive search for all possible user sets and optimal power allocation among users. In this work, we propose a novel method of selecting user sets for PF scheduling in downlink NOMA. Unlike conventional suboptimal methods, the proposed method is based on the mathematical characteristics of the PF metric. The proposed method can reduce the complexity when the maximum number of multiplexed users is three and four as well as two by judging whether a user set is worth multiplexing or not on the basis of a simple condition between the weakest two users within the set. Computer simulation showed that the proposed method can significantly reduce the average number of searched user sets while achieving almost the same total and cell-edge throughput as the optimal method.