Optimal Power Allocation and User Selection in Non-Orthogonal Multiple Access Systems

In this paper, we focus on the design of an optimal power allocation algorithm and an efficient user selection scheme for the downlink of a non-orthogonal multiple access (NOMA) system with power-domain user multiplexing at the transmitter side and successive interference cancellation (SIC) on the receiver side. We have derived an optimal power allocation solution in closed-form with the objective of maximizing the weighted sum rate subject to a maximum power constraint. The optimization problem is solved using the Karush-Kuhn-Tucker (KKT) conditions. A greedy user selection algorithm is used to schedule the best set of multiplexed users satisfying the criterion of maximizing sum proportional fairness (PF) metric. System-level simulation results are provided to assess and compare the performance gains of the proposed algorithm in terms of important key system performance indicators.