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. Systemlevel simulation results are provided to assess and compare the performance gains of the proposed algorithm in terms of important key system performance indicators.