Power Allocation for a Downlink Non-Orthogonal Multiple Access System

This letter investigates power allocation for a downlink non-orthogonal multiple access system with a base station and two users. We first formulate an optimization problem for the single-input single-output (SISO) case in terms of power allocation between two users for maximizing the sum capacity under a total power constraint and a quality-of-service condition that the minimum rate requirement of each user is satisfied. Two closed-form solutions are subsequently derived. Then we extend the SISO solutions to a multiple-input multiple-output scenario. As compared to the optimal and the suboptimal approach presented in a recent work, the proposed schemes achieve close sum capacity performance with much lower computational complexity.