Beamforming Design for Downlink Non-Orthogonal Multiple Access Systems

This paper investigates the beamforming design for multiple-input single-output nonorthogonal multiple access (NOMA) downlink systems. The NOMA beamforming design is formulated as a weighted sum rate maximization (WSRM) problem with decoding order constraints on the beamforming vectors and quality-of-service constraints for each user. We first investigate the feasibility of the NOMA WSRM problem and then propose an efficient method to achieve the optimized beamforming vectors. Furthermore, we consider the situation where user channels are homogeneous, which is particularly suitable for using NOMA. In this case, we show that the NOMA WSRM beamforming problem admits the favorable convexity and thus the optimal beamforming solution can be efficiently found. We further show that, with homogeneous channels, the optimal power allocation of the users can be analytically characterized.