In this paper we explore non-orthogonal multiple access (NOMA) inmillimeter-wave (mmWave) communications (mmWave-NOMA). In particular, weconsider a typical problem, i.e., maximization of the sum rate of a 2-usermmWave-NOMA system. In this problem, we need to find the beamforming vector tosteer towards the two users simultaneously subject to an analog beamformingstructure, while allocating appropriate power to them. As the problem isnon-convex and may not be converted to a convex problem with simplemanipulations, we propose a suboptimal solution to this problem. The basic ideais to decompose the original joint beamforming and power allocation probleminto two sub-problems which are relatively easy to solve: one is a power andbeam gain allocation problem, and the other is a beamforming problem under aconstant-modulus constraint. Extension of the proposed solution from 2-usermmWave-NOMA to more-user mmWave-NOMA is also discussed. Extensive performanceevaluations are conducted to verify the rational of the proposed solution, andthe results also show that the proposed sub-optimal solution achieveclose-to-bound sum-rate performance, which is significantly better than that oftime-division multiple access (TDMA).
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