  In this paper we explore non-orthogonal multiple access (NOMA) in
millimeter-wave (mmWave) communications (mmWave-NOMA). In particular, we
consider a typical problem, i.e., maximization of the sum rate of a 2-user
mmWave-NOMA system. In this problem, we need to find the beamforming vector to
steer towards the two users simultaneously subject to an analog beamforming
structure, while allocating appropriate power to them. As the problem is
non-convex and may not be converted to a convex problem with simple
manipulations, we propose a suboptimal solution to this problem. The basic idea
is to decompose the original joint beamforming and power allocation problem
into two sub-problems which are relatively easy to solve: one is a power and
beam gain allocation problem, and the other is a beamforming problem under a
constant-modulus constraint. Extension of the proposed solution from 2-user
mmWave-NOMA to more-user mmWave-NOMA is also discussed. Extensive performance
evaluations are conducted to verify the rational of the proposed solution, and
the results also show that the proposed sub-optimal solution achieve
close-to-bound sum-rate performance, which is significantly better than that of
time-division multiple access (TDMA).
