  This paper investigates the application of non-orthogonal multiple access in
millimeter-Wave communications (mmWave-NOMA). Particularly, we consider
downlink transmission with a hybrid beamforming structure. A user grouping
algorithm is first proposed according to the channel correlations of the users.
Whereafter, a joint hybrid beamforming and power allocation problem is
formulated to maximize the achievable sum rate, subject to a minimum rate
constraint for each user. To solve this non-convex problem with
high-dimensional variables, we first obtain the solution of power allocation
under arbitrary fixed hybrid beamforming, which is divided into intra-group
power allocation and inter-group power allocation. Then, given arbitrary fixed
analog beamforming, we utilize the approximate zero-forcing method to design
the digital beamforming to minimize the inter-group interference. Finally, the
analog beamforming problem with the constant-modulus constraint is solved with
a proposed boundary-compressed particle swarm optimization algorithm.
Simulation results show that the proposed joint approach, including user
grouping, hybrid beamforming and power allocation, outperforms the
state-of-the-art schemes and the conventional mmWave orthogonal multiple access
system in terms of achievable sum rate and energy efficiency.
