  This paper aims to study the utilization of non-orthogonal multiple access
(NOMA) in hybrid beamforming (HB) multi-user systems called HB-NOMA to serve a
large number of mobile users (MUs). First, a sum-rate expression for the
HB-NOMA problem is formulated. Second, a suboptimal algorithm is proposed to
maximize the sum-rate. Then, a lower bound for the achievable rate is derived
under the condition that the angle between the effective channel vectors of the
MU with the highest channel gain and other MUs located inside a cluster is
non-zero, which we denote by imperfect correlation. The lower bound indicates
that an inefficient MU cluster, can cause severe inter-cluster interference in
the network. To verify our findings, numerical simulations have been conducted.
