  In this paper, we study the energy efficiency (EE) maximization problem for
an uplink millimeter wave massive multiple-input multiple-output system with
non-orthogonal multiple access (NOMA). Multiple two-user clusters are formed
according to their channel correlation and gain difference, and NOMA is applied
within each cluster. Then, a hybrid analog-digital beamforming scheme is
designed to lower the number of radio frequency chains at the base station
(BS). On this basis, we formulate a power allocation (PA) problem to maximize
the EE under users' quality of service requirements. An iterative algorithm is
proposed to obtain the PA. Moreover, an enhanced NOMA scheme is also proposed,
by exploiting the global information at the BS. Numerical results show that the
proposed NOMA schemes achieve superior EE when compared with the conventional
orthogonal multiple access scheme.
