  In this letter, we investigate the energy efficiency (EE) problem in a
millimeter wave (mmWave) massive MIMO (mMIMO) system with non-orthogonal
multiple access (NOMA). Multiple two-user clusters are formulated according to
their channel correlation and gain difference. Following this, we propose a
hybrid analog/digital precoding scheme for the low radio frequency (RF) chains
structure at the base station (BS). On this basis, we formulate a power
allocation problem aiming to maximize the EE under users' quality of service
(QoS) requirements and per-cluster power constraint. An iterative algorithm is
proposed to obtain the optimal power allocation. Simulation results show that
the proposed NOMA achieves superior EE performance than that of conventional
OMA.
