Non-Orthogonal Multiple Access Based on Hybrid Beamforming for mmWave Systems

This paper aims to study the utilization of non-orthogonal multiple access (NOMA) in hybrid beamforming (HB) multiuser systems, called HB-NOMA, to serve a large number of mobile users. First, a sum-rate expression for the HB-NOMA problem is formulated. Second, an effective algorithm is proposed to maximize the sum-rate. Then, a lower bound is derived under two cases: i) the angle between the effective channel vectors of the MU with the highest channel gain and other MUs located inside a cluster is zero (or close to it) which is denoted by perfect correlation, and ii) this angle is non-zero, which we denote by imperfect correlation. For the second case, the relationship between the effective channels of two HB-NOMA users is modeled. The lower bound for the first case indicates that only the presence of inter-cluster interference and the use of an analog precoder negatively impact the sum-rate. However, in the case of imperfect correlation, which is more realistic, an inefficient MU cluster, can cause severe intra-cluster interference in the network. To verify our findings, numerical simulations have been conducted.