Performance Analysis of Hybrid Beamforming Precoders for Multiuser Millimeter Wave NOMA Systems

This paper proposes a low-complexity and energy efficient sub-connected structure (SCS) hybrid beamforming (HBF) and fully-connected structure (FCS) HBF for non-orthogonal multiple access (NOMA) systems operating under line of sight (LOS) and non-line of sight (NLOS) millimeter wave (mmW) environments. The mathematical model to maximize sum-rate and energy efficiency (EE) of these two HBF-NOMA configurations is formulated to corroborate our simulation results. We aim to maximize both the sum-rates and EEs of the SCS-HBF-NOMA and FCS-HBF-NOMA systems via successive interference cancellation-zero forcing (SIC-ZF) and phased-zero forcing (P-ZF) schemes, respectively. Associated performance analysis under New York University (NYU) mmW channel model is investigated for both two and four users per cluster. It is found that our proposed SCS-HBF-NOMA generally yields higher EE than the FCS-HBF-NOMA in both LOS and NLOS links having high signal to noise ratio (SNR) regime. Moreover, the proposed multistream HBF-NOMA leverages spatial multiplexing to attain higher sum-rates than the single stream counterpart. The employment of $\text{3}\,\text{bit}$ quantization is capable of optimizing the sum-rate for energy efficient finite resolution HBF-NOMA systems. Results also manifest that SCS-HBF structure with reduced antennas in its sub-array beamforms a wider beam capable of mitigating against the performance degradation on the account of channel imperfection in NOMA system. Furthermore, it is found that the number of users per cluster should be carefully selected for optimal performance of HBF-NOMA system. Lastly, space complexity of the HBF-NOMA scheme is found to be the lowest in P-ZF FCS-HBF-NOMA.