Optical True Time Delay-Based Hybrid Beamforming for Millimeter-Wave NOMA-MIMO Systems

A low-cost millimeter-wave NOMA-MIMO system by exploiting optical true time delay-based hybrid beamforming is proposed, where only one weight control unit is required. The obtained results show the proposed scheme achieves high spectral efficiency.

1. Introduction

Millimeter-wave (mmWave) communications considerably increase the data rate in wireless systems, while they suffer from high path losses [1]. To this end, mmWave communications must exploit the multiple-input multiple-output (MIMO) technology [2]. Due to the spatial sparsity of mmWave channels, hybrid beamforming (HBF) can provide near-optimal performance by using a few radio frequency (RF) chains. Specifically, the sub-connected HBF(SC-HBF) is more energy-efficient compared to the fully connected HBF [3]. Moreover, nonorthogonal multiple access (NOMA), a key technology for 5G and beyond 5G networks, has advantages in spectrum efficiency and massive connectivity [4]. In NOMA-MIMO systems, multiple clusters beamforming is crucial to reduce inter-beam interference and improve performance because multiple users are served in the same resource block simultaneously [5].