MIMO Hybrid Beamforming: Performance Assessment in Macrocells and HetNets

Introduction

Recently, millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) systems have emerged as a promising solution to enhance the network capacity and coverage of the new generation cellular networks (Marzetta, October 2010; Rusek, 2013; Hoydis, 2013; Busari, 2018). On the one hand, the mmWave can provide a considerable bandwidth; on the other hand, the significant gain of the massive arrays can compensate for the attenuation of the mmWave channel. Traditional MIMO-beamforming systems require a dedicated radio frequency (RF) chain for each antenna element to achieve optimal beamforming performance. However, in mmWave massive MIMO, the required number of radio frequency (RF) chains becomes impractical due to the expensive and power-hungry components such as variable gain power amplifiers, filters, mixers, and analog-to-digital/digital-to-analog converters (ADCs/DACs). A promising solution to this problem is reducing the number of radiofrequency (RF) chains by partitioning beamforming operations between the digital and RF domains, known as hybrid beamforming (HBF), while still achieving the near-optimal performance of the fully-digital beamforming systems with much-reduced hardware complexity (Sohrabi, F., Yu, W., 2016; El Ayach, O., 2014; Alkhateeb, A., 2014, 2015; Liang, L.,2014; Ni, W., 2017; Hefnawi, M., 2019; Kebede, T., 2022). In HBF, the RF analog beamformer is typically limited to applying phase shifters only to each array element, while digital beamforming with complex weighting vectors can be applied on each RF chain. Figure 1 shows a general hybrid configuration that connects N_a antenna elements to N_d RF chains, where N_d<N_a, using an analog RF beamforming matrix built from only phase shifters.

Figure 1.

Hybrid beamforming

Figure 2.

Architectures of analog beamformers: (a) Fully-connected; (b) partially-connected

Two widely-used analog beamformer architectures for hybrid beamforming are shown in Figure 2. The fully-connected hybrid beamforming structure of Figure 2(a) provides a high beamforming gain per transceiver—but with high complexity—by connecting each RF chain to all antennas through a network of N_d×N_a phase shifters. Figure 2(b), on the other hand, shows a partially-connected structure where each RF chain is connected to N_a/N_d number of sub-arrays. Such a structure has a lower hardware complexity at the price of reduced beamforming gain.

Hybrid beamforming design procedures differ from traditional MIMO beamforming systems and depend on the joint design of analog and digital beamformers. Typically, the joint design is achieved in two steps. The first step is selecting the best beams with the maximum SNR for a specific channel. Beam selection can be achieved by a codebook-based approach and implemented using the Butler matrix that produces beams in specified directions depending upon which ports are activated. Then in the next stage, digital beamformers such as maximum signal-to-interference plus noise ratio (SINR), Zero-Forcing (ZF), and minimum mean square error (MMSE) can optimize their weights based on the effective channels consisting of the cascade of the analog beamforming weights and the actual channel.

This Chapter reviews different HBF techniques for 5G and beyond systems. The basic HBF structures and their algorithm design are presented in the context of a point-to-point MIMO hybrid beamforming system. Then, the recently proposed HBF techniques for 5G heterogeneous networks (HetNets) and beyond is introduced. The access links, backhaul links, and relay-assisted links, where users and base stations are equipped with HBF-based massive arrays, are investigated. The benefit of HBF in the context of MIMO radar is also discussed in the case of covariance-based beamforming techniques.