Hybrid Beamforming for Millimeter Wave Massive Multiuser MIMO Systems Using Regularized Channel Diagonalization

This letter presents a novel hybrid analog-digital beamforming design for massive multiuser MIMO downlink transmission in sparse millimeter wave channels. The proposed hybrid regularized channel diagolnalization (HRCD) scheme combines analog (RF) phase-only beamforming with linear digital (baseband) precoding based on regularized channel diagonalization. Being a hybrid beamforming design utilizing simple non-iterative processing, HRCD provides a low complexity solution that is capable of outperforming several other hybrid schemes including those based on block diagonalization, as shown through simulations. Furthermore, it is also shown that the performance remains strong even with the use of finite-resolution phase shifters.

I. Introduction

Millimeter wave (mm-wave) communications and the massive MIMO technology hold the key to achieving the massive data rate requirements of the upcoming fifth generation (5G) mobile communication systems and beyond. The mm-wave spectrum offers the potential to fulfill the large bandwidth requirements of 5G services. However, transmission over the mm-wave channel poses some serious challenges as well such as heavy path losses and the sparse propagation characteristics, i.e., the availability of only a small number of channel paths between the base station (BS) and the user equipment (UE). Use of large-dimensional antenna arrays at the BS along with smaller UE arrays can provide the beamforming (BF) gain needed for acceptable signal-to-noise ratio (SNR) levels. Fortunately, the very short wavelengths of mm-waves make it a viable solution by allowing a large number of antennas to be packed within a tiny footprint [1].