Transmitter Side Beyond-Diagonal Reconfigurable Intelligent Surface for Massive MIMO Networks

This letter focuses on a transmitter or base station (BS) side beyond-diagonal reflecting intelligent surface (BD-RIS) deployment strategy to enhance the spectral efficiency (SE) of a time-division-duplex massive multiple-input multiple-output (MaMIMO) network. In this strategy, the active antenna array utilizes a BD-RIS at the BS to serve multiple users in the downlink. Based on the knowledge of statistical channel state information (CSI), the BD-RIS coefficients matrix is optimized by employing a novel manifold algorithm, and the power control coefficients are then optimized with the objective of maximizing the minimum SE. Through numerical results we illustrate the SE performance of the proposed transmission framework and compare it with that of a conventional MaMIMO transmission for different network settings.

I. Introduction

The impeccable merits of massive multiple-input multiple-output (MaMIMO) have effectuated it as the backbone of modern-day wireless networks [1], [2]. The ability of MaMIMO to serve a large number of users in the same time-frequency resource block by aggressively multiplexing them in the spatial domain has led to unprecedented levels of network coverage and spectral efficiency (SE) [1]. This aggressive spatial multiplexing is enabled by leveraging joint coherent transmission/reception and a large number of fully digital radio frequency (RF) chains at the base station (BS). However, as the number of RF chains increases inordinately the total energy consumption increases linearly with it. But, the data rates only grow logarithmically at maximum [2]. Such a situation is expensive and leads to energy inefficiency in the network. Therefore, there is a perpetual desire to find solutions that enhance the SE performance of a MaMIMO network for a small or moderate number of RF chains.