Beyond Diagonal RIS: Passive Maximum Ratio Transmission and Interference Nulling Enabler

TL;DR

This paper explores the use of beyond diagonal reconfigurable intelligent surfaces (BD-RIS) for passive multiuser beamforming, enhancing interference nulling and sum rate in MU-MISO systems with fewer elements and lower complexity.

Contribution

It introduces a novel BD-RIS design for passive beamforming that improves interference nulling and sum rate, reducing complexity compared to traditional methods.

Findings

BD-RIS outperforms D-RIS with fewer reflecting elements.

Passive interference nulling reduces BS complexity at high SNRs.

Passive MRT with ZF precoding achieves near-optimal sum rate.

Abstract

Beyond diagonal reconfigurable intelligent surfaces (BD-RIS) generalizes and goes beyond conventional diagonal reconfigurable intelligent surfaces (D-RIS) by interconnecting elements to generate beyond diagonal scattering matrices, which significantly strengthen the wireless channels. In this work, we use BD-RIS for passive multiuser beamforming in multiuser multiple-input-single-output (MU-MISO) systems. Specifically, we design the scattering matrix of BD-RIS to either maximize the sum received signal power at the users following maximum ratio transmission (MRT), or to nullify the interference at the users following zero forcing (ZF). Furthermore, we investigate uniform/optimized power allocation and ZF precoding at the base station (BS). Numerical results show that BD-RIS improves the interference nulling capability and sum rate with fewer reflecting elements (REs) compared to D-RIS. In addition, at moderate to high signal to noise ratios (SNRs), passive interference nulling reduces the complexity at the BS by relaxing the need for precoding or water-filling power allocation design. Furthermore, the passive MRT with ZF precoding achieves a tight sum rate performance to the joint design considering MU-MISO scenarios with many REs while maintaining low computational complexity and simplifying the channel estimation.