Joint Active and Passive Beamforming for RIS-aided MIMO Communications with Low-Resolution Phase Shifts

TL;DR

This paper proposes a novel joint active and passive beamforming method for RIS-assisted MIMO systems that effectively handles low-resolution phase shifts, improving achievable rates under hardware constraints.

Contribution

It introduces a new problem formulation that accounts for low-resolution phase shifts without explicit discrete constraints and develops an efficient iterative algorithm for large arrays.

Findings

Effective performance with low-resolution phase shifts

Significant rate improvements demonstrated in simulations

Algorithm scales well with large RIS panels

Abstract

Practical hardware limitations often impose a reduced number of available phase shifts at the elements of a reconfigurable intelligent surface (RIS). Most works often assume continuous phase-shits at the RIS elements for the transmit and passive beamforming optimization, which can lead to substantial performance loss. Therefore, to harvest the gains of RIS-assisted multi-stream multiple-input multiple-output (MIMO) communications under realistic phase shifts, this letter proposes a problem formulation for the maximization of the achievable rate over the transmit precoder and RIS elements, which avoids an explicit discrete constraint while still incorporating its effect. To efficiently tackle the resulting problem when considering large arrays and RIS panels, an iterative algorithm is derived which comprises a sequence of simple projections. Simulation results demonstrate that the proposed design can be very effective, especially with low-resolution phase-shifts.