Tight Bounds on the Optimal UL Sum-Rate of MU RIS-aided Wireless Systems

TL;DR

This paper introduces a novel channel separation technique to bound and optimize the uplink sum-rate in multi-user RIS-aided wireless systems, demonstrating tight bounds and robustness to phase quantization.

Contribution

It presents a new channel separation method for bounding sum-rate, along with a low-complexity optimization algorithm and analysis of quantization effects.

Findings

Derived tight upper and lower bounds on sum-rate.

Proposed an efficient alternating optimization algorithm.

Showed robustness of sum-rate to RIS phase quantization.

Abstract

The objective of this paper is to develop simple techniques to bound the optimal uplink sum-rate of multi-user RIS-aided wireless systems. Specifically, we develop a novel technique called \textit{channel separation} which provides a new understanding as to how the RIS phases affect the sum-rate. Leveraging channel separation, we derive upper and lower bounds on the optimal sum-rate. In addition, we propose a low-complexity alternating optimization algorithm to obtain near-optimal sum-rate results. Numerical results demonstrate the tightness of the bounds and show that the alternating optimization approach delivers sum-rate values similar to the results of a full numerical optimization procedure. Furthermore, in practical scenarios where hardware limitations cause the RIS phases to be quantized, our lower bound can still be applied and shows that the sum-rate is robust to quantization, even with low resolution.