Multi-RIS-Empowered Communication Systems: Capacity Analysis and Optimization

TL;DR

This paper derives asymptotic expressions for mutual information in multi-RIS communication systems, showing that statistical RIS optimization can significantly enhance performance even with modest system sizes and challenging channel conditions.

Contribution

It introduces novel asymptotic analysis methods for multi-RIS systems, enabling efficient metasurface optimization without brute-force methods.

Findings

Gaussian approximation for mutual information is accurate even for small systems.

RIS correlation benefits lead to significant capacity gains.

Metasurfaces can be optimized for robust communication without precise placement.

Abstract

In this chapter, using statistical physics methods, asymptotic closed-form expressions for the mean and variance of the mutual information for a multi-antenna transmitter-receiver pair in the presence of multiple Reconfigurable Intelligent Surfaces (RISs) are presented. While nominally valid in the large-system limit, it is shown that the derived Gaussian approximation for the mutual information can be quite accurate, even for modest-sized antenna arrays and metasurfaces. The above results are particularly useful when fast-fading conditions are present, which renders channel estimation challenging. The derived analysis indicates that, when the channel close to an RIS is correlated, for instance due to small angle spread which is reasonable for wireless systems with increasing carrier frequencies, the communication link benefits significantly from statistical RIS optimization, resulting in gains that are surprisingly higher than the nearly uncorrelated case. More importantly, the presented novel asymptotic properties of the correlation matrices of the impinging and outgoing signals at the RISs can be deployed to optimize the metasurfaces without brute-force numerical optimization. The numerical investigation demonstrates that, when the desired reflection from any of the RISs departs significantly from geometrical optics, the metasurfaces can be optimized to provide robust communication links, without significant need for their optimal placement.