Physics-Compliant Modeling and Scaling Laws of Multi-RIS Aided MIMO Systems

TL;DR

This paper develops a physics-compliant channel model for multi-RIS aided MIMO systems using multiport network theory, revealing significant discrepancies from traditional models and emphasizing the need for more accurate modeling in wireless communications.

Contribution

The study introduces a novel physics-compliant channel model for multi-RIS systems that accounts for structural scattering, impedance mismatch, and mutual coupling, improving accuracy over existing models.

Findings

Significant difference between physics-compliant and traditional models increases with RIS number.

Traditional models can overestimate channel gain by up to 93%.

More accurate models are essential for reliable RIS-assisted system design.

Abstract

Reconfigurable intelligent surface (RIS) enables the control of wireless channels to improve coverage. To further extend coverage, multi-RIS aided systems have been explored, where multiple RISs steer the signal via a multi-hop path. However, deriving a physics-compliant channel model for multi-RIS aided systems is still an open problem. In this study, we fill this gap by modeling multi-RIS aided systems through multiport network theory, and deriving a channel model accounting for impedance mismatch, mutual coupling, and structural scattering. The derived physics-compliant model differs from the model widely used in literature, which omits the RIS structural scattering. To quantify this difference, we derive the channel gain scaling laws of the two models under line-of-sight (LoS) and multipath channels. Theoretical insights, validated by numerical results, show an important discrepancy between the physics-compliant and the widely used models, increasing with the number of RISs and multipath richness. In a multi-hop system aided by four 128-element RISs with multipath channels, optimizing the RISs using the widely used model and applying their solutions to the physics-compliant model achieves only 7% of the maximum channel gain. This highlights how severely mismatched channel models can be, calling for more accurate models in communication theory.