Phase Selection and Analysis for Multi-frequency Multi-user RIS Systems Employing Subsurfaces

TL;DR

This paper introduces a novel subsurface RIS design for multi-user systems that optimizes performance in LoS conditions, reduces complexity, and extends to arbitrary channels, with analytical solutions for mean SNR and rate bounds.

Contribution

It develops a new subsurface RIS design based on single-user optimal design, applicable to arbitrary channels, with analytical performance metrics and robustness analysis.

Findings

Optimal subsurface design enhances LoS performance.

Design is robust to non-LoS conditions.

System significantly reduces complexity and channel estimation needs.

Abstract

In this paper, we analyse the performance of a reconfigurable intelligent surface (RIS) aided system where the RIS is divided into subsurfaces. Each subsurface is designed specifically for one user, who is served on their own frequency band. The other subsurfaces (those not designed for this user) provide additional uncontrolled scattering. A new subsurface RIS design is developed based on the optimal single-user design for a pure line-of-sight (LoS) base station (BS) to RIS channel. This is also extended to arbitrary BS-RIS channels. For our method, exact closed form solutions for the mean SNR and a mean rate upper bound are derived for the BS-RIS LoS scenario. For each user, the designed subsurface performs optimally in LoS conditions and is remarkably robust to non-LoS conditions. The system design drives down complexity to extremely low levels, reducing RIS design and receiver processing complexity and reducing the channel estimation requirements. We also quantify the complexity-performance trade-off for the new design relative to multi-user approaches.