Optimal Grouping Strategy for Reconfigurable Intelligent Surface Assisted Wireless Communications

TL;DR

This paper provides an analytical framework for optimizing the grouping of RIS elements in wireless systems, showing how to maximize achievable rate and improve performance, especially in high mobility or high power scenarios.

Contribution

It derives a closed-form upper bound for the achievable rate and identifies the optimal grouping strategy for RIS elements based on channel conditions.

Findings

Optimal grouping depends on channel coherence time and transmit power.

Coarser grouping is beneficial in high mobility or high power scenarios.

Optimal grouping outperforms simple 'On-Off' schemes.

Abstract

The channel estimation overhead of reconfigurable intelligent surface (RIS) assisted communication systems can be prohibitive. Prior works have demonstrated via simulations that grouping neighbouring RIS elements can help to reduce the pilot overhead and improve achievable rate. In this paper, we present an analytical study of RIS element grouping. We derive a tight closed-form upper bound for the achievable rate and then maximize it with respect to the group size. Our analysis reveals that more coarse-grained grouping is important-when the channel coherence time is low (high mobility scenarios) or the transmit power is large. We also demonstrate that optimal grouping can yield significant performance improvements over simple `On-Off' RIS element switching schemes that have been recently considered.