Beyond Max-SNR: Joint Encoding for Reconfigurable Intelligent Surfaces

TL;DR

This paper explores joint encoding strategies for RIS-assisted communication, demonstrating that adaptive RIS configurations encoding information outperform traditional max-SNR schemes, with practical layered encoding enabling effective decoding.

Contribution

It introduces an information-theoretic framework for joint encoding in RIS systems and proposes a layered signaling strategy that surpasses fixed RIS configurations.

Findings

Joint encoding in RIS systems increases capacity.

Layered encoding enables practical successive decoding.

Adaptive RIS configurations outperform max-SNR schemes.

Abstract

A communication link aided by a Reconfigurable Intelligent Surface (RIS) is studied, in which the transmitter can control the state of the RIS via a finite-rate control link. Prior work mostly assumed a fixed RIS configuration irrespective of the transmitted information. In contrast, this work derives information-theoretic limits, and demonstrates that the capacity is achieved by a scheme that jointly encodes information in the transmitted signal as well as in the RIS configuration. In addition, a novel signaling strategy based on layered encoding is proposed that enables practical successive cancellation-type decoding at the receiver. Numerical experiments demonstrate that the standard max-SNR scheme that fixes the configuration of the RIS as to maximize the Signal-to-Noise Ratio (SNR) at the receiver is strictly suboptimal, and is outperformed by the proposed strategies at all practical SNR levels.