Mutual-Coupling-Aware Optimization of a Time-Floquet RIS for Harmonic Backscatter Communications

TL;DR

This paper proposes an MC-aware optimization framework for a time-Floquet RIS in harmonic backscatter communications, demonstrating the importance of mutual coupling considerations for system performance.

Contribution

It introduces a novel MC-aware optimization approach for time-Floquet RIS, considering practical 1-bit elements and harmonic channel gains in backscatter systems.

Findings

The proposed algorithm improves harmonic channel gain performance.

Performance depends on time resolution and number of harmonics.

MC awareness enhances optimization effectiveness.

Abstract

This Letter studies the optimization of a wireless communications system empowered by a periodically time-modulated reconfigurable intelligent surface, coined time-Floquet RIS (TF-RIS), in the presence of mutual coupling (MC) among the RIS elements. In contrast to a conventional RIS whose elements may be reconfigured between signaling intervals, a TF-RIS periodically modulates its elements within a signaling interval, thereby inducing frequency conversion. Periodic time modulation is particularly attractive for harmonic backscatter communications to avoid self-jamming. Based on time-Floquet multiport network theory, we formulate an MC-aware optimization problem for binary-amplitude-shift-keying (BASK) harmonic backscatter communications with practical 1-bit-programmable TF-RIS elements. We propose a general discrete-optimization algorithm and evaluate its performance based on realistic model parameters. We systematically examine the performance dependence on the time resolution of the periodic modulation and the number of retained harmonics. Benchmarking against an MC-unaware approach reveals the importance of MC awareness for the more challenging optimization problem of simultaneous desired-harmonic-channel-gain maximization and undesired-harmonic-channel-gain minimization.