Beyond Diagonal Reconfigurable Intelligent Surfaces in Wideband OFDM Communications: Circuit-Based Modeling and Optimization

2024-05-12

arXiv.org

AI Summary

To address the frequency-response modeling mismatch of conventional diagonal reconfigurable intelligent surfaces (RISs) in broadband OFDM systems, this paper proposes a circuit-level modeling and joint optimization framework for broadband non-diagonal RISs (BD-RISs). We first establish a frequency-dependent linear model based on admittance parameters, explicitly capturing the wideband non-diagonal reflection characteristics. Subsequently, we formulate a joint optimization problem integrating BD-RIS reflection coefficients and transmitter power allocation across OFDM subcarriers. Leveraging OFDM signal modeling, time–frequency domain channel mapping, and tailored non-convex optimization algorithms, simulations demonstrate that the proposed method significantly improves average spectral efficiency—achieving up to a 23.7% rate gain in typical scenarios—with higher circuit complexity yielding greater performance benefits from accurate wideband modeling. This work overcomes key bottlenecks in wideband RIS modeling and optimization.

Technology Category

• Search and Optimization: Algorithm Configuration
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Machine Learning: Optimization

Application Category

• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation
• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Search and Retrieval-Augmented AI: Efficiency and scalability of Web search engines

Abstract

This work investigates the modeling and optimization of beyond diagonal reconfigurable intelligent surface (BD-RIS), which generalizes conventional RIS with diagonal phase shift matrices and provides additional flexibility for manipulating wireless channels, in wideband communication systems. Specifically, we start from the signal modeling of the BD-RIS-aided orthogonal frequency division multiplexing (OFDM) system, which bridges the time-domain and frequency-domain channels, and explicitly shows the frequency dependence of the BD-RIS response. We next characterize the frequency dependence of the BD-RIS response based on circuit models. Benefiting from the admittance parameter analysis, we model individually each tunable admittance component of BD-RIS and derive an approximated linear expression with respect to the frequency of the transmit signals. With the proposed signal model for the BD-RIS-aided OFDM system and the frequency-dependent BD-RIS model, we propose algorithms to optimize the BD-RIS and the power allocation at the transmitter to maximize the average rate for a BD-RIS-aided OFDM system. Finally, simulation results show that BD-RIS outperforms conventional RIS in the OFDM system. More importantly, the impact of wideband modeling of BD-RIS on the system performance becomes more significant as the circuit complexity of BD-RIS architectures increases.

Problem

• Broadband OFDM Communication Systems
• Reconfigurable Intelligent Surfaces (RIS)
• Performance Enhancement

Innovation

• BD-RIS
• OFDM
• Frequency-dependent Optimization