Reconfigurable Intelligent Surfaces 2.0: Beyond Diagonal Phase Shift Matrices

Abstract

This work studies the wideband modeling and beamforming design of beyond diagonal reconfigurable intelligent surface (BD-RIS), which generalizes and goes beyond conventional RIS with diagonal phase shift matrices to achieve enhanced channel gain. Specifically, we investigate the response of BD-RIS in wideband systems by going back to its hardware circuit realizations. We propose a novel wideband model which has simple expressions while capturing the response variations of BD-RIS for signals with different frequencies. With this wideband model, we propose a BD-RIS design algorithm for an orthogonal frequency division multiplexing system to maximize the average rate over all subcarriers. Finally, we provide simulation results to evaluate the performance of the proposed design and show the importance of wideband modeling for BD-RIS.

Closed-Form Global Optimization of Beyond Diagonal Reconfigurable Intelligent Surfaces

“…More recently, beyond diagonal RIS (BD-RIS), i.e., the set of generalized RIS architectures with scattering matrices not restricted to being diagonal [3], has emerged to break through the limitation of diagonal scattering matrices, and shown notable gains in improving performance in wireless systems, such as rate splitting multiple access (RSMA) [4], dualfunction radar-communication (DFRC) [5], and mobile edge computing (MEC) systems [6]. In the framework of BD-RIS, the conventional RIS is categorized as single-connected RIS, and further extended to fully-connected RIS by connecting all the RIS elements through tunable impedance components [2].…”