Beyond Diagonal Reconfigurable Intelligent Surfaces: From Transmitting and Reflecting Modes to Single-, Group-, and Fully-Connected Architectures

TL;DR

This paper introduces a comprehensive RIS model called BD-RIS that unifies various modes and architectures, and develops algorithms to optimize system performance, demonstrating significant improvements over traditional RIS configurations.

Contribution

The paper proposes a novel BD-RIS model that generalizes existing RIS architectures and modes, enabling joint optimization for enhanced wireless communication performance.

Findings

Fully- and group-connected RIS outperform single-connected RIS in sum-rate.

Hybrid RIS surpasses reflective/transmissive RIS with the same architecture.

BD-RIS effectively unifies different modes and architectures for better system design.

Abstract

Reconfigurable intelligent surfaces (RISs) are envisioned as a promising technology for future wireless communications. With various hardware realizations, RISs can work under different modes (reflective/transmissive/hybrid) or have different architectures (single/group/fully-connected). However, most existing research focused on single-connected reflective RISs, mathematically characterized by diagonal phase shift matrices, while there is a lack of a comprehensive study for RISs unifying different modes/architectures. In this paper, we solve this issue by analyzing and proposing a general RIS-aided communication model. Specifically, we establish an RIS model not limited to diagonal phase shift matrices, a novel branch referred to as beyond diagonal RIS (BD-RIS), unifying modes and architectures. With the proposed model, we develop efficient algorithms to jointly design transmit precoder and BDRIS matrix to maximize the sum-rate for RIS-aided systems. We also provide simulation results to compare the performance of BD-RISs with different modes/architectures. Simulation results show that under the same mode, fully- and group-connected RIS can effectively increase the sum-rate performance compared with single-connected RIS, and that hybrid RIS outperforms reflective/transmissive RIS with the same architecture.