A Family of Hybrid Beyond-Diagonal RIS Architectures: Design and Performance Analysis

TL;DR

This paper introduces a family of hybrid beyond-diagonal reconfigurable intelligent surface architectures that combine active and passive elements to enhance signal-to-noise ratio efficiently.

Contribution

It proposes a novel hybrid BD-RIS design framework that integrates active and passive elements, deriving a closed-form SNR-maximizing solution and analyzing three specific architectures.

Findings

Hybrid BD-RIS architectures achieve equal or higher SNR than diagonal RISs.

Fewer reflect-type amplifiers are needed for the same or better performance.

Numerical results validate the effectiveness of the proposed architectures.

Abstract

Beyond-diagonal reconfigurable intelligent surfaces (BD-RISs) extend conventional diagonal RISs by allowing inter-element coupling, thereby enlarging the set of attainable scattering matrices and improving the achievable signal-to-noise ratio (SNR). On the other hand, hybrid active/passive RISs use reflect-type power amplifiers in a fraction of the elements to alleviate the multiplicative path loss. In this paper, we bring these two ideas together and introduce a \emph{family of hybrid BD-RIS architectures}, in which the surface is partitioned into two reflecting subsurfaces (RSs), each adopting either a passive or an active group-connected BD-RIS design. We derive a closed-form SNR-maximizing solution that combines, for every BD-RIS group, Takagi's factorization of a certain complex symmetric matrix with an optimal per-group amplification factor that satisfies the reflect-power budget. Three architectures within the proposed family (active/passive, fully-connected-active/sub-connected-active, and sub-connected-active/sub-connected-active hybrid BD-RIS) are studied. Numerical results in a single-input single-output (SISO) link with blocked direct path show that the proposed hybrid BD-RIS architectures attain the same or higher receive SNR than their diagonal counterparts while using significantly fewer reflect-type amplifiers.