Non-Reciprocal Beyond Diagonal RIS: Multiport Network Models and Performance Benefits in Full-Duplex Systems

TL;DR

The paper introduces a multiport-network framework to model BD-RIS in full-duplex systems and derives simplified, physically consistent channel models that explicitly incorporate structural scattering. It shows that non-reciprocal BD-RIS can outperform reciprocal designs by enabling asymmetric uplink/downlink beams and joint optimization across spatial directions, with formal conditions and a matrix-projection solution for Θ in the non-reciprocal case. Through case studies and extensive simulations, the work demonstrates how structural scattering can yield up to fourfold gains in LOS scenarios and highlights the critical role of accurate BD-RIS modeling in design. The findings suggest significant practical opportunities for non-reciprocal BD-RIS in future wireless networks, while outlining directions for STAR-RIS integration, sensing-communications tradeoffs, and hardware-aware optimization.

Abstract

Beyond diagonal reconfigurable intelligent surfaces (BD-RIS) is a new advance in RIS techniques that introduces reconfigurable inter-element connections to generate scattering matrices not limited to being diagonal. BD-RIS has been recently proposed and proven to have benefits in enhancing channel gain and enlarging coverage in wireless communications. Uniquely, BD-RIS enables reciprocal and non-reciprocal architectures characterized by symmetric and non-symmetric scattering matrices. However, the performance benefits and new use cases enabled by non-reciprocal BD-RIS for wireless systems remain unexplored. This work takes a first step toward closing this knowledge gap and studies the non-reciprocal BD-RIS in full-duplex systems and its performance benefits over reciprocal counterparts. We start by deriving a general RIS aided full-duplex system model using a multiport circuit theory, followed by a simplified channel model based on physically consistent assumptions. With the considered channel model, we investigate the effect of BD-RIS non-reciprocity and identify the theoretical conditions for reciprocal and non-reciprocal BD-RISs to simultaneously achieve the maximum received power of the signal of interest in the uplink and the downlink. Simulation results validate the theories and highlight the significant benefits offered by non-reciprocal BD-RIS in full-duplex systems. The significant gains are achieved because of the non-reciprocity principle which implies that if a wave hits the non-reciprocal BD-RIS from one direction, the surface behaves differently than if it hits from the opposite direction. This enables an uplink user and a downlink user at different locations to optimally communicate with the same full-duplex base station via a non-reciprocal BD-RIS, which would not be possible with reciprocal surfaces.

Figures (12)

• Figure 1: Diagram of RIS aided full-duplex systems.
• Figure 2: Illustrative examples of non-reciprocal architectures: (a) 2 elements interconnect with each other via an isolator or gyrator; (b) 3 elements interconnect with each other via a circulator.
• Figure 3: Illustration of channel blocks in (\ref{['eq:simplified_h2']}).
• Figure 4: Transmission model with one single-antenna full-duplex base station, one RIS, and two full-duplex users.
• Figure 5: The impinging and reflected beam patterns of reciprocal and non-reciprocal BD-RISs without structural scattering ($\phi_{BI} = \frac{\pi}{6}$, $\phi_{R_DI} = \frac{\pi}{2}$, $N_I=16$). Top: $\phi_{IT_U} = \frac{2\pi}{3}$; bottom: $\phi_{IT_U} = \frac{\pi}{2}$.