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Beyond-Diagonal RIS Prototype and Performance Evaluation

Jean Tapie, Matteo Nerini, Bruno Clerckx, Philipp del Hougne

TL;DR

This work reports the first experimental prototype of a reflective BD-RIS with tunable inter-element connections and validates its performance in a rich-scattering environment using a Virtual VNA-based multi-port measurement framework. By modeling the end-to-end channel as $H = S_{RT} + S_{RS}(S_L^{-1} - S_{SS})^{-1} S_{ST}$ and benchmarking against open-circuit and default RIS configurations, the study demonstrates that reconfigurable coupling between RIS elements yields meaningful KPI gains. Hardware constraints, such as tri-diagonal connectivity and limited load programmability, strongly influence achievable improvements and the relative advantage of BD-RIS over conventional RIS, while mutual-coupling awareness during optimization shows variable importance depending on constraint looseness. The results underscore the practicality of BD-RIS concepts and highlight key hardware design trade-offs for future research and deployment of reconfigurable inter-element connectivity in wireless channels.

Abstract

We present the first experimental prototype of a reflective beyond-diagonal reconfigurable intelligent surface (BD-RIS), i.e., a RIS with reconfigurable inter-element connections. Our BD-RIS consists of an antenna array whose ports are terminated by a tunable load network. The latter can terminate each antenna port with three distinct individual loads or connect it to an adjacent antenna port. Extensive performance evaluations in a rich-scattering environment validate that inter-element connections are beneficial. Moreover, we observe that our tunable load network's mentioned hardware constraints significantly influence, first, the achievable performance, second, the benefits of having inter-element connections, and, third, the importance of mutual-coupling awareness during optimization.

Beyond-Diagonal RIS Prototype and Performance Evaluation

TL;DR

This work reports the first experimental prototype of a reflective BD-RIS with tunable inter-element connections and validates its performance in a rich-scattering environment using a Virtual VNA-based multi-port measurement framework. By modeling the end-to-end channel as and benchmarking against open-circuit and default RIS configurations, the study demonstrates that reconfigurable coupling between RIS elements yields meaningful KPI gains. Hardware constraints, such as tri-diagonal connectivity and limited load programmability, strongly influence achievable improvements and the relative advantage of BD-RIS over conventional RIS, while mutual-coupling awareness during optimization shows variable importance depending on constraint looseness. The results underscore the practicality of BD-RIS concepts and highlight key hardware design trade-offs for future research and deployment of reconfigurable inter-element connectivity in wireless channels.

Abstract

We present the first experimental prototype of a reflective beyond-diagonal reconfigurable intelligent surface (BD-RIS), i.e., a RIS with reconfigurable inter-element connections. Our BD-RIS consists of an antenna array whose ports are terminated by a tunable load network. The latter can terminate each antenna port with three distinct individual loads or connect it to an adjacent antenna port. Extensive performance evaluations in a rich-scattering environment validate that inter-element connections are beneficial. Moreover, we observe that our tunable load network's mentioned hardware constraints significantly influence, first, the achievable performance, second, the benefits of having inter-element connections, and, third, the importance of mutual-coupling awareness during optimization.
Paper Structure (9 sections, 2 equations, 3 figures)

This paper contains 9 sections, 2 equations, 3 figures.

Table of Contents

  1. Introduction
  2. System Model
  3. Experimental Setup and Procedure
  4. Performance Evaluation
  5. Key performance indicators
  6. Benchmarking
  7. Optimization
  8. Results
  9. Conclusion

Figures (3)

  • Figure 1: Experimental BD-RIS-parametrized rich-scattering radio environment. Bottom: Schematic of the BD-RIS and illustration of the $\mathbf{S}_\mathrm{L}$ realization corresponding to the switch setting in the schematic.
  • Figure 2: Measured scattering characteristics of the tunable load network (left and middle column) and the rich-scattering radio environment (right column, showing three selected wireless channels from the blocks $\mathcal{RT}$, $\mathcal{ST}$ and $\mathcal{SS}$).
  • Figure 3: Performance evaluation regarding the KPIs defined in Sec. \ref{['subsec_KPI']} for our BD-RIS prototype and the benchmarks defined in Sec. \ref{['subsec_benchmarks']}. Each displayed point is the average over the KPIs achieved for 201 frequency points and all possible choices of TX and RX among the available ones (see Sec. \ref{['subsec_optimization']}).