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Movable Signals with Dual-Polarized Fixed Intelligent Surfaces: Beyond Diagonal Reflection Matrices

Matteo Nerini, Bruno Clerckx

TL;DR

The paper analyzes dual-polarized intelligent surfaces and compares reconfigurable RIS with movable signals using fixed surfaces (FIS). By deriving closed-form power expressions and averaging over channel realizations, it shows movable signals with FIS outperform RIS by at least fourfold, and that beyond-diagonal FIS further enhances performance in opposite-polarization scenarios. The results provide explicit scaling laws and gains for diagonal and beyond-diagonal configurations across same- and opposite-polarization cases, highlighting practical, hardware-light benefits of frequency reconfiguration. The work offers a foundation for polarization-aware surface designs and frequency-domain optimization in future wireless systems.

Abstract

This paper investigates wireless systems aided by dual-polarized intelligent surfaces. We compare reconfigurable intelligent surface (RIS), which adjust their reflection matrices, with movable signals operating with fixed intelligent surface (FIS), which adjust the signal frequency while the surface properties remain fixed. For both RIS and FIS, we consider surfaces with a diagonal reflection matrix, named diagonal RIS/FIS, and surfaces with a reflection matrix not limited to being diagonal, named beyond-diagonal RIS/FIS. Movable signals with FIS always outperform RIS, achieving at least a fourfold gain. When transmitter and receiver polarizations differ, beyond-diagonal FIS further enhances performance.

Movable Signals with Dual-Polarized Fixed Intelligent Surfaces: Beyond Diagonal Reflection Matrices

TL;DR

The paper analyzes dual-polarized intelligent surfaces and compares reconfigurable RIS with movable signals using fixed surfaces (FIS). By deriving closed-form power expressions and averaging over channel realizations, it shows movable signals with FIS outperform RIS by at least fourfold, and that beyond-diagonal FIS further enhances performance in opposite-polarization scenarios. The results provide explicit scaling laws and gains for diagonal and beyond-diagonal configurations across same- and opposite-polarization cases, highlighting practical, hardware-light benefits of frequency reconfiguration. The work offers a foundation for polarization-aware surface designs and frequency-domain optimization in future wireless systems.

Abstract

This paper investigates wireless systems aided by dual-polarized intelligent surfaces. We compare reconfigurable intelligent surface (RIS), which adjust their reflection matrices, with movable signals operating with fixed intelligent surface (FIS), which adjust the signal frequency while the surface properties remain fixed. For both RIS and FIS, we consider surfaces with a diagonal reflection matrix, named diagonal RIS/FIS, and surfaces with a reflection matrix not limited to being diagonal, named beyond-diagonal RIS/FIS. Movable signals with FIS always outperform RIS, achieving at least a fourfold gain. When transmitter and receiver polarizations differ, beyond-diagonal FIS further enhances performance.

Paper Structure

This paper contains 18 sections, 26 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Dual-Polarized RIS/FIS-Aided Channel Model and Problem Formulation
  3. Channel Model
  4. Problem Formulation
  5. RIS
  6. Movable Signals with FIS
  7. Same Polarization at the Tx and Rx
  8. Diagonal RIS
  9. Beyond-Diagonal RIS
  10. Diagonal FIS
  11. Beyond-Diagonal FIS
  12. Opposite Polarization at the Tx and Rx
  13. Diagonal RIS
  14. Beyond-Diagonal RIS
  15. Diagonal FIS
  16. ...and 3 more sections

Figures (3)

  • Figure 1: Dual-polarized RIS/FIS-aided system where the transmitter and receiver have opposite polarization.
  • Figure 2: (a) A phase shifter with phase $\pi$ as an example of a $2$-port network with scattering matrix $[[0,-1]^T,[-1,0]^T]$, and (b) implementation of the $n$th group of the BD-FIS in Section \ref{['sec:oppo-BDFIS']}.
  • Figure 3: Gain of BD-RIS, D-FIS, and BD-RIS over D-RIS as a function of $\chi$, when the transmitter and receiver have opposite polarization.