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

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.

💡 Research Summary

The paper investigates wireless communication systems assisted by dual‑polarized intelligent surfaces, comparing two fundamentally different approaches: (i) a reconfigurable intelligent surface (RIS) whose reflection matrix Θ can be dynamically adjusted based on channel state information, and (ii) a fixed intelligent surface (FIS) whose reflection matrix is pre‑optimized and kept constant while the transmitted carrier frequency (or wavelength λ) is varied – a technique referred to as “movable signals”. Both approaches are examined under two surface designs: a diagonal reflection matrix (D‑RIS/D‑FIS) and a beyond‑diagonal (unitary) matrix (BD‑RIS/BD‑FIS).

The system model assumes a single‑input single‑output (SISO) link with N dual‑polarized elements placed on a uniform linear array. The overall channel is expressed as h = h_R Θ h_T − h_R h_T, where the first term represents the controllable reflected component and the second term captures the specular (structural scattering) component that is independent of Θ. The dual‑polarization is modeled through power‑imbalance vectors p_R and p_T that contain the inverse cross‑polarization discrimination (XPD) factor χ (0 ≤ χ ≤ 1). When the transmitter and receiver share the same polarization, the effective channel entries are