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Non-Reciprocal Reconfigurable Intelligent Surfaces

Jiaqi Xu, Haoyu Wang, Rang Liu, Josef A. Nossek, A. Lee Swindlehurst

Abstract

In contrast to conventional RIS, the scattering matrix of a non-reciprocal RIS (NR-RIS) is non-symmetric, leading to differences in the uplink and the downlink components of NR-RIS cascaded channels. In this paper, a physically-consistent device model is proposed in which an NR-RIS is composed of multiple groups of two-port elements inter-connected by non-reciprocal devices. The resulting non-reciprocal scattering matrix is derived for various cases including two-element groups connected with isolators or gyrators, and general three-element groups connected via circulators. Signal models are given for NR-RIS operating in either reflecting-only or simultaneously transmitting and reflecting modes. The problem of NR-RIS design for non-reciprocal beamsteering is formulated for three-element circulator implementations, and numerical results confirm that non-reciprocal beamsteering can be achieved with minimal sidelobe power. We also show that our physically consistent NR-RIS architecture is effective in implementing channel reciprocity attacks, achieving similar performance to that with idealized NR-RIS models.

Non-Reciprocal Reconfigurable Intelligent Surfaces

Abstract

In contrast to conventional RIS, the scattering matrix of a non-reciprocal RIS (NR-RIS) is non-symmetric, leading to differences in the uplink and the downlink components of NR-RIS cascaded channels. In this paper, a physically-consistent device model is proposed in which an NR-RIS is composed of multiple groups of two-port elements inter-connected by non-reciprocal devices. The resulting non-reciprocal scattering matrix is derived for various cases including two-element groups connected with isolators or gyrators, and general three-element groups connected via circulators. Signal models are given for NR-RIS operating in either reflecting-only or simultaneously transmitting and reflecting modes. The problem of NR-RIS design for non-reciprocal beamsteering is formulated for three-element circulator implementations, and numerical results confirm that non-reciprocal beamsteering can be achieved with minimal sidelobe power. We also show that our physically consistent NR-RIS architecture is effective in implementing channel reciprocity attacks, achieving similar performance to that with idealized NR-RIS models.

Paper Structure

This paper contains 12 sections, 4 theorems, 22 equations, 9 figures.

Table of Contents

  1. Introduction
  2. Physically Consistent NR-RIS Models
  3. Two-Element Group with Isolator
  4. Two-Element Group with Gyrator
  5. Three-Element Group with Circulator
  6. Signal Model and Non-Reciprocal Beamsteering
  7. Non-Reciprocal STAR Mode
  8. Non-Reciprocal Reflecting Mode
  9. Numerical Results
  10. Beamsteering
  11. Channel Reciprocity Attack
  12. Conclusions

Key Result

Theorem 1

The non-symmetric scattering matrix of a two-element group connected by an isolator is given by:

Figures (9)

  • Figure 1: Schematic illustration of an NR-RIS connected $N$-element group.
  • Figure 2: Schematics of two-element NR-RIS groups.
  • Figure 3: A three-element NR-RIS connected via a circulator.
  • Figure 4: Illustration of non-reciprocal beamsteering using NR-STAR-RISs.
  • Figure 5: Illustration of non-reciprocal beamsteering using NR-RISs.
  • ...and 4 more figures

Theorems & Definitions (8)

  • Theorem 1
  • proof
  • Theorem 2
  • proof
  • Theorem 3
  • proof
  • Corollary 1
  • proof