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RIS Empowered Near-Field Covert Communications

Jun Liu, Gang Yang, Yuanwei Liu, Xiangyun Zhou

TL;DR

The near-field communications can achieve a higher covert rate than the far-field covert communications in general, and still realize covert transmission even if Willie is located at the same direction as Bob and closer to the XL-RIS.

Abstract

This paper studies an extremely large-scale reconfigurable intelligent surface (XL-RIS) empowered covert communication system in the near-field region. Alice covertly transmits messages to Bob with the assistance of the XL-RIS, while evading detection by Willie. To enhance the covert communication performance, we maximize the achievable covert rate by jointly optimizing the hybrid analog and digital beamformers at Alice, as well as the reflection coefficient matrix at the XL-RIS. An alternating optimization algorithm is proposed to solve the joint beamforming design problem. For the hybrid beamformer design, a semi-closed-form solution for fully digital beamformer is first obtained by a weighted minimum mean-square error based algorithm, then the baseband digital and analog beamformers at Alice are designed by approximating the fully digital beamformer via manifold optimization. For the XL-RIS's reflection coefficient matrix design, a low-complexity alternating direction method of multipliers based algorithm is proposed to address the challenge of large-scale variables and unit-modulus constraints. Numerical results unveil that i) the near-field communications can achieve a higher covert rate than the far-field covert communications in general, and still realize covert transmission even if Willie is located at the same direction as Bob and closer to the XL-RIS; ii) the proposed algorithm can enhance the covert rate significantly compared to the benchmark schemes; iii) the proposed algorithm leads to a beam diffraction pattern that can bypass Willie and achieve high-rate covert transmission to Bob.

RIS Empowered Near-Field Covert Communications

TL;DR

The near-field communications can achieve a higher covert rate than the far-field covert communications in general, and still realize covert transmission even if Willie is located at the same direction as Bob and closer to the XL-RIS.

Abstract

This paper studies an extremely large-scale reconfigurable intelligent surface (XL-RIS) empowered covert communication system in the near-field region. Alice covertly transmits messages to Bob with the assistance of the XL-RIS, while evading detection by Willie. To enhance the covert communication performance, we maximize the achievable covert rate by jointly optimizing the hybrid analog and digital beamformers at Alice, as well as the reflection coefficient matrix at the XL-RIS. An alternating optimization algorithm is proposed to solve the joint beamforming design problem. For the hybrid beamformer design, a semi-closed-form solution for fully digital beamformer is first obtained by a weighted minimum mean-square error based algorithm, then the baseband digital and analog beamformers at Alice are designed by approximating the fully digital beamformer via manifold optimization. For the XL-RIS's reflection coefficient matrix design, a low-complexity alternating direction method of multipliers based algorithm is proposed to address the challenge of large-scale variables and unit-modulus constraints. Numerical results unveil that i) the near-field communications can achieve a higher covert rate than the far-field covert communications in general, and still realize covert transmission even if Willie is located at the same direction as Bob and closer to the XL-RIS; ii) the proposed algorithm can enhance the covert rate significantly compared to the benchmark schemes; iii) the proposed algorithm leads to a beam diffraction pattern that can bypass Willie and achieve high-rate covert transmission to Bob.
Paper Structure (19 sections, 2 theorems, 49 equations, 8 figures, 5 algorithms)

This paper contains 19 sections, 2 theorems, 49 equations, 8 figures, 5 algorithms.

Table of Contents

  1. Introduction
  2. System Model
  3. System Description
  4. Channel Model
  5. Signal Model
  6. Detection Performance Analysis at Willie
  7. Detection of Covert Communications at Willie
  8. Analysis of DEP at Willie
  9. Joint Beamforming Design for Covert Communication
  10. Problem Formulation
  11. Hybrid Beamforming Design at Alice
  12. Stage I: Fully-Digital Beamformer Design
  13. Stage II: Hybrid Beamformer Design
  14. Reflection Coefficient Design at XL-RIS
  15. Overall Algorithm and Complexity Analysis
  16. ...and 4 more sections

Key Result

Theorem 1

The equivalent channels in near-field XL-RIS-assisted communication systems do not satisfy asymptotic orthogonality, and the channel vectors are also not collinear.

Figures (8)

  • Figure 1: Illustration of an XL-RIS empowered near-field covert communication system.
  • Figure 2: Deployment of an XL-RIS empowered near-field covert communication system.
  • Figure 3: Convergence behaviors of the proposed algorithm.
  • Figure 4: Normalized heat maps for beam steering, beam focusing, and beam diffraction.
  • Figure 5: Covert rate versus minimum DEP requirement of Willie, $\xi$.
  • ...and 3 more figures

Theorems & Definitions (4)

  • Theorem 1
  • proof
  • Corollary 1
  • proof