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Stacked Intelligent Metasurfaces for Near-Field Multi-User Covert Communications

Ahmed M. Benaya, Ali A. Nasir, Khaled M. Rabie, Daniel B. da Costa

Abstract

Reconfigurable intelligent surfaces have emerged as a cutting-edge technology for next-generation wireless communications that are capable of reconfiguring the wireless environment using a large number of cost-effective reflecting elements. However, a significant body of prior studies has focused on single-layer surfaces that lack the capability of significantly mitigating inter-user interference. Moreover, previous studies mostly consider far-field operation and neglect working in the near-field region. In this paper, we propose a stacked intelligent metasurfaces (SIM)-assisted near-field multi-user multiple-input-single-output covert communication system. More specifically, we have a multi-antenna base station that is assisted with a SIM to serve multiple single-antenna users in the presence of multiple single-antenna wardens. We aim at optimizing the beamfocusing vectors at the BS and SIM phase shift matrices to maximize the sum covert rate under maximum transmit power budget constraint, quality-of-service (QoS) constraint for all users, and covertness constraint. Since the formulated problem is highly non-convex due to the coupling between the variables, we adopt alternating optimization to tackle it, where we divide the problem into beamfocusing sub-problem and SIM phase shift sub-problem, which are solved alternately until convergence. We leverage successive convex approximation (SCA) to solve the two sub-problems. Additionally, we formulate the SIM phase shift sub-problem using the widely adopted projected gradient ascent (PGA) method for comparison purposes. The conducted simulations reveal that the SCA-based algorithm outperforms the existing PGA-based algorithm as well as other benchmarks in terms of the achieved sum covert rate, demonstrating its consistent performance and robustness under various system parameter configurations.

Stacked Intelligent Metasurfaces for Near-Field Multi-User Covert Communications

Abstract

Reconfigurable intelligent surfaces have emerged as a cutting-edge technology for next-generation wireless communications that are capable of reconfiguring the wireless environment using a large number of cost-effective reflecting elements. However, a significant body of prior studies has focused on single-layer surfaces that lack the capability of significantly mitigating inter-user interference. Moreover, previous studies mostly consider far-field operation and neglect working in the near-field region. In this paper, we propose a stacked intelligent metasurfaces (SIM)-assisted near-field multi-user multiple-input-single-output covert communication system. More specifically, we have a multi-antenna base station that is assisted with a SIM to serve multiple single-antenna users in the presence of multiple single-antenna wardens. We aim at optimizing the beamfocusing vectors at the BS and SIM phase shift matrices to maximize the sum covert rate under maximum transmit power budget constraint, quality-of-service (QoS) constraint for all users, and covertness constraint. Since the formulated problem is highly non-convex due to the coupling between the variables, we adopt alternating optimization to tackle it, where we divide the problem into beamfocusing sub-problem and SIM phase shift sub-problem, which are solved alternately until convergence. We leverage successive convex approximation (SCA) to solve the two sub-problems. Additionally, we formulate the SIM phase shift sub-problem using the widely adopted projected gradient ascent (PGA) method for comparison purposes. The conducted simulations reveal that the SCA-based algorithm outperforms the existing PGA-based algorithm as well as other benchmarks in terms of the achieved sum covert rate, demonstrating its consistent performance and robustness under various system parameter configurations.

Paper Structure

This paper contains 15 sections, 1 theorem, 51 equations, 8 figures, 2 tables, 2 algorithms.

Table of Contents

  1. Introduction
  2. Related Work
  3. Motivation and Contribution
  4. System Model
  5. System Description and Signaling model
  6. Near-Field Channel Model
  7. Detection Performance Analysis
  8. Optimization Problem Formulation and Solution
  9. Beamfocusing Vectors Optimization
  10. SIM Phase Shift Optimization
  11. Per Layer SCA-based Phase Shift Design
  12. PGA-based Phase Shift Design
  13. Overall Algorithm and Complexity Analysis
  14. Simulation Results
  15. Conclusion

Key Result

Theorem 1

The partial derivatives of $\frac{\partial \gamma_k}{\partial \theta_n^l}$ is given by: where and

Figures (8)

  • Figure 1: SIM-assisted multi-user multi-warden near-field downlink covert communication system.
  • Figure 2: Convergence performance of the proposed system for different number of Layers.
  • Figure 3: Sum rate performance of the proposed system against the maximum transmit power as compared with benchmarks.
  • Figure 4: Sum rate performance of the proposed system against the number of transmit antennas as compared with benchmarks.
  • Figure 5: Sum rate performance of the proposed system against the number of covert users as compared with benchmarks.
  • ...and 3 more figures

Theorems & Definitions (1)

  • Theorem 1