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Beyond-Diagonal RIS For Enhanced Secrecy and Sensing Gains in Secure ISAC Networks: An Optimization Framework

Elmehdi Illi, Marwa Qaraqe

Abstract

Integrated sensing and communication (ISAC) has been receiving a notable interest as an energy- and spectrum-efficient enabler for simultaneous communication and sensing. Notably, reconfigurable intelligent surfaces (RIS) is among the key technologies enabling robust communication and sensing, particularly in environments without a line-of-sight (LoS). Recently, a new type of RIS, called beyond-diagonal RIS (BD-RIS), has drawn attention, offering additional degrees of freedom in controlling the propagation medium. In this paper, a novel secure BD-RIS-aided ISAC scheme is proposed and evaluated. The scheme is applicable to a multi-user multi-target ISAC network, where a dual-functional radar-communication (DFRC) base station (BS) simultaneously serves multiple downlink users and senses various targets that aim to eavesdrop on the legitimate signal transmitted to the users. The presence of a BD-RIS enables circumventing the absence of the LoS link and ensures secure transmission and sensing. To this end, an optimization problem is formulated aiming at maximizing a weighted sum of per-target reflected powers, subject to secrecy and transmit power constraints. Thus, by virtue of an alternating optimization (AO)- and Riemannian conjugate gradient-based approach, local optima for the BD-RIS scattering matrix, transmit signal beamforming matrices, and artificial noise covariance matrix are obtained. Numerical results highlight (i) the notable sensing gains of the BD-RIS-aided design with respect to its diagonal RIS (D-RIS)-based baseline and (ii) the improved secrecy-sensing trade-off, whereby the BD-RIS can ensure an increasing system secrecy without degrading the per-target reflected power.

Beyond-Diagonal RIS For Enhanced Secrecy and Sensing Gains in Secure ISAC Networks: An Optimization Framework

Abstract

Integrated sensing and communication (ISAC) has been receiving a notable interest as an energy- and spectrum-efficient enabler for simultaneous communication and sensing. Notably, reconfigurable intelligent surfaces (RIS) is among the key technologies enabling robust communication and sensing, particularly in environments without a line-of-sight (LoS). Recently, a new type of RIS, called beyond-diagonal RIS (BD-RIS), has drawn attention, offering additional degrees of freedom in controlling the propagation medium. In this paper, a novel secure BD-RIS-aided ISAC scheme is proposed and evaluated. The scheme is applicable to a multi-user multi-target ISAC network, where a dual-functional radar-communication (DFRC) base station (BS) simultaneously serves multiple downlink users and senses various targets that aim to eavesdrop on the legitimate signal transmitted to the users. The presence of a BD-RIS enables circumventing the absence of the LoS link and ensures secure transmission and sensing. To this end, an optimization problem is formulated aiming at maximizing a weighted sum of per-target reflected powers, subject to secrecy and transmit power constraints. Thus, by virtue of an alternating optimization (AO)- and Riemannian conjugate gradient-based approach, local optima for the BD-RIS scattering matrix, transmit signal beamforming matrices, and artificial noise covariance matrix are obtained. Numerical results highlight (i) the notable sensing gains of the BD-RIS-aided design with respect to its diagonal RIS (D-RIS)-based baseline and (ii) the improved secrecy-sensing trade-off, whereby the BD-RIS can ensure an increasing system secrecy without degrading the per-target reflected power.

Paper Structure

This paper contains 18 sections, 28 equations, 8 figures, 1 table, 1 algorithm.

Table of Contents

  1. Introduction
  2. Background
  3. Related Work
  4. Motivation and Contributions
  5. Notations
  6. System and Channel Model
  7. Key Performance Indicators
  8. Eavesdropping Assumptions and Secrecy Evaluation Metric
  9. Sensing Model Evaluation Metric
  10. Optimization Framework and Algorithmic Solution Procedure
  11. Optimization Problem Description
  12. Alternative Convex Representation
  13. Proposed Alternating Optimization-based Optimization Scheme
  14. Optimal BD-RIS$\ $Scattering Matrix $\mathbf{\Psi }$ For Given $\left\{ \overline{\mathbf{Q}}_{k}\right\} _{k=1}^{K}$ and $\mathbf{R}_{z}$
  15. Optimal $\left\{ \overline{\mathbf{Q}}_{k}\right\} _{p=1}^{P}$ and $\mathbf{R}_{z}$ For Given $\mathbf{\Psi }$
  16. ...and 3 more sections

Figures (8)

  • Figure 1: Considered System model.
  • Figure 2: Evolution of the weighted sum of reflected powers vs. $G_{\mathrm{it}}$ for three $\gamma _{D}^{(\min)}$ values.
  • Figure 3: The RIS beampattern for the considered BD-RIS-aided ISAC scheme in comparison with a D-RIS-aided baseline one.
  • Figure 4: Evolution of achievable SC compared of the minimal preset level $C_{\mathrm{S}}$ for different $M$ values.
  • Figure 5: Evolution of the weighted sum of reflected powers vs. $J_{\mathcal{T}}$ for different $M$ values.
  • ...and 3 more figures

Theorems & Definitions (1)

  • Remark 1