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

2026-04-06

AI Summary

This work addresses the challenge of jointly enhancing security and sensing performance in integrated sensing and communication (ISAC) systems under non-line-of-sight conditions with multiple users and targets. To this end, it introduces for the first time beyond-diagonal reconfigurable intelligent surfaces (BD-RIS) into secure ISAC and proposes a joint optimization framework for the BD-RIS scattering matrix, transmit beamforming, and artificial noise. By leveraging alternating optimization and Riemannian conjugate gradient methods, the scheme maximizes target reflection power while satisfying secrecy rate and transmit power constraints. Owing to the additional degrees of freedom afforded by the non-diagonal structure of BD-RIS, the proposed approach significantly improves sensing gain and simultaneously enhances system secrecy performance, outperforming conventional diagonal RIS-based schemes.

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.

Problem

• secure ISAC
• beyond-diagonal RIS
• secrecy
• sensing
• eavesdropping

Innovation

• Beyond-Diagonal RIS
• Secure ISAC
• Alternating Optimization
• Riemannian Conjugate Gradient
• Artificial Noise