A Sensing-Based Two-Phase Secure ISAC Framework

This paper develops a two-phase secure integrated sensing and communication (ISAC) framework, where a dual-functional transmitter (Alice) detects the existence of a potential eavesdropper (Eve) as a sensing target in the first phase. In the second phase, Alice adapts her secure transmission strategy based on the sensing outcome of the first phase. A distinct aspect of our proposed two-phase framework is the derivation of the overall effective secrecy rate across both phases. This is achieved by seamlessly incorporating the a priori probability of Eve’s presence, the sensing false alarm rate, and the sensing detection probability directly into the effective secrecy rate as weighting coefficients. For system design, we aim to maximize the overall effective secrecy rate subject to Alice’s average transmit power remaining below a predetermined threshold. To address the resulting non-convex design problem, an algorithm based on semi-definite relaxation (SDR) and double-layer optimization is proposed. Our numerical results show that the proposed two-phase secure ISAC framework significantly enhances the secrecy rate compared to a conventional physical layer security (PLS) system if Eve is present with a low probability.