Fundamental Limits of Bistatic Integrated Sensing and Communication under Sensing Information Leakage Constraint

Integrated sensing and communication (ISAC) systems aim to unify sensing and communication within a shared waveform, offering benefits in spectrum and hardware efficiency. However, this integration raises critical security concerns, particularly regarding the leakage of sensing-related information due to the broadcast nature of wireless transmissions. While prior work has explored ISAC under data security constraints, securing sensing information remains a key open challenge. In this work, we address this gap by studying a two-receiver state-dependent bistatic ISAC model with a sensing eavesdropper. We characterize the fundamental tradeoff among communication capacity, sensing distortion, and sensing information leakage. Specifically, we derive an inner bound using an input-constrained randomized encoding scheme and propose a new outer bound that incorporates leakage constraints. Under a reversely-degraded channel condition, we further establish the exact optimal tradeoff. Numerical results validate our scheme and illustrate the complex interactions among capacity, distortion, and leakage in secure ISAC systems.