How to enhance the ISAC security in cell-free networks?

This paper investigates a secure cell-free integrated sensing and communication (ISAC) system designed to simultaneously localize a potential eavesdropper (Eve) and ensure secure communication for legitimate user equipment (UEs). Unlike existing solutions that focus primarily on direction estimation, a distributed architecture employs multiple access points (APs) that collaboratively transmit communication signals and omnidirectional sensing signals for localization. We present analytical closed-form expressions for the signal-to-interference-plus-noise ratios (SINRs) at the legitimate UEs and Eve, facilitating a theoretical evaluation of the system's secrecy spectral efficiency (SSE). Furthermore, we derive a tractable closed-form expression for the localization SINR after matched filtering and subsequently develop the corresponding Cramér-Rao lower bound (CRLB) for the received signal strength (RSS)-based localization, which is validated through Monte-Carlo simulations using the GaussNewton method. To optimally enhance the communication security performance and sensing accuracy, we formulate and solve an optimization problem, allocating power subject to a total transmit power and localization SINR constraints. Numerical results demonstrate the effectiveness of the proposed approach, highlighting significant improvements in both the secure communication performance and localization accuracy within cell-free ISAC architectures.