From Sensing to Secrecy: Active Eavesdropper Localization in Massive MIMO ISAC

Abstract

This paper investigates a secure massive multiple-input multiple-output (mMIMO) integrated sensing and communication (ISAC) system and proposes a framework capable of precisely localizing an active eavesdropper (Eve) while simultaneously ensuring secure communication for legitimate user equipment (UEs). Compared with existing ISAC studies that primarily focus on sensing in the downlink phase, the proposed design leverages the uplink and downlink structure of time-division duplexing (TDD)-based ISAC systems by explicitly exploiting uplink training information to assist downlink localization. In the uplink, pilot-based channel estimation is performed in the presence of a pilot spoofing attack (PSA), enabling reliable detection of Eve’s angle of arrival (AoA). This coarse AoA estimate is then used in the downlink to form a dedicated sensing region that achieves precise, multipath-resilient direct localization of the Eve. Closed-form expressions for the signal-to-interference-plus-noise ratios (SINRs) at the legitimate UEs and Eve are derived, facilitating a theoretical evaluation of the system’s secrecy spectral efficiency (SSE). A tractable closed-form expression for the position error bound (PEB) is also derived. To jointly optimize sensing and communication performance, we propose a power allocation problem that maximizes the SSE while satisfying a given mainlobe-to-average-sidelobe ratio (MASR) threshold and power constraints. Numerical results demonstrate that the proposed joint uplink and downlink ISAC framework achieves substantially improved localization accuracy in multipath scenarios and significantly enhances secrecy performance, compared to conventional secure ISAC systems.