Passive Sensing for Multiple Vehicles in Bi-Static ISAC Systems

Integrated sensing and communications (ISAC) is emerging as a transformative 6G technology. By employing bistatic passive sensing between base stations (BSs), ISAC shows significant potential for intelligent transport systems (ITS), enabling real-time measurements of multiple vehicles across large areas without extensive hardware modifications. However, bistatic sensing faces significant challenges due to clock asynchronism. Existing phase offset elimination schemes are often limited by insufficient resolution or the introduction of undesired interference. To address these challenges, we propose a novel joint Doppler-delay (DD) estimation scheme that accounts for clock asynchronism, comprising four key components: (1) snapshot augmentation for covariance matrix estimation; (2) a spectrumweighted integrated periodogram-multiple signal classification (SWIP-MUSIC) algorithm for composite timing offset (CTO) estimation; (3) residual phase offset estimation and alignment; and (4) a periodogram-based joint DD estimation algorithm. Simulation results validate the robustness of the proposed method in multi-target scenarios, highlighting its potential to advance traffic monitoring and other sensing applications in the 6G era.