  Integrated sensing and communication (ISAC) is a potential technology of the
sixth-generation (6G) mobile communication system, which enables communication
base station (BS) with sensing capability. However, the performance of
single-BS sensing is limited, which can be overcome by multi-BS cooperative
sensing. There are three types of multi-BS cooperative sensing, including
cooperative active sensing, cooperative passive sensing, and cooperative active
and passive sensing, where the multi-BS cooperative passive sensing has the
advantages of low hardware modification cost and large sensing coverage.
However, multi-BS cooperative passive sensing faces the challenges of
synchronization offsets mitigation and sensing information fusion. To address
these challenges, a non-line of sight (NLoS) and line of sight (LoS) signal
cross-correlation (NLCC) method is proposed to mitigate carrier frequency
offset (CFO) and time offset (TO). Besides, a symbol-level multi-BS sensing
information fusion method is proposed. The discrete samplings of echo signals
from multiple BSs are matched independently and coherent accumulated to improve
sensing accuracy. Moreover, a lowcomplexity joint angle-of-arrival (AoA) and
angle-of-departure (AoD) estimation method is proposed to reduce the
computational complexity. Simulation results show that symbol-level multi-BS
cooperative passive sensing scheme has an order of magnitude higher sensing
accuracy than single-BS passive sensing. This work provides a reference for the
research on multi-BS cooperative passive sensing.
