  This paper investigates how to achieve integrated sensing and communication
(ISAC) based on a cell-free radio access network (CF-RAN) architecture with a
minimum footprint of communication resources. We propose a new passive sensing
scheme. The scheme is based on the radio frequency (RF) fingerprint learning of
the RF radio unit (RRU) to build an RF fingerprint library of RRUs. The source
RRU is identified by comparing the RF fingerprints carried by the signal at the
receiver side. The receiver extracts the channel parameters from the signal and
estimates the channel environment, thus locating the reflectors in the
environment. The proposed scheme can effectively solve the problem of
interference between signals in the same time-frequency domain but in different
spatial domains when multiple RRUs jointly serve users in CF-RAN architecture.
Simulation results show that the proposed passive ISAC scheme can effectively
detect reflector location information in the environment without degrading the
communication performance.
