  In this paper, we establish an integrated sensing and communication (ISAC)
system based on a distributed semi-passive intelligent reflecting surface
(IRS), which allows location sensing and data transmission to be carried out
simultaneously, sharing the same frequency and time resources. The detailed
working process of the proposed IRS-based ISAC system is designed, including
the transmission protocol, location sensing and beamforming optimization.
Specifically, each coherence block consists of two periods, the ISAC period
with two time blocks and the pure communication (PC) period. During each time
block of the ISAC period, data transmission and user positioning are carried
out simultaneously. The estimated user location in the first time block will be
used for beamforming design in the second time block. During the PC period,
only data transmission is conducted, by invoking the user location estimated in
the second time block of the ISAC period for beamforming design.
{\color{black}Simulation results show that a millimeter-level positioning
accuracy can be achieved by the proposed location sensing scheme, demonstrating
the advantage of the proposed IRS-based ISAC framework. Besides, the proposed
two beamforming schemes based on the estimated location information achieve
similar performance to the benchmark schemes assuming perfect channel state
information (CSI), which verifies the effectiveness of beamforming design using
sensed location information.
