  This paper explores the potential of the intelligent reflecting surface (IRS)
in realizing multi-user concurrent communication and localization, using the
same time-frequency resources. Specifically, we propose an IRS-enabled
multi-user integrated sensing and communication (ISAC) framework, where a
distributed semi-passive IRS assists the uplink data transmission from multiple
users to the base station (BS) and conducts multi-user localization,
simultaneously. We first design an ISAC transmission protocol, where the whole
transmission period consists of two periods, i.e., the ISAC period for
simultaneous uplink communication and multi-user localization, and the pure
communication (PC) period for only uplink data transmission. For the ISAC
period, we propose a multi-user location sensing algorithm, which utilizes the
uplink communication signals unknown to the IRS, thus removing the requirement
of dedicated positioning reference signals in conventional location sensing
methods. Based on the sensed users' locations, we propose two novel beamforming
algorithms for the ISAC period and PC period, respectively, which can work with
discrete phase shifts and require no channel state information (CSI)
acquisition. Numerical results show that the proposed multi-user location
sensing algorithm can achieve up to millimeter-level positioning accuracy,
indicating the advantage of the IRS-enabled ISAC framework. Moreover, the
proposed beamforming algorithms with sensed location information and discrete
phase shifts can achieve comparable performance to the benchmark considering
perfect CSI acquisition and continuous phase shifts, demonstrating how the
location information can ensure the communication performance.
