IRS Aided Millimeter-Wave Sensing and Communication: Beam Scanning, Beam Splitting, and Performance Analysis

Integrated sensing and communication (ISAC) has attracted growing interests for enabling the future 6G wireless networks, due to its capability of sharing spectrum and hardware resources between communication and sensing systems. However, existing works on ISAC usually need to modify the communication protocol to cater for the new sensing performance requirement, which may be difficult to implement in practice. In this paper, we study a semi-passive intelligent reflecting surface (IRS) aided millimeter-wave (mmWave) ISAC system by exploiting the established beam scanning operation for simultaneous mmWave communications and sensing. First, we propose a two-phase ISAC protocol, consisting of beam scanning and data transmission. Specifically, in the beam scanning phase, the semi-passive IRS finds the optimal beam for reflecting signals from the base station to a communication user via its passive elements and, meanwhile, directly estimates the angle of a nearby target based on echo signals from the target using its active sensing elements. In the data transmission phase, the sensing accuracy is further improved by leveraging the data signals via possible IRS beam splitting. Next, we derive the achievable rate of the communication user as well as the Cramér-Rao bound and the approximate mean square error of the target angle estimation. Finally, extensive simulation results are provided to verify our analysis as well as the effectiveness of the proposed scheme.