  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 new intelligent
reflecting surface (IRS) aided millimeter-wave (mmWave) ISAC system by
exploiting the distinct beam scanning operation in mmWave communications to
achieve efficient sensing at the same time. First, we propose a two-phase ISAC
protocol aided by a semi-passive IRS, consisting of beam scanning and data
transmission. Specifically, in the beam scanning phase, the IRS finds the
optimal beam for reflecting signals from the base station to a communication
user via its passive elements. Meanwhile, the IRS directly estimates the angle
of a nearby target based on echo signals from the target using its equipped
active sensing element. Then, 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 Cram\'er-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.
