  Besides improving communication performance, intelligent reflecting surfaces
(IRSs) are also promising enablers for achieving larger sensing coverage and
enhanced sensing quality. Nevertheless, in the absence of a direct path between
the base station (BS) and the targets, multi-target sensing is generally very
difficult, since IRSs are incapable of proactively transmitting sensing beams
or analyzing target information. Moreover, the echoes of different targets
reflected via the IRS-established virtual links share the same directionality
at the BS. In this paper, we study a wireless system comprising a multi-antenna
BS and an IRS for multi-target sensing, where the beamforming vector and the
IRS phase shifts are jointly optimized to improve the sensing performance. To
meet the different sensing requirements, such as a minimum received power and a
minimum sensing frequency, we propose three novel IRS-assisted sensing schemes:
Time division (TD) sensing, signature sequence (SS) sensing, and hybrid TD-SS
sensing. First, for TD sensing, the sensing tasks are performed in sequence
over time. Subsequently, a novel signature sequence (SS) sensing scheme is
proposed to improve sensing efficiency by establishing a relationship between
directions and SSs. To strike a flexible balance between the beam pattern gain
and sensing efficiency, we also propose a general hybrid TD-SS sensing scheme
with target grouping, where targets belonging to the same group are sensed
simultaneously via SS sensing, while the targets in different groups are
assigned to orthogonal time slots. By controlling the number of groups, the
hybrid TD-SS sensing scheme can provide a more flexible balance between beam
pattern gain and sensing frequency. Moreover, ...
