  Intelligent reflecting surface (IRS) is a promising technology being
considered for future wireless communications due to its ability to control
signal propagation. This paper considers the joint active and passive
beamforming problem for an IRS-assisted radar, where multiple IRSs are deployed
to assist the surveillance of multiple targets in cluttered environments.
Specifically, we aim to maximize the minimum target illumination power at
multiple target locations by jointly optimizing the active beamformer at the
radar transmitter and the passive phase-shift matrices at the IRSs, subject to
an upperbound on the clutter power at each clutter scatterer. The resulting
optimization problem is nonconvex and solved with a sequential optimization
procedure along with semedefinite relaxation (SDR). Simulation results show
that IRSs can help create effective line-of-sight (LOS) paths and thus
substantially improve the radar robustness against target blockage.
