  This work addresses the problem of intelligent reflecting surface (IRS)
assisted target sensing in a non-line-of-sight (NLOS) scenario, where an IRS is
employed to facilitate the radar/access point (AP) to sense the targets when
the line-of-sight (LOS) path between the AP and the target is blocked by
obstacles. To sense the targets, the AP transmits a train of uniformly-spaced
orthogonal frequency division multiplexing (OFDM) pulses, and then perceives
the targets based on the echoes from the AP-IRS-targets-IRS-AP channel. To
resolve an inherent scaling ambiguity associated with IRS-assisted NLOS
sensing, we propose a two-phase sensing scheme by exploiting the diversity in
the illumination pattern of the IRS across two different phases. Specifically,
the received echo signals from the two phases are formulated as third-order
tensors. Then a canonical polyadic (CP) decomposition-based method is developed
to estimate each target's parameters including the direction of arrival (DOA),
Doppler shift and time delay. Our analysis reveals that the proposed method
achieves reliable NLOS sensing using a modest quantity of pulse/subcarrier
resources. Simulation results are provided to show the effectiveness of the
proposed method under the challenging scenario where the degrees-of-freedom
provided by the AP-IRS channel are not enough for resolving the scaling
ambiguity.
