When Does Multipath Improve Imaging Resolution?

In this paper, we characterize resolution limits for imaging in electromagnetic spectrum where multipath is commonly encountered, e.g., spectrum often used for wireless communication. We analyze a passive system configuration with an aperture of fixed spatial extent sampling fields backscattered from an imaging scene consisting of both line-of-sight (LOS) and non-line-of-sight (NLOS) paths. We characterize the resolution limits using the degrees of freedom (DoF) metric. We make progress towards answering the question: when does multipath offer gains in imaging resolution over LOS-only propagation? Prior theoretical and empirical analysis offer seemingly contradictory answers to this question. On the one hand, prior theoretical analysis suggests that multipath does not improve resolution beyond LOS-only propagation. However, numerical simulations of multipath-exploiting imaging suggest otherwise. We show that the prior results correspond to two extreme operational regimes, under which multipath is equivalent to either LOS-only or NLOS-only propagation. Our DoF analysis unifies prior results, and establishes that: (i) multipath captures the best of both LOS-only and NLOS-only propagation, and (ii) under certain geometric configurations of scatterers, multipath can offer significant resolution gains over LOS-only propagation.