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.
