  This paper analyzes monostatic sensing by a user equipment (UE) for a setting
in which the UE is unable to resolve multiple targets due to their interference
within a single resolution bin. It is shown how sensing accuracy, in terms of
both detection rate and localization accuracy, can be boosted by a
reconfigurable intelligent surface (RIS), which can be advantageously used to
provide signal diversity and aid in resolving the targets. Specifically,
assuming prior information on the presence of a cluster of targets, a RIS beam
sweep procedure is used to facilitate the high resolution sensing. We derive
the Cram\'er-Rao lower bounds (CRLBs) for channel parameter estimation and
sensing and an upper bound on the detection probability. The concept of
coherence is defined and analyzed theoretically. Then, we propose an orthogonal
matching pursuit (OMP) channel estimation algorithm combined with data
association to fuse the information of the non-RIS signal and the RIS signal
and perform sensing. Finally, we provide numerical results to verify the
potential of RIS for improving sensor resolution, and to demonstrate that the
proposed methods can realize this potential for RIS-assisted high resolution
sensing.
