  Exploiting wavefront curvature enables localization with limited
infrastructure and hardware complexity. With the introduction of reconfigurable
intelligent surfaces (RISs), new opportunities arise, in particular when the
RIS is functioning as a lens receiver. We investigate the localization of a
transmitter using a RIS-based lens in close proximity to a single receive
antenna element attached to reception radio frequency chain. We perform a
Fisher information analysis, evaluate the impact of different lens
configurations, and propose a two-stage localization algorithm. Our results
indicate that positional beamforming can lead to better performance when a
priori location information is available, while random beamforming is preferred
when a priori information is lacking. Our simulation results for a moderate
size lens operating at 28 GHz showcased that decimeter-level accuracy can be
attained within 3 meters to the lens.
