  Intelligent metasurfaces are one of the favorite technologies for integrating
sixth-generation (6G) networks, especially the reconfigurable intelligent
surface (RIS) that has been extensively researched in various applications. In
this context, a feature that deserves further exploration is the frequency
scattering that occurs when the elements are periodically switched, referred to
as Space-Time-Coding metasurface (STCM) topology. This type of topology causes
impairments to the established communication methods by generating undesirable
interference both in frequency and space, which is worsened when using wideband
signals. Nevertheless, it has the potential to bring forward useful features
for sensing and localization. This work exploits STCM sensing capabilities in
target detection, localization, and classification using narrowband downlink
pilot signals at the base station (BS). The results of this novel approach
reveal the ability to retrieve a scattering point (SP) localization within the
sub-centimeter and sub-decimeter accuracy depending on the SP position in
space. We also analyze the associated detection and classification
probabilities, which show reliable detection performance in the whole analyzed
environment. In contrast, the classification is bounded by physical
constraints, and we conclude that this method presents a promising approach for
future integrated sensing and communications (ISAC) protocols by providing a
tool to perform sensing and localization services using legacy communication
signals.
