Sensing with Sensors-Embedded RISs

The target sensing/localization performance is fundamentally limited by the line-of-sight link and severe signal attenuation over long distances. This chapter considers a challenging scenario where the direct link between the base station (BS) and the target is blocked due to the surrounding blockages and leverages the reflecting intelligent surface (RIS) with some active sensors, termed as sensors-embedded or semi-passive RIS, for localization. To be specific, the active sensors receive echo signals reflected by the target and apply signal processing techniques to estimate the target location. We derive the corresponding Cramér–Rao bound (CRB) under the point target case and extended target case. In particular, the relationships of the CRB with the number of frames for RIS beam adjustments, number of RIS reflecting elements, and number of sensors are theoretically analyzed and demystified. Simulation results show that the proposed semi-passive RIS architecture provides sub-meter level positioning accuracy even over a long localization range from the BS to the target.