RIS-aided Location Estimation Based on MVDR in Near-Field Conditions

In this paper, we propose a reconfigurable intelligence surface (RIS)-aided location system. Through such a system, we exploit that near-field (NF) effects are still significant at long distances in mm-wave bands (i.e., up to the Fraunhofer distance) for large RISs. Specifically, we leverage the wavefront curvature, which depends on the impinging direction of arrival and the distance to the RIS, to locate emitting sources. As the location procedure cannot directly access the signals at a passive RIS, our method uses the signals reflected by the RIS that impinge on an antenna array located close to the RIS. By processing the signals the array collects, we estimate the spatial power density in a given region using a minimum variance distortionless response (MVDR) estimator. As shown in the paper, this technique can precisely estimate the positions of a set of sources within the NF region of the RIS. Even though the signals that are processed are those collected by the antenna array and not the signals at the RIS, the proposed system can provide a much higher location accuracy than a system with only the antenna array without RIS. We also extend the approach to scenarios with multiple RIS-array pairs.