Multi-RIS-based Target Positioning in Passive Radars

Reconfigurable intelligent surfaces (RISs)-enabled integrated sensing and communication (ISAC) communication systems can be used for efficient target localization in modern communication systems. A passive radar (PR) is employed in the ISAC setup to sense the target positions. Existing literature employs a single RIS for angle of arrival (AoA) estimation followed by target localization using a geometrical approach. Typically, at least two AoA estimates are required to perform positioning using the traditional angulation approach. This work proposes a multi-target positioning system by considering multiple RISs and a PR since RISs are typically inexpensive in nature and do not require any coordination between them. A novel beamformer is designed such that the PR suitably combines received signals from access point (AP), targets and RISs to perform effective beamforming. Subsequently, AoA estimation is performed using batch Normalized Least Mean Squares (NLMS) algorithm followed by target position estimation using the angulation approach. The performance of the proposed target localization scheme is evaluated in terms of Normalized Mean Squared Error (NMSE), Successful Recovery Probability (SRP), Root Mean Position Error (RMPE) and Cumulative Distribution Function (CDF) and is also compared with the baseline performance of no RIS. Simulation results demonstrate an improvement in the detection accuracy with an increase in the number of RIS elements and PR antennas.