Active RIS-Aided Wireless Localization System with Power Splitting

This paper investigates an active reconfigurable intelligent surface (RIS)-aided mmWave system, in which the base station (BS) proactively sends a positioning reference signal to localize the mobile station (MS). In particular, due to the propagation nature of mmWave, the direct link between the BS and the MS is usually blocked. Thus, the reference signal cannot directly reach the MS but can do so through the enhanced reflections provided by multiple active RISs in the surroundings. To harness the benefits of active RIS-aided wireless localization system, the multiple active RISs are designed to reflect the reference signal in a time-division manner, and the reflection coefficient matrix of each active RIS is designed via a codebook method. Then, based on the reflected reference signal, a root MUSIC-based angle of departure (AoD) estimation algorithm is proposed for the MS to estimate the AoD from each active RIS. After that, the MS is able to calculate its location with the estimated AoDs and the positions of the active RISs. In addition, the power splitting between the BS and the active RISs is considered to further improve the positioning accuracy when the total power consumption is given. The optimal power splitting ratio is obtained by minimizing Cramér-Rao lower bound of the AoD. Simulation results show the superiority of our proposed localization scheme and the significance of optimizing the power splitting ratio in the considered active RIS-aided system.