Asynchronous Hybrid RIS-assisted Near-field Localization

This paper investigates the near-field localization performance based on the hybrid reconfigurable intelligent surface (RIS), which contains both passive and active elements and is capable of simultaneously reflecting and receiving signals. The asynchronous scenario, where the transmitter and the receiver are not perfectly synchronized, is considered. The position error bound (PEB) of the hybrid RIS-assisted localization in the asynchronous scenario is derived, and the PEB expressions in different scenarios are compared. Moreover, we show that turning only a small part of the elements into active ones can significantly improve the PEB performance. We then investigate the design of the phase shift coefficients for not only the passive reflection elements but also the phase shifters connected to the active elements. The proposed Focnear scheme can efficiently improve the PEB performance compared with the random phases, and it can provide a good initial point for further optimizing the phase profiles of the hybrid RIS. Besides, we also show that increasing the interval between adjacent active elements can generally improve the PEB performance.