Indoor Localization and Synchronization Using Dual RIS in Multipath Environments

This paper addresses reconfigurable intelligent surface (RIS)-assisted indoor localization and synchronization in the presence of multipaths. Considering the far field condition, direct range estimation becomes infeasible and there exists clock offset in the system, compounding the difficulty for a single RIS to accomplish user equipment (UE) positioning and synchronization. To tackle this challenge, a dual-RISs system is proposed. The extra degrees of freedom it offers can effectively resolve the problem It uses initial RIS phase design to separate components from dual RISs. Then, atomic norm minimization is employed to reconstruct the separated received signals, and 2D-MUSIC with single-snapshot estimates the angles-of-departure (AODs) of the UE and scatterers. After removing angle terms, root-MUSIC estimates the time-of-arrival (TOA). The UE's position is derived via least squares using AODs from dual RISs. Combining the UE's position with LOS path TOAs yields the clock offset. Scatterer positions are obtained using geometric relationships with the UE's position, NLOS path TOAs, and clock offset. Channel parameters are refined via maximum likelihood estimation. Simulation results prove the method's effectiveness. © 2025 IEEE.