Multi-RIS-Enabled 3D Sidelink Positioning

Positioning is expected to support communication and location-based services in the fifth/sixth generation (5G/6G) networks. With the advent of reflective reconfigurable intelligent surfaces (RISs), radio propagation channels can be controlled, making high-accuracy positioning and extended service coverage possible. However, the passive nature of the RIS requires a signal source such as a base station (BS), which limits the positioning service in extreme situations, such as tunnels, dense urban areas, or complicated indoor scenarios where 5G/6G BSs are not accessible. In this work, we show that with the assistance of (at least) two RISs and sidelink communication between two user equipments (UEs), the absolute positions of these UEs can be estimated in the absence of BSs. A two-stage 3D sidelink positioning algorithm is proposed, benchmarked by the derived Cramér-Rao bounds. The effects of multipath and RIS profile designs on positioning performance are evaluated, and localization analyses are performed for various scenarios. Simulation results demonstrate the promising positioning accuracy of the proposed BS-free sidelink communication system. Additionally, we propose and evaluate several solutions to eliminate potential blind areas where positioning performance is poor, such as removing clock offset via round-trip communication, adding geometrical prior or constraints, as well as introducing more RISs.