Positioning with Dual Reconfigurable Intelligent Surfaces in Millimeter-Wave MIMO Systems

The combination of reconfigurable intelligent surface (RIS) and millimeter-wave (mm-wave) multiple-input multiple-output (MIMO) has drawn much attention for future wireless networks. In this paper, the RISs are used to assist the base station (BS) to localize the user equipment (UE) in the mmwave MIMO systems, where the reference signals sent by the UE are measured by the BS and the information of the direct UE-BS path and the reflection paths via RISs are utilized in the localization. In particular, the delay difference between the direct path and reflection path as well as the angles of each path are used to construct the triangulation needed in positioning. Therein, a two-stage positioning method with dual RISs is proposed, where in the first stage one suitable RIS is chosen to reflect signals and in the second stage location information is estimated. In particular, the optimal phase shifts of the RIS elements are obtained and the optimal RIS selection criterion is derived. Numerical results show that the proposed method achieves a localization accuracy up-to 10-510-4 meter.