Localization in the Near Field of a RIS-Assisted mmWave/subTHz System

The low hardware cost makes ultra-large (XL) reconfigurable intelligent surfaces (RIS) an attractive solution for enabling the intelligent electromagnetic environment, but it brings the challenge of near-field propagation channels. In this paper, we consider the propagation feature of the spherical wavefront in the near field of the millimeter-wave/sub Terahertz (mmWave/subTHz) localization system with the assistance of a RIS. The localization problem is investigated based on the derived second-order Fresnel approximation of the near-field channel model. In addition, the RIS training phase shifts and pilots are carefully designed to increase the channel rank so that the channel covariance matrix can be efficiently estimated. Simulation results validate the proposed near-field channel approximation and the localization algorithm.