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The low hardware cost makes ultra-large (XL) reconfigurable intelligentsurface (RIS) an attractive solution for the performance enhancement oflocalization and communication systems, but it results in near-fieldpropagation channels, especially for the high-frequency communication systems,e.g. mmWave and sub-terahertz (subTHz) systems.  This makes localization and channel estimation in the near field of theRIS-aided subTHz system more challenging.  In this paper, we consider the spherical wavefront propagation in the nearfield of the subTHz system with the assistance of a RIS.  A near-field channel estimation and localization (NF-JCEL) algorithm isproposed based on the derived second-order Fresnel approximation of thenear-field channel model.  To be specific, we first decouple the user equipment (UE) distances andangles of arrival (AoAs) through a down-sampled Toeplitz covariance matrix, sothat the vertical and azimuth AoAs in the array steering vectors can beestimated separately with low complexities.  Then, the UE distance can be estimated by the simple one-dimensional search,and the channel attenuation coefficients are obtained through the orthogonalmatching pursuit (OMP) method.  Simulation results validate the superiority of the proposed NF-JCEL algorithmto the conventional far-field algorithm, and show that higher resolutionaccuracy can be obtained by the proposed algorithm.

RIS-Aided Near-Field Localization and Channel Estimation for the Sub-Terahertz System