Low-Overhead Channel Estimation and Beamforming with Near/Far-Field Connection for Extra-Large RIS Communications

To ensure high array gain, RIS is evolving towards the extra-large RIS (XL-RIS). However, XL-RIS encounters significant challenges in high pilot overhead. In this paper, we consider a downlink XL-RIS communication system, which is used for assisting the data transmission from a single base station (BS) to multiple user equipments (UEs). We first propose a beam training strategy by turning on only a few RIS elements to reduce pilot overhead, so that the RIS can determine the UE areas. Then, the position information of the UEs is able to be obtained by estimating the far-field channels between the UEs and RIS. After that, all elements are activated for high- throughput data transmission, in which the near-field XL-RIS beamforming is performed based on the connection between the near-field and the far-field channels that we disclose with the positional information of UEs, ultimately forming multiple beams directed at different UEs with optimal power allocation.