Codebook design and beam training for extremely large-scale RIS: Far-field or near-field?

Reconfigurable intelligent surface (RIS) is more likely to develop into extremely large-scale RIS (XL-RIS) efficiently boost the system capacity for future 6G communications. Beam training an effective way acquire channel state information (CSI) XL-RIS. Existing beam schemes rely on far-field codebook. However, due large aperture of XL-RIS, scatters are be in near-field region The codebook mismatches model. Thus, existing scheme will cause severe performance loss XL-RIS assisted To solve this problem, we propose efficient by designing match Specifically, firstly design considering cascaded array steering vector Then, optimal codeword obtained exhausted procedure. reduce overhead, further a hierarchical and corresponding scheme, where different levels sub-codebooks searched turn with reduced size. Simulation results show proposed outperform scheme.