Distance-Dependent Beam Split Aided Low-Overhead Near-field Wideband Beam Training

In extremely large-scale multiple input multiple output (XL-MIMO) systems for future 6G communications, codebook-based beam training stands out as a promising technology to acquire channel state information. Due to sharp increase in the number of antennas, the transition of electromagnetic propagation from the far-field to the near-field introduces extremely high overhead when employing exhaustive search in both angle and distance domain. To reduce the overwhelming overheads, existing method has been proposed to utilize the dispersed directional beams produced by controllable beam split to simultaneously search multiple directions in a distance ring in wideband communications. However, the method still requires exhaustive search in the distance domain and thus cannot achieve low-overhead beam training for XL-MIMO. To address the problem, achieving a transition from distance-independent beam split to distance-dependent beam split, we propose a near-field beam training method where both different angles and distances can be searched in a pilot simultaneously, and thus a few pilots are capable of covering the whole two-dimensional space for wideband XL-MIMO. Numerical simulations are displayed to verify the performance of the proposed low-overhead method.