XL-MIMO promises to provide ultrahigh data rates in Terahertz (THz) spectrum. However, the spherical-wavefront wireless transmission caused by large aperture array presents huge challenges for channel state information (CSI) acquisition. Two independent parameters (physical angles and transmission distance) should be simultaneously considered in XL-MIMO beamforming, which brings severe overhead consumption and beamforming degradation. To address this problem, we exploit the near-field channel characteristic and propose one low-overhead hierarchical beam training scheme for near-field XL-MIMO system. Firstly, we project near-field channel into spatial-angular domain and slope-intercept domain to capture detailed representations. Secondly, a novel spatial-chirp beam-aided codebook and corresponding hierarchical update policy are proposed. Theoretical analyses and numerical simulations are also displayed to verify the superior performances on beamforming and training overhead.

翻译：XL-MIMO有望在太赫兹频谱中提供超高速率。然而，大孔径阵列引起的球面波前无线传输给信道状态信息获取带来了巨大挑战。独立参数（物理角度和传输距离）需同时考虑于XL-MIMO波束赋形中，这导致严重的开销消耗和波束赋形性能下降。针对该问题，本文利用近场信道特性，提出一种面向近场XL-MIMO系统的低开销分层波束训练方案。首先，我们将近场信道投影至空间角度域和截距斜率域以捕获详细表征。其次，提出一种新型空间啁啾波束辅助码本及对应的分层更新策略。理论分析与数值仿真均验证了该方法在波束赋形和训练开销方面的优越性能。