The natural integration of extremely large antenna arrays (ELAAs) and terahertz (THz) communications can potentially achieve Tbps data rates in 6G networks. However, due to the extremely large array aperture and wide bandwidth, a new phenomenon called "near-field beam split" emerges. This phenomenon causes beams at different frequencies to focus on distinct physical locations, leading to a significant gain loss of beamforming. To address this challenging problem, we first harness a piecewise-far-field channel model to approximate the complicated near-field wideband channel. In this model, the entire large array is partitioned into several small sub-arrays. While the wireless channel's phase discrepancy across the entire array is modeled as near-field spherical, the phase discrepancy within each sub-array is approximated as far-field planar. Built on this approximation, a phase-delay focusing (PDF) method employing delay phase precoding (DPP) architecture is proposed. Our PDF method could compensate for the intra-array far-field phase discrepancy and the inter-array near-field phase discrepancy via the joint control of phase shifters and time delayers, respectively. Theoretical and numerical results are provided to demonstrate the efficiency of the proposed PDF method in mitigating the near-field beam split effect.Finally, we define and derive a novel metric termed the "effective Rayleigh distance" by the evaluation of beamforming gain loss. Compared to classical Rayleigh distance, the effective Rayleigh distance is more accurate in determining the near-field range for practical communications.

翻译：超大规模天线阵列（ELAAs）与太赫兹（THz）通信的自然融合有望在6G网络中实现Tbps级数据速率。然而，由于极大的阵列孔径和宽带宽，一种称为“近场波束分裂”的新现象随之出现。该现象导致不同频率的波束聚焦于不同的物理位置，从而造成显著的波束成形增益损失。为应对这一挑战性问题，我们首先利用分段远场信道模型来近似复杂的近场宽带信道。在该模型中，整个大型阵列被划分为若干小型子阵列。虽然无线信道在整个阵列上的相位差异被建模为近场球面波，但在每个子阵列内部的相位差异则被近似为远场平面波。基于此近似，我们提出了一种采用延迟相位预编码（DPP）架构的相位延迟聚焦（PDF）方法。所提出的PDF方法能够分别通过联合控制移相器和时延器，补偿阵列内的远场相位差异与阵列间的近场相位差异。理论与数值结果验证了所提PDF方法在缓解近场波束分裂效应方面的有效性。最后，我们通过评估波束成形增益损失，定义并推导了一种称为“有效瑞利距离”的新度量指标。相较于经典瑞利距离，有效瑞利距离在确定实际通信中的近场范围时更为精确。