  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.
