  Terahertz (THz) band owning the abundant multi-ten-GHz bandwidth is capable
to support Terabit-per-second wireless communications, which is a pillar
technology for 6G and beyond systems. With sub-millimeter-long antennas,
ultra-massive (UM) MIMO and intelligent surface (IS) systems with thousands of
array elements are exploited to effectively combat the distance limitation and
blockage problems, which compose a promising THz ultra-large antenna array
(ULAA) system. As a combined effect of wavelength and array aperture, the
resulting coverage of THz systems ranges from near-field to far-field, leading
to a new paradigm of cross-field communications. Although channel models,
communications theories, and networking strategies have been studied for
far-field and near-field separately, the unified design of cross-field
communications that achieve high spectral efficiency and low complexity is
still missing. In this article, the challenges and features of THz ULAA
cross-field communications are investigated. Furthermore, cross-field solutions
in three perspectives are presented, including a hybrid spherical- and
planar-wave channel model, cross-field channel estimation, and widely-spaced
multi-subarray hybrid beamforming, where a subarray as a basic unit in THz ULAA
systems is exploited. The approximation error of channel modeling accuracy,
spectral efficiency, and estimation error of these designs are numerically
evaluated. Finally, as a roadmap of THz ULAA cross-field communications,
multiple open problems and potential research directions are elaborated.
