  This article focuses on the near-field effect in terahertz (THz)
communications and sensing systems. By equipping with extremely large-scale
antenna arrays (ELAAs), the near-field region in THz systems can be possibly
extended to hundreds of meters in proximity to THz transceivers, which
necessitates the consideration of near-field effect in the THz band both for
the communications and sensing. We first review the main characteristics of the
near-field region in the THz bands. The signal propagation in the near-field
region is characterized by spherical waves rather than planar waves in the
far-field region. This distinction introduces a new distance dimension to the
communication and sensing channels, which brings new opportunities and
challenges for both THz communications and sensing. More particularly, 1) For
THz communications, the near-field effect enables a new mechanism for
beamforming, namely, beamfocusing, in the focusing region. Furthermore, in THz
multiple-input and multiple-output (MIMO) systems, the near-field effect can be
exploited to combat the multiplexing gain degradation caused by the sparse THz
channels. To address the near-field beam split effect caused by the
conventional frequency-independent hybrid beamforming architecture in THz
wideband communications, we propose a pair of wideband beamforming optimization
approaches by a new hybrid beamforming architecture based on true-time-delayers
(TTDs). 2) For THz sensing, joint angle and distance sensing can be achieved in
the near-field region. Additionally, the near-field beam split becomes a
beneficial effect for enhancing the sensing performance by focusing on multiple
possible target locations rather than a drawback encountered in communications.
Finally, several topics for future research are discussed.
