  Facilitated by rapid technological development of the near-space platform
stations (NSPS), near-space communication (NS-COM) is envisioned to play a
pivotal role in the space-air-ground integrated network for sixth-generation
(6G) communications and beyond. In NS-COM, ultra-broadband wireless
connectivity between NSPSs and various airborne/spaceborne platforms is
required for a plethora of bandwidth-consuming applications, such as NSPS-based
Ad hoc networking, in-flight Internet and relaying technology. However, such
requirement seems to contradict with the scarcity of spectrum resources at
conventional microwave frequencies, which motivates the exploitation of
terahertz (THz) band ranging from 0.1 to 10 THz. Due to huge available
bandwidth, the THz signals are capable of supporting ultra-high-rate data
transmission for NS-COM over 100 Gb/s, which are naturally suitable for the
near-space environment with marginal path loss. To this end, this article
provides an extensive investigation on the THz-band NS-COM (THz-NS-COM) from a
physical-layer perspective. Firstly, we summarize the potential applications of
THz communications in the near-space environment, where the corresponding
technical barriers are analyzed. Then the channel characteristics of THz-NS-COM
and the corresponding modeling strategies are discussed, respectively.
Afterwards, three essential research directions are investigated to surpass the
technical barriers of THz-NS-COM, i.e., robust beamforming for ultra-massive
antenna array, signal processing algorithms against hybrid distortions, and
integrated sensing and communications. Several open problems are also provided
to unleash the full potential of THz-NS-COM.
