  Near-field mobile terahertz (THz) communications is one of the candidate
enablers for high-rate wireless data exchange in sixth-generation (6G)
networks. However, operating in the THz near field brings both attractive
opportunities and severe challenges. Hence, it becomes of interest to explore
if it is possible to design a realistic mobile THz communication system without
working in the THz near field. To answer this question, a mathematical
framework is presented modeling a mobile THz link that works exclusively in the
far field. The study leads to an interesting theoretical conclusion: while the
actual frequency is of (almost) no interest, such a system must operate over a
limited bandwidth not exceeding a certain threshold. It is then numerically
shown that operating only in the far field imposes stringent limitations on
mobile THz communications, thus making them less attractive to prospective
high-rate services. In contrast, it is shown that a stationary THz link can
still be broadband even when staying exclusively in the THz far field. Hence,
broadband mobile THz communications MUST be near-field, while broadband
stationary THz links do not have to.
