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.

翻译：近场移动太赫兹通信是第六代（6G）网络中实现高速无线数据交换的候选使能技术之一。然而，在太赫兹近场中运行既带来了诱人的机遇，也带来了严峻的挑战。因此，探究能否设计一个不工作在太赫兹近场的实际移动太赫兹通信系统变得具有重要意义。为回答这一问题，本文提出一个数学框架，对仅工作在远场的移动太赫兹链路进行建模。研究得出一个有趣的理论结论：虽然实际频率（几乎）无关紧要，但此类系统必须在不超过特定阈值的有限带宽内运行。随后通过数值分析表明，仅工作在远场会对移动太赫兹通信施加严格限制，从而降低其对未来高速业务服务的吸引力。相比之下，研究表明，即使完全停留在太赫兹远场，固定太赫兹链路仍可实现宽带传输。因此，宽带移动太赫兹通信必须采用近场模式，而宽带固定太赫兹链路则不必如此。