Near-field 6G Networks: Why Mobile Terahertz Communications MUST Operate in the Near Field

TL;DR

This paper demonstrates that for high-rate 6G mobile terahertz communications, operating exclusively in the far field is fundamentally limited, making near-field operation essential for broadband mobile links, unlike stationary ones.

Contribution

It provides a mathematical framework showing the necessity of near-field operation for broadband mobile THz communications, highlighting fundamental limitations of far-field only systems.

Findings

Far-field mobile THz links are limited to narrow bandwidths.

Stationary THz links can achieve broadband in the far field.

Near-field operation is essential for mobile THz broadband communication.

Abstract

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.