On Near-Far-Field Boundaries in Wireless Systems

TL;DR

This paper presents a Maxwell's equations-based framework to analyze the transition between near-field and far-field regions in wireless systems, revealing limitations of traditional distance thresholds.

Contribution

It introduces a rigorous analytical framework for defining NF and FF boundaries and evaluates the accuracy of existing distance-based criteria.

Findings

Traditional distance thresholds are insufficient to predict the NF-FF transition.

The framework highlights caveats in commonly used NF and FF concepts.

Numerical experiments confirm the limitations of simple distance-based boundaries.

Abstract

Near-field (NF) multi-antenna wireless communication and sensing have attracted growing research interest in recent years. A core question in this area is how to determine whether a wireless system is operating in the NF or far-field (FF) region. In this work, we propose a framework grounded in Maxwell's equations to analyze the transition region between the NF and FF, following the IEEE definition that specifies where the NF ends and the FF begins. Using this framework, we (i) compare a variety of traditional and recently introduced single-letter distance thresholds, often referred to as near-far-field boundaries, and (ii) conduct numerical experiments with both single- and multi-antenna wireless systems and with analytical models as well as full-wave electromagnetic simulations. Our results indicate that all of the considered single-letter distance thresholds are insufficient to predict the transition region between the NF and FF regions. Moreover, we highlight several important caveats associated with frequently (and recently) used NF and FF concepts.