A wideband generalization of the near-field region for extremely large phased-arrays

TL;DR

This paper extends the concept of the near-field region for extremely large phased arrays to wideband systems, deriving new bounds and metrics to better understand beamforming behavior in near-field and wideband conditions.

Contribution

It introduces a frequency-selective far-field boundary and a fundamental upper bound on the product of array size and bandwidth for wideband near-field systems.

Findings

The far-field threshold increases with frequency away from the center.

A fundamental upper bound on array aperture and bandwidth product is derived.

Numerical results show the impact on 5G NR band bandwidths.

Abstract

The narrowband and far-field assumption in conventional wireless system design leads to a mismatch with the optimal beamforming required for wideband and near-field systems. This discrepancy is exacerbated for larger apertures and bandwidths. To characterize the behavior of near-field and wideband systems, we derive the beamforming gain expression achieved by a frequency-flat phased array designed for plane-wave propagation. To determine the far-field to near-field boundary for a wideband system, we propose a frequency-selective distance metric. The proposed far-field threshold increases for frequencies away from the center frequency. The analysis results in a fundamental upper bound on the product of the array aperture and the system bandwidth. We present numerical results to illustrate how the gain threshold affects the maximum usable bandwidth for the n260 and n261 5G NR bands.