Near-Field Signal Processing: Unleashing the Power of Proximity

TL;DR

This survey consolidates state-of-the-art near-field signal processing across radar, communications, acoustics, ultrasound, and optics, emphasizing spherical NF wavefronts and range-dependent beampatterns that diverge from far-field assumptions. It surveys NF-specific algorithms for DoA localization, NF/FF source separation, coherent-signal handling, channel estimation with sparse polar-domain dictionaries, and NF beamforming/beam-focusing, including wideband beam-squint compensation and ISAC on shared NF platforms. Key contributions include NF signal models, NF-aware parameter estimation techniques (MUSIC, cumulants, calibration-based methods), and cross-domain insights on calibration, smoothing, and dictionary design to manage NF effects. The findings highlight practical implications for ELAA-enabled 6G+ systems, high-bandwidth NF sensing, and NF-optics imaging, while outlining open challenges such as beam-squint, coherence, and NF calibration that motivate future interdisciplinary research.

Abstract

After nearly a century of specialized applications in optics, remote sensing, and acoustics, the near-field (NF) electromagnetic propagation zone is experiencing a resurgence in research interest. This renewed attention is fueled by the emergence of promising applications in various fields such as wireless communications, holography, medical imaging, and quantum-inspired systems. Signal processing within NF sensing and wireless communications environments entails addressing issues related to extended scatterers, range-dependent beampatterns, spherical wavefronts, mutual coupling effects, and the presence of both reactive and radiative fields. Recent investigations have focused on these aspects in the context of extremely large arrays and wide bandwidths, giving rise to novel challenges in channel estimation, beamforming, beam training, sensing, and localization. While NF optics has a longstanding history, advancements in NF phase retrieval techniques and their applications have lately garnered significant research attention. Similarly, utilizing NF localization with acoustic arrays represents a contemporary extension of established principles in NF acoustic array signal processing. This article aims to provide an overview of state-of-the-art signal processing techniques within the NF domain, offering a comprehensive perspective on recent advances in diverse applications.

Figures (7)

• Figure 1: EM field regions of an antenna of size $A$ for the signal wavelength $\lambda$. The reactive NF is immediately adjacent to the antenna and becomes negligible at $R_1 =\lambda / \pi$. Beyond this boundary lies the radiative NF, itself split into two regions. The first part, known as the aperture zone, extends from $R_1$ to $R_2 = 0.62\sqrt{A^3/\lambda}$. The second part of the radiative NF is the Fresnel region, stretching from $R_2$ to $R_3=2 A^2 / \lambda$.
• Figure 2: Applications of NF signal processing on radar, communications, acoustics, ultrasound and optics.
• Figure 3: MSE for the difference of NF and FF signal model for a ULA of $N=256$ elements on various frequencies.
• Figure 4: Array beampattern for the mixture of FF beamforming and NF beamfocusing.
• Figure 5: Channel estimation and SE performance. (a) NMSE versus pilot length for the LS and OMP-based channel estimators. (b) Average SE per user with exact NF channels and mismatched FF channels.