Special Issue on Near-Field Signal Processing: Communications, Sensing, and Imaging

Multichannel signal processing technologies are moving toward the deployment of small and densely packed sensors yielding extremely large aperture arrays (ELAAs) in order to provide higher angular resolution and beamforming gain. In particular, technologies are moving beyond the fifth-generation (5G) networks, wherein the adoption of ELAAs or surfaces and the exploitation of higher-frequency bands, e.g., terahertz (THz), push the electromagnetic diffraction field from the far-field region to the near field. With the extended array aperture and smaller wavelength, the signal wavefront at the receiver is no longer planar when the receiver is in the near field, i.e., it is closer to the transmitter than the Fraunhofer distance. In particular, the Fraunhofer distance is  (2D2 )/λ , where D is the array aperture and  λ  is the wavelength. The near-field region can also be regarded as reactive and radiative parts. While the former indicates the region very close to the antenna up to the Fresnel distance...