Spectrum Management in Near-Field Multi-User Wireless Communications

Employing massive antenna arrays and reconfigurable intelligent surfaces together with recent exploration of higher spectral bands (i.e., mmWave and sub-terahertz frequencies) has unleashed an unprecedented territory where receivers located up to tens of meters from the base station can be within its near-field. In this regime, the typical assumption of far-field channels and planar wave-fronts can fail in practical wireless network scenar-ios. Indeed, in almost all prior work on spectrum management and spatial-spectral reuse, the impact of near-field propagation has been ignored. This article discusses the opportunities and challeng-es in spectrum management specific to near-field wireless communications. First, this article provides a comprehensive view of the interference models and their differences in near vs far fields. Second, this article illustrates the opportunities presented by exploiting new types of beams that are only available in the near field for multi-user interference and spectrum sharing. Notably, the near-field characteristics are viewed as an opportunity to enhance spatial-spectral efficiency rather than a problem to overcome. Finally, this article provides new trends in digital signal processing for wideband near-field communications and open research challenges.