Identifying a Near-Field Propagation Model for System-Level Research of 6G+ Terahertz Networks

Conventional cellular systems, including 5G operating in millimeter wave (mmWave, 30−100GHz), serve users located in the far-field of the base station antenna. One of the distinctive properties of future beyond 6 G cellular terahertz band (0.3−3THz) systems is the need to support mobile users located in both near- and far-fields. In the near-field, the received signal strength depends not only on the distance between BS and user equipment (UE) but also on the concrete coordinates of the UE and may fluctuate drastically even for displacements on the order of a wavelength. As a result, near-field propagation models are significantly more complex than far-field models, which makes system-level analysis of such prospective systems challenging. The aim of this paper is to provide a comprehensive review of nearfield propagation modeling principles and to identify a model that offers a practical compromise between accuracy and analytical tractability. Our results indicate that the best candidate nearfield propagation models for system-level research is the spherical model. Even though such a model is provided in terms of sums of components for phased antenna arrays, it allows for simple approximations for a specific set of carrier frequencies. From the accuracy point of view, it preserves the qualitative behavior of the Hertzian model, which is crucial for near-field communications.