Cross Far- and Near-Field Wireless Communications in Terahertz Ultra-Large Antenna Array Systems

The Terahertz (THz) band with abundant multiten-GHz bandwidth is capable of supporting terabit-per-second wireless communications, which is a pillar technology for 6G and beyond systems. With sub-millimeter-long antennas, ultra-massive (UM)-MIMO and reflective intelligent surface (RIS) systems with thousands of array elements can be exploited to effectively combat the distance limitation and blockage problems of a promising THz ultra-large antenna array (ULAA) system. As a combined effect of wavelength and array aperture, the resulting coverage of THz systems ranges from near-field to far-field, leading to a new paradigm of cross-field communications. Although channel models, communications theories, and networking strategies have been studied for far-field and near-field separately, the unified design of cross-field communications that achieve high spectral efficiency and low complexity is still missing. In this article, the challenges and features of THz ULAA cross-field communications are investigated. Furthermore, cross-field solutions in three perspectives are presented, including a hybrid spherical- and planar-wave channel model, cross-field channel estimation, and widely-spaced multi-subarray hybrid beamforming, where a subarray as a basic unit in THz ULAA systems is exploited. The approximation error of channel modeling accuracy, spectral efficiency, and estimation error of these designs are numerically evaluated. Finally, as a roadmap of THz ULAA cross-field communications, multiple open problems and potential research directions are elaborated.

Introduction

The boosting wireless traffic demands inspire increasing research attention to the exploration of future six-generation (6G) and beyond communication systems. To break the bottleneck of limited bandwidth, the new spectrum in the Terahertz (THz) band, which ranges from 0.1 to 10 THz and possesses abundant continuous bandwidth over tens of GHz, is regarded as a pillar technology to meet the terabits per second (Tb/s) data rates for 6G communication systems [1]. On the downside, as one major challenge, the THz wave suffers from severe propagation losses caused by large free space attenuation and strong molecular absorption, which thus limit the communication distance [2]. Moreover, being reliable on the line-of-sight (LoS) transmission due to diffuse scattering, signal propagation in the THz band is prone to be blocked by various obstacles in the communication channel.