Cross Near- and Far-Field Beamforming for Wideband Multi-User Terahertz Communications

Terahertz (THz) and sub-THz communications are visioned as the key technologies for the next generation mobile networks (6G). However, the sub-millimeter wavelength of THz and the enlarged array aperture of ultra-massive MIMO increase the Rayleigh distance, thereby making the working coverage of THz systems cross the near-field and far-field. Meanwhile, the beam split issue arisen in THz beamforming significantly impacts the system performance. Although the beam split issue in the near-field and far-field has been studied separately, it still remains a significant challenge for seamless cross-field communications. In this letter, we propose a novel wideband THz cross-field communication scheme for multiple users. An alternating optimization algorithm is proposed to maximize the sum-rate, which decomposes the original non-convex problem into three subproblems. The solutions to these subproblems are obtained by successive convex approximation, fractional programming and majorization minimization techniques. Simulation results demonstrate that our proposed scheme can achieve cross-field communication effectively.