Implementation and Experimental Evaluation of Nullforming-Based Interference Reduction Scheme for HAPS-Terrestrial Spectrum Sharing

Spectrum sharing with terrestrial networks (TNs) is a challenging task in wireless communications using high-altitude platform stations (HAPSs). Various methods for HAPS-terrestrial spectrum sharing have been proposed. However, few studies have verified their effectiveness using a real payload system for HAPSs. This study proposes a novel interference reduction scheme based on nullforming techniques for a cylindrical array antenna, which uses angular information of HAPS users instead of their channel state information. In addition, a 5G-based service link evaluation system, which includes a 36-element cylindrical array antenna and the proposed precoder, has been implemented, and a field trial using this system was conducted to evaluate the interference reduction performance of the proposed scheme in a real rural environment. The experimental results show that the proposed scheme can mitigate the interference with TNs by approximately 20 dB relative to the main lobe level of user-specific beams in a rural scenario.