Field Trials and Evaluations of Nullforming-Based Spectrum Sharing Between Aerial Platforms and Terrestrial Networks on 5G FDD Bands

Interference management for aerial platforms is an essential technique for the spectrum sharing with terrestrial base stations (TBSs). In contrast to the extensive academic researches on the spectrum sharing, few studies have evaluated the spectral efficiency of an actual system. This study proposes a nullforming scheme for aerial base stations (ABSs) having a cylindrical array antenna (CAA). The proposed scheme mitigates the interference with TBSs and automatically adjusts the null directions based on the current position of an ABS. Moreover, a 5G-based spectrum sharing system is implemented, where an ABS with a 36-element CAA and a single TBS use a common frequency band of 1.8 GHz. The results measured in field trials demonstrate that the proposed scheme reduces the interference power by approximately 18 dB. Even though the ABS on a tethered balloon flying at an altitude of 250 m moves constantly, the measured throughput of the TBS increases up to 2.6 times that of a conventional scheme without nullforming because of the significant interference reduction.