Spectrum sharing between a HAPS and terrestrial base station

SoftBank Corp. has successfully conducted a field trial using its cylindrical antenna for high-altitude platform station (HAPS) stratospheric-based wireless communication systems at Hokkaido Spaceport in Japan. The trial successfully demonstrated the use of nullforming technology to achieve spectrum sharing between a HAPS and a terrestrial base station.

SoftBank is currently conducting R&D on spectrum sharing to enable the deployment of communication services that use the same frequency for HAPS and terrestrial base stations. While assigning dedicated frequencies to HAPS and terrestrial base stations separately would eliminate radio wave interference and enable the provision of high-quality communication networks, doing this also means the utilisation of multiple frequencies, which are finite.

Accordingly, SoftBank is considering the use of cylindrical antennas as antennas for ‘service links’ — which handle data transmission and reception between HAPS and communication devices — and is developing a nullforming technology, which significantly suppresses radio wave emissions in specified directions to suppress interference. By preventing interference, it is possible to achieve spectrum sharing between HAPS and terrestrial base stations, and thereby utilise spectrum effectively.

In this field trial, SoftBank placed a terrestrial base station within the communication area of a high-altitude tethered aerostat equipped with a cylindrical antenna (‘airborne base station’). The cylindrical antenna comprises antenna elements attached both horizontally and vertically which can be controlled individually, making it possible to control the beam three-dimensionally in any desired direction. It can compensate for pitching, rolling, ascending, descending and other aircraft movements, making it possible to fix communication areas from the stratosphere.

The cylindrical antenna used in the field trial.

Mobile Device A was placed within the communication area of the airborne base station while Mobile Device B was positioned in a geographically close location within the communication area of the terrestrial base station. The same frequency was used for both the airborne base station and the terrestrial base station, and the communication speeds of Mobile Device A and Mobile Device B were measured based on whether nullforming technology was applied. Furthermore, when comparing communication speeds based on the presence or absence of nullforming technology, the radio waves from the airborne station were halted, and the communication speeds of Mobile Device B were measured in an environment where there was no radio wave interference from the terrestrial base station.

Through the field trial, it was observed that the application of nullforming technology improved the communication speeds of Mobile Device B without significantly degrading the communication speed of Mobile Device A. Furthermore, by applying nullforming technology, interference between both base stations was reduced, and it was confirmed that Mobile Device B achieved communication speeds equal to those in an environment without radio interference when the radio waves from the airborne base station were halted to prevent interference.

The field trial thus confirmed the feasibility and effectiveness of spectrum sharing between airborne base stations and terrestrial base stations using nullforming technology in actual outdoor environments, representing a significant achievement for enabling the effective usage of spectrum in networks that use existing base stations and HAPS. Using the knowledge and data obtained through this field trial, SoftBank will continue to work to realise the practical use of HAPS and utilise spectrum efficiently.

Top image: The high-altitude tethered aerostat used in the field trial.