The answer, say the companies, is a cylindrical antenna and a rotating connector.
Together they can widen the coverage of a single aircraft from 100km across to 200km – effectively quadrupling the area served.
“The cylindrical antenna comprises antenna elements attached both horizontally and vertically, and these can be controlled individually, making it possible to control the beam three-dimensionally in any desired direction,” say the companies.
HAPSMobile – based on an abbreviation for high altitude platform station – has been working on flying base stations for some time. Last year it successfully transmitted 4G mobile signals – including a video call – from an uncrewed solar-powered aircraft 19km above New Mexico.
But the research programme identified a challenge: “that the communication area projected to the ground cannot be fixed because the footprint shifts as the aircraft turns”.
SoftBank and HAPSMobile say they have a solution.
“By using digital beamforming, the technology fixes the footprint on the ground by shifting the direction of radio beams as the aircraft rotates. The cylindrical antenna comprises antenna elements attached both horizontally and vertically, and these can be controlled individually, making it possible to control the beam three-dimensionally in any desired direction.”
The cylindrical antenna can also compensate for other movements in the uncrewed aircraft, including pitching, rolling, ascending and descending.
The companies add that the antenna’s digital beamforming technology means they can control the shape and direction of radio beams, to concentrate on locations with high population densities and high communication traffic.
“This technology optimises the shape and direction of radio beams depending on peripheral user distribution wherever a HAPS is deployed, and it enhances the communication speeds of each mobile terminal and improves overall network efficiency.”
HAPSMobile’s original work was conducted with the help of Loon, a sister company of Google in the Alphabet group. Loon tried using solar-powered balloons to bring coverage to remote areas of east Africa, but the group abandoned the idea in January 2021.
However one company that will be watching with interest is Deutsche Telekom, which last year demonstrated high-speed 4G mobile transmission from a remotely powered aircraft in a project to test ways to expand rural coverage.
The company worked with London-based Stratospheric Platforms (SPL), a remotely powered aircraft specialist in which Deutsche Telekom is the largest shareholder.
SPL said last year that it is working on a platform using a hydrogen fuel cells, which it says can produce 49kW of power. The platform will have a wingspan of 60m, equivalent to that of a Boeing 747, but weighs only 3.5 tonnes, said SPL.
One of the major problems with the Loon balloon idea is that the flying base station was solar powered and did not carry much battery capacity – so Kenyan customers were cut off not long after sunset, and had to wait until morning.
SoftBank and HAPSMobile said they are aware of this issue. SoftBank set up its next-generation battery lab in June 2021 to promote research and development of next-generation batteries with high energy densities that are lightweight and safe, with a view to applying them to HAPS and other areas.
