SoftBank announced that it has developed and successfully tested an aerostat base station system – essentially a tethered high-altitude balloon with a cellular base station payload – with US-based Altaeros Energies.
The field trial, conducted in May at Hokkaido Spaceport, combined SoftBank’s footprint fixation technology and Altaeros’ ST-Flex high-altitude autonomous tethered aerostat to achieve stable, wide-area network coverage.
Softbank has used tethered-balloon wireless relay systems in the past to restore telecoms networks in the event of a disaster. The new High Altitude Platform Station (HAPS) system with Altaeros (in which Softbank is an investor) is able to operate at higher altitudes, carry heavier communication payloads, and achieve wider area network coverage with greater communication stability.
Aerostats – which have been used for temporary wireless communications for decades – are not to be confused with ideas like Google’s Project Loon that proposed turning weather balloons into floating base stations for rural connectivity. Aerostats are tethered, and fly at a lower altitude. However, they traditionally required a team of people to launch and operate, making them impractical and expensive as long-term commercial wireless broadband alternatives.
Altaeros’ ST-Flex aerostat is designed to fly autonomously using an AI-based autopilot system, which means no team is required. ST-Flex can circle at altitudes of up to 305m, and carry heavy communication payloads of up to 60kg.
Footprint fixation technology – which SoftBank is developing with its subsidiary HAPSMobile to enable stable connectivity regardless of airborne motion – is designed to tackle a key challenge with HAPS platforms. The footprint beamed to the ground isn’t fixed because the HAPS vehicle usually flies in a circle – that means cells rotate as the aircraft turns, which in turn causes frequent handovers and affects reception strength levels, thereby impacting connection quality.
Softbank says that by using its proprietary cylindrical antenna (a multi-element phased array antenna) with digital beamforming control, its footprint fixation technology secures the cell footprint on the ground by controlling the direction of radio beams as the airborne vehicle rotates.
During the field trial in Hokkaido, the ST-Flex system was tethered at an altitude of 249m, and provided stable line-of-sight wide-area network coverage of a few tens of kilometers in highly variable weather conditions, including gale-force winds.
Furthermore, Softbank says, even when the altitude, pitch and yaw of the aerostat equipped with the base station changed due to changes in wind speed and direction, there were no handovers for mobile devices and signal reception levels were unchanged.