SoftBank successfully delivered end-to-end 5G connectivity to standard smartphones using an airborne base station, in a live field trial conducted in June, above Hachijō Island, Japan. The company used a light aircraft flying at 3,000 meters to simulate a high-altitude platform, linking ground infrastructure to devices via a 26 GHz feeder connection and a 1.7 GHz service link.
Unlike most “direct-to-device” projects that rely on satellites in low Earth orbit, SoftBank’s approach uses High-Altitude Platform Stations (HAPS) that hover in the stratosphere, about 20 kilometers up. That altitude brings big advantages like lower latency and fewer of the power and Doppler challenges that present challenges to orbital links. According to SoftBank, the service link in the trial operated on the 1.7 GHz band, which is already supported by most 5G phones sold around the world.
The aircraft-mounted payload used beamforming to create six directional cells fixed in place on the ground, even as the aircraft flew a circular holding pattern overhead. The system automatically shifted beam coverage every 60 degrees of rotation, emulating the behavior of a future stratospheric platform while proving that stable cellular coverage is possible with real-world mobility.
SoftBank’s radio stack links a mmWave backhaul from ground to aircraft with a sub-2 GHz link from aircraft to user, stitching together the entire 5G core chain. That design means HAPS nodes can act as intelligent base stations, not just repeaters or relays. The company also validated Doppler correction, automatic power control, and adaptive beam tracking, all of which are required for commercial service in the sky.
Compared to satellite-to-cell systems like AST SpaceMobile’s LEO-based platform, which previously demonstrated a 5G call from space, HAPS can cover wide areas with lower path loss and better spectrum reuse. And thanks to recent ITU decisions at WRC-23, operators like SoftBank can now deploy HAPS in terrestrial mobile bands, including 700 MHz, 850 MHz, 1.7 GHz, and 2.5 GHz.
SoftBank’s work could lead to 5G service in places where towers don’t reach, like disaster zones, offshore waters, or remote islands. The company hasn’t said when it will scale this to a full commercial launch, but the technical foundation is here, at least in theory.
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