Skies over Tokyo are thick with air traffic these days amid an influx of international tourists. But one plane recently helped revive the dream of airborne Internet access for all. Researchers in Japan announced on 28 May that they have successfully tested 5G communications equipment in the 38 gigahertz band from an altitude of 4 kilometers.

The experiment was aimed at developing an aerial relay backhaul with millimeter-wave band links between ground stations and a simulated High-Altitude Platform Station (HAPS), a radio station aboard an uncrewed aircraft that stays aloft in the stratosphere for extended periods of time. A Cessna flying out of Chofu Airfield in western Tokyo was outfitted with a 38 GHz 5G base station and core network device, and three ground stations were equipped with lens antennas with automatic tracking.

With the Cessna as a relay station, the setup enabled communication between one ground station connected to the 5G terrestrial network and a terrestrial base station connected to a user terminal, according to a consortium of Japanese companies and the National Institute of Information and Communications Technology.

“We developed technology that enables communication using 5G [New Radio] by correctly directing 38 GHz beams toward three ground stations while adapting to the flight attitude, speed, direction, position, altitude, etc. during aircraft rotation,” said Shinichi Tanaka, a manager in broadcaster SKY Perfect JSAT’s Space Business Division. “We confirmed that the onboard system, designed for the stratosphere, has adequate communication and tracking performance even under the flight speed and attitude fluctuations of a Cessna aircraft, which are more severe than those of HAPS.”

The sharpest beam width of the ground station antenna is 0.8 degrees, and the trial demonstrated a tracking method that always captures the Cessna in this angular range, Tanaka added.

A Cessna [top left] carried a 38 GHz antenna [top right] during a flight, functioning as a 5G base station for receivers on the ground [bottom right]. The plane was able to connect to multiple ground stations at once [illustration, bottom left].NTT Docomo

Millimeter wave bands, such as the 38 GHz band, have the highest data capacity for 5G and are suited for crowded venues such as stadiums and shopping centers. When used outdoors, however, the signals can be attenuated by rain and other moisture in the atmosphere. To counter this, the consortium successfully tested an algorithm that automatically switches between multiple ground stations to compensate for moisture-weakened signals.

Unlike Google’s failed Loon effort, which focused on providing direct communication to user terminals, the HAPS trial is aimed at creating backhaul lines for base stations. Led by Japan’s Ministry of Internal Affairs and Communications, the experiment is designed to deliver high-speed, high-capacity communications both for the development of 5G and 6G networks as well as emergency response. The latter is critical in disaster-prone Japan—in January, communication lines around the Noto Peninsula on the Sea of Japan were severed following a magnitude-7 earthquake that caused over 1,500 casualties.

“This is the world’s first successful 5G communication experiment via the sky using the Q-band frequency,” said Hinata Kohara, a researcher with mobile carrier NTT Docomo’s 6G Network Innovation Department. “In addition, the use of 5G communication base stations and core network equipment on the aircraft for communication among multiple ground stations enables flexible and fast route switching of the ground [gateway] station for a feeder link, and is robust against propagation characteristics such as rainfall. Another key feature is the use of a full digital beamforming method for beam control, which uses multiple independent beams to improve frequency utilization efficiency.”

Doppler shift compensation was a challenge in the experiment, Kohara said, adding that the researchers will conduct further tests to find a solution with the aim of commercializing a HAPS service in 2026. Aside from SKY Perfect JSAT and NTT Docomo, the consortium includes Panasonic Holdings, known for its electronics equipment.

The HAPS push comes as NTT Docomo announced it has led another consortium in a $100 million investment in Airbus’ AALTO HAPS, operator of the Zephyr fixed-wing uncrewed aerial vehicle. The solar-powered wing can be used for 5G direct-to-device communications or Earth observation, and has set records including 64 days of stratospheric flight. According to Airbus, it has a reach of “up to 250 terrestrial towers in difficult mountainous terrain.” Docomo said the investment is aimed at commercializing Zephyr services in Japan, including coverage of rural areas and disaster zones, and around the world in 2026.