On May 28, Japanese researchers announced a successful test of 5G communications equipment in the 38 gigahertz band from an altitude of 4 kilometers. This experiment was focused on developing an aerial relay backhaul using millimeter-wave band links between ground stations and a simulated High-Altitude Platform Station (HAPS), a radio station aboard an uncrewed aircraft that remains in the stratosphere for extended periods.
The test involved a Cessna aircraft, equipped with a 38 GHz 5G base station and core network device, flying out of Chofu Airfield in western Tokyo. Three ground stations, equipped with lens antennas featuring automatic tracking, participated in the experiment. The setup enabled communication between a ground station connected to the 5G terrestrial network and a terrestrial base station linked to a user terminal.
Shinichi Tanaka, a manager in SKY Perfect JSAT’s Space Business Division, stated, “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. 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 ground station antenna achieved a sharpest beam width of 0.8 degrees, and the trial successfully demonstrated a tracking method that consistently captured the Cessna within this angular range. Millimeter wave bands, such as the 38 GHz band, offer the highest data capacity for 5G and are ideal for crowded venues like stadiums and shopping centers. However, these signals can be attenuated by rain and other atmospheric moisture. To mitigate 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 project, which aimed at providing direct communication to user terminals, this HAPS trial focuses on creating backhaul lines for base stations. Led by Japan’s Ministry of Internal Affairs and Communications, the experiment aims to deliver high-speed, high-capacity communications for the development of 5G and 6G networks, as well as for emergency response—a crucial need in disaster-prone Japan. For instance, in January, a magnitude-7 earthquake severed communication lines around the Noto Peninsula, resulting in 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 NTT Docomo’s 6G Network Innovation Department. “Using 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.”
Kohara noted that Doppler shift compensation was a challenge during the experiment, and further tests will be conducted to find a solution, with the goal of commercializing a HAPS service by 2026. The consortium includes SKY Perfect JSAT, NTT Docomo, Panasonic Holdings, and other notable companies.
This HAPS initiative aligns with NTT Docomo’s announcement of leading a $100 million investment in Airbus’ AALTO HAPS, operator of the Zephyr fixed-wing uncrewed aerial vehicle. The solar-powered Zephyr 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, Zephyr can reach up to 250 terrestrial towers in challenging mountainous terrain. Docomo aims to commercialize Zephyr services in Japan, providing coverage for rural areas and disaster zones, as well as globally by 2026.
