SoftBank Corp. Develops High-Capacity 6-Cell Capable HAPS Payload, Delivers 5G Connectivity From The Sky In Field Trial

• Newly developed payload integrates service-link and feeder-link equipment
• Payload successfully achieves wide-area and stable connectivity, with view to implementation in future HAPS commercial services

SoftBank Corp. (President & CEO: Junichi Miyakawa, "SoftBank") today announced it newly developed a 6-cell capable communications payload for High Altitude Platform Station (HAPS) stratospheric-based wireless communications that successfully delivered 5G connectivity from the sky to the ground in a field trial. The newly developed payload integrates equipment for a service link, which connects a HAPS platform with smartphones and other mobile devices, and a feeder link, which connects a HAPS platform to a ground gateway (a station connected to a base station on the ground).

In a field trial conducted in June 2025 on Hachijō Island, Tokyo, SoftBank mounted the newly developed payload on a light aircraft that flew at an altitude of 3,000m to relay connectivity. The configuration enabled SoftBank to successfully demonstrate end-to-end 5G communications between a base station and mobile device, as well as footprint fixation technology^*1 in a service area consisting of 6 cells. Based on the results of this field trial, SoftBank proved its newly developed payload can provide wide-area and stable communication services from the sky.

Building on the results of this field trial, SoftBank will make improvements to the newly developed payload, with the aim of achieving higher capacity and implementing the payload in HAPS commercial services.

Field trial background
SoftBank is researching and developing multi-cell capable payloads for HAPS platforms to realize wide-area coverage of up to 200km in diameter and high-capacity wireless communications. To date, SoftBank has successfully conducted a series of field trials to test key technologies, including the utilization of a cylindrical antenna^*2 that functions as a service-link antenna, footprint fixation^*3, area optimization^*4 and spectrum sharing^*5. To operate HAPS-based services, it is necessary to integrate both the service-link and feeder-link antennas into an aerial vehicle. SoftBank's newly developed payload can support 6 cells of area coverage and used a configuration that integrates equipment for the service link and feeder link, enabling the payload to successfully deliver 5G connectivity from the sky in a field trial. A portion of the field trial was conducted as part of "R&D on Technologies for HAPS Mobile Broadband Communications" (JPJ012368C07701), a proposal selected in 2023 by the National Institute of Information and Communications Technology ("NICT") in Japan for its "Innovative ICT Fund Projects for Beyond 5G/6G" program.

Field trial overview
In the field trial, SoftBank mounted the newly developed 6-cell capable payload onto a light aircraft that simulated a HAPS aerial vehicle. By relaying the feeder link (26 GHz band) between the gateway and the aircraft, as well as the service link (1.7 GHz band) between the aircraft and a mobile device, SoftBank conducted a field trial that achieved end-to-end 5G communication between a base station connected to a ground gateway and a mobile device. SoftBank also demonstrated footprint fixation in a 6-cell coverage area on the ground formed from an altitude of 3,000m in this trial.

System configuration

• The light aircraft circled over Hachijō Island at an altitude of 3,000m while relaying wireless uplink and downlink signals between the base station and a mobile device.
• The feeder-link equipment was equipped with functions including beam tracking, signal reception level compensation^*6, and Doppler shift^*7 correction, and wireless relay of 6-cell signals using radio waves in the 26 GHz band.
• Service-link equipment performed footprint fixation for each cell through digital beamforming control, and, by using radio waves in the 1.7 GHz band, formed a circular communication area on the ground consisting of 6 cells spaced every 60^° in azimuth, radiating from the service-link antenna (cylindrical antenna) mounted on the underside of the aircraft.

Field trial results
Using the above system configuration, SoftBank evaluated wide-area footprint fixation performance and throughput between the base station and mobile device at long range. Measurements of the signal strength reception level within the communication area confirmed that, along the 360^° azimuth circumference centered on the circling position of the light aircraft, the cell with the highest signal strength reception switched every 60^°. This indicates that even when the aircraft's position and orientation changed due to circling, the coverage area for each cell remained fixed on the ground.

In addition, using a mobile device to measure throughput, an average downlink speed of approximately 33 Mbps was achieved at a location 15 km from the center of the light aircraft's circling path. This confirmed that end-to-end 5G communication via the service link and feeder link is possible even at the edge of the coverage area. At a point 15km away from the center of where the aircraft circled, the elevation angle to the aircraft at an altitude of 3,000m was approximately 11 degrees. Since this is equivalent to the elevation angle to a HAPS at an altitude of 20km observed from a point 100km away (the edge of its coverage area), SoftBank confirmed that communication can be maintained even under conditions equivalent to the periphery of a HAPS coverage area.

SoftBank will leverage the results and know-how obtained from this field trial to further improve the payload, with the aim of achieving higher capacity and implementing the payload in HAPS commercial services.

^*1 Footprint fixation technology is a technology that fixes a communication area (footprint) for each cell formed on the ground by changing the direction of radio waves emitted from a communications payload mounted on a HAPS platform as it rotates or changes roll, pitch and yaw.

^*2 For more details, see SoftBank Corp.'s notice dated September 2, 2021: SoftBank Corp. and HAPSMobile Accelerate High Altitude Platform Station (HAPS) R&D to Provide Stable and High-quality Connectivity for Future Commercial Services.

^*3 For more details, see SoftBank Corp.'s press release dated June 22, 2022: SoftBank Corp. Successfully Tests High-altitude Tethered Balloon Base Station System that Utilizes Footprint Fixation Technology.

^*4 For more details, see SoftBank Corp.'s press release dated December 20, 2023: SoftBank Corp. Successfully Tests Coverage Area Optimization Technology that Realizes Communication Capacity Maximization for HAPS.

^*5 For more details, see SoftBank Corp.'s press release dated June 26, 2024: SoftBank Corp. Successfully Demonstrates Nullforming Technology to Achieve Spectrum Sharing Between HAPS and Terrestrial Base Stations.

^*6 Signal reception level compensation is a control function that maintains a constant signal reception level by adjusting for variations in the radio propagation path caused by tilt or movement of the aircraft.

^*7 Doppler shift is a phenomenon in which the movement of a moving object (either the transmitter or the receiver) causes a frequency misalignment with radio waves, thereby affecting communication quality.