SoftBank Corp. (“SoftBank”) and its subsidiary HAPSMobile Inc. (“HAPSMobile”) are advancing research and development (R&D) activities in preparation for commercial services that deliver connectivity from the stratosphere with stable and high-quality network coverage using High Altitude Platform Station (HAPS) technologies. The companies’ advanced technology research achievements include developing a cylindrical antenna and a rotating connector.
HAPSMobile is developing Sunglider, a HAPS solar-powered unmanned aircraft system (UAS), to provide telecommunications connectivity from the stratosphere to the ground while flying in a circle. One challenge is that the communication area (footprint) projected to the ground cannot be fixed because the footprint shifts as the aircraft turns. To solve this problem, researchers at SoftBank and HAPSMobile developed a cylindrical multi-beam phased array antenna, or cylindrical antenna, that fixes the footprint. By using digital beamforming*1, the technology fixes the footprint on the ground by shifting the direction of radio beams as the aircraft rotates. The cylindrical antenna comprises antenna elements attached both horizontally and vertically, and these can be controlled individually, making it possible to control the beam three-dimensionally in any desired direction. In addition to adapting to the yaw direction of the UAS, the cylindrical antenna can compensate for pitching, rolling, ascending, descending and other aircraft movements. In this way, the technology makes it possible to fix communication areas from the stratosphere.
A single HAPS UAS can cover a wide area or diameter of up to 200 km. Accordingly, it can be assumed that within this wide service area, communication needs will vary by location. For example, densely populated areas will have different capacity requirements than mountainous areas that are sparsely populated. By using a cylindrical antenna with digital beamforming technology, the shape and direction of radio beams can be controlled to focus on locations with high population densities and high communication traffic to achieve what is called the automated optimization of communication service areas. This technology optimizes the shape and direction of radio beams depending on peripheral user distribution wherever a HAPS is deployed, and it enhances the communication speeds of each mobile terminal and improves overall network efficiency. This technology is also expected to be used in communication service areas where there are changes dynamically in user distribution, such as when a disaster occurs or when there is a large-scale event. This research was recognized by the Japan’s Institute of Electronics, Information and Communication Engineers (IEICE) and received the Young Researcher’s Award*2 in March 2021.
Automated optimization of communication coverage area using a cylindrical antenna
Both the extension of communication service areas and increasing communication capacity will be required when deploying commercial HAPS services. Accordingly, SoftBank and HAPSMobile are conducting R&D to enhance communication capacity by extending digital beamforming using cylindrical antennas, and applying Massive MIMO*3, a technology used in 5G, to further increase communication capacity. Cylindrical antennas can also cover wider areas than the planar antennas typically used in Massive MIMO, and at the same time enable uniform communication quality and higher capacity. This research was recognized by IEICE and received the Active Research Award*4 in May 2021.
Massive MIMO using a cylindrical antenna
To provide stable connectivity from the stratosphere to the ground, stabilizing the communications area position is an important factor. Since a UAS circles above in the stratosphere, the communication antenna direction also shifts as the aircraft turns. As a result, the “moving cell issue” occurs, meaning it is difficult to secure a fixed communication area and provide stable connectivity. To solve this problem, SoftBank and HAPSMobile developed a rotating connector as another footprint fixation technology. The rotating connector is a device that is attached between the communication antenna and the radio housed in the communication equipment (payload) of the aircraft. Since the contact point of the cable that connects the two can rotate endlessly, the direction of the communication antenna can be fixed, even as the aircraft shifts. By utilizing the rotating connector, cable twisting can be prevented, even when multiple antennas are mounted and each of them are paired with multiple radios. By fixing the direction of the communication antennas, the moving cell issue can be solved to provide stable communication services.
In preparation for commercial HAPS services, SoftBank and HAPSMobile are conducting R&D in a variety of areas. In addition to conducting research on communication areas and network quality, both companies are working to enhance Sunglider's durability in the demanding conditions of the stratosphere, extend its flight times to enable even longer flight durations and reduce communication equipment (payload) weights. As part of these efforts, SoftBank established the "SoftBank Next-generation Battery Lab” in June 2021 to promote the research and development of next-generation batteries with high energy densities that are lightweight and safe, with a view to applying them to HAPS and other areas.
On the global stage, SoftBank and HAPSMobile are founding members of the HAPS Alliance, an industry association that comprises telecommunications, technology, aviation, and aerospace companies, as well as public and educational institutions. Working closely with Alliance members, SoftBank and HAPSMobile are engaging regulatory authorities in various countries, building a HAPS ecosystem and promoting the standardization and interoperability of HAPS technologies.
HAPSMobile has a proven track record of achievements that include flying its own UAS, Sunglider, in the stratosphere for an extended period of time and successfully making video calls with LTE connectivity delivered from the stratosphere. Both SoftBank and HAPSMobile possess advanced technology and insights related to HAPS. HAPS is also a key part of SoftBank’s Non-terrestrial Network solutions portfolio that provides connectivity from space and the stratosphere. Both companies will continue to work to build global communication networks and prepare for commercial HAPS services.