Japanese Consortium Achieves World's First Demonstration of 5G Communication from Altitude of 4km Using 38GHz Band

By simultaneously connecting and relaying any two ground stations that formed the backhaul line via the Cessna onboard station, it was possible to establish a data link between the two ground stations: one connected to the 5G terrestrial network (ground stations 1 & 2) and the other connected to the terrestrial base station communicating with the terminal (ground station 3). This setup demonstrated the backhaul line of the 5G terrestrial network via an aerial relay (dotted line in figure).

By applying this technology to the backhaul line between the base station communicating with the terminal and the end-to-end communication core network, end-to-end communication to the user terminal was achieved. In addition, handover between the aerial relay backhaul base station (base station 1) and the terrestrial backhaul base station (base station 2) was demonstrated.

To verify these two points, the end-to-end throughput between core communication network and the communication terminal was measured. Measurements were taken in both directions: from the end-to-end communication core network to the communication terminal (downlink) and from the terminal to the end-to-end communication core network (uplink).

The graph below illustrates the time variation of downlink throughput. In section 1, the aerial relay backhaul line was established, allowing data transmission from the end-to-end communication core network to the terminal. In section 2, data was transmitted over the terrestrial backhaul line. When transitioning from section 1 to section 2, the handover was performed without interruption of data communication. Furthermore, when transitioning to section 3, which was reconnected to the aerial relay backhaul, the handover was performed without any interruption in data communication. These results met the initial verification objective. It is also worth noting that the above verification was also successful in the uplink.

The trial focused on specific technical developments by replicating the flight conditions of HAPS in the stratosphere, some 20km above ground.
 

When using a fixed-wing model for HAPS, it is critical for the HAPS-mounted antenna to have precise control of the beam direction and to minimize frequency fluctuations due to the Doppler effect, especially when communicating at high frequencies in the millimeter-wave band.

Within this R&D framework, a technology for 5G NR communication was successfully demonstrated using the HAPS-mounted antenna and communication equipment in a flight environment, accurately directing the 38GHz band beam to three ground stations. This technology is adaptable to various flight conditions, adjusting to the aircraft's attitude, speed, direction, position, and altitude as it turns. 

Due to the effects of stratospheric winds and varying weather conditions, HAPS must continually change its trajectory. The development of high-precision tracking technology is essential for ground station antennas and communication equipment to accurately and continuously track the HAPS with the extremely narrow and sharp 38GHz band antenna beam. The ground station antenna used in the trial had a minimum beamwidth of 0.8 degree, enabling a tracking method that consistently tracked the Cessna aircraft within this angular range.

To integrate HAPS into 5G and 6G mobile networks, flexible line switching, and control will be necessary between the existing terrestrial network and the HAPS-based stratospheric network. The trial verified the handover between the terrestrial backhaul base station and the aerial relay backhaul base station using the 5G NR standard in the 38GHz band. The trial achieved seamless handover with no interruption in data transmission when the user equipment moved across the coverage areas of both base stations. This success demonstrated the feasibility of flexible link control technology that coordinates between the terrestrial network and the HAPS network.

Radio waves in the 38GHz band can be significantly weakened by rain, requiring practical countermeasures such as site diversity. The trial successfully tested an algorithm that automatically compensates for rain-induced attenuation, such as site diversity, in sync with the 5G network, making adjustments according to changes in link quality caused by rain.

It was confirmed that all initial development objectives were met. In addition, with a view to the future deployment of HAPS, the trial provided measurement data that will support the practical application and standardization of HAPS and identify areas for further technical development and performance enhancement. In the future, the four partners plan to use these results to realize 5G communication services from the stratosphere via HAPS, thus contributing to the realization of NTN.

 

The purpose of this R&D is to develop a HAPS system that can provide timely, high-speed, and high-capacity communication lines to meet the demand for backhaul lines to base stations in various applications expected in the era of 5G Evolution & 6G, and also for disaster response and recovery, using the broadband frequency of the millimeter wave band (Q/V band; 36.0-42.5GHz band centered on 38.0-39.5GHz) and HAPS. Another purpose is to improve the frequency utilization efficiency of HAPS communication using the millimeter-wave band and to establish elemental technologies for the practical application of HAPS systems.

To achieve the above, the development targets include a millimeter-wave communication device to be mounted on a HAPS and a corresponding ground station. Even when the HAPS position and flight attitude change, wireless communication must be ensured by controlling the direction of radio wave transmission while reducing interference from radio waves from other terrestrial systems. In addition, technologies such as switching between multiple ground stations (site diversity) will be used to ensure connectivity equivalent to that of mobile phone backhaul lines using the conventional stationary satellite system in Japan, even in the millimeter-wave band where the effect of rain attenuation can be significant.

The target for frequency utilization efficiency is to improve the throughput from around 3 bits/symbol to 4 bits/symbol, or more than 1.1 times the throughput of geostationary satellite systems. 

This R&D is being conducted as part of the Ministry of Internal Affairs and Communications' 'Research and Development for the Expansion of Radio Wave Resources (JPJ000254)’.