- World's first achievement of 2 Tbit/s free-space optical communication using small optical terminals that can be mounted on satellites and High Altitude Platform Stations (HAPS)
- Maintained stable optical communication between two types of small terminals separated by 7.4 km in an urban environment with atmospheric turbulence
- Marked a major step forward in the practical application of Non-Terrestrial Networks (NTN) for Beyond 5G/6G
Abstract
The National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.) has successfully demonstrated 2 Tbit/s Free-Space Optical (FSO) communication using small optical communication terminals that can be mounted on satellites and HAPS, marking a world first for this technology.
This experiment involved horizontal free-space optical communication between two types of small portable optical terminals developed by NICT: a high-performance FX (Full Transceiver) installed at NICT Headquarters (Koganei, Tokyo) and a simplified ST (Simple Transponder) installed at an experimental site 7.4 km away (Chofu, Tokyo). Despite the difficult conditions of an urban environment with atmospheric turbulence that disrupts laser beams, the system maintained a stable total communication speed of 2 Tbit/s via Wavelength Division Multiplexing (WDM) transmission of 5 channels (400 Gbit/s each). This is the first time in the world that terabit-class communication has been realized using terminals miniaturized enough to be mounted on satellites or HAPS.
Moving forward, NICT plans to further miniaturize the terminals for implementation onboard a 6U CubeSat. NICT aims to conduct free-space optical communication demonstrations at speeds of up to 10 Gbit/s between a Low Earth Orbit (LEO) satellite (altitude approx. 600 km) and the ground in 2026, and between a satellite and HAPS in 2027. Through these experiments, NICT will demonstrate compact, ultra-high-speed data communication capabilities and pave the way for the realization of Beyond 5G/6G Non-Terrestrial Networks (NTN).
Background
Free-Space Optical (FSO) communication, which transmits laser light through space without optical fibers, is attracting attention as a fundamental technology supporting high-capacity communication between the ground, the sky, and space. While demonstrations of FSO exceeding Tbit/s speeds have been advancing, primarily in Europe, previous experiments utilized large, stationary equipment in laboratory-style configurations. These configurations face challenges in meeting size and weight constraints for mounting on mobile platforms such as satellites or HAPS and in maintaining stable communication in fluctuating environments. Furthermore, in Asia, there have been no reports of FSO demonstrations exceeding the terabit level, with speeds reported to reach at most around 100 Gbit/s.
Achievements
The terminals used in this demonstration were designed for integration into microsatellites, including CubeSats. They meet size and weight constraints, distinguishing them from conventional laboratory-style configurations that use large stationary equipment. To achieve miniaturization, NICT strictly adhered to a design policy that fits within the severe Size, Weight, and Power (SWaP) constraints of CubeSats. NICT implemented three approaches:
Development of custom-designed components (e.g., a 9 cm-class telescope meeting optical quality requirements for the space environment).
Redesign and modification of commercial components (e.g., a miniaturized fine steering mirror improved to handle high-power laser beams in a vacuum).
Active utilization of existing components (e.g., repurposing high-speed optical transceivers for data centers and incorporating them into modems).
By implementing these approaches, NICT was able to reduce the size, weight, and power consumption of the entire device while maintaining all the required functions and minimizing the burden on the platform.
Additionally, to handle dynamic environments assumed for mobile operation, NICT implemented high-precision alignment using coarse acquisition and fine tracking. NICT also implemented its proprietary Beam Divergence Control (BDC) technology, which dynamically adjusts the laser beam divergence according to link conditions. This design, enabling stable communication in mobile environments, is a key feature of these terminals and distinguishes them from conventional fixed-station experimental equipment.
Furthermore, the developed terminals allow for flexible selection of configuration (ST or FX) and modem type (10 Gbit/s type or 100 Gbit/s type) based on communication requirements, as well as adaptive operation according to link conditions via internal adjustment functions.
This demonstration was achieved by overcoming technical challenges for mounting on mobile platforms—such as miniaturization of optical systems and high-precision, flexible beam control—through the development of novel functions such as variable transmission speeds and variable beam widths tailored to the communication environment. It represents a significant step toward the practical realization of Beyond 5G/6G Non-Terrestrial Networks.
Future Prospects
As the next step, NICT is preparing for a new experimental campaign in 2026 in which the small optical communication terminals (ST and FX) will be mounted on mobile platforms to simulate realistic links involving satellites and HAPS. In these experiments, NICT plans to verify the performance of the coarse acquisition and tracking system and the fine tracking system while both communicating terminals are in motion, demonstrating the feasibility of a multi-terabit optical backbone under dynamic conditions for Non-Terrestrial 6G Networks.
Simultaneously, NICT is working on a CubeSat mission scheduled for launch in 2026, aiming to verify a gimbal-less FX terminal (called CubeSOTA) combined with a 10 Gbit/s modem in orbit. While the CubeSat form factor cannot yet accommodate the power and volume of a 2 Tbit/s modem, NICT is proceeding with the miniaturization and environmental hardening of multi-Tbit/s modems for future on-orbit demonstrations. NICT aims to realize optical communication links in the multi-Tbit/s range between satellites, HAPS, and ground stations within the next 10 years.
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