characteristics such as large capacity, high condentiality, and low power consumption. If current radio wave communications can be replaced by optical communications, then we can not only realize larger capacity commu-nication links, but we will also contribute to solving the shortage of radio wave resources.With ETS-9 we plan to realize a 10 Gbps class communication link between a geo-stationary orbit and the ground, and we are promoting the development of HIgh speed Communication with Advanced Laser Instru-ment (HICALI) for that purpose. HICALI has a 15 cm diameter antenna and a 1550 nm (C-band) wavelength laser, which conforms to the international standards discussed by the Consultative Committee for Space Data System (CCSDS) standardization body. We also aim to establish a screening process for space-based usage of optical network devices used on the ground. On the other hand, light is signicantly impacted by the atmosphere and clouds, so disturbances to or shielding of the optical signal will lead to deteriorated commu-nication quality even if a large-capacity com-munication link can be constructed. As such, we are also conducting research on adaptive optics technologies that reduce the effects of atmospheric uctuations and site diversi-ty technologies that switch between optical ground stations situated in multiple locations.■Research on networks connecting very large numbers of communication satellitesThere have been satellite constellation sys-tems planned in recent years that can provide a global communication network with large numbers of communication satellites, and it is expected that such large-scale satellite com-munication systems in the future will have sat-ellites with different orbits, frequencies, sizes, and power, etc. Additionally, it is necessary to optimize the network to always provide a stable communication link according to tem-porally varying user numbers and their com-munication requests, the number of satellites that can be used, and the propagation state of their electromagnetic waves.The author proposed and developed a model for efciently operating a large-scale satellite communication system and a control scheme for dynamically optimizing the entire network (Figure 3). With this model, satellites with different orbits and different frequencies can be added to the system in the same frame-work. Furthermore, it is possible to avoid fre-quent changes in network structure that are a burden to satellite communication operators.Based on this model, optimal control schemes, and operational scenarios for large-scale satellite communication systems, the ef-fectiveness of the proposed method was con-rmed via simulation, and the author believes that performance can be further improved even if the system expands and the number of satellites gradually increases [3].■Future prospectsIn addition to the technologies introduced here, at NICT, we are also conducting re-search on integrating satellite communications and 5G, and we are considering using ETS-9 as a demonstration testbed. By realizing satel-lite communication systems that can provide large-capacity and exible communication links, we believe that user communication re-quests will be possible anywhere, on aircraft, ships, remote islands, deserts, mountains, and even planets. In order to realize such a world, NICT will continue promoting research on new satellite communication systems and will disseminate the results.CCommon communication subsystemFigure 1 ETS-9 communication missions (main subsystems being developed by NICT) Figure 3 Image of network optimizations for large-scale satellite communication systemsA: Simulation of communication satellite beam placementFigure 2 Simulation results using the frequency exibility control algorithm B: Simulation results of frequency exibility function9NICT NEWS 2021 No.1
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