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The utilization of drones as flying base stations (BSs) in the sixth-generation (6G) cellular networks has attracted much attention recently. In this context, free-space optical (FSO) systems could be deployed to provide high-capacity fronthaul/backhaul links between drones and ground BSs. Particularly, a drone-to-ground FSO communication link can be established by equipping the drone with a modulating retro-reflector (MRR) to modulate the incoming optical beam and reflect the modulated beam back along the same path. This helps to alleviate stringent pointing requirements from the drone while satisfying the limited size, weight, and power (SWaP) consumption requirements. Nevertheless, the underlying physical channel effects of this aerial retro-reflected FSO system have never been experimentally explored in the literature. In this paper, we report, for the first time, the experimental channel statistics of a drone-to-ground retro-reflected FSO link, offering practical insights into the angle-of-arrival (AoA) fluctuations at the receiver, the channel coherence time, the probability of fade, the level crossing rate, the average fade duration, and the time-frequency channel characteristics. Our results are expected to serve as practical sources of reference for the theoretical performance analyses and engineering designs of drone-based retro-reflected FSO systems.

In recent years, the necessity of free-space optical (FSO) communications has increased as a method for realizing high-speed communications between satellites and the ground. However, one disadvantage of FSO communications is the significant influence of the atmosphere. Specifically, FSO communications cannot be utilized under certain atmospheric conditions, especially in the presence of clouds. One of the solutions to this problem is the site diversity technique, which makes it possible to select a given ground station with better atmospheric conditions among a number of fixed ground stations. The other solution is to prepare a ground station that can be moved to a place with better atmospheric conditions. We applied the latter method and developed a transportable optical ground station in NICT. We utilize a realistic telescope diameter, which is about 30 cm at the maximum, capable of being set up quickly, and with a pointing accuracy of about 100 µrad. In addition, it is necessary to prepare a fine-pointing optical system that performs tracking with about 1/10 of the pointing accuracy of the telescope. In this paper, we report the results of the first performance test of the transportable optical ground station in NICT.

LEO-to-GEO intersatellite links using laser communications bring important benefits to greatly enhance applications such as downloading big amounts of data from LEO satellites by using the GEO satellite as a relay. By using this strategy, the total availability of the LEO satellite increases from less than 1% if the data is downloaded directly to the ground up to about 60% if the data is relayed through GEO. The main drawback of using a GEO relay is that link budget is much more difficult to close due to the much larger distance. However, this can be partially compensated by transmitting at a lower data rate, and still benefiting from the much-higher link availability when compared to LEO-to-ground downlinks, which additionally are more limited by the clouds than the relay option. After carrying out a feasibility study, NICT and the University of Tokyo started preparing a mission to demonstrate the technologies needed to perform these challenging lasercom links. Furthermore, to demonstrate the feasibility of this technique, an extremely-small satellite, i.e. a 6U CubeSat, will be used to achieve data rates as high as 10 Gbit/s between LEO and GEO. Some of the biggest challenges of this mission are the extremely low size, weight and power available in the CubeSat, the accurate pointing precision required for the lasercom link, and the difficulties of closing the link at such a high speed as 10 Gbit/s., Comment: 5 pages, 10 figures, 3 tables

In recent years, the necessity of satellite-to-ground optical communication has increased as a method for realizing higher-speed communications between satellites and the ground. However, one disadvantage of free-space optical (FSO) communication is the significant influence of the atmosphere. FSO communications cannot be utilized under certain atmospheric conditions, such as cloudy skies. One of the solutions to this problem is site diversity, which makes it possible to select a given ground station with better atmospheric conditions among a number of fixed ground stations. The other solution is to prepare a ground station that can be moved to a place with better atmospheric conditions. In this paper, we present the development of a transportable optical ground station currently being researched in NICT. In order to be transportable, it is necessary to build a system capable of travelling on public roads, installable in every place, and ready to be loaded on relatively-light trucks. For this purpose, a realistic telescope diameter is about 30 cm at the maximum, capable of being set up quickly, and with a pointing accuracy of about 100 μrad. In addition, it is necessary to prepare a fine-pointing optical system that performs tracking with about 1/10 of the pointing accuracy of the telescope. In this research, we will develop the base of the transportable optical ground station using the knowledge of mobile astronomical telescopes. With respect to tracking, we will develop a smaller and lighter fine-tracking system based on NICT’s previous experience. If necessary, we plan to develop an adaptive-optics system for correcting atmospheric disturbances to improve the fiber-coupling efficiency of the communication laser beam.

In National Institute of Information and Communications Technology (NICT) of JAPAN, an ultra high speed optical satellite communication equipment onboard the engineering test satellite IX has been developing. The satellite is planned to be launched to geosynchronous orbit in 2021. In this project, we are aiming for ultra high-speed data transmission at the world's highest level of 10 [Gbps] for both uplink and downlink between optical ground stations and geosynchronous satellite. This paper outlines the optical communication mission, the scheduled optical communication experiment, the examination of HICALI and the ground based system at the present time - the outline of the development situation is also explained.

Recently, satellite broadband communication services using Ka-band are emerging all over the world, some requiring capacities in excess of 100 Gbps. With the radio bandwidth resources becoming exhausted, high-speed optical communications can be used instead to achieve ultra-broadband communications. The National Institute of Information and Communications Technology (NICT) in Japan has over 20 years of experience in R&D of space laser communications with missions such as the Engineering Test Satellite VI (ETS-VI), OICETS, and SOCRATES/SOTA. We are currently developing a laser communication terminal named “HICALI”, aiming to achieve 10 Gbps-class space communications with a 1.5 μm-band laser beam between optical ground stations (OGSs) and the next generation high throughout satellite called ETS-IX with a hybrid communication system using radio and optical frequencies, which will be launched into the geostationary orbit in 2021. Moreover, we have studied laser communication terminals for terrestrial networks, as an alternative wireless system to radio frequency (RF) band. In 2014, we developed a terrestrial free-space optical communications network facility, named INNOVA (IN-orbit and Networked Optical ground stations experimental Verification Advanced testbed). Many demonstrations have been conducted to verify the feasibility of sophisticated optical communications equipment in orbit. We have conducted a feasibility study of a laser communication terminal for next-generation space networks following the above R&D trends in space communication networks, which is a high-speed, secure, small, and scalable laser communication terminal for optical ground stations (OGSs) and satellites or airborne terminals. In this paper, we describe the plan of NICT to develop a scalable laser communication terminal for next-generation space networks.

The experiment results on the inter-orbit laser communications between OICETS and a geostationary satellite and the results of two kinds of orbit-to-ground laser communications between OICETS and ground stations are summarized. The geostationary satellite for the inter-orbit demonstrations is the European Space Agency's geostationary satellite, ARTEMIS, and the ground stations for the orbit-to-ground demonstrations are of the National Institute of Information, and Communications Technology (NICT) in Japan and the German Aerospace Center (DLR), respectively. The descriptions of those experiments contain some statistically analyzed results as well as data samples measured during the demonstrations. The authors present the overview of these demonstration progresses and discuss on the results.

The tracking and pointing performance of the Laser Utilizing Communications Equipment (LUCE) equipped on the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) is presented. The operation characteristics of LUCE observed in the ground-satellite communication demonstrations are focused on. Since the angular movement of LUCE's optical antenna required in the ground-satellite trials exceeds the specification demanded in the inter-satellite communications, the marginal performance for proper tracking and pointing can be observed. During the ground-satellite communication trials, the ground stations found periodical discontinuances in the optical link. By looking at the LUCE's telemetries, the cause of the repeated breaks is revealed.

We present the results of the on-orbit free-space laser communications between the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) and the Advanced Relay and Technology Mission (ARTEMIS) geostationary satellite. We first introduce the history of developing OICETS and add some descriptions on characteristics of OICETS, where the performance of the optical communication terminal named the Laser Utilizing Communications Equipment (LUCE) is also stated. We summarize the on-orbit investigation results of LUCE's function and finally make a report on the inter-orbit laser communication experiment carried out by ARTEMIS and OICETS.

Next-Generation LEO System (NeLS) Research Center is now conducting continuous effort to demonstrate feasibility of key technologies for optical inter-satellite links in space. Evaluation of critical components for the NeLS optical terminal, such as Wide-range FPM, RX-collimator combined with a fine tracking sensor and devices for optical receiver, were carried out using trial models. In this paper, performance evaluation results are presented including mechanical environmental test and radiation test.

This paper presents outline of the optical terminal for Next-generation LEO System (NeLS) in-orbit demonstration, which will be conducted as part of Phase 2 of NeLS project. Two small satellites are assumed to launch into GTO orbit changing distance between them from 500km to 3000km. Acquisition and tracking experiments with a star or planet and 2.4Gbps data transmission between two SmartSat is also planned. The design of optical terminal is briefly presented.