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Beyond-5G (B5G) technology plays a pivotal role in the next generation of communication infrastructure to support the future Society 5.0, a concept introduced in the 5th Basic Plan for Science and Technology by the Japanese Cabinet to define the long-term growth strategy for reconciling economic development with the resolution of social issues through the promotion of science and technologies. Free-space laser communication is a key element in boosting the data transmission capabilities required for B5G applications. The NICT will complete in 2024 the first fully functional prototypes of a series of miniaturized laser-communication terminals for multiple platforms. These terminals are designed to adapt to a wide range of requirements to address scenarios where laser communications can offer a competitive, enhanced solution compared to existing technologies. This paper provides an overview of these terminals' capabilities and the plans for their functional validation, as well as preliminary data from the first full-system tests. A number of innovations integrated into the terminals are introduced, such as the manufacture of the smallest miniaturized EDFA with integrated HPA and LNA and full space qualification to date, the first-ever integration of a beam-divergence control system in a practical communication terminal, the development of the most compact Tbit/s-class modem prototype documented in the literature, and the smallest gimbal design integrated in a lasercom terminal. Furthermore, this paper outlines the mid-term plans for demonstration in the most significant realistic scenarios, emphasizing the use of High-Altitude Platform Stations (HAPSs) and ultra-small satellites. [ABSTRACT FROM AUTHOR]

Free-space optical (FSO) communications can offer high-capacity transmission owing to the properties of the laser beams. However, performance degradation caused by atmospheric turbulence is an urgent issue. Recently, the application of polar codes, which can provide capacity-achieving error-correcting performance with low computational cost for decoding, to FSO communications has been studied. However, long-distance and real-field experiments have not been conducted in these studies. To the best of our knowledge, this study is the first to present the experimental results of polar-coded transmission over 7.8-km FSO links. Using experimental data, we investigated the performance of polar codes over atmospheric channels, including their superiority to regular low-density parity-check codes. We expect that our results will offer a path toward the application of polar codes in high-speed optical communication networks including satellites. [ABSTRACT FROM AUTHOR]

The Japanese National Institute of Information and Communications Technology (NICT) is currently developing a high-performance laser-communication terminal for CubeSats aimed at providing a high-datarate communication solution for LEO satellites requiring transmission of large volumes of data from orbit. A key aspect of the communication system is a high-power optical amplifier capable of providing enough gain to the transmitted signals to be able to close the link on its counterpart's receiver with the smallest impact in terms of energy and power on the CubeSat's platform. This manuscript describes the development of a miniaturized 2-W space-grade 2-stage erbium-doped fiber amplifier (EDFA) compatible with the CubeSat form factor, showing the best power-to-size ratio for a space-qualified EDFA to the best of the authors' knowledge. Performance results under realistic conditions as well as full space qualification and test are presented, proving that this module can support short-duration LEO-ground downlinks as well as long-duration intersatellite links. [ABSTRACT FROM AUTHOR]

A ground-to-space laser communications experiment was conducted to verify the optical interfaces between a laser communications terminal in an optical ground station and at optical payload onboard a geostationary satellite 38 0011 km away The end-to-end optical characteristics such as intensity, sensitivity wavelength, polarization, and the modulation scheme of optical signals as well as acquisition sequences of the terminals were tested under fairly good atmospheric conditions. The downlink's, bit error rate was on the order of [10.sup.-10] in spite of atmospheric turbulence. Atmospheric turbulence-induced signal fading increased the uplink bit error rate, the best value of which was 2.5 x [10.sup.-5] because the turbulent layer near the earth surface affects the uplink signal more than it does the downlink one. The far-field optical antenna patterns were measured through the ground-to-satellite laser links. The long-term statistics of the optical signal data is in good agreement with the calculated joint probability density function due to atmospheric turbulence and pointing jitter error effects, which means the stationary stochastic process can be applied to not only the static link analysis but also the dynamic link performance of the optical communications link. The equivalent broadened optical beam pattern should be used for the fading analysis even though the atmospheric coherence length is larger than the antenna diameter or the optical beam diameter of the transmitter. From these results, a more accurate dynamic link design of the optical communications link can be performed that would be useful for system designers, especially for designers of commercial systems. Index Terms--Atmospheric turbulence, free-space laser communications, long-term statistics, optical ground station, probability density function, random pointing jitter.

Free-space optics (FSO)-based, low-earth orbit (LEO) satellite-assisted Internet of vehicles (IoVs) has recently attracted research efforts worldwide. For the majority of Internet applications, transmission control protocol (TCP) is one of the most widely deployed protocols. Nevertheless, a complete understanding of the TCP performance over FSO-based satellite-assisted IoVs networks is still lacking in the literature. This is a highly challenging issue, as a consequence of the non-trivial interdependencies between transmission losses caused by the dynamics/uncertainty of atmospheric last-mile channels and the buffer overflows on the Internet, as well as their joint impact on the TCP congestion control mechanisms. This paper offers a comprehensive analytical framework that allows assessing the TCP throughput performance under the joint impact of congestion losses occurred in the Internet and transmission errors at the last-mile link from the satellite-to-vehicle. Internet access for emerging unmanned aerial vehicles (UAVs) is considered while the analysis can be also applied to other kinds of vehicles, which, in fact, require simpler channel models. The link-layer incremental redundancy hybrid automatic repeat request (IR-HARQ) protocol is employed at satellite-to-UAV channels to further improve the TCP throughput performance. Numerical results provide non-trivial findings and insights for the TCP performance in satellite/FSO-based IoVs, in which optimal UAV’s parameters are chosen to maximize the TCP throughput. [ABSTRACT FROM AUTHOR]

The demand for satellite communication increases with digitalization and globalization in the era of 5G and beyond; therefore, research and development of high-throughput satellites (HTSs) to increase the communication capacity and improve the flexibility of satellite communication systems is underway. Additionally, digital beamforming (DBF), which can control the directivity of multiple beams through high-speed digital signal processing, has received significant attention as a technology to improve the flexibility of resource allocation with HTSs. The principal control parameters in the DBF-based HTS, including the transmission power, beam gain, and placement location of each beam, cause alterations in the positional characteristics of the total throughput in the coverage area of an HTS. However, there are no power resource allocation models with dual control of these control parameters, based on the above positional characteristic. This study clarifies the effect of such control parameters on the total throughput in the coverage area of an HTS. We demonstrate a method for improving the flexibility of resource allocation to satisfy the geographic distribution of traffic requirements by constructing a power resource allocation model with a DBF-based fusion control in the HTS communication system. We evaluated the effectiveness of our proposed method through a simulation analysis using exponential annealing. [ABSTRACT FROM AUTHOR]

Free-space optical communications in space can realize broadband and large-capacity communications using smaller and lower-power-consumption onboard equipment. Recently, many mega-constellation systems have been planned and launched from various countries. Although the frequency spectra will soon be exhausted, optical communications have potentially wider bandwidths in the optical spectrum and are a viable alternative. Because optical communications do not have regulatory restrictions or licensing issues, this technology can revolutionize the space system architecture. This article describes the recent trends and future prospects of space laser communications. [ABSTRACT FROM AUTHOR]

Satellite communication systems have attracted considerable attention recently because they can communicate in various conditions, including terrestrial, airspace, and marine terrain. In addition, high-throughput satellites (HTSs) that allow high-speed and large-capacity communication are currently being launched. However, the allocation of communication resources to each beam is fixed in conventional HTSs, which exhibit low flexibility under state changes. This includes requests to the satellite communication systems or environmental changes around the systems. Therefore, the use of a digital channelizer in a satellite communication system can allocate frequency resources to each beam. However, even if a digital channelizer is used, conventional frequency resource allocation methods that consider inter-beam interference do not use the same frequency resource as the adjacent beam. As a result, the frequency resources cannot be used effectively. To alleviate this issue, we propose a frequency resource allocation method with inter-beam interference so that the satellite communication system can allocate frequency resources more flexibly. In addition, we extend the conventional flexibility analysis model such that it quantifies the flexibility more accurately. Finally, the effectiveness of the proposed method is demonstrated under state changes of the satellite communication system using the extended flexibility analysis model. [ABSTRACT FROM AUTHOR]

Special Section Editors Szymon Gładysz, Morio Toyoshima, Miranda van Iersel, Linda Thomas introduce the Special Section on Free Space Optical Communications. [ABSTRACT FROM AUTHOR]

Recent years have seen the development of a satellite communication system called a high-throughput satellite (HTS), which enables large-capacity communication to cope with various communication demands. Current HTSs have a fixed allocation of communication resources and cannot flexibly change this allocation during operation. Thus, effectively allocating communication resources for communication demands with a bias is not possible. Therefore, technology is being developed to add flexibility to satellite communication systems, but there is no system analysis model available to quantitatively evaluate the flexibility performance. In this study, we constructed a system analysis model to quantitatively evaluate the flexibility of a satellite communication system and used it to analyze a satellite communication system equipped with a digital channelizer. [ABSTRACT FROM AUTHOR]

The recent rapid growth of satellite-constellation programmes for remote sensing and communications, enabled by the availability of small-sized and low-cost satellites, has provided impetus for the development of high-capacity laser communication (lasercom) in space. Quantum-limited communication can enhance the performance of lasercom and is also a prerequisite for the intrinsically hack-proof secure communication known as quantum key distribution. Here, we report a quantum-limited communication experiment between a microsatellite (48 kg, 50 cm cube) in low Earth orbit and a ground station. Non-orthogonal polarization states were transmitted from the satellite at a 10 MHz repetition rate. On the ground, by post-processing the received quantum states with ∼0.146 photons per pulse, clock data recovery and polarization reference-frame synchronization were successfully achieved, even under remarkable Doppler shifts. The quantum states were discriminated by a receiver with four photon counters, with a quantum bit error rate below 5%, validating the applicability of our technology to satellite-to-ground lasercom and quantum key distribution. [ABSTRACT FROM AUTHOR]

In order to address the high throughput requested for both downlink and uplink satellite to ground laser links, adaptive optics (AO) has become a key technology. While maturing, application of this technology for satellite to ground telecommunication, however, faces difficulties, such as higher bandwidth and optimal operation for a wide variety of atmospheric conditions (daytime and nighttime) with potentially low elevations that might severely affect wavefront sensing because of scintillation. To address these specificities, an accurate understanding of the origin of the perturbations is required, as well as operational validation of AO on real laser links. We report here on a low Earth orbiting (LEO) microsatellite to ground downlink with AO correction. We discuss propagation channel characterization based on Shack-Hartmann wavefront sensor (WFS) measurements. Fine modeling of the propagation channel is proposed based on multi-Gaussian model of turbulence profile. This model is then used to estimate the AO performance and validate the experimental results. While AO performance is limited by the experimental set-up, it proves to comply with expected performance and further interesting information on propagation channel is extracted. These results shall help dimensioning and operating AO systems for LEO to ground downlinks. [ABSTRACT FROM AUTHOR]