Dimitar Kolev

This paper presents an experimental and numerical evaluation of advanced FEC with interleaving on a GEO satellite-to-ground laser downlink between JAXA's LUCAS and NICT Okinawa OGS. This evaluation compares DVB-S2 and 5G NR LDPC over multiple interleaving depths, utilizing received packets and turbulence data acquired in November 2023. The experiments show that DVB-S2 consistently decodes error-free for pre-FEC BERs degraded by turbulence, outperforming 5G NR LDPC and Reed–Solomon with multiple interleaving depths. The simulations, reproducing the measured atmospheric channels, corroborate the trend and indicate that deeper interleaving is required to compensate and avoid error floors. These results provide practical design guidance for selecting FEC and interleaving depth for satellite-to-ground laser links under turbulence conditions.

This paper presents seasonal analysis of atmospheric channel model for GEO satellite-to-ground laser links using LUCAS and NICT Okinawa OGS. Our research work experimentally reveals that received power data measured from three campaigns (during November to December 2021, during September 2022, and during November 2023) show distinct seasonal characteristics and are consistently well-fitted to a combined Lognormal and Beckmann (LNBM) distribution in warm and cool seasons. Coherence times also exhibit seasonal dependence and are generally longer for a large aperture telescope due to aperture averaging. Seasonal characteristics comparison also highlights that the wind speed is strongly associated with the seasonal atmospheric channel models in both seasons. In addition, this paper proposes the predictive workflow for link design of satellite-to-ground laser links.

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.

This paper describes experimental analysis of atmospheric channel model of Geostationary Earth Orbit (GEO) satellite-to-ground optical link by utilizing the Laser Utilizing Communication Systems (LUCAS) onboard the optical data relay GEO satellite. Our research work examines the effect of misalignment fading and various atmospheric turbulence conditions. These analytical results clarify that atmospheric channel model is well fitted to theoretical distributions with misalignment fading under various turbulence regimes. We also evaluate several atmospheric channel characteristics, including coherence time, power spectral density and probability of fade, in various turbulence conditions.

In this paper, we introduce our vision for the future non-terrestrial networks beyond 5G and discuss the possible implementation scenarios for free-space optical communication technology in it, especially regarding the satellite communications segment. We show and discuss some of the latest system developments globally in the field of optical communications for small satellites. Together with the latest projects in the National Institute of Information and Communications Technology (NICT) of Japan for payloads for small moving platforms, a specific trend in the field is clearly defined. We further show our newly developed ground station for the experiments with our High-speed Communication with Advanced Laser Instrument (HICALI) payload and discuss the main challenges in a satellite to ground optical link and our specific solution providing preliminary test results of the performance.

Free-space optical (FSO) systems are compulsory to realize high capacity and interference-free communication links from low-Earth orbit (LEO) satellite constellations as well as spacecraft and space stations to the Earth. To be integrated with high-capacity ground networks, the collected portion of the incident beam should be coupled into an optical fiber. To accurately evaluate the signal-to-noise ratio (SNR) and bit-error rate (BER) performance metrics, the probability density function (PDF) of fiber coupling efficiency (CE) must be determined. Previous studies have experimentally verified the CE PDF for a single-mode fiber, however, there is no such investigation for the CE PDF of a multi-mode fiber (MMF) in a LEO-to-ground FSO downlink. In this paper, for the first time, the CE PDF for a 200-μm MMF is experimentally investigated using data from an FSO downlink from the Small Optical Link for International Space Station (SOLISS) terminal to a 40-cm sub-aperture optical ground station (OGS) supported by a fine-tracking system. An average CE of 5.45 dB was also achieved given that the alignment between SOLISS and OGS was not optimal. In addition, using the angle-of-arrival (AoA) and received power data, the statistical characteristics such as channel coherence time, power spectral density, spectrogram, and PDFs of AoA, beam misalignments, and atmospheric turbulence-induced fluctuations are revealed and compared with the state-of-the-art theoretical background.

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.

National Institute of Information and Communications Technology (NICT) has initiated several R&D projects related to satellite laser communications. In this paper we present some of the recent activities in the field, including the development of the optical ground station for geostationary orbit satellite-to-ground optical feeder link experiments with High-speed Communication with Advanced Laser Instrument (HICALI) and the optical bench preliminary experiment results with stars, planets and international collaboration with the OSIRIS payload from the German Aerospace Center (DLR). Furthermore, we present two main fields for future research - deep learning implementation for cloud recognition for site-diversity technology, and space optical communications with Cubesats.

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.