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 paper describes the recent trends and future prospects of space laser communications.

The polarization characteristics of an artificial laser source in space were measured through space-to-ground atmospheric transmission paths. An existing Japanese laser communication satellite and optical ground station were used to measure Stokes parameters and the degree of polarization of the laser beam transmitted from the satellite. As a result, the polarization was preserved within an rms error of 1.6°, and the degree of polarization was 99.4±4.4% through the space-to-ground atmosphere. These results contribute to the link estimation for quantum key distribution via space and provide the potential for enhancements in quantum cryptography worldwide in the future.

Free-space laser communication systems use optical-fiber-based technology such as optical amplifiers, receivers, and high-speed modulators. In these systems using single-mode fibers, the fiber coupling efficiency is one of the most significant issues to be solved. Optimum relationships between a focused optical beam and mode field size of the optical fiber in the presence of random angular jitter are discussed in relation to fiber-coupled optical systems. Maximum fiber coupling efficiency is analytically derived with the optimum Airy disk radius normalized by the mode field radius as a function of random angular jitter. The fade level of fiber-coupled signals at desired fade probability is investigated. It is shown that the average bit error ratio significantly degrades with the random angular jitter normalized by the mode field radius larger than about 0.3 when the Airy disk size is optimally selected.

The fading channel model for generating a random time-varying signal based on the atmospheric turbulence spectrum for space-to-ground laser links is discussed. The temporal frequency characteristics of the downlink are theoretically derived based on the von Karman spectrum. The rms wind speed based on the Bufton wind model is used as the transverse wind velocity, which makes the simulation simple. The time-varying signal is generated as functions of the receiver aperture diameter and the rms wind speed. The simulated result of the time-varying signal is presented and compared with the gamma-gamma distribution based on the scintillation theory in a moderate-to-strong-turbulence regime.

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.

The use of free-space optical (FSO) systems as secure transmission media has recently attracted research efforts worldwide. However, their secrecy performance may be compromised by the presence of an adversarial eavesdropper. In addition, misalignments between transceivers could severely affect the legitimate FSO channel and increase eavesdropping risks. This paper, for the first time, offers a complete framework to analyze the impact of an eavesdropper’s location on the secrecy performance of terrestrial FSO systems under generalized misalignments and atmospheric turbulence conditions. Particularly, the probability density functions (PDFs) of the eavesdropping channel are newly developed and presented in closed-form expressions. Capitalizing on the derived PDFs, secrecy performance metrics in the physical-layer security (PLS) and intensity modulation/direct detection (IM/DD) continuous-variable quantum key distribution (CV-QKD) systems can be analytically analyzed, incorporating all combined effects of the atmospheric turbulence, transceiver misalignments, receiver noises, and the eavesdropper’s location. Monte-Carlo (MC) simulations are also implemented to corroborate the analytical results.