The temporal characteristics of the free space optical communication (FSOC) turbulence fading channel are essential for analyzing the bit error rate (BER) performances and compiling the rationale of adaptive signal processing algorithms. However, the investigation is still limited since the majority of temporal sequence generation fails to combine the autocorrelation function (ACF) of the FSOC system parameters, and using the simplified formula results in the loss of detailed information for turbulence disturbances. In this paper, considering the ACF of engineering measurable atmospheric parameters, we propose a continuous-time FSOC channel fading sequence generation model that obeys the Gamma-Gamma (G-G) probability density function (PDF). First, under the influence of parameters such as transmission distance, optical wavelength, scintillation index, and atmospheric structural constant, the normalized channel fading models of ACF and PSD are established, and the numerical solution of the time-domain Gaussian correlation sequence is derived. Moreover, the light intensity generation model obeying the time-domain correlation with statistical distribution information is derived after employing the rank mapping, taking into account the association between the G-G PDF parameters and the large and small scales turbulence fading channels. Finally, the Monte Carlo numerical method is used to analyze the performances of the ACF, PDF, and PSD parameters, as well as the temporal characteristics of the generated sequence, and the matching relationships between these parameters and theory, under various turbulence intensities, propagation distances, and transverse wind speeds. Numerical results show that the proposed temporal sequence generation model highly restores the disturbance information in different frequency bands for the turbulence fading channels, and the agreement with the theoretical solution is 0.999. This study presents essential numerical simulation methods for analyzing and evaluating the temporal properties of modulated signals. When sophisticated algorithms are used to handle FSOC signals, our proposed temporal sequence model can provide communication signal experimental sample data generating techniques under various FSOC parameters, which is a crucial theoretical basis for evaluating algorithm performances.

In an all-optical double-hop free-space optical communication system, the outage probability and bit error rate are analyzed using a composite channel. The model involves atmospheric attenuation, atmospheric turbulence, pointing error, and fiber coupling efficiency. Based on analysis of the channel model and amplifier spontaneous emission noise, the outage probability and bit error rate are obtained. For an all-optical double-hop link, fiber coupling efficiency has an important impact on outage probability, especially at relatively short total link length. By compensating for three terms of wavefront distortions, the system outage probability significantly decreases. The communication performance is further improved by optimizing the receiving aperture diameter and beam width.

The application of ultra-high-bandwidth free-space optical (FSO) communication has become a widely researched hotspot. In remote areas without network coverage, the low altitude platform (LAP), such as airships, drones, etc., can provide a low-cost and rapid deployment emergency FSO communication platform covering hundreds of kilometers and has immeasurable application prospects. In our work, we experimentally demonstrated a line rate of 112 Gbit/s FSO communication at a distance of 104.8 km over Qinghai Lake. By adopting the methods of polarization multiplexing (PM), high-order quadrature amplitude modulation (n-QAM), automatic gain optical amplification (AGOA), etc., we have achieved a long-haul and high-bandwidth laser communication in atmospheric link with the optimum bit error rate (BER) of 3E-3 (zero-BER after error-correction) for a time-length of 50 s. It is particularly emphasized that the AGOA effectively can suppress the fluctuation of received optical power caused by atmospheric turbulence scintillation effect in Qinghai Lake, which solved the difficulty of detection-demodulation with ultra-high-speed laser signals. In addition, the efficient soft decision forward error correction (SD-FEC) also improved the stability of the communication link, which played a crucial role in the success of this experiment. Our design of non-adaptive optical (AO) system is very suitable for application in small-scale LAP and will potentially become an important foundation of the large-capacity nonterrestrial network (NTN) based on LAP. To the best of our knowledge, this experiment verified the feasibility of single-wavelength (1556.15 nm) 112 Gbit/s high-bandwidth laser signal transmission with a long-haul of 104.8 km in low-altitude horizontal atmospheric link for the world’s first time. To a large extent, our research findings also have great application value in the field of low-altitude economy.

In this paper, an atmospheric structure constant $C_n^2$ model is proposed for evaluating the channel turbulence degree of atmospheric laser communication. First, we derive a mathematical model for the correlation between the atmospheric coherence length $r_0$, the isoplanatic angle ${\theta }_0$ and $C_n^2$ using the Hufnagel–Valley (HV) turbulence model. Then, we calculate the seven parameters of the HV model with the actual measured $r_0$ and ${\theta }_0$ data as input quantities, so as to draw the $C_n^2$ profile and the ${\theta }_0$ profile. The experimental results show that the fitted average $C_n^2$ contours and single-day $C_n^2$ contours have superior fitting performance compared with our historical data, and the daily correlation coefficient between the single-day computed ${\theta }_0$ contours and the measured ${\theta }_0$ contours is up to 87%. This result verifies the feasibility of the proposed method. The results validate the feasibility of the proposed method and provide a new technical tool for the inversion of turbulence $C_n^2$ profiles.

The multi-hop amplify-and-forward free-space optical communication (FSOC) system is studied in random fog using the I-function, considering Gamma-Gamma atmospheric turbulence and Beckmann pointing error. Outage probability, average bit error rate and average ergodic channel capacity are obtained. Channel-state-information assisted relay performs better than fixed-gain relay under high transmitted power. Increasing the hop number significantly improves the performance. More hops are needed in medium fog than in light fog to achieve the same performance. In addition, on a single-hop link, the influence of fog channel on system performance is dominant, while atmospheric turbulence intensity, normalized jitter standard deviation and normalized boresight error have little effect on the system performance. However, on a multi-hop link, atmospheric turbulence intensity, normalized jitter standard deviation and normalized boresight error have serious effects on system performance. Compared with correcting the normalized boresight error, compensating the normalized jitter standard deviation greatly improves the multi-hop FSOC system performance. Furthermore, optimizing beam width can further improves the performance. To ensure good communication, the system should select a low-order modulation scheme.

The previous researches on signal compensation technology based on feedforward neural network (FNN) are all in underwater channel and fiber channel. In this paper, the signal compensation technology based on FNN is applied to 8Gb/s 4-PAM indoor free space optical communication (FSOC) system for the first time. Under 7% forward error correction (FEC) threshold, the FNN algorithm is compared with direct detection and traditional LMS filtering algorithm. The FNN-based method can significantly improve the receiver sensitivity and improve the performance of the communication system.