The satellite-to-ground communication system is a significant part of future space communication networks. The free-space optical (FSO) communication technique is a prospective solution for satellite-to-ground communication. However, atmospheric optical turbulence is a major impairment in FSO communication systems. In this paper, to improve the performance and flexibility of a satellite-to-ground laser communication system, we put forward a novel modulation format identification (MFI) technique for an FSO communication system based on a convolution neural network (CNN). The results indicate that our CNN model can blindly and accurately identify the modulation format with classification accuracy up to 99.98% for random channel condition, including the strength of turbulence and signal-to-noise ratio (SNR) of additive Gaussian white noise (AWGN) ranging from 10dB to 30dB. Moreover, the CNN demonstrated robustness against atmospheric optical turbulence and suggested immunity to additive noise. Therefore, the proposed methodology proved to be a viable solution in the application of an FSO communication simulation channel, which can easily deal with the scene of fast modulation format switching and accurate identification to satisfy system requirements. Therefore, we hope this scheme can find a practical implementation in satellite-to-ground optical wireless systems.

The Gram–Schmidt orthogonalization procedure (GSOP) and Löwdin symmetric orthogonalization procedure (SYOP) are the two mainstream algorithms for the compensation of phase mismatch in an imperfect optical 90° hybrid. In this paper, we put forward an algorithm switching orthogonalization procedure (ASOP) according to the quality of in-phase and quadrature signals based on the Q value of the eye diagram with less computation. If the quality of the in-phase and quadrature signals has a significant difference, we use the GSOP and select the signal branch with better quality as the initial reference vector for orthogonalization. If they are of about the same quality, then we use the SYOP. We present computer simulations for a coherent free-space optical (FSO) quadrature phase-shift keying (QPSK) communication system and demonstrate the system improvement that can be achieved using the ASOP. Finally, we also show that the proposed ASOP scheme can contribute to the frequency offset and phase estimation of the FSO system in the environment of atmospheric turbulence.

Probabilistic shaping (PS) is a promising technique to approach the Shannon limit. In this paper, we design a practical coded modulation scheme based on PS to improve the capacity of coherent free-space optical (FSO) links with quadrature amplitude modulation (QAM), where the fading channel follows the Gamma-Gamma distribution. The aim of this paper is to optimize the probability mass function (PMF) of the QAM signal points to achieve the maximum channel capacity. Due to the complexity of the objective function, the heuristic algorithm was employed to solve the optimization problem. To the best of the authors’ knowledge, the closed-form pairwise error probability (PEP) is first derived with the non-uniform signals under the turbulence channel. In addition, we measure the average symbol error rate (SER) and post-FEC bit error rate (BER) by the Monte Carlo simulation method. The numerical simulation results of both capacity and BER show that the proposed PS scheme is better than the uniform distribution. The post-FEC BER results show that the proposed PS scheme provides significant gains compared with the uniform scheme.