The performance of coherent free-space optical (CFSO) communication with phase modulation is limited by both phase fluctuations and intensity scintillations induced by atmospheric turbulence. To improve the system performance, one effective way is to use digital phase estimation. In this paper, a CFSO communication system with quadrature phase-shift keying modulation is studied. With consideration of the effects of log-normal amplitude fluctuations and Gauss phase fluctuations, a two-stage $M$th power carrier phase estimation (CPE) scheme is proposed. The simulation results show that the phase noise can be suppressed greatly by this scheme, and the system symbol error rate performance with the two-stage $M$th power CPE can be three orders lower than that of the single-stage $M$th power CPE. Therefore, the two-stage CPE we proposed can contribute to the performance improvements of the CFSO communication system and has determinate guidance sense to its actual application.

In this paper, On-Off Keying (OOK) free-space optical (FSO) communication with coherent detection and double adaptive detection thresholds (CD-DADT) is proposed. The 3-bit encoding and double adaptive detection thresholds presented in the scheme can significantly improve the system performance, and there is no need of acquiring the instantaneous channel state information (CSI) and probability density function (PDF) of turbulence model. In order to enhance the accuracy of the threshold even further, decision-aided method is offered to set the second detection threshold. The PDFs of the detection thresholds and the average bit error rate (BER) of the system are theoretically derived. Numerical studies show that the performance of the system is comparable to the performance of the idealized adaptive detection system and the loss of signal-to-noise ratio (SNR) performance is only 1 dB at a BER of 10-9 over a lognormal turbulence channel with normalized standard deviation of irradiance σ=0.25 and phase noise with normalized variance $\sigma;_{\oslash}^{2}$ 2=0.07. Hence, this scheme can contribute to the performance improvement of the FSO system and its practical realization.

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.

In this paper, we present a high-speed visible light communication (VLC) system utilizing a high-fidelity white light source based on red, green, blue, and yellow (RGBY) four-wavelength division multiplexing (WDM). The proposed white light source integrates red, green, and blue laser diodes (RGB LDs) with a yellow diode-pumped solid-state laser (Y-DPSSL) to achieve simultaneous illumination and data transmission. A total transmission capacity of 16 Gbps is achieved by encoding the RGB LDs via 16 quadrature amplitude modulation discrete multitone modulation (16-QAM DMT) data. The received bit error rates (BERs) are 1.6 × 10⁻⁴ for the red LD, 1.1 × 10⁻⁴ for the green LD and 2.9 × 10⁻³ for the blue LD, all of which are less than the forward error correction (FEC) threshold of 3.8 × 10⁻³. To address the demand for high-fidelity illumination, the Y-DPSSL in the white light source is switched on to provide yellow-orange luminescence and then fill the red-green spectral gap. By adjusting the power ratio of RGBY lasers, a white light with a correlated color temperature (CCT) of 4195 K, a color rendering index (CRI) of 82.4 and Commission Internationale de l'Eclairage (CIE) coordinates of (0.3784, 0.3976) is obtained. These values are superior to the currently reported tricolor-laser-based white light source for illumination. This work paves the way toward the technological development of high-fidelity white light illumination while simultaneously achieving high-speed VLC.