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.

In this paper, the design of an atmospheric turbulence simulation system for free-space optical (FSO) communication is proposed. The system can accurately simulate repeatable atmospheric turbulent fading channels. It has a dynamic execution range of 30 dB with an execution rate of 1 MHz. The execution accuracy of the system is higher than 0.1 dB. In addition, the reliable TCP/IP protocol is used for communication, which ensures the convenience and versatility of the system. Experiments are carried out to investigate the performance of the system, and the results prove its superiority. Hence, the atmospheric turbulence simulation system can effectively simulate the influence of atmospheric turbulence channels on FSO communication systems.

In this study, we propose a non-data-aided algorithm of the cycle slip self-correcting carrier phase estimation (CSSC-CPE) which mitigates the cycle slips caused by blind CPE in the coherent wireless optical communication (WOC) system. The CSSC-CPE uses the output of CPE for cumulative averaging and selects the difference $\delta $ between two cumulative average segments as the discriminant parameter. The location and direction of cycle slips are determined by identifying the position and sign of the peak value of $\delta $ . The optimal thresholds for cycle slip detection could be derived from the probability density function (PDF) of $\delta $ obtained by calculation and deduction. Finally, numerical simulations and indoor experiments are carried out. The results show that CSSC-CPE can effectively eliminate cycle slips under weak turbulence condition. Compared with relative non-data-aided cycle slip correction algorithms, the CSSC-CPE achieves a better performance in suppressing the phase noise generated by atmospheric turbulence and laser linewidth, which enhances the accuracy of the cycle slip identification and lowers the SNR requirement when the cycle slip is not allowed.

In this paper, a new, to the best of our knowledge, scheme is proposed to mitigate the atmospheric turbulence effect in coherent free-space optical (FSO) communications with 8-quadrature amplitude modulation by employing amplitude compensation and phase recovery. The amplitude compensation and phase recovery algorithm in the scheme can significantly improve system performance without acquiring instantaneous channel state information or probability density function of a turbulence model. Numerical studies show that the bit error rate of the proposed scheme is four orders of magnitude lower than that of the system without any algorithm over a lognormal turbulence channel with normalized standard deviation of irradiance σ = 0.25 and phase noise with normalized variance σ ϕ 2 = 0.07 , when the frequency offset f o = 20 M H z , combined linewidth Δ f = 10 k H z , and average signal-to-noise ratio γ = 20 d B . Experiments are also carried out to investigate the performance of the scheme, and the results prove its superiority. Hence, this scheme can contribute to practical realization of the FSO system.