| With the rapid development of global information technology and the increasing demand for wireless communication with large-capacity and high-speed data transmission,free space optical(FSO)communication has gained wide attention and application in specific fields such as satellite laser communication with its advantages of high bandwidth,low power consumption and spectrum license-free.The polarization multiplexing Mary quadrature amplitude modulation(PM-MQAM)scheme based on coherent reception has become an effective solution for long-range,highspeed,and large-capacity FSO communications with its high receiver sensitivity and spectral efficiency.However,the power budget of coherent FSO communication system is very tight due to atmospheric attenuation,beam divergence and atmospheric turbulence,especially the atmospheric turbulence effect can seriously deteriorate the system performance and may lead to link interruption,while diversity is considered as one of the effective techniques to suppress the atmospheric turbulence effect.In this thesis,we design a PM-MQAM-FSO communication system scheme based on spatial diversity and related digital signal processing(DSP)damage compensation algorithm to suppress atmospheric turbulence effect and improve receiver sensitivity.The details of the research are as follows.1.By analyzing and estimating various link losses in FSO communication system,a 10GBaud PM-MQAM-FSO spatial diversity reception simulation system with fiber coupling efficiency is built,and the system performance is simulated and studied in terms of turbulence intensity,merging method,number of diversity channels,transmission distance and reception aperture.In order to better describe the effect of atmospheric turbulence on optical intensity,phase and fiber coupling efficiency,the system uses a phase screen model to simulate the turbulent channel.The simulation results show that the spatial diversity receiver system can effectively suppress the atmospheric turbulence effect and improve the receiver sensitivity of the coherent FSO communication system.2.To address the problem that the relative phase difference between multiple branches in a coherent FSO spatial diversity system degrades the diversity gain,a spatial diversity reception scheme based on the optimal branch block phase correction is proposed.After frame synchronization and frequency offset pre-compensation for each diversity branch,block phase correction is performed with the optimal branch as the reference to compensate for the relative phase difference between different branches introduced by atmospheric turbulence and lasers.The simulation results of 40Gbps PM-16QAM-FSO spatial diversity reception system show that the proposed scheme can effectively compensate the relative phase difference between branches,improve the diversity gain and reduce the outage probability of the system.An offline experimental platform is also built to verify the effectiveness of the proposed scheme.Simulation and experimental results show that at the FEC threshold of 3.8 x 10-3,compared with single-branch reception,the diversity system cannot work properly when no phase correction is performed;when the optimal branch block phase correction is used,the receiver sensitivity of the diversity system with 2-branch reception under strong turbulence condition(@Cn2=3 × 10-12 m-2/3)improved by 5.22 dB and 4.88 dB,respectively.3.To address the contradiction between the high performance and low complexity requirements of mainstream blind frequency offset estimation algorithms,a joint processing algorithm for frame synchronization and frequency offset estimation based on training sequences is proposed.By designing training sequences with different structures in X and Y polarizations,so that the frequency offset estimation can be accomplished while ensuring the frame synchronization performance.The simulation results of the 10GBaud PM-4/16QAM-FSO spatial diversity reception system show that the proposed algorithm can achieve similar estimation accuracy and larger estimation range with lower complexity than the frequency offset estimation algorithm based on the Fourth-power Fast Fourier Transform(4th-FFT).Meanwhile,compared with the conventional training sequence-based frequency offset estimation algorithm,under the FEC threshold of 3.8 × 10-3,the receiver sensitivity of the proposed algorithm-based PM-4/16QAM-FSO spatial diversity system with 2branch reception is improved by 1.43 dB and 2.54 dB under weak turbulence condition(&Cn3=1e-16m-2/3),respectively;and by 2.09 dB and 3.41 dB under strong turbulence condition(@Cn2=1e-14m-2/3),respectively. |
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