Reseaches On Several Key Technologies For Free Space Optical Communication Based On Digital Coherent Detection

Free space optical communication is a way of communication using light as carrier to transmit information in space.At present,there are two main communication schemes in space optical communication: the intensity modulation/direct detection(IM/DD)and the multi order modulation/coherent detection.Compared with IM/DD system,the main advantages of the multi order modulation/coherent detection system are: Compared with the IM/DD system,the main advantage of the multi modulation / coherent detection system is that it's detection sensitivity is high,and it can reach the level of the approximate quantum noise limit.Compared with the direct detection system,the theoretical sensitivity limit is increased by at least 20 d B.The modulation mode is flexible,in addition to amplitude modulation,frequency modulation and phase modulation can be performed;in addition,coherent detection also supports polarization multiplexing,which makes coherent optical communication has higher single wavelength spectrum utilization.The channel selectivity is very good,and it can realize intensive wave division multiplexing and the channel interval can be as small as 1GHz to achieve ultrahigh capacity information transmission.More importantly,with the help of high speed digital signal processing technology,the distortion compensation of the signal can be realized in the receiving end of the system.Therefore,coherent detection has more advantages in free space high speed and long distance transmission,and it has wide application prospects.It is the development direction of free space optical communication.In this paper,the research significance,research progress and key technologies of free space coherent optical communication system are summarized,and the turbulence characteristics of atmospheric channel are introduced.At the same time,starting from the principle of coherent detection,the key technologies of coherent optical communication system are summarized,and some digital signal processing algorithms for coherent receivers are studied in detail.Therefore,this paper focuses on the study of space optical communication based on digital coherent detection technology.The main work is as follows:1.The characteristics of atmospheric channel including atmospheric attenuation effect and atmospheric turbulence effect are introduced.According to the mathematical model of the intensity scintillation,the VPI optical communication simulation software is used to design the logarithmic normal and Gamma-Gamma intensity scintillation simulation module.This paper focuses on the analysis of the method of accepting or rejecting to generate the random distribution of Gamma-Gamma,and the correctness of the scintillation module under weak turbulence is verified by measured data from outfield.It provides a channel simulation environment for subsequent building of coherent detection simulation environment.The influence of atmospheric turbulence on the received SNR of coherent optical communication system is also analyzed.A mathematical model of SNR under the condition of plane wave weak turbulence is obtained.The model takes into account the influence of the intensity and the variance of phase fluctuation on the system performance.2.The modulation and demodulation technology of atmospheric coherent optical communication is studied,including the basic principle and noise analysis of coherent reception technology.The principle of frequency offset estimation and phase correction algorithm to compensate the frequency and phase of the signal is emphatically described.3.The electrical part of the transmitting and receiving terminals of the atmospheric digital coherent detection system is designed,and the basic components of each part of the system are analyzed.The appropriate devices are selected for the key technology points involved in the atmospheric coherent detection system,and the key parameters are introduced.4.Orthogonal normalization,frequency offset estimation and phase correction algorithms for digital signal processing of digital coherent detection are analyzed.The Gram-Schmidt orthogonalization and L?wdin symmetric orthogonalization are introduced.On the basis of two algorithms of orthogonalization and symmetric orthogonalization,a scheme of algorithm switching is designed in this paper.The above two orthogonal methods can be well combined so that the signal can reach a better orthogonal state.In the phase correction part,an improved two-stage phase correction algorithm is proposed for weak turbulence conditions.The algorithm can effectively improve the system's tolerance to laser linewidth and turbulence disturbance.5.The BPSK/QPSK atmosphere coherent detection simulation system is built by using VPI simulation software,and the principle of BPSK modulation signal generation and the error rate performance of QPSK system under different conditions are analyzed.6.We designed and built an experimental platform for free space coherent optical communication.We carried out the indoor optical transmission experiment of 2.5Gbps QPSK modulation,and the performance of the system under different receiving power is verified.This paper has done related research and practical work in the aspects of atmospheric channel,coherent detection theory,system analysis,back-end processing algorithm and experimental platform construction,which has important guiding significance and reference value for the design of atmospheric coherent optical communication system.