Research On Digital Signal Processing Technology In Free-Space Coherent Optical Communication System

Free Space Optical Communication(FSO)is a communication method which uses laser as information carrier and atmosphere as channel.It has the advantages of large capacity,good confidentiality,strong anti-interference ability and rich frequency resources.At present,FSO technology is mainly divided into two communication systems: intensity modulation/direct detection(IM/DD)and multi-order modulation/coherent detection.The multi-order modulation/coherent detection system is widely used in the existing optical communication terminals due to its higher detection sensitivity,more flexible modulation mode and better channel selectivity,and it is also an important research direction of the future FSO technology.However,in the atmospheric communication link,in addition to the noise introduced by the transceiver of the laser communication system,the attenuation of the light intensity caused by the atmospheric scattering and absorption effects,as well as the fluctuation of the light intensity and phase caused by the atmospheric turbulence effect,will have an adverse impact on the communication performance of the system.Therefore,the influence of atmospheric channel has become another important factor limiting the application of FSO system.With the development of high-speed digital signal processing technology,the signal distortion caused by the atmospheric channel and the transceiver can be compensated by the digital signal processing algorithm at the receiver,which is undoubtedly of great help to improve the communication capacity,extend the communication distance and enhance the anti-interference ability of the system.In this paper,the research significance and progress of FSO technology are described firstly.Then,based on the structure and realization principle of coherent optical communication system,the influence of atmospheric channel on the communication performance of the system is analyzed.Finally,the application of digital signal processing technology in free space coherent optical communication system is deeply studied.The main work of this paper is as follows:1.The characteristics of the atmospheric channel are introduced,including the attenuation effect caused by atmospheric scattering and absorption,and the turbulence effect caused by the random change of atmospheric refractive index.The light intensity and phase fluctuation caused by the atmospheric turbulence effect are mainly studied.The light intensity scintillation models under different turbulence intensity conditions are given.Through the outdoor experiments,the records of the measured data under the condition of weak turbulence scintillation model is verified,which provides the basis for the simulation system construction and parameter selection of free space coherent optical communication.2.The composition of the transmitter and receiver of the free space coherent optical communication system is introduced.The principle of the electro-optic modulator at the transmitting end as well as the homodyne detection,heterodyne detection and phase diversity detection system at the receiving end are analyzed.The SNR of various detection methods in the coherent optical communication system are analyzed and compared.3.The digital signal processing module and processing flow in the free space coherent optical communication system are studied.The orthogonal normalization algorithm,clock synchronization algorithm,frequency estimation algorithm and phase recovery algorithm are analyzed.Aiming at the problem that the phase noise introduced by atmospheric turbulence will affect the accuracy of frequency estimation,a new algorithm of frequency estimation based on zoning weighting is proposed.Finally,the feasibility of the algorithm and the performance of frequency offset estimation under different SNR and phase fluctuations are verified by simulation.4.The generation process of cycle slip is analyzed.It is pointed out that the phase noise caused by atmospheric turbulence is one of the important reasons for cycle slip.In this paper,The commonly used methods to reduce or correct cycle slip are studied,and their advantages and disadvantages are compared.A piecewise difference cycle slip correction method is proposed for free space coherent optical communication system,and the optimal thresholds of the algorithm is determined by theoretical derivation and calculation.Finally,the performance of the algorithm is analyzed by simulation under different noise sources.5.The error code and cycle slip test system which can realize cycle slip counting are designed.The QPSK free space coherent optical communication link is built by the Optisystem optical communication system simulation software,and the system parameter setting is determined based on the outdoor experimental data.The proposed zoning weighting frequency offset estimation method,piecewise difference cycle slip correction method and the error code and cycle slip test system are inlined into the simulation system through the self built module in the simulation software.The feasibility of the algorithm and the performance of the test system are simulated and analyzed.Finally,the indoor free space optical link was built through the instruments and equipment in the laboratory,and the coherent optical communication experiment of 2.5Gbps QPSK signal with and without simulated weak turbulence was realized respectively,and the BER performance of the system under the two conditions was analyzed.