Research On Spatial Diversity Technology Of Coherent Free-space Optical Communication Based On Receiver Array

Free-space optical communication is a kind of communication technology which uses laser as optical carrier to transmit videos,images and other information in free-space medium,such as vacuum or atmosphere.It combines the advantages of traditional wireless communication and optical fiber communication.It has been widely concerned and applied in the military and aerospace fields because of its superiority of high speed,large capacity,wide bandwidth and strong security.In addition,it has great potential to be used to solve the "last kilometer" problem in all-optical communication network.However,the free-space optical communication system is seriously influenced by the atmospheric channel,resulting in the transmission signal distortion,performance degradation,and even communication system interruption.In this paper,a low-cost and high-performance diversity receiving scheme is proposed based on the single-photodetector coherent receiver and spatial diversity technology.The performance of the system is improved from the structure and algorithm,which is verified by numerical simulation and experiment.Firstly,starting from the system structure and basic theory of free space optical communication system,this paper briefly introduces the structure principle of coherent optical communication system,the modulation and demodulation technology of signal and digital signal processing technology.In this paper,the physical mechanism and influence of the attenuation effect and turbulence effect in the atmospheric channel are described,the commonly used atmospheric turbulence models are summarized and analyzed,and the principle of phase screen method to simulate atmospheric turbulence is introduced.Secondly,based on the phase screen theory,combined with the principle of the optical transmitter and the single-photodetector direct-detection coherent receiver,and fully considering the noise interference and other factors,the full link simulation model of free space optical communication system is established.In this paper,through the analysis of the atmospheric turbulence channel,the propagation evolution of laser beam in the atmospheric turbulence channel is simulated,and the influence of some important parameters such as the atmospheric turbulence intensity and transmission distance is compared and analyzed.Then,the process of spatial light coupling into optical fiber system is simulated by using Fraunhofer diffraction theory and mode field matching principle.After that,the performance of optical communication system,which is based on single-photodetector direct-detection coherent receiver,is compared under different atmospheric turbulence conditions.And the simulation results show that it can achieve the performance similar to the zero difference coherent receiver.Finally,this paper uses a single-photodetector direct-detection coherent receiver array to effectively improve the performance of free space optical communication system through atmospheric turbulence channel.The full-link simulation model is applied to simulate the improvement effect of bit error rate of the receiver array compared with that of single receiver,and the system gain provided by equal gain combining algorithm and maximum ratio combining algorithm is compared and analyzed.Then,the performance improvement effect of 1 × 2 coherent receiver array is verified by experiment,the equal gain algorithm is used to combining the signals.When the bit error rate is less than 0.01,diversity reception can bring 1?3 dB gain to the optical communication system.