| In recent years,with the rapid development of my country’s high-throughput satellite technology,satellite-based deep space exploration,air-to-ground meteorological monitoring,marine environment monitoring,satellite network positioning and other explorations have been more and more widely used.A large number of satellites Images and monitoring data need to be transmitted back to the monitoring station in time,so people have higher requirements for satellite-to-ground communication.Traditional satellite-to-ground communication uses microwave communication,which has the problems of small bandwidth,high delay,and low rate.Satellite-to-earth communication technology is developing in the direction of large-capacity,high-speed FSO,but the light intensity scintillation effect brought by atmospheric turbulence has seriously affected the receiver sensitivity of the FSO system and advanced code modulation technology is one of the key technologies to improve the light intensity flicker effect.Probabilistic Shaping technology,as a branch of code modulation technology,can alleviate the effect of light intensity scintillation.The paper mainly designs the low-complexity probability shaping scheme in the FSO coding system;and optimizes the calculation method of LLR.The main work of the paper is as followsFirst,the paper established a 40Gbps BICM-PM-16QAM coded modulation FSO simulation system based on the Optilux optical communication component package and Matlab programming.Since the system includes modules such as bit interleaving,LDPC encoding and decoding,and coherent receiving DSP compensation,the paper chooses a less complex Gamma-Gamma statistical channel model to simulate the influence of atmospheric turbulence.The paper simulates the link loss and system bit error rate of the free space optical communication system under different atmospheric conditions.The simulation results show that the simulation system established in the paper can reflect the performance of the 40Gbps BICM-PM-16QAM coded modulation communication system under the real atmospheric channel.Second,the probability shaping scheme of high-order modulation system uses CCDM,which complexity is high。According to the characteristics of Gallager constellation mapping,the paper maps the two-dimensional uniformly distributed bit stream to Gallager constellation points according to the non-uniform mapping table to achieve low-complexity probability shaping.Two different mapping tables can be designed to get 18.8%and 37.5%probability shaping constellations with redundancy,and the complexity is only 19.3%and 20.2%of the constant component distribution matcher.Furthermore,a 40Gbps PS-BICM-PM-16QAM-FSO simulation system and indoor offline experiments are used to verify the designed scheme.In a weakly turbulent environment(Cn2=10-15),compared to the uniform distribution scheme,the receiver sensitivity gain is 0.35dB and 1.05dB(@BER=10-6).Third,aiming at the deterioration of FSO system receiver sensitivity under strong turbulence,the paper designs and simulates and analyzes the diversity reception PS-BICM-PM-16QAM-FSO system scheme.Furthermore,the paper designs a LLR calculation method based on a shallow neural network,which can further improve the receiver sensitivity compared to the traditional LLR approximate calculation.The LLR Net is composed of 2 neural unit input layers,16 neural unit hidden layers,and 4 neural unit output layers.The LLR Net model is obtained by training with a random source of±3σ standard deviation generated on the 16QAM plane.The paper simulates and analyzes the influence of different diversity numbers on the performance of the system.The simulation results show that the 2-diversity system can obtain a receiver sensitivity gain of 5.6dB(@Post BER=10-4)compared to a single-transmit and single-receive system;the 4-diversity system can obtain a receiver sensitivity gain of 15.3dB(@Post BER=10-4).At the same time,compared with the traditional LLR approximate calculation,LLR Net can further bring a 0.08dB receiver sensitivity gain. |
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