| In recent years,with the rapid development of information network technology,the demand for information flow has increased rapidly,and it is necessary to establish a high-speed,large-capacity,and wide-coverage integrated air-space-ground communication network system.At present,the data transmission between satellites and between satellites and ground is mainly the communication using microwave as the carrier.Due to the limitation of microwave frequency resources and technology,it is difficult to greatly increase the rate of satellite communication using microwave as the carrier.However,satellite laser communication technology provides a novel solution for realizing high-speed data transmission between satellites and land due to the advantages of small equipment,strong anti-interference ability,strong confidentiality,and large communication rate improvement space.Therefore,satellite laser communication has become a research hotspot in the field of satellite communication.Among them,the research of laser communication technology between satellites and ground is the key to solving the problem of high-speed data exchange between the satellites and the ground.Because the laser communication between the satellites and the ground needs to pass through the atmospheric channel,the laser signal is inevitably affected by atmospheric turbulence and atmospheric scattering during transmission,resulting in a decline in communication reliability.Therefore,in view of the transmission characteristics of the atmospheric channel,the research on the optimization scheme of the transmission performance of the satellite-to-ground laser communication is the research focus of the realization of stable and reliable high-speed satellite-to-ground laser communication.The adaptive coding and modulation technology can dynamically adjust the coding and modulation mode of the signal according to the change of the wireless channel state,and seek a balance between communication reliability and efficiency,and can optimize the transmission performance of satellite-to-ground laser communication,which has become a research hotspot at home and abroad.Based on the study of the satellite-to-ground laser communication transmission mechanism,this thesis focuses on the satellite-to-ground laser communication system based on cyclic neural network channel estimation algorithm,receiving end signal modulation format identification method and joint multi-aperture reception adaptive coding modulation scheme.The main research work and innovations of the thesis are as follows:1.A satellite-to-ground laser communication channel estimation method based on Long Short-Term Memory(LSTM)cyclic neural network is proposed.In this method,using the LSTM network's "strong memory"characteristic of time series,under different intensities of atmospheric turbulence channel conditions such as weak turbulence,medium turbulence,and strong turbulence,a large number of satellite-to-ground laser communication system training data sets are obtained through simulation.The training data set is composed of a large number of receiver signal sequences with known optical signal-to-noise ratios,and the signal sequence value and the corresponding average optical signal-to-noise ratio are used as the input layer for LSTM model training.After training and fitting the model,the optical signal-to-noise ratio of the channel can be estimated by inputting the received signal.Compared with the traditional channel estimation method Least square estimation(LS),the channel estimation method based on LSTM network has higher estimation accuracy,and the average mean-square error(MSE)is about 0.01,MSE can be reduced to below 0.001 in the case of high signal-to-noise ratio.The channel estimation method realizes the accurate estimation of the optical signal-to-noise ratio of the satellite-to-ground laser communication channel,and provides a solution to the channel estimation problem in the ACM scheme.2.A satellite-to-ground laser communication channel estimation method based on Long Short-Term Memory(LSTM)cyclic neural network is proposed.In this method,using the LSTM network's "strong memory"characteristic of time series,under different intensities of atmospheric turbulence channel conditions such as weak turbulence,medium turbulence,and strong turbulence,a large number of satellite-to-ground laser communication system training data sets are obtained through simulation.The training data set is composed of a large number of receiver signal sequences with known optical signal-to-noise ratios,and the signal sequence value and the corresponding average optical signal-to-noise ratio are used as the input layer for LSTM model training.After training and fitting the model,the optical signal-to-noise ratio of the channel can be estimated by inputting the received signal.Compared with the traditional channel estimation method Least square estimation(LS),the channel estimation method based on LSTM network has higher estimation accuracy,and the average mean-square error(MSE)is about 0.01,MSE can be reduced to below 0.001 in the case of high signal-to-noise ratio.The channel estimation method realizes the accurate estimation of the optical signal-to-noise ratio of the satellite-to-ground laser communication channel,and provides a solution to the channel estimation problem in the ACM scheme.3.Ajoint AMC scheme for satellite-to-ground downlink is proposed.Since multi-aperture reception technology can suppress atmospheric turbulence,this paper combines multi-aperture reception technology with MPSK coherent modulation and LDPC coding to propose a joint AMC scheme,and design an AMC decision switching method based on the feedback channel The feedback information provided by the system dynamically changes the coding efficiency and modulation order,and when the system adopts low coding efficiency and the lowest order modulation,the bit error rate is still not guaranteed,adaptively adopts multi-aperture reception to further improve system performance and ensure Reliable transmission of the system under poor channel conditions.After simulation and analysis,compared with systems with fixed coding and modulation formats and ordinary adaptive modulation systems,under the set error rate threshold,the lower limit of the optical signal-to-noise ratio of ordinary adaptive coding and modulation schemes meeting the communication error rate conditions is 10dB,and The lower limit of the optical signal-to-noise ratio of the joint AMC scheme is 8dB.The scheme proposed in this paper shows a better average bit error rate. |
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