| Space laser communication has the outstanding characteristics of high communication rate,good confidentiality,good security,strong anti-interference ability and strong antiinterception ability,and occupies an important position in the future integrated network of air,sky and earth.It can also be used in inter-terrestrial communication,inter-river communication,earthquake and disaster mitigation communication,power communication,emergency communication and fibre optic access network,etc.When the space laser communication system is used in the atmospheric environment,the absorption,scattering,turbulence and other phenomena of the atmosphere will have an impact on the performance of the laser communication system,especially the atmospheric turbulence caused by the light intensity flicker,spot expansion and other effects to improve the performance of the communication system,which will cause a series of reactions such as reduced detection sensitivity,higher bit error rate,increased probability of interruption,shorter communication distance,reduced system capacity,reduced efficiency of fiber coupling,etc.The response.In order to improve the performance of atmospheric space laser communication systems,optical information reception and processing technology research has been carried out,and the main research content completed includes the following aspects.(1)Aiming at the influence of atmospheric channels on the performance of laser communication systems,the optical characteristics of absorption,scattering and turbulence are analysed,the fading effects of different weather conditions such as rain,snow and fog on laser transmission are simulated,and the effects of turbulent flicker on laser communication systems are focused on in terms of communication bit error rate,average channel capacity and interruption probability.At the same time,a real-time measurement method of turbulent flicker index is proposed,which can complete the measurement of turbulent channel parameters without affecting the communication and provide a reference and basis for the research of real-time anti-turbulence fading technology.(2)A composite cascade method of "chapter dynamic coupling + gain control +channel coding" is established to complete the reception and processing of optical information,and a system model is constructed to achieve dynamic adjustment of detection gain and channel coding parameters with dynamic turbulence real-time information as the feedback control quantity to improve the reception performance of the system under turbulent channels.First,this thesis proposes an energy-weighted laser chapter-action coupling algorithm based on atmospheric turbulence affecting beam jitter resulting in reduced spatial light-tosingle-mode fiber coupling efficiency.The deviation direction of the spot is solved by vector weighting the coupling into single-mode fiber energy,the convergence step is automatically adjusted according to the standard deviation coefficient of the chapterdynamic single-turn coupling energy,and the offset between the fiber and the spot center is actively compensated in real time by using a fast reflector.The effects of the chapter-action spot size,chapter-action scan radius,chapter-action scan sampling points and other factors on the coupling efficiency are simulated,and the effects of the laser communication system parameters on the fiber coupling efficiency under atmospheric turbulence are also simulated.And an experimental system is built to test the coupling performance of the energyweighted laser chapter-motion coupling algorithm under different perturbations.The experimental results show that the algorithm can realize the active scanning,capturing and alignment of the signal beam to the single-mode fiber position,and the coupling efficiency reaches 63% under 1mrad perturbation.Then,this thesis proposes a real-time countermeasure technique of atmospheric turbulence fading with closed-loop regulation of gain factor of avalanche photodetector(APD)for the phenomenon of random jitter of detection signal and data judgment and demodulation error caused by atmospheric turbulence scintillation effect.Based on the realtime measured turbulence scintillation index as the feedback quantity,a functional conversion relationship between the scintillation index and the gain factor is established,and a communication reception system is designed for real-time regulation of the detector gain factor with the turbulence strength.At the same time,laser communication experiments with 1km and 6.5km communication distance and 2.5Gbps communication rate under medium and weak turbulent fading channels were carried out to verify that the APD gain closed-loop regulation method has a significant improvement effect on the energy fading of medium and weak turbulence,and the experimental results show that the method effectively improves the communication system BER by more than two orders of magnitude and improves the stability and reliability of the laser communication system under atmospheric turbulent channels.The experimental results show that the method effectively improves the BER of the communication system by more than two orders of magnitude and improves the stability and reliability of the laser communication system in the atmospheric turbulence channel.Finally,this thesis addresses the problem of degraded communication data transmission performance of laser communication systems due to atmospheric turbulence fading.In this thesis,an adaptive coding scheme applicable to different atmospheric turbulence intensity is studied.Based on the simulation analysis of the coding performance of different channels,LDPC coding is selected for the design of the adaptive coding scheme.Meanwhile,the effects of parameters such as the number of iterations,quantization bit width and normalization factor on the LDPC decoding performance are simulated.The LDPC rate-compatible compiled code algorithm that can be applied to the atmospheric turbulent channel is studied for the different coding performance exhibited by LDPC coding at different code rates.Based on the principle of bidirectional atmospheric channel reciprocity,the LDPC code rate adaptive coding technique for atmospheric turbulent channels based on signal-to-noise ratio estimation is established,and the adaptive coding performance is simulated by Monte Carlo method,and the results show that the LDPC coding for adaptive channels improves the communication error performance more significantly under weak turbulence,but not significantly for medium and strong turbulence.(3)Experiments on the application of airship-airship and airship-ground inter-laser communication were carried out under the near-ground atmospheric channel,and the turbulence profile,communication performance,platform adaptability and other parameters were measured.The experiments verified the differences between the uplink and downlink turbulence channels,and provided theoretical support for the design and development of atmospheric laser communication systems.At the same time,it was verified that based on the measurement of turbulence channel flicker index as a feedback quantity to adjust the reception performance of the laser communication system in real time,the communication rate of 2.5Gbps and BER of 1E-7 orders of magnitude were achieved under the condition that the flying altitude of the airship was 1km and the communication distance was 12 km,and the 5G core network transmission test was conducted based on the results of this test.The research results of this paper have important reference value for the design of laser communication system under atmospheric turbulence and the development of atmospheric laser communication system. |
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