| Space laser communication is a communication method in which laser light is used as a carrier wave and carries information for wireless transmission.Due to its advantages such as high transmission rate,no electromagnetic interference,and long transmission distance,it has gradually become a research hotspot in the field of communication.However,due to the influence of atmospheric turbulence,the laser beam carrying the modulated information will be affected by various effects during the transmission process,which increases the difficulty of the traditional demodulation scheme and reduces the overall performance of the system.The blind optimization technology can optimize the channel characteristics only by the prior information of the signal at the receiving end without using additional training sequences,so that the output sequence is as close to the transmission sequence as possible,and it has certain advantages in the field of laser communication.Therefore,this paper designs a blind optimization demodulation scheme combining synchronization technology and improved blind likelihood demodulation algorithm for the needs of laser communication pulse position modulation(PPM)demodulation system under atmospheric turbulence.The laser communication system verifies the performance of the scheme.The main research work of the paper is as follows:1)The transmission characteristics of PPM modulated laser signal and spatial channel are analyzed.Firstly,the PPM signal is analyzed from the aspects of modulation principle,energy transmission efficiency,power spectral density,channel capacity and bit error rate;then the modulation waveform of the PPM signal is simulated;Finally,the theoretical and simulation analysis of the transmission characteristics of the laser signal in the free space channel is carried out,which provides theoretical guidance for the subsequent research on the scheme of suppressing the atmospheric turbulence effect.2)The key technologies of PPM demodulation based on blind optimisation are investigated,mainly including synchronisation techniques and demodulation algorithms.A modified phase-locked loop scheme with the addition of a filter is proposed for the problem of clock jitter when the traditional phase-locked loop method is used to synchronise the PPM signal time slots,and simulations are carried out to verify this.A symbolic synchronization scheme with a synchronization state controller is proposed to maintain a stable average synchronization time of about 2 frames;a blind likelihood demodulation algorithm is introduced and further optimized to address the problem of poor demodulation performance of PPM signals by conventional hard judgement algorithms in atmospheric channels;finally,algorithm simulations are carried out to show that the improved blind likelihood demodulation algorithm has a better overall performance and has a certain degree of suppression capability for atmospheric turbulence effects.3)The blind optimized PPM modulation and demodulation communication system is built for experimental verification,and the atmospheric turbulence pool is used to simulate the real turbulent environment.Firstly,the comparison experiment between the synchronization scheme in this paper and the traditional scheme is carried out.The experimental results show that the receiver sensitivity of the improved synchronization scheme is improved by about 1dbm compared with the traditional scheme;Then on this basis,PRBS code transmission experiments of different demodulation algorithms are carried out.The experimental results show that the blind optimized demodulation scheme combined with the proposed synchronization scheme and the improved blind likelihood demodulation algorithm can accurately demodulate the PPM signal.The bit error characteristics are basically consistent with the conclusions of simulation experiments.At the same time,the experiments also prove the rationality and effectiveness of the system scheme design. |
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