On Medium-long-wave Communications With MPPSK Scheme

The Medium-Long wave(MLW)is the electromagnetic wave that refers to the frequency range of 200kHz?1 MHz.It covers the low-frequency part of the medium wave to the high-frequency part of the long-wave.When MLW spread at the type of ground-wave,the propagation direction can be changed with the curvature of the earth's surface.It can also penetrate a certain depth of sea water and rock.Because of the advantage of low propagation loss,invisibility,anti-interception and anti-nuclear capability,MLW has become an important means in "mountain pass".However,the bandwidth of MLW is very narrow,so the transmission rate is low and signal in receiver is very weak,MLW can only be used as a supplement to ensure a minimum means of emergency communication.Therefore,in this thesis,the M-ary Position Phase Shift Keying(MPPSK)modulation is applied to MLW communication to achieve better system performance.Firstly,the MLW channel characteristics are introduced,including the propagation modes(ground wave and sky wave)and properties.By analyzing the interference of atmospheric noise in the MLW communication system,a mathematical model for the statistical characteristics of the noise is established,and the simulation results are given.To investigate the main factor of causing the MLW channel to be band-limited,the transmitting antenna is discussed,and a simulation model is established in MATLAB.Secondly,the traditional MPPSK system is introduced,including the MPPSK modulated signal,power spectrum characteristics and evolution of impact filter.By analyzing the waveform of MPPSK modulated signal,the difference between "0" code and "non "0" code is founded.Then,the full transition version of MPPSK modulation is described and selected in the MLW communication system.According to the requirements of the MLW communication,the parameters of MPPSK modulation are determined to improve its low transmission rate and bandwidth utilization.Then,the BER performance of the envelope detection and the coherent demodulation for the full transition MPPSK modulated signal are compared by simulation results.On the basis of coherent demodulation,the correlation coefficient discrimination is presented to relieve the inter-symbol interference.Simulation results show that it can achieve better performance than traditional amplitude integral decision in MLW communication system.Moreover,as the MPPSK signals are seriously damaged,the nonlinear noise suppression methods are introduced,and the demodulation performance of full transition MPPSK system is further improved by the adaptive clipper.Finally,the method of channel coding is used to enhance the performance of MLW communication system,the paper briefly introduces two typical error correcting coding schemes,LDPC coding and Reed-Solomon coding.Through the simulation and comparison of the two encoding methods,we choose a better way to improve the performance of MLW communication system.At the same time,interleaving method is adopted to random burst pulse interference error,and the system performance index is finally achieved.