A Stochastic Channel Model Of Plasma Sheath For Hypersonic Vehicle In Near Space

When a hypersonic vehicle is flying in the atmosphere at hypersonic speed,a plasma sheath covering the vehicle is generated.The plasma sheath attenuates the energy of communication signal and influences the quality of the signal.In addition,the electromagnetic signal is affected by the scatterers and multipath effect is produced.The coupling of plasma sheath and the multipath effect makes the hypersonic vehicle integrated channel environment more complex.The existing communication system is difficult to adapt to the abominable communication environment.The communication interruption always occur and result in a "black-out" phenomenon.Inadequate understanding of the comprehensive channel characteristics for hypersonic vehicle is the main reason that the “black out” problem have not be overcome.In this thesis,the ultimate goal is to establish the comprehensive channel model for hypersonic vehicle.Four aspects,the equivalent medium model of plasma sheath,the theoretical model of plasma sheath channel,the experimental verification of plasma sheath channel model and the geometry-statistics of hypersonic vehicle integrated channel,are studied in this thesis.1.The formation mechanism of the plasma sheath electron density dynamics is studied in depth,and the multi-scale model of the equivalent medium of plasma sheath with time and space is established.Considering the influence of different time varying factors,the flight conditions affecting the plasma parameters and the fluid conditions inside the sheath are classified into large scale,mesoscale and small scale factors respectively.The functional relationship between electron density and flight attitude and flow field parameters is established.The mechanism of electron density variation at different scales is analyzed in detail,and a multi time scale model of electron density is established.In this thesis,the statistical law of small scale changes of electron density is emphatically analyzed.Based on this,a multi scale physical model of plasma sheath is established.The calculation method of electromagnetic wave propagation in time-varying medium is studied.The whole channel of typical RAMC flight in the whole flight time is calculated.The amplitude and phase of the plasma channel in time domain,the standard deviations of the amplitude and phase,correlation coefficient between amplitude and phase and the skewness and frequency of variation are also calculated.The channel characteristics such as channel coherence time and coherent bandwidth are analyzed as well.2.A statistical model of small channel for plasma sheath is established,and the mathematical expression of statistical parameters is derived.The propagation mechanism of electromagnetic wave in plasma sheath is studied in depth,and the approximate linear relationship between attenuation coefficient and phase shift coefficient and small scale changes of electron density is established.The probability density functions of amplitude and phase,correlation coefficient between amplitude and phase,and autocorrelation functions of amplitude and phase for plasma sheath are derived.A method for generating the time series of small scale coefficient of variation of electron density in plasma sheath is proposed based on Fourier transformation and order matching.By comparing the theoretical probability density functions of amplitude and phase,correlation coefficient between amplitude and phase and autocorrelation functions of amplitude and phase,the effectiveness of the method is proved.3.A sliding correlation wideband measurement method based on CAZAC sequence for plasma sheath channel is proposed.Combined with the plasma sheath channel environment,the effect of channel detection method under different parameters is analyzed,and the optimal parameter selection scheme is given.The plasma generator DPSE,shock tube and rocket exhaust plume are comprehensively analyzed.The experimental verification scheme of small scale channel model for plasma sheath is established,and the parameters such as probability density functions of amplitude and phase,correlation coefficient between amplitude and phase and autocorrelation functions of amplitude and phase are verified experimentally.4.A geometric statistical model of hypersonic vehicle integrated channel is established.Based on the statistical model of the plasma sheath small scale channel,the coupling relationships between the spatial multipath channel and the plasma sheath channel in the uplink integrated channel and the downlink integrated channel are analyzed.Based on two different coupling mechanisms,the mathematical expressions of the probability density function and the power spectral density function of phase and amplitude are given respectively.A synthetic channel time series generation method for hypersonic vehicle,which can simulate the two coupling processes of high-speed mobile channel and plasma sheath channel,is proposed,which can provide channel input for hypersonic vehicle communication simulation.In summary,this thesis deepens the cognition of plasma sheath channel and hypersonic vehicle integrated channel.The established statistical model of plasma sheath and the geometric statistical model of hypersonic vehicle integrated channel can accurately and comprehensively characterize the plasma sheath channel and the hypersonic vehicle integrated channel.It can provide a theoretical basis for the proposed method of adaptive communication,provide the channel model input for hypersonic vehicle communication system evaluation,and improve the efficiency of the adaptive communication method.It is of great significance to alleviate the “black out” problem.