Research On The Transmission Characteristics Of Broadband Modulated Signals In Near Space Plasma Environment

Near space is the main activity area for various hypersonic vehicles.It has significant strategic value and development potential in the fields of aerospace and military.Near space has a complex electromagnetic environment,which can have various impacts on the vehicles involved.One of the main manifestations is the hypersonic plasma sheath and the communication "blackout" phenomenon.Plasma sheath has high temperature,high pressure and high speed,which make its electromagnetic environment extremely complex.The wireless signals would be absorbed and reflected or other effects when transmitting in plasma sheaths.This would cause serious attenuation and distortion of signals,and even be completely unable to be received by the vehicle.This will result in communication interruption,which called communication "blackout".The "blackout" phenomenon is the most prominent manifestation of the complexity of the near space electromagnetic environment.It is also a century long challenge in aerospace engineering.Countries including China have invested a lot of effort in these researches.Many researchers provided a serious of solutions to mitigate the impact of blackout.Using low-frequency(LF)or terahertz(THz)communications attach the attention of academic circle in recent years.The low-frequency signal has high power and long wavelength,it could penetrate the plasma sheath using the Skin Effect,using existing base stations would greatly reduce the construction costs.THz communication has a higher transmissivity by increasing the operating frequency of communication systems to be higher than the cutoff frequency of the plasma sheath.It also can compensate for the disadvantages of LF communication on low communication rate and only supporting one-way communication.In addition,broadband modulation technology has significant application potential in today’s increasingly scarce spectrum resources,it has great meaning that analyzing the characteristics of broadband signals in complex electromagnetic environments in near space.There are many researchers have analyzed those manners,but further details are needed on the physical mechanisms and influencing factors of the electromagnetic environment characteristics,signal transmission and attenuation characteristics in the plasma sheath.Focus on these issues,this dissertation takes the electromagnetic environment of plasma sheath at 3 KHz-300 KHz and 100 GHz-300 GHz as the starting point to analyze the transmission characteristics of signals under the complex electromagnetic environment of the plasma sheath.The relationship between the transmission coefficient of LF in the plasma sheath and the flight speed or antenna position are analyzed.The possibility of using "magnetic window" and the impacts of external magnetic field strength are analyzed,too.Based on that,two applications of using extending LF station and building specialized servo LF base station are put out and analyzed.Besides,the characteristic of the channel capacity of THz signals in the plasma sheath is analyzed in this study.The model of the additive noise and multiplicative interference of hypersonic vehicle plasma sheath is built and analyzed.The impacts of flight speed,antenna position,signal frequency and the time-varying characteristic on the channel capacity are analyzed.Based on that,a fast prediction model of using flight speed to predict the channel capacity of THz plasma sheath channel is built.This model could significantly reduce the calculation time and resources for calculating the channel capacity of THz plasma sheaths channel,which make real-time and full process prediction of channel capacity possible.At last,the present study models and analyzes the characteristic of broadband modulation signals in near space plasma which has a wider electron density and a broader communication system working frequency band.This model is based on the electronic motion equation and extends the Appleton-Hartree formula,which describes the transmission characteristics of electromagnetic waves in plasmas.This model extends the single-tone signals to broadband signals and analyzes the nonlinear dispersion characteristics of broadband amplitude modulation communication signals in plasma.The researches of this study provide a new method for the electromagnetic environment in the plasma sheath of hypersonic and re-entry vehicles,as well as the characteristics and physical mechanisms of communication channels.