Transmission Characteristics Of EHF Communication Signal In Time-varying Plasma Sheath

In the near space(20-100 km),plasma sheath will be generated around the reentry hypersonic vehicle.Plasma sheath is a complex electromagnetic environment.Affected by the complex electromagnetic environment and channel transmission characteristics,the signal will be reflected and absorbed,eventually attenuated,or even completely shielded,forming a "blackout" phenomenon.At present,the methods to alleviate the "blackout" effect that attract wide attention mainly include extremely high frequency(EHF)communication and "magnetic window method".For EHF communication,there are two common ways: one is to increase the signal frequency as much as possible to improve the transmission of EHF signal in the plasma sheath,but the attenuation of high-frequency signal in the atmosphere is relatively large;The second is to use auxiliary methods to reduce the required EHF signal frequency and reduce the attenuation of EHF signal in the atmosphere,such as combining the "magnetic window method".In terms of EHF communication,research has solved the problem of EHF signal propagation in the static plasma sheath.However,the propagation performance of EHF signal in the dynamic time-varying plasma sheath is still poorly understood.In addition,the existing research results of the "magnetic window method" show that the external enhanced magnetic field can improve the signal transmission.However,how does the magnetized plasma sheath affect the communication performance? Is the signal modulation mode constrained and selective by the magnetized plasma sheath? The understanding of these problems is basically blank.The existing work on the research idea of integrated EHF communication and "magnetic window method" mainly focuses on the physical level:research on the influence of onboard antenna position,flight altitude,flight speed,electromagnetic wave incidence angle,atmospheric environment,magnetic field and other factors on the transmission rate of EHF signals,and rarely conduct in-depth research from the perspective of communication,such as not considering what impact will the phase shift of transmission signals,signal modulation methods,signal coding methods,BER performance and other factors have on the communication system to mitigate the "blackout",and these factors have a huge impact on the communication system to mitigate the "blackout".In view of the above problems,this paper established a time-varying model of the plasma sheath of the re-entry hypersonic vehicle,on this basis,the temporal and spatial variation characteristics of the EHF modulation channel in the time-varying plasma sheath of the re-entry hypersonic vehicle under specific sailing conditions were studied,the EHF communication combined with the "magnetic window method" to alleviate the "blackout" scheme was proposed from the aspects of modulation mode,carrier frequency,coding mode,onboard antenna position,etc.,and a reasonable signal system suitable for reentry vehicles was designed to improve the new method of using EHF signal and "magnetic window method" to weaken the influence of“blackout” on communication.The main research results are as follows:1.The dynamic model of plasma sheath of reentry hypersonic vehicle obtained by solving the single fluid multi-component model was deeply studied,and the plasma sheath structure under specific navigation conditions was simulated.By studying the time-varying characteristics of various parameters of the plasma sheath,such as electron density,mass density,temperature,electron collision frequency and plasma frequency,the influence of the available frequency band and the installation position of the antenna on the communication performance was analyzed.The results show that the EHF frequency band is completely feasible for the reentry vehicle communication.When there is enough space on the side of the vehicle to install the antenna,better performance can be achieved when the antenna is installed in the area near the tail.When the antenna is installed in the tail,better communication performance can be achieved when the angle between the channel and the bottom of the vehicle is set at 50 ° to 60 °.2.The propagation models of EHF electromagnetic wave in the time-varying non magnetized plasma sheath of blunt coned reentry vehicle and sharp coned reentry vehicle were studied.The time-varying characteristics of transmissivity and phase shift of transmission signal of EHF signal in the side of blunt coned reentry vehicle and sharp coned reentry vehicle and in the tail channel of blunt coned reentry vehicle were analyzed respectively.The results show that for blunt coned reentry vehicle,the transmissivity of EHF signal in side channel increases with the antenna on the vehicle moving backward,the transmissivity of the EHF signal in the tail channel increases with the increase of the angle between the channel and the bottom of the vehicle.For the sharp coned reentry vehicle,the transmissivity of EHF signal in the side channel increases with the backward movement of the onboard antenna at the early stage of sheath evolution,and the transmissivity of EHF signal in the channel near the middle of the side channel is the largest at the later stage.In a specific channel,the transmissivity of EHF signal increases with the increase of carrier frequency.The time-varying characteristics of the phase shift of EHF transmission signal in the side channel of blunt coned and sharp coned reentry vehicle and in the tail channel of blunt coned reentry vehicle are different,and there is no significant rule.3.The communication simulation system of EHF signals modulated by2ASK/2PSK/NC-2FSK/4QAM/8PSK/8QAM/16 QAM in time-varying plasma sheath was established.The time-varying characteristics of BER of EHF signals modulated by the above methods in the side and tail time-varying plasma sheath channels of blunt cone reentry vehicles were studied,The BER time-varying characteristics of EHF signals with basic modulation(2ASK/2PSK/NC-2FSK)propagating in the side time-varying plasma sheath channel of the sharp coned reentry vehicle were studied.It is found that the lower base phase-shift modulation(2PSK/4QAM)combined with the carrier frequency of 140 GHz or 225 GHz can achieve better BER performance.At the same time,the influence of LDPC,LT and RAPTOR codes suitable for transmission in the plasma sheath on the propagation characteristics of EHF signals in the time-varying plasma sheath was also analyzed.The results show that RAPTOR codes have the best communication BER performance.4.The propagation model of electromagnetic wave in the magnetized plasma sheath was studied,the propagation characteristics of EHF electromagnetic wave with different frequencies in the magnetized plasma sheath were analyzed,the BER characteristics of EHF signals with different modulation modes in the time-varying magnetized plasma sheath channel at the side of the blunt cone reentry vehicle were studied,and the selection and restriction of the modulation mode by the magnetized plasma sheath were studied.The results show that,in a specific channel and carrier frequency,the transmissivity of left-handed circularly polarized EHF electromagnetic waves increases with the increase of magnetic field strength,while that of righthanded circularly polarized EHF electromagnetic waves is just the opposite.The influence of magnetic field on the transmissivity of left/right circularly polarized EHF electromagnetic waves decreases with the increase of carrier frequency and the backward shift of antenna position.In general,for 2ASK and 2PSK with single polarization wave,BER performance with left circular polarization is better than that with right circular polarization.In addition,due to the influence of phase shift,the BER performance of EHF signal is not always better with the increase of magnetic field strength.The 4QAM using left and right circularly polarized modes as orthogonal channels can still be used in the magnetized plasma sheath when the vehicle speed is 6550 m/s.However,as the speed of the vehicle increases,the plasma parameters become larger.The external magnetic field helps to alleviate the "blackout" and improve the communication performance.However,at this time,the transmissivity of EHF electromagnetic wave of right-handed circular polarization mode may be suppressed to less than-10 d B with the increase of magnetic field.In this case,the orthogonal modulation involving right-handed circular polarization mode may not be suitable for use in magnetized plasma sheath.To sum up,the plasma sheath of reentry hypersonic vehicle was regarded as a medium with spatio-temporal continuous change characteristics,and the propagation characteristics of EHF signals in the time-varying plasma sheath were systematically studied in this paper.The technical scheme combining EHF communication and "magnetic window method" to alleviate the "blackout" problem has been improved.The reference design scheme combining modulation mode,carrier frequency,coding mode and carrier antenna position is given.The results of this paper provide an easyto-implement engineering reference scheme for further alleviating the problem of "blackout".