Communication Signal Transmission Characteristics And Performance Evaluation Of Low-Frequency Electromagnetic Waves In Plasma Sheath

The high-density plasma sheath may interrupt the telemetry,tracking,and control（TT&C）subsystem between the vehicle and the ground stations at the microwave band when the reentry vehicle traverses the near space domain at high speed,causing the"radio blackout"phenomenon of communication failures.Exploring emergency communication solutions to mitigate the influence of the plasma sheath with the high electron density has a lot of application potential,where the research topic of expanding new frequency band windows has been attracting attention.The novel frequency window,where the frequency of the low-frequency（LF）electromagnetic（EM）wave is significantly smaller than the plasma frequency and the collision frequency（ω_c<<ω_p&v_e）,can be one of the potential breakthroughs in recent years.According to the LF EM wave propagation theory,the attenuation characteristics of the magnetic field of LF EM waves differ from those of the electric field when the LF EM wave penetrates the electrically small plasma sheath,and the attenuation characteristics of the magnetic field are much lower than that of the electric field.In severe blackout conditions,the band window of LF EM waves may provide a solution to maintain emergency communication.However,there have been no successful domestic or international applications of LF EM waves for hypersonic vehicle communication in the blackout.The bottleneck is that the propagation characteristics and transmission characteristics of LF communication signals are still unknown,i.e.,there is a lack of in-depth understanding of the dynamic plasma communication channel and ground experiment technology at the band of LF EM waves,which limits the research,verification,and application of LF EM wave communication under plasma sheath.This thesis aims to explore the bottleneck problem of LF EM wave communication throughout plasma sheath by expanding the stochastic model to characterize dynamic plasma channel at the LF band,proposing a shock tube experimental apparatus for LF EM wave signal measurement,and addressing the difficulty of ground experiment technology at the LF band.Based on the above innovations,this thesis focuses on the analysis and comparison of transmission characteristics of LF communication signals,the ground shock tube experiment,and the performance evaluation of LF communication links.This thesis improves the theoretical foundation and verification methodologies for the application of LF EM wave communications.The following are the specific contributions of this thesis:1.The stochastic model for characterizing the dynamic plasma channel is extended to the LF band.The propagation model of the magnetic field of LF EM waves in the dynamic plasma is merged with the colored-Gaussian time-varying electron density model to obtain the channel stochastic model of LF EM waves propagating in dynamic plasma.This stochastic model of the dynamic plasma channel at the LF band is not identical to the ones used in the microwave band.The first-order statistical properties of the dynamic plasma channel at the LF band are given,including the probability density function（PDF）of amplitude,the PDF of phase shift,and the power spectral density（PSD）.The stochastic model of the dynamic plasma channel is extended to the LF band.On this basis,the bit error rate（BER）evaluations of typical communication algorithms used in LF communication systems are conducted out.The simulation results show that even though the LF EM waves have the potential to transmit through the plasma with the high electron density Ne=10¹⁵/cm³,the BER performance of the typical LF communication algorithms will deteriorate,when the channel characteristics of dynamic plasma at the LF band are considered.The BER performance is Time-Frequency Modulation>Frequency Modulation>Phase Modulation.When the electron density fluctuation range reaches 20%(the fluctuation range is±2×10¹⁴/cm³),to maintain the BER at P_b=10^-5,the performance of the QPSK algorithm against electron density is lower than Ne=10¹³/cm³;The performance of the 4-FSK algorithm against electron density is near to Ne=10¹⁵/cm³,but the needed signal-to-noise ratio（SNR,denoted by E_b/N₀）for the 4-FSK reaches E_b/N₀=13.1 d B;The performance of the frequency shift chirp modulation（FSCM）algorithm against electron density can reache to Ne=10¹⁵/cm³,the needed SNR for the FSCM algorithm is only E_b/N₀=6.1 d B,and the BER performance is not nearly deteriorated.These results suggest that the FSCM algorithm,which represents two-dimensional orthogonal linear time-frequency modulation technology,can be used as the preferable modulation algorithm for LF EM wave communications.2.A shock tube experimental apparatus is proposed for LF EM wave signal transmission measurement.Aiming at addressing the difficulties of shock tube experiments at the LF band,including the focusing difficulty caused by LF wide beams,the shielding difficulty caused by LF strong diffraction,and the cut-off waveguide effect caused by quartz windows,a novel improved shock tube experimental apparatus for investigating LF EM waves is proposed.A device for generating the round flow field in the shock tube is developed,which eliminates the requirement for focusing antennas and shielding structures compared with in typical experimental methods at the microwave bands.A signal measurement system for LF EM wave enveloped by dynamic plasma flow field over the complete 360 angular range was established,which realizes the decoupling measurement of the signal transmission characteristics of the electric field/magnetic field of the LF EM wave.The frequency-division-multiplex（FDM）experimental method is presented to quantitatively measure the signal transmission characteristics of the LF EM wave,which addresses the inconsistency of plasma parameters in multiple shock tube experiments.Several signal transmission experiments of LF EM waves were conducted on the shock tube.The experimental results show that（1）Under the actual dynamic plasma flow field,the shielding effectiveness theory of LF EM waves was verified,which predicts that the attenuation of the LF electric field is different from that of the LF magnetic field;（2）The attenuation of the LF magnetic field signal（1-30 MHz）in the actual dynamic plasma flow field with the peak electron density Ne=2.03×10¹³/cm³ is only 0.15 d B to 12.88 d B.In comparison,TT&C signals at the S band experience serious blackout with attenuation of more than 40 d B under this plasma state;（3）The fluctuation range of phase shift of the LF magnetic field signal can reach 160°,causing the PSD to broaden to 280 k Hz,which is consistent with the channel characteristics of the dynamic plasma channel at the LF band.（4）The transmission performance of typical LF communication signals is in the following order:FSCM>4-FSK>QPSK,which is consistent with the theoretical trend of the BER analysis for the LF communication algorithms.3.The communication link of the LF EM waves for the hypersonic vehicle was evaluated in several aspects.There is no actual data available for reference at home or abroad in the scenario of the LF EM waves ground-air communication of the hypersonic vehicle,there is no actual data available at home and abroad for reference.This thesis choses a commercial near-space sub-orbital vehicle to conduct a preliminary measurement experiment of the LF magnetic field,in order to evaluate the communication link performance in future flight applications of the LF communications,such as frequency selection,link budget,communication distance,and so on.On the vehicle’s metal skin surface,the key parameters of the magnetic field strength and the noise floor of the magnetic field are obtained.As a result,the measured results required to evaluate the SNR of the LF magnetic field signal during the flight test are also provided.During the flight test,a conformal miniaturized LF magnetometer is designed and loaded on the near-space vehicle,confirming the possibility of sensing LF magnetic field on the metal skin surface of a near-space vehicle.The flight test results show that the noise spectral density（NSD）of the background LF magnetic field is about 16 f T/√Hz,which is induced by the secondary magnetic field interference from the current on the metal skin surface and is different from that in the typical microwave communications.Finally,to comprehensively evaluate the communication link,this thesis utilizes the noise floor measured in flight test as a main basis and combines it with the sky-wave propagation model of LF EM waves.Communication parameters such as time,range,preferred frequency range,ray path basic transmission loss,plasma transmission loss,and link budget are provided.The evaluation results demonstratet hat the LF communications system can ensure a link budget of 30d B under the plasma with the high electron density Ne=10¹⁵/cm³,when the LF communications system chooses the 10-100 meter wave（such as the typical 7-10 MHz in the short-wave band）and utilizes the FSCM algorithm.The LF communication link’s channel capacity is in the tens of kbps range,which can meet the small-quantity critical uplink requirements for emergency communications.The LF communications system has the ability of over-the-horizon communication over 1000 kilometers.In terms of theory,the results of this thesis contribute to a better knowledge of the communication signals transmission characteristics of LF EM wave in the dynamic plasma,as well as a theoretical basis for designing a LF communication algorithm suitable for the plasma sheath.In terms of experiments,the results of this thesis not only develop the shock tube experimental apparatus and method for the LF EM wave,but also recognize the background noise properties in the actual flight test,which provides a preliminary evaluation and testing method for the LF communication systems design.It presents important theoretical basis for strengthening further use of the LF EM waves to mitigate the"radio blackout",which is the bottleneck in the development of hypersonic vehicles.