Research On Adaptive Coding Technology In Plasma Sheath Transmission Environment

In recent years,near-space platforms become the researching hotspot of major world powers.As an important carrier for the development and utilization of near space,hypersonic vehicle has attracted more and more attention from all over the world due to its fast flight speed,rapid orbit transfer and global fast arrival.However,when the aircraft is flying at high speed,its surface will be covered with a layer of plasma sheath,which will interfere with the normal signal transmission between the aircraft and the ground station.At present,there are three schemes to solve the problem of poor communication: reducing the transmission rate,increasing the transmission frequency and reducing the electron density of plasma sheath by physical intervention.However,the dynamic characteristics of plasma parameters and the difficulty of establishing an effective feedback loop in the channel result in the above schemes having some application limitations in the plasma sheath channel.Through theoretical research and experimental verification,it is found that in the initial emergence of plasma sheath,the link is not completely interrupted.Due to the high bit error rate of the traditional communication method,the link is in a “unsteadily state”.In addition,it is found that there are many "communication windows" in the channel due to the dynamic jitter of plasma in the time domain and the non-uniform distribution in the spatial domain,which makes it possible to extend the reliable communication time under the plasma sheath.In view of the above characteristics of plasma sheath,the following researches are carried out from the coordination of space-time domain and new technology of adaptive coding:First,the influence of the plasma sheath on electromagnetic wave transmission is analyzed from the theoretical aspect.Second,the estimation method of channel capacity in plasma sheath transmission environment is studied.Finally,according to the space-time characteristics of the plasma sheath channel,three new adaptive coding forms are proposed,which are classification fountain code,short-frame fountain code and space-time fountain code.The main research contents and contributions of this paper are as follows:1.According to the fact that it is difficult to establish an effective feedback loop for plasma sheath channel,a channel capacity estimation method based on the Voltage Standing Wave Ratio(VSWR)of transmitting antenna is proposed.In the stationary plasma sheath model,the reflection coefficient and transmission coefficient of the electromagnetic wave at the plasma sheath are theoretically derived,and the negative correlation between them is used as the basis to establish the relationship between the VSWR of transmitting antenna and the channel capacity.Through theoretical analysis,simulation and experimental verification,the relationship between VSWR and channel capacity under different frequency ratios is verified;In the dynamic plasma sheath model,the transmission of angle modulation signal in the plasma channel(the electron density of plasma obeys sinusoidal jitter)is analyzed as an example,and the dynamic plasma channel model is established.Besides,the influence of plasma dynamics on angle modulation signal is analyzed from the perspective of channel capacity.2.In the large-scale model,the electron density of the plasma sheath changes slowly.Therefore,in order to improve the reliability of key information transmission,a communication method with classification fountain code is proposed.This paper studies the method of unequal error protection based on mixed degree distribution function,and discusses the adjustment rules of information classification when the channel conditions change.Furthermore,the transmission model of classification fountain code is established in the telemetry channel of the reentry vehicle.Simulation and experimental results show that the classification fountain code can achieve the optimal transmission of key information in the plasma sheath.3.There are many "communication windows" in the mesoscale dynamic jitter model of plasma sheath.At the same time,such "communication windows" also exist in the plasma sheath regulated by physical means.In this paper,an adaptive short-frame fountain code communication method is proposed based on the maximum utilization of channel time domain resources.Moreover,the particularity of plasma channel with "communication windows" and the adaptability of short-frame fountain code to this channel are analyzed.After discussing the encoding and decoding methods of short-frame fountain code,the relationship between coding parameters and "communication windows" and the influence of each parameter on communication performance are derived.In particular,the adaptive adjustment rules of parameters of short-frame fountain code when VSWR changes are discussed.The transmission model of short-frame fountain code can then be established in telemetry channel of reentry vehicle.Simulation and experimental results show that the short-frame fountain code can effectively transmit information by using the time slot resources of plasma sheath in time domain.4.In the space domain,there are many "communication windows" in the channel due to the difference in the distribution of the plasma sheath on the windward and leeward sides of the aircraft.In order to make full use of the limited airspace channel resources that cannot be used by conventional communication methods,this paper proposes an adaptive space-time fountain code communication method based on classification fountain code and short-frame fountain code.The channel capacity of the multi-antenna system under the plasma sheath is analyzed.This paper discusses not only the coding mode of space-time fountain code,but also the adaptive switching between the diversity and multiplexing of the space-time fountain code in the transmission link without feedback loop.Meanwhile,a multi-channel downlink model in the plasma sheath environment is established.Simulation results show that the space-time fountain code can provide effective information transmission with the slot resources on multiple links.In summary,the research results of this paper provide new ways to make full use of the resources in space-time domain and solve the problem of “unsteadily states” in the plasma sheath channel.