Research And Implementation Of Adaptive Coding Under Plasma Sheath

When the spacecraft is flying in the near space at hypersonic speed,a thick "plasma sheath" will be generated on the surface of the aircraft.The plasma sheath will absorb,reflect and scatter the incident electromagnetic waves,and cause strong interference to the measurement and control communication signals,which will attenuate the radio signals transmitted between the aircraft and the ground station,resulting in increased bit error rate of the communication and serious Sometimes even the communication link is interrupted,forming a "black barrier" phenomenon.Existing research and experiments show that increasing the communication carrier frequency to be higher than the plasma oscillation frequency can reduce the attenuation of the communication signal by the plasma to a certain extent,but the bit error rate of the communication is still very large.In the near space measurement and control communication,a reliable communication system is essential for the normal operation of the aircraft,especially the manned spacecraft.A good communication system provides an important guarantee for the life safety of the space pilot and the national property.However,in actual aircraft measurement and control communications,the importance of information varies,and some information is always more important than other information.For example,the flight heading,flight attitude,and other information of the aircraft are more important,and they need to be given priority to ensure reliable transmission.Fountain code as a new type of error correction code has the advantages of no code rate and forward error correction,and is suitable for unstable channels such as plasma sheaths.In this paper,we start with the coding of fountain codes and combine the measurement and estimation results of plasma channels to improve the LT coding scheme and the degree distribution function of fountain codes.An adaptive coding scheme is designed and the experimental demonstration is carried out in parallel.This article mainly completes the following aspects: 1.Starting from the perspective of communication modulation system,the modulation system of a typical measurement and control communication system was designed and implemented under the software radio platform,namely GMSK modulation and DQPSK modulation.The impact on two different incident measurement and control communication signals.2.Aiming at the large-scale changing plasma channel characteristics and the impact on the communication signal,the fountain code LT coding is introduced,and an adaptive communication strategy is proposed.The basic basis of adaptation is to measure the voltage standing wave of the launch antenna of the aircraft in real time More recently,the channel is estimated and divided,and the results are fed back to the transmitter in real time.The transmitter adaptively adjusts the coding strategy based on the feedback results to give priority to ensuring the reliability of high-priority information transmission.At the same time,for dynamic plasma channels,pseudo-code and interleaving coding technologies were developed,and a combined coding scheme of "fountain code LT code + check + pseudo code + interleaving" was designed.3.According to the established adaptive communication system model,the coding strategy is adjusted in real time by improving the fountain code LT coding scheme.And under the software radio platform GNU Radio,the hierarchical LT coding,check code,pseudo code and interleaving coding were implemented and tested.At the same time,combined with the hardware platform USRP,an end-to-end adaptive communication principle prototype was built on the ground.4.The plasma channel was simulated under the self-developed glow discharge device in the laboratory and the existing channel simulator.The adaptive communication system was tested in different frequency bands(S/C/X band),different modulation systems and different information Grade experiment.The results of this paper show that under the same coding scheme and plasma channel conditions,the GMSK modulation scheme is superior to the DQPSK modulation scheme.The adaptive coding scheme for large-scale transform plasma channels can give priority to the reliable transmission of important information to a certain extent.However,the reliable transmission of all information cannot be guaranteed,nor can the "black barrier" phenomenon be eliminated.The combined coding scheme can effectively use the time slot resources of the dynamic plasma channel.Compared with the single coding scheme,it can effectively improve the reliability of information transmission.