Research On The Effect Of Plasma To Digital Microwave Communications

When the spacecraft enters the atmosphere at a speed of more than ten times, the surface material of the spacecraft and the surrounding air are ionized, forming a plasma sheath. This plasma field makes the electromagnetic wave of communication signal transmission attenuate significantly, and even causes the interruption of communication between the spacecraft and the earth, which will cause serious consequences. The research on the effects of plasma to digital microwave communication is proposed based on the above background..This thesis introduces the transmission characteristics of electromagnetic wave in plasma and the mathematical model of the plasma channel, and simplifies the model according to the actual communication. Finally, the attenuation constant and phase constant of the plasma are measured by the shock tube, and the time-varying plasma channel model used in this paper is established based on the above experimental data and the Gauss white noise.In the plasma channel, the thesis simulates the the bit error rates(BER) of the 2FSK, 2PSK, QAM change with the signal to noise ratio(SNR) and symbol rate, and compares the BER of different modulation under the same symbol rate, SNR condition. It can be seen that the 2PSK performance is the best, 2FSK less, and QAM has the highest error rate.In view of the low utilization ratio of 2PSK and 2FSK, this paper presents a new hybrid modulation method, based on frequency and phase. This method makes the frequency and phase of the modulated signal have different sequence information at the transmitter. In the receiver, demodulate the original two sets of sequences by the different demodulation algorithms for frequency and phase. According to the method, this paper introduces the detailed theoretical derivation of the principle of modulation and demodulation, and the modulation and demodulation simulation are also given, which verify the correctness of the method with high frequency band utilization ratio.Finally, this paper establishes the time-varying plasma channel based on the experimental data of shock tube, and analyzes the anti-noise performance of 2FSK, 2PSK, QAM, hybrid modulation with the different SNR and symbol rate. From the simulation results, it is concluded that 2PSK is the best, 2FSK less, and QAM has the highest error rate. For the new modulation presented in this paper, the error rate of the phase, frequency modulation are higher than 2PSK, 2FSK. Therefore, this modulation will have a good application prospect in the low speed symbol transmission. Therefore select the appropriate symbol rate to take advantage of the high frequency band utilization ratio, realizing the information transmission.