Research On Modulation And Demodulation Technology For High Dynamic Environment

In recent years,as the demand for satellite communications and high-speed train communications continues to expand,the research on reliable communications in highly dynamic environments is becoming more and more urgent.Due to the large relative movement of the transmitter and receiver in a high dynamic environment,a large Doppler shift exists in the signal received by the receiver,which will make it difficult for the receiver to correctly recover the signal and directly affect the system performance,therefore,research on reliable communication in Doppler shift environments is of great significance.At present,although the literature proposes to track the carrier frequency by means of a phase-locked loop,the carrier frequency often exceeds the capture range of the PLL due to too much Doppler shift in a high dynamic environment,which makes frequency estimation cannot be performed accurately.Therefore,the paper focuses on the modulation Doppler shift modulation and demodulation techniques.Firstly,Doppler Effect of mobile radio channels was introduced in the paper,and then the effect of Doppler shift on phase-shift keying signals was analyzed.Then,the common modulation and demodulation techniques against Doppler shift are summarized and analyzed by simulations.Finally,the two types of countermeasures against Doppler shift based on Differential Phase Keying(DPSK)modulation and Double-Differential Phase Keying(DDPSK)modulation and demodulation are deeply studied.The main research contents and innovations are as follows:Although the traditional multi-symbol differential detection(MSDD)algorithm improves the error detection performance of differential detection to a certain extent,its performance decays rapidly under the condition of large Doppler shift.Aiming at the problem,the Pilot-Assisted Sequence Detection(PASD)algorithm based on single difference signal is proposed.The algorithm takes the frequency offset into consideration for maximum likelihood detection of multiple symbols and estimates the frequency shift introduced in the system by adding pilot symbols before each frame of data so that when there is Doppler shift in the system Bit error performance is hardly affected.The simulation results show that the proposed algorithm can resist the large Doppler frequency shift.Compared with the traditional multi-symbol detection based on single-difference signal,the proposed algorithm can significantly improve the bit error rate(BER)while the overhead is less than 0.5%.Although double differential phase keying modulation is hardly affected by the Doppler shift and can improve the error performance by multi-symbol detection,its practical use is limited due to the high computational complexity of multi-symbol detection.To solve this problem,This paper proposes Reduced-complexity differential recursive detection(RC-DRD)algorithm based on multi-symbol detection to reduce the complexity of differential recursive detection.On the one hand,this algorithm uses the received N consecutive signals and the first N-1 symbols that have been detected to iteratively detect the current symbol to solve the problem of repeated detection of multi-symbol detection,thereby greatly reducing the computational complexity of MSDD base on DDPSK.On the other hand,the proposed algorithm avoids Continuous detection error and improves the error performance of the system by comparing the results of traditional double differential demodulation for every N-1 consecutively detected symbols with the corresponding symbol interval.The simulation results show that the proposed RC-DRD algorithm is almost independent of the Doppler shift.Compared with the traditional multi-symbol detection based on double-difference modulation,the performance of the code is significantly improved while the computational complexity is significantly reduced.Finally,the article concludes with a summary of the full-text work and puts forward the prospect of future research work.