| As a form of modern warfare,electronic countermeasures mainly carried out between the communication and reconnaissance of the two sides of the battle.In addition to ensuring the normal communication of the friendly side,it is also necessary to decipher the communication of the enemy.Among all the links to obtain the enemy’s communication information,the modulation identification and demodulation of the signal is the most important part.Information acquisition can be realized only by correctly identifying the type of the transmitted signal.The identification algorithm of digital modulated signal and demodulation technology nology are studied in this thesis.Through the research of identification algorithm and the analysis of identification effect,the identification of digital modulated signal is completed,and then the demodulation of digital modulated signal is realized.The main content and work of this thesis are summarized as follows:(1)Summarize and sort out the modulation identification and demodulation technology of communication signal,expound the modulation and demodulation principle and identification and demodulation method of digital modulation signal.In addition,its waveform diagram,spectrum diagram and constellation diagram are simulated.Combined with the way to realize demodulation,the basics of synchronization are introduced.(2)A modulation identification algorithm for instantaneous information suitable for high SNR,and a modulation identification algorithm for high-order cumulant and wavelet suitable for low SNR are designed.The modulation identification algorithm based on instantaneous information first analyzes the characteristics of the digital modulation signal,extracts the instantaneous amplitude,the absolute value standard deviation of the instantaneous frequency,etc.as characteristic parameters.Based on the different information of the signal extracted from each characteristic parameter,combine with the decision tree classifier,select the appropriate decision threshold value,and design the process of identification and classification.The simulation results show that when the signal-to-noise ratio is higher,the recognition rate of the signal is about 91% or more.Then,in order to improve the recognition rate of digital modulated signals,the modulation identification algorithm of high-order cumulant is used to study it.By using the advantages of high-order cumulant,the characteristic parameters are extracted by analyzing the second-order,fourthorder and other different-order cumulants.When the recognition effect of some signals is not ideal,the wavelet transform is used to improve it.The simulation results show that the algorithm can obtain a higher recognition rate when the signal-to-noise ratio is low.Through the comparative analysis of the two algorithms,both of them can be applied to different SNR environments.(3)The hardware platform of digital modulation signal recognition and demodulation is designed and built based on FPGA.First,combined with the instantaneous characteristics of the signal,the hardware realization of digital modulation signal recognition is completed.Secondly,after the classification and identification is completed,the modules that realize digital modulation signal demodulation are verified.This thesis designs the bit synchronization and carrier synchronization modules involved in signal demodulation,and uses Verilog language to complete the behavior-level verification of each module,thus the synthesis of each module can be completed.Finally,through the demodulation of the input signal and the analysis of the waveforms of each module,the modulation identification mode is judged.The running results show that the designed experimental platform can complete the identification and demodulation of digital modulated signals,which proves the correctness of the design. |
