Research On OFDM Modulation Recognition And Parameter Blind Estimation

Modulation signal recognition plays a key role in a variety of civil and military applications,such as cognitive radio,spectrum monitoring.In actual wireless communication,multipath channels can cause distortion of the signal,and signal recognition becomes more challenging.Orthogonal frequency division multiplexing(OFDM)in the multipath channel transmission of information due to the advantages of high spectral efficiency and high transmission rate,etc,is one of the main alternatives for the contemporary and the next generation wireless communication systems.So it is of great theoretical and practical significance to study the modulation recognition and parameter estimation of OFDM signals.Therefore,the recognition of different OFDM standard protocol signal,and OFDM parameter estimation in multipath channels environment,based on non-cooperative communication,are researched in this thesis.The main work of this thesis are as follows:(1)Aiming at the limitations of the prior information of the transmitted signal constellation in the cost function of the blind equalization algorithm such as the constant modulus algorithm(CMA)and the modified constant modulus algorithm(MCMA),in order to apply the MCMA algorithm to the scene of non-cooperative communication modulation recognition,the cost function and iterative step size are improved to obtain the VNMCMA algorithm which can be applied to non-cooperative communication conditions.The simulation results show that the constellation of the equilibrium is more clear,and it can effectively reduce residual internal symbol interference,and the VNMCMA algorithm is used to blindly equalize the OFDM subcarriers,which can effectively improve the influence of multipath.(2)The method of detecting OFDM signal based on high order cumulant is studied.According to the actual communication,the receiver can only receive the Gauss white noise at a certain time,then the high order cumulant of Gauss white noise is very close to the value of OFDM.To solve this problem,high order cumulants and cyclic prefix are used to detect OFDM signals and estimate partial parameters in this thesis,the effectiveness of the algorithm is verified by simulation and analysis.(3)The method of identifying OFDM effective sub carrier based on constellation clustering is improved.Subtractive clustering is sensitive to the radius of the domain,and fuzzy clustering requires a pre-specified number of clusters,its sensitivity to the initial center and fuzzy factor,and the diversity of validity function.In order to solve these problems,the improved algorithm can dynamically determine the number of clusters and the radius of the subtractive clustering domain,and replace the initial clustering center of FCM with its clustering center,and combine the Xie-Beni indicators and the relative radius of the constellation after clustering to evaluate the rationality of clustering,it can automatically determine the optimal number of clusters and the type of modulation.The simulation results show that the improved algorithm can more accurately identify the modulation mode of the signal,have better clustering performance,reduce the number of iterations to achieve convergence of fuzzy clustering and extend the range of identifiable signals when the signal-to-noise ratio is greater than or equal to 13 dB,and after the blind equalization,it is recognized that there is a signal-to-noise gain of about 2 dB,compared to the direct identification.