Channel Estimation Algorithm In Fast Time-Varying OFDM Systems

With the advantages of fast transmission rate and high frequency spectrum utilization,orthogonal frequency division multiplexing?OFDM?technology is widely used in wireless communication.Channel estimation is the basis of channel equalization,detection and decoding in OFDM systems.The performance of channel estimation has a direct effect on the accuracy of the OFDM signals.But in fast time-varying environment,the faster the moving velocity of the mobile station,the bigger the Doppler shift,which leads to spectrum broadening and inter-carrier interference?ICI?.Therefore,we need to propose a suitable channel estimation algorithm,which can effectively depress the ICI and further improve system performance.This paper mainly studies the channel estimation algorithm in fast time-varying OFDM systems.The contents and the major contribution of this paper are listed as follows.Firstly,the characteristics of the wireless channel and the basic principle of OFDM systems are introduced,and the channel estimation algorithm based on the pilot is studied.Then the effect of pilot structure on the estimation performance in different environments is analyzed,and the better performance of comb pilot structure in fast time-varying OFDM systems is verified by simulation.Secondly,in order to solve the problem that the common OFDM channel estimation algorithms are mainly for sample-spaced channels and cannot suppress the noise in the channel impulse response?CIR?within the length of cyclic prefix,an improved DFT-based channel estimation algorithm is proposed.The algorithm obtains the distribution of CIR by using the energy growth rate function to depress the noise of CIR within the length of the cyclic prefix.Then through iterations,inter-carrier interference?ICI?and noise are further suppressed.Simulated with the least square?LS?channel estimation algorithm,the traditional channel estimation algorithm based on discrete Fourier transform?DFT?and the channel estimation algorithm based on DFT with the threshold,the system performance of the proposed algorithm improves 3-5dB when the bite error rate is 10^-2.Thirdly,considering that the channel estimation algorithm based on cluster analysis has the edge error and the non-sample-spaced channels could not be well estimated,a channel estimation algorithm based on K-means algorithm is proposed.With the characteristics of the non-sample-spaced channels' CIR distribution,the CIR is divided into the signal part and the noise part by constant discrimination and iterative update of the cluster centers.Simulated with LS channel estimation algorithm,the channel estimation algorithm based on cluster analysis and theimproved DFT-based channel estimation algorithm in the non-sample-spaced channels,the system performance of the proposed algorithm improves 5-7dB when the Doppler shift is 200 Hz and the bite error rate is 10^-2.In the sample-spaced channels,although the system performance of the proposed algorithm is a little worse than the improved DFT-based algorithm,but still better than other two algorithms,which verifies the certain universality of the algorithm.Last,in order to improve the estimation performance of the non-sample-spaced channels in fast time-varying OFDM systems,a new channel estimation algorithm is proposed.The algorithm combines complex exponential basis expansion model and the feedback of DFT,which is built according to the fraction taps channel approximation?FTCA-CE-BEM-DFT?.The channel parameters are first estimated with the complex exponential basis expansion model based on the fraction taps channel approximation?FTCA-CE-BEM?.Then the feedback is obtained by the suppression of ICI,and the noise could be depressed by DFT according to the CIR distribution characteristics of non-sample-spaced channels.The algorithm effectively depresses the ICI,noise and CIR leakage.Simulated with the algorithm based on complex exponential basis expansion model?CE-BEM?,the algorithm combines complex exponential basis expansion model and the feedback of DFT?CE-BEM-DFT?and the FTCA-CE-BEM algorithm,the system performance of the proposed algorithm is best.Compared with FTCA-CE-BEM algorithm,it improves 3dB when the bite error rate is 10^-2,and improves 4 dB when the channel mean square error is 10^-3.