Fast Time-varying Channel Estimation And Design In High-speed Mobile Environment

In recent years,human's life in high-speed mobile environment such as highways and high-speed rail becomes more and more common.As a result,the attentions of traffic safety issues and communication needs in this environment are emerging.The problems brought by high-speed environment to orthogonal frequency division multiplex(OFDM)systems are supposed to be solved in this paper.Mostly,fast time-varying channel estimation and carry frequency offset(CFO)estimation in OFDM systems under high-speed mobile environment are studied.The completed works are listed as follows:1.The impacts brought by high-speed mobile environment to channel estimation in OFDM systems are analyzed.In order to eliminate inter carrier interference(ICI)in OFDM system,which is generated by time-varying channel under vehicular environment,an improved time-varying channel estimation scheme is proposed.A generalized complex-exponential basis expansion model(GCE-BEM)is used to approximate the time-varying channel,and coefficients of the model are estimated via least square(LS)algorithm.Besides,the improved channel estimation scheme utilizes the data symbols to construct pilots as well as the correlation characteristics between channels within two adjacent symbols.Corresponding channel models of the vehicular environment are built according to the actual measurement data,and the modified time-varying channel estimation scheme based on this models is implemented by computer simulation.Simulation results show that the proposed method improves the bit error rate(BER)performance of the system compared with the initial method and presents higher spectrum efficiency.2.The influence brought by high-speed mobile environment to CFO estimation in OFDM systems is analyzed.An improved CFO estimation method based on phase difference is proposed.In this method,OFDM training symbol signals within adjacent sampling time are took the conjugate to alleviate the effect from fast time-varying channel,and the relationship between the results of conjugate and CFO is used to estimate CFO,for which the value of training symbol in time domain is set as "1".Simulation results show that the mean square error(MSE)performance of the improved method is better than traditional MOOSE algorithm,especially in higher signal to noise ratio(SNR),and the superiority is enhancing with channel time variety increasing.Because traditional joint CFO and channel estimation method based on maximum likelihood estimation(MLE)is not adapted to high-speed mobile environment,a modified CFO estimation method based on MLE algorithm is researched.CFO estimation is based on the training sequence in preamble in the modified method,and training symbol value in time domain is set as "1",which can reduce the computational complexity.Polynomial basis expansion model(P-BEM)is used to approximate the time-varying channel,and likelihood function about CFO and sum of the model coefficients is established so that CFO can be estimated accurately by MLE algorithm.Simulation results confirmed the effectiveness of the modified method in CFO estimation under high-speed mobile environment.