Doppler Shift Estimation In OFDM Mobile Communication System

After entering the 90s,the communication technology has gone through a period of rapid development.And Communication Technology has started form 2G to 3G and owned long term evolution for 4G.Because of the progress of Communication Technology,it must come a popularity of intelligent terminal.OFDM technology is the most widely used and it has good performance in many high speed data transmission scenarios.With the arrival of the era of the acceleration for the whole people,the Doppler shift caused by high moving speed is inevitable.However the maximum Doppler frequency is an important parameter.It also can be used in interleaving technique,handoff decision and resource allocation.The accuracy of maximum Doppler frequency estimation will affect the performance of the entire communication system.So the Doppler shift estimation has important practical significance.In this paper the development status of mobile communication is briefly introduce.The advantage and disadvantage of OFDM system and the OFDM symbol structure feature are summarized.The three classical Doppler frequency estimation methods are discussed and the characteristics of those algorithms in OFDM system are analyzed.Due to the simulation analysis of the algorithm,the channel is needed,so we need to analyze the characteristics of the land mobile communication channel,at the same time the Rayleigh fading channel with Jakes power spectrum is modeled and the simulation as well as performance is discussed.Based on the theoretical of the autocorrelation function(ACF)algorithm,which means after the periodic signal is transmitted through the channel,the relation between the autocorrelation function and the maximum Doppler shift value satisfies the characteristics of the first class of Bessel's function.And the deficiency of the cyclic prefix auto correlation function algorithm is analyzed.The algorithm actually uses the cyclic prefix of the OFDM symbol which is a simple copy at the end of the OFDM symbol,so the cyclic prefix can be regarded as a pseudo periodic signal.Because of that the correlation function of the pseudo periodic signal can be used to obtain the maximum Doppler frequency shift.Due to the limited length of cyclic prefix,that method is easily affected by the noise.So the estimation error of the maximum Doppler shift value is very high.And in the relevant operation,the relevant time value is also very small.Then in the use of the Bessel function from the autocorrelation function to solve the maximum Doppler frequency shift value will bring greater inverse error.In this paper,according to the deficiency of cyclic prefix algorithm,based on the principle of ACF algorithm,two improved algorithms are proposed respectively.The first improved method is to use the S&C synchronization symbol to estimate the Doppler shift.Because the data amount of the S&C synchronization symbol is far larger than the length of the cyclic prefix,so when the relevant operation is made,the anti-noise ability of the Doppler shift estimation is enhanced.But the algorithm still has the same disadvantage of using the cyclic prefix which the relative time is very short.The second improved method is to use PUSCH channel which the periodic signal is transmitted and can be used to estimate Doppler shift.This method not only increases the amount of data involved in related operations,but also has a large correlation time.So in the conventional velocity range,the dynamic range of the maximum Doppler shift value can be extended almost to the entire first monotone interval of the Bessel's function,which the nonlinear influence of Bessel's function on Doppler shift value is reduced.The estimated performance is better than that of the CP method,especially in the low signal to noise ratio.