Research On Doppler Frequency Estimation Technology For OFDM Communications

OFDM is a special multi-carrier modulation technology, which has a high spectralefficiency, and can effectively combat multipath fading and impulse noise. In recentyears, OFDM has aroused widespread attention, and has been used in Digital VideoBroadcasting (DVB), Asymmetric Digital Subscriber Line systems (ADSL), WLANand LTE plans, and is expected to become the mainstream technology of the fourthgeneration mobile communication system. However, OFDM technology is sensitive tothe frequency deviation.Especially in high-speed mobile environment, it is prone toproduce Doppler shift and Doppler spread, which would damage OFDM systemsorthogonality between subcarriers, results in serious Inter-Carrier Interference (ICI),andthen results in deterioration of system performance. Therefore, the study about Dopplershift estimation in OFDM systems is essential.This paper supports a more in-depth research on the Doppler shift estimationtechniques, the main as follows:In this paper, the instantaneous value of the Doppler frequency shift estimation hasbeen studied,including the Doppler frequency shift estimation algorithm based onrepetitive structure train symbols and the Doppler frequency shift estimation algorithmbased on the total conjugate symmetry train symbols.The research results show that inthe Doppler channel, the two algorithms have a high level of timing synchronizationaccuracy, and can get accurate estimation results by estimating the instantaneous valueof the Doppler frequency shift and the carrier oscillator frequency offset as a wholeone,and then make up the frequency offset,and finally improve the performance of theOFDM system. But in the multipath and Doppler channel, the two algorithms’estimation accuracy of the timing and frequency offset are so low that it will be hard toestimate the instantaneous value of the Doppler frequency shift.Subsequently, the paper studies the mobile channel’s maximun Doppler shiftestimation techniques. First of all, a cyclic prefix-based Doppler shift estimationalgorithm has been anlysized, in which the paper calculates the correlation function ofthe existing cyclic prefix that based on the OFDM symbols to obtain the maximum Doppler frequency shift. Simulation results show that the greater the signal to noiseratio, the closer the estimation to the true value, and the greater the frequency, the higherthe estimation accuracy. When the carrier frequency offset exists bettwen the receiverand transmitter, the Doppler frequency offset estimation performance of the twoalgorithms is analyzed.The research results show that the estimated value is sum ofthe carrier frequency offset and Doppler frequency offset,which can not be separate.What’ more, the greater the signal to noise ratio, the more realistic the estimated valueof the Doppler shift to the true one,which means that the carrier frequency offsetrobustness of the algorithm is poor.