Study On The Doppler Shift Estimation Of OFDM System In High-speed Mobile Environment

Orthogonal frequency division multiplexing (OFDM) is a kind of multi-carrier modulation technology in wireless environment. OFDM has become a key technology that is applied in high-speed mobile communications system for its many advantages, such as, high frequency spectrum efficiency and the effectiveness in countering multi-path fading. However, OFDM system is sensitive to frequency offset which will destroy the orthogonality among sub-carriers so that inter-carrier interference (ICI) is brought in and the system performance degrades greatly. The effect is more obvious under high-speed mobile environment where the time-varying characteristic of channel, Doppler shift and Doppler spread are more obvious. Therefore, the research on the application of OFDM in high-speed mobile environment to improve its ability of countering Doppler effect has important theoretical significance and practical value.This paper focuses on the algorithm of Doppler shift estimation. First, the maximum Doppler shift estimation in mobile channel is studied. And an estimation algorithm which gains the information of Doppler shift through the relation between the autocorrelation function of the same parts shared by cyclic prefix and OFDM symbols and Doppler shift is analyzed, this algorithm is based on the autocorrelation function of the cyclic prefix is analyzed. Then, a Doppler shift estimation algorithm based on transform domain is introduced, which gains the information of Doppler shift by inserting pilot symbols in OFDM symbols and analyzing the spectrum of the received signals in pilot transform domain. Finally, a new algorithm base on channel estimation is proposed by using Orthogonal Variable Spreading Factors (OVSF) combined with the estimation algorithm based on cyclic prefix. Compared with the original algorithms, this algorithm has higher accuracy of estimation, simple structure and is easy to implement, which brings better performance.At last, a simplified Doppler diversity receiver for OFDM system is studied, it can not only guarantee the performance of system but also improve the real-time capability and decrease the complexity of calculation. The simulation results show that Doppler diversity technology can decrease the BER obviously and improve the system performance.