Study On Doppler Spread Estimation For OFDM Systems

Orthogonal Frequency Division Multiplexing(OFDM) is the key technology of B3G and 4G mobile communication system. The most significant advantage is its high spectrum efficiency and it also can greatly eliminate the impact of multi-path propagation of wireless channel. With the rapid development of the hardware technique in recent years, OFDM has grown to be a main candidate technology for high speed data transmission. While in the meantime, the main shortcoming of OFDM technology is its sensitivity to frequency offset, especially under high-speed movement environment where Doppler shift and Doppler spread are more perceptible. Doppler shift can destroy the orthogonal characteristic between sub-carriers in OFDM system and bring serious inter-carrier interference (ICI) which decrease the performance of system greatly. In this thesis the exactly estimation methods of Doppler shift are mainly studied.First of all in this thesis, the technology of the maximum Doppler shift estimation in mobile channel is studied, an algorithm for the estimation based on the cyclic prefix of OFDM symbol is analyzed, this algorithm calculates the correlation function of the existing cyclic prefix of OFDM symbol to achieve the information of the maximum Doppler shift. And then we have analyzed, studied a Doppler spread estimation algorithm which based on the transform domain, it analyzes the received pilot transform domain signal spectrum directly to get the doppler shift information. Furthermore, Doppler diversity technology is also studied, we consider the Doppler spread as the source of frequency domain diversity, by this way it can not only overcome the adverse impact of Doppler spread but also improve the performance of system through diversity. 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. The simulation results also show the Doppler diversity technology can decrease the BER obviously and improve the system performance in mobile environment.