Research On Diversity Technologies For Broad-band OFDM System Under High-speed Mobile Environment

Because of the high spectrum efficiency, ease of implementation (via Fast Fourier Transform) and so on, orthogonal frequency division multiplexing (OFDM) technology has become the key technology of high-speed data transmission, and has been adopted by the fourth generation mobile communication. But OFDM systems are very sensitive to frequency deviation, particularly in high-speed mobile environment. Because the user moves fast, the channel's time-varying characteristics is obvious, the Doppler shift and Doppler spread are significant. These cause the frequency offset, thus the orthogonality between OFDM sub-carriers is destroyed, which results in inter-carrier interference (ICI) and greatly reduces the system performance. In this thesis we focus on the Doppler effects which affect OFDM significantly, and study key technologies of OFDM in high-speed mobile communications.In this thesis we study the physical properties and mathematical model of high-speed wireless channels, describe the characteristics of several fading channels, and analyze the impact of time-varying channel on signal spectrum. We analyze several common fading channels'function, and mainly focus on the key techniques of OFDM systems under high-speed mobile environment. Then we introduce the OFDM systems' basic principle and diversity techniques, give OFDM time domain and frequency domain model, and discuss the OFDM systems' advantages and disadvantages. Finally we introduce diversity techniques and several commonly used methods. The advantages and disadvantages of diversity are analyzed, and the general rules and characteristics of diversity techniques are given.The main word of this thesis is Doppler diversity technology. We take the Doppler spread as a frequency diversity resource, which not only overcomes the impact of Doppler spread on OFDM systems, but also improves the performance of system through diversity. At the receiver, we study Doppler diversity for OFDM systems under high-speed mobile environment, analyze a simplified Doppler diversity technology for OFDM systems, and discuss the Doppler diversity technology for MIMO-OFDM systems. Simulation results show the Doppler diversity techniques combine with transmitter diversity can improve the system performance obviously. In the transmitter, according to the high-speed train's feature, we propose Doppler diversity transmitter, and propose the scheme of future high-speed broadband OFDM mobile communication, which is based on the scheme of TD-SCDMA high-speed railway and solves the application problem of Doppler diversity technique. The simulation results show this technology has a good performance.