Study On Key Technologies Of OFDM In High-Speed Mobile Communications

Orthogonal Frequency Division Multiplexing (OFDM) has high spectrum efficiency and capability to mitigate the side-effect of multipath propagation. So it has been regarded as a key technology for high data rate transmission. But OFDM is sensitive to frequency offset, especially under high-speed mobile environment where Doppler shift and Doppler spread are obvious. Frequency offset destroys the orthogonality between OFDM sub-carriers, which brings about inter-carrier interference (ICI), resulting in performance degradation. This dissertation focuses on the Doppler effects which influence OFDM significantly and studies key technologies of OFDM in high-speed mobile communications. The content includes:Firstly, this dissertation studies the Doppler shift estimation. Utilizing the signal envelope, Doppler shift estimation technology for Rician channel is studied. And a corresponding modified algorithm is proposed. For high-speed railway environment, with a novel Rician channel model, the paper puts forward a new Doppler shift estimation algorithm using the envelope statistical properties of received signals.Doppler spread destroys the orthogonality between OFDM sub-carriers. But at the same time, Doppler spread brings usable resources. Doppler diversity at the receiver is studied for Doppler spread is obvious in high-speed mobile environments. Then a novel simplified Doppler diversity technology for OFDM is proposed. Following that, the optimal configuration of the simplified Doppler diversity receiver is put forward.At the transmitter, in order to utilize the resources resulting from Doppler spread, the duration of transmission signal is increased, and then time selectivity can be increased. In this case, time diversity can be obtained. For combined OFDM and CDMA systems, a new overlapped time selective transmit signaling is designed, by which the diversity can be achieved and transmission efficiency will not be reduced. The corresponding received signal processing is given out.In multiple antennas OFDM systems, there are ICI due to Doppler shift and co-channel interference (CCI) that exists in multiple antennas systems. A joint interference cancellation technique is proposed for Space Frequency Block Code (SFBC)-OFDM system, which can restrains ICI and CCI at the same time and improves...