ICI Cancellation Techniques For OFDM Systems Under High Mobility Environment

The developments of high-speed railway put forward higher requirement to railway mobile communications system. Compared with normal mobile communications environment, the most prominent problem of high mobility environment is the impact of Doppler shift on communication. Orthogonal frequency division multiplexing (OFDM) has good performance in multi-path delay resilient with high frequency spectrum efficiency, and has become the key technology of next-generation mobile communications system. However, OFDM is sensitive to frequency offset and phase noise. Studies show that time variation of the channel will be enhanced and Doppler shift and Doppler spread will become obvious with the increased mobility. Frequency offset destroys the orthogonality between OFDM sub-carriers, which brings about inter-carrier interference (ICI), resulting in performance degradation. Therefore, study the application of OFDM technology in high mobility environment, and increase its resistance to Doppler effect has important theoretical and practical value.Doppler effect reflects the time-varying characteristics of wireless channel. Time-varying channel can provide time diversity, also known as the Doppler diversity. Doppler diversity considers Doppler spread as a way of diversity, and improves the system performance by enhancing the signal energy. Doppler diversity methods can be summarized into three kinds, transmitter Doppler diversity, receiver Doppler diversity, and joint transmitter and receiver Doppler diversity. Maximum diversity transmission can be achieved by adopting precoder at the transmitter and equalizer at the receiver. Applying Doppler diversity in OFDM system can combat the effect of Doppler spread on system performance. The principle of Doppler diversity is studied systematically, and based on the basis expansion model, maximum diversity transmission under time selective channel is simulated In view of the structure of tall Vandermonde precoder, a Doppler diversity scheme appropriate for OFDM system is designed. Compared with non-coding system, Doppler diversity scheme in OFDM can achieve diversity gain, and larger diversity order can get more diversity gain.With respect to the Doppler shift in OFDM system, ICI cancellation algorithms can be adopted to reduce its effect to system performance. ICI cancellation methods include frequency domain equalization, time domain windowing, ICI self-cancellation algorithm, and so on. In OFDM system, when the normalized frequency shift is not too large, simple ICI self-cancellation algorithm can be used to decrease the effect of ICI. The basic principle of ICI self-cancellation algorithm is analyzed in detail, and the performance of several ICI self-cancellation algorithms are compared by simulation. Based on existing ICI self-cancellation algorithm, an improved ICI self-cancellation algorithm is proposed. Simulation results show that the performance of this algorithm is better than previous relative algorithms when the normalized frequency is not large.