The Research Of Inter-Carrier Interference Cancellation Algorithm For MIMO-OFDM Systems In High-Speed Railway Environment

With the rapid development of the high-speed railway and the modern communications technology, broadband access technology for the high-speed railway also raised new challenges. In 2010, China Railway Bureau proposed that we would research the time division duplex LTE-R broadband mobile communication system independently in the seventh world high-speed railway congress. The system is based on orthogonal frequency division multiplexing(OFDM) technology. OFDM is an effective transmission scheme to provide high-rate services due to its appealing properties such as high spectral efficiency and robustness against multipath fading channels. Therefore, it is great significant to investigate OFDM communications technology in the high-speed railway environment. However, OFDM systems require to maintain strict orthogonality between subcarriers, in the mobile environment, especially the ultra-high speed mobile environment, the doppler frequency offset destroys the orthogonality between subcarriers and causes inter-carrier interference(ICI). As a result, the communications performance may be degraded significantly. Therefore, it has being more attention during the current years to solve the ICI issue of OFDM systems in the high-speed railway environment.Firstly, we simply summarize the traditional channel model and high-speed railway channel model, and the corresponding MIMO-OFDM systems are introduced. The ICI specific expression is derived, which leads to the main issues studied in this article. The main issues are to analyze and eliminate the ICI.Secondly, the natural reason of causing ICI is time-varying channel by investigating the high-speed railway channel model and ICI expression. Further, analysis shows that the influencing factors to ICI are rice K factor, multipath number and doppler frequency offset. Carrier-to-interference ratio(CIR) as an evaluated standard to the ICI level, quantitative analysis that CIR as a function of rice K factor, multipath number and doppler frequency offset respectively is implemented, besides, considering the actual scenarios, we take simulations that the desired signal power and ICI power are affected by the changing of train speed, respectively.Finally, two ICI cancellation algorithms are proposed: 1、joint cancellation algorithm of cyclic prefix and zero-forcing equalizer; 2、adaptive phase rotated conjugate cancellation algorithm. Where, joint cancellation algorithm of cyclic prefix and zero-forcing equalizer is the use of cyclic prefix achieves the ICI self-cancellation, effectively improves the CIR, receiving signals are detected by zero-forcing equalizer. Adaptive phase rotated conjugate cancellation algorithm is a two-path transmission self-cancellation scheme. Conjugate transmission are employed on the two-path transmitting signals, conjugate receive and a group of additional rotated phases are employed on the two-path receiving signals. Maximizing the CIR by adjusting the group of additional rotated phases. An adaptive ICI mitigation algorithm is proposed by updating the group of additional rotated phases per two OFDM symbol periods.In this article, the three aspects of work are a complete study including the ICI caused reason, ICI influencing factors and ICI cancellation methods. These study will provide a strong theoretical foundation and technical support for the future railway communications technology.