Researches On Channel Estimation Of OFDM System In High Speed Environment

Orthogonal Frequency Division Multiplexing (OFDM) technique is a key technology of the new generation mobile wireless communication system, having the advantages of effectively combat the frequency selective channel, high frequency efficiency and suitable for high speed transmission. In OFDM system, channel estimation is indispensable for coherent demodulation, and its performance plays an important role in the total system performance. For high speed environment, the channel is becoming time selective, so how to estimate the time varying channel in high speed environment is the problem we are preparing to research.Although channel estimation for OFDM system in high speed environment has been widely researched and gained lots of achievements, the channel estimation mechanisms still need attention and improvements. Besides, the schemes under nonideal situation, for example when carrier frequency offset exists, need more research. This dissertation is supported by the subproject "Key technology research and application for high speed railway broadband communication based on TD-LTE" from major national science and technology project "A new generation of broadband wireless mobile communication network". Some channel estimation schemes suitable for high speed environment are proposed and some achievements have been gained.The main issues of this dissertation are: Chapter1mainly discusses the characteristics of wireless channel, the main purpose of channel estimation and the main problems need to be solved in channel estimation of high speed OFDM system. And the main structure and content of this dissertation is introduced to lay the first stone for later research on high speed channel estimation.Chapter2proposes optimized schemes for linear time varying channel model and basis expansion model separately. The linear time varying channel estimation method is introduced, including the principle and the performance. Besides, the iterative method for improving the performance of linear time varying channel estimation is researched, which can effectively eliminate the effect of inter carrier interference. Because the complexity of the iterative linear time varying channel estimation method is relatively high, a method that can effectively reduce the complexity is proposed. Besides, the principle of basis expansion model is introduced. An optimized generalized complex exponential basis expansion model is proposed, with the coefficient relative to the maximum Doppler frequency, making the maximum frequency of basis function less than the maximum Doppler frequency, which can effectively eliminate the effect of high-frequency noise and improve the channel estimation performance. The sensitivity to Doppler frequency estimation error of different basis expansion models is researched and a channel estimation model based on basis expansion model is proposed which can adaptively adjust coefficient to suit different situations.Chapter3presents a kind of pilot named superimposed training which is superimposed on data sequence and doesn’t occupy bandwidth resources, so it enables higher bandwidth efficiency. A time varying channel estimation method based on superimposed training is proposed. Besides, the inter carrier interference cancellation method is discussed and a superimposed training which can eliminate inter carrier interference by its own structure is presented. Based on this superimposed training, a time varying channel estimation method in proposed and a dual-iteration detection method is designed to further eliminate the inter carrier interference.Chapter4analyzes the effect of carrier frequency offset to the performance and because the existed channel estimation research are mainly conducted based on the assumption of frequency synchronous, a joint estimation scheme of carrier frequency offset and time varying channel is proposed to improve the performance.Chapter5summarizes the dissertation and points out the directions of the future research.