Study On Synchronization Of Timing And Frequency Technology For OFDM

OFDM system is one basic technique of the future wireless communication. It is a Multi-Carrier Modulation technique with high performance. It has high utilization of frequency spectrum, good capability of countering inter-symbol interference and multi-path fading, It is also adaptive to high-rate data transmission in wireless channel.The OFDM system is sensitive to synchronization error. The orthogonality between the sub-carries is destroyed by frequency offset, that will cause inter-carrier interference (ICI), the timing error also introduces inter-symbol interference (ISI). Thus studying the synchronization techniques of OFDM has signality to improve the system performance. The main work will be described as followings in this paper:Firstly, the background, basic principle, structure and the system mode of the OFDM are introduced in this thesis, the orthogonality between the sub-carrier for OFDM system, the realization of OFDM system based on IDFT/DFT and the structure of OFDM symbol with Cyclic Prefix (CP) are described, and the effects of symbol timing error and the carrier frequency offset on the performance of OFDM system are discussed and deduced in theory.Secondly, the ML algorithm based on CP in being is analyzed, the method of maximum likelihood estimation is adopted to jointly estimate symbol timing and carrier frequency offset, and some improved algorithms that have been existed are also analyzed. Base on the analysis, an improved ML algorithm is proposed, the fore feedback structure which is used to optimize the ML algorithm. Simulation results show that the improved algorithm is better than the traditional algorithm.In the end, the characteristic of training symbol and the principle for SC algorithm and improved algorithm are researched. The disadvantage and advantage are analyzed about the classic timing synchronization algorithms based on training symbol. An improved training symbol with symmetric and repetitive structure is proposed, and the estimate range of the frequency offset is enlarged by increasing the number of repetitive sequence and shortening the interval of repetitive sequence, and a new training symbol weighted by the pseudo-noise (PN) sequence is designed. The symmetric property of the training symbol and the correlation property of the PN weighted factor are utilized in the timing offset estimation to improve the accuracy. Simulation results show that the improved joint timing and frequency synchronization algorithm is better than the conventional SC algorithm in the accuracy of timing synchronization and the estimate range of the frequency offset.