Timing And Frequency Synchronization Algorithm For OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a kind of multi-carriermodulation technology with high spectrum efficiency and multi-path fading resistanceability, so the OFDM system is suitable for high speed wireless transmission. But theOFDM technology has a shortcoming that it is highly sensitive to synchronization error. Themutually orthogonal sub-carriers will bring inter-carrier interference when they aredestroyed by the frequency offset, and the timing error would introduce inter-symbolinterference. Thus it is very important to research the symbol timing synchronization and thecarrier frequency synchronization techniques for OFDM system.At first, the academic research background and significance, the recent status of theOFDM synchronization technology and the basic principle of the OFDM system are brieflyintroduced in this paper. The influence to the system performance of the symbol timingoffset and carrier frequency offset are analyzed in detail, and then these influence factors arealso analyzed by simulation to emphasize the importance of the synchronization.Secondly, the advantages and the disadvantages of the timing synchronization andcarrier frequency synchronization estimation algorithms are compared by analysis andsimulation through the MATLAB software. The advantages and shortcomings of themaximum likelihood algorithm based on the cyclic prefix are analyzed in great detail inblind estimation algorithm which does not include auxiliary information. The algorithmdoes not need additional overhead, and it will not affect the bandwidth of the system, but itcould only estimate carrier frequency offset in a small range. The synchronization process ofthe Schmidl&Cox algorithm based on training sequence is researched in the algorithmwhich is based on the additional auxiliary information. The estimation processes of theMinn algorithm and the Morelli algorithm to improve the original algorithm are analyzedfor the defects of the algorithm. And the synchronization performance of the improvedalgorithm is compared with the original algorithm by the simulation.A new time and frequency synchronization algorithm is proposed on the basis of the aboveanalysis of the synchronization algorithm based on training sequence. The new trainingsequence of the improved algorithm has a similar structure with the training sequence of theMinn algorithm. But the weighting factors which have a good correlation are introduced.The correlation function of the cyclic prefix and the tail data of the symbol are combinedwith the original symbol decision function for eliminating the effects of the side-lobes. Sothe accuracy of symbol timing estimation is improved. To calculate the carrier frequencyoffset, the improved algorithm utilizes the differential relationship of the frequency domaindata and uses only one training symbol that the carrier frequency offset could be estimated in a wide range. The simulation of the estimating performance of the algorithm proves thatthe improved algorithm has a lower estimation error than the Schmidl&Cox algorithm, andthe synchronization performance is improved.