Research On And Simulation Of Synchronization Technique In OFDM Wireless Communication System

Orthogonal Frequency Division Multiplexing(OFDM)is a kind of multi-carrier technology with high spectrum efficiency, which can robust the multi-path fading and inter-symbol interference efficiently and is considered as the key technology for the next generation mobile communication system. However OFDM is very sensitive to time and frequency synchronization errors, especially the frequency synchronization errors, so it has higher demand for synchronization. In this thesis, the principle of OFDM system is described, and the timing and frequency synchronization,frequency tracking and phase compensation are researched and simulated with matlab on the basis of analyzing the influence of the synchronization errors to the system performance. The major achievements are outlined as follows:At the beginning of this thesis, the whole OFDM transmission system is simulated with matlab in ideal channel and the signal transmitted is analyzed in time and frequency domain, and the process of the modulation-demodulation is researched too. And next introduces the work of putting up the OFDM equivalent baseband system based on the model of wireless fading channel and AWGN noise.Secondly, a better timing and frequency synchronization algorithm of OFDM has been introduced, in which Chu sequence is used as training sequence and a new preamble is designed, and timing synchronization is accomplished only one step by using a new timing metric. Meanwhile, the integral and decimal frequency offset can be obtained in time domain, thus reduce the complexity of the system. In multi-path fading channel, by improving the timing metric, the first channel path can be detected exactly. The results of the simulation show that the proposed method can meet the demands of the system synchronization in low SNR and multi-path fading channel.Thirdly, a method of setting the threshold of timing metric reasonably in multi-path fading channel is proposed. By using Monte-Carlo simulation, the probability distribution of timing metric can be obtained, while the analytical expression is uncertain, and then the suitable threshold to minimize the missing and false detection probabilities of frame timing synchronization.Finally, the frequency tracking used with the correlation with cyclic prefix and phase compensation based on pilots are analyzed and researched. Because of the residual synchronization errors existing in OFDM transmission system after timing and frequency synchronization, frequency tracking is needed to limit the residual frequency error in a permit zone. Furthermore, phase error in system results in constellation whole rotation, and phase estimation and compensation in frequency domain basically mitigate the influence of phase error.