Frequency Synchronization And Channel Estimation For OFDM Underwater Acoustic Communication

Ocean channel is such a complex environment that it varies in space, time and frequency. Narrow band, multipaths and fading are the main obstacles in reliable high data rate communication. So it is a hot topic in that how to make the communication performance well in underwater environment for the moment. The Orthogonal Frequency Division Multiplexing (OFDM) is an attractive parallel transmission method, which has high utilization of the frequency spectrum and high data rate, and can combat intersymbol interference (ISI) brought by multipaths effect without complex channel equalization. But because of the sensitivity to the carrier frequency-offset, very small frequency offset will destroy the performance of the OFDM system,badly. So in this paper, frequency synchronization and channel estimation on OFDM is investigated. And an underwater acoustic communication system based on OFDM is designed and simulated. The simulating results indicated that the system and the techniques are reliable. Main work and achievements are summarized as follows:1) High data rate OFDM underwater acoustic system is designed based on the study of basic theory, and the flow chart and simulation models for every functional part are built up. That contains Doppler compensation, symbol timing synchronization, carrier frequency-offset compensation and channel estimation.2)The question of Doppler shift brought by relactively movement of transmitter and receiver is solved. A Rayleigh channel existing Doppler expand model is founded, and a Doppler shift estimation method of diference detection in frequency domain is adopted. Over the method simulation in the channel model, we can see that the estimation effect of this method is good, especially in low SNR, which is suiltable to underwater acoustic communication.3) The symbol timing synchronization method combining the cyclic prefix(CP) energy difference and set correlation is adopted. This method can eliminate the effect of carrier frequency-offset on symbol timing synchronization and can improve the performance of the system.