Preliminary Implementation Of The OFDM Underwater Acoustic Communication System Based On FPGA And Research On Doppler Compensation

Shallow water channel is such a complex environment that it varies with time, space and frequency. Narrow channel band, severe multipaths and frequency dispersion are the main obstacles in reliable high data rate underwater acoustic communications. Now it is a hot topic in the field of underwater acoustic communication.The Orthogonal Frequency Division Multiplexing(OFDM)is an attractive parallel multicarrier transmission technology which has high utilization of the frequency spectrum and can combat intersymbol interference (ISI) caused by multipaths transmission without complex channel equalization. OFDM is now used in underwater acoustic communications. However, the output of the OFDM system is the superposition of the subchannel signals, among which the orthogonality strict requirs. Doppler shift is one of the characteristics of time-varying for the underwater acoustic channel. Doppler shift would changes the frequency and phase of signal,which would inevitably undermines the orthogonality between the subcarriers in the OFDM system and introduce Inter-Carriers Interference(ICI),while phase noise would degrades the performance,so that the performance of the system declines.The focus of this thesis is on the Doppler shift estimation and compensation algorithm for wideband OFDM underwater acoustic communication system.The thesis reviews the developments of underwater acoustic communications at first and then introduces the characteristics of the underwater acoustic channels, proposes the shallow water acoustic channel model based on ray theory. The channel model is analyzed theoretically and the reasonability of the model is verified. The performances of an underwater acoustic communication system based on OFDM are simulated using the shallow water acoustic channel model. In order to compensate the effect of the Doppler shift, this thesis investigates the Doppler effects of underwater acoustic wideband signal and proposes an efficient Doppler estimation and compensation technique for underwater acoustic communications. Computer simulation results show that the proposed approach can reduce the Doppler effect significantly which is caused by the movement between transmitter and receiver. At last the paper implements the simulation of the OFDM underwater acoustic base system on FPGA.