Research On Robust Communication Technology Of OFDM In Time-Varying Underwater Acoustic

In recent years, as orthogonal frequency division multiplexing (OFDM) technology withhigh spectrum efficiency, strong ability of overcoming multi-path and impulse noise, theability of high-speed transmission and simple implementation, OFDM becomes one of theresearch priorities and hotspots in high-speed underwater acoustic communication. OFDM issensitive to Doppler and the underwater acoustic channel is time-varying, frequency-varyingand space-varying. The strong multipath and fast time-varying of underwater acoustic channelcause the performance of OFDM descend seriously. This restricts the application of OFDM inunderwater acoustic communication. So how to realize the robust communication of OFDMin time-varying underwater becomes important.In this thesis, key technologies of OFDM are researched mainly for the time-varyingunderwater acoustic communication. The works done as follows:1. The influence of time-varying underwater acoustic channel on OFDM system isanalyzed. The modeling and simulation of time-varying underwater acoustic channel aregiven which lay the foundation for the later study.2. Doppler estimation and compensation of OFDM are researched. First, the framestructure of OFDM suitable for underwater acoustic communication is designed corroding tocharacteristic of underwater acoustic channel. Then different wideband Doppler estimatedmethods are analyzed and compared and extraction in frequency domain method tocompensate Doppler is proposed. The narrowband Doppler is measured by null subcarrierswhich does not need additional spending and saves the power. The algorithms are validatedby lake experiments. The results show that the two step approach to mitigating the channelDoppler effect removed the ICI caused by Doppler shift perfectly.3. The channel estimation and equalization of OFDM in time-varying channel areresearched. As the underwater acoustic channel is sparse, the MP algorithm is applied in thejoined channel estimation of multipath and Doppler shift. Through the simulation and lakeexperiment, the advantages and shortcomings of MP channel estimation and LS channelestimation performance are analyzed and compared.4. The channel estimation of OFDM based on PTRM is researched. PTRM channelestimation and LS channel estimation performance are analyzed and compared by simulationin different channels which is proved by lake experiments.