The Study Of The Key Technologies For High-Speed Shallow Water Acoustic Communication Based On OFDM

In traditional single-carrier coherent underwater acoustic communication, adaptive decision feedback equalizer coupled with phase tracking methods is adopted. As the communication rate is increasing, symbol duration is becoming shorter, and equalizer is more complex with channel delay spread, the application of traditional methods is hampered in high-speed underwater acoustic communication.Orthogonal frequency division multiplexing (OFDM) has been successfully used in broadband wireless communication over radio channel. Because the performances such as high spectrum efficiency, strong anti-multipath and impulse noise ability, high-speed transmission capacity under efficient bandwidth utilization, and simple implementation, OFDM becomes one of the research priorities and hotspots in high-speed underwater acoustic communication.The most difficult problems in realizing high-speed underwater acoustic communication are limited bandwidth, strong multi-path and rapid time-varying in channel structure. Considering the high noise background in the marine environment, so far the underwater acoustic channel has become the most difficult wireless communication channel. Therefore, how to realize high-speed and reliable underwater acoustic communication, especially in shallow water, is a worldwide problem.In this thesis, key technologies and applications of OFDM are researched mainly for the shallow underwater acoustic communication. The works done as follows:1. Coded OFDM (COFDM). Application of Turbo code is studied in underwater acoustic OFDM system, and the performances of Turbo code are compared with the popular convolutional code and Reed-Solomon (RS) code. Then LDPC code, of which the excellent performances are re-recognized through studying the Turbo code, is explored primarily and validated by simulating and experimenting in the pool and lake test.2.Doppler estimation and compensation in mobile OFDM underwater acoustic communication system. Two algorithms based on CP-OFDM are designed which are block Doppler estimation and frequency measurement method based on single-frequency signal. Also the experiment about mobile underwater acoustic communication on OFDM is done, the two algorithms are validated, and the advantages and disadvantages as well as the development are analyzed.3.Channel estimation and compensation. Analysis and Comparison of the different types of channel estimation and compensation techniques are made out, the principles of how to choose the estimation and compensation methods are pointed out. For different guard intervals (CP-OFDM, ZP-OFDM), capabilities are compared, principles of choosing guard intervals are given. Finally, considering the sparse features in shallow acoustic channel, a channel estimation and compensation method based on the combination of DFT and LS is given, and the improved algorithm performances are validated through simulation and experiments.4. Spatial diversity in OFDM underwater acoustic communication. Three kinds of diversity combining methods are analyzed, which are the Maximum Ratio Combining (MRC), Equal Gain Combining (EGC) and Selection Combining (SC). Different spatial diversity techniques combined OFDM are analyzed in computational complexity and error-rate performance. At last, Pre-DFT-MRC, Post-DFT-MRC, Post-DFT-EQU, Post-DFT-SC are simulated and experimented in pool test. Through the comparison of computational complexity, simulation and experiment results, the principles of choosing spatial diversity algorithms are given.