| The twenty-first Century is the century of the ocean. With the continuous exploration of the ocean, underwater acoustic communication, which is the leading technology of ocean developing, draws more and more attentions from human beings. Underwater acoustic communication means to achieve the signal transmission in the underwater environment, including texts, pictures and something else. However, due to the specificity of underwater acoustic channel, there are still some differences between underwater acoustic communication and terrestrial wireless communication, which leads to a lot of mature technologies used in wireless communication are not applicable or need to improve and optimized in underwater acoustic communication. So how to select and improve a variety of technologies to achieve high-speed, reliable and long-range underwater acoustic communication has been one of the key researches.Orthogonal frequency-division multiplexing (OFDM) technology modulated signals by a number of orthogonal subcarriers. This technology can overcome frequency selective fading effectively against multi-path effect of the underwater acoustic channel and improve the the bandwidth efficiency. So the research of OFDM technology used in underwater acoustic communication is also increasing in recent years. However, the subcarriers of OFDM technology are sensitive to frequency offset, while the Doppler effect of underwater acoustic channel is serious, which will affect the system performance seriously. Therefore, it is very important to design the improved solution to cater to the technology.Multiple input and multiple output (MIMO) technology is a new favorite of wireless communication. Not only applied in the third generation mobile communication (3G), but also become one of the key technology in 4G. MIMO technology can increase channel capacity greatly and improve system performance without increase the bandwidth or transmitted power. This has an important significance to the underwater acoustic channel which is the band-limited one. So the fusion of MIMO technology and OFDM technology that applies in underwater acoustic communication can not only against multi-path effect and inhibit the frequency selective fading, but also increase channel capacity and improve the speed rate of transmission.At first, this paper introduces the specificity of underwater acoustic channel and the current status of underwater acoustic communication. Analysis the difficulties brought by the Doppler Effect, multi-path effects and band-limited feature of underwater acoustic communication. It also studies the theory knowledge of MIMO technology and OFDM technology separately and carries on the research and experiment simulating of the key technology of MIMO-OFDM underwater acoustic communication system. The paper analysis the impact on the performance to underwater acoustic communication system from both theory and simulation.Then the paper studies the impact on the system from the Doppler frequency offset and how to estimate and compensate. It analysis the reason why the Doppler frequency offset will cause serious impact on MIMO-OFDM underwater acoustic communication system by theoretical. In addition, the paper presents an improved algorithm, which is improved based on the matched autocorrelation algorithm proposed by Sean F.Mason. This algorithm consider to reducing the number of parallel autocorrelation to reduce the computation and improve the efficiency of the system. Then achieve the complete Doppler frequency offset compensation by the second compensation.Finally, the paper shows that the algorithm can achieve the estimation and compensation of the Doppler frequency offset on the basis of reducing the complexity of the receiver of the system. In addition, the paper put forward the key factors that influence the effect which makes it more convenient for the adjustment and improvement of the experimental method according to the different actual situations. It will accommodate different conditions and requirements preferably. |
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