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Research on OFDM key technology of underwater acoustic communication in shallow water environment

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Tutor: ZhangYan
School: China Ship Research Institute
Course: Underwater Acoustics
Keywords: OFDM,synchronization errors,Doppler compensation,synchronization technique,compr
CLC: TN929.3
Type: Master's thesis
Year:  2013
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Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme is widely usedin high speed underwater communication, since its high spectral utilization efficiency androbustness against multipath. While taking into account the synchronization error and Dopplereffect, however the performance of OFDM might degrade significantly. In this paper, we firstanalyze the impact of the synchronization error and Doppler drift on OFDM. Then wecomprehensively study the key techniques to overcome these negative effects. The workincludes:1. The features of shallow water acoustic channel and the Doppler effect are studied. A fullOFDM communication system is designed, which includes scrambling, frequency interleaving,peak to average ratio reduction technique and error correcting coding. Then we compare theperformance among three different kinds of peak to average ratio reduction techniques.2. We analyze the effects of synchronization on channel equalization. The severesynchronization error might cause the aliasing in comb-pilot least square (LS) algorithm basedchannel estimation. Hence, in this paper, we propose a second synchronization algorithm toaddress the problem from linear frequency modulation correlation algorithm. The presentedalgorithm is tested in lake experiments. The results show that the accuracy of LS channelestimation is significantly improved if there is synchronization error.3. We comprehensively compared the performance of several Doppler estimationalgorithms in tank experiment, which include linear resampling algorithm (to compensatewideband Doppler), null sub-carriers frequency offset estimation algorithm (to compensatenarrowband Doppler) and compressed sensing channel estimation algorithm (to suppress theinter-carrier interference caused by Doppler spread). 4. Lake experiment shows that the recursive channel estimation algorithm is very sensitiveto the accumulated synchronization error, which is caused by Doppler effect, because theaccumulated synchronization errors bring in phase rotation. An adaptive blind channelestimation and phase compensation algorithm is analyzed. The differential channel equalizationalgorithm also tends to be effected by phase rotation, The simplified Doppler phasecompensation algorithm which is proposed here can clearly explain and solve this problem. Thisis verified in lake experiment.The results of this paper, demonstrate that these algorithms can improve the robustness ofthe OFDM acoustic communication, and it will have a practical implication for UnmannedUnderwater Vehicles (UUVs) which use acoustic for communication in the complex shallowwater environment.
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