| There are great successes of underwater targets in terms of maneuverability and invisibility,which bring great challenges to traditional underwater detection techniques with sonar array.Compared with the traditional sonar array with fixed shape,the flexible version can be established based on cooperative MIMO scheme.However,the performance of such an array configuration is influenced by the imperfect clock synchronization among the underwater nodes.Different from underwater acoustic communication,magnetic induction(MI)communication transmits information through induced magnetic field,which has the characteristics of low signal propagation delay and stable transmission channel.In this thesis,magnetic induction communication is utilized to synchronize underwater nodes of cooperative MIMO array.Furthermore,the DOA estimation scheme is studied based on the established cooperative MIMO system.Firstly,the basic principles of magnetic induction communication and communication model are analyzed,as well as the transmission characteristics.Aiming at the problem of clock synchronization of underwater nodes,an underwater cooperative MIMO system model based on MI synchronization is established.The advantage of this system is that the signal propagation delay of MI is smaller and the bandwidth is larger,as a result,the synchronization error is much smaller than that of underwater acoustic communication.Based on the master-slave synchronization strategy,the synchronization error of the system is analyzed,and the simulation results verify the advantages of MI synchronization.Secondly,following the framework if compressed sensing,the direction finding scheme of cooperative MIMO is studied.Each node applies compressive sampling to the received data,which is send to the center node subsequently.The advantage of this scheme is that it can achieve a high resolution with less sampled data,thus the power consumption during data transmission phase is reduced,which implies time expand of underwater working.Based on the signal model,the effect of synchronization error is analyzed and simulated.The simulation results show that the algorithm has higher resolution than that of MUSIC algorithm,and the accuracy of direction-of-arrival(DOA)estimation is higher in the MI synchronization mode.Finally,considering the complex underwater environment and synchronization error,a robust beamforming technique for the cooperative MIMO system is studied.Given that underwater signals obey sub-Gaussian distribution,the beamforming algorithm based on the minimum deviation criterion is studied,as well as the effect of synchronization error.The beamforming algorithm can be described as an optimization problem,which can be solved by the standard interior point method,but the computational complexity is high.Considering the energy consumption limitation of underwater nodes,the descent algorithm based on Newton's method is used to solve the problem efficiently.The simulation results show the effectiveness of beamforming algorithm in the MI synchronization mode.There are 25 figures,3 tables and 82 references in this thesis. |
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