Underwater Acoustic Communications Based On Multiband OFDM

With the increasing marine activities,human have paid more and more attention to the oceans.Relevant marine technologies have also received more and more attention from industry and academia.As an important means of underwater long-distance wireless communications,acoustic wave has important research value.However,the complex channel conditions in the underwater environment have caused great challenges to the development of underwater acoustic communications.Such as multipath effect,doppler effect,long delay and narrow bandwidth make the underwater acoustic signal be distorted easily.These factors affect the performance of underwater acoustic communications.This paper studies the covert underwater acoustic communications based on multi-subband orthogonal frequency division multiplexing(OFDM),and also designs a software-defined underwater acoustic modem with reconfigurability.Finally,we developed two experimental platforms.The specific research contents are as follows:Covert underwater acoustic communications technology can fuse communications signal with the marine environment to prevent third-party detection.This technology can guarantee communications security.The hidden ability of the communications system of the military command system is very important.This paper builds a system model of covert underwater acoustic communications based on multi-subband OFDM.This scheme increases the gain of signal processing at the receiver by extending the signal symbol,and finally achieves good communications performance with low signal-to-noise ratio.This paper shows the signal processing flow of the transmitter and receiver.In the signal estimation module,we chose basis expansion model(BEM)to model the double selective fading underwater acoustic channel.This paper calculates the estimation algorithm of the weight coefficient of BEM basis function.This paper compares the performance of least squares estimator,linear minimum mean square error estimator and best linear unbiased estimator.Finally,we show the simulation results.This paper combines the theory of underwater acoustic communications with actual engineering.we simplify and modify the OFDM technology used in covert communications so that it can match the computing performance of the embedded processor.Than we design and implement an underwater acoustic modem based on software radio technology.This paper presents the complete physical layer transmitter and receiver signal processing flow.This paper selects a linear frequency modulation sequence as the preamble part,and the signal is detected by the receiver with matched filtering.The signal part uses OFDM technology,the receiver uses BEM for channel estimation,and the estimated channel is used for channel equalization.Finally,we implemented the entire algorithm on the embedded board.In this paper,we present the relevant design parameters and verify them through simulation and experimentation.This paper builds two sets of underwater acoustic communications experimental platforms with the underwater acoustic modem as the core.We implement a cross-media communications system based on a water acoustic modem.We solved the problem of the inability to transfer data between underwater and aerial equipment and we provide solutions for the sharing of information between underwater sensors and air sensors.In addition,we have integrated the underwater acoustic modem into our self-developed "Iwinsub-18 Underwater Wizard" unmanned underwater vehicle.We can remotely control it through the underwater acoustic signal to perform actions such as forward and backward,ascending dive,and so on.For the above two systems,we have given the design of the hardware structure,software interface,key technologies,etc.,and demonstrated the experimental results.