Underwater Acoustic Communication Node Implementation Based On OFDM And Research On The Time Varying Channel

With the development of oceans in recent years,the importance of underwater acoustic communication has been highlighted,and the rate and reliability of underwater acoustic communication have been increased.Orthogonal frequency division multiplexing?OFDM?is used as a high rate modulation technique has been implicated into the field of underwater acoustic communications more and more.However,some of the underwater acoustic channels have severe multi-channel effects and temporal variability,which we call the double selective channel?DSC?.The time-varying characteristic of the Doppler shift and the propagation induces OFDM subcarriers?ICI?,which affects the performance of the OFDM underwater acoustic communication system.In this paper,the channel estimation and equalization technology of the underwater acoustic OFDM system under DSC is studied,and the basic work of OFDM system on DSP is done.Firstly,the basic principle of OFDM is introduced and the detailed explanation of DSC is given.On this basis,the mathematical model of OFDM system based on DSC is established.Because of the complexity of channel estimation and equalization of OFDM system under DSC,a kind of basis extended model?BEM?is used to approximate DSC,so the mathematical model of the system is simplified and the parameters needed to be estimated are greatly reduced.Secondly,a channel estimation method based on linear minimum mean square error?LMMSE?is introduced by using BEM to approximate DSC.This method makes full use of the elements of the channel frequency domain matrix.At the same time,three kinds of channel equalization methods based on LMMSE are given,namely,frequency domain windowing iterative equalization,frequency domain elimination method iterative equalization and time domain iterative equalization,which are given in different Doppler and different The performance of each equalization mode under the method of simplified method is selected,and the optimal equalization is selected and the performance of the equalization mode under different conditions is analyzed according to the simulation results.When snr is 15 dB,the ber is less than 10^-3.Furthermore,considering the sparseness of the underwater acoustic channel,the distribution of the average power and time-varying channels of the sparse DSC in each path is introduced,and the average power of the sparse channel is estimated by using the constant amplitude zero autocorrelation?CAZAC?And gives the simulation results of the method to illustrate its effectiveness.Then,using the average power of each path of the estimated channel and considering the DSC of the underwater acoustic sparseness,the channel estimation and channel equalization are carried out for the sparse DSC by combining the channel estimation method and the optimal channel equalization method described in the previous chapter.The simulation results of different conditions are compared to discuss the effect of channel estimation and equalization method under different conditions.When snr is15 dB,the ber is less than 10^-4.Finally,after the OFDM system simulation of the underwater acoustic sparseness DSC,the basic OFDM system is implemented by DSP.In this part,the hardware platform is introduced,and then the basic OFDM system theory,such as up sampling and PWM modulation,is simulated and analyzed.After the theory is designed and implemented,the signal transmission and reception are considered.Then,we make a experiment by DSP in the pool to verify the validity of OFDM MODEM.