Research On Time-varying Doppler Compensation Of Underwater Acoustic OFDM

Acoustic waves are currently the only underwater information carriers that can be transmitted remotely.Underwater acoustic channels have characteristics such as narrow bandwidth,multipath strength,high noise,and random variation in space-time-frequency.Therefore,it is necessary to achieve high-speed and accurate underwater communication between nodes raletively hard.Orthogonal frequency division multiplexing(OFDM)technology has the advantages of suppressing multipath effects and high frequency utilization.It is suitable for high-speed underwater communication.However,Doppler frequency changes caused by relative movement between communication nodes will cause Subcarrier interference in OFDM systems affects the performance of OFDM communication systems.This paper first studies the basic principle of OFDM and analyzes the orthogonality between subcarriers in time domain and frequency domain.Secondly,the doppler effect caused by the relative motion of receiver and transmitter is studied and the influence of doppler frequency shift on the orthogonality of subcarriers in OFDM system is analyzed.At the same time this paper caused by collecting originated relative motion interference sub-channels,studied the EMD adaptive equalization algorithm,the algorithm using the coherent detection method to separate the signal center frequency deviation of the component,and using the PEMD(Pilot FFT)decomposition coherent detection after processing the signal frequency components,and then estimate the signal frequency deviation caused by doppler,adaptive equalization algorithm is then used to residual doppler compensation.This paper proposes a doppler suppression algorithm based on WFFT(wavelet FFT).This method takes channel decomposition and reconstruction as a general idea and USES a set of predetermined functions to decompose channels.In the reconstruction process,according to the idea of function approximation,this set of predetermined functions is used to approximate the channel function.At the same time,differential coherence detection technology and adaptive stochastic gradient algorithm are used to obtain tap coefficients in the process of signal reconstruction,which has good pilot performance and channel tracking ability.Finally,the algorithm is simulated,and the feasibility of the algorithm is preliminarily verified by a pool experiment.