Research On Anti-multipath Time Synchronization Of OFDM Underwater Acoustic Communication System

With the increasing ocean exploration in recent years,there is an increasing demand for reliable underwater acoustic communications.However,the underwater environment is sophisticated,and the underwater acoustic channel has serious multipath effect,large delay spread,significant Doppler effect and strong time-varying characteristics.Orthogonal Frequ ency Division Multiplexing(OFDM)has been widely used in underwater acoustic communication s in recent years due to its good anti-multipath fading capability and high frequency band utilization.The OFDM communications system has certain requirements for time synchronization: that is,the estimation of the synchronization time needs to fall within the non inter symbol interference interval.However,the underwater acoustic channel has more significant Doppler effect and serious multipath effect,and the first arriving path does not necessarily have the strongest energy,so that the time synchronization method of the terrestrial OFDM wireless communications system cannot be directly applied to the underwater acoustic OFDM communications system.In this paper,based on the analysis of the influence of time synchronization on OFDM communications system,an anti-multipath time synchronization method based on hyperbolic FM signal is proposed.This method constructs a correlation window,and correlates it with the matched filtered output of the received hyperbolic FM signal to obtain the normalized correlation coefficient.When the first path energy is low,it still has the large correlation coefficient.Therefore,it is easier to find the position of the first path signal from the correlation coefficient.In addition,since the hyperbolic FM signal has Doppler insensitivity,the time synchronization method also has certain anti-Doppler performance.Experimental and simulation analysis show that the synchronization method further reduces the bit error rate compared with the traditional method when Doppler are evident and the multipath effect is serious.Based on the above methods,this paper proposes a Doppler compensation and advance processing method of synchronization time to further improve the synchronization performance.The Doppler compensation process is an improvement which aims at the compensation of the delay coupling effect of the hyperbolic FM signal.In this method,the estimated Doppler factor is used to compensate and correct the synchronization time offset caused by the Doppler effect.The synchronization time advance processing method is to ensure that the synchronization time falls within the time interval of no inter-code interference when the channel path arrives in clusters.In addition,the Doppler effect of the underwater acoustic channel will cause the time structure of the received signal to change.At this time,the arrival time of the first path may not be the ideal synchronization time.This method can also effectively reduce the inter-symbol interference in that case.Simulation and experiments show that the improved correlation window-based time synchronization method is superior to the traditional time synchronization method in the case of strong Doppler effect and path cluster arrival,which effectively improves the reliability of communications.