Research On Synchronization And Doppler Compensation Of OFDM Based On Underwater Communication

With the development of the information age people are not only satisfied with the communication on the land,there are more and more people devoting to the study of the deep space and the deep ocean communication,in the future,the communication will connect with all the ocean,land and space,we can authentically achieve the communication whenever and wherever.Not only the frequency of underwater acoustic communication is very limited,but also the medium of water which used to spread the message is always varying in different time and region.Sound transmission in water is slow,the water is flowing without stopping,biological noise in water and scattering of surface relief cause the environment of the underwater acoustic channel is very complex and the underwater communication is very different with the terrestrial communication,so the technology of the terrestrial communication can not be directly used in the underwater communication,to make some changes according to the characteristics of underwater acoustic channel is very important.In this paper,we will focus on the most precise synchronization and Doppler compensation,we will expand the research of the underwater communication based on OFDM contain the theoretical simulation and the experimental data processing.In this paper,we combined the convolution channel encoding with OFDM in order to improve the reliability of communication,the anti-interference from the multipath effect of underwater acoustic channel has been enhanced by used the digital modulation of QPSK,and added the training sequence and pilot to complete the synchronization,channel estimation,Doppler compensation and other issues on the receiver,the design of the launch data had make full use of the anti-interference,high speed and parallel transmission and high efficiency of bandwidth and other characteristics of OFDM to improve the problem of bandwidth and time-varying channel of underwater acoustic channel.The whole structure of transmitting data have been verified by the design of receiving module,the simulation results showed that the BER is between 10-4 and 10-5,the BLER of this design through modulation and demodulation is between 10-2and 10-3.In June 30,2015,we achieved an experiment of underwater communication at the Qiandao Lake.We used this launch data to transmit for the underwater acoustic communication,then,we collected so many experiment data from different distance and different time in order to research the performance of eachreceiving module and test the advantages and disadvantages of the design.This paper mainly aims at the two problems,which are synchronous and Doppler frequency shift,which are faced with the serious inaccuracy of underwater acoustic communication.The research mainly compared the different methods of synchronization and Doppler compensation.In the process of synchronization,the method of cross-correlation with the training sequence is superior to the method of auto-correlation with the received signal,but in practically the method of auto-correlation is more easily to achieve;in the process of Doppler compensation for received data demodulation,the training sequence is divided into coarse estimation and fine carrier estimation,and the effect will be better,which also can effectively reduce the error rate of the system,improve the performance of communication system,achieve the reliable data transmission.