Application Of OFDM Doppler Compensation Based On Null-subcarriers In Underwater Acoustic Communication

The OFDM(orthogonal frequency-division multiplexing) in underwater communicationhas been more widely used in high-speed underwater acoustic communication, for theadvantage of its high spectrum utilization and effective anti-multipath ability. While itsperformance is directly affected by the precision of time and frequency accuracy due to meetthe strictly orthogonal between the subcarriers, there is great practical significance and meritin project to investigate the technique synchronizing the frequency. A Doppler estimationalgorithm that can be easily applied to engineering is proposed by using null subcarriers andparabola fitting combination in this paper, and the experimental data processing results on thelake verify the feasibility and effectiveness of the algorithm.This method requires to insert null subcarriers in the data transmitted to estimate theDoppler factors, the pilot signal is also required in the methods to increase the performance ofthe entire communication system by channel estimation. The Doppler estimation methodbased on null subcarriers regard the energy at the null subcarriers as the objective function,using different Doppler factors to search the energy of the null subcarriers by DFT, thevelocity corresponding to the lowest place of the energy is the relative moving velocity. Sincethe null subcarrier energy curve is approximated as a parabolic shape when it is near thelowest point, we can fit the parabola by knowing three point of the null subcarriers energynear the lowest point, the lowest point of the parabola is corresponding to the speed of theestimated relative moving speed. The two methods use two different ways to find the nullsubcarrier energy near lowest point, the first is to use greater interval to find the lowest point,the second method is to use the copy correlation method, the LFM signal is coupled at thehead and tail, based on the calculation of the data compression ratio calculate the relativemoving speed, then fitting a parabola. This paper verifies the feasibility of the two algorithmsby simulation of both Doppler estimation algorithm and analyzes the advantages and theapplicable conditions of the Doppler estimation algorithms. Finally, the processing of the lakeexperimental data verifies the value of the application of two algorithms.