Research On Channel Compensation Techniques For Underwater Acoustic OFDM

In recent years,as people’s demand for marine exploration has increased,the demand for information transmission in underwater communication system is increasing,and the reliability of information transmission for underwater communication system is also increasing.However,the spectrum of underwater acoustic communication is very precious.There is space-time-frequency variation,strong multipath,severe Doppler effect and noise interference in underwater acoustic communication channel,and these will cause signal distortion.Based on underwater acoustic OFDM,this paper focuses on channel prediction technique,channel precoding technique and impulse noise interference elimination technique to improve the system performance.Simulated results and experimental results both show that the proposed OFDM system with channel prediction technique,channel precoding technique or impulse noise interference elimination technique has a better performance.The main contributions of this paper are as follows.(1)Channel prediction in time domain based on sparse channel for underwater acoustic OFDM is studied.Considering the acoustic channel is time-varing and there is delay for the CSI feedback to the transmitter,the channel prediction technique for underwater acoustic communication system is studied in this paper.The underwater acoustic channel is sparse in time domain,so compared to prediction in frequency domain,prediction in time domain can greatly reduces the computation.In this paper,the channel predication based on sparse channel is studied to provide stable CSI to the transmitter.Simulated results and experimental results both show that the proposed OFDM system with channel prediction has a better performance.(2)Precoding based on channel prediction for underwater acoustic OFDM is studied.In order to compensate the effect of underwater acoustic channel in underwater acoustic communication system,the receiver needs to do channel estimation and equalization.But it increases the complexity by doing channel estimation and equalization.In this paper,channel precoding technique is applied in underwater OFDM communication system to reduce the complexity of the receiver and caompensate the effect of underwater acoustic channel and improve the reliability of underwater OFDM communication system.Simulated results and experimental results both show that the proposed OFDM system with precoding has a better performance.OFDM with non-linear precoding has a better performance than the OFDM system with linear precoding.(3)Impulse noise and large scale single frequency noise mitigation for underwater acoustic OFDM is studied.The elimination of impulse noise interference from ocean is studied in this paper.This paper mainly focuses on the mitigation of impulse noise in time domain and large scale single frequency noise.Impulse noise and large scale single frequency noise have effect on the received signal which will increase the bit error rate and reduce the performance of the underwater acoustic communication system.Compressed sensing technique is used in this paper for impulse noise and large scale single frequency noise mitigaiton.Separate method and joint method are both used to mitigate the impulse noise and large scale single frequency noise.Take the energy of the null subcarriers as a cost function,impulse noise and large scale single frequency noise are eliminated and mitigated iteratively.Simulated results and experimental results both show that the proposed OFDM system with impulse noise mitigation has a better performance.