| With the increase of ocean exploration activities,underwater acoustic communication technology has been paid more and more attention.The ocean is a complex environment,mainly because of the sea water as the medium,the acoustic wave as the carrier,which makes the delay spread caused by multipath effect and Doppler effect caused by the relative motion of the receiver and transmitter more obvious.Underwater acoustic communication channel has the characteristics of space-time-frequency selective fading.thus it can be seen,underwater acoustic communication faces many technical difficulties.The biggest difference between underwater acoustic communication and wireless communication on land is that land wireless communication uses electromagnetic wave to transmit,while underwater acoustic communication uses sound wave to transmit.The propagation speed of sound wave in sea water is much different from that of electromagnetic wave in air,so the Doppler effect is much more serious.In addition,in shallow water environment,there is a serious multipath effect.The non-uniformity of Doppler in each path leads to the deterioration of the performance of underwater acoustic communication system,and destroys the timing synchronization performance of the system at the receiver.In the traditional underwater acoustic communication environment,it is impossible to distinguish the underwater acoustic multipath signals effectively.Instead,the same Doppler factor is used to compensate the multipath signals,which leads to the serious degradation of the system performance.In this paper,a space division multi-stage iterative timing synchronization algorithm based on hyperbolic frequency modulation(HFM)signal is proposed.Firstly,the multipath signal is divided into different subspaces according to the receiving angle range by using the orthogonal projection technology,and then the data is processed in the subspace,and the Doppler pre compensation is carried out for the signal in each subspace The Doppler signal is compensated by the residual subspace of each iteration.The simulation results show that,compared with the traditional single path Doppler compensation system and single Doppler compensation system,the Doppler compensation algorithm based on orthogonal angle domain subspace projection and multi-stage iteration can estimate and compensate Doppler in underwater acoustic multipath environment more accurately,and the performance of the system can be effectively improved.Orthogonal frequency division multiplexing,is one of the most important technologies in wireless communication.Because of its strong anti fading ability and high spectrum utilization,OFDM is widely used in various application scenarios.In addition,it also has excellent inter symbol interference(ISI)resistance.However,OFDM technology also has some shortcomings.Frequency offset and phase noise can easily destroy the orthogonality between subcarriers,but it happens that OFDM technology is very sensitive to these noises.Channel estimation technology is also an important step in underwater acoustic communication.This paper will study the channel estimation algorithm based on OFDM system.The traditional underwater acoustic channel estimation algorithm can not distinguish the influence of multipath.In this paper,the algorithm based on orthogonal subspace projection is used to project the received signal into several orthogonal subspaces,and then the channel estimation of the signal in each subspace is carried out.Firstly,the channel estimation algorithm based on base extension(BEM)is used to obtain the initial value of channel estimation,and then the channel estimation algorithm based on BEM is used Under the minimum mean square error criterion,the channel estimation value is updated by using the initial channel estimation value and detection symbol until the algorithm converges.Simulation results show that the channel estimation algorithm proposed in this paper has good performance in the case of high SNR. |
PDF Full Text Request