Research Of Channel Time-Varying Suppression Technique In Underwater Communication

With the development and exploration of the ocean,underwater acoustic communication technology has developed rapidly in the past decade.However,underwater acoustic channels,as the physical medium for underwater sound waves,are considered as one of the most challenging wireless channels in nature.The complexity of underwater acoustic channels is dominated by the characteristics of ocean environment,which is mainly restricted by three factors: Compared with electromagnetic waves,the speed of sound waves is slower in the water(1500m/s);The path loss will be with the signal frequency,as a result,the available bandwidth is severely limited.As well as the time-varying multipath delay and Doppler spread,these factors lead to many technical difficulties in underwater acoustic signal processing.Firstly,the capacity of underwater acoustic channel may be very limited,depending on the distance of the communication.However,the low center frequency of the signal is also not negligible.Thus,the underwater acousitc communication system is essentially a broadband system.The traditional signal processing methods for narrorband are not directly applicable to underwater acoustic communications.Secondly,underwater acoustic communication systems based on single carrier usually adopt decision feedback equalizer to resist intersymbol interference caused by the delay spread,but the complexity of the equalizer design increases with the increase of the communication rate.Orthogonal frequency division multiplexing(OFDM)technology,as a lowcomplexity high-speed communications technology,has strong capacity to resist multipath and noise.However,the disadvantage of the OFDM technology is very sensitive to frequency offset.Due to the time-varying acoustic channels,data transmission with significant Doppler effects and the subcarriers with different frequency offsets which destroy the orthogonality between the subcarriers and cause the intercarrier interference.Therefore,precise Doppler compensation techniques and channel estimation techniques are crucial for achieving high-speed,robust underwater acoustic communication systems.In this paper,the time-varying suppression technology of underwater acoustic channel is mainly focused on these two aspects.The following are the main research contents of this paper.In this paper,the Doppler suppression technique in underwater acoustic communication is studied.Due to the slow propagation velocity of acoustic wave,Doppler spread in underwater acoustic communication system is more serious than that in radio system,and different paths of multipath channel have different Doppler effect.The traditional Doppler compensation technique usually may not distinguish the different Doppler spread of multipath signals,resulting in Doppler spread can not be effectively compensated.In order to solve this problem,this paper presents a receiver design based on the orthogonal angle domain subspace projection algorithm.Firstly,the channel is modeled according to the characteristics of underwater acoustic channel and the channel input-output relationship is deduced.Then the angle domain resolution of the array receiver is used to divide the received multipath signal into several subspace signals.The Doppler estimation and compensation of the subspace signals are respectively performed by using the information of the angular domain in different subspaces.The simulation results verify that the proposed method can estimate and compensate different Doppler spreads of different propagation paths more effectively than traditional methods,and the system performance is effectively improved.The performance of an underwater OFDM system also depends on the performance of the channel estimation.Due to the sparseness of the underwater acoustic channel,the compressed sensing is used for channel estimation,and an accurate estimation result can be obtained even when the pilot number is small.This paper analyzes several commonly algorithms based on compressed sensing theory,and derives the system model of channel estimation by using the sparsity of delay-Doppler parameters.Then,based on the clustered sparseness of underwater acoustic channels,A low complexity channel estimation algorithm based on block orthogonal matching tracking(BOMP)is proposed.In order to solve the problem of high computational complexity of traditional recovery algorithm,this paper first notices a large number of repeated computation in inner product computation.The idea of block FFT computation is used to reduce the complexity of inner product computation.Second,by selecting multiple matching blocks in each iteration of the algorithm,the convergence speed of the algorithm is faster,which can reduce the number of iterations of the algorithm.The simulation results show that when the number of matching blocks is less than 3,the performance of the algorithm is not lost,but the complexity of the algorithm is greatly reduced compared with the original algorithm.