| The underwater acoustic channel is harsh and complex,with the characteristics of limited bandwidth and serious multipath effect.Orthogonal frequency division multiplexing technology is widely used in the field of underwater acoustic communication due to its strong anti-multipath effect and high spectral efficiency.However,there are many challenges to achieve high-speed underwater acoustic OFDM communication in a complex underwater environment,especially accurate channel estimation.Therefore,this paper will focus on the research of channel estimation in underwater acoustic OFDM communication system.This paper analyzes and introduces several classic channel estimation algorithms from two perspectives: frequency domain and time domain.However,since the underwater acoustic channel is a time-varying channel and the transmission delay is large,the channel estimation for the purpose of adaptive modulation in the underwater acoustic OFDM communication system will face the issue of timeliness.Based on the above-mentioned problem,this paper proposes a channel correction-based channel estimation method.Specifically,after preliminary channel estimation,the channel information is further corrected according to the tracked channel changes,thereby improving the accuracy of channel estimation and solving the timeless problem that channel estimation faces.Our main research contents are as follows:1.We introduce the fundamentals of the underwater acoustic OFDM communication system model.This paper analyzes the channel characteristics of underwater acoustic channels including transmission loss,environmental noise,multipath effects and Doppler effects.Based on these characteristics,this paper build a time-varying underwater acoustic multipath channel model.Subsequently,the other modules of the system and key technologies such as guard interval and cyclic prefix are introduced.It is found that OFDM can be used to combat the multipath effects and limited bandwidth of the underwater acoustic channel.2.We investigate the channel estimation algorithm and its performance in underwater acoustic OFDM system.Channel estimation plays a crucial role in underwater acoustic OFDM systems.Therefore,this paper will introduce several classic channel estimation algorithms.Least square algorithm and linear minimum mean square error algorithm are two commonly used frequency domain channel estimation algorithms in OFDM system.Such algorithms assume that the channel is dense,and the sparseness of the underwater acoustic channel cannot be utilized.Therefore,the performance of frequency domain channel estimation algorithms in the underwater acoustic OFDM system is usually poor.Since the sparsity of underwater acoustic channels is manifested in the channel impulse response,time-domain channel estimation algorithms,especially compressed sensing algorithms that can make good use of channel sparsity,are more widely used in underwater acoustic OFDM systems.3.We investigate the timeliness of channel estimation.When channel estimation serves the adaptive modulation of the sender,the estimated channel needs to be fed back to the sender.Due to the slow transmission speed of acoustic signals in underwater environments,the underwater acoustic channel has changed when the estimated information reaches the origin.Therefore,it is necessary to correct the feedback estimation information before adaptive modulation.In response to this,this paper introduces several classic algorithms that can be used for channel correction.Through the above research,this paper provides a channel correction-based channel estimation algorithm to solve the time-efficiency problem faced by channel estimation.This scheme integrates the channel estimation and channel correction modules so that the estimation result can be adjusted adaptively with the change of the channel.At the same time,the scheme takes advantage of the sparsity of the underwater acoustic channel to eliminate some excess noise.Simulation results show that the proposed scheme improves the NMSE performance of channel estimation from 2.3 to about 0.5.It can be seen from the results that the channel estimation method improves the accuracy of estimation,so that the real-time performance of the channel estimation is guaranteed.When the estimated result is used for adaptive modulation,the system resources can be allocated as best as possible,thereby improving the overall system performance. |
