| The vast ocean is the frontier of human exploration of the mysteries of nature,and underwater acoustic communication technology is an effective means to explore the ocean.Underwater acoustic channel is not only a channel with serious multipath effect,but also a random channel with fast time-frequency variation.It is a complex channel with serious reverberation,serious doppler and limited bandwidth.Facing the harsh marine environment,it is difficult to realize reliable communication of underwater information,which brings severe challenges to the research in the field of water sound.Spread spectrum communication technology has the ability to overcome the the disadvantages of complex channels,it can realize reliable transmission of information under a low signal-to-noise ratio.Time-reversal mirror technology can use the physical characteristics of underwater acoustic channel to adaptively match acoustic channel without any prior environmental knowledge.It can suppress inter-code interference by recombining multi-path signals and reduce the influence of channel fading through spatial focusing.Rake receiver achieves the utilization of multipath energy by diversity,reduces multipath interference and improves the signal-to-noise ratio of the system.In this paper,in order to realize the low bit error rate communication of underwater system,we study the time reversal mirror and Rake receiver in spread spectrum communication system.Firstly,based on the theoretical basis of the spread spectrum system,this paper analyzes the anti-interference and anti-multipath principle of the spread spectrum system.The underwater acoustic spread spectrum has strong resistance to single-tone and multi-tone interference in the underwater acoustic multipath environment.The direct sequence spread spectrum communication system has high transmission reliability,but its communication rate is relatively low,which cannot meet the underwater high rate communication requirements.In order to break through the limitation of spread spectrum gain on communication rate,the principle of M-ary spread spectrum,CSK spread spectrum and M-ary CSK spread spectrum communication method is analyzed.They use the characteristics of spread spectrum sequence to improve the transmission rate of underwater acoustic system.Then,aiming at the multipath characteristics of underwater acoustic channel,the underwater acoustic spread spectrum communication system based on time reversal mirror is studied.In order to meet the requirements of low-power consumption of underwater communication,the time-reversal mirror for one-way transmission is mainly studied,and theprinciple and focus gain of single-element time-reversing mirror are mainly analyzed.The structure of the channel is obviously different under different seabed topography.The more complex the channel multipath structure,the better the focusing performance of the time reversal mirror.In order to improve the system performance,it is necessary to select the appropriate detection signal in the passive time reversal mirror,while the virtual time reversal mirror needs accurate channel estimation.The MP and BP algorithms in channel estimation are studied to improve the performance of the time-spreading system.The simulation results show that the time reversal mirror can equalize the channel,reduce the inter-symbol interference,and the spread spectrum communication system with the time reversal mirror can significantly improve the reliability of information transmission.Finally,Rake receiving technology is an effective means to overcome multipath fading.The Rake receiver is combined with the spread spectrum communication system to improve the signal-to-noise ratio of the received signal.A time-reversed Rake receiver is proposed in the spread spectrum system,which can reduce the complexity of the system under certain conditions and its performance can be verified. |
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