Study On Underwater Acoustic Spread Spectrum Communication Based On HFM Signal

In recent years,the severe environmental damage and shortage of resources have caused people to turn their attention to the ocean.With the increasing activity of various countries in the fields of submarine energy exploration,underwater military and marine surveys,the development of underwater communication technology has been promoted.The characteristics of acoustic wave is low transmission attenuation and long transmission distance,which makes underwater acoustic communication technology the main means of underwater wireless communication.However,the severe time-varying,multipath and Doppler effects of underwater acoustic channels have brought great challenges to underwater acoustic communication.Spread-spectrum communication occupies a place in underwater acoustic communication technology due to its advantages of strong anti-interference ability,antiattenuation and anti-multipath,but its large transmission bandwidth conflicts with the available bandwidth of underwater acoustic channel,which easily leads to lower data transmission rate.Therefore,how to increase the transmission rate of information in underwater acoustic channels and achieve reliable transmission with low signal-to-noise ratio is the key to the research of underwater acoustic spread spectrum communication.This paper studies and analyzes the underwater acoustic spread spectrum communication based on the Hyperbolic Frequency Modulated(HFM)signal.The main work is summarized as follows:1.We analyzed the causes and effects of the underwater acoustic channel characteristics in detail,and introduced the Linear Frequency Modulated(LFM)signal and its time-bandwidth product how to influence the despreading at the receiving end of the Chirp Spread Spectrum(CSS)system.What's more,we mainly introduced the principle and realization of the Sweep Spectrum Carrier(S2C)scheme of LFM signal applied to underwater acoustic communication in this paper,also its advantages and disadvantages are analyzed.2.The pulse compressibility and doppler tolerance of the HFM signal is simulated and analyzed in this paper.It mainly includes: comparing the HFM signal after the Doppler scale transformation with the LFM signal,and proving the advantage of the HFM signal in combating the broadband Doppler effect.At the same time,we explored the influence of the spread spectrum period and bandwidth on the pulse compressibility of the HFM signal,and the correlation between the up HFM and down HFM signals whose frequency variation rate are opposite to each other.3.For the purpose of improving the data transmission rate and ensuring the reliability of communication,Frequency Division Multiplexing and up-down HFM signal is used to propose a new underwater acoustic spread-spectrum communication scheme based on up-down HFM signal and sub-band interleaving.Matlab simulation analysis is carried out in Multi-Scale Multi-Lag(MSML)underwater acoustic channels.Not only we compared the bit error rate performance and the data transmission rate of the scheme longitudinally in the aspects of timebandwidth product and whether the spreading period exceeds the channel delay,but also compared the proposed scheme with other existing schemes to verify the effectiveness of the scheme,which proves that the proposed scheme has a good bit error rate performance.4.we experimented above scheme in the actual underwater acoustic channel environment that included a short-distance pool experiment and a long-distance Wanlu Lake experiment,and designed the signal frame structure and frame synchronization processing of the measured scheme.Moreover,the experiment is analyzed from the aspects of channel environment,synchronization processing,doppler estimation,bit error rate performance and so on.The experimental results show that the scheme proposed in this paper also has good bit error rate performance in actual underwater acoustic channels,and has certain feasibility.