Research On AUV Underwater Acoustic Communication System

With the development and utilization of marine resources,Autonomous Underwater Vehicle(AUV)is increasingly and widely used as an important unmanned underwater vehicle in fields,i.e.,marine bathymetric surveys,marine resource exploration and so on.Underwater acoustic communication(UAC)is employed to control the AUV platform as well as the real-time transmission of AUV data.The UAC technology is an important technical premise for AUV to perform underwater operations.UAC channels have inherent disadvantages such as long time delay,strong multi-path,high noise level,and limited bandwidth available.Moreover,the AUV system features violent noise fluctuations,a considerable maneuvering range,and a high reliability requirement.Therefore,there are more demanding requirements for the development of an underwater acoustic communication system for AUVs.The thesis presents studies on the following technologies concerning AUV's UAC system based on the highly reliable frequency hopping frequency shifted keying(FH-FSK)communication:the joint detection to linear frequency modulation(LFM)synchronization signal based on copy correlation and chirp rate estimation,channel equalization technology for underwater acoustic frequency-hopping communication system,AUV underwater acoustic frequency-hopping communication principle and protocol design,and application of Q-ary LDPC in FH-FSK system.A robust synchronization detection technology is a premise for the communication system.The AUV UAC system,which adopts the external synchronization method,uses high-autocorrelation LFM signals as the synchronization signals.The thesis analyzes the noise interferences affecting the synchronization performance of the AUV UAC system.Experimental results show that the AUV underwater acoustic communication system's background noises fluctuate sharply and non-stationary noises tend to lead to false alarms of the copy-correlation detector,according to which the demodulator carries out demodulation operations with a high error rate,with the result that the communication link is broken.Therefore,the thesis proposes a joint detection method for LFM synchronization signals based on copy correlation and chirp rate estimate.Synchronization signals are jointly detected by the rapid location function of the copy-correlation and the LFM parameter estimate function based on fractional Fourier transform.Simulation and experimental results show that the proposed method has a high resistance to false alarms incurred by non-stationary noises as well as the high ability to detect synchronization signals at a certain signal-noise ratio(SNR).The underwater acoustic communication channel's multi-path spread creates a strong inter-symbol interference(ISI)at the receiving end and brings about a certain SNR loss.The AUV UAC system based on the FH-FSK has a certain capability to resist the multi-path interference.However,the multi-path interference is still a problem because of the impacts of long multi-path spread and limited bandwidth.This thesis presents an FH-FSK channel equalization method that is based on compressive channel sensing and time reversal mirror.Taking advantage of sparsity feature of the underwater acoustic channel,the equalization method estimates the underwater acoustic channel by introducing the sparse signal recovery algorithm within the compressive sensing framework and carries out the virtual time reversal mirror processing of the estimated channel and received FH-FSK signals.Simulation and experimental results show that when the channel conditions such as AUV hovering are relatively stable,the proposed method can effectively compress the multi-path spread of the underwater acoustic channel and enable energy focusing.In a word,the proposed method can improve ability of frequency hopping UAC system to resist the multi-path interferences.With a sever multi-path spread and a serious bandwidth limitation in the underwater acoustic channel.The FH-FSK system provides a relatively robust communication under complex channel conditions.The thesis presents an analysis of the impacts of multi-path interferences on the underwater acoustic FH-FSK communication system as well as a parameter design method for the FH-FSK system resisting multi-path interferences.On one hand,the AUV UAC system shall be very reliable in data transmission for special task.On the other hand,the AUV platforrm's extensive maneuvering range and violent fluctuations of the noises from AUV and its mother ship lead to changes in transmission environment,as well as the channel structure and input SNR.Consequently,the AUV underwater acoustic communication system is required to have a high capability to adapt to the changed transmission environment.The thesis puts forward a link adaptation based multi-rate UAC protocol on the basis of the underwater acoustic FH-FSK system.The protocol enables a cooperative work according to the fixed frame period,the status of the task within which is determined by the surface unit through setup.In addition,a closed loop,which is made up by the data named setting up frame,is employed to accomplish the self-adaptation transmission rate adjustment and automatic repeat request(ARQ)function.Thus,this protocol can improve the capability to adapt to the changed transmission environment as well as its reliability in the underwater acoustic communication system.Besides,an analysis of the specific self-adaptive rate adjustment method and a description of the implementation procedure of the protocol are made.The protocol has been proven to be effective by lake experiments.Multi-path interferences and frequency selective fading of an underwater acoustic channel will increase the average bit error rate of FH-FSK system.In addition,the received signals tend to be subjected to time-selective fading due to the movement of the AUV.Therefore,it will occur a series of burst errors or even a communication interruption.The two above factors reduce the reliability of the AUV frequency hopping communication system.This thesis presents a Q-ary low density parity check(LDPC)based FH-FSK system.Simulation and experimental results show that compared with the convolution code based FH-FSK system,the binary-LDPC-based FH-FSK system has a stronger anti-interference ability.An analysis of the main factors affecting the performance of the Q-ary LDPC is made.Simulation and experimental results show that the Q-ary LDPC based FH-FSK system is more likely to resist the interference when compared with the binary LDPC based FH-FSK system.