Research On Key Technologies Of Underwater Acoustic Spread Spectrum Communication Between Multi-UUV In Patrol Mode

Unmanned underwater vehicle(UUV)has gradually become the main force in all kinds of marine operations.Due to the use of non-cable control mode,when the potential multi-UUV in the water is far apart,they can only achieve reliable information transmission through underwater acoustic communication.Spread spectrum underwater acoustic communication has strong anti-jamming ability,high confidentiality and flexible code division multiple access ability,and can deal with most marine channel environments.The requirement of receiving signal-to-noise ratio of spread spectrum communication is not high,and it can still meet the communication requirements when the source level of UUV is limited.Therefore,spread spectrum underwater acoustic communication is usually selected as a means of information exchange when multi-UUV patrol tasks are carried out.However,the multipath effect caused by reflection and scattering and the Doppler effect caused by the relative motion between UUV bring challenges to spread spectrum underwater acoustic communication,in which the carrier phase hopping caused by Doppler effect seriously affects the decoding performance of the system.This paper will start with the communication requirements of multi-UUV patrol mode,focusing on mobile underwater acoustic spread spectrum communication and related multipath and Doppler equalization technology.In order to study the characteristics of underwater acoustic channel and analyze its influence on mobile underwater acoustic communication,this paper first studies the characteristics of underwater acoustic channel in shallow sea,and the effects of propagation loss,multipath effect,Doppler effect and environmental noise on signal transmission are analyzed.Then the virtual source method is used to model the multipath channel,and the shallow sea mobile multipath channel model is obtained after adding Doppler offset.Secondly,the mobile underwater acoustic communication method based on direct spread spectrum system is studied,and the two Doppler estimation methods are analyzed and compared.Code division multiple access(CDMA)can meet the communication needs of multi-users,and the introduction of spread spectrum codes also makes the system have a certain multipath tolerance,but the anti-Doppler ability of DS-SS system is poor.For the sake of the problem,this paper studies the Doppler estimation methods based on auxiliary signal,including delay correlation method and time-shift frequency estimation method.After simulation verification,the two methods are compared in terms of Doppler estimation performance.On the basis of direct spread spectrum system,this paper deeply studies the receiver algorithm based on differential spread spectrum system,and compares the performance of three receiver algorithms: differential detector,differential correlation detector and correlation energy detector in processing received signals.through simulation,it is found that the correlation energy detector greatly improves the influence of multipath effect on communication signals.In order to solve the problem of small Doppler tolerance of the algorithm,the Doppler equalization method based on symbol correlation is studied,and the correlation energy detector is improved.Through software simulation,it is proved that the improved system can still work normally under the condition of large Doppler.Finally,in order to alleviate the computational pressure of Doppler compensation on the receiver,the spread spectrum system based on M-ary coding is studied.Compared with the traditional spread spectrum system,the M-ary spread spectrum system does not need Doppler compensation for the data.At the same time,the introduction of MSS also makes the single symbol length signal can carry more information,which improves the data transmission rate of the spread spectrum system.In order to solve the problem of low decoding efficiency,the two-dimensional search method is optimized in three steps,and the optimized search method greatly improves the decoding speed of the system.Through the software simulation,it is proved that the M-ary coded spread spectrum system is feasible,and the three-step optimization method greatly shortens the processing time on the premise of ensuring the decoding accuracy,which highlighting its practical value.