Research On Mobile Spread Spectrum Underwater Acoustic Communication And Multiple Access Technology

The reverberant and time-varying nature of the underwater acoustic(UWA)channel(referred to as a doubly spread channel)poses many obstacles to reliable communications,which is one of the most complex radio channel.Spread spectrum UWA communication is featured with anti-interference ability,multiple-access ability,high security and low signal noise ratio(SNR)working condition,which can effectively deal with the interference of complex UWA channel and is a preferred means of high quality UWA communication.However,there are still some problems in the practical application of spread spectrum UWA communication.Carrier phase fluctuation caused by the relative motion and the complexity of the marine environment seriously affects the performance of spread spectrum UWA communication.Hence,this paper will focus on receiver algorithms design and related spread spectrum technology to ensure and further improve the quality of spread spectrum UWA communication.Firstly,mobile spread spectrum UWA communication is studied in this paper,including direct-sequence spread spectrum(DSSS)communication,cyclic shift keying(CSK)spread spectrum communication,traditional M-ary spread spectrum communication and combination spread spectrum communication.Differential correlation detector(DCD),differential energy detector(DED)and M-ary energy detector(MED)are proposed for low carrier phase fluctuation interference,respectively,whose decoding performance has been analyzed by computer simulation.These detectors are insensitive to carrier phase fluctuation by taking advantage of the relationship of carrier phase between adjacent spread symbols,which can effectively ensure the reliability of spread spectrum UWA system.Improved DED,double differential correlation detector(DDCD),decoding differential spread spectrum sequence detector(De-DSSD)and Segment M-ary energy detector(S-MED)are further proposed for fast carrier phase fluctuation caused by relative motion between transducer and receiver.Moreover,a Doppler estimation method based on spread spectrum sequence is proposed,which can track large time-varying Doppler.The above proposed algorithms are also analyzed and verified by computer simulation.Secondly,the DS-CDMA UWA communication system is studied.The multiple access interference(MAI)in UWA communication is much more complex than that in radio communication,especially for the near-far problem in UWA communication,making the practical application of DS-CDMA UWA communication very difficult.Zero interference cancellation spatial division multiple access(ZIC-SDMA)technology based on single vectorsensor is proposed for DS-CDMA UWA communication:(1)using a fast Walsh–Hadamard transform(FWHT)based on the interferer's code sequence,the interference signal is enhanced and removed by coordinate zeroing;(2)using each user's azimuth estimated by single vector active average sound intensity algorithm to achieve vector directional combination communication,which can bring vector processing gain for desired user and suppress undesired users' interference;(3)using time reversal processing to focus desired user and shield undesired users' interference by taking advantage of physical characteristics of UWA channels.The combination of zero interference cancellation processing,single vector combinations directional communication and single element time reversal processing can effectively deal with the complex MAI,which can significantly improve system performance.Thirdly,MIMO spread spectrum UWA communication system is studied.DS-CDMA combined with MIMO technology is proposed,using the passive-phase conjugation(PPC)combined with proposed detectors(such as DCD,DED,and so on).In addition,tow MIMO frequency domain equalization algrithoms(focus-shield weight vector and inverse weight matrix)based on PPC frequency domain model are proposed by reasonable modeling MIMO system.Focus-shield weight can focus the desired signal and shield undesired signals and inverse weight matrix can achieve separation and equalization of all signals,theoretically.Time reversal processing,fouse-shield vector processing and invse matrix processing are analyzed by computer simulation.The simulation results point out:(1)time reversal processing has good stability which is not limited by the number of array elements;(2)focus-shiled weight vector processing is better than time reversal processing for desired signal with strong interference signals,which requires the number of receiving array being greater than or equal to the number of sending array;(3)inverse weight matrix achieve a good balance and separation for each transmitted signals when the number of receiving array is greater than the number of sending array,whose performance will be better than the above tow algorithms at high SNR.Group M-ary spread spectrum coding based MIMO-CDMA UWA communication is further proposed based on cyclic shift spreading code mapping algorithm and r-combination mapping algotithm,aiming at further improving the communication rate.Finally,the above proposed algorithms are verified by at-lake data,at-sea data and under-ice data in this paper and specific application details of algorithms are described during the data processing.A sliding correlation based DCD scheme is proposed in DCD data processing.Improved DDCD is proposed in mobile data with long code and narrowbandwidth processing,which can achieve zero error decoding without any Doppler compensation.Vector MIMO spread spectrum UWA communcation system based on inverse weight matrix is proposed during MIMO data processing,which can significantly improve system performance by using output signal of velocity channel of vector sensors.High-quality spread spectrum communication under complicated interference conditions,6users DS-CDMA communication with significant near-far problem,7 users DS-CDMA communication with large multipath interference and 4?4 vector MIMO spread spectrum communication are achieved in different water areas.