Research On Underwater Acousitc Spread Spectrum Multiple Access Communication

All attention has been highly paid to underwater multi-user communication due to itstremendous potential in civil and millitarial uses. The interest in underwater communicationnetworks has emerged with the development of reliable acoustic modems and unmannedunderwater viehcles, which together open a range of possibilities for remote underwaterexploration.However, with the characteristics of time-varying, space-varing, strong multipathinterference, narrow available frequency band and high environment noise, underwateracoustical channel appears to be extremely severe, which slows down the development ofunderwater acoustical communication. Since spread spectrum communication is featured withstrong anti-multipath, anti-interference, secret-keeping and communication web composingability, spread spectrum communication can not be replaced by any other communicationthemes in underwater acoustical channel.Time reversal mirror (TRM) is of great pragmatics since it will match the acousticchannel automatically without any priori knowledge and requires little channel estimationaccuracy. In multi-user communication system, the weak correlation between differentcommunication knots determined by time-varying and space-varying characteristics of UWAchannel is utilized to suppress the interference of non-expect-users. In order to improve theaccuracy of synchronization, higher order Gold sequences can be used as synchro head afterdifference coding, along with the benefit that the carrier can be removed effectively at thereceiving terminal.Sound field information and azimuth of target can be obtained by Vector sensor. Themost common methods to estimate azimuth with self-directivity of vector sensor are averagesound intensity method and complex sound intensity method, however, for the samefrequency band multi-users, theoretical limitation for these methods is only two users. Activeaverage sound intensity approach, which measures azimuths of multi-users simultaneouslywith the correlative characteristics of pseudo-random code in spread spectrum communication,is proposed in this paper. With the estimated azimuth, vector combination can be generated toadjust the directivity of vector sensor, achieve multi-user beam communication, inhibitmulti-path interference, enhance processing gain and lower error rate. Simulation andexperiment for same frequency band spread spectrum multi-user communication testify thefeasibility and utility of active average sound intensity approach. Since different spread spectrum signals are somehow related, interference betweendifferent users can not be ignored, which will be particularly severe as the order of Goldsequence relatively small and the amount of user grows. With the utility of array beamforming, interference between different users can be decreased. Effectiveness of ConventionalBeam Forming (CBF) and Steered Minimum Variance (STMV) in simulation is compared inthis paper.