The Synchronization And Adaptive Equalization Technique In Underwater Acoustic Direct Sequence Spread Spectrum Communication

Direct sequence spread spectrum is considered for underwater acoustic communication networks.Unlike in the majority of spread-spectrum radio systems,intersymbol interference cannot be neglected,which can spans tens of symbol intervals even hundreds of,and time variability of the channel requires that receiver adaptation be performed at the chip,rather than the bit rate.Adaptive decision-feedback equalization,which has successfully been used for single-user underwater communications,is not directly applicable to spread-spectrum signals because of the delay in the despreading process and the lack of reliable chip decisions.To overcome this problem,a receiver is proposed which feedback hypothesized,rather-than the actual decisions.Numerical examples demonstrate the receiver's ability to cope with time varying channel distortions and preserve the processing gain when conventional,symbol-rate adaptive methods fail.To test and verify the algorithms mentioned above,this paper use the Bellhop software to produce the South Sea's January channel(including shallow and deep)and G Proakis's channel B to simulate the receiver's performance,what's more,the Songhua Lalke's data is processed and analyzed in the end.The simulation results show that the hypothesis-feedback equalization algorithm update the filter coefficients using different ways,the RLS algorithm has a better performance than LMS algorithm,but at the cost of higher system complexity,which limits its application.Coupled with the phase-locked loop(PLL)and doppler compensation techniques,the equalizer can track time-varying channels more efficiently.Fractional space equalizer has lower doppler sensitivity than symbol space equalizer.Larger spreading gains can lead to larger SNR gain,however,the performance of system decrease because of the time variation of acoustic channels.We can adopt high rank modulation techniques to improve the communication rate,nevertheless,high rank modulation systems are sensitive to phase shift.Compare to RAKE receiver,CHF receiver is better suited to serve the needs of time varying channels,which exhibit spectrum nulls within the signal bandwidth.RAKE receiver which has a lower complexity regards the channel response as time invariant.The Songhua Lake's data processing results shows that CHF receiver can cope with a majority of the underwater acoustic channel,by employing a particular way,we can gain the optimal parameters of the receiver.However,it shows its limitations for the burst interference or fast changing environment.