The Doppler Compensation And Channel Equalization Of Sweep-Spread Modulation In Underwater Acoustic Communication

The biggest obstacle that underwater acoustic communication technology lags far behind of radio communication technology is the particularity of underwater acoustic channel.Underwater acoustic channel usually presents a serious and time-varying characteristics,namely delay spread and Doppler spread. Anti-channel doubly spread effect has been a focus for underwater acoustic communication researchers.Conventional open-loop technology can only compensate the average Doppler distortion over the whole data frame, which will degrade the performance of receiver or fail completely if the Doppler changes largely over the data frame.Conventional closed-loop technology is able to compensatethe time-varying Doppler, but the error propagation caused by the mismatched hard decision feedback will reduce the accuracy of the Doppler compensation,ultimately limit the performance of the system. In order to mitigate the error propagation in closed-loop technology, a receiver based on a bidirectional closed-loop technology consisting of one normal Decision Feedback Equalizer (DFE) and one time reversal DFE is proposed,which can improve the performance of the underwater acoustic communication system. Using the single carrier modulation, the performance advantages over existing methods are validated with output SNR of equalizer and Bit Error Rate (BER) through both simulations and lake trial data analysis. It is shown that under very hard conditions (rich multipath, BPSK/QPSK modulation and relative speed up to 6 knots) proposed receiver has about 1dB output SNR gain of equalizer under a symbol rate of 1 ksymbols/s, system performance is improved significantly.The Inter-Symbol Interference (ISI) due to multipath delay can substantially controlled by a new method based on the implementation of a Sweep-Spread Carrier (S2C), such carrier consists of a succession of sweeps which cause permanent rapid fluctuation of signal frequency.The method provides significant processing improvement enabling clear separation of multipath arrivals by converting their time delays into their frequency reallocations.This paper analyzes the suitable conditions and how to select sweep-spread modulation parameters in underwater acoustic multipath channels by extensive simulations.The simulation results show that the sweep-spread modulation can relieve ISI under certain values, but it does not completely eliminate. This is due to the complexity of underwater acoustic channel. Thus the equalizer is used to suppress the influence of the residual ISI. Another challenge sweep-spread modulation technology faced is how to estimate and compensate the Doppler spread when there is a relative movement between the transmitter and receiver. The feasibility and effectiveness of the open-loop and closed-loop Doppler estimate compensation receivers under sweep-spread modulation scheme are verified through both simulation and lake trial.