Iterative double-differential M-ary phase-shift keying direct-sequence spread-spectrum receiver in underwater acoustic channels.

Abstract

In the underwater acoustic channel, the conventional M-ary phase-shift keying (MPSK) direct-sequence spread-spectrum (DSSS) systems are sensitive to time-varying Doppler shifts. To address this issue, this paper proposes an iterative multi-channel double-differential MPSK DSSS receiver in which a per-survivor processing (PSP) method is incorporated in a soft-input/soft-output convolutional decoder to jointly estimate the data sequences and the time-varying Doppler shifts for each symbol. This PSP method embeds data-aided Doppler shift estimation within the structure of the Bahl-Cocke-Jelinek-Raviv algorithm, ensuring that the Doppler shift estimation is unaffected by the information bits. To estimate and track Doppler shifts, a symbol-by-symbol cross correlation method is employed, which is then used for dynamic phase compensation using a linear prediction model to enhance decoding performance in time-varying channels. Additionally, a multi-channel diversity combiner using soft information combines the branch metrics from each channel. Numerical simulation results demonstrate that the proposed receiver method exhibits superior performance in time-varying channels. Finally, the performance of the proposed receiver is validated through experiments conducted in shallow-water horizontal channels and deep-sea vertical channels.