Research on Modulation Methods for Underwater Acoustic Communications

This dissertation explores the advanced modulation schemes for underwater acoustic (UWA) communication through two major contributions: a new Turbo Decision Feedback Equalizer and Decoder (TDFED) for the Orthogonal Time-Frequency Space (OTFS) system, and the Data Dithering Reuse (DDR) method for cross-evaluation of UWA modulation schemes with post-experimental data.The proposed TDFED for the OTFS system employs time-domain feedforward and feedback filters to equalize the received OTFS signals directly without using the delay-Doppler domain approach that is commonly used for OTFS in RF communications. For an OTFS system with $N$ Doppler and $M$ delay grids, a set of $N$ feedforward and feedback equalizers are used in parallel so that the computational complexity is trackable. With low-complexity IPNLMS channel estimation and soft Low-Density Parity Check (LDPC) decoding, the proposed TDFED achieves outstanding performance in heavy Doppler and multipath UWA environments. Extensive lake experiments reveal that the OTFS system significantly reduces the precision requirements of Doppler compensation algorithms, achieving notable improvements in Bit Error Rate (BER) compared to single carrier coherent modulation (SCCM) and orthogonal frequency division multiplexing (OFDM).
The DDR method enables post-experimental cross-evaluation between SCCM and OFDM by reusing the original experimental scheme (OES) data to preserve the nonstationary properties of the UWA channels. This method applies dithering and reverse dithering to transmitted and received data, facilitating the evaluation of new modulation schemes. Field experiments using multiple-input multiple-output (MIMO) measurements demonstrate that the DDR method provides more accurate BER predictions than the existing residual prediction error (RPE)-enhanced simulations.