Underwater Acoustic Communications: Algorithms for Delay-Scale Spread Wideband Channels
In wideband wireless communication systems, the relative motion of the transmitter, receiver, or scatterers in the medium causes the Doppler effect, which stretches or compresses the transmitted waveforms, resulting in inter-symbol interference and a consequent severe performance degradation. To counter this, specialized transmitter and receiver architectures are needed for energy- and spectrally-efficient communications in channels characterized by path-dependent delays and time-scales. In this thesis, we develop and evaluate improved receiver side signal processing algorithms for two existing modulation schemes used in wideband Underwater Acoustic (UWA) communications. We also propose new modulation schemes suited for wideband delay and scale spread UWA channels. In the first part of the thesis, for the well known Orthogonal Frequency Division Multiplexing (OFDM) waveform, we develop a two-stage iterative algorithm at the receiver that alternates between sparse channel estimation and data detection. Specifically, we consider the sequence of observations from partial interval demodulators (PIDs) using a partial-length Fast Fourier Transform (FFT). We show that the PID outputs help in tracking the channel by providing additional measurements to estimate the Inter-Carrier Interference (ICI) due to the Doppler spread. We also derive the Cram ́er-Rao lower bound on the mean squared error in channel estimation, and empirically show that the two-stage algorithm meets the bound at high SNR. Next, we develop a new Bayesian-inspired data detection algorithm in the context of sweep spread carrier (S2C) communication – a practically successful waveform used in some commercial underwater acoustic modems. The existing schemes for data detection – based on the gradient heterodyne receiver – are only effective when the path delay and Doppler spread are moderate. Based on the principle of variational Bayes’ inference, we present a new variational soft symbol decoding (VSSD) algorithm. In harsh UWA channels where the existing S2C receivers completely fail, or must compromise on the data rate to maintain the bit error rate (BER) performance, the VSSD algorithm successfully recovers the data symbols, even at low signal-to-noise ratios (SNRs). We then turn to developing a new modulation scheme for the wideband doubly spread channel, namely, Orthogonal Delay Scale Space (ODSS), from first principles. The scheme pre-processes the information symbols using a 2D ODSS transform, which performs a discrete Fourier transform on the frequency axis and inverse Mellin transform on the Mellin variable axis, to obtain the transformed symbols in the delay-scale domain. These transformed symbols are mounted onto ODSS modulation waveforms to generate the signal to be transmitted. The pre-processing step spreads the symbols in the delay-scale domain, which in turn improves the bit error rate compared to Orthogonal Time Frequency Space (OTFS) and OFDM in wideband time-varying channels. More importantly, since the ODSS modulation renders the channel matrix near-diagonal, it performs well even under low-complexity subcarrier-by-subcarrier equalization in the delay-scale domain followed by symbol recovery in the Mellin-Fourier domain. Finally, we develop a novel Variable Bandwidth Multicarrier (VBMC) waveform comprising of multiple subcarriers that are constructed from chirp pulses. The chirps occupy progressively increasing, frequency-dependent bandwidth from the lower to upper frequency edge of the communication band. Due to this, the subcarriers maintain their near mutual orthogonality even after passing through a delay and scale spread channel. We compare the performance of VBMC with existing waveforms using a generic framework for modeling delay-scale spread channels that we develop for the first time in this thesis. Overall, this thesis develops advanced receiver processing techniques and novel modulation schemes that greatly outperform the state-of-the-art in wideband delay-scale spread channels, while matching their performance in more benign channels.