Adaptive Orthogonal Chirp Division Multiplexing (OCDM) for Underwater Acoustic Communications

Abstract

This thesis investigates adaptive orthogonal chirp division multiplexing (OCDM) techniques for underwater acoustic (UWA) communications. UWA channels are characterized by extremely limited bandwidth, long multipath delay spreads and time-varying Doppler shifts. These impairments make reliable communication challenging, particularly when strict energy and spectral efficiency constraints must be satisfied. OCDM is a linear chirp modulation technique based on the Fresnel transform that can exploit multipath diversity and has been shown to outperform orthogonal frequency division multiplexing (OFDM) under multipath fading channels. By employing adaptive OCDM with optimized power allocation, we aim to improve spectral efficiency and robustness in doubly dispersive channels. Two peer‑reviewed conference papers form the core of this thesis. The first develops a power‑allocation scheme for OCDM‑based UWA communications to maximize the signal‑to‑noise ratio (SNR) using zero‑forcing (ZF) and minimum-mean-square-error (MMSE) equalizers via convex and fractional programming. The second proposes a weighted SNR optimization framework for UWA integrated sensing and communication (ISAC) using OCDM. In both scenarios, we apply a novel diagonal power‑allocation matrix to balance communication and sensing SNRs. Together, these contributions advance the state of the art in UWA communications by addressing key challenges in bandwidth‑constrained, Doppler‑distorted channels and demonstrating the versatility of OCDM for both communication and sensing applications.