Underwater wireless communication system

Abstract

Underwater communication systems (UWC) have been a research focus since the mid-20th century, driven by the need for effective communication in underwater environments for applications such as environmental monitoring, underwater exploration, and defense operations. Among the various UWC methods, underwater radio frequency (UWRF) communication is significantly limited by high attenuation, making it impractical for further development. Consequently, researchers have turned their attention to underwater acoustic communication (UWAC) for long-range transmission and underwater optical communication (UWOC) for short/medium-range applications. Recent advancements in UWAC have trended towards the use of Multiple Input Multiple Output (MIMO) technology to enhance data rates and communication reliability. However, conventional UWAC systems rely on transducers, which make MIMO configurations bulky and impractical. To address this issue, there is a strong motivation to investigate the integration of fiber acoustic sensing into underwater acoustic communication technology, which offers advantages such as small size, insulation, and resistance to electromagnetic interference. However, traditional fiber optic sensors with tunable narrow linewidth laser sources, such as FBG, interferometers, FP cavities, and micro resonance cavities, must be phase-locked to achieve high sensitivity and stability. This requirement complicates the setup and makes the detected signal prone to distortion. It is essential to investigate alternative acoustic optical fiber-based detection techniques that provide high sensitivity and the ability for multiplexing. On the other hand, light-emitting diode (LED)-based UWOC systems are preferred over laser diode (LD)-based systems due to their relaxed alignment requirements and auxiliary lighting capabilities. Nevertheless, LED-based systems encounter nonlinear distortion, with modulation nonlinearity of the LED playing a predominant role, resulting in substantial performance deterioration. Equalization algorithms, such as Volterra-based equalizers, are frequently adopted. However, the integration of the Volterra feedforward equalizer (VFFE) with the decision feedback equalizer (DFE), known as VDFE, remains susceptible to error propagation in the case of incorrect symbol decisions. As a result, it is essential to find alternative equalization techniques that can ensure good system performance. In light of these challenges, this thesis focuses on a time-stretched self-coherent detection (TSSCD)-based underwater acoustic communication (UWAC) system for long-range acoustic signal transmission and an LED-based underwater optical communication (UWOC) system for medium-range transmission. The proposed schemes have been successfully demonstrated experimentally, and their performance has been evaluated. The 2.2-Mbps TSSCD-based UWAC system achieved a noise equivalent pressure (NEP) of 0.53 Pa/Hz^1/2 and applied amplitude modulation with the PAM-4 format on a sinusoidal ultrasonic signal with a frequency of 11 MHz. Additionally, a nonlinear weighted decision feedback equalizer (NWDFE) was introduced and demonstrated in a 520-Mbps PAM-4 blue-LED-based UWOC system for the first time, improving the data rate by approximately 9% while maintaining similar complexity compared to VFFE-DFE. The research study shall help to advance underwater communication technology.