SIMULATION, MODELLING AND DESIGN OF IMPROVED UNDERWATER OPTICAL WIRELESS COMMUNICATION SYSTEM

Abstract

Underwater Optical Wireless Communication (UOWC) has emerged as a cutting-edge alternative to traditional acoustic communication systems for high-speed, short-range data exchange in aquatic environments. Unlike acoustic methods, which suffer from high latency and low bandwidth, UOWC offers significant advantages in terms of speed, security and spectral efficiency. This makes it particularly suitable for applications such as underwater sensor networks, remotely operated vehicles, ocean monitoring and exploration missions. However, the practical realization of UOWC systems is far from straightforward, due to the complex and often harsh nature of the underwater environment. There are numerous technical challenges that undermine the performance and reliability of UOWC links. One of the most critical is severe signal fading, caused by absorption and scattering in water, which limits communication range and increases bit error rates. Dynamic underwater conditions, such as salinity and temperature variations, introduce non-linear distortions and temporal signal fluctuations. Additionally, beam wandering, transmitter-receiver misalignment and pointing errors often caused by platform mobility or underwater currents further degrade signal quality. Turbulence-induced channel fading introduces randomness to signal strength and traditional modulation or coding schemes often fall short in compensating for such deep fades. Moreover, underwater nodes are often energy-constrained, limiting the feasibility of complex signal processing or adaptive communication protocols. This thesis addresses these challenges through a multi-pronged approach, contributing a set of novel physical-layer models and system-level enhancements aimed at improving link robustness, error performance, and spectral efficiency across various underwater conditions. To combat turbulence and signal fading, novel MIMO and SIMO-based UOWC architectures were proposed and evaluated under different detection schemes. The 2×2 MIMO model demonstrated strong performance, reducing outage probability to 9.9×10−3 and achieving ergodic capacity up to 16 Nats/sec/Hz, even in strong turbulence. vii To address the problem of range limitations and signal attenuation, a dual-hop relaying scheme using fixed-gain relays was introduced, modelled with the Málaga distribution to better capture underwater turbulence effects. This system achieved a significant capacity improvement up to 35.75 Nats/sec/Hz under weak pointing conditions while also improving bit error rate and received signal power. To reduce decoding errors in turbulent channels, Reed–Solomon (RS) coded UOWC systems were developed and analysed. The RS (127,106) code, when modulated with BPSK, reduced decoding error probability from 1.6×10−4 (uncoded) to as low as 1.4×10−27, ensuring near error-free communication in extreme turbulence. Lastly, to build intelligence into the receiver, the modulation classification performance of digital schemes like Phase Shift Keying (PSK) and Quadrate Amplitude Modulation (QAM) was evaluated. QPSK was found to deliver perfect classification at high SNR, while 8-PSK offered a balance between spectral efficiency and classification accuracy in moderate conditions. These solutions directly target the limitations of traditional UOWC systems by introducing spatial diversity, adaptive relaying, advanced coding, and intelligent signal interpretation. Each method was rigorously tested across a variety of SNR levels, turbulence conditions and detection mechanisms to ensure practical viability. In conclusion, this work lays a strong foundation for building intelligent, robust and high-performance UOWC systems that can adapt to the random and challenging underwater environment. By integrating resilience, flexibility and intelligence into the system design, this research contributes significantly to the future of underwater communication whether for marine research, disaster monitoring, autonomous robotics, or secure subsea networks.