High-speed underwater optical wireless communication system models investigation: from design to application
Creators: name: Fang, Chengwei
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400605
subject: Optical fibre communication systems and technologies
classificationCode: 400909
subject: Photonic and electro-optical devices, sensors and systems (excl. communications)
classificationCode: 510204
subject: Photonics, optoelectronics and optical communications

description: Underwater optical wireless communication (UOWC) systems have been widely researched to achieve high speed and secure wireless communications. Compared with conventional underwater wireless communication systems based on acoustic waves, UOWC systems have the key advantages of higher data rates and lower latency. However, due to the complex underwater environment, UOWC systems have limited transmission distance and communication reliability. To investigate and optimize UOWC systems, an accurate theoretical model to characterize the performance of UOWC systems is important. Therefore, the main focus of this thesis is to establish accurate UOWC system models to provide the framework and foundation for further in vestigations. These accurate models are developed from the basic theory and verified via extensive simulations. Moreover, these accurate models can be employed to investigate the system performance in challenging underwater channels and provide optimized solutions for practical scenarios.

This thesis first presents the fundamental overview and current progress of the UOWC systems. The previous UOWC models mainly focused on the line-of-sight (LOS) link considered constant filter transmittance for simplicity, and we established an accurate LOS UOWC system model, incorporating the previously overlooked filter transmittance with incident angles. The proposed model was verified by MonteCarlo(MC)simulations. Results show that our proposed model which considered various incident angles can provide fluctuation of received signal power and solar noise than previous simple models. These fluctuations can further impact the practical signal-noise-ratio (SNR) and bit-error-rate (BER) performance of UOWC systems.

In addition, LOS UOWC systems face the limitation that the signal light can be easily blocked by marine biology or complex underwater topography. To overcome this challenge, we further establish an accurate theoretical model for non-line-of-sight (NLOS) UOWC systems, which use the water surface to reflect signal light. This model is also validated by MC simulations. Results show that our proposed model can capture the changes of received powers of both signal and background lights with various transmitter and receiver alignments and orientations, which typically affect the SNR and BER performance of UOWCsystems.

Following that, the advanced NLOS UOWC models are employed to build a framework incorporating wavy surfaces. Most previous NLOS UOWC studies have assumed a flat water surface or general sine or cosine wave model for simplicity, which leads to inaccurate performance modeling of the NLOS UOWC system. In this thesis, the NLOS UOWCsystem model considering the Pierson wave model incorporating wind speed was built and investigated. Results show that the theoretical results agree well with MC simulations, which verify the accuracy of our proposed model with a wavy surface. Moreover, wavy water surfaces not only result in higher channel losses but also lead to channel delays, which further impact the practical SNR performance.

Finally, to improve system performance under strong wind, the multiple-input-multiple-output  (MIMO) principle is considered in NLOS UOWC systems with complex wavy surfaces. Results show that MIMO can significantly reduce the impact of the wavy surface to increase the probability of receiving strong signal power. Therefore, a more completed UOWC platform to obtain accurate performance theoretically and to overcome some limitations of wavy surfaces is achieved in this thesis, benefiting the future UOWC system development and optimization.