title: Hierarchical subcarrier intensity modulation for resilient optical wireless communication
Creators: name: Güler, Egecan
Publisher: name: The University of Edinburgh
Year: 2025
Subjects: subject: optical wireless communication
subject: OWC
subject: signalling techniques
subject: hierarchical signalling technique
subject: resilience
subject: subcarrier intensity modulation
subject: SIM
subject: underwater channel emulator
subject: pulse-based modulation scheme
subject: PAFsk-SIM

description: Optical wireless communication (OWC) has emerged as a promising technology to address the growing demand for high-speed wireless connectivity, offering vast unregulated bandwidth in the optical spectrum and inherent security advantages. However, the hostile nature of optical wireless channels, particularly in underwater environments, presents significant challenges for reliable data transmission. This thesis investigates the development and implementation of robust modulation techniques based on subcarrier intensity modulation (SIM) to overcome these challenges while maintaining practical implementation complexity. Through experimental evaluation in an underwater channel emulator, this work presents an empirical comparison of different modulation schemes under both still and turbulent water conditions. The results demonstrate that state-of-the-art pulse-based modulation schemes suffer severe performance degradation in turbulent conditions. On the other hand, SIM exhibits inherent resilience to turbulence-induced fading, achieving superior data rates across all testing conditions. The investigation is expanded through the implementation of advanced signal processing techniques. Compared with state-of-the-art bit and power loading optimisation algorithms, pairwise coding demonstrates remarkable performance, achieving a maximum data rate of 5.2 Gbps in still water conditions. These results establish SIM as a robust foundation for reliable OWC systems. Building upon these findings, a novel three-dimensional hierarchical SIM technique (PAFsk-SIM) is developed, presenting a distinctive approach to data transmission by simultaneously utilising the frequency, phase, and amplitude dimensions of the subcarrier signal. This hierarchical structure enables priority-based data transmission, with high-priority data encoded on the frequency dimension, medium-priority data on the phase, and low-priority data on the amplitude. A comprehensive theoretical framework is developed, including detailed error performance analyses across different channel conditions. The experimental validation demonstrates that encoding high and medium priority data on the frequency and phase of the subcarrier signal provides significant resilience against the negative effects of the OWC channel, while the amplitude dimension can be utilised to enhance spectral efficiency when channel conditions allow. This approach is evaluated across various hostile environments, including atmospheric turbulence, indoor multipath propagation and underwater channels, with notable success in maintaining reliable communication in turbulent conditions. The theoretical analysis is supported by extensive simulation results and experimental validation, demonstrating the resilience of this technique in various practical OWC channel conditions. The practical viability of the SIM-based techniques is demonstrated through the implementation of a real-time FSK-SIM system on an FPGA platform. The implementation achieves 8 Mbps transmission using 4-FSK-SIM, with thorough testing that demonstrates reliable performance in both back-to-back configurations and through a turbulent underwater channel. This successful hardware implementation validates the feasibility of the proposed modulation techniques for robust optical wireless communications and provides a foundation for the future development of more advanced real-time systems.