|dc.description.abstract||Bandwidth intensive applications such as high-definition video streaming, real-time video transmission, tactile communications, virtual reality (VR), augmented reality (AR) and the like are being developed for portable consumer devices connected wirelessly to the Internet. Increasing the bandwidth of the wireless connectivity has been the focus of much research. Optical wireless communications (OWC) system has emerged as a promising technique to provide high-speed wireless connections for data intensive applications as it can easily leverage today’s well-developed fibre based broadband access networks. The OWC technology is preferable over other alternatives including Wi-Fi, millimetre-wave (mm-wave), and ultra-wideband (UWB) systems in certain deployment scenarios. The optical band employed in OWC systems has a huge amount of unregulated bandwidth and is also immune to the interference from the radio frequency (RF) band. With these key advantages, OWC systems can be implemented in personal living and working spaces providing scalable bandwidth. In addition, they can also be deployed in areas where RF signals are carefully controlled and managed, such as in hospitals and airplanes. The thesis presents a systematic investigation of key functionalities of OWC – high bandwidth through modulation formats, multi-user access, and security in the physical layer.
Firstly, a comprehensive analytical system model is built for typical indoor OWC system with a general square QAM modulation format to achieve high bandwidth and by using cost-effective opto-electronic components. By using experiments based on 1.25 Gb/s – 5 Gb/s 4-QAM modulation format and 2.5 Gb/s – 10 Gb/s 16-QAM modulated system to verify, an relatively accurate analytical model was developed and the impacts of key system parameters on the system performance are thoroughly analysed using the analytical model, such as transmission optical power, laser RIN property, expanded beam waist, background light power and so on.
A multi-user access scheme named as time-slot coding (TSC) scheme is proposed. The function of the TSC scheme is experimentally demonstrated based on 4-QAM and 16-QAM modulation formats, respectively. The adaptive loading function employing both 4-QAM and 16-QAM compatible with the TSC scheme is also studied. Experimental results show that the satisfactory coverage is improved by a maximum of 61.2% compared to the TSC scheme employing 16-QAM alone. Furthermore, time-slot code misalignment due to non-ideal timing issues during code generation process is investigated both experimentally and analytically. In particular, the effect of the code overlapping ratio on the bit error rate (BER) performance of a general square QAM modulation format is analytically derived for typical indoor OWC systems. The experimental results match with analytical results with 4-QAM and 16-QAM modulation formats and show that the code misalignment tolerance can be more than 92.3% for 4-QAM and 26.9% for 16-QAM with received optical power levels greater than -19.7 dBm.
A novel physical layer mechanism is proposed for the provision of simultaneously secure connections for multiple users in indoor OWC systems. It is achieved by employing the TSC scheme together with chaotic phase sequence. The chaotic phase sequence applied to each symbol to secure the transmission is generated according to the logistic map, which based on a key set containing a constant parameter r and an initial value x1. The feasibility of showing the proposed mechanism is capable of providing OWC connections is proved by analytical study and experiments. Experimental results also show that adding the chaotic phase does not degrade the legitimate user’s signal quality. The robustness is also critically investigated and both analytical and experimental results indicate that the proposed scheme was robust and was able to maintain high communication security against eavesdropper’s high searching accuracy of 10-10 within the optical-wireless links. The time-slot code misalignment tolerance with chaotic phase is further studied through analytical model and experiments, where the agreement shows that 68.6% and 22.6% misalignment tolerance is achieved for 4-QAM and 16-QAM modulation formats, respectively, at the received optical power of over -20.2 dBm.||en_US