Enhanced energy and spectrum efficiency in visible light communications

In recent years, there has been a surge in data traffic, leading to the investigation of using optical frequencies in conjunction with radio frequency (RF) wireless communication systems. One such technology is visible light communication (VLC), which uses light-emitting diodes (LEDs) in the visible light spectrum to transmit data. VLC has gained popularity for short-range wireless connections due to its energy efficiency, low-cost, and wide availability of front-end devices. However, one of the main challenges in designing a VLC system is improving its energy and spectral efficiency. This thesis aims to investigate techniques and determine the most effective methods for enhancing the energy and spectral efficiency of VLC systems. The thesis examined methods for optimising the bias point of an LED to benefit from increasing bandwidth at higher driving current while minimising the resulting signal distortion. The approaches are based on allowing for some nonlinear distortion or reducing signal swing/signal-to-noise ratio (SNR) while benefiting from higher bandwidth at higher driving currents. A framework is presented to estimate the attainable capacity under both conditions. Simulation results showed that the optimal bias point does not lie in the middle of the dynamic range. This was verified through a PAM-based VLC experiment, which showed that the transmission rate can be increased by choosing the optimal bias current instead of the midpoint of the linear range. Subsequently, VLC with probabilistic shaping (PS) is studied to optimise the distribution of source symbols and improve system performance. In this study, the error performance of PS is analysed, and closed-form analytical expressions are provided. The results show that PS outperforms the conventional uniform distribution and significantly reduces the required SNR to achieve a certain error probability. To demonstrate the practical application of PS in VLC, it was implemented in conjunction with optical orthogonal frequency-division multiplexing (OFDM) modulation. This allowed for continuous and adaptive loading of information bits to the channel response, resulting in an efficient use of available modulation bandwidth and transmission rates close to the channel capacity limits. In the two experimental demonstrations, a single low-power LED and a wavelength-division multiplexing (WDM) system using three off-the-shelf LEDs were used to achieve bit rates of 1.13~Gbps and 10.81~Gbps, respectively, representing increases of 27.13\% and 25.7\% over the traditional bit-power loading technique. Finally, an alternative approach towards enhancing the energy of VLC systems is introduced using frequency shift chirp modulation (FSCM). The error performance of FSCM was analysed in different types of channels, and a proof-of-concept experiment was conducted to demonstrate its potential use in VLC systems. FSCM offers improved robustness in band-limited, frequency-selective channels compared to other modulation techniques. This makes it a promising choice for integrating into VLC systems, particularly in low-power and low-rate application scenarios.