Wavelength Division Multiplexed Visible Light Communication Performance under Illumination Constraints

2017-01-09T03:48:58Z (GMT) by Dilukshan Ajith Karunatilaka
Visible Light Communication (VLC) is able to utilize the immense unregulated bandwidth of the visible light spectrum for simultaneous data transmission and illumination. Wavelength Division Multiplexing (WDM) allows for higher aggregate data rates by independently modulating multiple colored light sources - typically red, green and blue for VLC. Receivers with optical filters corresponding to each WDM channels LED spectra are used to isolate individual channels with minimum interference. Orthogonal Frequency Division Multiplexing (OFDM) is a preferred modulation scheme for VLC, due to its high spectral efficiency, robustness against multi-path reflections, and simple equalizers. In this thesis, we model and evaluate the communication performance of WDM and OFDM-based VLC, and develop methods to improve this under various illumination constraints at both the transmitter and the receiver. <br>    OFDM-based VLC systems, despite their advantages are susceptible to biasing dependent non linear effects and clipping. Biasing levels and signal power should be optimized to mitigate these effects, maximize data dates and improve lighting quality. The effect of disparity in spectral properties and input-output relationships of WDM channels in these systems and the constraints imposed on communication and illumination has not been researched in depth. This thesis studies the impact of individual bias selection in an RGB luminaire on the aggregate data rate as well as the quality of light produced. An analytical model relating communication performance to illumination in an indoor environment considering both Line of Sight (LOS) and Non LOS (NLOS) signal propagation is developed. Using this model, the capacity of OFDM-based WDM-VLC systems was predicted under illumination constraints. <br>    The model was used to show that typical disparities in LED spectral properties impact the uniformity of communication performance at different correlated color temperatures (CCT). It was also shown that the main limitation in achieving uniform capacity over different CCTs is the lower power output around the wavelengths of red color. To mitigate this limitation in the red wavelengths, it is shown that an alternate WDM scheme utilizing additional unmodulated amber LED can increase the maximum capacity by 19.2% for warm white CCTs, 27.6%for neutral white CCTs, and 11.6% for cool white CCTs, while the light quality in terms of Color Rendering Index (CRI) is improved compared to conventional RGB-based WDM systems. Furthermore, the adoption of asymmetrically clipped optical OFDM (ACO-OFDM) can improve the communication capacity at low illuminance levels, compared to conventional DC Offset OFDM (DCO-OFDM). It was shown that combining both ACO-OFDM and DCO-OFDM under illumination constraints can provide an increased maximum capacity of 13.5% for warm white CCTs and 11.7% for neutral white CCTs over a wide illuminance span. The respective capacity gains are further increased by 24.3% and 21.5% in low SNR conditions which are observed for NLOS diffused links. <br>    The receiver optical filters which correspond to each WDM-LED channel spectra have a large impact on VLC system performance. Thin film optical filters are preferred for use in VLC due to their high transmissivity and ability to produce narrow passbands, but show a passband dependence on light angle of incidence. The impact of this effect on a WDM-based VLC system was evaluated. It was shown that by designing appropriate filter specifications, the SNR performance can be improved for indoor applications. By use of a novel field of view diversity receiver, an average SNR increase for red, green and blue was demonstrated, while reducing the difference between maximum and minimum SNR