Constellation design analysis of color shift keying for visible light communication

Abstract

This thesis presents following three investigations: 1. Color-Shift Keying-How Its Largest Obtainable "Minimum Distance" Depends on Its Preset Operating Chromaticity and Constellation Size: This investigation is explained in chapter 3, with some necessary background detailed in the preceding chapter. The investigation conceptualizes the constellation design problem by exploring parameters such as modulation sizes and perceived chromaticity. Constellation designing is carried out over a uniform grid developed on the entire CSK constellation region for all possible combinations of assumed modulation sizes and chromaticity values. The corresponding minimum distances are then related to specified combinations of modulation sizes and chromaticity values. Eventually, a mathematical relationship is developed between minimum distance of a constellation as a function of modulation size and chromaticity. Objective of this work is to allow a system designer to have a notion about the maximum attainable minimum distance from any possible combination of desired modulation size and chromaticity. This work has been published in IEEE/OSA Journal of Lightwave Technology (authors - the candidate, her chief supervisor and one other collaborator). 2. Color-Shift-Keying Constellation Design Enhanced by Exploiting the Human Vision's Tolerance of Light-Intensity Fluctuations: This investigation is carried out in chapter 4 where constellation designing is further explored by the adding a feature of "imperceivable fluctuations" in the output intensity sum of designed symbols. The added feature appends another degree of freedom to the designing process and results show an improvement in the minimum distance of the designed constellations. The proposed scheme shows an improvement of 56% for M = 32 with an intensity relaxation of 8%. From the assumed settings of chromaticity values, modulation sizes as well as intensity relaxation, constellation designing is carried on an even intense uniform grid, as compared to the one used in first investigation. The purpose of using denser grid is to increase confidence on the obtained constellation designs and the possibility of attaining globally maximum "minimum distance". The resultant designs are then related to the three parameters according to the minimum distance of the designs. The investigation concludes with the development of a mathematical relation for the minimum distance of a designed constellation as a tri-variate function of chromaticity, modulation size and intensity relaxation. 3. Color-Shift-Keying Constellation and Decoder Design Under Channel Randomness: The third and final investigation presented in chapter 5 modifies the basic optimal constellation design problem of CSK for random channel. The randomness in channel may be attributed towards different abnormalities for example non-linearities of the Light Emitting Diodes (LEDs), drift in the driving current of LEDs, presence of ambient light, cross-talk effects of Photo-Detectors (PDs) etc. The over-all noise becomes statistical in nature, varying at every time instant. The underlying assumption, for the work carried out in this investigation is that the statistical channel information is available both at transmitter as well as receiver. Based on this information, detailed derivation of constellation and decoder design equations is conducted and presented in chapter 5. The mathematical work is then preceded by results where it is shown that a system designed with random channel information performs better than a system designed with assuming ideal conditions, even if particular instances of channel are not available.