Passive Visible Light Communication

Abstract

As more and more devices have become wireless, the traditional Radio Frequency (RF) spectrum has become densely populated. To relieve this pressure, communication using light, visible light communication (VLC), has been proposed by many researchers. However, not all light sources can be modulated to transmit information. Therefore, the use of materials that can change their state between translucent and opaque has often been investigated in order to modulate light. ChromaLux is a recent system that uses such a material. It uses Liquid Crystal shutters to transmit information. However, instead of a single crystal, a stack of crystals is used, which exposes an extra transient region. This transient region shows several peaks that allow faster communication, but makes the system unstable. This thesis addresses several issues that ChromaLux experiences, in order to improve the performance. The system is changed from open-loop to closed-loop, creating a stable system that reaches higher speeds. However, in order to achieve this, part of the crystals is blocked, causing a lower Signal-to-Noise Ratio (SNR). For shorter distances up to 1 m, we are able to increase the data rate by 140 %, while at longer distances of up to 3 m we achieve an increase of up to 92 %. A second improvement which increases the SNR of the system, is the smart use of differential amplification. This allows communication at even greater distances of at least 5 meters.