Improving robustness and channel length of visible light based optical links for low-SNR transmitters

Abstract

The ever increasing demand for higher data rate and increased bandwidth has pushed radio frequency spectrum towards congestion. To fulfill this demand, a natural extension is towards the exploration of optical spectrum for communication. Fueled by mass adaptation of light emitting diodes (LEDs) for illumination purposes, mainly due to its energy efficient properties and high frequency switching characteristics, a new type of communication has emerged known as visible light communication (VLC). Recently, VLC has gained much attention not only for its utilization in indoor positioning and localization applications, but also it is paving the way towards outdoor applications related to intelligent transportation systems (ITS). Despite VLC's popularity as a complementary communication technology, it faces many challenges including the low data rate, limited communication distance and robustness to interference, specifically in multi-user environment representing real-world scenarios. Much work has been done to increase the data rate, however very few works found in literature deal with increasing the robustness and communication distance of a VLC link, both of which are critical parameters due to the visible nature of the link. In this work, we propose to address these challenges in four phases. In the first phase, we propose a polarization based transceiver methodology to mitigate channel interference thereby increasing the robustness of the optical link. Experimental results demonstrate a 32.6% more robust VLC link compared to the conventional transceiver under severe optical interference. In the next phase, we extend the work to support for multi transmitter environment. With additional capabilities of spatially separating interference sources, image sensor based VLC receivers have been studied. The main challenges include evaluation of shutter speed imposing restrictions on minimum SNR requirement of the transmitter and variable frame rate phenomenon resulting in inconsistent sampling intervals. To cater for these issues, an adaptive threshold under-sampled communication technique has been proposed that enables the receiver to sample at slower shutter speeds thereby increasing the communication distance for low-SNR transmitters. We extend this work to propose a hybrid phase-frequency modulation technique to establish a record 160 m long communication link, making it the longest communication distance achieved till date using visible light. In the third phase, we propose channel modeling of a camera communication system using perspective projection. Perspective projection provides better approximation of channel parameters compared to conventional pin-hole model. The work is extended to estimate the channel length of a MIMO camera communication system, which is a critical parameter specifically for distance-critical applications. In the last phase, we use the channel model to predict the behavior of the system in a real-world scenario where multiple transmitters and receivers would be operating simultaneously specifically in ITS applications. Either the transmitter, or the receiver, or both could be moving in a random fashion independently of each other. Vehicle vibration analysis was analyzed to quantify the extent of vibrations a transmitter, or a receiver, undergoes in a real-world environment. Based upon the channel model simulations, we propose a passive detection and tracking methodology to detect and track a moving transmitter, while maintaining a robust communication link between the transmitter and the receiver. The proposed methodology caters for the worst case vehicle vibrations, validated through experimental results.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology