Energy efficient sensing and networking : a visible light perspective

Abstract

While the traditional communication technique with radio frequency is getting its bottlenecks in terms of available spectrum and energy consumption, visible light has attracted a lot of research attention as a promising supplement of communication medium, since it is ubiquitous and necessary in our daily lives. Known as a “green” technology, Visible Light Communication (VLC) piggybacking on existing lighting system has been considered as a promising access technique for next-generation wireless systems (5G). However, whereas major research efforts in the field of VLC are pursuing higher data rate, exploring the potential of VLC employing Commercial Off-The-Shelf (COTS) devices and providing seamless connectivity for mobile users seem to be ignored. To this end, in this thesis, we focus on developing a practical VLC system which can offer roaming support for users with commonly handy devices, e.g. smartphones. In the first part, we design and implement CeilingTalk as an LED-Camera VLC broadcast system using COTS Light Emitting Diode (LED) luminaires as transmitters and smartphone cameras as receivers so that it can be fully hosted in a smartphone and is feasible for all possible indoor environments. CeilingTalk innovates in both encoding and decoding to achieve an adequate throughput for realistic applications. On one hand, it employs Raptor coding to allow multiple LED luminaires to transmit collaboratively so as to benefit both throughput and reliability. On the other hand, it presents a lightweight decoding scheme to handle the asynchrony (both spatial and temporal) in transmissions. Moreover, we analyze the impact of various parameters on the performance of CeilingTalk, in order to derive an analytical model for such VLC systems enabled by COTS devices and hence provide general guidance for future VLC deployments in larger scales. Finally, we conduct extensive field experiments to validate the effectiveness of our LED-Camera VLC model, as well as to demonstrate the promising performance of CeilingTalk: up to 1.0 kb/s at a distance of 5 meters. During we develop CeilingTalk, we notice that the lack of feedback from receivers in LED-Camera VLC results in extremely difficult to offer seamless connectivity to mobile users. We hereby exploit the sensing ability of an LED luminaire to detect user presence so as to provide possible feedback for VLC. To evaluate the possibility using LEDs to sense occupants, we innovate CeilingSee, a dedicated occupancy inference system free of heavy infrastructure deployments and user involvements in the second part of this thesis. In realizing CeilingSee, we first re-design the LED driver to leverage LED's photoelectric effect so as to transform a light emitter to a light sensor. In order to produce accurate occupancy inference, we then engineer efficient learning algorithms to fuse sensing data gathered by multiple LED luminaires. We build a testbed covering a 5mX6m office area; extensive experiments show that CeilingSee is able to achieve very high accuracy in occupancy inference. Finally, we propose RoCLight integrating CeilingTalk with CeilingSee to provide seamless connectivity for mobile VLC users. We innovate in converting part of the LED luminaires (already used as the VLC transmitters) into light sensors to have a proper feedback channel for roaming support. By sensing the disturbance to the ambient diffuse reflection change caused by user presence and mobility, RoCLight is able to adaptively handoff its transmissions from one luminaire to another, so as to keep up with user mobility and hence to offer an adequate roaming support. We implement RoCLight as a small-scale testbed with five LED luminaires spanning a length of 7.5 meters, and conduct extensive evaluations on the roaming support capability of RoCLight based on this testbed; all our experiments strongly demonstrate its promising performance.