Sunlight-based Passive VLC

Abstract

Nowadays,wireless connectivity is ubiquitous: humans use smartphones, smartwatches, laptops and other devices, while at the same time, the Internet of Things (IoT) is adding millions of connected objects. This large number of devices uses mainly the radio frequency (RF) spectrumfor communication. And a direct consequence of this exponential growth is the scarcity of free RF bands to cope with this demand.

To tackle this challenge, researchers have proposed using a different carrier: visible light. With Visible Light Communications (VLC), devices communicate with each other by modulating the intensity of their light-emitting diodes (LEDs) and demodulating it using light sensors. The key advantage of VLC is the utilization of the visible light spectrum, with free bands that do not interfere with traditional RF systems. Nonetheless, despite the efficiency of LED technology, luminaries still require several Watts to generate light. The need for this considerable amount of energy has triggered interest in a new research area: Passive VLC. The fundamental principle of Passive VLC is to exploit ambient light to create wireless links, thus reducing the energy required by transmitters to generate their own light.

Passive VLC is a promising area, but poses a daring challenge: modulate light without any control over the source. The research community has proposed using optical surfaces that block or reflect light dynamically as modulators, but these platforms provide limited data rates, ranging froma few tens of bps to a few kbps. Moreover, using the sun as the source of ambient light introduces another challenge: variations in position and intensity.

This dissertation aims to improve the performance of Passive VLC systems operating with sunlight, with a particular focus on increasing the data rate and resilience to the changing sun’s position.

Our first contribution is a short-range wireless link using a tiny screen as a transmitter and a camera as a receiver. The screen is a reflective surface, adapted to work with ambient light. The sunlight reaching the screen is modulated to transmit information to a smartphone’s camera, creating a stream of optical data. This screen-to-camera link using sunlight attains up to 10 kbps, ten times faster than previous similar systems, working from sunrise to sunset - independent of the sun’s position.

Inspired by the concept of Li-Fi, which combines illumination and VLC, our second contribution envisions the creation of a natural light bulb with wireless communication capabilities. Our design combines optical modulators, optical filters and sunlight collectors to track the sun’s position during the day and radiate modulated beams of sunlight in indoor scenarios. These beams of natural light provide illumination and communication and are the first to divide sunlight into two color channels to double the data rate.

Our third contribution proposes a novel link for robots to communicate using sunlight. We leverage a material used in solar technology, the Luminescent Solar Concentrator (LSC). An LSC surface absorbs light fromits top and emits it on its edges. We place LSCs on top of robots, together with liquid crystal cells (LCs), so sunlight arriving from the top can be modulated into data packets transmitted toward the edges. This novel communication systemallows task coordination between robots using sunlight.

Overall, this dissertation presents new Passive VLC systems focusing on applications that exploit the sun as the light source. Within this scenario, our focus has been to increase the data rate, with the first two contributions, and on making the systems resilient to the sun’s position, with all three contributions.