Every day, we are surrounded by billions of invisible bits transmitted by our phones, tablets, and laptops. Wireless communication is pervasive but hard to teach because radio waves (5G, WiFi, BLE) cannot be seen, and a sensory experience is key to learning. From a physics perspective, radio and light waves are similar, but our eyes only see light. Thus, we design a platform to teach the complex concept of wireless communication using light. With our platform, children can "see" how a device transmits information using a normal flashlight. Our design includes connections with robots, superheroes, and games to make a joyful learning experience. The platform was used by hundreds of school students at an international science fair. After a guided activity, 80% of the students reported understanding the basic idea of wireless communication and some students even identified advanced concepts, such as interference, playing on their own.@en

Communication technology has continually adapted to new challenges. RF communication, with its established infrastructure, supports technologies like Bluetooth, Wi-Fi, and LoRa. However, the rise of IoT devices has lead to problems in traditional RF communication such as increasing spectrum congestion and interference. In this context, Visible Light Communication (VLC) emerges as a promising alternative to traditional RF systems. VLC uses the unlicensed light spectrum, modulating light intensity at speeds invisible to the human eye to transmit data, which is then received and decoded by photodetectors. While RF communication can also utilize unlicensed bands, VLC operates in an entirely separate region that experiences fewer regulatory constraints and interference issues compared to the crowded RF spectrum.
VLC systems can be classified based on their light source into active VLC and passive VLC. Active VLC systems use LEDs to transmit data through rapid light modulation controlled by circuitry, enabling fast communication. A key advantage of active VLC is its dual-purpose function, serving both illumination and communication, thereby maximizing resource use. However, the illumination of LEDs is often obstructed by walls or furniture affecting the coverage of active VLC systems. On the other hand, passive VLC systems use sunlight for data transmission, offering a pervasive and energy-efficient communication method, as sunlight provides a continuous source of illumination without additional energy costs. Passive VLC systems use external modulating surfaces to modulate ambient light properties to transmit data but face challenges due to the variable nature of ambient light, leading to links with lower performance compared to active VLC.
In this thesis, we address the obstruction challenges of active VLC and the low performance of passive VLC through a comprehensive approach that combines novel hardware designs and strategic system integration.@en

Consider an enclosed area, such as a room without windows. During the day, artificial light can provide illumination and communication thanks to advances in Visible Light Communication (VLC). Artificial lighting, however, has some drawbacks compared to using daylight in enclosed spaces. First, using sunlight consumes less power. Second, the use of natural light improves the health and comfort of the occupants. We propose a system, dubbed Sol-Fi, to provide joint illumination and communication in enclosed spaces using sunlight. Sol-Fi relies on two main components: commercial sunlight collectors and a novel transmitter to modulate ambient light. The sunlight collectors utilize optical fibers to guide natural light from open to enclosed spaces, and our transmitter modulates the incoming light providing two novel features. First, to analyze the pros and cons of the optical devices used in the literature for ambient light communication, Sol-Fi examines the properties of Liquid Crystals (LCs) and Digital Micro Mirror Devices (DMDs). Second, to investigate the trade-off between single- and multi-band communication, Sol-Fi proposes an optical design that can modulate the entire spectrum or divide it into different (individually modulated) bands. Our evaluation shows that, depending on the number of bands (single or dual) and the type of modulator (LC or DMD), Sol-Fi provides a data rate between 0.8 to 80 kbps, a range between 0.5 to 5 m, and a field-of-view between 30° to 60°.@en

In this paper, we propose a new approach where drones attain accurate localization by fusing information from artificial lighting and their embedded inertial and barometer sensors. Our system is able to provide accurate drone localization without the use of radios, GPS or cameras. We evaluate our framework, dubbed Firefly, with a testbed consisting of four light beacons and a mini-drone. Our results show that Firefly allows locating the drone within a few decimeters of the actual position; and compared to two state-of-the-art positioning methods that rely solely on lighting information, Firefly can reduce the localization error by 50 % and 80%.. respectively.@en

We explore a new alternative for drones to gather information from sensors. Instead of using the traditional radio-frequency spectrum, whose broadcast nature makes it more difficult to poll specific objects, we utilize the light spectrum. In our system, the drone carries a light, and flies to an area that it is interested in polling. Only the sensor (tag) under the coverage of the light sends data back by backscattering the impinging light waves. Enabling this system poses two challenges. First, a reliable modulation method with light is required. The method must overcome noise dynamics introduced by the drone (mechanical oscillations), the object (backscattering effects) and the environment (interference from ambient light). Second, to facilitate the deployment of tags in pervasive applications, the design of the tag should be battery-less and have a small surface area. These requirements limit the amount of power available for reception, transmission and sensing, since the energy harvested by solar cells is proportional to their surface area. Regarding the first challenge, we show that the amplitude-based modulation methods used in state-of-the-art studies do not work in our scenario, and investigate instead a frequency-based approach. For the second challenge, we optimize the computation, reception and transmission of the tag to create a battery-less design that operates with frequency-modulated signals generated from light. We build a prototype for the drone and the tag, and test them under different lighting scenarios: dark, indoors, and outdoors with sunlight. The results show that, under standard indoor lighting, our system can attain a polling range of 1.1 m with a data rate of 120 bps, while the tag operates with small solar cells and consumes less than 1 mW.@en

There is a growing interest in exploiting ambient light for wireless communication. This new research area has two key advantages: it utilizes a free portion of the spectrum and does not require modifications of the lighting infrastructure. Most existing designs, however, rely on a single type of optical surface at the transmitter: liquid crystal shutters (LCs). LCs have two inherent limitations, they cut the optical power in half, which affects the range; and they have slow time responses, which affects the data rate. We take a step back to provide a new perspective for ambient light communication with two novel contributions. First, we propose an optical model to understand the fundamental limits and opportunities of ambient light communication. Second, based on the insights of our analystical model, we build a novel platform, dubbed PhotoLink, that exploits a different type of optical surface: digital micro-mirror devices (DMDs). Considering the same scenario in terms of surface area and ambient light conditions, we benchmark the performance of PhotoLink using two types of receivers, one optimized for LCs and the other for DMDs. In both cases, PhotoLink outperforms the data rate of equivalent LC-transmitters by factors of 30 and 80: 30 kbps & 80 kbps vs. 1 kbps, while consuming less than 50 mW. Even when compared to a more sophisticated multi-cell LC platform, which has a surface area that is 500 times bigger than ours, PhotoLink's data rate is 10-fold: 80 kbps vs. 8 kbps. To the best of our knowledge this is the first work providing an optical model for ambient light communication and breaking the 10 kbps barrier for these types of links.

@en

A recent development in wireless communication is the use of optical shutters and smartphone cameras to create optical links solely from ambient light. At the transmitter, a liquid crystal display (LCD) modulates ambient light by changing its level of transparency. At the receiver, a smartphone camera decodes the optical pattern. This LCD-To-camera link requires low-power levels at the transmitter, and it is easy to deploy because it does not require modifying the existing lighting infrastructure. The system, however, provides a low data rate, of just a few tens of bps. This occurs because the LCDs used in the state-of-The-Art are slow single-pixel transmitters. To overcome this limitation, we introduce a novel multi-pixel display. Our display is similar to a simple screen, but instead of using embedded LEDs to radiate information, it uses only the surrounding ambient light. We build a prototype, called SunBox, and evaluate it indoors and outdoors with both, artificial and natural ambient light. Our results show that SunBox can achieve a throughput between 2 kbps and 10 kbps using a low-end smartphone camera with just 30 FPS. To the best of our knowledge, this is the first screen-To-camera system that works solely with ambient light.

@en

Solar cells are mainly used as power sources, but can be used for sensing as well. We propose a novel indoor system that exploits solar cells to track people by monitoring the changes in light intensity caused by their shadows and reflections as they walk by. Our framework has three main components. First, we develop a simulator based on a ray-tracing model to determine how the solar cells should be positioned in the tracking environment to maximize the signal to noise ratio. Next, we apply changepoint detection methods to convert the (noisy) solar cell signal into a binary detection signal. Our detection method uses a Bayesian approach, which allows our system to work well in various environments, with natural and artifical light. Finally, the binary output from multiple solar cells is fused to track multiple targets. The tracking engine is based on a particle filter implementation based on the probability hypothesis density filter. This approach allows us to perform tracking without knowing the actual number of targets in the environment. To evaluate our framework, we build small tags that consist of a solar cell, a micro-controller and a wireless module, and deploy them in a real apartment. Ours results show that our system allows solar cells to track people under different lighting conditions, during day and night.

@en

Passive visible light communication (VLC) takes advantage of the pervasive nature of ambient light in our environment for wireless transmissions. The design of transmitters in passive VLC predominately uses liquid crystal displays (LCDs). While LCDs are an economical choice with low power consumption, they lack some key properties that are desirable for passive VLC. For example, LCDs absorb more than half of the incident light, leaving only a small portion to be used for communication. In addition, since the direction of ambient can change over time, the relative positions of the LCDs and receivers have to be changed constantly to maintain the correct alignment. To overcome these shortcomings, we propose the use of a novel transmitter with integrated optical fibres and digital micro-mirror devices (DMDs). DMDs are able to reflect up to 97% of the incident light, while the accompanying optical fibres aim to capture ambient light from various angles and guide them to the DMDs in a fixed direction. This design is a first step towards the goal of decoupling the direction of ambient light from the direction of the optical link, while achieving the same communication characteristics as LCDs with a much smaller device. We also design an App to allow users to easily interact with the system and our evaluation shows that the link can achieve a data rate of 1bps at a distance of 30cm.

@en

This work presents the design and experimental validation of a dual-band fully reconfigurable circularly polarized (CP) reflectarray (RA) antenna for satellite communication applications in the Ku-band. The proposed structure operates with the downlink and uplink beams at frequency bands of [10.8-11.8] and [14-15.4] GHz, respectively. Simultaneous and independent beam control is provided over each of the two bands. The constituent unit cell is composed of two interleaved circular loops of different sizes to address the two frequency bands. Each loop is loaded using four varactor diodes. The loops are symmetrically loaded in the transverse plane to provide an isotropic response suitable for CP applications. A phase range of more than 300° is achieved in both bands as the capacitive loading varies, with an average of 2 dB loss in the lower frequency band and 3 dB loss in the higher band. In this article, the response of the unit cell is studied through full-wave simulations and verified through quasi-optical (QO) measurements, and the fully tunable performance of the reflectarray is validated through measurements in a near-field anechoic chamber.

@en