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.

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.

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.

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.

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.