Radio-Frequency (RF) backscatter has emerged as a low-power communication technique. Backscatter systems either rely on active signal generators (spectrum efficient, but dedicated infrastructure) or existing ambient wireless transmissions (existing infrastructure, but spectrum inefficient). In this paper, we aim to make RF backscatter spectrum efficient and at the same time work with existing infrastructure. We propose to leverage the deployment of LiFi networks built upon LED bulbs for pervasive RF backscatter. We experimentally demonstrate that LiFi, which passively leaks RF signals, can be exploited as a radio carrier generator for low-power RF backscatter. We further design LeakageScatter, the first backscatter system operating in the ISM band and exploiting LiFi-leaked RF signals, without the need to actively generate the carrier wave. We customize the design of the loop at the LiFi transmitter, as well as the coil antennas at the tag and RF backscatter receiver, to optimize the system performance. We propose to opportunistically enable the oscillator of the backscatter tag in the software that could reduce the energy consumption on backscattering by up to 75%. Experimental results show that LeakageScatter achieves a backscattering distance up to 10 m and 18 m in indoor and outdoor scenarios, respectively, without using a dedicated RF carrier generator.

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Micromachined Ultrasonic Transducers (MUTs) are being explored as power converters in wirelessly powered biomedical implants. This paper investigates the role of mechanical support properties in piezoelectric MUTs (PMUTs) on their power conversion efficiency. For this purpose, a finite element model (FEM) of a PMUT array was developed and integrated with an equivalent circuit model (ECM). The study considered different mechanical support scenarios, from rigidly clamped to completely free. These were numerically analyzed and validated by impedance measurements and acoustic power transfer experiments on PMUT prototypes. The results show that reducing the mass of the mechanical support increases the Q factor, leading to a significant improvement in power conversion efficiency, with an efficiency increase factor of 5.6x from the clamped to the free case. This approach can potentially enhance overall power conversion efficiency, reduce the need for matching networks, and enable miniaturization in ultrasonically powered implants.@en

Visible Light Communication (VLC) has emerged in the last few years as a promising technology not only for high-speed communication but also for serving a new generation of Internet of Things (IoT) devices that may leverage the pervasive lighting infrastructures. Integrating VLC in lighting environments for IoT requires the design of networked and intelligent luminaries and new IoT devices, encompassing the development of innovative technologies and new algorithms. A common experimental platform is necessary to lower the entrance barriers of VLC and speed up the research development. In this article, we provide guidelines for prototyping VLC for IoT applications, assisted by the open-source platform OpenVLC. We also introduce the new development on OpenVLC, which guarantees support for more powerful LEDs and much longer distance (extending the communication distance from 6 m to 19 m), dimming adaption, among other features. Its low-cost, open-source, and open-hardware designs allow researchers in the community to swiftly adapt it to suit their research purposes.

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Light-based positioning systems (LPS) are gaining significant attention as a means to provide localization with cm accuracy. Many of these systems estimate the object position based on the received light intensity, and work properly in 'ideal' environments such as large open spaces without obstructions around the light-emitting diode (LED) and the receiver, where reflections are negligible. In more dynamic environments, such as indoor spaces with moving people and city roads with moving vehicles, materials cause a wide variety of reflections. This causes variations in the received light intensity and, as a consequence, gross localization errors in LPS. We propose a new multipath detection technique for improving LPS that does not require the knowledge of the channel impulse response and then, it is suited to be implemented in low-cost positioning receivers that use a single-pixel photodetector. To develop our technique, we (i) analyze the statistical properties of non-line-of-sight (NLOS) components, (ii) develop an automated testbed to study the reflections of different types of surfaces and materials, and (iii) design an algorithm to remove the NLOS components affecting the positioning estimate. Our experimental evaluation shows that, in complex environments, our methodology can reduce the localization error using LEDs up to 93%.

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LED luminaries are now deployed densely in indoor areas to provide uniform illumination. Visible Light Communication (VLC) can also benefit from this dense LED infrastructure. In this paper, we propose DenseVLC, a cell-free massive MIMO networking system enabled by densely distributed LEDs, that forms different beamspots to simultaneously serve multiple receivers. This is a cell-free system, as there is no notion of autonomous cells and transmitters cooperate to jointly serve the users. Given a power budget for communication, DenseVLC assigns the power budget among the distributed LEDs to optimize the system throughput and user fairness. We formulate an optimization problem to derive the optimal policy for the power allocation. Our insights from the optimal policies allow us to simplify DenseVLC's system design and propose a heuristic algorithm that can reduce the complexity by 99.96%. Besides, we propose a novel synchronization method using non-line-of-sight VLC to synchronize all the transmitters that will form a beamspot to serve the same receiver. We implement DenseVLC with off-the-shelf devices, solve practical challenges in the system design, and evaluate it with extensive and realistic experiments in a system of 36 transmitters and 4 receivers in an area of\:\text{m}\,\,\times 3\:\text{m}$. Our results show that DenseVLC can improve the average system throughput by 45%, or improve the average power efficiency by 2.3 times, while maintaining the requirement for uniform illumination. Finally, we demonstrate that DenseVLC is robust against blockage.

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We analyze and optimize the performance of a new type of channel that exploits sunlight for wireless communication. Recent advances on visible light backscatter have shown that if mobile objects attach distinctive reflective patterns to their surfaces, simple photosensors deployed in our environments can decode the reflected light signals. Although the vision is promising, only initial feasibility studies have been performed so far. There is no analysis on how much information this channel can transmit or how reliable the links are. Achieving this vision is a complex endeavour because we have no control over (i) the sun or clouds, which determine the amount and direction of light intensity, and (ii) the mobile object, which determines the modulated reflection of sunlight. We investigate the impact of the surrounding light intensity and physical properties of the object (reflective materials, size and speed) to design a communication system that optimizes the encoding and decoding of information with sunlight. Our experimental evaluation, performed with a car moving on a regular street, shows that our analysis leads to significant improvements across many dimensions. Compared to the state of the art, we can encode seven times more information, and decode this information reliably from an object moving three times faster (53km/h) at a range that is four times longer (4m) and with three times lower light intensity (cloudy day).

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LEDs in our buildings, vehicles, and consumer products are rapidly gaining visible light communication capabilities. LED links however are notorious for being unreliable: shadowing, blockage, mobility, external light, all of these issues can disrupt the connectivity easily. Therefore, unless a reliable and cost-efficient data link layer is designed, VLC will be confined to niche applications. In this paper, we reveal a reason for unreliable VLC: a single type of photodetector at the receiver cannot establish a reliable link. We show that the photodetectors with complementary properties, in terms of optical spectral response and field-of-view, are necessary to handle the wide dynamic range of optical noise (such as the sun and other unwanted light sources) and mobility of users. Motivated by our experimental observations, we design a reliable and adaptive receiver for VLC (REAL-VLC) for low-end communication systems, an inexpensive receiver that senses light with complementary photodetectors and configures itself (physical and data link layers) dynamically to maintain the communication link. We implement the hardware and the software of REAL-VLC in low-end platforms, and experimentally validate it in representative test scenarios and a proofof-
concept application that consists of mobile nodes maintaining a VLC link under various lighting and path conditions.@en

Visible light is gaining significant attention as a medium to achieve accurate relative localization. Most of the studies in the area focus on indoor positioning and rely on two important assumptions: (i) lights are static, and (ii) the receiver has line-of-sight with multiple lights. These requirements limit the application of localization methods in scenarios where nodes have a single light and are mobile, such as motorbikes or swarms of robots. In general, this particular type of scenarios (single lights moving on a plane) leads to under-determined localization systems where no unique solution can be found. We follow a holistic approach that includes theory, simulations, and experiments to overcome some of the limitations present in such type of scenarios. Our theoretical and simulation results show that if nodes are enhanced with sensors providing relative directions (such as compasses), we can derive dependencies in the system to obtain unique solutions. Our proof-of-concept implementation validates our model by showing that single lights can provide relative localization with high accuracy: an average error below 5 cm.@en

We introduce the OpenVLC1.2 platform for research in Visible Light Communication (VLC) systems. The platform builds on top of previous versions, that has attracted dozens of users from the research community. We maintain its advantages such as the support for communication with TCP/IP layers, software-based and programmable MAC and PHY layers, and low-cost front-end. In this new version, we make an effort to increase data rate and its overall performance. OpenVLC1.2 is available to the research community.@en

In this work, we propose a new communication system for illuminated areas, indoors and outdoors. Light sources in our environments --such as light bulbs or even the sun-- are our signal emitters, but we do not modulate data at the light source. We instead propose that the environment itself modulates the {ambient} light signals: if mobile elements `wear' patterns consisting of distinctive reflecting surfaces, single photodiode could decode the disturbed light signals to read passive information. Achieving this vision requires a deep understanding of a new type of communication channel. Many parameters can affect the performance of passive communication based on visible light: the size of reflective surfaces, the surrounding light intensity, the speed of mobile objects, the field-of-view of the receiver, to name a few. In this paper, we present our vision for a passive communication channel with visible light, the design challenges and the evaluation of an outdoor application where our receiver decodes information from a car moving at 18 km/h.@en