Emerging technologies like Fluid Antenna Systems (FAS) are important ingredients toward new generation communications. In particular, we focus on systems based on the reconfigurability capabilities of liquid antennas, which are a subset of FAS that use a liquid as their main conductor. Despite recent pertinent advances in the field, the reconfiguration speed was not directly addressed in the literature, as it is assumed to be very high in most works. However, it is a crucial parameter that defines the system’s capabilities and performance bounds. In addition, a few approaches to spatial correlation modeling are presented here that are also needed for a holistic performance evaluation. Hence, a numerical methodology is proposed to realistically model both parameters. Performance metrics are also addressed, which aid in identifying and comprehending the system’s strengths and weaknesses. The importance of these models and metrics is showcased by reconfiguring the FAS, proposing several algorithms and illustrating their features. As a result, we show that it is possible to obtain a realistic performance evaluation for FAS and improve currently available models and performance to further progress on this topic.

Solar cells have shown promising performance for Simultaneous Lightwave Information and Power Transfer (SLIPT). Most of literature has focused on applying this concept to battery-powered high-data rate communication. Nevertheless, its application is also particularly advantageous in the low-data rate regime for battery-free systems. However, we find that using the same solar cell causes both communication and harvesting performances degradation. This problem is particularly critical in battery-free systems for its size and power constraints. This paper reveals that the reason behind this problem is that solar cells are typically operated at their Maximum Power Point (MPP), which optimizes energy harvesting but is inefficient for communication. In order to address this finding we introduce, for the first time, the Maximum Communication Point (MCP), a new operating point that complements the traditional MPP by optimizing communication performance. We then propose a transformer-based SLIPT design that enables an accurate control of the impedance at the solar cell’s output and thereby allowing precise selection of its operating point. Extensive experiments show that our system provides communication gains of 5x in peak-to-peak received voltage when operating at the MCP, and harvested power gains of more than 3x when operating at the MPP, with respect to the best performing state-of-the-art design.

Visible light communication (VLC) is a technology that complements radio frequency (RF) to fulfill the ever-increasing demand for wireless data traffic. The ubiquity of light-emitting diodes (LEDs), exploited as transmitters, increases the VLC market penetration and positions it as one of the most promising technologies to alleviate the spectrum scarcity of RF. However, VLC deployment is hindered by blockage causing connectivity outages in the presence of obstacles. Recently, optical reconfigurable intelligent surfaces (ORISs) have been considered to mitigate this problem. While prior works exploit ORISs for data or secrecy rate maximization, this paper studies the optimal placement of mirrors and ORISs, and the LED power allocation, for jointly minimizing the outage probability while keeping the lighting standards. We describe an optimal outage minimization framework called JointMinOut and present solvable heuristics. We provide extensive numerical results and show that the use of ORISs may reduce the outage probability by up to 67% with respect to a no-mirror scenario and provide a gain of hundreds of kbit/J in optical energy efficiency with respect to the presented benchmark.

Visible light communication (VLC) complements radio frequency in indoor environments with large wireless data traffic. However, VLC is hindered by dramatic path losses when an opaque object is interposed between the transmitter and the receiver. Prior works propose the use of plane mirrors as optical reconfigurable intelligent surfaces (ORISs) to enhance communications through non-line-of-sight links. Plane mirrors rely on their orientation to forward the light to the target user location, which is challenging to implement in practice. This paper studies the potential of curved mirrors as static reflective surfaces to provide a broadening specular reflection that increases the signal coverage in mirror-assisted VLC scenarios. We study the behavior of paraboloid and semi-spherical mirrors and derive the irradiance equations. We provide extensive numerical and analytical results and show that curved mirrors, when developed with proper dimensions, may reduce the shadowing probability to zero, while static plane mirrors of the same size have shadowing probabilities larger than 65%. Furthermore, the signal-to-noise ratio offered by curved mirrors may suffice to provide connectivity to users deployed in the room even when a line-of-sight link blockage occurs.

Light bulbs have been recently explored to design Light Fidelity (LiFi) communication to battery-free tags, thus complementing Radiofrequency (RF) backscatter in the uplink. In this paper, we show that LiFi and RF backscatter are complementary and have unexplored interactions. We introduce PassiveLiFi, a battery-free system that uses LiFi to transmit RF backscatter at a meagre power budget. We address several challenges on the system design in the LiFi transmitter, the tag and the RF receiver. We design the first LiFi transmitter that implements a chirp spread spectrum (CSS) using the visible light spectrum. We use a small bank of solar cells for both communication and harvesting, and reconfigure them based on the amount of harvested energy and desired data rate. We further alleviate the low responsiveness of solar cells with a new low-power receiver design in the tag. We design and implement a novel technique for embedding multiple symbols in the RF backscatter based on delayed chirps. Experimental results with an RF carrier of 17dBm show that we can generate RF backscatter with a range of 92.1 meters/$\mu \text{W}$consumed in the tag, which is almost double with respect to prior work.

Visible light communication (VLC) is a growing technology that can complement radio frequency to fulfill the increasing demand for wireless data services. VLC exploits the already deployed illumination infrastructure based on light-emitting diodes to communicate data. While mirrors have been traditionally used for interior decoration, among other applications, to what extent can they be deployed to improve VLC performance? In this paper, we propose optimizing the placement of mirrors to minimize the outage area, a common problem in VLC when line-of-sight link blockage occurs. We compare this optimal performance with a single-rectangular mirror array, a common mirror shape that is found indoors. We also study the most helpful mirror placement under different conditions. We show that our outage minimization algorithm, MinOut, easily provides threefold outage reductions. Besides, we demonstrate that a single-rectangular mirror array with orientation mobility can provide similar or even better performance than an optimal distributed mirror placement without mobility.

While much research effort has been invested in long-range and low-power uplink communication for battery-free IoT networks, current deployments lack a scalable bidirectional communication infrastructure for data collection and processing with battery-free devices. To fill this gap, we introduce Lo W-Fi, a system architecture specifically designed to meet the requirements of battery-free IoT applications. We show the suitability of LoW-Fi for deploying monitoring systems for precision agriculture indoors. This sector is revolutionizing with the installation of smart greenhouses that require the constant monitoring of ambient parameters to ensure optimal conditions for the crops growth. Our system is implemented using commercial off-the-shelf devices, and it works at the intersection of WiFi and LiFi for downlink and RF backscatter for uplink, retaining the advantages of each technology and solving their practical limitations. We evaluate LoW-Fi performance in a real greenhouse, and the experimental results show that it can achieve an uplink (downlink) range of 45m(70m) with 0% BER. The aggregated data rate is up to 4.5 Mb/s.

In this paper, we demonstrate PassiveLiFi, a battery-free system that works at the intersection of Light-Fidelity (LiFi) and Radio-Frequency (RF) backscatter technologies to enable long-range and ultra-low power wireless communications. We show how the PassiveLiFi tag can transmit real measured data through an interactive demo, and we display the received data in a customized graphical user interface. Our demo shows the suitability of PassiveLiFi for implementing real IoT applications, such as monitoring systems for smart agrifood facilities (i.e., greenhouses and vertical farms).

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.

RGB LED bulbs have entered the market as a promising alternative to traditional phosphor coating LEDs to meet illumination standards. In this paper, we introduce and propose solutions to address the challenges of RGB LED lighting for the Internet of Things applications, performing communication, harvesting energy and sensing tasks. Through experiments, we demonstrate that we can use RGB LEDs for multiple tasks. We can achieve a bandwidth in the order of 50 kHz and distances of around 3.5 m using commercial RGB LED bulbs as transceivers, without using any dedicated photodetector. RGB LED links are composed of three main colours, and we show that red is the best colour both for communicating to another receiving RGB LED bulb, as well as for harvesting with a solar cell; green and blue can instead be exploited for standard-compliant lighting and/or sensing purposes. We evaluate the system in two proof of concepts and provide insights for operating RGB LEDs for multiple tasks.

Light bulbs have been recently explored to design Light Fidelity (LiFi) communication to battery-free tags, thus complementing Radiofrequency (RF) backscatter in the uplink. In this paper, we show that LiFi and RF backscatter are complementary and have unexplored interactions. We introduce PassiveLiFi, a battery-free system that uses LiFi to transmit RF backscatter at a meagre power budget. We address several challenges on the system design in the LiFi transmitter, the tag and the RF receiver. We design the first LiFi transmitter that implements a chirp spread spectrum (CSS) using the visible light spectrum. We use a small bank of solar cells for communication and harvesting and reconfigure them based on the amount of harvested energy and desired data rate. We further alleviate the low responsiveness of solar cells with a new low-power receiver design in the tag. Experimental results with an RF carrier of 17 dBm show that we can generate RF backscatter with a range of 80.3 meters/μW consumed in the tag, which is almost double with respect to prior work.

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%.

This work has been partially funded by the Spanish MECD FPU fellowship program granted to the author Borja Genovés Guzmán, the Catalan Government under Grant 2017-SGR-1479, and the Spanish Government under the national project ’TERESA-ADA’ with ID no. TEC2017-90093-C3-2-R and TEC2017-90093-C3-1-R (MINECO/AEI/FEDER, UE).

Visible Light Communications (VLC) is considered one of the most promising technologies to guarantee the future demand of user wireless data rate. Since a DC value is needed for illumination, Direct Current-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) is adopted as modulation technique in these environments. Channel estimation in VLC is quite different from the conventional radiofrequency (RF) counterparts because the indoor optical wireless channel model significantly differs from the traditional RF one. Previous proposals rely on dedicated pilot symbols to perform channel estimation what leads to a loss in efficiency. In this paper, we propose a novel approach of Superimposed Training (ST) for channel estimation in VLC scenarios.

Rescue tasks in coal mines are fundamental but often suppose the loss of rescuers lives, as currently they are carried out only by humans. TeleRescuer project expects to make this activity safer by developing a robot to be driven by rescuers from a safe area. It offers virtual teleportation providing to the operator an environment where he can interact with the reality perceived by the robot. In this paper, the design of the communication, vision and sensory systems are presented, giving an overview of all the elements that are included in the robot. The adaptation of these electronics to the hazardous environment is also explained. A prototype of an ad-hoc wireless network and results in terms of the transmission characteristics are shown.

Direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) signals used in visible light communications suffer from high peak-to-average-power ratio (PAPR) or cubic metric (CM). It strongly degrades the performance due to the great back-off necessary to avoid the clipping effect in the light-emitting diode. Thus, PAPR and CM reduction techniques become crucial to improve the system performance. In this paper, an adaptive network-based fuzzy inference system (ANFIS) is used to obtain efficient DCO-OFDM signals with a low power envelope profile. First, signals specially designed for DCO-OFDM with very low CM, as the ones obtained from the raw cubic metric (RCM)-active constellation extension method, are used to train the fuzzy systems in time and frequency domains. Second, after the off-line training, the ANFIS can generate a real-valued signal in a one-shot way with 8.9 dB of RCM reduction from the original real-valued signal, which involves a gain in the input power back off larger than 2.8 dB, an illumination-to-communication conversion efficiency gain of more than 35% and considerable improvements in bit error rate.