Visible light communication networks for iot and its applications

Resumen

Light is one of the fundamental elements that supports life. The energy that it provides has allowed life to flourish. Every single living organism on Earth depends on light to survive directly or indirectly. Thus, for millions of years, life has evolved, optimizing the use of light. While plants use it as an energy source, the evolution has provided animals with mechanisms to see part of the Electro Magnetic (EM) spectrum, usually in the form of eyesight. We use it to navigate through the environment, interact with each other, and to get an accurate representation of the world. Humans are no exception. This is why the discovery of fire by early humans is considered one of the vital elements in our evolution. It provided the energy to survive cold environments, allowed us to cook the food that speed-up the development of our brain, and gave us light to explore the unknown during dark times. During the next hundreds of thousands of years, the fire was the prime artificial light source. Inventions such as torches, candles, and oil lamps permitted to have controlled fires with low burning rates that allowed the spread of artificial light in all types of environments. Ancient greeks envisioned light as a tool for communicating. With a system of torches located on top of tall towers, they could communicate at vast distances relatively fast for the time. The main drawback of those systems was the maintenance and low communication rate, as the time to turn on and off the torches and assure that the message was received at the other end was high. The invention of incandescent light was one of the key elements that pushed citizens to include electrical infrastructure in their buildings. They worked by having a thin filament of a difficult to melt metal in semi-vacuum and making an electric current run through it, heat it, and make it glow. In the early XX century, this changed with the invention of the LED light. Unlike its incandescent counterpart, LEDs do not require heat to produce light. When an electric current flows through a p-n junction, the recombination of electrons with holes in a lower energy state generates a photon. The frequency of the photon, and therefore the frequency of the light, depends on the band-gap between electrons and holes. In order to obtain different gaps, the use of different materials is required Nevertheless, manipulating materials to obtain LEDs with such behavior requires high amounts of precision and technical improvement. This is why it took nearly 40 years since the invention of the first red LED to achieve a blue LED. Combining the technology of red, green and blue LEDs, the first white LED was created, and with it, a new era for efficient illumination. Creating light this way is much more energy efficient than heating a thin wire and with that, another advantage was found: A small time to change the properties of the light. This allows controlling the light fast enough to embed information on it at rates comparable to RF communications. Visible Light Communication(VLC) has emerged in the last years as a new way to communicate. Using the existing lighting infrastructure, it has great potential to provide high bandwidth and communication security, making it a strong alternative against conventional Radio Frequency (RF) communications. Although VLC addresses several of the problems that RF communications have for specific scenarios, its potential for Internet-of-Things (IoT) applications must still be unleashed. IoT deployments are, by nature, limited in some way. The limitation could be given by the hardware used, as the cost may need to be minimal to have dense realistic deployments; the energy available, which depends on the battery size of the device; and computing power available, which is given by the available energy and the processing power of the device, among others. This limitations respond to the scenario where the devices are deployed or the cost of deployment, which includes price of the device and price of the infrastructure needed. Therefore, there is an interest in studying the advantages, drawbacks, and limitations of integrating VLC in IoT scenarios with the constraints mentioned above. This is especially necessary in the case of real-life deployments, mainly if the devices used are multi-purpose, and they need to perform other tasks, such as sensing, in addition to communicating. The practical work done around VLC is divided into two groups. The first one is trying to push the limits of what is possible to do with VLC, reaching high data rates with customized hardware and/or very computation extensive signal processing done in both ends of the communication. Although this type of work is essential to push the limits of what is possible, in the majority of cases is far from being practical in real-life scenarios, as the number of resources needed both in hardware and in processing power makes it impractical or very costly. There are some spin-offs and companies such as PureLifi and Lvx Systems that already do deployments. However, they offer final products, with limited capabilities to optimize them for research-oriented work. The second group, where this work is located, acknowledges that not all the application scenarios require very high throughputs. For some applications, such as IoT, having lower throughputs is acceptable if the cost of deployment per link is small enough to make it affordable in realistic scenarios. In recent years, several platforms have emerged as the need to have a lower barrier for VLC research is evident. These platforms try to get the cost of the system down by reducing the amount of processing required, thus using simple modulation schemes and off-the-shelf components for their designs. The platform that we created will be presented at the end of this document. First of all, VLC deployments for IoT use the available dense lighting infrastructure to achieve communication on top of illumination in indoor scenarios. The advantage of such an approach is that it allows reusing the existing infrastructure, improving the coverage, and the energy consumed while reducing the cost of deployment. Although VLC is energy efficient, it consumes more than just illuminating for the same amount of average illuminating power. If the luminaries are not correctly controlled, energy could be wasted by transmitting from a luminary with little or no effect into the receiver, usually located far from the desired or objective IoT device. In this thesis, we explore the energy consumption of luminaries in dense deployments, and we propose an approach to optimize the Signal-to-Interference-plus-Noise Ratio (SINR) given an energy budget. In order to do so, we propose to coordinate the transmission done by several independent devices concurrently. We introduce a novel synchronization method that uses the Non Line-Of-Sight (NLOS) component of the signal to tackle this problem. We are able to achieve this because the first reflection of the light is still powerfull enough to trigger our . Our approach can improve the average system throughput by 45%, or improve the average power efficiency by 2.3 times, compared to existing solutions. Secondly, even if the required infrastructure follows the design presented above, IoT deployments still need to face one fundamental problem: power management at deployed mobile devices. Having batteries increases the price of the device, its size, the maintenance required, and the ecological impact that the product has. Removing the battery while still being able to operate under realistic circumstances would be desired. This would allow to have deployments that require little-to-none maintenance, are cheaper to manufacture, with smaller ecological footprint and that could allow to have smallre devices that could be placed in locations what no device could be placed in before. In this work, we study what limitations such a system has. We then propose a new communication scheme, combining VLC and RF backscattering, that allows having continuous end-to- end communication with a custom-designed battery-free device. We design the hardware, software, and protocol that optimizes each aspect of the system to decrease the power requirement of each component. Finally, we evaluate our system and show that it can run with consumptions as low as 95μW, transmit continuously at 500 bits/second, and achieve more than 20 meters on backscattering distance, even with blockage elements as glass and walls covering the Line-Of-Sight (LOS). Thirdly, we explore one of the multiple applications that the designed VLC systems for IoT allow to implement; device positioning. VLC has been used for device positioning because the dense deployments, high availability and directionality makes it a great candidate for localization, especially in indoor environments. The majority of the literature requires to have multiple transmitters and/or receivers to achieve localization. The objective of this work is to localize with the minimum amount of necessary hardware, which is critical for IoT applications. Nevertheless, the majority of the work on the literature assumes that at least some element of the localization system remains static. This would be true in scenarios with small mobility, leading to continues assumptions of the position of the device every few seconds, leading to possible errors when this assumption is not true. In vehicles, for example, it can not be assumed that this is the case, as the vehicles have the capacity of moving. In fact, localization becomes more critical if the user is moving, as for applications as automated driving it is fundamental that the system that drives the vehicle knows with a high certainty the localization of the vehicle as well as its surroundings. We investigate how VLC systems could be used for positioning in dynamic scenarios. Exploiting the fact that, in our scenario, the transmitter and receiver are relatively moving, we propose a mathematical solution that, just using one VLC transmitter and one receiver both equipped with a compass, computes the correct relative position. We then implement our solution in a modified version of OpenVLC and achieve accuracies with less than 5 cm of error. Nevertheless, in this work we assume that the NLOS is non-existant, which may not always be the case. As explained earlier, the NLOS component of the light, the reflections, may interfere with our communication. Depending on the sensitivity of the platform, the error obtained by the the NLOS component may be high enough to have localization problems. This is why we, finally, try to overcome the problem mentioned above of NLOS reflections for device positioning with a low resource consumption NLOS component detector. Similar work try to solve this problem computing the Channel Impulse Response (CIR), but for systems with limited resources this is impractical because: The VLC front-end may not be fast enough for acquiring required data for the CIR calculation, depending on the hardware used in the front-end or 2) IoT boards are not able to run computationally expensive algorithms in real-time. Both these limitations come from the amount of resources that we want to put into our system, which basically is the same as limiting the cost of the device and the amount of spent energy. In this thesis, we propose a solution that in complex environments, reduces the localization error using LEDs up to 93%. In order to perform experimental research in VLC for IoT, a research platform is needed. Usually, all the available platforms using VLC are an end-product that create point-to-point links. Unluckily, they do not allow to tune the properties of the link such as transmission power, encoding used, hardware front-end, interconnected with other devices and power measurements in an easy way to the user without having to hack the platform. In this thesis, we also present the latest version of an open-source, software- based, VLC platform, OpenVLC. OpenVLC was first introduced as part of the thesis of Dr. Qing Wang. During this thesis, the platform has been re-designed on both hardware and software. The throughput improved more than 23 times and the transmission distance increased by a factor of 4. In this thesis, OpenVLC, parts of it, or modified versions have been used as a framework to create new IoT systems and explore the practical side of VLC. As a summary, in this work, we explore how VLC can be leveraged for IoT deployments. We study the features of such real-world deployments from different perspectives in a variety of scenarios, and we show that realistic implementations of VLC systems are not only possible but doable, enabling new features that IoT developer can exploit. During this thesis, uncountable challenges were encountered. Some of them very small, some of them incredibly complex. As VLC is usually not used for IoT, the majority of the challenges came from trying to adapt the VLC technology to IoT environments. The lack of computational power, available energy and device's size forced us to think out-of-the-box to achieve VLC communication with limited resources. Moreover, creating solutions for applications with different requirements is even more challenging, as in this type of systems there is no universal solution. More details about the research challenges of this work are described below. The design of a custom VLC frontend: Designing the Hardware (HW) for the VLC frontend of the IoT devices supposes a challenge as it is required to have communication on top of illumination. As IoT devices usually require dense deployments, the design needs to be as low-cost as possible, maintaining the capabilities of the device. Balancing cost and functionality was one of the most significant challenges faced in this work. The available off-the-shelf hardware's specifications needed for VLC are not always specified in the datasheet, requiring to experiment with it before deciding if it fits in the design. Making it run in real-time: Our system wants to be affordable, which means that the available processing power is going to be limited. Nevertheless, the system should be deployable in real scenarios. This means that protocols and devices designed and implemented need to work in real-time with limited resources. Realistic scenarios: The main objective of our research is to be practical. Practicality can not be achieved if the results are only implemented in laboratories under strictly controlled conditions. Having the final user in mind: The main goal of any practical scientific study like this should be to generate a great impact on the research community and the society. The best way to achieve that is to make it easy for others to use your work. Making the tools and devices created easy to use for others adds another layer of complexity to the design and implementation of any system. This thesis is supported by 9 publications. 1 in Transaction on Networking (indexed in jcr). 2 in the tier-1 conference ACM CoNEXT, 1 in tier-1 conference ACM MOBISYS, 1 EEE SECON, 1 in IEEE WONS and another 1 in IEEE WF-IOT. One workshop paper and a demo are also presented During the development of the thesis, several pieces of code have been released as open-source. The majority of them are related to OpenVLC, but there are also parts of some deployments and demos. They are available for anyone that wants to recreate our experiments or used them as a base for their project related with VLC.