A recent technology known as transparent screens is transforming windows into displays. These smart windows are present in buses, airports and offices. They can remain transparent, as a normal window, or display relevant information that overlays their panoramic views. In this paper, we propose transforming these windows not only into screens but also into wireless transmitters. To achieve this goal, we build upon the research area of screen-to-camera communication. In this area, videos are modified in a way that smartphone cameras can decode data out of them, while this data remains invisible to the viewers. A person sees a normal video, but the camera sees the video plus additional information. In this communication method, one of the biggest disadvantages is the traditional screens’ power consumption, more than 80% of which is used to generate light. To solve this, we employ novel transparent screens relying on ambient light to display pictures, hence eliminating the power source. However, this comes at the cost of a lower image quality, since they use variable and out-of-control environment light, instead of generating a constant and strong light by LED panels. Our work–dubbed PassiveCam – overcomes the challenge of creating the first screen-to-camera communication link using passive displays. This paper presents two main contributions. First, we analyze and modify existing screens and encoding methods to embed information reliably in ambient light. Second, we develop an Android App that optimizes the decoding process obtaining a real-time performance. Our evaluation, which considers a musical application, shows a Packet Success Rate (PSR) of close to 90%. In addition, our real-time application achieves response times of 530 ms and 1071 ms when the camera is static and when it is hand-held, respectively.@en

The increasing number of portable electronics and Internet of Things (IoT) devices demand scalable, low-power, and versatile networks. The Radio Frequency (RF) band has long been the main carrier for communication technologies. For instance, Bluetooth, Wi-Fi, and LoRa are assigned specific RF bands and use matured techniques to avoid infringing other regions of the spectrum. However, the high number of RF devices make interference between those in the same band inevitable. Furthermore, the growing number of RF applications increases the demand for bandwidth, making it a costly resource. To solve these issues, researchers are exploring other parts of the electromagnetic spectrum. A wide, free, and prevalent candidate is visible light. In this regard, Light Emitting Diode (LED)s and lamps have been explored in building Visible Light Communication (VLC) platforms. However, all light emitting devices require power in the order of several watts, even the most power-efficient LEDs. This amount is difficult to afford by low-power and IoT devices. Therefore, a new communication paradigm has emerged in VLC, called passive VLC. In passive VLC, a platform, instead of generating light, modulates the light in the environment to transmit data. A passive transmitter relies on other light sources, such as the sun or a ceiling lamp; it modulates the already-generated light, and redirects it towards a receiver. Although very promising in lowering power consumption, this method is not as easy to apply. Modulating light out of its source is a big challenge. Until now, various technologies have been used for this purpose, including Liquid Crystal (LC)s, Digital Micro-Mirror Device (DMD)s, and piezoelectric modulators, each with downsides and advantages. Among these, the most power-efficient are LCs, consuming in the order of microwatts. Nevertheless, this appealing low power comes at a price: they have an inherently low bandwidth, where the typical LC has a maximum switching frequency of few hundreds of Hz. Motivated by their ultra low power prospects, we investigate LCs to overcome their disadvantages. We delve into their physical properties, use them as transmitters, and make systematic guidelines to increase the data rate of LC-based transmitters. As a result, we uncover new ways of operating LC cells. In addition, we bring the well-known concepts and methods of classical communications to the context of passive VLC, such as Multiple-Input Multiple-Output (MIMO), which has been proven to increase the data rate of a communication link. Lastly, besides technical enhancements, we investigate possible use cases of passive communications in people’s daily lives, so as to pave the way for the adoption of this nascent area. In short, we investigate the solutions to the low data rate of LCs, as well as their integration and acceptance as a novel method of communication.@en

Multiple-input multiple-output (MIMO) methods play a pivotal role in increasing the capacity of wireless communication systems, but they have not been analyzed systematically in the nascent area of passive communication with visible light (passive-VLC). The main challenge in passive-VLC is its low data rate. This limitation is caused by the slow switching speed of the most popular modulator used in the state-of-the-art: liquid crystal cells (LCs). Several studies use sophisticated modulation schemes with multiple LCs to increase the data rate. However, these efforts have only led to logarithmic improvements. A transmitter with a single LC can provide 1 kbps, and a transmitter with 64 LCs delivers 8 kbps: resulting in an efficiency of 125 bps per LC cell. Ideally, the capacity should increase linearly with the number of LCs.

We propose a general framework to achieve reliable MIMO communications with passive-VLC. Our approach, which has a theoretical and empirical foundation, has three desirable properties: (i) does not assume orthogonality of the individual channels (overcomes co-channel interference), (ii) can exploit multiple properties of light (polarization and color); and (iii) is agnostic to LC parameters (which some studies rely on). Our results show that a transmitter with 9 LCs increases its capacity almost linearly up to 9 channels, attaining 6.8 kbps (750 bps per LC) using the simplest modulation method in the SoA.@en

In recent years, the number of wireless applications has increased significantly, resulting in the radio bands becoming expensive and prone to interference. There is a new research area aiming at mitigating these issues by creating communication links using ambient light. This area, called passive-VLC, not only exploits the visible light frequencies, but does so with low-power transmitters. All the previous work in passive-VLC, however, forget about individual wavelength bands of light, and do not exploit its wide spectrum, reducing the potential channel capacity. In this paper, we propose a novel method to transmit and decode data, using liquid crystal cells that modulate and consider the full spectrum, and put it to the test by prototyping a multi-symbol communication link. The main contribution of our work is to show that passive-VLC can move from spectrum-agnostic to spectrum-aware modulation. We explore this new domain by making use of a novel type of receiver (i.e., a spectrometer) and uncovering the advantages and caveats of this spectrum-aware approach.

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To take advantage of Visible Light Communication (VLC) for low-power applications, such as IoT tags, researchers have been developing systems to modulate (backscatter) ambient light using LC shutters. Various approaches have been explored for single-pixel transmitters, but without following a principled approach. This has resulted in either relatively low data rates, short ranges, or the need for powerful artificial light sources. This paper takes a step back and proposes a more theoretical framework: ChromaLux. By considering the fundamental characteristics of liquid crystals (birefringence and thickness), we demonstrate that the design space is way larger than previously explored, allowing for much better systems. In particular, we uncover the existence of a transient state where the switching time can be reduced by an order of magnitude without lowering the contrast significantly, improving both range and data rate. Using a prototype, we demonstrate that our framework is applicable to different LCs. Our results show significant improvements over state-of-the-art single-pixel systems, achieving ranges of 50 meters at 1 kbps and with bit-error-rates below 1%.

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