Passive visible light communications

Abstract

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.