Visible light communication
[1] VLC can be used as a communications medium for ubiquitous computing, because light-producing devices (such as indoor/outdoor lamps, TVs, traffic signs, commercial displays and car headlights/taillights[3]) are used everywhere.
The history of visible light communications dates back to the 1880s in Washington, D.C., when the Scottish-born scientist Alexander Graham Bell invented the photophone, which transmitted speech on modulated sunlight over several hundred meters.
More recent work began in 2003 at Nakagawa Laboratory, in Keio University, Japan, using LEDs to transmit data by visible light.
In 2006, researchers from CICTR at Penn State proposed a combination of power line communication (PLC) and white light LED to provide broadband access for indoor applications.
[8] In July 2011 a presentation at TED Global[9] gave a live demonstration of high-definition video being transmitted from a standard LED lamp, and proposed the term Li-Fi to refer to a subset of VLC technology.
[10] Publications have been coming from Nakagawa Laboratory,[11] ByteLight filed a patent[12] on a light positioning system using LED digital pulse recognition in March 2012.
[17][18] Another recent application is in the world of toys, thanks to cost-efficient and low-complexity implementation, which only requires one microcontroller and one LED as optical front-end.
[22] In October 2014, Axrtek launched a commercial bidirectional RGB LED VLC system called MOMO that transmits down and up at speeds of 300 Mbit/s and with a range of 25 feet.
[23] In May 2015, Philips collaborated with supermarket company Carrefour to deliver VLC location-based services to shoppers' smartphones in a hypermarket in Lille, France.
[24] In June 2015, two Chinese companies, Kuang-Chi and Ping An Bank, partnered to introduce a payment card that communicates information through a unique visible light.
The modulation will thus be an alternating signal around the positive dc level, with a high-enough frequency to be imperceptible to the human eye.
Out of these three, FSK is capable of larger bitrate transmission once it allows more symbols to be easily differentiated on frequency switching.
This brings two main advantages for the Pulse-Based Transmission modulations: It can be implemented with a single high-power, high-efficiency, dc converter of slow response and an additional power switch operating in fast speeds to deliver current to the LED at determined instants.
In order to fix this problem, the modulation requires a compensation pulse that will be inserted on the data period whenever necessary to equalize the brightness overall.
However, the information encoded on the pulse width is easy to differentiate and decode, so the complexity of the transmitter is balanced by the simplicity of the receiver.
This decoding complexity mostly comes from the information being encoded at different rising edges for each symbol, which makes the sampling harder in a microcontroller.
In other words, CSK transmission maintains a constant time-averaged luminous flux, even as its symbol sequence varies rapidly in chromaticity.
