Visible light communication is a data communications variant which uses visible light between 400 and 800 THz. VLC is a subset of optical wireless communications technologies. The technology uses fluorescent lamps to transmit signals at 10 kbit/s, or LEDs for up to 500 Mbit/s over short distances. Systems such as RONJA can transmit at full Ethernet speed over distances of. Specially designed electronic devices generally containing a photodiode receive signals from light sources, although in some cases a cell phone camera or a digital camera will be sufficient. The image sensor used in these devices is in fact an array of photodiodes and in some applications its use may be preferred over a single photodiode. Such a sensor may provide either multi-channel or a spatial awareness of multiple light sources. VLC can be used as a communications medium for ubiquitous computing, because light-producing devices are used everywhere.
History
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. This pre-dates the transmission of speech by radio. More recent work began in 2003 at Nakagawa Laboratory, in Keio University, Japan, using LEDs to transmit data by visible light. Since then there have been numerous research activities focussed on VLC. In 2006, researchers from CICTR at Penn State proposed a combination of power line communication and white light LED to provide broadband access for indoor applications. This research suggested that VLC could be deployed as a perfect last-mile solution in the future. In January 2010 a team of researchers from Siemens and Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute in Berlin demonstrated transmission at 500 Mbit/s with a white LED over a distance of, and 100 Mbit/s over longer distance using five LEDs. The VLC standardization process is conducted within the working group. In December 2010 St. Cloud, Minnesota, signed a contract with Minnesota and became the first to commercially deploy this technology. In July 2011 a presentation at TED Global. 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. Recently, VLC-based indoor positioning systems have become an attractive topic. ABI research forecasts that it could be a key solution to unlocking the $5 billion "indoor location market". Publications have been coming from Nakagawa Laboratory, ByteLight filed a patent on a light positioning system using LED digital pulse recognition in March 2012. COWA at Penn State and other researchers around the world. 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. VLCs can be used for providing security. They are especially useful in body sensor networks and personal area networks. Recently Organic LEDs have been used as optical transceivers to build up VLC communication links up to 10 Mbit/s. 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. In May 2015, Philips collaborated with supermarket company Carrefour to deliver VLC location-based services to shoppers' smartphones in a hypermarket in Lille, France. 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. In March 2017, Philips set up the first VLC location-based services to shoppers' smartphones in Germany. The installation was presented at EuroShop in Düsseldorf. As first supermarket in Germany an Edeka supermarket in Düsseldorf-Bilk is using the system, which offers a 30 centimeter positioning accuracy can be achieved, which meets the special demands in food retail. Indoor positioning systems based on VLC can be used in places such as hospitals, eldercare homes, warehouses, and large, open offices to locate people and control indoor robotic vehicles. There is wireless network that for data transmission uses visible light, and does not use intensity modulation of optical sources. The idea is to use vibration generator instead of optical sources for data transmission.
Colour shift keying
Color shift keying, outlined in IEEE 802.15.7, is an intensity modulation based modulation scheme for VLC. CSK is intensity-based, as the modulated signal takes on an instantaneous color equal to the physical sum of three LED instantaneous intensities. This modulated signal jumps instantaneously, from symbol to symbol, across different visible colors; hence, CSK can be construed as a form of frequency shifting. However, this instantaneous variation in the transmitted color is not to be humanly perceptible, because of the limited temporal sensitivity in the human vision — the "critical flicker fusion threshold" and the "critical color fusion threshold", both of which cannot resolve temporal changes shorter than 0.01 second. The LEDs’ transmissions are, therefore, preset to time-average to a specific time-constant color. Humans can thus perceive only this preset color that seems constant over time, but cannot perceive the instantaneous color that varies rapidly in time. In other words, CSK transmission maintains a constant time-averaged luminous flux, even as its symbol sequence varies rapidly in chromaticity.
