![EM spectrum](https://upload.wikimedia.org/wikipedia/commons/f/f1/EM_spectrum.svg)
In
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, visible light communication (VLC) is the use of
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
(
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
with a
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of 400–800
THz/
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of 780–375
nm) as a
transmission medium
A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulate ...
. VLC is a subset of
optical wireless communications
Optical wireless communications (OWC) is a form of optical communication in which unguided visible, infrared (IR), or ultraviolet (UV) light is used to carry a signal. It is generally used in short-range communication.
OWC systems operating in ...
technologies.
The technology uses
fluorescent lamp
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...
s (ordinary lamps, not special communications devices) to transmit signals at 10
kbit/s
In telecommunications, data-transfer rate is the average number of bits (bitrate), characters or symbols (baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are multi ...
, or
LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
s for up to 500 Mbit/s over short distances. Systems such as
RONJA
RONJA (Reasonable Optical Near Joint Access) is a free-space optical communication system originating in the Czech Republic, developed by Karel Kulhavý of Twibright Labs and released in 2001. It transmits data wirelessly using beams of light ...
can transmit at full
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
speed (10 Mbit/s) over distances of .
Specially designed electronic devices generally containing a
photodiode
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
receive signals from light sources,
although in some cases a
cell phone camera or a
digital camera
A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile device ...
will be sufficient.
The image sensor used in these devices is in fact an array of photodiodes (pixels) and in some applications its use may be preferred over a single photodiode. Such a sensor may provide either multi-channel (down to 1 pixel = 1 channel) or a spatial awareness of multiple light sources.
VLC can be used as a communications medium for
ubiquitous computing
Ubiquitous computing (or "ubicomp") is a concept in software engineering, hardware engineering and computer science where computing is made to appear anytime and everywhere. In contrast to desktop computing, ubiquitous computing can occur using a ...
, because light-producing devices (such as indoor/outdoor lamps, TVs, traffic signs, commercial displays and car
headlights/taillights) are used everywhere.
History
The history of visible light communications (VLC) dates back to the 1880s in Washington, D.C., when the Scottish-born scientist
Alexander Graham Bell
Alexander Graham Bell (, born Alexander Bell; March 3, 1847 – August 2, 1922) was a Scottish-born inventor, scientist and engineer who is credited with patenting the first practical telephone. He also co-founded the American Telephone and Te ...
invented the
photophone
The photophone is a telecommunications device that allows transmission of speech on a beam of light. It was invented jointly by Alexander Graham Bell and his assistant Charles Sumner Tainter on February 19, 1880, at Bell's laboratory at 1325 ...
, 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
, mottoeng = The pen is mightier than the sword
, type = Private research coeducational higher education institution
, established = 1858
, founder = Yukichi Fukuzawa
, endowmen ...
,
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, using
LEDs
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
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
Power-line communication (also known as power-line carrier or PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers.
A wide range of power-line communicat ...
(PLC) 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
Siemens AG ( ) is a German multinational conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad.
The principal divisions of the corporation are ''Industry'', '' ...
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
IEEE 802.15.7 working group.
In December 2010
St. Cloud, Minnesota
St. Cloud is a city in the U.S. state of Minnesota and the largest population center in the state's central region. The population was 68,881 at the 2020 census, making it Minnesota's 12th-largest city. St. Cloud is the county seat of Stear ...
, signed a contract wit
LVXMinnesota and became the first to commercially deploy this technology.
In July 2011 a presentation at
TED Global
TED Conferences, LLC (Technology, Entertainment, Design) is an American-Canadian non-profit media organization that posts international talks online for free distribution under the slogan "ideas worth spreading". TED was founded by Richard Sau ...
. gave a live demonstration of
high-definition video
High-definition video (HD video) is video of higher resolution and quality than standard-definition. While there is no standardized meaning for ''high-definition'', generally any video image with considerably more than 480 vertical scan lines (No ...
being transmitted from a standard LED lamp, and proposed the term
Li-Fi
Li-Fi (also written as LiFi) is a wireless communication technology which utilizes light to transmit data and position between devices. The term was first introduced by Harald Haas during a 2011 TEDGlobal talk in Edinburgh.
Li-Fi is a light comm ...
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 (
OLED
An organic light-emitting diode (OLED or organic LED), also known as organic electroluminescent (organic EL) diode, is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light i ...
) 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
Kuang-Chi Group (; ) is a Chinese company that develops and invests in metamaterial, telecommunication, aerospace, smart-city, AI and digital health technologies.
Kuang-Chi was founded in 2010 by five Chinese PhD scholars, led by Ruopeng Liu. ...
and
Ping An Bank
Ping An Bank Co., Ltd. is a Chinese joint-stock commercial bank with its headquarters in Shenzhen. It primarily operates in Mainland China with a representative branch in Hong Kong. The bank offers services in retail and corporate banking, includi ...
, 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 (March 5 – 9 th). 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.
Modulation Techniques
In order to send data, a modulation of light is required. A modulation is the form in which the light signal varies in order to represent different symbols. In order for the data to be decoded. Unlike
radio transmission
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitt ...
, a VLC modulation requires the light signal to be modulated around a positive dc value, responsible for the lighting aspect of the lamp. 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.
Due to this superposition of signals, implementation of VLC transmitter usually require a high-efficiency, higher-power, slower response DC converter responsible for the LED bias that will provide lighting, alongside a lower-efficiency, lower-power, but higher response velocity amplifier in order to synthetize the required ac current modulation.
There are several modulation techniques available, forming three main groups: Single-Carrier Modulated Transmission (SCMT), Multi-Carrier Modulated Transmission (MCMT) and Pulse-Based Transmission (PBT).
Single-Carrier Modulated Transmission
The Single-Carrier Modulated Transmission comprises modulation techniques established for traditional forms of transmission, such as radio. A sinusoidal wave is added to the lighting dc level, allowing digital information to be coded in the characteristics of the wave. By keying between two or several different values of a given characteristic, symbols attributed to each value are transmitted on the light link.
Possible techniques are Amplitude Switch Keying (ASK), Phase Switch Keying (PSK) and Frequency Switch Keying (FSK). Out of these three, FSK is capable of larger bitrate transmission once it allows more symbols to be easily differentiated on frequency switching. An additional technique called Quadrature Amplitude Modulation (QAM) has also been proposed, where both amplitude and phase of the sinusoidal voltage are keyed simultaneously in order to increase the possible number of symbols.
Multi-Carrier Modulated Transmission
Multi-Carrier Modulated Transmission works on the same way of Single-Carrier Modulated Transmission methods, but embed two or more sinusoidal waves modulated for data transmission. This type of modulation is among the hardest and more complex to synthetize and decode. However, it presents the advantage of excelling in multipath transmission, where the receptor is not in direct view of the transmitter and therefore makes the transmission depend on reflection of the light in other barriers.
Pulse-Based Transmission
Pulse-Based transmission encompasses modulation techniques in which the data is encoded not on a sinusoidal wave, but on a pulsed wave. Unlike sinusoidal alternating signals, in which the periodic average will always be null, pulsed waves based on high-low states will present inherit average values. 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.
* Once the average value depends on the pulse width of the data signal, the same switch that operates the data transmission can provide dimming control, greatly simplifying the dc converter.
Due to these important implementation advantages, these dimming-capable modulations have been standardized in
IEEE 802.15.7, in which are described three modulation techniques: On-Off Keying (OOK), Variable Pulse Position Modulation (VPPM) and Color Shift Keying (CSK).
On-Off Keying
On the On-Off Keying technique, the LED is switched on and off repeatedly, and the symbols are differentiated by the pulse width, with a wider pulse representing the logical high '1', while narrower pulses representing logical low '0'. Because the data is encoded on the pulse width, the information sent will affect the dimming level if not corrected: for instance, a bitstream with several high values '1' will appear brighter than a bitstream with several low values '0'. 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. The lack of this compensation symbol could introduce perceived flickering, which is undesirable.
Because of the additional compensation pulse, modulating this wave is slightly more complex than modulating the VPPM. 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.
Variable Pulse Position Modulation
Variable Pulse Position also switches the LED on and off repeatedly, but encode the symbols on the pulse position inside the data period. Whenever the pulse is located at the immediate beginning of the data period, the transmitted symbol is standardized as logical low '0', with logical high '1' being composed of pulses that end with the data period. Because the information is encoded at the location of the pulse inside the data period, both pulses can and will have the same width, and thus, no compensation symbol is required. Dimming is performed by the transmitting algorithm, that will select the width of the data pulses accordingly.
The lack of a compensation pulse makes VPPM marginally simpler to encode when compared to OOK. However, a slightly more complex demodulation compensates for that simplicity on the VPPM technique. 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. Additionally, in order to decode the location of a pulse within the data period, the receptor must be somehow synchronized with the transmitter, knowing exactly when a data period starts and how long it lasts. These characteristics makes the demodulation of a VPPM signal slightly more difficult to implement.
Color Shift Keying
Color shift keying (CSK), outlined in IEEE 802.15.7, is an
intensity modulation In optical communications, intensity modulation (IM) is a form of modulation in which the optical power output of a source is varied in accordance with some characteristic of the modulating signal. The envelope of the modulated optical signal is an ...
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 (red/green/blue) 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
The flicker fusion threshold, critical flicker frequency (CFF) or flicker fusion rate, is a concept in the psychophysics of vision. It is defined as the frequency at which an intermittent light stimulus appears to be completely steady to the avera ...
" (CFF) and the "critical color fusion threshold" (CCF), both of which cannot resolve temporal changes shorter than 0.01 second. The LEDs’ transmissions are, therefore, preset to time-average (over the CFF and the CCF) 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
Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called ...
.
See also
*
Electric beacon
In navigation, a radio beacon or radiobeacon is a kind of beacon, a device that marks a fixed location and allows direction-finding equipment to find relative bearing. But instead of employing visible light, radio beacons transmit electromagnet ...
*
Fiber-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pref ...
*
Free space optics
*
Free-space optical communication
Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking. "Free space" means air, outer space, vacuum, or ...
*
IrDA
The Infrared Data Association (IrDA) is an industry-driven interest group that was founded in 1994 by around 50 companies. IrDA provides specifications for a complete set of protocols for wireless infrared communications, and the name "IrDA" also ...
—Same principle as VLC but uses
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
light instead of visible light
*
Li-Fi
Li-Fi (also written as LiFi) is a wireless communication technology which utilizes light to transmit data and position between devices. The term was first introduced by Harald Haas during a 2011 TEDGlobal talk in Edinburgh.
Li-Fi is a light comm ...
*
Optical wireless communications
Optical wireless communications (OWC) is a form of optical communication in which unguided visible, infrared (IR), or ultraviolet (UV) light is used to carry a signal. It is generally used in short-range communication.
OWC systems operating in ...
*
RONJA
RONJA (Reasonable Optical Near Joint Access) is a free-space optical communication system originating in the Czech Republic, developed by Karel Kulhavý of Twibright Labs and released in 2001. It transmits data wirelessly using beams of light ...
References
Further reading
*David G. Aviv (2006): Laser Space Communications, ARTECH HOUSE. .
External links
* IEE
802.15 WPAN Task Group 7(TG7) Visible Light Communication
{{Telecommunications
Optical communications
Telecommunications
Light