It is predicted that LEDs will be the ultimate light source in the near future powering indoor illumination, outdoor lamps, traffic signs, advertising displays, car headlights/taillights, etc. Li-Fi, or light fidelity, promises to double the utility of light-emitting diodes (LEDs) by using their light as a medium to deliver networked, mobile, and high-speed communication in a similar manner as Wi-Fi. Li-Fi has already achieved blistering speeds in the lab. Researchers at the University of Oxford have reached a new milestone in networking by using light fidelity (Li-Fi) to achieve bi-directional speeds of 224 gigabits per second (Gbps).
It works by switching LEDs on and off within nanoseconds to communicate data, which is too quick to be noticed by the human eye. At the receiving end Li-Fi uses a photodetector to receive signals and convert them into streamable content. Li-Fi pioneers pureLiFi already have two products on the market: Li-Flame Ceiling Unit to connect to an LED light fixture and Li-Flame Desktop Unit which connects to a device via USB, both aiming to provide light and connectivity in one device.
In future LiFi would make possible the extensive deployment of visible light communication for a wide range of short and medium-range communication applications including wireless, local, personal, and body area networks (WLAN, WPAN, and WBANs), vehicular networks, underwater networks and machine-to-machine (M2M) communication among others.
The Navy is interested in using Li-Fi to improve submarine communications, since radio waves travel poorly under water and current acoustic communications are slow. The underwater VLC in the blue/green spectral range (450 nm-550 nm) is able to achieve data speeds of hundreds of Mbps for short ranges (less than a hundred meter) complementing long range acoustic communication.
Short range, low reliability and high installation costs are the potential downsides of LiFi. Researchers are now making efforts towards developing heterogeneous networks incorporating both WiFi and LiFi to make the best of the pros of both VLC and WiFi. Operators say that 80% of the mobile traffic occurs indoors; therefore, the combination of LiFi and WiFi has great potential to be breakthrough technologies in future HetNets including the next generation (5G) mobile telecommunications systems
The system functions in the following way: a regular light-emitting diode acts as the signal source. It flashes at a high frequency and radiates light impulses. A photodetector receives the impulses and decodes them into an electrical signal. The signal is further deciphered and digital data is extracted. Altogether the light-emitting diode and the photodetector resemble a transmitter: light on is binary 1, light off is binary 0. This mode of data transmission cannot be detected by the human eye: the frequency with which the diodes flash is extremely high.
The Li-Fi technology is being developed into an omnipresent systems technology. They consist of application specific combinations of light transmitters, light receivers including solar cells, efficient computational algorithms and networking potential that can be deployed in an extensive range of communication scenarios and in a diversity of device platforms.
The components of LiFi system are
The light-emitting diode (LED) is a semiconductor device that emits light when an electric current is passed via it. Light is produced when the particles that carry the current (known as electrons and holes) combine together within the semiconductor material. The light is not explicitly bright, but in most LEDs it is monochromatic, occurring at a single wavelength. Preliminary LEDs manufacture only red light, but modern LEDs can manufacture various different colors, including red, green, and blue (RGB) light. Nowadays advances in LED technology have made it possible for LEDs to fabricate white light as well. A nice example of an LED is the led status indicators on your keyboard for NUM Lock, Scroll Lock and Caps Lock.
Using an array of LEDs and different colors, data rates in the range of hundreds or megabits per second can be generated, by flickering of LED light bulbs to create binary code (on = 1, off = 0), and is done at surpassing rates than the human eye can detect. The excess LEDs in your lamp, the excess data it can process.
A photodiode is a device that helps in the metamorphose of light into electrical current. This is made of semi-conductor material and containing a p-n junction, it is designed to function in reverse bias. The current is propagated in the photodiode when photons are absorbed and a very less amount of current is also propagated when there is no existing light. The Photodiodes is comprehensively used in the electronics industry, especially in detectors and wide bandwidth optical telecommunications systems.
The Image Sensor
An image sensor is an electronic, photosensitive device which transforms an optical image into an electronic signal. They are composed of millions of photodiodes and is used as an image receiver in digital imaging equipment. An image sensor is competent of reacting to the impact of photons, thus converting them into an electrical current that is then passed onto an analog-digital converter. At this time, the utmost common image sensors are digital charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) active pixel sensors. In a camera, a photo electronic image sensor transforms the light passing through the lens into per-photodiode charges of varying sizes.
LED, like any multimedia data, it is essential to modulate these into a carrier signal. This carrier signal consists of light pulses sent out at less intervals. The system used in optical wireless communication (OWC) technique is Optical Orthogonal Frequency Division Multiplexing (O-OFDM).
August 30, 2018 China developed a chipset for visible light communication.
National Digital Switching System Engineering (NDSC) and Dongguan Xinda Institute of Integrated Innovation collaboratively developed a commercial chipset for visible light communication with an aim to establish wireless communication where there is LED. The chipset is expected to provide ubiquitous and cheap access for 5G communication in indoor environment, high-speed digital system, underwater high-speed wireless networks and intelligent VR based family services. It is reported that the highest rate of the visible light communication chipset can be as high as Gbps.
Wu Jiangxing, an academician with the Chinese Academy of Engineering, said that the innovation of the visible light communication commercial-grade chipset would push forward the development of the visible light communication industry and application market.
Smart LED Bulbs Deliver Communication and Illumination
Researchers at Disney Research and ETH Zurich designed and implemented the system called EnLighting, that enables distributed and fully connected LED light bulbs to communicate through free space optics to interconnect devices within a room. The researchers added a system-on-a-chip (SoC) running an embedded version of Linux to each bulb, as well as photodiodes to enable sensing of incoming signals and an additional power supply for the added electronics.
“LED light bulbs mounted on the ceiling or in free-standing floor lamps easily cover a room, serving as illumination while at the same time creating a room-area network that allows data exchange between light-emitting devices,” said Markus Gross, vice president at Disney Research, adding that even if the bulb is switched off, it can still serve as a receiver of signals from those devices.
“Interconnecting appliances, sensors and a wide variety of devices into the Internet of Things has many potential benefits, but using radio links to do so threatens to make the radio spectrum an even scarcer resource,” said Gross. “Visible light communication networks conserve the radio spectrum, while also making it difficult to eavesdrop for anyone out of line of sight of the network.”
Optical wireless communication (OWC) uses infrared, visible or ultraviolet bands to enable wireless connectivity. With its powerful features such as high bandwidth, low cost and operation in an unregulated spectrum, OWC can be, in some applications, a powerful alternative to and, in others, complementary to the existing wireless technologies. OWC systems operating in the visible band (390-750 nm) are commonly referred to as visible light communication (VLC).
Ultra-parallel visible light communications (UP-VLC)
Harald Haas from University of Edinburgh, along with researchers from the Universities of -Cambridge, Oxford, St. Andrews, and Strathclyde are pursuing ultra-parallel visible light communication, which would use multiple colors of light to provide high-bandwidth linkages over distances of a few meters. In the lab, as part of the Ultra-Parallel Visible Light Communications Project (UP-VLC), Haas and colleagues have reached blisteringly fast 10Gbit/s data transmission speeds.
Here they developed micro-LED arrays to transmit 3.5Gbit/s via each red, green and blue micro-LEDs in parallel. They also applied novel spatial modulation orthogonal frequency divisional multiplexing so the micro-LED elements within the array could beam thousands of streams of light in parallel, multiplying the volume of data transmitted at any one time, reports Rebecca Pool in SPIE.
Yet many commercial LEDs use a phosphor coating to convert blue light to white light, and this coating limits how fast the devices can be modulated, slowing down data rates. Haas isn’t fazed, saying: “Even with slow, phosphor-coated LEDs, we can exploit parallelism in the spatial dimension.” “Laser LEDs have bandwidths up to 1 GHz, as opposed to 20 MHz for the phosphor-coated LEDs, so we can also encode data in the frequency domain to achieve fast data rates,” he says.
“LEDs have been the bottleneck in data rate so as part of this project, we wanted to develop a technology that would unlock the vast amount of data rates available in the visible light spectrum,” says Haas. “We’ve achieved 10Gbit/s with these LEDs, but can reach 100Gbit/s using red, green and blue laser diodes.” Haas’s team has also created the first receiver chip for Li-Fi with integrated avalanche photodiodes on CMOS. The 7.8-square-millimeter IC houses 49 photodiodes. In an avalanche photodiode, a single photon striking the receiver produces a cascade of electrons, amplifying the signal.
According to Haas, a detector-receiver array may have direct line of sight with a LED transmitter, but will also receive “residual” light that has reflected off surrounding walls, objects, the ground and ceiling. “If you block the strongest incoming ray, residual light still reaches the receiver, and a good photodetector on the receiver side will still make sense out of the weakest of signals,” he says.
“We can achieve the sensitivity we need with off-the-shelf avalanche photodiodes and PIN diodes, but colleagues at Edinburgh have pioneered single-photon avalanche diodes,” he adds. “These detect single photons, achieving sensitivities that are orders of magnitude higher than those from off-the-shelf avalanche photodiodes.”
“Recently, by integrating CMOS electronics with GaN based micro-LEDs, we have developed CMOS-controlled color-tunable smart displays. The color-tunable LED pixels in these displays have a modulation bandwidth of 100 MHz, thus providing simultaneously a wavelength-agile source for high-speed visible light communications.” The vision is built on the unique capabilities of gallium nitride (GaN) optoelectronics to combine optical communications with lighting functions, and especially on the capability to implement new forms of spatial multiplexing, where individual elements in high-density arrays of GaN based light emitting diodes (LEDs) provide independent communications channels, but can combine as displays. “We envisage ultra-high data density – potentially Tb/s/mm2 – arrays of LEDs driven via CMOS control electronics in novel addressing and encoding schemes and in compact and versatile forms.”
The emergence of VLC is in fact a result of recent development in solid state lighting technologies. New generations of LEDs have attractive features such as a long life expectancy, high tolerance to humidity, lower power consumption and reduced heat dissipation.
Metamaterials: Nanopatterned metamaterial boosts LED underwater communications
University of California, San Diego (UCSD) researcher Zhaowei Liu and colleagues have taken the first steps in developing high-modulation-rate blue and green LEDs for underwater optical communications. They have created a hyperbolic metamaterial (HMM), consisting of layers of Ag and Si patterned with a grating and covered with Rhodamine 6G dye, that boosts the spontaneous-emission rate of a fluorescent light-emitting dye molecule—Rhodamine 6G (R6G)—by a factor of 76, as well as increasing the emission intensity of the dye by a factor of 80.1. Hyperbolic metamaterials are able to achieve plasmonic resonance in which electrons oscillate collectively within a material which, when aligned with fluorescent emission, can amplify the emission.
Gallium nitride (GaN)-based blue- and green-emitting LEDs can achieve extremely high modulation rates, currently they achieve less than 1 GHz. The next step for the researchers will be to pair the nanostructured metamaterial with GaN-based LEDs to develop a better light source for communication purposes. They hope to increase the rate at which information can be sent underwater via optical channels, such as between ships and submarines, submarines and divers, underwater environmental sensors and unmanned underwater vehicles.
But Haas cautiously praised Liu’s results, saying that they could help solve a challenge in the Li-Fi industry if they deliver. Off-the-shelf bulbs are optimized for visible light, not for communications, and so it’s only possible to modulate their intensity comparatively slowly. Liu and colleagues’ blink rate-boosting materials could be a boon. “These devices are perhaps able to provide a step towards the results we would like to achieve,” Haas said
China Creates LiFi Nanomaterial
Most researchers of the cutting-edge technology use rare earth materials to provide the light for LiFi to transmit data. But the team of scientists in China has devised a safer, faster and cheaper alternative known as F-CDs, which is a fluorescent carbon nanomaterial. “Many researchers around the world are still working on this. We were the first to successfully create it using cost-effective raw materials such as urea with simple processing,” said Qu Songnan, an associate researcher at Changchun Institute of Optics, Fine Mechanics and Physics, the Chinese Academy of Sciences, which is spearheading the research. Qu noted that rare earth has a long lifespan which reduces the speed of LiFi transmission. However, F-CDs enjoy the advantage of faster data transmission speeds.
The technology can easily complement existing cellular and WiFi networks. In previous studies, carbon dots were limited to the emission of lights such as blue and green. The new nanomaterial that Qu’s team has developed can emit all light visible to the human eye, which is a breakthrough in the field of fluorescent carbon nanomaterial. Qu said this is significant for the development of LiFi, which he expects to enter the market in just six years. 2015 test by a Chinese government ministry showed that LiFi can reach speeds of 50 gigabytes per second, at which a movie download can be completed in just 0.3 seconds.
ASU’s white laser technology breakthrough
Arizona State University electrical engineering professor Cun-Zheng Ning and his team have invented world’s first white laser, which could revolutionize communications, lighting and displays, is being recognized as one of the top 100 breakthroughs of the year by Popular Science magazine.
Their Nano photonics device is based on nanoscale materials, semiconductor that produces the white laser is formed into three segments that generate red, green and blue lasers that combine to create a pure white light. Growing the semiconductor on a nanoscale was the key to cracking the problem. “Lasers are a hundred times faster than LEDs. That’s been demonstrated.”
LiFi based networking
But while point-to-point demonstrations have proven LiFi to be a serious alternative to RF-based communications, the technology’s real strength lies in networking. Detector-receiver arrays also house infrared diodes to provide an uplink connection back to the LED-based LiFi access point.
And by combining numerous micro-LED transmitters and receiver assemblies, the researchers can take LiFi beyond a straightforward point-to-point communications system, and create a multiple-input multiple-output (MIMO) transmission network. Transmitters and receivers can even be combined in a single unit to create a single-input, single-output link. As Haas emphasizes: “The LiFi communications system can serve many users. It allows multi-user access, has an uplink and downlink, and allows handover.”
“The coverage of each LED lamp is up to 10 square meters and when you leave that space, you are illuminated by another lamp, handover takes place, and you don’t lose wireless connectivity,” he adds. Haas is also confident ambient light will not interfere with transmissions as the receivers are only sensitive to the modulating LED light. “Even the 100 Hz flicker from an incandescent light bulb is not an issue, as our modulation frequency starts at 1 MHz,” he says. “As long as fluctuations are outside our modulation, these aren’t an issue.”