Visible and near-infrared light represents a readily available and very large source of wireless spectrum beyond the traditional radio spectrum. Enter LiFi, a technology promising speeds in excess of 100Gb/s delivered via solid-state lighting, such as light-emitting diodes (LEDs). This enables high-bandwidth data signals to be transmitted without wires, while not being subjected to or contributing to electromagnetic interference (EMI) below 3THz. The technology also has great potential in applications where security is a concern.
What’s more, LiFi is becoming a commercial reality, with Edinburgh-based firm PureLiFi offering a LiFi USB dongle and a LiFi-integrated luminaire. Other organisations have produced prototype devices, including the Fraunhofer Institute for Photonic Microsystems (IPMS). Researchers at Fraunhofer are working hard to develop and improve the technology, as is a team at the Technical University of Eindhoven (TU/e).
The concept was developed in the early 2000s after Professor Harald Haas – now the director of the LiFi Research and Development Centre in Edinburgh and chief science officer of PureLiFi – realised there was a problem with the amount of radio bandwidth currently available. ‘It was exactly at the cusp where we went from mobile telephones to multimedia devices,’ Haas recalls. ‘The full extent of the mobile internet wasn’t clear at the time. It was clear however that if mobile multimedia took off then the bandwidth in the radio domain wouldn’t be sufficient. That prediction is now coming true.’
Professor Harald Hass coined the term 'LiFi' in 2011
Haas and his colleagues began exploring how much data could be transmitted using the high-brightness white LEDs that were being developed for solid-state lighting applications – sending signals the same way that a television remote control transmits tiny amounts of data in the infrared.
‘The breakthrough was where we used a form of modulation that had already successfully been used in radios,’ said Haas. ‘We turned a disadvantage of OFDM [orthogonal frequency division multiplexing] – the high peak to average power ratio, also referred to as the crest factor – into an advantage for intensity modulation with LEDs. We had the first proof of concept paper in 2006; from then onwards, all the hero data-rate experiments in that field have been based on OFDM.’
Data transmission rates have since gone from being a couple of megabits to multiple gigabits per second, according to Haas, who with his colleagues was recently able to achieve 11Gb/s using a single LED. ‘This is faster than the soon to be commercialised next-generation WiFi system, WiGig,’ he remarks. OFDM has been the instrumental algorithm on the data encoding side to achieve these data rates. Haas also believes that data rates of 50Gb/s are conceivable soon by combining multiple LEDs of different wavelengths.
Let there be LiFi
Haas coined the term ‘LiFi’ at his 2011 TED Global presentation, where he introduced the idea of receiving ‘wireless data from every light’. LiFi, for Light Fidelity, transmits data through the extremely rapid intensity modulation of LEDs, which can be received by photo-sensitive detectors and demodulated into electrical signals that are readable by devices. These modulations occur at megahertz frequencies and are therefore imperceptible to the human eye.
The devices made by Haas’ firm use frequencies above 1MHz and up to 1GHz, depending on the source, which allows ambient light to be filtered out. ‘Even in the strongest 70,000 lux sunlight conditions, there will be negligible degradation of the LiFi signal,’ he said. The LED can also be dimmed to the point where it appears off, while still providing full functionality – a feature that allows the technology to be used at night.
Being transmitted through visible or near-infrared light, LiFi signals cannot pass through opaque objects, such as walls or clothing; however, this potential disadvantage can be turned into a benefit. Security is assured, as the signal cannot be intercepted from outside the room where the source is emitting. A recent qualitative study performed by Haas and his colleagues – for which they won an award at the International Conference on Communications – showed that LiFi’s ‘secrecy capacity’ is 20 times higher than WiFi’s.
The first commercial LiFi system, a desktop transceiver called ‘Li-1st’, was demonstrated by PureLiFi at the Mobile World Congress 2014 in Barcelona, Spain. Since then the company has developed the LiFi-X, a smaller system in the form of a USB dongle that receives signals from a LiFi-enabled luminaire in the ceiling. The ceiling module is bidirectional; a unique feature of LiFi is that it can transmit in both directions.
Lucibel’s ceiling luminaire delivers an optical LiFi signal to up to eight devices
LiFi-X enables a person to move between multiple LiFi-integrated luminaires without suffering loss in connectivity. Alternatively, two people in different rooms could also receive different data on the same frequency without interrupting each other’s signal. ‘We can re-use the same spectrum in a very dense way, and this is why we achieve these unprecedented data densities,’ explained Haas.
The LiFi-X also doesn’t need direct line of sight, as it can pick up surface reflections – if a smartphone with a receiver was pointing at the floor, it could still detect enough reflected signal from a transmitter in the ceiling to maintain the communication link, according to Haas.
The current LiFi-X module is the third generation system on an aggressive roadmap that will see the technology become smaller and smaller. This will ultimately lead to increased uptake of the technology, according to Haas, who believes mass-market adoption will come with further miniaturisation.
PureLifi also has an ongoing partnership with French firm Lucibel, which has developed an industrial LiFi-integrated LED luminaire that can be accessed by up to eight separate users. The luminaire incorporates Lucibel’s LiFi USB key, which uses the same technology as PureLifi’s LiFi-X, and currently offers bidirectional data communication up to 42Mb/s, with faster data rates expected in the future. ‘Our partnership with Lucibel … allows us to accelerate LiFi’s real world commercial implementation,’ said Alistair Banham, CEO of PureLiFi. ‘This is a step towards LiFi’s future as a 5G technology.’
Fraunhofer IPMS is taking an alternative approach to delivering LiFi. Its Gigadock and Hotspot systems use direct line of sight to deliver faster data rates; the Gigadock operates at around 12.5Gb/s, while the Hotspot delivers 1Gb/s. The Gigadock is an optical docking station that can replace cables and connectors over short distances, for example, between electrical printed circuit boards. Measuring only 5 x 5mm, the transceiver can easily be soldered onto the board, according to Dr Alexander Noack, team leader for LiFi development at Fraunhofer. It has a range approaching one metre, and the institute is looking to make it immune to vibrations, shocks, rotation and general movement. It plans to increase the speed of the Gigadock to 100Gb/s.
In contrast, the LiFi Hotspot enables a private, high-speed network to be established over distances up to 30m. As with the PureLiFi’s LiFi-X product, the data exchange of the Hotspot is bidirectional and limited to a defined area, making it possible to use the full bandwidth of each LiFi link without interfering with other hotspots in the network.
‘The Hotspot could be used in an open office environment where people are sitting around a table and wish to have a high data rate,’ explained Noack. ‘Everyone would have a dedicated private connection and could change connection whenever they want while maintaining a high data rate.’ The organisation also envisions LiFi being used to transfer data in scenarios where closed equipment operation is required, such as in a hospital operating room or in underwater environments.
Invisible signals
The chief benefit of working with LEDs today is that they will allow LiFi networks to be built into the existing lighting infrastructure. However, the lighting industry is pushing for a move from LEDs to laser-based lighting, according to Haas, a move that he and his colleagues would welcome because of the advantages that lasers can offer.
The principles of operation for LEDs and lasers are the same, but a laser has potential for a better performance. ‘Laser communication is another technology that is currently researched actively, as it could provide higher bandwidths, therefore higher communication speeds can be achieved compared to LEDs,’ explained TU/e researcher Dr Joanne Oh. ‘However, lasers inherently have a narrow beam. To maximise the capacity that can be provided by a laser, steering modules are being proposed by several research groups.’
PureLiFi’s LiFi-X is a USB dongle that can detect and receive LiFi signals
Oh and her colleagues at TU/e have developed a system based on optical antennas that can direct light rays of varying wavelengths using a pair of passive diffraction gratings. The system can be used to broadcast LiFi to multiple smartphones or tablets, and offer seamless handover when a device exceeds the range of an individual antenna. If further devices are then added to the network, they are individually assigned different optical wavelengths by the same antenna, so they are not required to share LiFi capacity.
The system conceived at TU Eindhoven uses infrared light with wavelengths of 1500nm and above; Using a proof-of-principle device, the researchers have managed to achieve a download speed of 42.8Gb/s over a distance of 2.5 metres. Light rays are used only to download in the demonstrator; uploads from connected devices are still performed using lower-capacity radio signals using radio communication in the 60GHz band.
‘The biggest challenge in enabling such a system is in the method of directing the communication links to multiple users,’ commented Oh. ‘The ideal beam-steering module is considered to be passive with no mechanical motions and low or no local powering needed, has high power efficiency, provides a large steering angle, supports bidirectionality, supports simultaneous steering of multiple beams, and has short response time. User terminals with limited or full mobility naturally require real-time alignment, tracking and fast-beam steering with large angular tuning range.’
The infrared system is part of the Beam-steered Reconfigurable Optical-Wireless System for Energy-efficient communication (BROWSE) project, funded by the European Research Council and headed by Ton Koonen, professor of broadband communication technology at TU/e, who expects it will still be five years or more before the new technology becomes available to consumers.
It’s an exciting area, and the wide range of potential applications makes LiFi an extremely promising technology, according to Haas. ‘Everywhere where there is a light, including cars, trains, planes, satellites and underwater vehicles, there is now communication capability,’ he said. ‘This is so important, as we are now at the start of the fourth industrial revolution, which is all about a data-centric economy. LiFi will create the underlying fabric of the future of wireless connectivity. It will be a key enabler for hundreds and thousands of these services. We may have seen big things with the smartphone, but this could be only around five per cent of what’s ahead of us when we connect everything around us,’ he concluded.
IEEE 802.11 launches light communications study group
The IEEE has formed the IEEE 802.11 Light Communications Study Group to work towards standards for light-based wireless local area networks (LANs), or Li-Fi as it is often known. The new study group will directly engage with manufacturers, operators and end users in consensus-building efforts and to create a Project Authorization Request (PAR) to develop a global communications standard.
With industry analysts like Gartner projecting the Internet of Things (IoT) will grow to 20 billion connected devices by 2020, light communication looks ideal for applications in EMI-challenged environments, such as hospitals, petrochemical plants, and airplanes, but also in secure environments where RF is not sanctioned.
‘In just a few short years, the interest in light communications has grown significantly and there is an enormous amount of valuable knowledge that vendors and operators can share as they work together to advance the technology globally,’ said Nikola Serafimovski, chair of the IEEE 802.11 Light Communications Study Group. ‘It’s an exciting time for the light communications market sector, as it is poised for substantial growth over the next five years.
'We look forward to broad participation under the auspices of the IEEE 802.11 Wireless LAN Working Group and the IEEE-SA as we work to develop the light communications market in line with industry needs, and to ensure best practices that drive market expansion.’