Visible-light communications

You've probably heard that the days of incandescent and fluorescent light bulbs are numbered. What you probably haven't heard is that the light-emitting diodes (LEDs) that will soon replace them will not only be helping light your way; they will also be broadcasting digital information to help your gadgets communicate with each other and the Internet.

Visible light communications (VLC) is the latest in a series of optical wireless communications technologies that can be traced back to the smoke signals and semaphore networks developed by native North Americans and the French, respectively, hundreds of years ago. More recently, infrared systems (using the IrDA standard) and satellite-satellite laser links have applied free-space optics to communications links. But, in general, wireless communications to date have been radio-based, and optical communications technologies have relied on fibre as the carrier medium.

Harmless waves

For all its numerous benefits, radio-based communications technology has its flaws.

Chief among them is the potential threat that RF waves pose to both the human body and electrical equipment, including the precision instruments used in hospitals and space stations. Regulations on power emission levels keep such threats under control, but they also mean that wireless transmissions can't take full advantage of the available RF spectrum.

Using the optical (or visible light) portion of the spectrum to wirelessly transmit information should be largely harmless, in terms of radiation damage and electromagnetic compatibility. It would also free up 300THz of unused bandwidth - a chunk of spectrum that dwarfs the 300GHz that RF represents.

"So you get to use unexploited parts of the spectrum while avoiding the typical interference problems that Wi-Fi equipment, for example, suffers from," says Dr Joachim Walewski, research scientist at Siemens Corporate Technology, Networks & Multimedia Communications.

"VLC uses different frequencies. But they are unlicensed, not unrestricted. There are restrictions regarding, for example, eye safety, but there are no licence requirements."

Walewski says another advantage of VLC is that it is visible: "This may sound obvious, but you can't see the radio waves emitted by Bluetooth. And when you start going towards high data rates, for example at the 60GHz frequency band, the emitted radio waves have beam-like propagation properties. In these cases, alignment starts to become an issue.

"A typical example is what happens with IrDA," he adds. "You need to guess where to point your devices to align them correctly. If this beam were visible, pointing in the right direction to establish the link wouldn't be a problem. Also, visibility gives you an intrinsic security element, because you can see if somebody puts a mirror into the light beam."

Coexistence

Earlier this year, Walewski and his Munich-based research team announced that they had transmitted digital data at up to 100Mbit/s using visible light.

To do that, the scientists built an array of high-performance white LEDs to act as the transmitter. At the other end, a photo sensor acted as the receiver that converted the signals back into electrical pulses to decode the information. Data was transferred over 1m, and the modulation technique consisted of simple variations in the intensity of the light emitted by the LEDs.

Siemens' decision to focus on LEDs is not coincidental. Apart from the advantages that the latest generation of LED lamps brings in terms of operating life, mechanical robustness, power consumption, energy efficiency and size, these light sources also have potential for coexistence. In other words, both ambient lighting and wireless broadband access could be supplied to any room in a house, college, office building or even aircraft cabin, from a single optical device.

"If the source you're using is the same as for ambient lighting, then you have all the advantages that result from piggybacking," Walewski explains. "You could argue that people are already paying for their lighting electricity, and, for almost no additional cost, they could address their in-house wireless communication needs using the same infrastructure."

Until it became evident that LEDs were destined to take a major role in the lighting industry, research on VLC had focused on fluorescent lighting. Some of the results can now be found in a number of products that are commercially available.

Professor Masao Nakagawa, from Keio University in Japan, is credited as the pioneer of VLC using LEDs, his first papers on the subject dating back to the early 2000s.

Walewski's team at Siemens launched its research project in 2006. The latest endorsement to the technology comes from the European Commission, which, in January 2008, established OMEGA, a research project with a consortium of 20 European partners from industry and the scientific community.

OMEGA's ambitious goal is to develop a global standard that would enable people to set up ultra-broadband home networks (operating at 1Gbit/s) without having to install any new home wiring. LED-powered VLC is a key technology to achieve this vision.

An OMEGA showroom is due to be built by 2010 at the offices of France Telecom in Paris, where the ceiling lighting will be broadcasting data at 100Mbit/s.

Meanwhile, in the US interest in the technology is also intensifying. The country's National Science Foundation has just approved a grant of $18.5m to help establish an Engineering Research Centre focused on VLC using next-generation LEDs. Based at the Rensselaer Polytechnic Institute in New York, the Centre will be staffed by a group of researchers from this and two other institutions, Boston University and the University of New Mexico. With the goal of developing 'the optical communication technology that would make an LED light the equivalent of a Wi-Fi access point', research will concentrate on the creation of the necessary devices and ideal networking architectures.

Lightning fast

According to Siemens' theore-tical research, data rates of more than 300Mbit/s could be achieved in ideal conditions. "I haven't done all the calculations, but my estimate is that a 1Gbit/s link shouldn't be impossible using a red laser over short distances," reckons Walewski.

"In terms of longer range, there has been a demonstration by the group of Professor Nakagawa, where they claim to have achieved several bits per second over 1km in daylight."

One obvious issue with VLC systems is the need to avoid interference from other visible light sources (such as lamps or the Sun), and multi-path interference from reflections. But a series of simple tricks can ensure the technology will work indoors and out.

"Just as you can take your IrDA link outside and it will still work, spectral filtering techniques can be used to make coloured LEDs perfectly capable of working outdoors without being affected by any significant interference issues," explains Walewski.

"If we take into account that fluorescent lighting modulates nowadays at around 20kHz- 30kHz with some overtones, modulating your LED at the same frequency in a room that uses fluorescent lighting would clearly create a big problem."

In an attempt to demonstrate the potential of VLC, Samsung recently built a prototype LED-powered USB dongle that can send and receive data to link a computer wirelessly with a smartphone or with another computer placed at a short distance.

According to Walewski, any small LED (such as those regularly found in flash memory sticks) can easily be turned into an IrDA-like communications device by making some minor modifications to its analogue driving circuit.

Looking into the future, Walweski believes that one of the first applications to embrace VLC once the technology becomes a commercial reality will be handheld devices.

"That's simply because of the sheer number and turnover rate of handheld devices," he argues. "There are something like three billion mobile subscribers in the world, while the average lifetime of a mobile phone is, say, three years. That means you have a turnover of one billion units per year.

"Mobile phones are also somewhat decoupled from other systems, so you don't have any potential conflicts like in the case of lighting. The next potential application might be laptops. Any high-volume product that has a high turnover rate will be of interest for pretty much every consumer electronics company."

Should VLC find its way into portable mobile devices, then any of the applications frequently suggested for other short-range wireless technologies, such as the near-field communications protocol, will be candidates here, too. E-payment, identity authentication and augmented-reality applications at museums are just some examples.

Other suggested uses for VLC include underwater communications, and car to car and road infrastructure to car communications links for intelligent transport systems. Most brake lights and even some headlights in new vehicles are moving to LED illumination, so there would be nothing to prevent using the same modulation techniques applied in ambient lighting LEDs to enable cars talk to each other about what they might expect a few metres down the road.