Visible light communication (VLC) uses light to provide faster, more secure communications than traditional radio waves.

Driving this research forward is the ever-increasing demand for bandwidth and flexibility in gigabit home networks, which will be needed for future Internet services. This demand will further grow as the roll-out of the National Broadband Network (NBN) hits its peak and services such as high-definition TV and telemedicine applications become increasingly popular.

Joachim Walewski works for Siemens AG’s corporate research and technologies department in Germany.

Siemens researchers recently improved on their own record for wireless data transfer using white LED light, in collaboration with the Heinrich Hertz Institute in Berlin, achieving a data transfer rate of up to 500Mbps, significantly bettering the previous record of 200Mbps.

Joachim says wireless data transport by means of light paves the way for new applications in the home as well as in industry and transportation.

“VLC is not ‘better’ than radio waves per se, actually it’s rather worse, but it offers some interesting properties, such as the use of an unlicensed and unregulated spectrum,” he says.

“Light is easily contained, which results in secure communication even without encryption and the same modulation bandwidth can be used in adjacent areas. It also offers high data rates at rather low emitting power when the light is bundled into a beam of low divergence.”

He also lists the intuitive alignment of sender and receiver, the dual use of already existing infrastructure, visualisation of the data link and potentially higher acceptance by the public than RF technologies as key selling points of the technology.

Using a white LED produced by the Siemens subsidiary Osram, the company’s researchers in Munich, Germany succeeded in transmitting data over a distance of up to 5m of empty space.

The data is directly transferred by modulating, via the power supply, the amount of light emitted by the LED. The researchers used an Ostar LED, one of the brightest LEDs on the market, which can be modulated at such a frequency that data transfer rates of up to 500Mbps are possible. Most impressively, the resulting changes in brightness remain imperceptible to the human eye.

This form of data transfer has a variety of potential applications. In the home, for example, it could represent a valuable addition to established wireless local area network (WLAN) technology. Increasingly, wireless networks are compromised by the fact that in many buildings the three independent WLAN frequency bands are multiply occupied, which leads to collisions among the data packets.

In a situation like this, visible light, as a currently unused and license-free medium, offers a suitable alternative. A further advantage is that this form of data transfer is impervious to interception. Only the photodetector that is positioned directly within the light cone is able receive the data. In other words, it is impossible to ‘tap’ the data transported in the light beam.

There is also a need for this type of data transfer in factory and medical environments, where in certain areas radio-borne transmission is either impossible or only a limited option. A further application is in the field of transportation, where LED stoplights or railroad signals could be used to transmit information to cars or trains.

The researchers were also able to show that a system combining up to five LEDs is capable of transferring data over longer distances at rates as high as 100Mbps. Once again, this has practical applications, since such a system could be used to transmit data via ceiling lights to a receiver mounted on a desk located anywhere within a room.

Engineers at Pennsylvania State University (Penn State), have also been researching the benefits of VLC and believe that light is better than radio waves when it comes to some wireless communications. Optical communications systems could provide faster, more secure communications with wider bandwidth and would be suitable for restricted areas like hospitals, aircraft and factories.

Sending information via light waves either in physical light guides or wirelessly is not new, but existing wireless systems either require direct line of sight or are diffused and have low signal strength. The researchers chose to take a different approach using multi-element transmitters and multi-branch optical receivers in a quasi-diffuse configuration.

The system uses a high-powered laser diode – a device that converts electricity into light – as the optical transmitter and an avalanche photo diode – a device that converts light to electricity – as the receiver. The light bounces off the walls and is picked up by the receiver.

Electrical engineering professor Mohsen Kavehrad says radio frequency systems do not require line of sight transmission, but can pass through some substances and so present a security problem. Light in a room without windows, however, will not escape the room, improving security, but also allowing the same frequencies to be used in adjacent rooms without interference. Multiple sensors could allow the light signal to pass from room to room or even from floor to floor. The system could also be set up to convert the signal to electricity, transfer it to another location and change it back to light.

“The safest security is physical layer security,” Mohsen says. “If you first have to break into the building before you can attack the network it makes it very difficult.”

He also says an optical system can operate in locations where radio frequency transmission would interfere with other equipment, especially in hospitals, aircraft and even some factories. Because this system is optical it will not interfere with the radio frequency emissions of navigation equipment, medical devices or factory control systems.

Optical transmissions can transfer sensor data and unlike radio frequency communications, can also distribute high-resolution images.

“One application for this system would be wireless projection of high definition television,” he says.

“Currently, two high definition broadcasts exceed the bandwidth of any radio system, but with a 1.6Gbps system, two HD channels could be broadcast.”

While this application in conference rooms could provide mobility for presentations, applications in aircraft and medical facilities are probably more important. Currently, wireless communications are difficult in these situations because radio frequency systems can interfere with equipment using radio frequency control or communications. An optical system can operate in the same space as a radio system without interference.

“As far as I know, these are the first set of measurements for indoor optical wireless links that show the feasibility of the highest bit rates with no line-of-sight. No radio system had comparable ability.”

The researchers from Penn State will continue to test optical systems, looking at visible and ultra violet light. They also believe that LED room lighting could be incorporated into the systems to provide a blanket communications network.

So how long will you have to wait before viable VLC solutions will be available to market?

For Joachim, when it comes to commercialisation most of the groundwork has been done (R&D, prototyping, and standardisation) and quite a handful of companies have looked into including the technology in their business plans.

“But predictions are hard, particularly when they concern the future.”