In a pilot project under the direction of Dr. Anagnostis Paraskevopoulos, a working group from the Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut, HHI fits out a conference room on the island of Mainau in Lake Constance with VLC technology. Their optical test WLAN in the conference hall will be used to test and refine this technology under real conditions. The Initiative BodenseeMobilfunk (Lake Constance wireless initiative) is committed to reducing radio emissions and provided the idea for the project on Mainau Island. The project is sponsored by the Ministry of the Environment, Climate Protection and the Energy Sector Baden-Württemberg.
Visible light communication is suitable for places where radio transmission could disrupt existing control systems, such as in aircraft or hospitals. Many other application scenarios are also conceivable: in museums or at trade fairs, information about exhibits could be transmitted from a ceiling lamp to visitors moving freely in the beam of light. In particular, VLC could offer an alternative to WLAN and Bluetooth in places with constant lighting, such as open-plan offices and factories.
The transmission principle of visible light communication is easy: a high-power LED is switched on and off very quickly using a modulator. These light impulses are captured by a photodiode in the receiver device (a laptop, tablet computer or cellphone) and converted into a string of electrical zeroes and ones – in other words, the language of computers.
This all happens too fast for the human eye to notice any flickering lights. It is also possible to transmit in both directions. Data can for example be up-streamed using an infrared return channel. VLC technology supports data rates of up to 1.25 gigabits per second. Some white LEDs have three light colors or light frequencies, which means they can achieve high-speed data connections of up to 3 gigabits per second – enough to run several video streams in HD quality at the same time.
With conventional radio transmission, network access can soon become overloaded if many people are accessing the internet via WLAN and all downloading large data volumes simultaneously. But with optical WLAN the data transfer rate always remains stable, thanks to the extra wavelengths in the white light. In order for VLC signals to be received, there has to be direct ‘visual contact’ between the transmitting LED and the receiver device. This drawback becomes an advantage, however, when it comes to protecting data: optically opaque surfaces such as walls prevent data flows from leaking to the outside world.
The Fraunhofer HHI researchers are now developing components for optical wireless data communication. One challenge they face is to further develop the modules for sending and receiving light signals; these have to be made a lot smaller before the technology can go into series production. In the future, it should take no more than a simple USB stick on a laptop or smartphone to receive data from a ceiling lamp.