Research Project Sends Data Wirelessly at High Speeds via Light
by George Lawton
Siemens scientists are working with the Heinrich Hertz Institute the Visible Light Communication (VLC) project. VLC uses white-light-emitting diode (LED) technology to transmit data at 500 Mbits per second over 5 meters. "This is much faster than other VLC work using this type of LED by a long shot," said Dominic O'Brien, an engineering science researcher at the University of Oxford. This beats the previous record of 200 Mbps set by the same group and is much faster than current Wi-Fi technologies, which operate at speeds up to 150 Mbps with 802.11n.
The researchers used Ostar white-LEDs designed for room lighting from Siemens' subsidiary, OSRAM Opto Semiconductors. This technology could allow ceiling lights to double as high-speed data broadcasting equipment, said Sebastian Randel, research scientist at Siemens. "Today's LEDs are slowly starting to replace conventional light bulbs," he said. "The idea is to upgrade the existing driver electronics systems to provide communications features on the transmitter side. Improvements are also needed for the receivers."
The VLC research started four years ago and was incorporated into the EU-funded Omega project that launched in 2008. Omega aims to increase wireless bandwidth for homes and offices to 1 Gbps, using VLC and radio frequency technologies. "The goal for VLC is to demonstrate 100 Mbps by modulation of ceiling lighting, and the work is going according to plan," said Jelena Vucic research scientist at the Heinrich Hertz Institute in Germany. VLC partners successfully demonstrated an VLC implementation earlier this year at the Omega Open Event 2010, operating at 84 Mbps and transmitting three parallel HD video signals.
Transmitting light indoors causes challenges from reflections and multipath interference as the light rays bounce off the walls and cause the signals to interfere with each other. The high data rates are only possible with a direct line of sight between the light and the receiver. Data can also be transmitted with the diffuse light that reflects off the walls for uses outside a line of sight, but the data rates are much lower.
Improved Modulation Techniques
Current white LEDs are built using blue LEDs that energize a layer of phosphorous material, causing it to glow white. The blue LED itself can be rapidly turned on and off, making it easy to modulate with a digital signal. However, the phosphor has a much slower response time, so the researchers use a filter on the receiver to isolate the blue light, which improves the data rate.
Efficient modulation of the LED also requires a special driver-amplifier design, which has been a major challenge for the project. "The main hindrance to achieving high data rates is the limited system bandwidth due to the LED itself or its driver," said Vucic, "even with blue filtering."
The simplest modulation scheme uses on-off keying, which turns the light rapidly on and off, but this only allows a data rate of 1 bit/second/Hz. To achieve higher data rates, the researchers have been investigating two separate modulation schemes: discrete multitone modulation (DMT) and multilevel pulsed-amplitude modulation (M-PAM). DMT systems have achieved link spectral efficiencies of up to 4 bits/second/Hz.
DMT lets the system modulate different subcarriers within the transmission band separately, which supports adaptation based on transmission band noise and interference. Through a process called bit-loading, DMT can carry more information on frequency slices with low-noise and throttle down the data rate on noisier parts of the spectrum. This isolates the impact of noise across the entire system.
M-PAM is spectrally efficient as well, but it's usually regarded as a baseband technique, which doesn't support bit-loading. Consequently, noise on one spectral slice impacts the entire transmission band.
VLC's Prospects
VLC offers several potential advantages over RF communications. It can provide more secure communications over a shorter range because walls and curtains can easily block light signals, whereas potential eavesdroppers can easily detect RF signals from outside buildings. The technology could be useful in factories and hospitals, in which radio transmissions are either impossible or limited owing to concerns about RF interference with critical equipment.
The new technology's main limitation is that it is primarily envisioned as a one-way broadcast technology. Bidirectional communications would have to occur over a different technology such as Wi-Fi.
Several efforts are under way to develop VLC standards. The IEEE 802.15 task group 7 is working on one standard. Meanwhile, the VLC Consortium in Japan is working to harmonize VLC technology with the existing IrDA standard for infrared-based communications so that? VLC transmitters and receivers could use existing IrDA optoelectronic modules with only slight modification.
Other VLC projects in Japan are focused on increasing VLC communication distances to hundreds of meters rather than the data rate. VLC could then support signaling between special stop lights and trains or beam digital information from special bill boards. The Japanese are also exploring the possible use of VLC to send position-tracking information to specially equipped shopping carts.
For more information on VLC, visit the following Web sites: the Visible Light Communication Consortium at www.vlcc.net, the 802.15 Task Group 7 at www.ieee802.org/15/pub/TG7.html, and the Omega Home Gigabit Access project: http://www.ict-omega.eu.
George Lawton is a freelance technology writer based in Guerneville, California. Contact him at glawton@glawton.com.
