Fibreless optical data transfer mooted

In the future, you might get your email through light bulbs.

The Visible Light Communications Consortium, a group of 15 IT manufacturers, most of them Japanese, is proposing using light emitting diodes -- which will increasingly become common for ordinary light fixtures and outdoor equipment like traffic lights -- to transmit data traffic at high speeds up to 10 meters.

With the technology, a person trapped in a building could hold up a cell phone to a ceiling light, and rescuers would be able to pinpoint his or her exact location. Similarly, cars could exchange information through headlights and taillights, and car computer systems could tell drivers if there were major stalls ahead.

Conceivably, entire movies could be shuttled from one TV to another in a few seconds through signals bouncing between the two screens.

Formed two years ago, the group is holding its first public demonstrations of the concept at CEATEC, a sprawling tech show taking place here this week.

"Although there are no concrete plans right now for making it into a standard, some companies are very serious," said Shinichiro Haruyama, a professor of information and computer science at Keio University.

The consortium is essentially trying to capitalise on the growing proliferation of LEDs and their improving capabilities, said Masao Nakagawa, the professor at Keio who first came up with the idea seven years ago.

LEDs function in a manner similar to the light source inside optical fiber. They emit light at a specific bandwidth: To harness the light, engineers create modulators that cut it up into data that can subsequently be interpreted as ones or zeroes by a computer.

Unlike fibre, LEDs, which emit bright light and consume little power, are expected to be everywhere soon. "In the next five to 10 years, most light bulbs will be replaced by LEDs, then fluorescent bulbs will follow," Nakagawa said.

Standard LEDs don't carry data traffic very well because they are currently optimised for brightness, he said. However, LEDs that have been designed with communication in mind will offer sharply better performance. Research shows that white LEDs can send data at 80Mbps, while red and green ones can shuttle data at 200Mbps and 500Mbps, respectively.

With volume production, any cost increases for optimising the tiny lights for communication pulses will largely evaporate, Nakagawa speculated. The system consists of two basic components: LED lights and receivers, small silicon chips that relay any messages from the LED to a bank of servers and vice versa.

While the technology sounds far out, the concept was minted long ago. Alexander Graham Bell conceived of a similar idea in 1880. The Photophone consisted of a mirror that vibrated when someone spoke. The vibrations were then captured by a subsequent receiver and turned into electrical impulses.

"It worked for 200 meters," said Nakagawa, adding that Bell predicted it would be one of his biggest achievements.

So why haven't optical technology companies tried this? Most have spent their time trying to wring more performance out of fibre. "Maybe the blind spot was invisible light. It is kind of an irony," Nakagawa said. "Very few people did research on the free space."

After Nakagawa came up with the idea, he contacted Sony, which then teamed him up with Haruyama, who at the time worked there. Sony and Keio collaborated on their own from 1999 to 2002, when other participants joined.

Haruyama asserted that light enjoys several advantages over other wireless communication technologies. Hackers can snare supposedly confidential radio traffic. If light is interfered with, the communication is broken. Privacy is also fairly easy to protect. Shutting a door or window will prevent a light source in another room from extracting data from a cell phone enabled with a communication LED.

The cell phone user also generally would have at least some control over the communication. If they don't want to be tracked, they can put the cell phone in a pocket.

Light can also accommodate more data streams. In some experiments, researchers have transferred data from 30,000 fairly tightly placed LEDs to a similar set of receivers. If radio transmitters get too closely packed together, interference results.

Demos at the show included a set of headphones that play a different musical track depending on the colour of the lights beaming on the wearer. In another demo prepared by NEC, a person held up a phone to a light, and their location in a building could be identified. People also played games by aiming their cell phone screens at targets on a TV a few feet away.

A lot of work remains. All of the prototypes, for instance, require a clean line of sight between the transmitter and receiver. Recent results in experiments at Keio, however, have shown that the system still works if the light bounces off other objects before getting intercepted by the receiver.