2012-02-02 11:33:53
A fast and cheap optical version of Wi-Fi is coming
Jan 28th 2012 | from the print edition
AMONG the many new gadgets unveiled at the recent Consumer Electronics Show in
Las Vegas was a pair of smartphones able to exchange data using light. These
phones, as yet only prototypes from Casio, a Japanese firm, transmit digital
signals by varying the intensity of the light given off from their screens. The
flickering is so slight that it is imperceptible to the human eye, but the
camera on another phone can detect it at a distance of up to ten metres. In an
age of Wi-Fi and Bluetooth, flashing lights might seem like going back to
sending messages with an Aldis lamp. In fact, they are the beginning of a fast
and cheap wireless-communication system that some have labelled Li-Fi.
The data being exchanged by Casio s phones were trifles: message balloons to be
added to pictures on social-networking sites. But the firm sees bigger
applications, such as pointing a smartphone at an illuminated shop sign to read
information being transmitted by the light: opening times, for example, or the
latest bargains.
Yet that is still only a flicker of what is possible. Last October a number of
companies and industry groups formed the Li-Fi Consortium, to promote
high-speed optical wireless systems. The idea is that light can help with a
looming capacity problem. As radio-based wireless becomes ubiquitous, more and
more devices transmitting more and more data are able to connect to the
internet, either through the mobile-phone network or through Wi-Fi. But there
is only a limited amount of radio spectrum available. Using light offers the
possibility of breaking out of this conundrum by exploiting a completely
different part of the electromagnetic spectrum, one that is already ubiquitous
because it is used for another purpose: illumination.
Lighten the darkness
To turn a light into a Li-Fi router involves modulating its output, to carry a
message, and linking it with a network cable to a modem that is connected to a
telephone or cable-broadband service, just like a Wi-Fi router. Incandescent
light bulbs and fluorescent tubes are not really suitable for modulation, but
they are yesterday s lighting technology. Tomorrow s is the light-emitting
diode. LEDs are rapidly replacing bulbs and tubes because they are more
efficient. And because they are semiconductor devices, tinkering with their
electronics to produce the flickering signals required for data transmission is
pretty straightforward, according to Gordon Povey, who is working on light
communication with Harald Haas and his colleagues at the University of
Edinburgh, in Britain.
The rate of data transfer is also good. Dr Povey s group is already up to 130
megabits a second (faster than some older Wi-Fi routers) over a distance of
about two metres, using standard LEDs. Dr Povey, who is also the boss of VLC, a
firm set up to commercialise the technology, thinks such devices should be able
to reach 1 gigabit per second (Gbps), and do so over greater range. Specially
constructed LEDs would be even faster. The Li-Fi consortium reckons more than
10 Gbps is possible. In theory, that would allow a high-definition film to be
downloaded in 30 seconds.
Dr Povey believes that adapting existing LEDs to work with the sensors and
light sources cameras, ambient-light detectors, screens, flashbulbs, torches
and so on already found in smartphones and similar devices will be the fastest
way to bring Li-Fi to market. VLC has already produced a smartphone app which
allows low-speed data transmission between a pair of iPhones. It has also made
an experimental optical transceiver that plugs into a laptop to receive and
send light signals. Later this year it will bring out Li-Fi products for firms
installing LED-lighting systems.
There are limitations to using light, of course. Unlike radio, light waves will
not penetrate walls. Yet for secure applications that could be a bonus. And
light bulbs some 14 billion of them around the world are almost everywhere and
often on. As they are gradually replaced by LEDs, every home, office, public
building and even streetlight could become a Li-Fi hotspot. Having a
line-of-sight connection with the LED in question would undoubtedly improve the
signal, but light reflected from walls or ceilings might often be enough. In
any case, having a good line of sight helps Wi-Fi as well. And spotting a
nearby light in order to sit next to it is certainly easier than finding the
location of a Wi-Fi router.
Communication, though, is a two-way street. That means the LEDs involved in
Li-Fi would need photodetectors to receive data. Some LED systems have such
sensors already (to know when to turn on at night). But even if LEDs are not
modified Dr Povey reckons hybrid systems are possible: data could be downloaded
using light but uploaded (typically a less data-intensive process) using radio.
In an office, for example, an LED-powered desk lamp could work as a Li-Fi
router, able to link up with any networked device placed on the desk.
A big advantage of light is that it can be used in areas which contain
sensitive equipment that radio signals might interfere with, such as aircraft
and operating theatres. LEDs in the ceiling of an airliner would not only allow
internet access but could also transmit films on demand to individual seats,
removing the need for lots of expensive and heavy cabling, thus saving airlines
fuel. That alone could be enough to, as it were, make this idea fly.
from the print edition | Science and technology