“We can thus, without a conducting wire as in electric
telephony, speak from station to station, wherever we can project a
beam of light” — Alexander Graham Bell, 1880.
One hundred and thirty years after Bell invented the photophone, a Professor at the University of Edinburgh is once again proposing using light as a means of data transmission.
With an estimated 5 billion cellular phones currently in existence worldwide, and the proliferation of data-hungry smartphones and tablet devices in recent years, the strain on the wireless networks that carry our data is growing.
As the increasingly-congested radio frequency (RF) spectrum bends under the weight of the data demands of the human race, more and more energy inefficient radio base stations are deployed. There are currently 1.4 million of these masts dotted around the globe, many of them diesel-powered.
Professor Harald Haas is Professor of Mobile Communications, at the Institute for Digital Communications. Based in a building named after Bell at the Edinburgh University, Professor Haas has been studying Visible Light Communication since 2004, in an attempt to solve what he sees as a data “bottleneck”, with a more energy-efficient method of wireless data transmission.
Although the idea of transmitting data via the visible light spectrum is not a new one, the development of the light-emitting diode, or LED light, has allowed Professor Haas to create technology that can transfer vast quantities of information across a spectrum 10,000 times wider than the radio frequency spectrum.
Professor Haas’ spatial modulation and SIM/OFDM (subcarrier-index modulation/orthogonal frequency division multiplexing) technologies allow the LED light to modulate at a rate so fast as to be imperceptible to the human eye but which can be picked up by receivers such as smartphone cameras at speeds of hundreds of megabits per second.
“If you use already-installed lighting equipment as an infrastructure”, explains Professor Haas, “we just piggy-back on the existing illumination functionality, and provide additional data communication. It’s data communication through illumination; two functionalities combined”.
“Our technology even works if you dim down the lights to a level where it appears to be off. You can still transmit data, so even during daylight it can work in buildings, so there’s a big energy-saving advantage.”
Visible Light Communication also has the added advantage that it can be used in areas where RF wireless communications are not permitted, such as in hospitals or in chemical plants or on oil rigs.
“Wireless communications is not available is hospitals. It’s not available in airplanes. However, there are many lights installed in hospitals, and there are many lights installed in an aircraft cabin. And even underwater, where radio frequency communication doesn’t work, light propagates underwater.”
Since radio frequencies penetrate walls, they are easy to intercept. For more secure data transmission, VLC could offer an alternative, whereby the data is only shared with whoever it is directed at.
“People with bad intentions can do all sorts of bad things. Light is harder to penetrate through walls and there is only data where there is light. We can see where we send the data to, because we see the light beam. So it’s much easier to control where the data is sent to, and it’s not lost in all directions. It’s a directed wireless transmission, and it is therefore, more secure.”
This strength is, in another sense, a weakness, as VLC can only operate where light can shine, and lacks the all-pervasiveness of radio frequency WiFi. For this reason, Professor Haas does not see it as a replacement for existing WiFi technology, but rather as an accompaniment.
“It is a complementary solution to the classical WiFi situation. Go in to a hotel at certain times and people are all using the WiFi in the hotel, it is painfully slow, it doesn’t work because there is only limited frequency spectrum.
“If you use light you can relieve some of the over-used spectrum so that in total there’s more data transmitted, so it’s complementing the RF. But we have these additional environments, underwater, intrinsically safe environments, hospitals and so on, where RF doesn’t work or isn’t allowed, but light would be workable there.”
Speeds have been recorded in lab environments of up to 500mb/s but Professor Haas is more concerned with test results garnered in real-life conditions
“If you record a number then you need to say what are the underlying conditions? What is the energy you invested? Is the room dark? Or, does it work in ambient light situations?
“At the moment we can run a practical demonstration in ambient light conditions with 100mb/second.
“We have unique technology which we call spatial modulation and SIM/OFDM, with that technology we achieved 600mb/sec theoretically. That’s what we’re developing now and we can even see a data rate of up to 1Gb, under practical, realistic situations, rather than artificial lab conditions”.
Light bulbs are all around us, and Professor Haas believes the essential infrastructure is in place for incorporating VLC, or “LiFi”, as it has been dubbed.
“You take the light bulb, we would integrate our technology which is a chip and a little bit of analog circuitry, very simple analog circuitry as compared to WiFi, because we don’t have an antenna. It’s analog circuitry and a digital chip that needs to be fitted to the light bulb. It’s not a major operation. The infrastructure’s already there.”
Professor Haas has been running a proof of concept project, funded by the Scottish Government for the previous eighteen months, and at the moment is developing a pre-production prototype which he hopes to have ready by the end of the year.
“Hopefully we will find a pilot customer, school, hotel, or private enterprise where we can install our technology in the first half of next year and after the middle of next year we will be able to have the technology available in a larger scale.”
Main picture: “Copyright (c) Peter Tuffy, The University of Edinburgh”