With preparations under way for a shift to solid-state Lighting based on high-output Leds, a proverbial light bulb has appeared above the heads of some forward-looking engineers. Their proposal: Why not switch the LEDs on and off so fast the eye cannot tell, in order to use them to transmit data too?
With enough advance work, every new led light fixture could also be wired into the network backbone, accomplishing ubiquitous wireless communications to any device in a room without burdening the already crowded RF bands. Visible light communications （VLC） is being refined by industry, standards groups and well-funded government initiatives. And the stakes are enormous, since the traditional lighting market is measured in trillions of dollars and the transition to solid-state has already begun. This year, Led lighting will account for more than a $1 billion market, with projected growth to about $7.3 billion by 2014, according to Strategies Unlimited.
Of course, solid-state lighting's priorities are to lower greenhouse gas emissions and users' utility bills, since led lamps use less power than today's standard lighting products. But the enormous size of the market has already prompted nearly every major electronics research organization to begin developing VLC applications.
Most of those apps will not attempt to replace other wireless technologies, such as Bluetooth, Wi-Fi, WiMAX and LTE, but will aim for niches that are not well served by RF wireless today-from hospitals and aircraft, where RF can interfere with signals in life-critical equipment, to robots that could navigate halls for mail delivery using virtual signposts in overhead lighting, or signage that could supply additional information when a phone camera is pointed it.
Japan's Visible Light Communications Consortium, with members Casio, NEC, Panasonic Electric Works, Samsung, Sharp and Toshiba as well as telecom carriers like NTT Docomo, was instrumental in stimulating the IEEE 802.15 Wireless Personal Area Network standards committee to add a “.7” effort to elevate visible light communications to the same wireless status as RF and infrared. The 802.15.7 committee just approved the current draft of the wireless VLC standard at the working group level, “but we still have a lot of comments to resolve,” said Rick Roberts, a scientist at Intel Labs （Portland, Ore.） who acts as technical editor for the IEEE 802.15.7 committee.
“The stimulus for getting IEEE involved is the ubiquitous deployment of LEDs. The technology is there for illumination, but if a wireless market is going to be there too, we know from past experience that there needs to be standardization for interoperability,” said Roberts. “Our standardization efforts started in 2008, and we are hoping to have the ink dry on the standard next year.”
According to Roberts, the primary objective for the 802.15.7 committee is to enforce a standard that puts illumination first and communications second. “Visible light communications is the only [wireless comms] signal you can see with the human eye, so it cannot be obtrusive,” he said. “For instance, it would not be good for remote controls, because people watch TV in darkened rooms; you don't want flashes of light being emitted by the remote. . . Using [LEDs] for communications cannot cause flicker, and [VLC] has to accommodate anything that people normally do with illumination sources, like dimming them.”
Via Wi-Fi
VLC is expected to yield a swath of apps that, while possible to perform with Wi-Fi or IR, could more conveniently or more safely be performed via visible light. Interference from and with neighboring radios, for example, can limit Wi-Fi, whereas visible light presents few interference problems; adjacent beams can pass through each other, as long as their destinations are distinct. For safety reasons, use of RF comms is forbidden in some places, such as in hospitals and on aircraft. VLC is a logical alternative in those instances, since LED illumination will already be available and since VLC would not interfere with mission-critical system signals. Yet VLC also promises high data capacity.
“Visible light communications will enable all kinds of new applications,” said Roberts. “My favorite is the smart LED sign that might [ordinarily] say 'Eat at Joe's,' but if you pull out your mobile device and point it at that sign you can download more information,” such as the eatery's address, its menu and maybe a coupon. “You have to use your imagination, but there are lots of new applications possible.”
Samsung is experimenting with including VLC with its LED-based backlit LCD flat-panel displays so that viewers can download everything from product information to Web site addresses. “We believe that LCD backlight communications is one of best applications for VLC, because LCD backlights are changing to LEDs anyway,” said Scott Birnbaum, VP of Samsung's LCD business.
Just as the IEEE was beginning its own standards efforts in 2008, the U.S. National Science Foundation saw the light and added VLC research under its Smart Lighting Engineering Research Center （ERC） initiative.
The Smart Lighting ERC is an $18.5 million, 10-year program involving more than 30 university researchers from institutions including Rensselaer Polytechnic Institute, Boston University and the University of New Mexico. “We are considering all of the things we can do with light, now that [society is] switching over to solid-state lighting,” said RPI professor Robert Karlicek, director of the smart-lighting research center. “We want to know what we can do that was never possible before, and identify what devices need to be created and what kind of systems architectures need to be employed to be able to realize everything that an advanced lighting system can do.”
Karlicek envisions making the illumination side of the lighting system smart too, by leveraging VLC to add environmental value to the lighting system itself. “We want to know what else can we do with LEDs to provide value to society as well as drive new opportunities,” he said. “For instance, I can envision indoor lighting fixtures that communicate with each other-from light to light-with low-bit-rate signals to normalize colors and provide uniformity.”
Smart Lighting ERC researchers seek to control all aspects of LED lighting-including its color, intensity, energy use, polarization and modulation-to yield apps that might range from using solid-state lighting to provide data communications-VLC-to controlling circadian rhythms or providing the most healthful form of light for the given time of day. The ERC is also investigating the use of visible light in biosensing, medical diagnostics and therapeutics.
Common connection
Smart Lighting ERC participant Boston University is focusing on the use of VLC for traditional data communications in unconventional places, such as on aircraft. VLC offers the ability to harness separate parallel data connections, either from different lines of sight or by virtue of multiplexing different frequencies of visible light over the same line of sight. Thus, everyone watching the same movie could share a common broadcast connection, or separate data streams could be fed to the devices of individuals watching different movies.
On the factory floor, the same capability could allow mobile robots to use VLC to navigate warehouses by checking their location using overhead lights and by communicating directly with each other to avoid collisions. Likewise, automobiles could keep tabs on their location by reading the coordinates being broadcast by traffic lights. Vehicle-to-vehicle VLC could help avoid collisions and prevent traffic jams.
BU professor Thomas Little, a lead researcher and associate director of the smart-lighting center, is experimenting with different modulation schemes, including encoders that use standard binary codes, non-return-to-zero encoders, pulse-code modulation and pulse-density modulation, all of which he claims can be made to work without flickering light, so long as data rates are above 900kHz. Little's group is also investigating how to transmit and receive signals reliably without having a direct line of sight, by using reflected signals but without incurring intermodulation interference.
“We want to make the installation of a network as easy as screwing in a light bulb,” said Little, whose lab to date has completed more than 40 prototypes, which are being evaluated by industrial partners.
The Smart Lighting lab at BU has several demonstrations already set up that show how light fixtures might be made to accommodate both the illumination function and the data communications function. Hardwired Ethernet signals could be routed from fixture to fixture, for instance, with LEDs modulating the data signals from the Ethernet installation.
“The problem that we are trying to address is how to provide high data rates at a very low cost so that it [VLC] can become a part of the lighting infrastructure,” said Little.
Nanowires as emitters
The signal from the user device, such as a smart phone or laptop, to the Ethernet hub could be made with visible light by building LED emitters to the device. But Wi-Fi could also be used for the return signal to the Ethernet hub and reap the same benefits, since the return signals from the user （via keystrokes, for example） will typically be low-bandwidth.
For its part, the University of New Mexico is concentrating on novel device architectures-basically ways of improving the efficiency and switching speeds of LEDs in pursuit of gigahertz bandwidths. Toward that end, Prof. Steve Hersee has invented a scalable process for manufacturing nanowire-based LEDs. The technique, which the university intends to license to industry, would enable a nanowire-based LED to be mass produced using the same materials as are used for traditional LEDs, but with an improved architecture, in which vertical columns of millions of nanowires would act as emitters.
“We use the same gallium nitride material, but instead of all the layers lying flat in a horizontal plane as in a normal LED, they coaxially wrap around the central nanowire, making the device much more efficient and allowing it to modulate at a much higher rate,” said Hersee. “This could be a game changer in solid-state lighting. Nanowires contain zero defects, compared with a million defects per square centimeter for conventional, horizontal films of gallium nitride in traditional LEDs.”
The nanowires, ranging from 100- to 500nm wide, are grown in vertical columns on the substrate to heights of 5- to 10lm. The first prototypes were just fabricated last month, but the university expects to have licensees lined up by the end of the year.
The NSF will sponsor a demonstration in July at RPI during which progress to date will be evaluated and a new set of milestones will be laid out for 2011. The NSF program culminates in 2018, by which time the effort hopes to have smart-lighting standards and technology ready to let every new solid-state lighting installation do double duty as a VLC hub.
“Now that we have LED lighting, we want to use it for more than just illumination,” said Hersee.
