Innovative ideas for realizing visible light communication using LED lighting

As society moves toward high-power LEDs based on solid-state lighting , a bold and innovative idea has emerged in the minds of some visionary engineers. Their suggestion is: why not let LEDs switch on/off fast enough that the human eye cannot distinguish, and use them to transmit data?

This proposal is the theoretical basis for visible light communication (VLC). With sufficiently advanced technology, each new LED fixture can also be wired into the backbone network , enabling ubiquitous wireless communication for any device in the room without burdening the already crowded RF bandwidth. Many industries, standards organizations, and well-funded government agencies are developing VLC. The prospects for VLC are very bright because the traditional lighting market is worth trillions of dollars and the transition to solid-state lighting has already begun. According to Strategies Unlimited, the LED lighting market will exceed $1 billion this year and is expected to grow to about $7.3 billion by 2014.

Of course, the focus of solid-state lighting is to reduce greenhouse gas emissions, because LED lights consume much less power than current standard lighting products. But the huge market has inspired nearly every major electronics research organization to invest in the development of visible light communication applications.

Most VLC applications are not intended to replace other wireless technologies, such as Bluetooth, Wi-Fi, WiMax and LTE . Their application targets are places where current RF wireless communications cannot be implemented, such as hospitals and aircraft, where RF may interfere with signals in life-critical equipment; robots , which can use virtual landmarks in their headlights to navigate and transmit information; and signs, which can provide additional information when a mobile phone camera is pointed at it.

Japan's Visible Light Communication Alliance, whose members include Casio, NEC, Panasonic Electric Engineering, Samsung, Sharp, Toshiba, and NTT Docomo, is working to promote the IEEE 802.15 Wireless Personal Area Network Standards Committee to add ".7" work in order to elevate visible light communication to the same wireless status as radio frequency and infrared. The 802.15.7 committee has just approved the current draft version of the wireless VLC standard at the working group level, "but we still have a lot of problems to solve." Intel Labs scientist and IEEE 802.15.7 committee technical editor Rick Roberts pointed out.

"The reason for the IEEE's interest is the widespread adoption of LEDs. Current LED technology is mainly used for lighting, but if the wireless market is also developed, we know from past experience that interoperability needs to be standardized," said Roberts. "Our standardization work began in 2008 and we hope to finalize the standard next year."

According to Roberts, the 801.15.7 committee's priority is to promote a standard that puts lighting first and communication second. "Visible light communication is the only 'wireless' signal that can be seen by the naked eye, so it can't be distracting to others," he said. "For example, visible light communication is not suitable for remote control because people usually watch TV in dimmed rooms. You don't want to see a flickering light from the remote control, do you? Using LEDs for communication does not cause flickering light, and visible light communication must adapt to the way people usually use lighting sources, such as light dimming."

Figure 1: Just as brake lights "tell" the driver to stop, VLC can send the same message to the engine control unit to avoid a collision.

VLC opens up space for creative applications

Visible light communication is expected to enable a range of new applications that can be achieved with Wi-Fi or infrared, but are more convenient or safer to achieve with visible light. For example, mutual interference with adjacent radio frequency signals may limit the use of Wi-Fi, but visible light basically does not have interference issues; adjacent beams can pass through each other as long as their destinations are different. For safety reasons, some places prohibit the use of radio frequency communications, such as hospitals and airplanes. Visible light communication is an ideal alternative technology in these occasions because LED lighting is already in use and visible light communication will not interfere with the signals of mission-critical systems. It also has the potential for high data capacity.

"Visible light communication enables all kinds of new applications," Roberts said. "One thing I particularly like is a smart LED sign that says ''''''''''''''''', but if you take out your mobile device and point it at the sign, you can also download more information - such as the restaurant's address, menu and even coupons. You have to use your imagination, and the new possibilities are really endless."

Samsung is experimenting with using VLC in LED-backlit LCD flat-panel displays
so that users can download everything from product information to website addresses. "We believe LCD backlight communication is one of the best applications for VLC because LCD backlights are moving to LEDs," said Scott Birnbaum, vice president of Samsung's LCD business unit. Just as the IEEE began its own standardization efforts in 2008, the National Science Foundation also took a fancy to "the light" and added VLC research projects to its Intelligent Lighting Engineering Research Center (ERC) program.

The Intelligent Lighting ERC is a $18.5 million, 10-year initiative involving more than 30 university researchers at multiple institutions, including Rensselaer Polytechnic Institute (RPI), Boston University and the University of New Mexico. "As society moves toward solid-state lighting, we are looking at everything we can do with LED light," said Robert Karlicek, PRI College Professor and Director of the Intelligent Lighting Research Center. "We want to know what we can do that we never thought was possible, and determine what kind of devices we need to create and what kind of system architectures we need to adopt to achieve more capabilities in an advanced lighting system."

Karlicek envisions making the lighting functions of lighting systems smarter, too, by using visible light communications to add environmental parameters to the lighting systems themselves. "We want to know what else we can do with LEDs to provide more value to society and new opportunities," he noted. "For example, I can imagine indoor lighting fixtures communicating with each other - from one lamp to another, using low-rate signals between them to standardize color and provide uniform, consistent light."

Intelligent lighting ERC researchers are seeking to control all aspects of LED lighting, including color, density, energy use, polarization and modulation, in order to form new applications. These new applications range from using solid-state lighting to providing data communications to control human circadian rhythms or provide the healthiest light at a specified time of day. ERC is also studying the use of visible light in biosensing, medical diagnosis and treatment.

Boston University, which is involved in the Smart Lighting ERC project, is focused on using VLC to enable traditional data communications in special situations, such as on airplanes. VLC can utilize multiple independent parallel data connections, such as connections from different lines of sight, or multiplex different frequencies of visible light on the same line of sight. In this way, everyone watching the same movie can share a common broadcast connection, or feed independent data streams to each viewer watching different movies.

On the factory floor, the same capability allows mobile robots to use VLC to navigate a warehouse, using overhead lights to check their position and communicating directly with each other to avoid collisions. Similarly, cars can stay on the right track by reading coordinates broadcast by traffic lights. Car-to-car VLC helps avoid collisions and prevent traffic jams.

Figure 2: Using LED lights for both lighting and communications helps enable ubiquitous computing, with independent data streams available to every device in the room.

Figure 3: Current wireless network deployments use Wi-Fi, which relies on network cables and access points, but future systems could leverage existing wired infrastructure to send data to LED lights and serve as new access points everywhere.

Figure 4: Using LED communications at airports can reduce ground conflicts because LEDs can provide signals between airport lighting infrastructure, ground vehicles, and aircraft.

VLC Implementation

Thomas Little , a professor at Boston University and a senior researcher and associate director of the Intelligent Lighting Center, is experimenting with different modulation schemes, including encoders using standard binary codes, non-return-to-zero encoders, pulse code modulation, and pulse density modulation. He claims that all of these schemes work without flickering light as long as the data rate is greater than 900kHz. Little's team is also studying how to reliably send and receive signals without direct line of sight, which requires the use of reflected signals without intermodulation interference.

"We want to make installing the network as easy as screwing in a light bulb," says Little, whose lab has built more than 40 prototypes so far, which are being evaluated by several industrial partners.

Boston University's Smart Lighting Lab has built several demonstrations that show how to make a light fixture have both lighting and data communication capabilities. For example, an Ethernet signal transmitted through the wire can be routed from one light fixture to another, and the data signal from the Ethernet device can be modulated by the LED.

"The problem we want to solve is how to provide high data rates at a low cost so that visible light communication can become part of the lighting infrastructure," Little said.

By installing an LED transmitter in the device, the signal from the user device (such as a smartphone or laptop) to the Ethernet hub can be implemented using visible light. However, Wi-Fi can still be used for the return signal to the Ethernet hub and gain the same advantages, because the return signal from the user (such as through keystrokes) is usually a low-bandwidth signal.

As part of its work, the University of New Mexico is focusing on innovative device architectures - working to increase the efficiency and switching speed of LEDs, aiming to achieve GHz bandwidths. So far, Professor Steve Hersee has invented a scalable process for manufacturing nanowire-based LEDs. The University of New Mexico plans to license this technology to industry, allowing them to mass-produce nanowire-based LEDs using the same materials as traditional LEDs, but with a much-improved architecture, where vertical columns of millions of nanowires will serve as emitters.

“We use the same gallium nitride material, but instead of all the layers lying flat in a horizontal plane like in a normal LED, they will be wrapped coaxially around a central nanowire, making the device much more efficient and allowing modulation at much higher rates,” Hersee noted. “This will fundamentally change solid-state lighting. The nanowire contains zero defects, whereas conventional horizontal gallium nitride films used in conventional LEDs have millions of defects per square centimeter.”

The nanowires range in width from 100nm to 500nm and can be grown 5 to 10um high in vertical columns on a substrate. The first prototypes were only recently produced, but the University of New Mexico hopes to have more companies licensed by the end of the year.

An NSF-sponsored expo at the RPI Institute in July will assess the state of the art so far and set new milestones for 2011. By 2018, the NSF hopes to have smart lighting standards and technology that will allow every new solid-state lighting fixture to do double duty as a visible light communications hub.

"Now that we have achieved LED lighting, we also hope that LED can achieve other uses besides lighting," Hersee pointed out.