As the society continues to develop high-power LEDs based on solid-state lighting, a bold and innovative approach emerges in the minds of some visionary engineers. Their advice is: Why not let the LED on/off switch so fast that the human eye can't tell, so use them to transmit data?
This proposal is the theoretical basis of visible light communication (VLC). With enough advanced technology support, each new LED luminaire can also be wired to the backbone network, enabling any device in the room to achieve ubiquitous wireless communication without increasing the already burdensome RF bandwidth burden. Many industries, standards organizations and government agencies that are heavily funded are developing visible light communications. The prospects for visible light communication are very broad, as the traditional lighting market has reached trillions of dollars and the transition to solid-state lighting has begun. According to Strategies Unlimited, the LED lighting market will exceed $1 billion this year and is expected to grow to approximately $7.3 billion by 2014.
Of course, the focus of solid-state lighting is to reduce greenhouse gas emissions, because LED lamps consume much less power than current standard lighting products. But the huge market has inspired almost every major electronic research organization to invest in the development of visible light communication applications.
Most visible light communication applications are not intended to replace other wireless technologies, such as Bluetooth, Wi-Fi, WiMax, and LTE. Applications are targeted at applications where current RF wireless communications cannot be implemented, such as hospitals and aircraft. Radio frequencies can interfere with signals in life-critical devices; robots -- they can navigate using virtual road signs in headlights for information transfer; signage -- provides additional information when the phone camera points at it.
Japan's visible light communication alliance members include Casio, NEC, Matsushita Electric Engineering, Samsung, Sharp, Toshiba and NTT Docomo. The alliance is working to promote the IEEE 802.15 Wireless Personal Area Network Standards Committee to increase the ".7" work, with a view to Visible light communication is upgraded to the same wireless state as RF and IR. 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," said Rick Roberts, an IntEL laboratory scientist and technical editor of the IEEE 802.15.7 committee.
"The reason for the interest of IEEE is the widespread popularity of LEDs. The current LED technology is mainly used for lighting, but if the wireless market is also developed, we know from the past experience that we need to standardize interoperability." Roberts said, " Our standardization work began in 2008 and is expected to finalize the standard next year."
According to Roberts, the primary task of the 801.15.7 committee is to implement the standards for lighting first and communication second. "Visible light communication is the only 'wireless communication' signal that can be seen with the naked eye, so it can't affect others. He pointed out, "For example, visible light communication is not suitable for remote control because people usually watch TV in darker indoors. You don't want to see the flashing light from the remote control? Communication with LEDs does not cause light to flicker, and visible light communication must adapt to the way people usually use lighting sources, such as light adjustment. â€
Figure 1: Just as the brake lights tell the driver to stop, the VLC can send the same message to the engine control unit to avoid collisions.
VLC opens up space for creative applications
Visible light communication is expected to form a series of new applications. Although it can be realized by Wi-Fi or infrared, it is more convenient or safer to realize by visible light. For example, mutual interference with adjacent RF signals may limit the use of Wi-Fi, while visible light is substantially free of interference problems; adjacent beams may pass through as long as their destinations are different. For safety reasons, RF communications such as hospitals and airplanes are prohibited in some places. Visible light communication is an ideal alternative to these applications because LED illumination is already in use, and visible light communication does not interfere with system signals that carry important tasks. At the same time, visible light communication also has the potential for high data capacity.
"Visible light communication can implement various types of new applications," Roberts said. "I especially like smart LED signs that can display ''cho's restaurant'', but if you take out the mobile device and point to this sign You can also download more information - such as the restaurant's address, menus, even coupons, etc. You have to expand your imagination, and there are a lot of new possible applications."
Samsung is experimenting with the use of visible light communication in LED-based backlit LCD flat panel displays so that users can download all information from product information to website addresses. "We believe that LCD backlight communication is one of the best applications for visible light communication, because LCD backlights are turning to LEDs," said Scott Birnbaum, vice president of Samsung's LCD business unit.
Just as IEEE began its own standardization efforts in 2008, the National Science Foundation also took a fancy to “this light†and added VLC research projects to its Intelligent Lighting Engineering Research Center (ERC) program.
Smart Lighting ERC is a $18.5 million 10-year program involving more than 30 university researchers at several colleges, including Rensselaer Polytechnic Institute (RPI), Boston University, and the University of New Mexico. “Because the whole society is moving towards solid-state lighting, we are considering everything that can be done with LED light energy,†said Robert Karlicek, professor of PRI Academy and director of the Intelligent Lighting Research Center. “We want to know which things we thought would never be possible. We can do things and determine what kind of equipment needs to be created and what kind of system architecture is needed to achieve more of an advanced lighting system."
Karlicek envisages that the lighting function of the lighting system is also made smarter by using visible light communication to add environmental parameters to the lighting system itself. “We want to know what else can be done with LEDs in order to provide more value to society and provide new opportunities.†He pointed out, “For example, I can imagine indoor lighting fixtures communicating with each other – from a light to Another lamp that uses low-rate signals to standardize colors and provide uniform light."
Intelligent lighting ERC researchers are looking to control all aspects of LED lighting, including color, density, energy usage, polarization and modulation to create new applications. These new applications range from using solid-state lighting to providing data communication to control the circadian rhythm of the human body, or providing the healthiest lighting at specified times of the day. ERC is also investigating the use of visible light in biosensing, medical diagnosis and treatment.
Boston University, which participates in the Smart Lighting ERC program, specializes in traditional data communications using visible light communications on special occasions, such as on airplanes. Visible light communication 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 viewing the same movie can share a public broadcast connection or feed a separate stream of data to individual viewers watching different movies.
On the factory floor, the same functionality allows mobile robots to navigate through the warehouse using visible light communication, check their position with headlights, and communicate directly with each other to avoid collisions. Similarly, the car can maintain the correct direction of travel by reading the coordinates of the traffic light broadcast. Visible light communication from the car to the car helps avoid collisions and prevents traffic jams.
Figure 2: LED lights are used for both lighting and communication to facilitate ubiquitous computing, and each device in the room can use a separate data stream.
Figure 3: The current deployment of wireless networks uses Wi-Fi, which relies on network cables and access points, but in the future, systems can use existing wired infrastructure to send data to LED lights and serve as new access everywhere. point.
Figure 4: Using LED communication at an airport can reduce ground conflicts because LEDs can provide signals between airport lighting infrastructure, ground vehicles, and aircraft.
The road to VLC implementation
Boston University professor Thomas Little is a senior researcher and associate director of the Intelligent Lighting Center. He is experimenting with different modulation schemes, including encoders using standard binary codes, non-return-to-zero encoders, pulse code modulation, and pulse density modulation. According to him, as long as the data rate is greater than 900 kHz, all of these schemes do not produce flicker when working. The team led by Little is also studying how to reliably send and receive signals without direct line of sight, which requires the use of reflected signals and at the same time does not produce intermodulation interference.
"We want to make the installation of the network as easy as screwing a light bulb," Little said. His lab has so far completed more than 40 prototypes that are being evaluated by some industrial partners.
Boston University's Intelligent Lighting Lab has built several demonstration devices that can be used to demonstrate how to make lighting fixtures with both lighting and data communication capabilities. For example, an Ethernet signal transmitted over a wire can be routed from one luminaire to another, and the LED modulates the data signal from the Ethernet device.
"The problem we want to solve is how to provide higher data rates at a very low cost, so that visible light communication can be a 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 achieved with visible light. However, Wi-Fi can still be used for backhaul signals to Ethernet hubs and achieve the same advantages, since the return signal from the user (eg via a button) is typically a low bandwidth signal.
As part of its work, the University of New Mexico is focusing on innovative equipment architectures that focus on improving efficiency and switching speed of LEDs to achieve GHz bandwidth. So far, Professor Steve Hersee has invented a scalable process for fabricating nanowire-based LEDs. The University of New Mexico is preparing to license this technology to the industry
“We use the same GaN material, but unlike ordinary LEDs, all layers lie flat on the horizontal surface, they will be wound around the central nanowire in a coaxial manner, making the device much more efficient and allowing Modulation is much higher at a higher rate," Hersee noted. "This will make a fundamental change in solid-state lighting. Nanowires contain zero defects, while traditional horizontal gallium nitride films used in conventional LEDs have millions of defects per square centimeter. ."
Nanowires vary in width from 100 nm to 500 nm and can grow to 5 to 10 um in vertical columns on the substrate. The first prototypes were only recently produced, but the University of New Mexico hopes to have more companies licensed before the end of the year.
An NSF-sponsored exhibition will be held at the RPI Academy in July this year, which will assess the latest technology to date and set a new milestone for 2011. NSF is scheduled to expire by 2018, when NSF hopes to have intelligent lighting standards and technologies, and to enable each new solid-state lighting device to perform dual tasks like a visible light communication hub.
"Since we have achieved LED lighting, we also hope that LEDs can achieve other uses than lighting," Hersee noted.
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