Wi-Fi at the Speed of Light
A wireless network that uses reflected infrared light instead of radio waves has transmitted data through the air at a speed of one gigabit per second–six to 14 times faster than the fastest Wi-Fi network. Such optical networks could provide faster, more secure communications and would be especially suitable for use in hospitals, aircraft, and factories, where radio-frequency transmission can interfere with navigation equipment, medical devices, or control systems. Another possible application is wireless networking for home theaters; a system that transmits data at 1.6 gigabits per second could broadcast two separate high-definition TV channels across a room, a capacity that exceeds the bandwidth of any existing radio system.
Penn State graduate student Jarir Fadlullah and Mohsen Kavehrad, professor of electrical engineering and director of the university’s Center for Information and Communications Technology Research, built and tested the experimental system. Their setup sent data across a room by modulating a beam of infrared light that was focused on the ceiling and picking up the reflections using a specially modified photodetector. The pair says that their measurements show the system could support data rates “well beyond” the one gigabit per second they are currently claiming.
“This probably will be the next generation wireless communications technology,” says Zhengyuan Daniel Xu, professor of electrical engineering at the University of California, Riverside. Xu is also the director of the UC-Light Center, a consortium of researchers working on wireless optical communications at different UC campuses. “Light will give you higher data rates than radio frequencies, and RF already has a very congested spectrum.”
Optical wireless networks could also offer less interference and greater security than radio-frequency networks, Kavehrad says. While radio signals pass through walls and doors, light does not, making it easier to reuse frequencies and more difficult to intercept transmissions. He also notes that unlike radio frequencies, the spectral region for all light–infrared, visible, and ultraviolet–is unregulated worldwide. This could make it easier to commercialize optical wireless networks.
Researchers have studied indoor optical communications since the late 1970s, when engineers at IBM Zürich built the first working system. The technology languished because the Internet was still in its infancy, and there was no demand for wireless broadband systems–though interest has picked up in the past few years.
Kavehrad’s demonstration is by far the highest speed that’s been demonstrated for an indoor wireless optical network, says Valencia M. Joyner, assistant professor of electrical and computer engineering at Tufts University. She notes that the transmission distances that he and Fadlullah achieved, and their use of diffuse light rather than a point-to-point optical system, are especially important. “There are a lot challenges in demonstrating the high-speed capabilities of indoor optical signals,” she says. “The fact that he was able to demonstrate a one-gigabit-per-second system with diffuse light is extremely significant. That drastically reduces the complexity of the transceiver system.”
Kavehrad and Fadlullah built the experimental system using a low-power infrared laser to prevent possible damage to the eyes or skin. They focused the light through a lens, creating an elliptical spot on the ceiling; they then used a high-sensitivity light detector called an avalanche photodiode to pick up the light reflected off the ceiling. They used a plastic holographic lens to collect enough reflected light from the ceiling spot and focus it on to the photodiode’s active area. By using the lens, Fadlullah and Kavehrad could transmit a one-gigabit-per-second optical signal across a room about eight meters long by four meters wide.
Free-space optical networks have previously been used to transmit broadband data over long distances, but the high power of the lasers and need for a clear line of sight and extremely precise alignment between the transmitter and receiver have limited their usefulness. The low-power, diffuse light approach that Kavehrad and Fadlullah chose doesn’t require such precise alignment and is much more practical for indoor communications. Kavehrad says that their system should work for visible and ultraviolet light as well as infrared.
Companies such as Intel, InterDigital, Siemens, Sony, Samsung, Mitsubishi, and Sanyo are all pursuing research on optical wireless networks, say Kavehrad and Xu. Several of these companies are members of the Infrared Data Association (IrDA), an industry organization that is developing technical standards for infrared wireless communications. IrDA recently announced the GigaIR standard for very short range, line-of-sight infrared communication links operating at one gigabit per second. And the IEEE 802.15 working group, which sets standards for wireless personal area networks, is working to create standards for wireless networks that use visible light, says Fadlullah.
Kavehrad says that “a lot of engineering has to happen” before optical wireless networks are a reality. He and Fadlullah used lasers, transmitters, and receivers not designed for communications in their experimental system; all of that equipment must be optimized for data networking. However, Kavehrad says, if development of white LEDs for indoor lighting continues at its present pace, it should be possible to have practical wireless optical networks within three years. “The main limiting factors will be industries and their politics, as well as consumer demand,” he says.
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