Penn State University engineers have shown that broadband, wireless, indoor, local area communication networks that rely on non-line-of-sight infrared (IR) signal transmission can offer low error rates as well as safe, low power levels.
‘Line-of-sight or point-to-point infrared signal transmission, which is used, for example, in television remote controls, is highly efficient at low power levels but suffers from the need for alignment between the transmitter and receiver,’ said Dr. Mohsen Kavehrad, professor of electrical engineering. ‘If someone ‘shadows’ or blocks the remote control beam while you’re trying to change the channel, the signal can’t get through.
‘On the other hand, non-line-of-sight transmission, which uses a broad diffuse beam, suffers less from shadowing but usually forfeits the power efficiency, broadband and low error rate values that infrared transmission can offer.’
Now, however, Kavehrad and his colleagues at Penn State’s Centre for Information and Communications Technology Research have developed a new link design that uses a multi-beam transmitter with a narrow field of view receiver.
The system is reported to have a bit-error rate of only one error per billion bits and uses milliwatt transmitted power levels.
To use the Penn State signalling scheme, for example, to form a local area network for a group of computers in a room, each machine is equipped with a low power infrared source and a holographic beam splitter.
The original low power beam is separated into several narrow beams, which strike the ceiling and walls at points that form an invisible grid throughout the entire volume of the room. Because the beams are also reflected at each of the strike points, they can be used to send or receive information.
Since the beams created by the splitter are narrow, narrow field-of-view receivers are used. Using a narrow field of view receiver makes it easier to filter out noise. In addition, receivers consisting of more than one element can insure continued coverage when some of the transmitter beams are blocked.
‘Others have attempted to develop local area networks with radio frequencies. However, indoors, radio frequencies can pose a radiation hazard,’ noted Kavehrad.
‘Infrared signals, on the other hand, pose no such hazard, especially at the low powers used by our system. However, since the sun is an infrared emitter, as well as fluorescent and incandescent bulbs, light coming in through windows or from artificial lighting can add background noise to the system. This noise, to some extent, can be filtered at the receivers.’
The Penn State team developed a framework for computer simulation under which properties of room, transmitter and receiver are quantified.
Using the simulation results, they showed that the system has a bit-error rate of only one error per billion bits in 99 percent of the coverage area at bit rates up to a few hundred megabits per second. In addition, the system uses transmitted power levels well below one Watt.
The wireless infrared communication system is being patented by the University.