Light beams can enable coded data transmission to wireless devices located inside closed spaces and provide internet multimedia services, such as video conferences, movies on demand and more
By Mohsen Coward
Electronics engineers have been dreaming for years about total connectivity - wireless data transfer to every person and every device, everywhere all the time. They've also made real strides toward that goal: more than two billion people now own cell phones, and hundreds of millions send and receive messages and files using laptops, PDAs, and other digital devices linked together via short-range radio networks (Wi-Fi).
Besides that, more and more Wi-Fi users enjoy the convenience of deploying portable wireless devices anywhere in the home and office. At the same time, manufacturers are integrating wireless communication capabilities into devices and devices that were previously considered stationary, to allow consumers to communicate with them remotely. These users are also increasingly interested in accessing broadband services without needing permanent wired connections. But in the range of radio frequencies in which Wi-Fi technology operates there is a lack of available bandwidth. Because of this, the speed of data transfer and the capacity of the channels are not enough to enable fast wireless access to multimedia services on the Internet such as browsing, video conferencing, TV broadcasts and movies on demand. Even the faster and longer-range radio systems, such as WiMAX, are not well suited for broadband communication inside buildings, because they are only able to serve a limited number of users within a closed space, and more importantly - they are not able to provide secure communication.
The wireless optical technology is an intriguing alternative. Instead of transmitting radio waves, short-range optical wireless networks transmit coded data using beams of white light or beams of invisible infrared light, similar to those of remote control devices. Optical systems can connect wireless digital devices to the Internet connection in the room and through it to the high-speed broadband network that serves the home or building. This rapidly evolving technology offers several advantages: its targeted, interference-free cells (also called basic service areas) allow multiple users virtually unlimited access to broadband. It also provides almost perfect security, because the light - unlike radio waves - does not pass through walls. A wireless optical system is particularly suitable for large business premises with a large number of broadband users close together, such as a factory or office floor that has modular workstations.
Data transmission through light
You may have heard about the "last mile" problem - the high cost of transferring broadband services in the section between the national infrastructure and the stationary users. On the other hand, the optical wireless technology grapples with the problem of the "last few meters" - the difficulty of transmitting broadband data from the connection point to the wireless devices themselves located inside the building.
Researchers tested the idea of internal optical communication as early as the early 80s, when researchers at the IBM branch in Zurich built the first system that actually worked. The technology was frozen for ten years because the Internet was still in its infancy, and the demand for wireless broadband technology had not yet arisen. However, the dizzying development of the Internet in recent years has changed everything.
Engineers describe the short-range wireless communication networks, which operate using light-emitting diodes (LEDs) in the infrared and visible light ranges, as "optical" systems because they transmit data using visible and invisible light waves (or photons) rather than long radio waves or microwaves More. The current wireless optical systems use infrared radiation included in the "optical" part of the electromagnetic spectrum, and its wavelengths are longer than those of visible light, but shorter than those of radio waves. The radiation is transmitted at a very low intensity that humans are unable to sense. When infrared light is emitted at higher intensities, we experience a sensation of heat.
Optical links work at their best when the transmitter is aimed directly at the receiver, as we know in the "point and click" systems in television remote controls and digital cameras. But such an arrangement is not practical for the purpose of connecting an entire office or granting access to the network in a public place such as an airport or a restaurant. To achieve full coverage of a room, the optical networks distribute the data-carrying light beams throughout the space. Beams of infrared light are reflected from the face of every surface - walls, tables, coffee machines and even the faces of the people in the room. The reflections created in this way are spread around the room, so you can point the receivers in any direction. There are some commercial infrared network products that use this method, but the reflected beams from the surfaces create an echo-like phenomenon that makes it difficult for the receiver to determine the accuracy of the data. The echoes can cause data loss and severely limit the speed of data transfer in the network.
Make infrared work
To address the echo problem, my research team at Penn State University developed a wireless optical system that sends multiple copies of the data in an array, or network, of pencil-narrow infrared light beams that fill the space of the room. These light beams transmit at low power and each of them repeats the same signals over and over again. The light beams connect all digital devices equipped with infrared receivers to the connection point to the network, and from there a physical connection to the high-speed data transfer infrastructure. As mentioned, the light beams repeatedly transmit the encrypted signals, and users can move around the room without disconnecting because when they lose contact with one of the beams, they immediately connect to another beam. The device receives multiple identical data at the same time, and this allows it to perform an error check through a simple comparison of the data from several beams and checking their accuracy. The network of beams as narrow as a pencil allows for the fast transfer of signals, at a rate of XNUMX gigabit per second, several hundreds of times the speed of a DSL modem, with few errors in the transfer. Such a system can make wireless broadband access within a building a remarkably easy matter.
We create the network of light beams by passing the coded infrared signal through a special holographic filter known as a "beam shaper", and it distributes them in the desired directions. To create the holographic filter, we first project, from two directions, an image of a grid onto a cheap, light-sensitive plastic sheet. To do this, we split the beam that contains the image into two using a semi-silvered mirror and recombine the light beams using two reflectors. This array illuminates the light-sensitive sheet with the same grid image from different angles and creates a three-dimensional image. When the wireless infrared transmitter sends a coded light beam through the holographic filter, many copies of the beam emerge from it, in the form of a three-dimensional grid.
The beam pattern we use depends on the shape of the room. It is possible to illuminate different areas as needed: in a fan, in a rectangular grid, in concentric circles and so on. For illustration purposes, spaces for general use, such as offices and factories, are often lit with uniform lighting, on the other hand, in an art museum you usually need focused beams that illuminate paintings and sculptures. In the same way, wireless optical networks will work optimally if they concentrate the beams mainly in the areas where most of the workers who use broadband are located and not in the places where there are fewer workers.
The wireless infrared receiver in the room is equipped with a similar holographic filter known as the "fly's eye". The filter helps collect the "answer" beams transmitted by the digital devices in the return path. It channels the signals received from many directions to separate light detectors and improves reception by combining the energy of all the light beams.
Wireless with light bulbs LED to build
The wireless optical systems based on infrared light will likely be replaced at some point by local area networks based on white LEDs, which offer even greater bandwidth and other advantages. LED technology is increasingly seen as a replacement for normal lighting, and it can also provide broadband data transmission at the same time.
White LED bulbs combine the low power consumption of fluorescent bulbs and their longevity with the eye-pleasing light spectrum of incandescent bulbs. According to industry experts, within a few years mass production of silicon chips emitting white light will begin. The production of LED bulbs using traditional methods used today to produce integrated circuits will reduce their price until they are cheap enough to replace the compact fluorescent bulbs, which today are considered the most economical means of lighting. What has not yet penetrated the general consciousness is that the white LED light technology that may one day illuminate rooms and other interior spaces very efficiently and at a very low cost, can at the same time also provide broadband wireless digital access to all the digital devices in that space that are properly equipped. When you turn on a white LED light, your wireless device will be able to receive broadband transmissions at the same time through the same light that appears to illuminate the room.
Unlike other light sources, it is easy to adapt the LED to the role of transmitter for wireless communication in visible light, an idea first proposed a few years ago by a team of researchers at Keio University in Japan. The fast response time of the LED allows the bulb to be turned off and on millions of times per second, or at a frequency of megahertz, in a kind of high-tech flag signaling. This makes it possible to modulate the visible light for coding wireless communication. According to the results of the initial experiments conducted by my research group, it is possible to modulate a white LED component, common in the market, to transmit signals at a frequency of up to 100 MHz. Such a high frequency signal is much too fast to be picked up by the human eye.
Wireless communication inside a building using white LEDs has several advantages over Wi-Fi networks and even over infrared light networks. Since the white LEDs may be installed in the future for indoor lighting, it will probably be easier to install a wireless system based on them than to install most other wireless systems. Apart from that, the weakening of the signal due to day-to-day objects in the room that block transmissions from point to point - a weakening that we call shadow - will be reduced to a minimum because the white LED lights will be placed all over the room anyway. Lamps installed on the ceiling will be especially useful because the chance that their light rays will be blocked is smaller. As with all optical systems, the white LED technology is not sensitive to interference from light signals of other colors and offers a huge bandwidth for communication.
It is important to note that the occupants of the room will be able to turn off the lights at night and still use their laptops and other devices, because even when the LEDs are "off" and dark, a low current power supply will still allow them to release enough "residual photons" for wireless communication. An alternative approach would be to design white LED systems that would also incorporate a cheap source of invisible light that would transmit the data when the lights in the room are off.
The researchers still need to solve some problems in the technology of short-range networks using white LEDs. A key step will be to design the system of repeating signals, with the help of which the wireless devices will communicate with the white LED lights and through them with the infrastructure systems for the transmission of information. For example, the engineers will be able to install transmitters in them (on electronic cards that can be connected to existing equipment) that will produce another, invisible wavelength (perhaps infrared). These light sources will send coded light beams to the white LED bulbs that will be equipped with small photodiode receivers. Alternatively, the entire system, both transmission and reception, could operate on one wavelength of visible light and take advantage of the fact that the LEDs "blink" at very high frequencies (most of the time they are on with very short pauses of switching off). The return signals from wireless devices can be transmitted to the receivers during the pre-set off pauses of the LED lights. Engineers call such a technique time division duplex (TDD). Whichever solution is chosen, the additional equipment will make the system somewhat more expensive.
The developers of white light systems must also take into account the possible negative effects, which have not yet been tested, of natural and artificial light that comes through windows or from other sources. Before working systems can be created, additional simulations and experiments must be performed to determine the best balance between indoor lighting and communication. Finally, the researchers will have to create effective techniques for coding, decoding, modulation and multiplexing of signals in the visible light field, which will be suitable for both illumination and communication.
Broadband in the electricity network
Both types of wireless optical technology could greatly benefit from a promising method for connecting broadband along the "last mile" to the stationary end users via the power lines (BoPL). With this method, data arrives at high speed through the low or medium voltage power grids to the sockets in the room. BoPL technology uses the cables of the existing home electrical network, and also adds the broadband transmission to them. Companies in Ohio, Texas and elsewhere in the US already offer BoPL service to consumers at prices similar to ADSL. Many users in Europe and Asia (for example, Spain, Sweden, Norway, the Netherlands, South Korea and Japan) already receive Internet services through BoPL because the structure of the power grids in these countries makes it easier to adapt. Small and inexpensive adapters that connect users to the electrical outlets in the wall enable this broadband optical-wireless "bridging". The adapters transmit the data via infrared lights to all the digital devices in the room equipped with optical receivers. In buildings that are lit with white LED lights, there will be no need for adapters.
My research team has shown that a system of white LEDs for lighting and high-speed wireless communication, combined with BoPL technology, can transfer data at a rate of up to a gigabit (billion bits) per second, faster than standard DSL (which is 4-2 megabytes -bit per second, meaning 4-2 million bits per second at most) or of the TV cables (which is about 50 megabits per second on average). This maximum rate is limited only by optical path differences in rooms of certain shapes and sizes, which can increase signal distortion. Repeated reception of the same message, if not properly processed, causes such distortions. On the other hand, if the engineers design the system properly, they will be able to keep such distortions at tolerable levels and even utilize the multiple copies to increase the quality of the broadband service transmission to the end users.
Whether their systems use infrared light or visible light, users of wireless digital devices will soon have a new way to ride the broadband into the future. The wireless optical technology is well adapted to serve as a bridge that will allow digital access from the network, through the last few meters, to the place where we live and work.
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[Head to Head]
Radio vs. Infrared
Systems with non-directional (or distributed) infrared technology, which scatter coded light by reflecting the light beams from surfaces in the room, offer advantages over systems that are based on radio frequencies for wireless broadband data transmission within rooms.
transfer speed
Radio: Maintaining safe power levels that will not harm the occupants of the room limits the maximum data transfer rate to a few hundred megabits per second.
Infrared: data transfer speeds of XNUMX gigabit per second.
Maximum bandwidth
Radio: Since radio signals sent on the same frequency block each other, the authorities monitor the transmission frequencies and thereby limit the available bandwidth.
Infrared: the light rays do not block each other. The possible bandwidth is therefore limited by the maximum rate at which the photodiodes can receive incoming data and prepare to receive additional data.
Security
Radio: Radio waves pass through walls and allow listening.
Infrared: light waves do not pass through walls, therefore limiting the possibility of ignition.
Decay due to multiple routes
Radio: When coded radio waves bounce off conductive surfaces, they often arrive at different times. Sometimes, the returns arrive with such a phase difference that they cancel each other out due to the interference phenomenon.
Infrared: the sensors in the active area of the photodiode receive the waves separately and average the incoming energy, so that no destructive interference can occur.
Main source of noise:
Radio: Interference from other users transmitting on the same frequency slows down the transmission speed.
Infrared: random signals from environmental light sources - the sun, lamps and so on - slow down the transmission speed.
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About the author
Stored in Coward (Kavehrad) holds the W. L. Weiss Chair in Electrical Engineering and serves as the founding director of the Center for Information and Communications Technology at Pennsylvania State University. Before joining academia, Coward worked at Bell Labs. He received his doctorate in electrical engineering from New York Polytechnic University in 1977. Coward, a member of the Institute of Electrical and Electronics Engineers (IEEE), enjoys reading and writing poetry.
In the topic:
Spot-Diffusing and Fly-Eye Receivers for Indoor Infrared Wireless Communications. G. Yun and M. Kavehrad in Conference Proceedings, IEEE Wireless Communications, June 1992.
Fundamental Analysis for Visible-Light Communication System Using LED Lights. T. Komine and M. Nakagawa in IEEE Transactions on Consumer Electronics, Vol. 50, no. 1, pages 100–107; February 2004.
Short-Range Optical Wireless Communications. Dominic C. O'Brien and Marcos Katz. Wireless World Research Forum (WWRF11), Oslo, June 2004.
Hybrid MV-LV Power Lines and White Light Emitting Diodes for Triple-Play Broadband Access Communications. M. Kavehrad and P. Amirshahi in Achieving the Triple Play: Technologies and Business Models for Success. International Engineering Consortium, 2005.
Transmission Channel Model and Capacity of Overhead Multi-Conductor Medium-Voltage Power-Lines for Broadband Communications. P. Amirshahi and M. Kavehrad. IEEE Consumer Communications and Networking Conference, Las Vegas, January 2005.
