OPTICAL WIRELESS COMMUNICATIONS
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t is well known that a technology becomes and usability constraints, hut also with eye safety ubiquitous when it is incorporated seamlessly constraints. Short-range optical wireless links within many products enteringour homes, when are usually power-budget-limited. The maturing of optical transmitter, receiver, it isinvisibleand yet useddaily indeviceswedepend on, without us even realizing the presence of and optics technologies, and understanding of free space channel intricacies have been invaluable the underlying specific technology. We can quote motor technology, microprocessors, and to overall system development. Indoors, for computers as examples. It is not untrue to say example, the ambient light noise intensity at the that to some extent optical wireless links are A~~~~~~ c, receiverfluctuatessignif~antlydependingon theproxalso ubiquitous. There are two or three infrared Boucouvnlns imityofthenoisesource tothedetector, andcouldarise (1R)remotecontrols inourhomes,so theyalready mainly from tungsten lamp sources, fluorescent influence our daily habits. We can all imagine lights, or diffusesunlight.Interference could occur from the frustration of most people living a day without IR conother products radiating 1R such as TV remote controls or trols and struggling to deal with the TV, DVD, or music IR music headphones, or from neighboring users, if there is center manually. no medium access control. The modem historyofopticalwirelesscommunications(free Outdoor terrestrial links are subjected to different chalspace optical links) predates that of fiber optics. For examlengessincethechannelissubjecttoweatherfluctudtionsaffectple, point-to-point communications using optical pipes with ingthe powerbudget, and, dependingonlocation, could perhaps he influenced by the sun. lenseswereextensivelybeingstudied at BellLaboratoriesprior to the development of optical fibers, and IR links have subEye safety in all situations is very important, and operatsequentlybenefitedfromfiberopticcomponentdevelopments, ing within the safety margins as specified by IEC and other although they do not compete for the same market. Optical regulations is part of system design; this can be a frustrating and often costly consideration. wirelesscommunicationsareinuse today inmanydiverseapplications, offering very-high-speed wireless links cost effecThe articles in this special issue cover three user scenartively. ios: IrDA short-range indoor, diffuse IR-LAN, and outApplication user models dictate the product specificadoors free space links with range on the order of 2 km. tion, and the design and engineering bf optical links which Eachscenariohasitsownintricaciesandisuniqueinitsown isuniquelytai1oredtotheproduct.Theproductvariantsofoptiright. Forthe IrDAusermodel, thechallengesarelowcost (under cai wireless links range from very distant intersatellite links to very short optical interconnects. $9,low hardware real estate, high data rate, very high In common with ather systems, the technology growth dynamic range, low power consumption, and low bit error depends on the user problems it successfully solves, technorate (BER) links. The diffuse IR LAN user model designs rely on the economic issues, and developmentshreakthroughs in constituent component technologies. Possible applications are emitted radiation being diffused by reflections in a uniform numerous, spanning from payment systems, TV remote manner fromwallsand ceilings.Thisisnot easilyacbieved,concontrol, Infrared Data Association (IrDA) ports for watchsideringreflectionlosses, and thereareanumberoffutureproes, printers, laptops, and mobile phones, to last milebroadband posals for multibeam diffusersand receiver diversitytechniques access and intersatellite links. for improving the performance of such systems without sacForhigh-volumeapplicationstherequirementsareforlow- rificing too much bandwidth. The cost of such systems is cost short-range high-dynamic-rangelinks,robustnesstoambihigher and is an important consideration ($50-$400). ent noise, interference, and availability of standards to The outdoors free space links are a few kilometers long allow interoperability between products. a n d require high receiver sensitivity, the use of lasers Thecomputerindustryhasforanumherofyearsdeveloped, instead of LEDs for transmitters, telescopic optics for creating and aligningifocusing the beam, and a means of comthrough the IrDA, standards for wireless data links. The main purpose of those standards is to offer low-cost reliable pensatingforchannelvariationduetoweatherconditionssuch connectivity between devices. IrDA has produced standards as rain, fog, and snow. The biggest challenge for outdoor links is offering cost-effective high link availability under all for 115.2 kbis, 4 Mbis, and 16 Mbis optical wireless links. IEEE 802.11 has also produced a wireless LAN specificaweather conditions. More expensive components of suitable specification are needed in this case, reflecting on the cost tion for optical wireless PHY. of such systems ranging from $1000 up to $10,000, dependIndoor remote control and interdevice connectivity therefore has proven to be a fertile market for optical wireing on the application and specification. This issue begins with an interesting comprehensive lowless. Optical wireless products must comply not only to cost level comparison of RF with IR since it helps clarify the dif-
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IEEE Wirelcss Communicatiom * April 2003
ferences between the two technologies. The recent growth of Bluetooth and 802.11 products with R F transceivers at the physical layer makes this comparison useful in positioning 1R and R F technologies. The modulation scheme is critical for efficient operation of I R links. Apart from bandwidth and power efficiency, short-rangeIrDAlinksmustoperatefromcontactto 1 m, which respondsto adynamicrange on theorder of50 dB. The receivers must cope with intersymhol interference and large variations of DC optical power via diffuse lights and other modulated optical sources causing R F interference in the signal band. Careful choice of encoding schemes is very important in helping to alleviate those problems and at the same time facilitate the clock extraction process. The purpose of the second article is to discuss various modulation schemes for short-range IrDA links, and introduces E-PPM as a possible future candidate for such links. Good short-range IR link engineering is an ingenious and lean process, more often a technical compromise in cost-effectively satisfyingproduct specification. IrDA has recently completed a standard on the use of IR in financial transactions and payment systems. The Infrared Financial Messagingstanddrd (IrFM) is a means tostandardize the way payments can he made using I R connections to point-of-sale devices. The mobile phonecould heused as aplatform for IrFM. If adopted widely i t will spread the use of lrDA technology dramatically. IrDA is also exploring ways t o standardize even faster standardinterfacesoffering 100Mh/soreven 1Gbisdatarates. The third article dealswith a study on how to optimize throughput at thc link layer and offers design rules for an adaptive link layer for any BER link. T h e proposed system adapts the window and frame sizes for maximum throughput. Such techniques may become useful in future very-high-data-rate lrDA products. Next, wemoveonfromtheIrDApoint-and-shoot usermodel to the indooropticalLA”ode1, where many user scan heconnected to each other without the need for alignment. A transmission from a user can he received by any other utilizing multiple reflections from t h e room walls and ceiling, thus emulating optical ether. This is a more difficult challenge; one of the issues is uniform distribution of the optical power across the room while maintaining a system capacity advantage over RF. T h e fourth article examines this challenge, and discusses multispot diffusing techniques and their potential for future high-capacity systems,
IEEE Wirelccs Communications April 2003
F o r the same diffuse LAN systems, the receiver rate adaptivity and angle diversity reception a r e new ideas examined in the fifth article. There is no shortage of ideas and research efforts o n how to deploy broadband connectivity t o the home in a cost-eff_ectiveway. Optical wireless is a credible solution offering low installation cost with minimum disruption and in a timely manner, compared to wired technologies, without regulatoryrestrictions. Thechallengeshere are the achievement of 99.999 percent link availability, especially in foggy outdoor conditions and building sway. The sixth article offers an excellent discussion of the issues and describes a hybrid IR/RF system solution for last mile applications. The seventh article argues the case for the use of spread spectrum techniquesfor indoor IRsystems, and proposes future IR high-data-rate systems using DSSS techniques to combat ambient light interference and multipath effects. The final article is a study ofthe possible applicationofmore advanced receiver techniques, explaining the principles of a receiver using wavelets and neural networks for the extraction of signal from noise and for more efficient bandwidth utilization of possible future IR receivers. It remains for me to thank the authors and reviewers for their great effort in producing this special issue. I hope you enjoy it, and that it may stimulate more creative research ideas in using this technology.
BIOGRAPHY ANTHONYC. BOUCOUVALAI IF1 (
[email protected]) graduated with a B.Sc. in electrical and electronic engineering from Newcastle upon Tyne university in 1978. He received his M.Sc. and D.I.C. degrees in communications engineering in 1979 from Imperial College. where he also received hirPh.D. degreeinfiberopticrin 1982. Subsequently. hejoined GECHirrt Research Center, and berameagroup leaderand divirionalchiefscientirtwarkingan fiber optic components. measurements. and seniors until 1987. when he joined Hewlett Packard Laboratories as project manager. At HP he worked in the areas of optical communication systems. optical networks. and instrumenta~ tion, until 1994, when he joined Bournemouth University. In 1996 he became a professor of multimedia communications and in 1999 became director of the Microelectronic$ and Multimedia Rerearch Center. His current rerearch interests lie in components. optical witelexs communications. Internet communications. optical fiber communications. multimedia communications. and human-computer interfaces, and he har published over 130 papers in the areas of fiber optics. optical fiber components. optical wireless communication5 and Internet communications. and HCI. He i5 a Fellow of the Royal Society for the Encouragement of Arts, Manufacturers and Commerce and a Fellow of IEE. He i s a member of the New York Academy of Sciences and ACM. He is an Editor of IFEF Wireless Communications and IEEE Transactions on Wireless Networks. and Secretary of the IEEE UK&RI Communications Chapter. He is on the Organizing Committee of the Internationalsymporium onc~mmunicati~nsyrtemr Netwarband Digital Signal Procesring, and a member of Technical Committees for numerous conferences. He can be reached a t h t t p : l l d e c . b o u r n e m o u t h . ~ ~ . ~ W ~ t ~ f f.htm ltb~~~~~~~l~~t~~~l
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