wireless in action - sys-con.com's archive of magazines - SYS-CON ...

3GIs 3G-TDDthe Future of the Wireless LAN?TIME DIVISION DUPLEXING OFFERS LONG-TERM ADVANTAGES FOR OPERATORS AND USERSbyDr. PrabhakarChitrapuPrabhakar Chitrapu, PhD, is principal engineer atInterDigital Communications Corp. He is also anadjunct professor at Villanova and Drexel Universities,where he teaches courses on wirelesscommunications, signal processing, andcommunication networks. Prabhakar receivedhis B.Tech. from IIT, Madras, India, an MS fromPhilips International Institute, Eindhoven, theNetherlands, and a PhD from Delft University,Delft, the Netherlands.@prabhakar.chitrapu@interdigital.comTDD technology is rapidly evolving as wirelessdata services become more popular and thepricing models less expensive and less confusing.Born in the mobile telecom arena, TDD is stillin development, but is considered to be a viableoption to WLANs for public hotspot high-speeddata applications.Consumer and business demand formobile data services and applicationshas emerged as a primary driver in thenext generation (3G) of wireless communications.In the current bridgingperiod between 3G and existing services,several technology alternatives are beingdeployed to address short-term demands fordata-centric communications, including wirelessLANs.The next phase of growth for WLANs – and themost lucrative – will be in the public domain.Whether in an airport, shopping mall, or coffeeshop, anyone with a device equipped with awireless LAN card will soon be able to access theInternet. Analysys Research in its report, “PublicWireless LAN Access: U.S. Market Forecasts2002–2007,” predicts that more than 21 millionAmericans will be using public wireless LANs in2007. It also forecasts an increase in the numberof hotspot locations, from 3,700 this year to41,000 by 2007. Similarly, Cahners-Instat Groupforecasts that the industry will grow from $1.1billion in 2000 to $5.2 billion by 2005.With significant growth predicted into thefuture, it’s no surprise that many serviceproviders/network operators are closely watchingthe public wireless hotspot market. Beforeany decisions are made on what technology todeploy to capitalize on this rapidly growing sector,operators should be careful to evaluate allpotential solutions. One option receiving considerableinterest as an alternative to computercentricWLAN technology is 3G-TDD (TimeDivision Duplexing) technology, a solution bornin the mobile telecom arena.The Basics of 3G-TDDTwo-way communication systems requireseparate channels – one for each direction oftraffic. Creating these directional channels in thisfashion is called duplexing. Frequency DivisionDuplexing (FDD) separates the channels intoseparate frequencies; Time Division Duplexing(TDD) separates the channels in time.Consequently, FDD requires paired spectrum,whereas TDD requires unpaired spectrum.Both FDD and TDD have been standardizedas third-generation or 3G mobile communicationstandards under the auspices of 3GPP (3rdGeneration Partnership Program), an interna-tional partnership of standards development organizations. ManyEuropean and Asian operators have acquired 3G spectrum, whichincludes both paired FDD and unpaired TDD bands. Generally, thepaired FDD spectrum is planned for providing wide area coverageand, in most cases, the unpaired TDD spectrum, a revenue-generatingasset, has not yet been leveraged.Comparing WLANs and 3G-TDDSo what characteristics make 3G-TDD a viable alternative toWLANs for public hotspot high-speed data applications? Let’s compareand contrast 3G-TDD with WLANs in the key areas of primaryconcern to service providers/network operators and end users:SPECTRUMWLANs operate in the unlicensed spectrum. As a result, otherproducts that transmit energy in the same frequency range – such as aBluetooth-enabled device – can potentially cause some degree ofinterference. This often has a detrimental effect on performance andthe user experience. In contrast, 3G-TDD is designed to operate in thelicensed spectrum, which is a controlled environment where operatorscan guarantee a certain level of service.The downside of course is that the availability of licensed spectrumis limited. However, as mentioned previously, many providersthat have purchased 3G licenses already possess paired (FDD) andunpaired (TDD) bands.MOBILITYComputer-centric WLANs offer “islands” of coverage. Problemscan arise when the user moves from one “island” to another. Whileindustry bodies are working to address the hand-off issue betweenthese “islands,” there is currently no standardized solution toenable comprehensive service roaming without any degradation inperformance.Coming from the mobile telecom environment, 3G-TDD hasbeen designed from the outset to accommodate the needs of themobile user through standards-based handoffs and tight synchronizationwith 3G-FDD. The synergistic relationship between thewide coverage area provided by 3G-FDD and the hotspot coverageof 3G-TDD provides a key advantage. Due to the seamless handoffsthat are possible between 3G-TDD and 3G-FDD systems, a userthat leaves a 3G-TDD coverage area and enters a 3G-FDD coveragearea will not experience any interruption in service, althoughthroughput will be reduced.NETWORK INTEGRATIONIn the public access arena, network integration is a fundamentalrequirement. To provide public access to a WLAN, the variousislands of coverage have to be tightly integrated into a back-endnetwork for common authentication, billing, and other basic functions.Another key driver for integration is the ability to provideseamless access to “always on” position-based multimedia services.While industry bodies are currently defining an industry standardfor integrating WLANs into the core network, it does not existtoday. In contrast, 3G-TDD, due in part to its mobile telecom heritage,has been designed from the outset to be tightly integratedwith a much larger core network. Indeed, this is part of the original3G-TDD specification. 3G-TDD can be used not only in conjunctionwith 3G-FDD, but also as a standalone technology to provideDSL-like service.MULTIMEDIA CAPABILITIESAs an IP-based, computer-centric solution, WLANs are optimizedfor data services, not real-time services such as voice. Workis progressing to address this by adding a radio resource managementsystem. In contrast, 3G-TDD has been designed to deliverboth voice and data dynamically. By establishing simultaneous circuit-switchedand packet-switched connections over the air interface,a user could talk and surf the Web at the same time with thesame device.Due to its mobile telephony heritage, 3G-TDD can also provideaccess to a wide range of high-revenue (for the service provider)and high-value (for the user) data-intensive services such as multimediamessaging services (MMS), synchronization of personal digitalassistants (PDAs), Internet chat, as well as picture, video, andmusic downloads. In addition, 3G-TDD can also provide continuousaccess to location-based services. Delivering these types ofservices is a much more difficult proposition for IP-based WLANsolutions.SECURITYIt is widely known that the current WLAN technology is far fromsecure – not only for demanding m-commerce applications, butalso for simple privacy of user data. In fact the common practicetoday is to disable the inbuilt WLAN security feature and relyentirely on application-level methods to ensure security. Whilestandardizations are currently under way to fix and improve WLANsecurity, industry acceptance and product availability will takemore time. 3G-TDD provides a high level of security to users aswell as network operators with proven techniques that have beenused in GSM systems all over the world.POWER CONSUMPTIONThe power requirements for WLAN devices are high per individualdevice, often reaching 100 milliwatts. This can be attributed totheir roots in a corporate computer-based environment wherepower restrictions were not an issue. Because 3G-TDD wasdesigned to be deployed in mobile handheld devices, great carewas taken to optimize power consumption. To reduce power consumption,3G-TDD devices can leverage “idle mode” or “sleepmode.” Currently, WLAN PCMCIA cards do not offer this option inan optimal standardized way.SCALABILITYAs a consequence of their limited mobility, WLANs are not particularlyscalable. As more and more cells are added, handoffsbecome an issue. And as more and more users are added, performance(throughput) is compromised. By comparison, 3G-TDD isvery scalable and delivers consistent performance over a widerange of coverage scenarios: in building (pico cell), urban (microcell), and suburban (macro cell, when used in conjunction with3G-FDD) settings.QUALITY OF SERVICE (QoS)QoS is determined by the consistency of the user experienceand throughput. The user experience with WLANs can bedescribed as unpredictable, especially when an individual movesin and out of coverage areas. In doing so, the user will see a significantshift in service quality and throughput, depending on the distancefrom the access point. As a result, it is virtually impossiblefor service operators to provide QoS guarantees.Because of the inherent power control of 3G-TDD and intelligencethat will be built into 3G-TDD devices, performance andthe user experience will not vary. As part of its standard, 3G-TDDincludes continuous and real-time data rate services overreserved connections. This enables service operators to guaranteeQoS.3G48 www.WBT2.com O C T O B E R 2 0 0 2O C T O B E R 2 0 0 2www.WBT2.com49