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Is 3G-TDD the Future of the Wireless LAN?

Is 3G-TDD the Future of the Wireless LAN?

TDD technology is rapidly evolving as wireless data services become more popular and the pricing models less expensive and less confusing. Born in the mobile telecom arena, TDD is still in development, but is considered to be a viable option to WLANs for public hotspot high-speed data applications.

Consumer and business demand for mobile data services and applications has emerged as a primary driver in the next generation (3G) of wireless communications. In the current bridging period between 3G and existing services, several technology alternatives are being deployed to address short-term demands for data-centric communications, including wireless LANs.

The next phase of growth for WLANs - and the most lucrative - will be in the public domain. Whether in an airport, shopping mall, or coffee shop, anyone with a device equipped with a wireless LAN card will soon be able to access the Internet. Analysys Research in its report, "Public Wireless LAN Access: U.S. Market Forecasts 2002-2007," predicts that more than 21 million Americans will be using public wireless LANs in 2007. It also forecasts an increase in the number of hotspot locations, from 3,700 this year to 41,000 by 2007. Similarly, Cahners-Instat Group forecasts that the industry will grow from $1.1 billion in 2000 to $5.2 billion by 2005.

With significant growth predicted into the future, it's no surprise that many service providers/network operators are closely watching the public wireless hotspot market. Before any decisions are made on what technology to deploy to capitalize on this rapidly growing sector, operators should be careful to evaluate all potential solutions. One option receiving considerable interest as an alternative to computer-centric WLAN technology is 3G-TDD (Time Division Duplexing) technology, a solution born in the mobile telecom arena.

The Basics of 3G-TDD
Two-way communication systems require separate channels - one for each direction of traffic. Creating these directional channels in this fashion is called duplexing. Frequency Division Duplexing (FDD) separates the channels into separate 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 standardized as third-generation or 3G mobile communication standards under the auspices of 3GPP (3rd Generation Partnership Program), an international partnership of standards development organizations. Many European and Asian operators have acquired 3G spectrum, which includes both paired FDD and unpaired TDD bands. Generally, the paired FDD spectrum is planned for providing wide area coverage and, in most cases, the unpaired TDD spectrum, a revenue-generating asset, has not yet been leveraged.

Comparing WLANs and 3G-TDD
So what characteristics make 3G-TDD a viable alternative to WLANs for public hotspot high-speed data applications? Let's compare and contrast 3G-TDD with WLANs in the key areas of primary concern to service providers/network operators and end users:

WLANs operate in the unlicensed spectrum. As a result, other products that transmit energy in the same frequency range - such as a Bluetooth-enabled device - can potentially cause some degree of interference. This often has a detrimental effect on performance and the user experience. In contrast, 3G-TDD is designed to operate in the licensed spectrum, which is a controlled environment where operators can guarantee a certain level of service.

The downside of course is that the availability of licensed spectrum is limited. However, as mentioned previously, many providers that have purchased 3G licenses already possess paired (FDD) and unpaired (TDD) bands.

Computer-centric WLANs offer "islands" of coverage. Problems can arise when the user moves from one "island" to another. While industry bodies are working to address the hand-off issue between these "islands," there is currently no standardized solution to enable comprehensive service roaming without any degradation in performance.

Coming from the mobile telecom environment, 3G-TDD has been designed from the outset to accommodate the needs of the mobile user through standards-based handoffs and tight synchronization with 3G-FDD. The synergistic relationship between the wide coverage area provided by 3G-FDD and the hotspot coverage of 3G-TDD provides a key advantage. Due to the seamless handoffs that are possible between 3G-TDD and 3G-FDD systems, a user that leaves a 3G-TDD coverage area and enters a 3G-FDD coverage area will not experience any interruption in service, although throughput will be reduced.

Network Integration
In the public access arena, network integration is a fundamental requirement. To provide public access to a WLAN, the various islands of coverage have to be tightly integrated into a back-end network for common authentication, billing, and other basic functions. Another key driver for integration is the ability to provide seamless access to "always on" position-based multimedia services.

While industry bodies are currently defining an industry standard for integrating WLANs into the core network, it does not exist today. In contrast, 3G-TDD, due in part to its mobile telecom heritage, has been designed from the outset to be tightly integrated with a much larger core network. Indeed, this is part of the original 3G-TDD specification. 3G-TDD can be used not only in conjunction with 3G-FDD, but also as a standalone technology to provide DSL-like service.

Multimedia Capabilities
As an IP-based, computer-centric solution, WLANs are optimized for data services, not real-time services such as voice. Work is progressing to address this by adding a radio resource management system. In contrast, 3G-TDD has been designed to deliver both voice and data dynamically. By establishing simultaneous circuit-switched and packet-switched connections over the air interface, a user could talk and surf the Web at the same time with the same device.

Due to its mobile telephony heritage, 3G-TDD can also provide access to a wide range of high-revenue (for the service provider) and high-value (for the user) data-intensive services such as multimedia messaging services (MMS), synchronization of personal digital assistants (PDAs), Internet chat, as well as picture, video, and music downloads. In addition, 3G-TDD can also provide continuous access to location-based services. Delivering these types of services is a much more difficult proposition for IP-based WLAN solutions.

It is widely known that the current WLAN technology is far from secure - not only for demanding m-commerce applications, but also for simple privacy of user data. In fact the common practice today is to disable the inbuilt WLAN security feature and rely entirely on application-level methods to ensure security. While standardizations are currently under way to fix and improve WLAN security, industry acceptance and product availability will take more time. 3G-TDD provides a high level of security to users as well as network operators with proven techniques that have been used in GSM systems all over the world.

Power Consumption
The power requirements for WLAN devices are high per individual device, often reaching 100 milliwatts. This can be attributed to their roots in a corporate computer-based environment where power restrictions were not an issue. Because 3G-TDD was designed to be deployed in mobile handheld devices, great care was taken to optimize power consumption. To reduce power consumption, 3G-TDD devices can leverage "idle mode" or "sleep mode." Currently, WLAN PCMCIA cards do not offer this option in an optimal standardized way.

As a consequence of their limited mobility, WLANs are not particularly scalable. As more and more cells are added, handoffs become an issue. And as more and more users are added, performance (throughput) is compromised. By comparison, 3G-TDD is very scalable and delivers consistent performance over a wide range of coverage scenarios: in building (pico cell), urban (micro cell), and suburban (macro cell, when used in conjunction with 3G-FDD) settings.

Quality of Service (QoS)
QoS is determined by the consistency of the user experience and throughput. The user experience with WLANs can be described as unpredictable, especially when an individual moves in and out of coverage areas. In doing so, the user will see a significant shift in service quality and throughput, depending on the distance from the access point. As a result, it is virtually impossible for service operators to provide QoS guarantees.

Because of the inherent power control of 3G-TDD and intelligence that will be built into 3G-TDD devices, performance and the user experience will not vary. As part of its standard, 3G-TDD includes continuous and real-time data rate services over reserved connections. This enables service operators to guarantee QoS.

An argument often made against 3G-TDD concerns the significant data rates associated with WLANs. At first glance, the WLAN peak data rate of 11Mbs/second (based on the 802.11b standard) is significantly faster than the 2Mbs/second maximum offered by 3G-TDD. However, these numbers bear further analysis. The WLAN signals occupy approximately four times more bandwidth (20MHz) than 3G-TDD signals(5MHz). So ideally, in 20MHz, 3G-TDD can support 4x2=8Mbps compared to 11Mbps.

Once the user moves away from the access point, the WLAN data rates are lowered to 5.5 and 2Mbps, with 2Mbps being typical at around 200 feet. In contrast, the 3G-TDD data rates are not lowered, but maintained irrespective of the user location in the cell due to power control. In other words, in the same 20MHz bandwidth, at cell boundaries, WLANs support 2Mbps whereas 3G-TDD supports the full 4x2=8 Mbps!

The actual data rates experienced by the WLAN users (referred to as throughput) are even lower due to three reasons: signaling/overhead bits, idle guard times, and collisions between data from multiple users. To illustrate these points, the 11Mbps data rate supported at locations close to the WLAN access points decreases to 6Mbps for one user, and to 2.5Mbps for 50 users (shared!). At the WLAN cell boundary, these rates fall to 2Mbps and 1Mbps respectively. In 3G-TDD, the stated data rates are not decreased because signaling/overhead bits and idle guard times are already taken into account and because data from multiple users is multiplexed in such a way that there will never be collisions. Figure 1 depicts these ideas.

WLANs are available now, while 3G-TDD is still under development. However, solutions for many of the shortfalls of WLANs are also currently under development. Technology trials for 3G-TDD systems should be under way in the second half of 2003. 3G-TDD - because it has been designed from the ground up to operate in a mobile environment - has the potential to provide a more future-proof solution than WLANs.

Product Cost
It is clear that 3G-TDD will be less expensive and cleaner to integrate into a multimode handset because of the high degree of harmonization between 3G-FDD and 3G-TDD. For example, radios, bandwidths and, most important, the protocol stack, can all be shared between 3G-FDD and 3G-TDD. Providing add-on multimode capabilities for WLAN devices will be more cumbersome due to the many disparate elements required. While technically feasible, in the cutthroat business of providing low-cost terminals, this could be important.

For operators currently evaluating WLANs or watching the public hotspot high-speed data market, 3G-TDD is clearly a future-proofed solution that is gaining recognition. Its mobile telecom heritage may in fact be superior for many applications in the emerging 3G wireless environment. Because billing, authentication, location-based services, and other valuable features are built into the 3G specification that 3G-TDD leverages and is built on, it has the potential to grow into a ubiquitous service that has long-term financial and operational advantages for service operators and users alike.

More Stories By Prabhakar Chitrapu

Prabhakar Chitrapu, PhD, is principal engineer at InterDigital
Communications Corp. He is also an adjunct professor at Villanova
and Drexel Universities, where he teaches courses on wireless
communications, signal processing, and
communication networks. Prabhakar received
his B.Tech. from IIT, Madras, India, an MS from Philips
International Institute, Eindhoven, the Netherlands, and a PhD from
Delft University,
Delft, the Netherlands.

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