Five reasons why TD-CDMA outshines WiMAX in wireless broadband
TD-CDMA has a number of technical advantages that make it a strong contender in the wireless broadband market. Here's a look at five of the key benefits TS-CDMA provides to a broadband wireless design.
By Chenwei Yan, UTStarcom, Inc.
Courtesy of CommsDesign
(02/14/2005 5:43 AM EST)
In today's complex broadband environment, demand is rising for ubiquitous wireless broadband coverage and for bigger and faster wide-area wireless pipes that can handle the bandwidth required for applications such as video broadcasting, Internet browsing, Voice over IP (VoIP), and more. Cellular operators and Internet Service Providers (ISPs) alike want a wireless connection solution that can also provide a migration path to full mobility similar to that offered by existing cellular systems. The availability of such a wireless broadband solution is especially important for operators that do not own cable or telephone access lines, since the solution will give them the opportunity to reach subscribers directly.
Two options supporting wireless broadband applications are time division-CDMA (TD-CDMA) and WiMAX. WiMAX has been hailed as the hot new metropolitan-area wireless standard, even though it is not yet complete and real products are still two to three years away. TD-CDMA complies with the 3G Partnership Project (3GPP) Universal Mobile Telecommunications Systems Time Division Duplexing (UMTS TDD) standard, and many operators have already deployed and are today generating revenue from TD-CDMA-based wireless networks. As part of 3GPP, TD-CDMA has the backing of a large international standards body and large number of operators and equipment vendors. WiMAX, on the other hand, is being developed by a small industry group.
Compared to WiMAX, TD-CDMA has a number of technical advantages that make it a strong contender in the wireless broadband market. Below, we'll show you five reasons why TD-CDMA may be a better option than WiMAX in broadband wireless designs.
1. Bigger Cells, Reduced Expenses
A wireless technology's cell coverage area is of key importance, since operators can reduce their initial capital expenditures if they can serve the same area with fewer base stations. TD-CDMA has a clear advantage here; cell coverage of up to several kilometers has been proven in major operator trials and commercial deployments. To achieve this coverage, TD-CDMA employs advanced power control mechanisms that allow the data throughput to gradually decrease as a terminal gets further and further away from the base station.
WiMAX calls for use of an orthogonal frequency-division multiplexing (OFDM)-based modulation technique, and coverage has not yet been proven in an actual network. Theoretical analysis shows that cell coverage is less than 280 meters outdoors using OFD multiple access (OFDMA), and less than 450 meters outdoors using OFDMA in the 2.6 GHz band.
2. Fixed Wireless Now, Mobile Later
The ability of a technology to support mobility -- even for operators that primarily wish to deploy a fixed wireless solution -- is important for two reasons. First, the ability to carry the PCMCIA modem anywhere is an important value-add differentiation and selling point for operators, even though most of the time the user is stationary. Second, a wireless broadband technology that supports mobility now, unlike one that supports only fixed applications, gives operators an easy migration path to mobile applications in the future.
Mobility and portability must be supported on the subscriber terminal, since traditionally fixed modems have a very different form factor than mobile devices. One approach to resolving this problem employs a detachable PCMCIA card that can be used with a range of fixed subscriber terminal boxes (CTEs). For stationary applications, the user can insert the PCMCIA card into a CTE, and for mobile applications, the user can remove the PCMCIA card from the CTE and insert it into a laptop computer. In the future, TD-CDMA and dual-mode handsets that take advantage of the mobile network will be available.
TD-CDMA subscriber terminals can support mobility by automatically detecting the signal strength of surrounding base station cells and informing the network of a better signal before the existing connection is broken. Automatic signal detection thus enables a transparent handoff as the user moves from the coverage area of one base station to another.
Mobility also requires the support of both the radio access network (RAN) and the core network and is much easier to achieve when the entire network is based on the same standard. TD-CDMA uses the same mature, core networks already widely deployed for UMTS/GSM, and handoff from one piece of equipment to another is handled by well-defined 3GPP standards and protocols.
The first release of WiMAX is defined only as a broadband RAN and does not support mobility. While it is claimed that later versions of WiMAX will support mobility, the lack of standardization makes it difficult to integrate WiMAX-based products from different vendors. Although some vendors say they are working on mobile WiMAX, industry observers are skeptical this will happen soon, if at all, especially as there is increasing recognition that UMTS-TDD already offers mobility as well as high speeds.
3. VoIP Supprt
Normal shared-channel implementation for packet data is great for achieving bandwidth efficiency but presents a big challenge for packetized voice traffic. TD-CDMA technology resolves this problem by supporting dedicated air interface channels for voice-over-IP (VoIP) traffic when a user voice session is set up, as well as by supporting end-to-end quality of service (QoS).
Dedicated bandwidth for voice packets over the air-interface is critical, as voice packets must be delivered with minimal latency to ensure sound quality. TD-CDMA, together with the 3GPP-defined core network QoS, ensures that neither the air-interface nor the core network becomes a bottleneck for voice traffic, traditionally a critical limitation for voice services over a shared packet data network. TD-CDMA VoIP is being demonstrated in live field deployments today.
WiMAX does not address VoIP yet, and QoS is an end-to-end effort that requires support from the subscriber modem to every piece of equipment in the network. Unlike 3GPP, WiMAX does not define the behavior of each piece of equipment in the network, making QoS a complex task.
4. Global Roaming
TD-CDMA supports global roaming, since the underlying 3GPP standards were developed for both cellular operators and traditional ISPs. Cellular operators' needs are addressed through support for Home Location Register (HLR) and Universal Subscriber Identify Module (USIM), which are defined for global roaming. ISP roaming needs are met via AAA RADIUS servers and HLR extension functions that can be built into TD-CDMA equipment.
WiMAX, however, does not address or define cellular operators' needs. Because it is being designed to meet the needs of ISPs, WiMAX supports roaming only via AAA RADIUS and does not have well-defined protocols for global roaming. 5. Frequency Band Flexibility
TD-CDMA has the edge over WiMAX in providing the operator with as much flexibility as possible in using available frequency band. TD-CDMA operates in 1.9 and 2 GHz UMTS-TDD band, as well as the 2.5 and 3.4 GHz bands. WiMAX, on the other hand, cannot operate in the UMTS band. Furthermore, European governments may restrict the 2.5 GHz band to UMTS-based technology only in the near future. Thus, TD-CDMA has a much bigger addressable market than WiMAX.
N=1 re-use is also a key part of frequency flexibility and a natural characteristic of all CDMA technologies. With N=1 re-use, operators need to support only one times the frequency spectrum required. WiMAX requires in-band subchannel re-use or frequency hopping if N=1 is used. Both techniques reduce network capacity. In most cases, N=3 reuse is necessary with WiMax and would require operators to purchase more spectrum.
Clear Advantage
While it is difficult to compare all aspects of TD-CDMA and WiMAX technologies in a brief article, even a bird's eye view of some of the issues reveals certain major differences. The clear advantage goes to TD-CDMA, a proven technology that is available now for cellular operators and ISPs alike. Also, because TD-CDMA is part of the 3GPP umbrella, it is fully interoperable with existing UMTS-based networks and provides a migration path to mobile applications, enabling far more flexible use of available spectrum as well as the QoS required by voice applications. These advantages add up to a compelling brief for TD-CDMA as the technology best suited to implement wireless broadband solutions worldwide.
About the Author
Chenwei Yan is a product line manager at UTStarcom, Inc. Chemwei can be reached at chenwei.yan@utstar.com.
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