Tech Scan

TD-SCDMA suitable for countries with defined TDD bands

Pradeep Chakraborty

Time-division-synchronous code-division multiple access (TD-SCDMA), a 3G technology co-developed by Siemens AG and the China Academy of Telecommunication Technology (CATT) is said to be the only technology suitable for time-division duplex (TDD) bands, and could well emerge as the surprise package in China, should it get the government's nod.

TD-SCDMA has already been approved as a global standard by the ITU and standardized in the 3GPP, and is not in a minority. It is considered more spectrally efficient for both asymmetrical and symmetrical data services and quite capable of dealing with 'hotspot scenarios.' Should it be deployed in China as a global standard, addressing all cell sizes, the necessary effects of scale would be available for operators' worldwide.

Three essential technological aspects set TD-SCDMA apart from other 3G technologies. The TDD transmission mode uses the same frequency band for uplink and downlink. Next, joint detection -- a mathematical operation performed in the receiver, minimizes the multiple access interference by subtracting all signals from other connections from the received signal, thus improving the processing gain and increasing the receiver's sensitivity and the loading factor of the cell. Finally, the smart antenna arrays used on the base station perform real beam steering. It means, the base station determines the terminal's position in a cell during the uplink and redirects energy toward the terminal in downlink. This is performed in every timeframe, making it an adaptive process.

Convergence Plus caught up with Marco Principato, who is responsible as director for partnering and promoting TD-SCDMA at Siemens Information & Communication Mobile (ICM) Group, in Munich, Germany. Excerpts from an interview:

Convergence Plus: Why is TD-SCDMA considered more spectrally efficient than the other 3G technologies?

Marco Principato: Spectrum efficiency denotes how well a specific technology uses the radio spectrum. It is given in Kbps/MHz/cell, in order to allow the comparison between different technologies. Furthermore, since the spectrum efficiency is affected by parameters such as buildings, velocity of the subscriber, the general physical environment, several other scenarios are considered as well.

The 3GPP Group has defined a number of scenarios in order to make a fair analysis. Spectrum efficiency data is determined through extensive system simulation campaigns based on mathematical models of a real network. TD-SCDMA has shown to achieve high spectrum efficiency, achieved through the combined use of TDD as a transmission mode, joint detection, smart antennas and the absence of soft handoffs.

Convergence Plus: What role does the smart antenna play in TD-SCDMA?

Marco Principato: Smart antennas are among the basic technologies of a TD-SCDMA system. Here, a difference should be made between a diversity and a switched beam antenna, having fixed propagation characteristics [these are already being used in GSM networks for a long time now] and real-adaptive, phased-array smart antennas, which are employed in TD-SCDMA. Such antennas allow terminals to be followed as they move within the cell, very much like a spotlight following an artist on a stage. Smart antennas bring about gains in capacity and range.

Convergence Plus: But W-CDMA and cdma2000/1x proponents say performance of such networks can be accelerated using smart antennae? Would this make those technologies on par with TD-SCDMA?

Marco Principato: First, one has to differentiate between real smart antennas [such as those used by TD-SCDMA] and what are essentially diversity antennas. Smart antennas adapt to changing conditions by analyzing the uplink signal from the terminal to the base station in order to determine its position. This information is used to adapt and transmit the downlink signal to the terminal. In a TDD technology such as TD-SCDMA, uplink and downlink occur in the same frequency band. Since radio signals are strongly affected by the frequency at which they operate, the measurements made in the uplink in a TDD system can be efficiently used for the downlink.

This is not the case in an FDD system! The uplink and downlink signals are transmitted in two separate bands, which have different propagation properties, so that what gets measured in the uplink does not necessarily apply in the downlink. As a basic concept, smart antennas are better suited for a TDD system than for an FDD system. In fact, smart antennas have already had an extensive success in PHS systems - also a TDD technology - but they are very rare in GSM - an FDD system. Furthermore, there are certain implementation aspects, which make smart antennas easier and cheaper to implement in a TDD system than in an FDD system.

Convergence Plus: Why should operators go for a TDD technology such as TD-SCDMA when W-CDMA and cdma2000 are FDD technologies?

Marco Principato: The choice of an operator for opting for a TDD or FDD technology is driven by many factors - foremost by the availability of unpaired spectrum. Beyond that, the choice is determined by which technology better suits the current and the future environment expected in the market.

For example, the European operators have both TDD and FDD licenses. But the North American operators are in a different situation: they do not have additional spectrum specific for 3G, but will need to deploy 3G in their current PCS licenses - which consist of paired spectrum only. However, should the FCC make additional spectrum available, it is certainly easier to find unpaired slices of spectrum than paired bands of sufficient bandwidth necessary for the FDD technologies.

Convergence Plus: Is there then a compelling case for TDMA operators to take the migration path via the TD-SCDMA route, instead of the other 3G technologies?

Marco Principato: Most TDMA operators are choosing GSM as a first step. This can bring them to W-CDMA as well as to TD-SCDMA. The evolution path (FDD or TDD or, as in Europe, both) is determined by the spectrum availability.

Convergence Plus: Will there be enhanced versions of W-CDMA and cdma2000 1x/3x by the time TD-SCDMA sees light of day?

Marco Principato: No technology is static! You know that cdma2000 will evolve from the 2.5G technology of 1x to the 3G technology of 1xEV-DV. Currently, no evolution to 3x is being seriously considered. Similarly W-CDMA and TD-SCDMA, both part of the UMTS standard and themselves evolutions of GSM/GPRS 2.5G technology, will evolve through IP based RANs and HSDPA, both included in Release 5. A Release 6 of UMTS is already planned as well.

Convergence Plus: If GSM carriers opt for this technology, wouldn't it be a major threat to W-CDMA?

Marco Principato: No. GSM carriers cannot simply choose one or the other (or both), but have to follow the restrictions implied in the licenses. In Europe, for example, most operators have both FDD and TDD licenses. W-CDMA is being deployed in the FDD licenses, and deployments in the TDD bands will be made when optimizing the network configurations in order to support higher subscriber densities - such as in urban centers - and higher data rates. These deployments will probably take place in Europe in the 2005-2006 timeframe.

The situation in China is a little different, in that the regulator has not yet defined how the 3G spectrum will be licensed, nor how many licensees there will be, or how much spectrum will be used for FDD licenses and how much for TDD licenses. Indeed, in China, an operator could be put in the position, depending on which licenses are made available, to deploy TD-SCDMA or W-CDMA for their entire networks [in this sense they are competitors], or to deploy both, like the Europeans. The key factor is in any case the spectrum license.

Convergence Plus: How does TD-SCDMA match up with public WLANs?

Marco Principato: WLAN has recently attracted quite a bit of attention. However, first we should be careful not to lump an "anytime-anywhere-technology" and a "sometime-somewhere-technology" together. As an unlicensed technology, it has to share the spectrum with a lot of other devices, such as Bluetooth, microwaves and so on. An operator offering (and charging) a public WLAN service will never be able to really guarantee a service quality.

TD-SCDMA, operating in a licensed environment, has all the characteristics an operator needs in order to offer a consistent, reliable service. Furthermore, TD-SCDMA is fully integrated into GSM and W-CDMA networks under many points of view: administration, billing, services, and even terminals. WLAN is centered round data-exclusive services [with a laptop or a PDA], whereas TD-SCDMA is an integral part of the design optimization of an entire network, for both voice and data services. In fact, a user moving from a W-CDMA cell to a TD-SCDMA cell will not notice anything.

Convergence Plus: What is the current status regarding the adoption of the technology in China?

Marco Principato: There continues to be strong interest on behalf of the Chinese authorities and the operators. Together with our partner Datang Mobile, we have successfully passed the first phase of MII's (Ministry of Information Industry) official 3G tests - called MTNet tests, and are continuously demonstrating the advances and the performance achieved by the technology. The Chinese government has not yet issued a decision concerning the adoption of a particular technology, so the operators are de facto not allowed to make decisions right now. However, the successful results of the tests and the demonstrations on our trial network in Beijing make us feel confident that TD-SCDMA will be successfully adopted in China.

Convergence Plus: When are going to see TD-SCDMA rolled out in China?

Marco Principato: As mentioned, we have already "rolled out" a small test network in Beijing. Trial networks with customers will be deployed as soon as the Chinese authorities issue trial licenses to the various operators - incumbents and greenfields. We expect that this will take place within some months. Large-scale commercial network deployments will be possible in the middle of 2003, again pending the issuing of operating licenses to the operators.

Convergence Plus: Where else can TD-SCDMA be deployed, besides China?

Marco Principato: TD-SCDMA can be deployed in all those countries that have defined TDD bands. Such countries include all of Europe, most of Asia-Pacific (Japan and Korea have not awarded TDD licenses, other countries, like China, have not yet awarded any licenses). Siemens is in fact offering TD-SCDMA information on demand to operators. However, deployments here will not take place until the 2005-2006 timeframe.

Convergence Plus: If the above does not happen, will there be a case of the technology locking itself out?

Marco Principato: Siemens is focusing on China at the moment, as this will have a major impact on other countries. We are confident of being successful in China.