TDD to Play a Critical Role in 3G Designs
10/26/99 By: Marc Barberis, Synopsys
In my last article, I presented an overview of the history of the 3G standardization
efforts, starting with local organizations such as ETSI and
ARIB and leading to a global standard with the emergence of
the global 3G (G3G) specification (see The History of
3G—Sorting Through the Standards Battles). We have seen
that the current G3G standard includes three modes: two
CDMA modes (direct sequence and multi carrier) and one
time-division CDMA (TD-CDMA). This month, I want to address
the role of the TD-CDMA mode in 3G designs.
It is interesting to note that while requirements for the future 3G systems are
clear (data rate, interoperability, environment, etc.), many aspects of the
deployment are still very fuzzy at best: which application to support, which mode
to deploy, what will be the typical user equipment (UE) capabilities, etc. Among
the most heavily debated questions is certainly the usage and application domain
of the time division duplexing (TDD) mode versus the frequency division duplexing
(FDD) mode.
TDD weaknesses
The direct-sequence CDMA (DS-CDMA) mode is considered the most applicable
to medium size or larger cells. As such, it is expected to be the backbone of the
first 3G systems and a lot of effort is currently being devoted to building
DS-CDMA user equipment and base stations.
TDD development, on the other hand, is expected to await the 2000 release of the
specifications. This delay has been sparked by some concerns that have been
raised about TD-CDMA systems. In particular, concerns are being tabled
regarding the need for synchronization between TDD as well as the need for
efficient dynamic channel allocation (DCA) procedures in order to minimize the
interference level between two neighboring base stations. In addition, it is widely
accepted that TDD user equipment requires the use of more complex receivers
than the classical rake receiver, such as joint detectors.
TDD strengths
Despite their shortcomings, TD-CDMA systems do provide some advantages over
FDD-based CDMA approaches. One of the major advantages of TDD technology
over FDD technology is the use of unimpaired bands. This means that TDD-based
systems do not require for each uplink frequency a symmetrical downlink
frequency as does FDD.
Currently, two paired bands of 60 MHz bandwidth have been reserved both in
Europe and in Japan, together with a smaller section of unpaired bandwidth. It is,
however, not unlikely that paired bands will be more difficult to find in the future,
thus making the TDD mode potentially increasingly important.
In addition to its unimpaired band use, TDD offers other benefits to developers of
3G systems. In particular, TDD systems offer ability to deal nicely with
asymmetrical traffic.
If we look at Internet browsing as being one of the major applications driving the
move to 3G, it appears that the amount of downlink traffic is likely to exceed on
average the amount of uplink data by a significant factor. In these applications,
the FDD mode allocates as much bandwidth to the uplink and the downlink
directions. Some industry members will argue that this approach provides an
inefficient use of spectrum.
By providing an asymmetrical scheme, TDD can devote more capacity to the
downlink direction, which bears the most traffic. By doing this, industry
professionals claim that the TDD approach more efficiently uses spectrum.
Business issues
Let's put down the technology issues now and turn to the business issues
surrounding TD-CDMA systems. In the emerging 3G world, there will be two
possibilities for picocell transmission: unlicensed and licensed. Unlicensed
operation means that operators are not involved. On the other hand, some
spectrum packages proposed to operators in some countries do include unpaired
bands where TDD is to be deployed. Hence, operators are interested in being
able to deploy TDD base stations provided that:
the additional cost is commensurate with the additional traffic that can be
accommodated or rather with the additional potential revenue, and
dual-mode FDD/TDD terminal are widely available, which can hand-over
communications between these 2 modes.
A possible licensed application of TDD would then be to alleviate the congestion
at hot spots like shopping malls or airports. It is also conceivable to see TDD
cells deployed where more asymmetrical traffic is expected.
To conclude this discussion on TDD versus FDD modes, I wanted to mention two
other emerging standards:
the Bluetooth initiative, which provides high data rate transmission with a
very short range, which could potentially compete with some (unlicensed)
TDD applications, and
the EDGE standard, which provides transmission up to 384 kb/s and
represents an easy upgrade from GSM systems, hence looking as a
promising short-term alternative to FDD systems, especially in the many
countries where GSM is largely deployed.
Significant efforts are being invested in developing both of these standards. When
they are complete, they could possible change the face of the TDD versus FDD
debate in the 3G market.
About the author:
Marc Barberis is a staff engineer at Synopsys in Mountain View, CA, where he is
currently involved in the development of the Synopsys 3G products. He has
worked among others on the design of receivers for 2G wireless. He received an
MS in electrical engineering from Ecole Nationale Superieure des
Telecommunications, France. He can be reached at barberis@synopsys.com.
Edited by Robert Keenan
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