UMTS-TDD is a mobile data network standard built upon the UMTS 3G cellular mobile phone standard, using a TD-CDMA,
TD-SCDMA, or other 3GPP-approved, air interface that uses Time Division Duplexing to duplex spectrum between the up-link and down-link. While a full implementation of UMTS, it is mainly used to provide Internet access in circumstances similar to those where WiMAXmight be used. UMTS-TDD is not directly compatible with UMTS: a device designed to use one standard cannot, unless specifically designed to, work on the other, because of the difference in air interface technologies and frequencies used.
TD-CDMA is the primary air interface used by UMTS-TDD. It uses increments of 5MHz of spectrum, with each slice divided into 10ms frames containing fifteen time slots (1500 per second). The time slots are allocated in fixed percentage for downlink and uplink.
Code Division Multiple Accessis used within each time slot to multiplex streams from or to multiple transceivers. [citeweb|title=UMTS World TD-CDMA information|url=http://www.umtsworld.com/technology/tdcdma.htm|publisher=umtsworld.com|accessdate=2008-02-28]
TD-CDMA is an
IMT-20003G air interface, classified as IMT-TD Time-Division, and is standardized in UMTS by the 3GPPas UTRA TDD-HCR. TD-CDMA is closely related to W-CDMA, and provides the same types of channels where possible. W-CDMA's HSDPA/ HSUPAenhancements are also implemented under TD-CDMA. [citeweb|title=IPWireless Ships First Commercial 3GPP Chipset with Full HSDPA Implementation|url=http://www.ipwireless.com/news/press_020805.html|publisher=ipwireless.com|accessdate=2008-02-28]
An alternative air interface for UMTS-TDD is
TD-SCDMA, which uses 1.6MHz slices of spectrum, and is standardized in UMTS by the 3GPP as UTRA TDD-LCR.
CEPTallocated the 2010-2020MHz range for a variant of UMTS-TDD designed for unlicensed, self-provided use. [citeweb|title=ERC/DEC/(99)25 EU Recommendation on UMTS TDD|url=http://www.ero.dk/documentation/docs/doc98/official/pdf/DEC9925E.PDF|publisher=ero.dk|accessdate=2008-02-28] Some telecom groups and jurisdictions have proposed withdrawing this service in favour of licensed UMTS-TDD, [citeweb|title=Award_of_available_spectrum:_2500-2690_MHz,_2010-2025_MHz_and_2290-2300_MHz|url=http://www.ofcom.org.uk/consult/condocs/2ghzawards/2ghzawards.pdf|publisher=ofcom.org.uk|accessdate=2008-02-28] due to lack of demand, and lack of development of a UMTS TDD air interface technology suitable for deployment in this band.
Comparison with UMTS
Ordinary UMTS uses a
W-CDMAair interface technology and Frequency Division Duplexing, meaning that the up-link and down-link transmit on different frequencies. UMTS is usually transmitted on frequencies assigned for 1G, 2G, or 3Gmobile telephone service in the countries of operation.
UMTS-TDD uses time division duplexing, allowing the up-link and down-link to share the same spectrum. This allows the operator to more flexibly divide the usage of available spectrum according to traffic patterns. For ordinary phone service, you would expect the up-link and down-link to carry approximately equal amounts of data (because every phone call needs a voice transmission in either direction), but Internet-oriented traffic is more frequently one-way. For example, when browsing a website, the user will send commands, which are short, to the server, but the server will send whole files, that are generally larger than those commands, in response.
UMTS-TDD tends to be allocated frequency intended for mobile/wireless Internet services rather than used on existing cellular frequencies. This is, in part, because TDD duplexing is not normally allowed on cellular,
PCS/PCN, and 3G frequencies. TDD technologies open up the usage of left-over unpaired spectrum.
Europe-wide, several bands are provided either specifically for UMTS-TDD or for similar technologies. These are 1900MHz and 1920MHz and between 2010MHz and 2025MHz. In several countries the 2500-2690 MHz band (also known as MMDS in the USA) have been used for UMTS-TDD deployments. Additionally, spectrum around the 3.5GHz range has been allocated in some countries, notably Britain, in a technology-neutral environment.
UMTS-TDD has been deployed for public and/or private networks in at least nineteen countries around the world, with live systems in, amongst other countries, Australia, Czech Republic, France, Germany, Japan, New Zealand, South Africa, the UK, and the USA. [citeweb|title=UMTS-TDD Alliance list of Deployments|url=http://www.umtstdd.org/deployments.html|publisher=umtstdd.org|accessdate=2008-02-28]
Deployments in the US thus far have been limited. It has been selected for a public safety support network used by emergency responders in New York, [citeweb|title=Northrop Grumman Wins $500 Million New York City Broadband Mobile Wireless Contract|url=http://www.ipwireless.com/news/press_091206.html|publisher=ipwireless.com|accessdate=2008-02-28] but outside of some experimental systems, notably one from
Nextel, thus far the WiMAX standard appears to have gained greater traction as a general mobile Internet access system.
A variety of Internet-access systems exist which provide broadband speed access to the net. These include
WiMAXand HIPERMAN. UMTS-TDD has the advantages of being able to use an operator's existing UMTS/GSM infrastructure, should it have one, and that it includes UMTS modes optimized for circuit switching should, for example, the operator want to offer telephone service. UMTS-TDD's performance is also more consistent. However, UMTS-TDD deployers often have regulatory problems with taking advantage of some of the services UMTS compatibility provides. For example, UMTS-TDD spectrum in the UK cannot be used to provide telephone service, though the regulator OFCOMis discussing the possibility of allowing it at some point in the future. Few operators considering UMTS-TDD have existing UMTS/GSM infrastructure.
Additionally, the WiMAX and HIPERMAN systems provide significantly larger bandwidths when the mobile station is in close proximity to the tower.
Like most mobile Internet access systems, many users who might otherwise choose UMTS-TDD will find their needs covered by the ad hoc collection of unconnected
Wifiaccess points at many restaurants and transportation hubs, and/or by Internet access already provided by their mobile phone operator. By comparison, UMTS-TDD (and systems like WiMAX) offers mobile, and more consistent, access than the former, and generally faster access than the latter.
3G, 4G, IMT-2000
* [http://www.umtstdd.org/ UMTS TDD Alliance]
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