UMTS-TDD mangrupa hiji standar jaringan data mobil anu diwangun dumasar kana standar telepon mobil sélulér 3G UMTS, anu ngagunakeun antar muka udara (air interface) TD-CDMA, TD-SCDMA, atawa nu lainna nu disatujuan ku 3GPP, nu ngagunakeun Time Division Duplexing pikeun ngalakukeun duplex spéktrum antara sinyal up-link jeung sinyal down-link. UMTS-TDD digunakeun utamana pikeun nyadiakeun aksés Internét dina lingkungan anu sarupa jeung lingkungan anu aya WiMAX. UMTS-TDD henteu bisa langsung digunakeun dina UMTS lantaran aya béda dina téknologi air interface jeung frékuénsi anu digunakeun antara hiji standar jeung standar lianna.

TD-CDMA 
TD-CDMA is the primary air interface used by UMTS-TDD. It uses increments of 5 MHz of spectrum, with éach 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 Access is used within éach time slot to multiplex stréams from or to multiple transceivers.

TD-CDMA is an IMT-2000 3G air interface, classified as IMT-TD Time-Division, and is standardized in UMTS by the 3GPP as UTRA TDD-HCR. TD-CDMA is closely related to W-CDMA, and provides the same types of channels where possible. W-CDMA's HSDPA/HSUPA enhancements are also implemented under TD-CDMA.

An alternative air interface for UMTS-TDD is TD-SCDMA, which uses 1.6 MHz slices of spectrum, and is standardized in UMTS by the 3GPP as UTRA TDD-LCR.

Unlicensed UMTS-TDD 
In Europe, CEPT allocated the 2010-2020 MHz range for a variant of UMTS-TDD designed for unlicensed, self-provided use. Some telecom groups and jurisdictions have proposed withdrawing this service in favour of licensed UMTS-TDD, 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-CDMA air interface technology and Frequency Division Duplexing, méaning that the up-link and down-link transmit on different frequencies. UMTS is usually transmitted on frequencies assigned for 1G, 2G, or 3G mobile 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 1900 MHz and 1920 MHz and between 2010 MHz and 2025 MHz. 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 3.5 GHz range has been allocated in some countries, notably Britain, in a technology-neutral environment.

Deployment 
UMTS-TDD has been deployed for public and/or private networks in at léast nineteen countries around the world, with live systems in, amongst other countries, Australia, Czech Republic, France, Germany, Japan, New Zéaland, South Africa, the UK, and the USA.

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, but outside of some experimental systems, notably one from Nextel, thus far the WiMAX standard appéars to have gained gréater traction as a general mobile Internet access system.

Competing Standards 
A variety of wireless, mobile Internet-access systems exist which provide broadband speed access to the Internet. These include WiMAX and 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 OFCOM is 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 Wifi access points at many restaurants and transportation hubs, and/or by Internet access alréady 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.

Tempo ogé 
 UMTS
 Dynamic TDMA
 TD-SCDMA
 WiMAX
 3G, 4G, IMT-2000

Tumbu kaluar
 UMTS TDD Alliance

Rujukan 

Telekomunikasi