UMTS-TDD: Béda antarrépisi

'''UMTS-TDD''' ngarupakeunmangrupa 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 is the primary air interface used by UMTS-TDD. It uses increments of 5MHz5&nbsp;MHz of spectrum, with eachéach 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 eachéach time slot to multiplex streamsstréams from or to multiple transceivers.<ref>[http://www.umtsworld.com/technology/tdcdma.htm UMTS World TD-CDMA information]</ref>

An alternative air interface for UMTS-TDD is [[TD-SCDMA]], which uses 1.6MHz6&nbsp;MHz slices of spectrum, and is standardized in UMTS by the 3GPP as UTRA TDD-LCR.

In Europe, [[CEPT]] allocated the 2010-2020MHz2020&nbsp;MHz range for a variant of UMTS-TDD designed for unlicensed, self-provided use.<ref>[http://www.ero.dk/documentation/docs/doc98/official/pdf/DEC9925E.PDF ERC/DEC/(99)25 EU Recommendation on UMTS TDD], Annex 1, points 5 and 6</ref> Some telecom groups and jurisdictions have proposed withdrawing this service in favour of licensed UMTS-TDD,<ref>[http://www.umts-forum.org/servlet/dycon/ztumts/umts/Live/en/umts/MultiMedia_Consultations_RSC-Questionnaire-Band-2010-2025M.pdf UMTS Forum on RSC Questionaire concerning future of 2010-2025MHz band] (PDF)</ref><ref>[http://www.ofcom.org.uk/consult/condocs/2ghzawards/2ghzawards.pdf OFCOM report on IMT-2000 spectrum]</ref> due to lack of demand, and lack of development of a UMTS TDD air interface technology suitable for deployment in this band.

Ordinary UMTS uses a [[W-CDMA]] air interface technology and [[Frequency division duplex|Frequency Division Duplexing]], meaningméaning 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 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 [[Mobile phone|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 1900MHz1900&nbsp;MHz and 1920MHz1920&nbsp;MHz and between 2010MHz2010&nbsp;MHz and 2025MHz2025&nbsp;MHz. In several countries the 2500-2690 &nbsp;MHz band (also known as MMDS in the USA) have been used for UMTS-TDD deployments. Additionally, spectrum around the 3.5GHz5&nbsp;GHz 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 leastléast nineteen countries around the world, with live systems in, amongst other countries, Australia, Czech Republic, France, Germany, Japan, New ZealandZéaland, South Africa, the UK, and the USA.<ref>[http://www.umtstdd.org/deployments.html UMTS-TDD Alliance list of Deployments]</ref>

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,<ref>[http://www.ipwireless.com/news/press_091206.html Northrop Grumman Wins $500 Million New York City Broadband Mobile Wireless Contract]</ref> but outside of some experimental systems, notably one from [[Nextel]], thus far the WiMAX standard appearsappéars to have gained greatergréater traction as a general mobile Internet access system.

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 alreadyalréady 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.