Time-division multiple access: Difference between revisions

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'''Time-division multiple access''' ('''TDMA''') is a [[channel access method]] for [[shared-medium network]]s. It allows several users to share the same [[frequency channel]] by dividing the signal into different time slots.<ref name=Zander>{{cite book|author1=Guowang Miao|author-link=Guowang Miao|author2=Jens Zander|author3=Ki Won Sung|author4=Ben Slimane|title=Fundamentals of Mobile Data Networks|publisher=[[Cambridge University Press]]|isbn=978-1107143210|year=2016}}</ref> The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its [[channel capacity]]. '''Dynamic TDMA''' is a TDMA variant that dynamically reserves a variable number of time slots in each frame to variable bit-rate data streams, based on the traffic demand of each data stream.

TDMA is used in the digital [[2G]] [[cellular communication networks|cellular systems]] such as [[Global System for Mobile Communications]] (GSM), [[IS-136]], [[Personal Digital Cellular]] (PDC) and [[iDEN]], and in the [[Digital Enhanced Cordless Telecommunications]] (DECT) standard for [[portable phone]]s. TDMA was first used in [[communications satellite|satellite communication]] systems by [[Western Union]] in its [[Westar|Westar 3]] communications satellite in 1979. It is now used extensively in satellite communications,<ref>{{cite conference|last1=Maine|first1=K.|last2=Devieux|first2=C.|last3=Swan|first3=P.|date=November 1995|url=https://www.researchgate.net/publication/3622510|title=Overview of IRIDIUM satellite network|conference=WESCON'95|page=483|publisher=IEEE}}</ref><ref>{{cite conference|last1=Mazzella|first1=M.|last2=Cohen|first2=M.|last3=Rouffet|first3=D.|last4=Louie|first4=M.|last5=Gilhousen|first5=K. S.|date=April 1993|title=Multiple access techniques and spectrum utilisation of the GLOBALSTAR mobile satellite system|conference=Fourth IEE Conference on Telecommunications 1993|pages=306–311|publisher=IET}}</ref><ref>{{cite conference|last=Sturza|first=M. A.|date=June 1995|url=https://www.researchgate.net/publication/4672931|title=Architecture of the TELEDESIC satellite system|conference=International Mobile Satellite Conference|volume=95|page=214}}</ref><ref>{{cite web|url=https://www.ctu.cz/sites/default/files/cs/download/oznamene_typy_rozhrani/orbcomm-rozhrani_02_06_2010.pdf|title=ORBCOMM System Overview}}</ref> [[combat-net radio]] systems, and [[passive optical network]] (PON) networks for upstream traffic from premises to the operator.

TDMA is a type of [[time-division multiplexing]] (TDM), with the special point that instead of having one [[transmitter]] connected to one [[Receiver (radio)|receiver]], there are multiple transmitters. In the case of the ''[[uplink]]'' from a [[mobile phone]] to a [[base station]] this becomes particularly difficult because the mobile phone can move around and vary the ''timing advance'' required to make its transmission match the gap in transmission from its peers.

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* Non-continuous transmission makes handoff simpler

* Slots can be assigned on demand in dynamic TDMA

* Less stringent power control than [[code-division multiple access|CDMA]] due to reduced intra cell interference

* Higher synchronization overhead than CDMA

* Advanced [[Equalization (communications)|equalization]] may be necessary for high data rates if the channel is "frequency selective" and creates [[Intersymbol interference]]

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=== 3G systems ===

{{Expand section|date=November 2014}}

In the context of 3G systems, the integration of Time-Division Multiple Access (TDMA) with [[Code-division multiple access|Code-Division Multiple Access]] (CDMA) and Time-Division Duplexing (TDD) in the [[UMTS|Universal Mobile Telecommunications System]] (UMTS) represents a sophisticated approach to optimizing spectrum efficiency and network performance.<ref>{{Cite book|url=https://books.google.com/books?id=zLz-CwAAQBAJ&q=3G+systems+are+primarily+based+upon+CDMA&pg=PT24|title=Principles of Modern Wireless Communication Systems|last=Jagannatham|first=Aditya K.|publisher=McGraw-Hill Education|year=2016|isbn=9789339220037}}</ref>

UTRA-FDD (Frequency Division Duplex) employs CDMA and FDD, where separate [[Spectral band|frequency bands]] are allocated for uplink and downlink transmissions. This separation minimizes interference and allows for continuous data transmission in both directions, making it suitable for environments with balanced traffic loads.<ref name=":0">{{Cite journal |title=3G mobile systems |url=https://link.springer.com/content/pdf/10.1007/0-306-47795-5_3.pdf |format=pdf |journal=[[Springer Nature]] |language=English |publisher=Springer, Boston, MA |page=45–89 |doi=10.1007/0-306-47795-5_3 |isbn=978-0-306-47795-9 |url-access=subscription}}</ref>

UTRA-TDD (Time Division Duplex), on the other hand, combines CDMA with TDMA and TDD. In this scheme, the same frequency band is used for both uplink and downlink, but at different times. This time-based separation is particularly advantageous in scenarios with asymmetric traffic loads, where the data rates for uplink and downlink differ significantly. By dynamically allocating time slots based on demand, UTRA-TDD can efficiently manage varying traffic patterns and enhance overall network capacity.<ref name=":0" /><ref>{{Cite web |title=ETSI TS 136 214 V14.3.0 (2017-10) |url=https://www.etsi.org/deliver/etsi_ts/136200_136299/136214/14.03.00_60/ts_136214v140300p.pdf}}</ref>

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== In wired networks ==

The [[ITU-T]] [[G.hn]] standard, which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables) is based on a TDMA scheme. In [[G.hn]], a "master" device allocates "Contention-Free Transmission Opportunities" (CFTXOP) to other "slave" devices in the network. Only one device can use a CFTXOP at a time, thus avoiding collisions.

[[FlexRay]] protocol which is also a wired network used for [[safety-critical system|safety-critical]] communication in modern cars, uses the TDMA method for data transmission control.

== Comparison with other multiple-access schemes ==