Editing Time-division multiple access

{{Short description|Channel access method for networks using a shared communications medium}}
{{About|the channel access method|the obsolete U.S. mobile-telephone standard that uses TDMA to control channel access|Digital AMPS}}
{{more citations needed|date=August 2011}}

[[Image:Tdma-frame-structure.png|thumbnail|350px|TDMA frame structure showing a data stream divided into frames and those frames divided into time slots]]

'''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 digital [[2G]] [[cellular communication networks|cellular systems]] such as [[Global System for Mobile Communications]] (GSM), [[IS-136]], [[Personal Digital Cellular]] (PDC) and [[iDEN]], in the Maritime [[Automatic Identification System]],<ref>{{cite web|title=USCG How IAS Works|url=https://www.navcen.uscg.gov/how-ais-works |website=How IAS Works|access-date=10 March 2025}}</ref> and in the [[Digital Enhanced Cordless Telecommunications]] (DECT) standard for [[portable phone]]s. TDMA was first used in [[satellite communication]] systems by [[Western Union]] in its [[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.

== Characteristics ==
* Shares single carrier frequency with multiple users
* Non-continuous transmission makes handoff simpler
* Slots can be assigned on demand in dynamic TDMA
* Less stringent power control than [[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]]
* [[Cell breathing (telephony)|Cell breathing]] (borrowing resources from adjacent cells) is more complicated than in CDMA
* Frequency/slot allocation complexity
* Pulsating power envelope: [[Electromagnetic interference|interference]] with other devices

== In mobile phone systems ==
=== 2G systems ===
Most 2G cellular systems, with the notable exception of [[IS-95]], are based on TDMA. [[GSM]], [[D-AMPS]], [[Personal Digital Cellular|PDC]], [[iDEN]], and [[Personal Handy-phone System|PHS]] are examples of TDMA cellular systems.

In the GSM system, the synchronization of the mobile phones is achieved by sending timing advance commands from the base station which instruct the mobile phone to transmit earlier and by how much. This compensates for the speed-of-light [[propagation delay]]. The mobile phone is not allowed to transmit for its entire time slot; there is a [[guard interval]] at the end of each time slot. As the transmission moves into the guard period, the mobile network adjusts the timing advance to synchronize the transmission.

Initial synchronization of a phone requires even more care. Before a mobile transmits there is no way to know the offset required. For this reason, an entire time slot has to be dedicated to mobiles attempting to contact the network; this is known as the [[random-access channel]] (RACH) in GSM. The mobile transmits at the beginning of the time slot as received from the network. If the mobile is near the base station, the propagation delay is short and the initiation can succeed. If, however, the mobile phone is just less than 35&nbsp;km from the base station, the delay will mean the mobile's transmission arrives at the end of the time slot. In this case, the mobile will be instructed to transmit its messages starting nearly a whole time slot earlier so that it can be received at the proper time. Finally, if the mobile is beyond the 35&nbsp;km cell range of GSM, the transmission will arrive in a neighbouring time slot and be ignored. It is this feature, rather than limitations of power, that limits the range of a GSM cell to 35&nbsp;km when no special extension techniques are used. By changing the synchronization between the uplink and downlink at the base station, however, this limitation can be overcome.{{citation needed|date=November 2014}}

=== 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]] (CDMA) and time-division duplexing (TDD) in the [[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 [[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 |journal=[[Springer Nature]] |date=2002 |language=English |publisher=Springer, Boston, MA |pages=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>

The combination of these technologies in UMTS allows for more flexible and efficient use of the available spectrum, catering to diverse user demands and improving the adaptability of 3G networks to different operational environments.<ref name=":0" />

== 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]] communication in modern cars, uses the TDMA method for data transmission control.

== Comparison with other multiple-access schemes ==
In radio systems, TDMA is usually used alongside [[frequency-division multiple access]] (FDMA) and frequency-division duplex (FDD); the combination is referred to as FDMA/TDMA/FDD. This is the case in both GSM and IS-136 for example. Exceptions to this include the [[DECT]] and [[Personal Handy-phone System]] (PHS) micro-cellular systems, [[UMTS-TDD]] UMTS variant, and China's [[TD-SCDMA]], which use time-division duplexing, where different time slots are allocated for the base station and handsets on the same frequency.

A major advantage of TDMA is that the radio part of the mobile-only needs to listen and broadcast for its own time slot. For the rest of the time, the mobile can carry out measurements on the network, detecting surrounding transmitters on different frequencies. This allows safe inter-frequency [[handover]]s, something which is difficult in CDMA systems, not supported at all in [[IS-95]] and supported through complex system additions in [[Universal Mobile Telecommunications System]] (UMTS). This in turn allows for co-existence of [[microcell]] layers with [[macrocell]] layers.

CDMA, by comparison, supports "soft hand-off" which allows a mobile phone to be in communication with up to 6 base stations simultaneously, a type of "same-frequency handover". The incoming packets are compared for quality, and the best one is selected. CDMA's "cell breathing" characteristic, where a terminal on the boundary of two congested cells will be unable to receive a clear signal, can often negate this advantage during peak periods.

A disadvantage of TDMA systems is that they create [[electromagnetic interference|interference]] at a frequency that is directly connected to the time slot length. This is the buzz that can sometimes be heard if a TDMA phone is left next to a radio or speakers.<ref>{{cite web | url=http://www.eetimes.com/design/microwave-rf-design/4019057/Minimize-GSM-buzz-noise-in-mobile-phones | title=Minimize GSM buzz noise in mobile phones | publisher = EETimes | date=July 20, 2009 | access-date=November 22, 2010}}</ref>  Another disadvantage is that the "dead time" between time slots limits the potential bandwidth of a TDMA channel. These are implemented in part because of the difficulty in ensuring that different terminals transmit at exactly the times required. Handsets that are moving will need to constantly adjust their timings to ensure their transmission is received at precisely the right time because as they move further from the base station, their signal will take longer to arrive. This also means that the major TDMA systems have hard limits on cell sizes in terms of range, though in practice the power levels required to receive and transmit over distances greater than the supported range would be mostly impractical anyway.

== Advantages of TDMA ==
TDMA (time-division multiple access) is a communication method that allocates radio frequency (RF) bandwidth into discrete time slots, allowing multiple users to share the channel in a sequential manner. This approach not only improves spectrum efficiency compared to analog systems but also offers several specific advantages that enhance communication quality and system performance.<ref>{{Cite web |title=What is Time Division Multiple Access (TDMA)? |url=https://www.techtarget.com/searchnetworking/definition/TDMA |access-date=2024-10-28 |website=Networking |language=en}}</ref>

=== Advantages of TDMA ===

# '''Enhanced spectrum efficiency''': TDMA maximizes the use of available bandwidth by allowing multiple users to share the same channel without overlapping. Each user is assigned a specific time slot, ensuring that the channel's capacity is fully utilized, thereby increasing overall system efficiency.
# '''Reduction of intersymbol interference''': By assigning non-overlapping time slots to users, TDMA significantly reduces the risk of intersymbol interference. This interference occurs when signals from adjacent symbols overlap, leading to distortion and communication errors. The clear separation of time slots ensures that each symbol is transmitted distinctly, enhancing the reliability and clarity of the signal.
# '''Elimination of guard bands''': Since adjacent channels in TDMA do not interfere with one another, there is no need for guard bands—unused frequency ranges that typically separate channels to prevent interference in other systems. This absence of guard bands allows for more efficient use of the available spectrum, providing additional capacity for more users.<ref>{{Cite journal |last1=Kaur |first1=Amritpreet |last2=Kaur |first2=Guneet |date=2017-03-15 |title=The Enhanced ECC Approach to Secure Code Dissemination in Wireless Sensor Network |url=http://dx.doi.org/10.5120/ijca2017913237 |journal=International Journal of Computer Applications |volume=161 |issue=7 |pages=30–33 |doi=10.5120/ijca2017913237 |issn=0975-8887}}</ref>
# '''Flexible rate allocation''': TDMA supports dynamic allocation of time slots, allowing the system to adapt to varying user demands. Users can be assigned multiple time slots based on their data transmission needs, which can vary due to factors such as call duration or data requirements. This flexibility optimizes resource usage and can improve overall user experience.
# '''Low battery consumption''': Unlike FDMA (frequency-division multiple access), which requires continuous transmission, TDMA operates in a noncontinuous manner. Each transmitter can be turned off when not in use, leading to significant power savings. This is particularly advantageous for mobile devices, as it prolongs battery life and reduces the need for frequent recharging.
# '''Simplified implementation''': The time-based nature of TDMA simplifies the implementation of synchronization mechanisms between users. As users take turns using the channel, the system can more easily manage timing and coordination compared to more complex methods like CDMA (code-division multiple access), where signals overlap.<ref>{{Citation |title=Multiple access techniques: FDMA, TDMA, CDMA; system capacity comparisons |date=2004-12-16 |work=Mobile Wireless Communications |pages=137–160 |url=http://dx.doi.org/10.1017/cbo9780511811333.007 |access-date=2024-10-28 |publisher=Cambridge University Press|doi=10.1017/cbo9780511811333.007 |isbn=978-0-521-84347-8 |url-access=subscription }}</ref>
# '''Scalability''': TDMA systems can be scaled effectively to accommodate a growing number of users. As demand increases, additional time slots can be introduced without the need for significant changes to the existing infrastructure, making it easier to expand the network capacity.
# '''Improved quality of service (QoS)''': With the ability to assign specific time slots and manage user access dynamically, TDMA can enhance the overall quality of service. This can lead to reduced latency and increased throughput, ensuring that users experience reliable and efficient communication.

=== Disadvantages of TDMA ===

# '''Guard intervals''': To prevent interference between adjacent TDMA slots, guard intervals must be added. These intervals, typically ranging from 30 to 50 microseconds, serve as buffers to ensure that transmissions do not overlap. However, this requirement for extra time means that the overall throughput of the system can be reduced, as valuable time is spent in guard intervals rather than transmitting data. This is particularly problematic in cellular networks where time and energy efficiency are paramount.<ref>{{Cite journal |last1=Nguyen |first1=Kien |last2=Golam Kibria |first2=Mirza |last3=Ishizu |first3=Kentaro |last4=Kojima |first4=Fumihide |date=2019-02-14 |title=Performance Evaluation of IEEE 802.11ad in Evolving Wi-Fi Networks |journal=Wireless Communications and Mobile Computing |volume=2019 |pages=1–11 |doi=10.1155/2019/4089365 |doi-access=free |issn=1530-8669}}</ref>
# '''Energy consumption''': While TDMA allows for some energy savings by turning off transmitters during idle periods, the inclusion of guard intervals can offset these benefits. The need for synchronization and the overhead associated with managing time slots can lead to increased energy consumption, particularly in scenarios where numerous users are competing for access to the channel. This can be a critical issue for mobile devices that rely on battery power.
# '''Synchronization challenges''': TDMA requires precise synchronization between all users to ensure that each user transmits within their designated time slot. This can complicate system design and implementation, especially in dynamic environments where users may frequently join or leave the network. Maintaining synchronization becomes increasingly difficult as the number of users grows, leading to potential disruptions and communication errors if not managed effectively.
# '''Limited data rates''': TDMA generally provides medium data rates compared to other multiple access techniques like CDMA (code-division multiple access). This limitation arises from the fixed time slot allocation, which can restrict the amount of data that can be transmitted in a given timeframe. As a result, users with higher data requirements may experience slower transmission speeds, leading to potential dissatisfaction and reduced performance for data-intensive applications.
# '''Moderate system flexibility''': TDMA offers moderate flexibility in terms of user allocation and data transmission rates. Unlike CDMA, which allows for a more dynamic and adaptive use of bandwidth, TDMA's fixed time slot assignment can lead to inefficiencies. In scenarios where user demand fluctuates significantly, the rigid structure of TDMA may result in underutilization of resources, as not all time slots may be filled during periods of low demand.<ref>{{Citation |title=Multiple access techniques: FDMA, TDMA, CDMA; system capacity comparisons |date=2004-12-16 |work=Mobile Wireless Communications |pages=137–160 |url=http://dx.doi.org/10.1017/cbo9780511811333.007 |access-date=2024-10-28 |publisher=Cambridge University Press|doi=10.1017/cbo9780511811333.007 |isbn=978-0-521-84347-8 |url-access=subscription }}</ref>
# '''Latency issues''': Due to the time-sharing nature of TDMA, users may experience increased latency. When multiple users are connected, each must wait for their designated time slot to transmit data. In applications that require real-time communication, such as voice calls or video conferencing, this added delay can affect the quality of service, leading to lag and reduced responsiveness.
# '''Scalability constraints''': While TDMA can accommodate a growing number of users by adding more time slots, this scalability is limited by the need for synchronization and the fixed nature of time slot assignments. As user demand increases, the system may face challenges in maintaining performance levels without significant investment in infrastructure upgrades or more complex management systems.<ref>{{Cite book |last=Le Gouable |first=R. |chapter=Performance of MC-CDMA systems in multipath indoor environments. Comparison with COFDM-TDMA system |date=2000 |title=First International Conference on 3G Mobile Communication Technologies |chapter-url=http://dx.doi.org/10.1049/cp:20000018 |publisher=IEE |volume=2000 |pages=81–85 |doi=10.1049/cp:20000018|isbn=0-85296-726-8 }}</ref>

== Dynamic TDMA ==
In '''dynamic time-division multiple access''' ('''dynamic TDMA'''), a [[scheduling algorithm]] 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. Dynamic TDMA is used in:
* [[HIPERLAN/2]] broadband radio access network.
* [[IEEE 802.16a]] WiMax
* [[Bluetooth]]
* Military Radios / [[Tactical Data Link]]
* [[TD-SCDMA]]
* [[ITU-T]] [[G.hn]]
* [[Network simulator|Simulation of TDMA / DTMA links]]
* [[Multimedia over Coax Alliance|MoCA]]

== See also ==
* {{anl|Duplex (telecommunications)}}
* [[Link 16]]{{snd}}NATO military tactical data exchange network that uses TDMA

== References ==
{{Reflist}}

{{Channel access methods}}
{{Authority control}}

{{DEFAULTSORT:Time-Division Multiple Access}}
[[Category:Radio resource management]]
[[Category:Media access control]]