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]], 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.

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# '''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.

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# '''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>