Time division multiple access - AbsoluteAstronomy.com

This article is about the channel access method. The name "TDMA" is also commonly used in the United States to refer to Digital AMPS
Digital AMPS
IS-54 and IS-136 are second-generation mobile phone systems, known as Digital AMPS . It was once prevalent throughout the Americas, particularly in the United States and Canada. D-AMPS is considered end-of-life, and existing networks have mostly been replaced by GSM/GPRS or CDMA2000...

, which is an obsolete mobile telephone standard that uses TDMA to control channel access.

Time division multiple access (TDMA) is a channel access method

Channel access method

In telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same multi-point transmission medium to transmit over it and to share its capacity...

for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. 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

Channel capacity

In electrical engineering, computer science and information theory, channel capacity is the tightest upper bound on the amount of information that can be reliably transmitted over a communications channel...

. TDMA is used in the digital 2G

2G is short for second-generation wireless telephone technology. Second generation 2G cellular telecom networks were commercially launched on the GSM standard in Finland by Radiolinja in 1991...

cellular systems such as Global System for Mobile Communications

Global System for Mobile Communications

GSM , is a standard set developed by the European Telecommunications Standards Institute to describe technologies for second generation digital cellular networks...

(GSM), IS-136, Personal Digital Cellular

Personal Digital Cellular

Personal Digital Cellular is a 2G mobile telecommunications standard developed and used exclusively in Japan.After a peak of nearly 80 million subscribers to PDC, it had 46 million subscribers in December 2005, and is slowly being phased out in favor of 3G technologies like W-CDMA and CDMA2000...

(PDC) and iDEN

Iden

For other uses of the word, see Iden The Iden was an English automobile manufactured from 1904 until 1907. Designed by George Iden, formerly of Daimler, they were four-cylinder 10/17 hp and 25/35 hp shaft-driven cars; each came with "Idens's frictionless radial gearbox"....

, and in the Digital Enhanced Cordless Telecommunications

Digital Enhanced Cordless Telecommunications

Digital Enhanced Cordless Telecommunications , usually known by the acronym DECT, is a digital communication standard, which is primarily used for creating cordless phone systems...

(DECT) standard for portable phones. It is also used extensively in satellite

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

systems, combat-net radio

Combat-net radio

In telecommunication, a combat-net radio is a radio operating in a network that provides a half-duplex circuit and uses either a single radio frequency or a discrete set of radio frequencies when in a frequency hopping mode....

systems, and PON

Passive optical network

A passive optical network is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 16-128. A PON consists of an optical line terminal at the service provider's central...

networks for upstream traffic from premises to the operator. For usage of Dynamic TDMA packet mode communication, see below.

TDMA is a type of Time-division multiplexing

Time-division multiplexing

Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...

, with the special point that instead of having one transmitter

Transmitter

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

connected to one receiver

Receiver (radio)

A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...

, there are multiple transmitters. In the case of the uplink
Uplink
A telecommunications link is generally one of several types of information transmission paths such as those provided by communication satellites to connect two points on earth.-Uplink:...

from a mobile phone

Mobile phone

A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

to a base station

Base station

The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...

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.

TDMA 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
Code division multiple access
Code division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access...

due to reduced intra cell interference
Higher synchronization overhead than CDMA
Advanced equalization
Equalization
Equalization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...

may be necessary for high data rates if the channel is "frequency selective" and creates Intersymbol interference
Intersymbol interference
In telecommunication, intersymbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable...
Cell breathing
Cell breathing (telephony)
In CDMA-based mobile telephone systems, the effect of radio interference from other mobile transmitters in the same cell or coverage area is very marked and has a special name, cell breathing....

(borrowing resources from adjacent cells) is more complicated than in CDMA
Frequency/slot allocation complexity
Pulsating power envelope: Interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...

with other devices

2G systems

Most 2G cellular systems, with the notable exception of IS-95, are based on TDMA. GSM, D-AMPS, PDC, iDEN, and PHS

Personal Handy-phone System

The Personal Handy-phone System , also marketed as the Personal Access System and commercially branded as Xiaolingtong in China, is a mobile network system operating in the 1880–1930 MHz frequency band, used mainly in Japan, China, Taiwan, and some other Asian countries and...

are examples of TDMA cellular systems. GSM combines TDMA with Frequency Hopping

Frequency-hopping spread spectrum

Frequency-hopping spread spectrum is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver...

and wideband transmission to minimize common types of interference.

In the GSM system, the synchronization of the mobile phones is achieved by sending timing advance commands from the base station which instructs the mobile phone to transmit earlier and by how much. This compensates for the propagation delay resulting from the light speed velocity of radio waves. The mobile phone is not allowed to transmit for its entire time slot, but there is a guard interval

Guard interval

In telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another. These transmissions may belong to different users or to the same user ....

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 actually know the offset required. For this reason, an entire time slot has to be dedicated to mobiles attempting to contact the network (known as the RACH

Rach

Rach may refer to:* Random Access Channel , a feature of mobiles or other wireless devices* Sergei Rachmaninoff, the composer. "Rach" is a colloquial short form of his surname, most commonly used to refer to some of his compositions, such as "Rach 2" "Rach 3" * Rachel...

in GSM). The mobile attempts to broadcast at the beginning of the time slot, as received from the network. If the mobile is located next to the base station, there will be no time delay and this will succeed. If, however, the mobile phone is at just less than 35 km from the base station, the time delay will mean the mobile's broadcast arrives at the very end of the time slot. In that case, the mobile will be instructed to broadcast its messages starting nearly a whole time slot earlier than would be expected otherwise. Finally, if the mobile is beyond the 35 km cell range in GSM, then the RACH 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 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.

3G systems

Although most major 3G systems are primarily based upon CDMA, time division duplexing (TDD), packet scheduling (dynamic TDMA) and packet oriented multiple access schemes are available in 3G form, combined with CDMA to take advantage of the benefits of both technologies.

While the most popular form of the UMTS 3G system uses CDMA and frequency division duplexing (FDD) instead of TDMA, TDMA is combined with CDMA and Time Division Duplexing in two standard UMTS UTRA

TDMA in wired networks

The ITU-T

ITU-T

The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

G.hn

G.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...

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

G.hn

G.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...

, 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

FlexRay

FlexRay is an automotive network communications protocol developed by the . It is designed to be faster and more reliable than CAN and TTP, but is also more expensive...

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

Frequency-division multiple access

Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels. It is particularly commonplace in satellite communication. FDMA, like...

(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 PHS

Personal Handy-phone System

micro-cellular systems, UMTS-TDD

UMTS-TDD

UMTS-TDD, an acronym for Universal Mobile Telecommunications System - time-division duplexing , is a 3GPP standardized version of UMTS networks that use UTRA-TDD. UTRA-TDD is a UTRA that uses time-division duplexing for duplexing. While a full implementation of UMTS, it is mainly used to provide...

UMTS variant, and China's TD-SCDMA

TD-SCDMA

Time Division Synchronous Code Division Multiple Access or UTRA/UMTS-TDD 1.28 Mcps Low Chip Rate , is an air interface found in UMTS mobile telecommunications networks in China as an alternative to W-CDMA. Together with TD-CDMA, it is also known as UMTS-TDD or IMT 2000 Time-Division .The term...

, 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 handovers, something which is difficult in CDMA systems, not supported at all in IS-95

IS-95

Interim Standard 95 is the first CDMA-based digital cellular standard by Qualcomm. The brand name for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA-95....

and supported through complex system additions in Universal Mobile Telecommunications System

Universal Mobile Telecommunications System

Universal Mobile Telecommunications System is a third generation mobile cellular technology for networks based on the GSM standard. Developed by the 3GPP , UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the CDMA2000 standard set for...

(UMTS). This in turn allows for co-existence of microcell

Microcell

A microcell is a cell in a mobile phone network served by a low power cellular base station , covering a limited area such as a mall, a hotel, or a transportation hub. A microcell is usually larger than a picocell, though the distinction is not always clear...

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 interference

Electromagnetic interference

Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...

at a frequency which is directly connected to the time slot length. This is the buzz which can sometimes be heard if a TDMA phone is left next to a radio or speakers. 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.

Dynamic TDMA

In dynamic time division multiple access, 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