Time-division multiple access
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Time-division multiple access (TDMA) is a
TDMA is used in the digital
TDMA is a type of
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 may be necessary for high data rates if the channel is "frequency selective" and creates Intersymbol interference
- Cell breathing (borrowing resources from adjacent cells) is more complicated than in CDMA
- Frequency/slot allocation complexity
- Pulsating power envelope: interference with other devices
In mobile phone systems
2G systems
Most 2G cellular systems, with the notable exception of
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 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 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; this is known as the random-access channel (RACH) 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. [citation needed]
3G systems
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Although most major 3G systems are primarily based upon
While the most popular form of the
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
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
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 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.[7] 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 (dynamic TDMA), a
- HIPERLAN/2broadband radio access network.
- IEEE 802.16aWiMax
- Bluetooth
- Military Radios / Tactical Data Link
- TD-SCDMA
- ITU-T G.hn
- Simulation of TDMA / DTMA links
See also
- Duplex (telecommunications) – Communication flowing simultaneously in both directions
- Link 16 – NATO military tactical data exchange network that uses TDMA
References
- ISBN 978-1107143210.
- ^ Maine, K.; Devieux, C.; Swan, P. (November 1995). Overview of IRIDIUM satellite network. WESCON'95. IEEE. p. 483.
- ^ Mazzella, M.; Cohen, M.; Rouffet, D.; Louie, M.; Gilhousen, K. S. (April 1993). Multiple access techniques and spectrum utilisation of the GLOBALSTAR mobile satellite system. Fourth IEE Conference on Telecommunications 1993. IET. pp. 306–311.
- ^ Sturza, M. A. (June 1995). Architecture of the TELEDESIC satellite system. International Mobile Satellite Conference. Vol. 95. p. 214.
- ^ "ORBCOMM System Overview" (PDF).
- ISBN 9789339220037.
- ^ "Minimize GSM buzz noise in mobile phones". EETimes. July 20, 2009. Retrieved November 22, 2010.