Time division multiple access

From Wikipedia, the free encyclopedia

This article is about the medium access technology. The name "TDMA" is also commonly used in America to refer to a specific second generation (2G) mobile phone standard, more properly referred to as IS-136 or D-AMPS

Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. It allows several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own timeslot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only the part of its bandwidth they require. TDMA is used in the digital 2G 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 phones. It is also used extensively in satellite systems, and combat-net radio systems. For usage of Dynamic TDMA packet mode communication, see below.

TDMA frame structure showing a data stream divided into frames and those frames divided into timeslots.

TDMA is a type of Time-division multiplexing, with the special point that instead of having one transmitter connected to one 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.

[edit] TDMA features

• 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 is necessary for high data rates
• Cell breathing (borrowing resources from adjacent cells) is more complicated than in CDMA
• Frequency/slot allocation complexity
• Pulsating power envelop: Interference with other devices

[edit] TDMA in 2G cellular systems

Most 2G cellular systems, with the notable exception of IS-95, are based around TDMA. GSM, D-AMPS, PDC, and PHS are examples of TDMA cellular systems. GSM combines TDMA with Frequency Hopping and wideband transmission to reduce interference, this minimizes 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 propagation delay as the speed of radio waves is the same as light (finite). The mobile phone is not allowed to transmit for its entire timeslot, but there is a guard interval at the end of each timeslot. 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 timeslot has to be dedicated to mobiles attempting to contact the network (known as the RACH in GSM). The mobile attempts to broadcast at the beginning of the timeslot, 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 35km from the base station, the time delay will mean the mobile's broadcast arrives at the very end of the timeslot. In that case, the mobile will be instructed to broadcast its messages starting nearly a whole timeslot 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 neighboring timeslot and be ignored. It is this feature, rather than limitations of power which limits the range of a GSM cell to 35 kilometers 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.

[edit] TDMA in 3G Cellular Systems

Most major 3G systems are primarily based upon CDMA. However, Time Division duplexing and multiple access schemes are available in 3G form, sometimes combined with CDMA to take advantage of the benefits of both technologies.

While the most popular form of the UMTS 3G GSM system uses CDMA instead of TDMA, TDMA is combined with CDMA and Time Division Duplexing in two standard UMTS UTRA modes, UTRA TDD-HCR (better known as TD-CDMA), and UTRA TDD-LCR (better known as TD-SCDMA). In each mode, more than one handset may share a single time slot. UTRA TDD-HCR is used most commonly by UMTS-TDD to provide Internet access, whereas UTRA TDD-LCR provides some interoperability with the forthcoming Chinese 3G standard.

[edit] 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 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 timeslot. 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 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 interference at a frequency which is directly connected to the timeslot length. This is the irritating buzz which can sometimes be heard if a GSM phone is left next to a radio or speakers. Another disadvantage is that the "dead time" between timeslots limits the potential bandwidth of a TDMA channel. These are implemented in part because of the difficulty 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.

[edit] Dynamic TDMA

In dynamic time division multiple access, a scheduling algorithm dynamically reserves a variable number of timeslots 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
• The Packet radio multiple access (PRMA) method for combined circuit switched voice communication and packet data.
• TD-SCDMA

[edit] See also

• Channel access methods
• Time-division multiplex (TDM)
• Duplex (telecommunications) (FDD,TDD)