TDMA, which stands for time division multiple access, is a method of channel access developed specifically for shared medium networks, such as radio broadcast. It is a form of time-division multiplexing; however, instead of having one transmitter connected to one receiver, TDMA systems may connect multiple transmitters to a receiver.
Several users are able to share the same frequency channel through the method of dividing the signal into different timeslots, and then having the users transmit data in rapid succession using individual timeslots. This way, only a fraction of the bandwidth required is used as multiple stations share the same frequency channel.
Satellite systems, combat-net radio systems, second generation (2G) systems such as Interim Standard 136 (IS-136), Digital Advanced Mobile Phone System (D-AMPS), Global System for Mobile Communications (GSM), Personal Digital Cellular (PDC) and iDEN, as well as the Digital Enhanced Cordless Telecommunications (DECT) standard for wireless telecommunications all made use of TDMA.
The features of TDMA include the following:
- single carrier frequency is shared among multiple users;
- dynamic TDMA enables the option of slots being assigned on demand;
- handoff is made simpler through non-continuous transmission;
- reduced intra cell interference gives TDMA less stringent power control then code division multiple access (CDMA);
- higher synchronization overhead than CDMA;
- and frequency/slot allocation complexity.
A variant of TDMA known as Dynamic TDMA utilizes a scheduling algorithm which dynamically reserves a variable number of timeslots per frame to variable bit rate data streams, basing this on each data stream’s traffic demand.
There are various technologies that make use of Dynamic TDMA, such as the High Performance Radio LAN (HIPERLAN) standard, IEEE 802.16a (also known as WiMax), Bluetooth, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) and packet radio multiple access (PRMA).
Radio stations, as well as GSM and IS-136, tend to use TDMA with FDMA and frequency division duplex (FDD) in a combination known as FDMA/TDMA/FDD. The mobile only needs to listen and broadcast for its own timeslot, leaving it free to carry out measurements on the network for the rest of the time. This advantage of TDMA allows the mobile phone to detect surrounding transmitters on different frequencies; it also allows safe inter frequency handovers, and consequently, the co-existence of macrocell layers and microcell layers, unlike CDMA systems and IS-95. Universal Mobile Telecommunications System (UMTS) does allow this, but only through complex system additions.
TDMA systems do have a disadvantage in that interference is created at a frequency that is directly proportional to the timeslot length, causing an irritating buzzing noise that may be heard if a GSM-enabled mobile phone is placed next to a radio or speakers.
There is also the "dead time" between timeslots, implemented due to the difficulty of ensuring that different terminals transmit their data at their precise timeslots, so the "dead time" allows them room for error." Dead time limits the potential bandwidth of a TDMA channel.