Glossary definition of Time Division Multiple Access

Also known as: TDMA

Time Division Multiple Access (TDMA) is a digital transmission technique used for mobile phone communications, in which a frequency channel is divided into a sequence of time slots, and each user is allocated a unique time slot within the sequence. The callers are assigned time slots as they are needed, and their signals are interleaved within the one channel as the sequence is transmitted. For example, with GSM each channel is divided into 8 time slots, so that eight different subscribers can potentially use it simultaneously. Because the time slots are being switched very rapidly, the participants in a conversation are not aware of the fact that the signal has been chopped into short pieces. The original call is re-assembled by selecting the correct time slots at the receiving end.

Using TDMA in this way, multiple users can share the available channels without the risk of any interference between the simultaneous calls. There is actually an improvement in security, as it is unlikely that a third party will be able to tune into the time slots being used by a particular subscriber. TDMA also improves the network capacity, and therefore the efficiency, because more users can share the same frequency channels.

Time Division Multiple Accessis one of the three main multiplexing techniques that enable users to share the radio spectrum, and in the case of TDMA the receiver will discriminate between the signals by selecting the particular time slot that carries the desired signal. Alternative multiple access techniques are FDMA and CDMA, in which the receivers discriminate between signals by using, respectively, different frequency channels or different codes. However, in practice TDMA and CDMA are always found in combination with FDMA, i.e. TDMA or CDMA are used to increase capacity on a channel within an FDMA system.

TDMA is the principal technology used in 2G (second generation) digital cellular networks, and different versions of it are used by D-AMPS (USA, IS-136), GSM (Europe), PDC (Japan) and the DECT cordless phone system. In fact, in regions such as the USA, the second generation phone network is often referred to as 'TDMA'.

TDMA systems can also be enhanced by EDGE, to provide higher data transmission rates.