Time-division multiple access: Difference between revisions

Revision as of 18:06, 13 October 2021

Revision as of 18:08, 13 October 2021

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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 [[Personal Handy-phone System]] (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 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[[handover]]s, 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.