Time division multiple access

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Time Division Multiple Access (TDMA) is a technology for shared medium (usually radio) networks. It allows several users to share the same frequency by dividing it into different time slots. Used in the GSM, PDC and IDEN digital cellular standards, among others.

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, which uses the TDMA technique to timeshare the bandwidth of the carrier wave.

The two different uses of this term can be confusing. TDMA (the technique) is used in the GSM standard. However, TDMA (the standard, i.e. IS-136) has been competing against GSM and systems based on CDMA modulation for adoption by the carriers.

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 offset required to make its transmission match the gap in transmission from it's peers.

In the GSM system, the synchronisation of the mobile phones is achieved by sending timing offset commands from the base station which instructs the mobile phone to transmit earlier or later. The mobile phone is not allowed to transmit for its entire timeslot, but there is a guard period at the beginning and end of the timeslot. As the transmission moves into the guard period, the mobile network adjusts the timing offset to re-center the transmission.

Initial synchronisation 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 it's messages starting 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 neighbouring time slot 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 tricks are used. By changing the syncronisation between the uplink and downlink at the base station, however, this limitation can be overcome.

A major advantage of TDMA is that the radio part of the mobile only needs to listen and broadcast for it's 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 UMTS. This in turn allows for co-existence of microcell layers with macrocell layers.

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 irritating buzz which can sometimes be heard if a GSM phone is left next to a radio.