What is TDMA

TDMA stands for ‘Time Division Multiple Access’. It is a technology that allows multiple network nodes to share a single channel between them by allowing the nodes to transmit or receive in different time slots. An example of a system where this technique is employed is the cellular networks of Global System for Mobile (GSM). 

To grasp this technique in a better way, suppose that eight mobile units in a cellular network need to share a single frequency channel in order to uplink voice information to the Base Station. Through the control channels of the Base Station, each mobile unit will be assigned a different time slot on the channel in which it can transmit its data to the Base Station. For example: – If say ’t’ seconds is the period of one time slot, then the first mobile unit will transmit in the time 0 to t seconds, the second unit will transmit in time t to 2t seconds, the third unit will transmit in time 2t to 3t seconds and so on.  At 8t seconds when all the mobile units have finished transmitting which is known as the end of one TDMA frame, the first unit will get its turn to transmit again followed by the other units in the same order. The same scheme will be used when the mobile units are receiving information from the Base Station while sharing a single channel.

Then a question arises, why aren’t there any intervals heard between voice when a mobile unit is receiving data on a TDMA channel? Well the fact is that the duration of the time slots used is very short usually of the order of a few microseconds. So the intervals between the consequently received slots are so small that the human ear can’t resolve them and the speech is heard as one continuous stream.

In practical systems, the allocation of TDMA channels is done on dynamic basis according to the requirements of the network nodes. Also there is a guard period used between consecutive slots to avoid intermixing of data.  And then there are two schemes of Duplexing that can be used in a TDMA system, frequency division multiplexing (FDD) where uplink and downlink are on different frequency channels and time division multiplexing (TDD) where uplink and downlink are on the same frequency channel in different time slots.

The use of TDMA not only increases the capacity of a channel but also allows the mobile units to scan control channels during the duration in which they remain idle. This forms the basis of the implementation of Mobile Initiated Handoff (MAHO) which is a feature of GSM networks.  It also eliminates the need to use duplexers in mobile transceivers because of a time interval between transmission and reception in this system.

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