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Time Division Multiple Access (TDMA)

Module by: Richard Baraniuk

Summary: Briefly explains time division multiple access.

TDMA is a classical scheme for sharing a communications channel among several users. It is used in T1-T3 lines for trunking multiple telephone lines over a single telephone cable and also in several cell phone standards like GSM (European cellular). As the name suggests, TDMA involves sharing in time (as opposed to frequency in FDMA), where each user takes a turn with complete control over the channel. This scheme can be compared to normal conversation where one person speaks while the others listen, then that person finishes, another speaks, and so on.

Figure 1: This is a representation of TDMA's sharing in the time domain. Each shape is a different user controlling the channel for that interval. The guard intervals are used to guarantee there is no interference.
Figure 1 (tdma1.jpg)

Obviously, with TDMA's widespread use, it does have some advantages. Among these advantages are the fact that it is a simple and intuitive concept. This may be the reason it is so common today (since it was in use before CDMA existed). It also does allow for high bit rate bursts since each user has complete control of the channel for short periods, which is very nice for many applications.

This scheme does have some disadvantages also, however. Basically, it works nicely when there is one entity (base station) that is in complete control of everybody using the channel; it must keep track of who uses what and when. This is fairly reasonable in a cellular environment where one base station can regulate all of the hand held devices in its cell. Without this control, we are left with a situation similar reporters at a press conference, where everybody is yelling to get everybody else's attention and nobody can understand anybody.

A point worth noting here is that this system works because each user's transmission is orthogonal to all of the others'. This is due to the fact that all transmissions are disjoint in time.

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