In any cellular telecommunications or mobile phone system, it is necessary to have a scheme that enables several multiple users to gain access to it and use it simultaneously. These methods are known as multiple access schemes.

There are three main multiple access schemes that are in use at the moment:

* Frequency Division Multiple Access - FDMA


* Time Division Multiple Access - TDMA


* Code Division Multiple Access - CDMA


FDMA
FDMA is the most straightforward of the multiple access schemes that have been used. As a subscriber comes onto the system, or swaps from one cell to the next, the network allocates a channel or frequency to each one. In this way the different subscribers are allocated a different slot and access to the network. As different frequencies are used, the system is naturally termed Frequency Division Multiple Access. This scheme was used by all analogue systems.

TDMA
The second system came about with the transition to digital schemes. Here digital data could be split up in time and sent as bursts when required. As speech was digitised it could be sent in short data bursts, any small delay caused by sending the data in bursts would be short and not noticed. In this way it became possible to organise the system so that a given number of slots were available on a give transmission. Each subscriber would then be allocated a different time slot in which they could transmit or receive data. As different time slots are used for each subscriber to gain access to the system, it is known as time division multiple access. Obviously this only allows a certain number of users access to the system. Beyond this another channel may be used, so systems that use TDMA may also have elements of FDMA operation as well.

CDMA
The final scheme uses one of the aspects associated with the use of direct sequence spread spectrum. It can be seen from the article in the cellular telecoms area of this site that when extracting the required data from a DSSS signal it was necessary to have the correct spreading or chip code, and all other data from sources using different orthogonal chip codes would be rejected. It is therefore possible to allocate different users different codes, and use this as the means by which different users are given access to the system.

The scheme has been likened to being in a room filled with people all speaking different languages. Even though the noise level is very high, it is still possible to understand someone speaking in your own language. With CDMA different spreading or chip codes are used. When generating a direct sequence spread spectrum, the data to be transmitted is multiplied with spreading or chip code. This widens the spectrum of the signal, but it can only be decided in the receiver if it is again multiplied with the same spreading code. All signals that use different spreading codes are not seen, and are discarded in the process. Thus in the presence of a variety of signals it is possible to receive only the required one.

In this way the base station allocates different codes to different users and when it receives the signal it will use one code to receive the signal from one mobile, and another spreading code to receive the signal from a second mobile. In this way the same frequency channel can be used to serve a number of different mobiles.

Situation today
Although the latest cellular telecommunications systems use CDMA as their basis, elements of TDMA and FDMA are also used. Both the major schemes, UMTS and CDMA2000 have a limit on the number of users who are able to use a single channel. In some instances two or more channels may be allocated to a particular cell. This means that the system still uses an element of FDMA.

Additionally UMTS incorporates some timeslots, and this means that the scheme uses elements of TDMA.
While CDMA is currently the dominant technology, both the other forms of access scheme are still in evidence, not just in legacy technologies, but utilised as part of the main access scheme in the latest 3G systems.