Basic
One of the important aims of telecom is to provide sharing of communication media between multiple users.
Say for example, there are 10 transmission frequencies available for transmission:
1) We can provide two way communication between 10 users with these frequencies, 2 frequencies per 2 user, one in each direction. This is Frequency Division Multiple Access (diagramatically shown below).
2) Another way to assign time slots in a frequency or range of frequencies. This Time Division Multiple Access (diagramatically shown below).
3) To improve above situation, let us say it is possible to assign time slots in frequency as below, we can increase the maximum number of users that can be supported as shown below.
Popular example of TDMA with FDMA is GSM air interface. A variant of FDMA popular with latest 4G technologies is OFDMA in which separation/sharing is achieved using Orthogonal frequencies. OFDMA is discussed in later article.
Let us look at the problem from alternative point of view.
Let say that it is possible to make above complete time-frequency spectrum to all users and differentiation between two users is made with a help secret code. A real like example would be a restaurant full of different language speaking people. A person will only be able to "understand" only those conversations which are in his language (even if he can "hear" all conversations !). This method popularly known as Code Division Multiple Access or CDMA.
© Copyright Samir Amberkar 2010
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