And now users could either be given the all the frequency bands, but for a time given, then we get time division multiple access, or the users can be given a particular frequency for the whole time, we get FDMA. Or the users could be given, like here, at some time one frequency, and at another time another frequency, another time another frequency, giving us the FT-matrix, or the CDMA code division multiple access.
Similarly, one could also consider space into account and get what is known as the space/time and space/frequency, or the last one can even combine all the three and you can get SFT, giving a large number of multiple access. But then, obviously, complexity will be quite more.

Now quickly I wish to compare the three basic techniques of the multiple access, namely the frequency division, the time division and code division. In fact, they are being used in the cellular mobile communications.
FDMA, as you can see, is where I subdivide the given frequency band into a smaller band and each user is assigned a separate frequency band for the whole time. It is a simplest, obviously, it is best suited for analogue signals, and that is why it was used for the analogue systems and other analogue similar systems, each user has his own frequency band, as you can see. Of course you require bandwidth between the two, lowing the efficiency of the utilisation, the receivers have to tune the separate or desired frequencies, and that can be done with the help of filters only. And that is a great disadvantage, FDMA being based on filters, which are quite bulky in nature. People do not want now FDMA to be used in cellular mobile communications. Number of frequency is again limited, due to the given number, to the given spectrum, they can be simply calculated by this, that is a transmission bandwidth. Numbers of users, in case of the analogue cellular systems, where 25 MHz is the spectrum allocated and 30 KHz per channel you have roughly 833 channels, or 833 users could be supported by this.
But this is not enough, if you want to have more number of users, obviously you have to reuse the frequencies in the cellular concept, and because of the reuse of the frequency, you will have a problem of the carrier to interference ratio and then you have the capacity of the cells by this relationship.