TDMA, FDMA and CDMA

A novice to digital communication system may wonder what are the meanings of these terminologies.

Let's start from the "M" in the middle, Multiplexing. In telecom system, multiplexing is a term used to refer to a process where multiple signals are combined into one signal over a shared medium. The reason of multiplexing is to share a medium (or channel in terms of information theory), for example, a wire connection, or a radio frequency, which is often limited resource.

It should now easy to understand the initials: "T" - time, "F" - frequency or "C" - code are just different ways of multiplexing.

TDM - channel is divided into sub-channels by time slots. A typical example is the congestion reduction strategy in Beijing, today is my turn, tomorrow is yours :-)
FDM - channle is divided into sub-channels by frequencies - by modulating baseband signals onto different frequency carriers. A typical example is dividing the road into lanes, cars can drive along without interfering each other on diff. lanes (ideally :-)
CDM - channel is divided into sub-channels by scrambling codes. Difficult to understand? Not at all. An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can communicate.

What's the "A" then? - Relation to multiple access:

A multiplexing technique may be further extended into a multiple access method or channel access method, for example TDM into Time-division multiple access (TDMA) and statistical multiplexing into carrier sense multiple access (CSMA). A multiple access method makes it possible for several transmitters connected to the same physical medium to share its capacity. Multiplexing is provided by the physical layer, while multiple access also involves a media access control protocol, which is part of the data link layer.