# 8.2 Frequency Division Multiple Access (FDMA)

## Basic system operation

Used extensively in the early telephone and wireless multi-user communication systems, frequency division multiplexing of users is perhaps the most intuitive form of resource sharing. If a channel, such as a cable, has a transmission bandwidth W Hz, and individual users require B Hz to achieve their required information rate, then the channel in theory should be able to support W/B users simultaneously by using bandpass modulation, and placing each user in an adjacent slot of the available bandwidth. Immediately, we see that the efficiency of frequency multiplexing is governed by how effectively the transmission bandwidth is constrained by each user (the value of alpha in the root raised cosine filter, for example). It is also dependent on how good (selective) the 'de-multiplexing' system is at filtering out the modulation corresponding to each user. |

With frequency division multiplexing, the data rate and hence modem design for each user remains unchanged by the requirement to operate a multi-user system, and the only additional circuitry is for frequency conversion to the assigned slot. The user will typically be assigned the frequency slot for the duration of the message.

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