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. |
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