# How Frequency Division Multiplexing works

1. Jul 2, 2012

### justwild

I know that FDM actually uses a division of total frequency of the carrier channel to separate into several smaller frequency channels. I want to know that whether these small channels are actually having their own waves over the communication medium or the carrier wave is modulated in such a way that it works for all the smaller channels?
If they are having their own waves over the communication channel then please explain how can two waves travel over the same medium.
I always assume signals to be like AC current...how can two AC current travel over the same wire!!!

2. Jul 2, 2012

### phinds

I don't know about FDM, but frequency multiplexing in general is trivial ... just add two sine waves together to see what I mean. It's one wave but filters can separate out the two frequencies.

3. Jul 2, 2012

### Ibix

The same principle is at work as when Newton split and recombined light with a prism. Conceptually, what you do in FDM is shine a red laser, a green laser, and a blue laser into a prism such that they are combined into one white(ish) light which you feed in to a fiber-optic cable. At the other end of the cable you feed the beam into a prism, which splits it into the red, green and blue beams again. Modulate the beams and hey presto! It's quite straightforward, seen that way (any actual telecoms engineers reading this will be turning purple because I'm plain ignoring a lot of very important stuff - but that's the gist of it).

I suspect what's making your head spin is thinking that a wave plus a wave is just another wave. That's true enough, but (if the component waves are different frequencies) the sum is a different shaped wave, and the precise shape depends on the frequencies of the component waves - in fact, is uniquely determined by the frequencies of the component waves. By using a mathematical technique called a Fourier Transform (or any one of various processes that implement it, explicitly or otherwise, such as prisms, diffraction, and electronic filters) you can separate out the components and get your separate signals back.

4. Jul 2, 2012

### CWatters

Either works. It depends on the system..

A simplest example is FM radio. Each FM radio station is allocated a limited frequency band from a larger slice of the spectrum. Each is transmitted independantly.

Another example would be an analogue cable TV system. In this case the signals are combined in a multiplexer before being launched onto the cable.

There is no problem sending many sound or radio waves through the air. Sending them down a wire is no different. Something at the receiving end has to select only the wanted channels. This would typically be a set of band pass filters.