How is frequency multiplexing implemented?
I understand that in time division multiplexing the feed of each channel just uses its own time interval for sending out data over the same medium.
But how can you mix different frequencies into a single medium? Wouldn't that just cause static ie: "white-noise".
Assuming this is in the context of computer comms (i.e. WiFi or ADSL) then what you're looking for is called OFDM (Orthogonal Frequency Division Multiplexing) which is specifically designed for carrying digital data.
The Wikipedia link above can do a far better explanation than I can, but in simple terms the data is chunked up and then lots of individual sub-carriers are used to transmit those chunks.
The result isn't white noise, because that term means noise spread evenly across the spectrum. With OFDM there'll still be identifiable peaks in the spectrum of the received signal which can be detected and demultiplexed.
Are you looking for something more than this as an answer?
EDIT: based on your edit. No this would not produce any more whitenoise than the airwaves do now. Take TV and Radio stations. All are using the air as the transmission medium, but each device receiving the "data" tunes to a specific frequency and all others are ignored.
In the case of frequency multiplexing, each device has multiple tuners and logically joins the received "data" into a single stream.
There's a very succinct diagram showing how three separate source\user signal pairs can be carried on a single medium in this National Instruments page. Fundamentally each individual channel is allocated a frequency band, with each set of bands occupying separate chunks of the RF spectrum. Because of this they can all be carried on the same shared medium at the same time and the band pass filters at the receiving end allow them to be separated. The same principle applies whatever frequency range or media you want to use. There are many ways to implement this so if you are interested in how it works at the physical level for something in particular then you will have to be a lot more specific.
As far as putting these channels to use and carrying signals\data there are quite a few variations in technique depending on how the signal itself is carried within the band assigned to the carrier - the Wikipedia article that Scott linked to is as good a place as any to to start digging into those.
White noise has a flat continuous power spectrum over the entire frequency range, ie, containes a continuous range of frequencies...
A simple for of frequency multiplex is all the channels on your radio, all the radio broadcasts are modulated on different carriers and transmitted over the same medium (the atmosphere).
In Frequency Multiplexing, data is simply sliced up, and the sliced up bits all modulated onto a different carrier - and again, as long as the medium has the ability to pass all the carrier frequencies, you can demux them and put all the bits of data together.
