How Frequency Division Multiplexing works

[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!

 

[phinds]

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!

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.

 

[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 into 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 anyone 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.

 

[CWatters]

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?

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.

..please explain how can two waves travel over the same medium

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.

 

[sardar]

Frequency Division Multiplexing (FDM) works by dividing the available frequency spectrum of a carrier channel into smaller frequency channels, each of which can carry a separate communication signal. This allows multiple signals to be transmitted simultaneously over the same communication medium.

In FDM, the carrier wave is modulated in such a way that it can carry multiple signals at different frequencies. Each of the smaller frequency channels is then allocated to a specific signal, which is then transmitted using its own wave over the communication medium.

To understand how two waves can travel over the same medium, it is important to first understand the concept of interference. When two waves of the same frequency are superimposed on each other, they can either amplify or cancel each other out, depending on their relative phase. In FDM, the signals are carefully selected and modulated at different frequencies to avoid interference and allow them to coexist on the same medium.

In terms of your analogy to AC current, it is important to note that FDM is not limited to electrical signals. It can also be used for other types of signals, such as radio waves or optical signals. In these cases, the signals are not actually traveling over the same wire, but rather through the same medium (e.g. air or fiber optic cable).

In conclusion, FDM allows for efficient use of the available frequency spectrum by dividing it into smaller channels for simultaneous transmission of multiple signals. These signals are carefully modulated and selected to avoid interference and allow them to coexist on the same communication medium.