Editing Frequency-division multiplexing (section)

== Principle ==
[[File:Screenshot-fdm-75-random-am.png|thumb|The passband of an FDM channel carrying digital data, modulated by QPSK [[quadrature phase-shift keying]].]]
The multiple separate information (modulation) signals that are sent over an FDM system, such as the video signals of the television channels that are sent over a cable TV system,  are called [[baseband]] signals.   At the source end, for each frequency channel, an [[electronic oscillator]] generates a ''[[carrier signal|carrier]]'' signal, a steady oscillating waveform at a single [[frequency]] that serves to "carry" information. The carrier is much higher in frequency than the baseband signal.  The carrier signal and the baseband signal are combined in a [[modulator]] circuit.  The modulator alters some aspect of the carrier signal, such as its [[amplitude]], [[frequency]], or phase, with the baseband signal, "[[Piggybacking (data transmission)|piggybacking]]" the data onto the carrier.

The result of [[modulation|modulating]] (mixing) the carrier with the baseband signal is to generate sub-frequencies near the [[carrier frequency]], at the sum (''f''<sub>C</sub> + ''f''<sub>B</sub>) and difference (''f''<sub>C</sub> − ''f''<sub>B</sub>) of the frequencies.  The information from the modulated signal is carried in [[sideband]]s on each side of the carrier frequency.  Therefore, all the information carried by the channel is in a narrow band of frequencies clustered around the carrier frequency, this is called the [[passband]] of the channel.

Similarly, additional baseband signals are used to modulate carriers at other frequencies, creating other channels of information.  The carriers are spaced far enough apart in frequency that the band of frequencies occupied by each channel, the passbands of the separate channels, do not overlap.  All the channels are sent through the transmission medium, such as a coaxial cable, optical fiber, or through the air using a [[radio transmitter]].   As long as the channel frequencies are spaced far enough apart that none of the passbands overlap, the separate channels will not interfere with each other. Thus the available bandwidth is divided into "slots" or channels, each of which can carry a separate modulated signal.

For example, the [[coaxial cable]] used by [[cable television]] systems has a bandwidth of about 1000 [[MHz]], but the passband of each television channel is only 6&nbsp;MHz wide, so there is room for many channels on the cable (in modern [[digital cable]] systems each channel in turn is subdivided into subchannels and can carry up to 10 digital television channels).

At the destination end of the cable or fiber, or the radio receiver,  for each channel a [[local oscillator]] produces a signal at the carrier frequency of that channel, that is mixed with the incoming modulated signal.  The frequencies subtract, producing the baseband signal for that channel again.   This is called [[demodulation]].   The resulting baseband signal is filtered out of the other frequencies and output to the user.