Frequency-division multiplexing

frequency division multiplexingFDMfrequency division multiplexfrequency-divisionmultiplexingfrequencyfrequency division multiplexedfrequency-frequency-division multiplexfrequency-domain multiplexing
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal.wikipedia
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DSL modem

ADSL modemDSL modemsDSL
FDM is also used by telephone systems to transmit multiple telephone calls through high capacity trunklines, communications satellites to transmit multiple channels of data on uplink and downlink radio beams, and broadband DSL modems to transmit large amounts of computer data through twisted pair telephone lines, among many other uses.
With ADSL, the modem and the DSLAM communicate by a protocol called discrete multitone modulation (DMT), which is a form of frequency division multiplexing.

Cable television

cablecable TVcable channel
Another example is cable television, in which many television channels are carried simultaneously on a single cable.
Many channels can be transmitted through one coaxial cable by a technique called frequency division multiplexing.

Modulation

modulatedmodulatordigital modulation
The carrier signal and the baseband signal are combined in a modulator circuit.
Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband channels (several different carrier frequencies).

Telecommunication

telecommunicationscommunicationstelecom
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal.
This system of dividing the medium into channels according to frequency is called "frequency-division multiplexing".

Radio

radio communicationradio communicationswireless
The most natural example of frequency-division multiplexing is radio and television broadcasting, in which multiple radio signals at different frequencies pass through the air at the same time.

Baseband

baseband signalbase bandcellular baseband
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.
Frequency division multiplexing (FDM) allows an analog telephone wire to carry a baseband telephone call, concurrently as one or several carrier-modulated telephone calls.

Carrier wave

carrier frequencycarriercarrier signal
At the source end, for each frequency channel, an electronic oscillator generates a carrier signal, a steady oscillating waveform at a single frequency that serves to "carry" information.
The purpose of the carrier is usually either to transmit the information through space as an electromagnetic wave (as in radio communication), or to allow several carriers at different frequencies to share a common physical transmission medium by frequency division multiplexing (as in a cable television system, for example).

Local oscillator

LOlocal-oscillator (LO)oscillator
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.
They are also used in many other communications circuits such as modems, cable television set top boxes, frequency division multiplexing systems used in telephone trunklines, microwave relay systems, telemetry systems, atomic clocks, radio telescopes, and military electronic countermeasure (antijamming) systems.

12-channel carrier system

12 channel carriercarrier systems
See 12-channel carrier system.
In the U.S. telephone network, the 12-channel carrier system was an early frequency-division multiplexing system standard, used to carry multiple telephone calls on a single twisted pair of wires, mostly for short to medium distances.

Orthogonal frequency-division multiplexing

OFDMCOFDMOFDM system comparison table
Since the late 20th century digital subscriber lines (DSL) have used a Discrete multitone (DMT) system to divide their spectrum into frequency channels.
OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method.

Single-sideband modulation

SSBsingle sidebandsingle-sideband
Those cables didn't allow such large bandwidths, so only 12 voice channels (double sideband) and later 24 (single sideband) were multiplexed into four wires, one pair for each direction with repeaters every several miles, approximately 10 km.
SSB was also used over long distance telephone lines, as part of a technique known as frequency-division multiplexing (FDM).

L-carrier

L-3L3 coaxial carrierLong Distance telephone
For long distance telephone connections, 20th century telephone companies used L-carrier and similar coaxial cable systems carrying thousands of voice circuits multiplexed in multiple stages by channel banks.
Starting in 1911, telephone networks used frequency-division multiplexing to carry several voice channels on a single physical circuit, beginning with the first Type C carrier in that year, which heterodyned three voice channels stacked on top of one voice circuit.

Wavelength-division multiplexing

DWDMwavelength division multiplexingWDM
An analogous technique called wavelength division multiplexing is used in fiber-optic communication, in which multiple channels of data are transmitted over a single optical fiber using different wavelengths (frequencies) of light.
The term WDM is commonly applied to an optical carrier, which is typically described by its wavelength, whereas frequency-division multiplexing typically applies to a radio carrier which is more often described by frequency.

Passband

pass bandpass-bandpassband signal
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.

Balanced line

balancedbalanced pairbalanced circuit
For shorter distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier.
Telephone trunk lines, and especially frequency division multiplexing carrier systems, are usually 4-wire circuits rather than 2-wire circuits (or at least they were before fibre-optic became widespread) and require a different kind of cable.

Time-division multiplexing

TDMtime division multiplexingtime slot
Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM.

Communication channel

channelchannelscommunications channel
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal. Where frequency-division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA).

AN/UCC-4

The AN/UCC-4, or UCC-4, was a solid state frequency division multiplexer manufactured by Lenkurt in 1969.

Repeater

digipeaterrepeatersrelay
Those cables didn't allow such large bandwidths, so only 12 voice channels (double sideband) and later 24 (single sideband) were multiplexed into four wires, one pair for each direction with repeaters every several miles, approximately 10 km.
They are also used in trunklines that transmit multiple signals using frequency division multiplexing (FDM).

Frequency-division multiple access

FDMAfrequency division multiple accessFDD
Where frequency-division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA).
Frequency-division multiplexing (FDM) is also distinct from FDMA.

Bandwidth (signal processing)

bandwidthbandwidthssignal bandwidth
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal.

Frequency band

frequency rangebandsfrequency bands
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal.

Optical fiber

fiber opticfiber opticsfiber-optic
An analogous technique called wavelength division multiplexing is used in fiber-optic communication, in which multiple channels of data are transmitted over a single optical fiber using different wavelengths (frequencies) of light. This allows a single transmission medium such as a cable or optical fiber to be shared by multiple independent signals.

Broadcasting

broadcastbroadcasterbroadcasters
The most natural example of frequency-division multiplexing is radio and television broadcasting, in which multiple radio signals at different frequencies pass through the air at the same time.

Communications satellite

satellite communicationssatellitecommunication satellite
FDM is also used by telephone systems to transmit multiple telephone calls through high capacity trunklines, communications satellites to transmit multiple channels of data on uplink and downlink radio beams, and broadband DSL modems to transmit large amounts of computer data through twisted pair telephone lines, among many other uses.