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Intelecommunications,frequency-division multiplexing (FDM) is a technique by which the totalbandwidth available in acommunication medium is divided into a series of non-overlappingfrequency bands, each of which is used to carry a separate signal. This allows a single transmission medium such as a microwave radio link, cable oroptical fiber to be shared by multiple independent signals. Another use is to carry separate serial bits or segments of a higher rate signal inparallel.
The most common example of frequency-division multiplexing isradio and television broadcasting, in which multiple radio signals at different frequencies pass through the air at the same time. Another example iscable television, in which many television channels are carried simultaneously on a single cable. 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 broadbandDSL modems to transmit large amounts of computer data throughtwisted pair telephone lines, among many other uses.
An analogous technique calledwavelength division multiplexing is used infiber-optic communication, in which multiple channels of data are transmitted over a singleoptical fiber using differentwavelengths (frequencies).
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 calledbaseband signals. At the source end, for each frequency channel, anelectronic oscillator generates acarrier signal, a steady oscillating waveform at a singlefrequency 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 amodulator circuit. The modulator alters some aspect of the carrier signal, such as itsamplitude,frequency, or phase, with the baseband signal, "piggybacking" the data onto the carrier.
The result ofmodulating (mixing) the carrier with the baseband signal is to generate sub-frequencies near thecarrier frequency, at the sum (fC +fB) and difference (fC −fB) of the frequencies. The information from the modulated signal is carried insidebands 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 thepassband 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 aradio 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, thecoaxial cable used bycable television systems has a bandwidth of about 1000MHz, but the passband of each television channel is only 6 MHz wide, so there is room for many channels on the cable (in moderndigital 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 alocal 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 calleddemodulation. The resulting baseband signal is filtered out of the other frequencies and output to the user.
Forlong distance telephone connections, 20th century telephone companies usedL-carrier and similarcoaxial cable systems carrying thousands of voice circuits multiplexed in multiple stages bychannel banks.
For shorter distances, cheaperbalanced pair cables were used for various systems includingBell System K- and N-Carrier. Those cables did not allow such large bandwidths, so only 12 voice channels (double sideband) and later 24 (single sideband) were multiplexed intofour wires, one pair for each direction withrepeaters every several miles, approximately 10 km. See12-channel carrier system. By the end of the 20th century, FDM voice circuits had become rare. Modern telephone systems employ digital transmission, in whichtime-division multiplexing (TDM) is used instead of FDM.
Since the late 20th century,digital subscriber lines (DSL) have used aDiscrete multitone (DMT) system to divide their spectrum into frequency channels.
A once commonplace FDM system, used for example inL-carrier, uses crystal filters which operate at the 8 MHz range to form a Channel Group of 12 channels, 48 kHz bandwidth in the range 8140 to 8188 kHz by selecting carriers in the range 8140 to 8184 kHz selectingupper sideband this group can then be translated to the standard range 60 to 108 kHz by a carrier of 8248 kHz. Such systems are used in DTL (Direct To Line) and DFSG (Directly formed super group).
132 voice channels (2SG + 1G) can be formed using DTL plane the modulation andfrequency plan are given in FIG1 and FIG2 use of DTL technique allows the formation of a maximum of 132 voice channels that can be placed direct to line. DTL eliminates group and super group equipment.
DFSG can take similar steps where a direct formation of a number of super groups can be obtained in the 8 kHz the DFSG also eliminates group equipment and can offer:
Both DTL and DFSG can fit the requirement of low density system (using DTL) and higher density system (using DFSG). The DFSG terminal is similar to DTL terminal except instead of two super groups many super groups are combined. A Mastergroup of 600 channels (10 super-groups) is an example based on DFSG.
FDM can also be used to combine signals before final modulation onto a carrier wave. In this case thecarrier signals are referred to assubcarriers: an example isstereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal.An analogNTSC television channel is divided intosubcarrier frequencies for video, color, and audio.DSL uses different frequencies for voice and forupstream anddownstream data transmission on the same conductors, which is also an example offrequency duplex.
Where frequency-division multiplexing is used as to allow multiple users to share a physicalcommunications channel, it is calledfrequency-division multiple access (FDMA).[1]
FDMA is the traditional way of separatingradio signals from differenttransmitters.
In the 1860s and 70s, several inventors attempted FDM under the names ofacoustic telegraphy and harmonic telegraphy. Practical FDM was only achieved in the electronic age. Meanwhile, their efforts led to an elementary understanding of electroacoustic technology, resulting in theinvention of the telephone.
