Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM)/FDM and TDM 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. This allows a single transmission medium such as a cable or optical fiber to be shared by multiple independent signals.
Another use is to carry separate serial bits or segments of a higher rate signal in parallel.
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. Another example is cable 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 broadband DSL modems to transmit large amounts of computer data through twisted pair telephone lines, among many other uses. 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 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, 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" the data onto the carrier. The information from the modulated signal is carried in sidebands 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.
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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. 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.
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. For shorter distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier.
See channel carrier system. By the end of the 20th Century, FDM voice circuits had become rare. Since the late 20th century digital subscriber lines DSL have used a Discrete multitone DMT system to divide their spectrum into frequency channels. The concept corresponding to frequency-division multiplexing in the optical domain is known as wavelength-division multiplexing. DTL eliminates group and super group equipment. FDM can also be used to combine signals before final modulation onto a carrier wave.
An analog NTSC television channel is divided into subcarrier frequencies for video, color, and audio. DSL uses different frequencies for voice and for upstream and downstream data transmission on the same conductors, which is also an example of frequency duplex.
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. FDMA is the traditional way of separating radio signals from different transmitters.
In the s and 70s, several inventors attempted FDM under the names of acoustic 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 the invention of the telephone.
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