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frequency division multiplexing

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Frequency-division multiplexing (FDM)

A typical analog Internet connection via a twisted pair telephone line requires approximately three kilohertz (3 kHz) of bandwidth for accurate and reliable data transfer. Twisted-pair lines are common in households and small businesses. But major telephone cables, operating between large businesses, government agencies, and municipalities, are capable of much larger bandwidths. Suppose a long-distance cable is available with a bandwidth allotment of three megahertz (3 MHz). This is 3,000 kHz, so in theory, it is possible to place 1,000 signals, each 3 kHz wide, into the long-distance channel. The circuit that does this is known as a multiplexer. It accepts the input from each individual end user, and generates a signal on a different frequency for each of the inputs. This results in a high-bandwidth, complex signal containing data from all the end users. At the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high-bandwidth cable. When FDM is used in a communications network, each input signal is sent and received at maximum speed at all times. This is its chief asset. However, if many signals must be sent along a single long-distance line, the necessary bandwidth is large, and careful engineering is required to ensure that the system will perform properly. In some systems, a different scheme, known as time-division multiplexing, is used instead.


 FDM is possible when the useful bandwidth of the medium excess the required bandwidth of signals to be transmission • (BWmedium >> BWsignal) • Example: voice signal is transmitted via an optical fiber – A number of signals are transmitted simultaneously if each signal is modulated onto a different carrier frequency, and the carrier frequencies are sufficiently separated that the bandwidths of the signals do not overlap.

Some  of the important things why we are using FDM

Useful bandwidth of medium exceeds required bandwidth of channel

Each signal is modulated to a different carrier frequency

Carrier frequencies separated so signals do not overlap (guard bands)

e.g. broadcast radio

Channel allocated even if no data

Multiplexing

sources are fed into a multiplexer

– the multiplexer modulates each signal onto a different frequency

– each modulated signal requires a certain bandwidth centered around its carrier frequency, referred to as a channel

– The modulated signals are summed to produce a composite signal


Problems of FDM

– Crosstalk

• occur if the spectra of adjacent component signals overlap

– Inter modulation noise

• on a long link, the nonlinear effects of amplifiers on a signal in one channel could produce frequency components in other channels

No. of voice channel        Bandwidth                                     Spectrum

Group                               12 48 kHz                                          60-108 kHz

Super group                      60 240 kHz                                      312-552 kHz

Master group 300             2.52 MHz                                        564-3084 kHz

Jumbo group 3600           16.984 MHz                            0.564 - 17.584 MHz

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