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Frequency Division Multiplexing Guide

Multiplexing involves transmitting multiple signals over a single transmission path by combining or dividing the signals. Frequency division multiplexing (FDM) allocates different frequency slots within the overall bandwidth to different channels. Each information signal occupies its own frequency slot simultaneously. At the receiver, the signals are separated using bandpass filters tuned to the different carrier frequencies. FDM allows for efficient use of bandwidth by stacking narrowband channels on top of each other in the frequency domain.

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77 views7 pages

Frequency Division Multiplexing Guide

Multiplexing involves transmitting multiple signals over a single transmission path by combining or dividing the signals. Frequency division multiplexing (FDM) allocates different frequency slots within the overall bandwidth to different channels. Each information signal occupies its own frequency slot simultaneously. At the receiver, the signals are separated using bandpass filters tuned to the different carrier frequencies. FDM allows for efficient use of bandwidth by stacking narrowband channels on top of each other in the frequency domain.

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Vaibhav Nata
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© © All Rights Reserved
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MULTIPLEXING

o Multiplexing – methods of transmitting more than one signal along a


single transmission path/stream i.e many to one.
o Demultiplexing – separate the stream back into its component
transmission i.e one to many.
o Path– refers to the physical link.
o Channel – refers to a portion that carries a transmission between a
given pair of devices. One path can have many channels.

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MULTIPLEXING
o Two common form of multiplexing are Frequency Division
Multiplexing(FDM) and Time Division Multiplexing(TDM).
o Advantages:
n Increase number of channels so that more info can be transmitted
n Save cost by using one channel to send many info signals

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Frequency Division Multiplexing (FDM)
o FDM – multiple sources that originally occupied the same frequency
spectrum are each converted to different frequency band and
transmitted simultaneously.
o FDM is an analog technique – the information entering an FDM
system must be analog. If the source is digital, it must be
converted to analog before being frequency-division multiplexed.
o Split the total channel bandwidth into several smaller channels of
different frequencies.
o Different signal travel over the medium concurrently.
o Guard bands keep the modulated signals from overlapping and
interfering with one another.
o Modulation is used to lift the centre freq of the baseband signal up
into a preassigned freq slot.

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FDM

q In communication systems, Frequency Division Multiplexing (FDM)


is a method in which each signal (channel) is allocated a frequency
slot within the overall line/transmission bandwidth.
q In other words the total available frequency bandwidth on the
transmission line is divided into frequency channels and each
information signal occupies one of these channels
q The signal will have exclusive use of this frequency slot all the time
(i.e. each subscriber occupies his/her own slot).
FDM of Three Voiceband Signals
The bandwidth of a voice signal
is taken to be 4kHz, with an
effective spectrum of 300 to
3400Hz.
If this signal is used to amplitude
modulate a 64 kHz carrier, the
spectrum becomes the modulated
signal which has a bandwidth of 8
kHz, extending from 60 to 68kHz.

To make efficient use of bandwidth,


we transmit only the lower sideband.
If three voice signals are used to
modulate carriers at 64, 68, and 72
kHz, the spectrum output is as
shown.
Frequency Division Multiplexing (FDM)
A number of signal, mi(t), i=1..,n are to be
multiplexed onto the same Tx medium.

Each signal mi(t),is modulate onto a


carrier fi, refers as subcarrier.
Modulated signals are then summed to
produce a composite signal mb(t).

Figure (b) shows the result – signal mi(t)


is shifted to be centered of fi.

fi must be chosen so that the BW of the


various signals do not overlap i.e channel
must be separated by unused BW (guard
band).

At the Rx end, the FDM signal is


demodulated to retrieved mb(t), which is
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then passed through n BPF.a
With FDM, each
narrowband channels are
stacked on top of one
another in the frequency
domain.

Figure shows a simple


FDM system where
four 5kHz channels are
frequency-division
multiplexed into a
single 20kHz combined
channel.

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