COMMUNICATION SYSTEM EEEB453 Chapter 7(Part I) MULTIPLEXING.

COMMUNICATION SYSTEM EEEB453Chapter 7(Part I)

MULTIPLEXING

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MULTIPLEXING Multiplexing – methods of transmitting more than one

signal along a single transmission path/stream i.e many to one.

Demultiplexing – separate the stream back into its component transmission i.e one to many.

Path– refers to the physical link. 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

Two common form of multiplexing are Frequency Division Multiplexing(FDM) and Time Division Multiplexing(TDM).

Two variations of these basic methods are frequency division multiple access (FDMA) and time division multiple access (TDMA)

Another form of multiple access is known as code-division multiple access (CDMA)

Advantages: Increase number of channels so that more info can be

transmitted Save cost by using one channel to send many info

signals

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Frequency Division Multiplexing (FDM)

FDM – multiple sources that originally occupied the same frequency spectrum are each converted to different frequency band and transmitted simultaneously.

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.

Split the total channel bandwidth into several smaller channels of different frequencies.

Different signal travel over the medium concurrently. Guard bands keep the modulated signals from

overlapping and interfering with one another. Modulation is used to lift the centre freq of the baseband

signal up into a preassigned freq slot.

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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 then passed through n BPF.a

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

With FDM, each narrowband channels are stacked on top of one another in the frequency domain.

Figure (a) shows how a group is formed with A-type channel bank.

Each voice band channel is bandlimited with an antialising filter prior to modulating the channel carrier.

Figure (b) shows the output spectrum.

Analog Hierarchy

▀ Figure shows first stages of telephone mux▀ Group multiplexer takes 12 voice ch and puts them on

subcarriers at 64, 68,…108kHz using LSB.▀ The resulting spectrum extends 48kHz starting at 60kHz.▀ Five such carriers are combined by LSB on subcarriers at 420,

468,… 612kHz to produce a supergroup from 312 to 552kHz8

Analog Hierarchy

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FDM in Telephone System The original signal voice is in 300 to 3000Hz range.

The voice is used to modulate subcarrier. Each subcarrier is on different frequency.

These subcarriers are then added together to form a single channel.

Balance Modulator

Voice

fc = 60kHz

Ch 12

fc = 104kHz

Ch 1

BPF

BPF

fc = 64kHz

Ch 11 BPF

fv

fv+fc

fv-fc fv-fc

56 – 64kHz60 – 64kHz

Selects USB

100– 108kHz

DSBSC

SSBSC

104– 108kHz

0 - 4 kHz

0 - 4 kHz

0 - 4 kHz

Linear Mixer

Voice signal amplitude modulates 1 of 12 ch’s in the 60 to 108kHz range.

The carrier freq begin at 60kHz with a spacing of 4kHz.(slightly higher than the highest typical freq of voice)

Output of the balance modulator – DSBSC. The output of the filter is the SB containing the original voice signal.

All 12 SSB signals are then summed in a linear mixer to produce a single frequency multiplexed signal – basic group.

Basic group freq spectrum for FDM telephone mux system is shown below.

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FDM in Telephone System

60 72 76 80f (kHz)

64 68 84 88 92 96 100 104 108

123456789101112 Channel No.

4kHz

Carrier frequencies

If more than 12 voice channels are needed, multiple basic groups are used.

Example 1– A cable TV service uses a single coaxial cable with bandwidth of 860 MHz to transmit multiple TV signals to subscribers. Each TV signal is 6 MHz wide. How many channels can be carried?

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Example 2– For a particular telephone company, the first sub carrier frequency is at 60 kHz and the total bandwidth is 96 kHz. Design a FDM system, given a general rule of 12 channels per basic group and 4 kHz per channel applies to the design.i. How many basic groups are required?ii. Draw the circuit diagram of your designiii. Draw the frequency spectrum of your multiplexed system

Solution

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i. Given BWtotal = 96kHz;12 channels/basic group 1 channel = 4 kHz, then 12 channels = 12x4 = 48kHz/basic group Thus 96/48 = 2 basic group

ii. Circuit Diagram

Voicefc = 60kHz

Ch 12

fc = 104kHz

Ch 1

BPF

BPF

Linear Mixer

fc = 108kHz

Ch 24

fc = 152kHz

Ch 13

BPF

BPF

Linear Mixer

Linear Mixer

f (kHz)60 104 108 152

48 kHz 48 kHz

iii. Frequency Spectrum

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