Download Principles of Electronic Communication Systems

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Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 10
Multiplexing and Demultiplexing
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Topics Covered in Chapter 10
 10-1: Multiplexing Principles
 10-2: Frequency-Division Multiplexing
 10-3: Time-Division Multiplexing
 10-4: Pulse-Code Modulation
 10-5: Duplexing
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10-1: Multiplexing Principles
 Transmitting two or more signals simultaneously can
be accomplished by running multiple cables or setting
up one transmitter-receiver pair for each channel, but
this is an expensive approach.
 A single cable or radio link can handle multiple signals
simultaneously using a technique known as
multiplexing.
 Multiplexing permits hundreds or even thousands of
signals to be combined and transmitted over a single
medium.
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10-1: Multiplexing Principles
 Multiplexing is the process of simultaneously
transmitting two or more individual signals over a
single communication channel.
 It increases the number of communication channels so
that more information can be transmitted.
 An application may require multiple signals.
 Cost savings can be gained by using a single channel
to send multiple information signals.
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10-1: Multiplexing Principles

Four communication applications that would be
prohibitively expensive or impossible without
multiplexing are:
1. Telephone systems
2. Telemetry
3. Satellites
4. Broadcasting (radio and TV)
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10-1: Multiplexing Principles
Fig. 10-1: Concept of multiplexing.
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10-1: Multiplexing Principles
 The two most common types of multiplexing are
1. Frequency-division multiplexing (FDM)
 Generally used for analog information.
 Individual signals to be transmitted are assigned
a different frequency within a common
bandwidth.
2. Time-division multiplexing (TDM)
 Generally used for digital information.
 Multiple signals are transmitted in different time
slots on a single channel.
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10-1: Multiplexing Principles
 Another form of multiple access is known as code-
division multiple access (CDMA).
 Widely used in cell phone systems to allow many
subscribers to use a common bandwidth
simultaneously.
 Uses special codes assigned to each user that can be
identified.
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10-2: Frequency-Division
Multiplexing
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 In frequency-division multiplexing (FDM), multiple
signals share the bandwidth of a common
communication channel.
 All channels have specific bandwidths.
 A wide bandwidth can be shared for the purpose of
transmitting many signals at the same time.
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10-2: Frequency-Division
Multiplexing
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Transmitter-Multiplexers
 In an FDM system, each signal to be transmitted feeds
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a modulator circuit.
The carrier for each modulator (fc) is on a different
frequency.
The carriers are equally spaced from one another.
These carriers are referred to as subcarriers.
Each input signal is given a portion of the bandwidth.
The FDM process divides up the bandwidth of the
single channel into smaller, equally spaced channels,
each capable of carrying information in sidebands.
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10-2: Frequency-Division
Multiplexing
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Fig. 10-2: The transmitting end of an FDM system.
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10-2: Frequency-Division
Multiplexing
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Transmitter-Multiplexers
 The modulator outputs containing the sideband
information are added algebraically in a linear mixer.
 The resulting output signal is a composite of all the
modulated subcarriers.
 This signal can be used to modulate a radio transmitter,
or can itself be transmitted over a single channel.
 The composite signal can also become one input to
another multiplexed system.
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10-2: Frequency-Division
Multiplexing
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Receiver-Demultiplexer
 In an FDM system, a receiver picks up the signal and
demodulates it, recovering the composite signal.
 The composite signal is sent to a group of bandpass
filters, each centered on one of the carrier frequencies.
 Each filter passes only its channel and rejects all others.
 A channel demodulator then recovers each original
input signal.
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10-2: Frequency-Division
Multiplexing
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Figure 10-4: The receiving end of an FDM system.
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10-2: Frequency-Division
Multiplexing
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FDM Applications: Telemetry
 Sensors in telemetry systems generate electrical
signals that change in some way in response to
changes in physical characteristics.
 An example of a sensor is a thermistor, a device used
to measure temperature.
 A thermistor’s resistance varies inversely with
temperature.
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10-2: Frequency-Division
Multiplexing
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FDM Applications: Telemetry
 The thermistor is usually connected into a resistive
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network, such as a voltage divider or bridge.
The thermistor is also connected to a DC voltage
source.
The result is a DC output voltage which varies in
accordance with temperature.
This voltage is transmitted to a remote receiver for
measurement, readout, and recording.
The thermistor becomes one channel of an FDM
system.
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10-2: Frequency-Division
Multiplexing
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FDM Applications: Telemetry
 The varying direct or alternating current changes the
frequency of an oscillator operating at the carrier
frequency.
 Such a circuit is called a voltage-controlled
oscillator (VCO) or subcarrier oscillator (SCO).
 Most VCOs are astable multivibrators whose frequency
is controlled by the input from the signal conditioning
circuits.
 A system that uses FM of the VCO subcarriers as well
as FM of the final carrier is called FM/FM.
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10-2: Frequency-Division
Multiplexing
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Figure 10-5: An FDM telemetry transmitting system.
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10-2: Frequency-Division
Multiplexing
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Figure 10-7: An FM/FM telemetry receiver
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10-2: Frequency-Division
Multiplexing
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FDM Applications: Telemetry
 On the receiving end of a telemetry system, an FM
demodulator reproduces the original composite
multiplexed signal, which is then fed to a demultiplexer.
 The demultiplexer divides the signals and reproduces
the original inputs.
 The output of the first FM demodulator is fed
simultaneously to multiple bandpass filters, each of
which is tuned to the center frequency of one of the
specified subchannels.
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10-2: Frequency-Division
Multiplexing
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FDM Applications: Telemetry
 Each filter passes only its subcarrier and related
sidebands and rejects all the others.
 The demultiplexing process essentially uses filters to
sort the composite multiplex signal back into its original
components.
 The output of each filter is the subcarrier oscillator
frequency with its modulation.
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10-2: Frequency-Division
Multiplexing
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FDM Applications: Telemetry
 These signals are then applied to FM demodulators.
Also known as discriminators, these circuits take the
FM signal and recreate the original dc or ac signal
produced by the transducer.
 The original signals are measured or processed to
provide the desired information from the remote
transmitting source.
 In most systems, the multiplexed signal is sent to a data
recorder where it is stored for possible future use.
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10-2: Frequency-Division
Multiplexing
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FDM Applications: Telephone Systems
 For decades, telephone companies used FDM to
send multiple telephone conversations over a
minimum number of cables.
 The original voice signal, in the 300- to 3000-Hz
range is used to modulate a subcarrier.
 Lower sideband (LSB) SSB AM was used.
 Each subcarrier is on a different frequency. Those
subcarriers are added together to form one channel.
 The FDM system has been replaced by an all-digital
time multiplexing (TDM) system.
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10-2: Frequency-Division
Multiplexing
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FDM Applications: Cable TV
 In a cable TV system, TV signals, each in its own 6-
MHz channel, are multiplexed on a common coaxial or
fiber-optic cable and sent to homes.
 Each 6-MHz channel carries the video and voice of the
TV signal.
 Coaxial and fiber-optic cables have an enormous
bandwidth and can carry more than one hundred TV
channels.
 Many cable TV companies also use their cable system
for Internet access.
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10-2: Frequency-Division
Multiplexing
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FDM Applications: FM Stereo Broadcasting
 In recording original stereo, two microphones are used
to generate two separate audio signals.
 Two microphones pick up sound from a common
source, such as voice, but from different directions.
 The separation of the two microphones provides
sufficient differences in the two audio signals to provide
a realistic reproduction of the original sound.
 FDM techniques are used to transmit these
independent signals by a single transmitter.
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10-3: Time-Division Multiplexing
 In FDM, multiple signals are transmitted over a single
channel, each signal being allocated a portion of the
spectrum within that bandwidth.
 In time-division multiplexing (TDM), each signal
occupies the entire bandwidth of the channel.
 Each signal is transmitted for only a brief period of
time.
 TDM can be used with both digital and analog signals.
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10-3: Time-Division Multiplexing
Figure 10-12: The basic TDM concept.
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10-3: Time-Division Multiplexing
 Sampling an analog signal creates pulse-amplitude
modulation (PAM).
 The analog signal is converted to a series of constantwidth pulses whose amplitude follows the shape of the
analog signal.
 The original analog signal is recovered by passing it
through a low-pass filter.
 In TDM using PAM, a circuit called a multiplexer
(MUX or MPX) samples multiple analog signal
sources; the pulses are interleaved and transmitted
over one channel.
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10-3: Time-Division Multiplexing
Figure 10-13: Sampling an analog signal to produce pulse-amplitude modulation.
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10-3: Time-Division Multiplexing
PAM Multiplexer
 The simplest time multiplexer operates like a single-pole
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multiple-position mechanical or electronic switch.
It rapidly, sequentially samples multiple analog inputs.
The switch arm dwells momentarily on each contact.
This allows the input signal to be passed to the output.
It then switches quickly to the next channel, allowing
that channel to pass for a fixed duration.
The remaining channels are sampled in the same way.
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10-3: Time-Division Multiplexing
Figure 10-14: Simple rotary-switch multiplexer.
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10-3: Time-Division Multiplexing
PAM Multiplexer
 Four different analog signals can be sampled by a PAM
multiplexer. In the following slide of Figure 10-15:
 Signals A and C are continuously varying analog
signals.
 Signal B is a positive-going linear ramp.
 Signal D is a constant DC voltage.
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10-3: Time-Division Multiplexing
Figure 10-15: Four-channel
PAM time-division
multiplexer.
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10-3: Time-Division Multiplexing
PAM Multiplexer: Commutator Switches
 Multiplexers in early TDM/PAM telemetry systems used
a form of rotary switch known as a commutator.
 One complete revolution of the commutator switch is
referred to as a frame. During one frame, each input
channel is sampled one time.
 The number of frames completed in 1 second is called
the frame rate.
 Multiplying the frame rate by the number of samples per
frame yields the commutation rate or multiplex rate,
which is the basic frequency of the composite signal
transmitted over the communication channel.
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10-3: Time-Division Multiplexing
PAM Multiplexer: Electronic Multiplexers
 In practical TDM/PAM systems, electronic circuits are
used instead of mechanical switches or commutators.
 The multiplexer itself is usually implemented with FETs.
 FETs are nearly ideal on/off switches and can turn off
and on at very high speeds.
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10-3: Time-Division Multiplexing
Figure 10-16: A time-division multiplexer used to produce pulse-amplitude modulation.
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10-3: Time-Division Multiplexing
Demultiplexer Circuits
 Once the composite signal is received, it must be
demodulated and demultiplexed.
 The signal is picked up by the receiver.
 The signal is sent to an FM demodulator that recovers
the original PAM data.
 The PAM signal is then demultiplexed into the original
analog signals.
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10-3: Time-Division Multiplexing
Figure 10-18: A PAM demultiplexer.
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10-3: Time-Division Multiplexing
Demultiplexer Circuits
 The main problem encountered in demultiplexing is
synchronization.
 For the PAM signal to be accurately demultiplexed, the
clock frequencies at the receiver demultiplexer and the
transmitting multiplexer must be identical.
 The sequence of the demultiplexer must also be
identical to that of the multiplexer.
 Such synchronization is usually carried out by a special
synchronizing pulse included as a part of each frame.
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10-3: Time-Division Multiplexing
Demultiplexer Circuits: Clock Recovery and Frame
Synchronization
 The clock for a demultiplexer is typically derived from
the received PAM signal through a clock recovery
circuit.
 After clock pulses of the proper frequency have been
obtained, the multiplexed channels must be
synchronized.
 This synchronization is achieved by using a special
synchronizing (sync) pulse applied to one of the input
channels at the transmitter.
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10-3: Time-Division Multiplexing
Figure 10-19: Two PAM clock recover circuits: (a) Closed loop. (b) Open loop.
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10-3: Time-Division Multiplexing
Figure 10-20: Frame sync pulse and comparator detector.
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10-3: Time-Division Multiplexing
Figure 10-21: Complete PAM demultiplexer.
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10-4: Pulse-Code Modulation
 The most popular form of TDM uses pulse-code
modulation (PCM).
 With pulse-code modulation, multiple channels of
digital data are transmitted in serial form.
 Each channel is assigned a time slot in which to
transmit one binary word of data.
 The data streams from the various channels are
interleaved and transmitted sequentially.
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10-4: Pulse-Code Modulation
PCM Multiplexers
 When PCM is used to transmit analog signals, the
signals are sampled with a multiplexer.
 The signals are then converted by an A/D converter into
a series of binary numbers.
 Each number is proportional to the amplitude of the
analog signal at various sampling points.
 These binary words are converted from parallel to serial
format and then transmitted.
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10-4: Pulse-Code Modulation
PCM Multiplexers
 At the receiving end, the various channels are
demultiplexed.
 The original sequential binary numbers are recovered
and stored in a digital memory.
 They are then transferred to a D/A converter that
reconstructs the analog signal.
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10-4: Pulse-Code Modulation
Figure 10-22: A PCM system.
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10-4: Pulse-Code Modulation
PCM Demultiplexers
 At the receiving end, the PCM signal is demultiplexed
and converted back into the original data.
 If the PCM signal has modulated a carrier and is being
transmitted by radio, the RF signal will be picked up by
a receiver and demodulated.
 The original serial PCM binary waveform is recovered
and fed to a shaping circuit to clean up and rejuvenate
the binary pulses.
 The original signal is then demultiplexed by a digital
demultiplexer using AND or NAND gates.
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10-4: Pulse-Code Modulation
Figure 10-24:A PCM receiver-demultiplexer.
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10-4: Pulse-Code Modulation
Benefits of PCM
 PCM is reliable, inexpensive, and highly resistant to
noise.
 The transmitted binary pulses all have the same
amplitude and can be clipped to reduce noise.
 Even when signals have been degraded because of
noise, attenuation, or distortion, the receiver only has to
determine whether a pulse was transmitted.
 PCM signals are easily recovered and rejuvenated.
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10-4: Pulse-Code Modulation
Digital Carrier Systems
 The most widespread use of TDM is in the telephone
system.
 Years ago, the telephone companies developed a
complete digital transmission system called the Tcarrier system.
 The T-carrier system defines a range of PCM TDM
systems with progressively faster data rates.
 The physical implementations of these systems are
referred to as T-1, T-2, T-3, and T-4.
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10-4: Pulse-Code Modulation
T-1 Systems
 The most commonly used PCM system is the T-1
system for transmitting telephone conversations by
high-speed digital links.
 The T-1 system multiplexes 24 voice channels onto a
single line using TDM techniques.
 Each serial digital word (8-bit words, 7 bits of
magnitude, and 1 bit representing polarity) from the 24
channels is transmitted sequentially.
 Each frame is sampled at an 8-kHz rate, producing a
125-μs sampling interval.
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10-4: Pulse-Code Modulation
T-Carrier Systems, and T-2, T-3, and T-4 Systems
 T-1 systems transmit each voice signal at a 64-kbp/s
rate. They are also used to transmit fewer than 24
inputs at a faster rate.
 T-2 systems are not widely used except as a stepping
stone to form DS3 signals.
 T-1 and T-3 lines are widely used by business and
industry for telephone service as well as for digital data
transmission.
 T-2 and T-4 lines are rarely used by subscribers, but
they are used within the telephone system itself.
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10-5: Duplexing
 Duplexing is the method by which two-way
communications are handled.
 Half duplexing means that the two stations
communicating take turns transmitting and receiving.
 Full duplexing means that the two stations can send
and receive simultaneously.
 There are two ways to provide duplexing:
 Frequency-division duplexing (FDD)
 Time-division duplexing (TDD).
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10-5: Duplexing
 FDD is the simplest way to provide full duplex.
 FDD uses two separate channels, one for sending and
another for receiving.
 The big disadvantage of this method is the extra
spectrum space required.
 However, most cell phone systems use this method
because it is the easiest to implement and the most
reliable.
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10-5: Duplexing
Figure 10-27: Frequency-division duplexing (FDD).
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10-5: Duplexing
 Time-division duplexing (TDD) means that signals are
transmitted simultaneously on a single channel by
interleaving them in different time slots.
 Each time slot may contain one data word, such as 1
byte from an A/D converter or a D/A converter.
 The primary benefit of TDD is that only one channel is
needed, saving spectrum space and cost.
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10-5: Duplexing
 The TDD method is harder to implement.
 The key to making it work is precise timing and
synchronization between transmitter and receiver.
 Special synchronizing pulses or frame sequences are
needed to constantly ensure that timing will not result
in collisions between transmit and receive.
 Several of the newer third-generation cell phone
systems may use TDD.
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10-5: Duplexing
Figure 10-28: Time-division duplexing (TDD).
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