This document discusses key concepts in communication systems including: 1) The basic elements of a communication system including the information source, transmitter, channel, and receiver. 2) Types of communication including simplex, full duplex, and half duplex as well as analog and digital signals. 3) Modulation and multiplexing which allow signals to be transmitted efficiently over a channel by modifying the signal or combining multiple signals.

1. UNIT – 1 Introduction to Communication System
Block schematic of communication system, Simplex and duplex systems,
Modes of communication: Broadcast and point to point communication,
Necessity of modulation, Classification of modulation, sampling
theorem and pulse analog modulation, multiplexing: TDM, FDM.

2. Block schematic of communication system

3. Introduction
Elements of Communication System:
Communication: It is the process of conveying or
transferring
another.
information from one point to
(Or)
It is the process of establishing connection or
between two points for information exchange.
link

4. Elements of Communication System:
Information source:
The message or information to be communicated
originates in information source.
Message can be words, group of words, code, data,
symbols, signals etc.
Transmitter :
The objective of the transmitter block is to collect
the incoming message signal and modify it in a
suitable fashion (if needed), such that, it can
to
be
the
transmitted via the chosen channel
receiving point.

5. Elements of Communication System:
Channel :
Channel is the physical medium which connects the
transmitter with that of the receiver.
The physical medium includes copper wire, coaxial
cable, fibre optic cable, wave guide and free
space or atmosphere.
Receiver:
The receiver block receives the incoming modified
version of the message signal from the channel
and processes it to recreate the original (non-
electrical) form of the message signal.

6. Signal, Message, Information
Signal:
It is a physical quantity which varies with respect to
time or space or independent or dependent
variable.
(Or)
It is electrical waveform which carries information.
Ex: m(t) = Acos(ωt+ϕ)
Where, A= Amplitude or peak amplitude(Volts)
w = Frequency ( rad/sec)
ϕ = Phase (rad)

7. Types
Analog or Continuous
Digital Signal
of Signals
Signal
•
•
Analog or Continuous Signal: If the amplitude of
or
of
signal continuously varies with respect to time
if the signal contains infinite number
amplitudes,
signal.
it is called Analog or continuous

8. Types of Signals
Digital Signal: If the signal contains only two
discrete amplitudes, then it is called digital signal.
• With respect to communication, signals are
classified into,
Baseband signal
Bandpass signal
•
•
Baseband signal: If the
zero
signal contains zero
frequency or near to frequency, it is called
baseband signal.
Ex: Voice, Audio, Video, Bio-medical signals etc.

9. Types of Signals
Bandpass signal: If the signal contains band of
frequencies far away from base or zero, it is called
bandpass signal.
Ex: AM, FM signals.
Message: It is sequence of symbols.
Ex: Happy New Year 2020.
Information: The content in the message is called
information. It is inversely proportional to
probability of occurrence of the symbol.
• Information is measured in bits, decits, nats.

10. Limitations of Communication
Technological Problems:
System
•
To implement communication systems, Tx, Rx, channel
hardware.
complex.
are required which requires
Communication system is expensive and
Bandwidth & Noise:
•
The effect of noise can be reduced by providing
more bandwidth to stations but due to this less
number of stations can only be accommodated.
Signal to Noise Ratio (SNR):Noise should be low to
•
increase channel capacity but it is an unavoidable
aspect of communication system.

18. 1-3 Types of Electronic Communication
Electronic communications are classified according to whether they are (1) one-way
(simplex) or two-way (full duplex or half duplex) transmissions and (2) analog or digital
signals.
Simplex
The simplest way in which electronic communication is conducted is one-way com-
munications, normally referred to as simplex communication. Examples are shown
in Fig. 1-3. The most common forms of simplex communication are radio and TV broad-
casting. Another example of one-way communication is transmission to a remotely con-
trolled vehicle like a toy car or an unmanned aerial vehicle (UAV or drone).
Simplex
communication
Figure 1-3 Simplex communication.
Antenna
Propeller
Motor
Camera
Radio control box
(b) Remote control
6 Chapter 1
set
TV
TV
transmitter
(a) TV broadcasting

19. Figure 1-4 Duplex communication. (a) Full duplex (simultaneous two-way). (b) Half duplex (one way at a time).
Microphone Microphone
Speaker
Full Duplex
The bulk of electronic communication is two-way, or duplex communication. Typical
duplex applications are shown in Fig. 1-4. For example, people communicating with one
another over the telephone can talk and listen simultaneously, as Fig. 1-4(a) illustrates.
This is called full duplex communication.
Duplex
communication
Full duplex
communication
Half Duplex
The form of two-way communication in which only one party transmits at a time is
known as half duplex communication [see Fig. 1-4(b)]. The communication is two-way,
but the direction alternates: the communicating parties take turns transmitting and receiv-
ing. Most radio transmissions, such as those used in the military, fire, police, aircraft,
marine, and other services, are half duplex communication. Citizens band (CB), Family
Radio, and amateur radio communication are also half duplex.
Half duplex
communication
AnalogSignals
An analog signal is a smoothly and continuously varying voltage or current.
Some typical analog signals are shown in Fig. 1-5. A sine wave is a single-frequency
analog signal. Voice and video voltages are analog signals that vary in accordance with
the sound or light variations that are analogous to the information being transmitted.
Analog
signal
Digital Signals
Digital signals, in contrast to analog signals, do not vary continuously, but change
in steps or in discrete increments. Most digital signals use binary or two-state codes.
Some
Digital
signal
Figure 1-5 Analog signals. (a) Sine wave “tone.” (b) Voice. (c) Video (TV) signal.
7
Introduction to Electronic Communication
(a) (b) Sync pulse
Sync pulse
Light variation
along one
scan line
of video
(c)
Earphone
Telephone system
Telephone Telephone
The medium or channel
Microphone Speake
(a) Full duplex (simultaneous, two-way)
Transceiver Transceiver
TX TX
RX RX
r
(b) Half duplex (one way at a time)

20. Figure 1-6 Digital signals. (a) Telegraph (Morse code). (b) Continuous-wave (CW)
code. (c) Serial binary code.
Mark Mark Mark
(a)
Dot Dash Dot
The letter R
(b)
5 V
0 V
(c)
examples are shown in Fig. 1-6. The earliest forms of both wire and radio
communica-
tion used a type of on /off digital code. The telegraph used Morse code, with its system
of short and long signals (dots and dashes) to designate letters and numbers. See Fig. 1-
6(a). In radio telegraphy, also known as continuous-wave (CW) transmission, a sine
wave signal is turned off and on for short or long durations to represent the dots and
dashes. Refer to Fig. 1-6(b).
Data used in computers is also digital. Binary codes representing numbers, letters, and
special symbols are transmitted serially by wire, radio, or optical medium. The most com-
monly used digital code in communications is the American Standard Code for Information
Interchange (ASCII, pronounced “ask key”). Fig. 1-6(c) shows a serial binary code.
Many transmissions are of signals that originate in digital form, e.g., telegraphy
messages or computer data, but that must be converted to analog form to match the
transmission medium. An example is the transmission of digital data over the tele-
phone network, which was designed to handle analog voice signals only. If the
digi- tal data is converted to analog signals, such as tones in the audio frequency
range, it can be transmitted over the telephone network.
Analog signals can also be transmitted digitally. It is very common today to
take voice or video analog signals and digitize them with an analog-to-digital (A /D)
converter. The data can then be transmitted efficiently in digital form and processed by
computers and other digital circuits.
ASC
II
1-4 Modulation and Multiplexing
Modulation and multiplexing are electronic techniques for transmitting information effi-
ciently from one place to another. Modulation makes the information signal more
compatible with the medium, and multiplexing allows more than one signal to be trans-
mitted concurrently over a single medium. Modulation and multiplexing techniques are
basic to electronic communication. Once you have mastered the fundamentals of these
techniques, you will easily understand how most modern communication systems
work.
Modulatio
n
Multiplexin
g
GOODTOKNOW
Multiplexing has been used
in the
music industry to create
stereo
sound. In stereo radio, two
signals
are transmitted and
received—
one for the right and one
for the
left channel of sound.
(For more
information on
multiplexing,
Baseband Transmission
Before it can be transmitted, the information or intelligence must be converted to an
electronic signal compatible with the medium. For example, a microphone changes voice
signals (sound waves) into an analog voltage of varying frequency and amplitude. This
signal is then passed over wires to a speaker or headphones. This is the way the
telephone system works.
A video camera generates an analog signal that represents the light variations along
one scan line of the picture. This analog signal is usually transmitted over a coaxial cable.
Binary
8 Chapter 1
1 0 1 0 1 1 0 1 0 0 1
Space Mark on; Space off Space

21. data is generated by a keyboard attached to a computer. The computer stores the data
and
processes it in some way. The data is then transmitted on cables to peripherals such as a
printer or to other computers over a LAN. Regardless of whether the original
information or intelligence signals are analog or digital, they are all referred to as
baseband signals.
In a communication system, baseband information signals can be sent directly and
unmodified over the medium or can be used to modulate a carrier for transmission
over the medium. Putting the original voice, video, or digital signals directly into the
medium is referred to as baseband transmission. For example, in many telephone and
intercom systems, it is the voice itself that is placed on the wires and transmitted over
some dis- tance to the receiver. In most computer networks, the digital signals are
applied directly to coaxial or twisted-pair cables for transmission to another computer.
In many instances, baseband signals are incompatible with the medium. Although
it is theoretically possible to transmit voice signals directly by radio, realistically it is
impractical. As a result, the baseband information signal, be it audio, video, or data, is
normally used to modulate a high-frequency signal called a carrier. The higher-
frequency carriers radiate into space more efficiently than the baseband signals
themselves. Such wireless signals consist of both electric and magnetic fields. These
electromagnetic sig- nals, which are able to travel through space for long distances, are
also referred to as radio-frequency (RF) waves, or just radio waves.
Baseband
transmission
Carrie
r
Radio-frequency (RF)
wave
Broadband Transmission
Modulation is the process of having a baseband voice, video, or digital signal modify
another, higher-frequency signal, the carrier. The process is illustrated in Fig. 1-7. The
information or intelligence to be sent is said to be impressed upon the carrier. The
carrier is usually a sine wave generated by an oscillator. The carrier is fed to a circuit
called a modulator along with the baseband intelligence signal. The intelligence signal
changes the carrier in a unique way. The modulated carrier is amplified and sent to
the antenna for transmission. This process is called broadband transmission.
Consider the common mathematical expression for a sine wave:
Broadband
transmission
υ5 Vp sin (2πft 1 θ) or υ5 Vp sin (ωt 1 θ)
where υ5 instantaneous value of sine wave voltage
Vp 5 peak value of sine wave
f 5 frequency, Hz
ω 5 angular velocity 52πf t
5 time, s
ωt 52πft 5angle, rad (360° 5 2π rad)
θ 5 phase angle
Figure 1-7 Modulation at the transmitter.
Antenna
or
9
Introduction to Electronic Communication
Baseband signal
Microphone Amplifier Modulated carrier
Voice Modulator
other
intelligence
High-frequency carrier
oscillator
Power amplifier

22. 1-3 Types of Electronic Communication
Electronic communications are classified according to whether they are (1) one-way
(simplex) or two-way (full duplex or half duplex) transmissions and (2) analog or digital
signals.
Simplex
The simplest way in which electronic communication is conducted is one-way com-
munications, normally referred to as simplex communication. Examples are shown
in Fig. 1-3. The most common forms of simplex communication are radio and TV broad-
casting. Another example of one-way communication is transmission to a remotely con-
trolled vehicle like a toy car or an unmanned aerial vehicle (UAV or drone).
Simplex
communication
Figure 1-3 Simplex communication.
Antenna
Propeller
Motor
Camera
Radio control box
(b) Remote control
6 Chapter 1
set
TV
TV
transmitter
(a) TV broadcasting

23. Figure 1-4 Duplex communication. (a) Full duplex (simultaneous two-way). (b) Half duplex (one way at a time).
Microphone Microphone
Speaker
Full Duplex
The bulk of electronic communication is two-way, or duplex communication. Typical
duplex applications are shown in Fig. 1-4. For example, people communicating with one
another over the telephone can talk and listen simultaneously, as Fig. 1-4(a) illustrates.
This is called full duplex communication.
Duplex
communication
Full duplex
communication
Half Duplex
The form of two-way communication in which only one party transmits at a time is
known as half duplex communication [see Fig. 1-4(b)]. The communication is two-way,
but the direction alternates: the communicating parties take turns transmitting and receiv-
ing. Most radio transmissions, such as those used in the military, fire, police, aircraft,
marine, and other services, are half duplex communication. Citizens band (CB), Family
Radio, and amateur radio communication are also half duplex.
Half duplex
communication
AnalogSignals
An analog signal is a smoothly and continuously varying voltage or current.
Some typical analog signals are shown in Fig. 1-5. A sine wave is a single-frequency
analog signal. Voice and video voltages are analog signals that vary in accordance with
the sound or light variations that are analogous to the information being transmitted.
Analog
signal
Digital Signals
Digital signals, in contrast to analog signals, do not vary continuously, but change
in steps or in discrete increments. Most digital signals use binary or two-state codes.
Some
Digital
signal
Figure 1-5 Analog signals. (a) Sine wave “tone.” (b) Voice. (c) Video (TV) signal.
7
Introduction to Electronic Communication
(a) (b) Sync pulse
Sync pulse
Light variation
along one
scan line
of video
(c)
Earphone
Telephone system
Telephone Telephone
The medium or channel
Microphone Speake
(a) Full duplex (simultaneous, two-way)
Transceiver Transceiver
TX TX
RX RX
r
(b) Half duplex (one way at a time)

24. Figure 1-6 Digital signals. (a) Telegraph (Morse code). (b) Continuous-wave (CW)
code. (c) Serial binary code.
Mark Mark Mark
(a)
Dot Dash Dot
The letter R
(b)
5 V
0 V
(c)
examples are shown in Fig. 1-6. The earliest forms of both wire and radio
communica-
tion used a type of on /off digital code. The telegraph used Morse code, with its system
of short and long signals (dots and dashes) to designate letters and numbers. See Fig. 1-
6(a). In radio telegraphy, also known as continuous-wave (CW) transmission, a sine
wave signal is turned off and on for short or long durations to represent the dots and
dashes. Refer to Fig. 1-6(b).
Data used in computers is also digital. Binary codes representing numbers, letters, and
special symbols are transmitted serially by wire, radio, or optical medium. The most com-
monly used digital code in communications is the American Standard Code for Information
Interchange (ASCII, pronounced “ask key”). Fig. 1-6(c) shows a serial binary code.
Many transmissions are of signals that originate in digital form, e.g., telegraphy
messages or computer data, but that must be converted to analog form to match the
transmission medium. An example is the transmission of digital data over the tele-
phone network, which was designed to handle analog voice signals only. If the
digi- tal data is converted to analog signals, such as tones in the audio frequency
range, it can be transmitted over the telephone network.
Analog signals can also be transmitted digitally. It is very common today to
take voice or video analog signals and digitize them with an analog-to-digital (A /D)
converter. The data can then be transmitted efficiently in digital form and processed by
computers and other digital circuits.
ASC
II
1-4 Modulation and Multiplexing
Modulation and multiplexing are electronic techniques for transmitting information effi-
ciently from one place to another. Modulation makes the information signal more
compatible with the medium, and multiplexing allows more than one signal to be trans-
mitted concurrently over a single medium. Modulation and multiplexing techniques are
basic to electronic communication. Once you have mastered the fundamentals of these
techniques, you will easily understand how most modern communication systems
work.
Modulatio
n
Multiplexin
g
GOODTOKNOW
Multiplexing has been used
in the
music industry to create
stereo
sound. In stereo radio, two
signals
are transmitted and
received—
one for the right and one
for the
left channel of sound.
(For more
information on
multiplexing,
Baseband Transmission
Before it can be transmitted, the information or intelligence must be converted to an
electronic signal compatible with the medium. For example, a microphone changes voice
signals (sound waves) into an analog voltage of varying frequency and amplitude. This
signal is then passed over wires to a speaker or headphones. This is the way the
telephone system works.
A video camera generates an analog signal that represents the light variations along
one scan line of the picture. This analog signal is usually transmitted over a coaxial cable.
Binary
8 Chapter 1
1 0 1 0 1 1 0 1 0 0 1
Space Mark on; Space off Space

25. data is generated by a keyboard attached to a computer. The computer stores the data
and
processes it in some way. The data is then transmitted on cables to peripherals such as a
printer or to other computers over a LAN. Regardless of whether the original
information or intelligence signals are analog or digital, they are all referred to as
baseband signals.
In a communication system, baseband information signals can be sent directly and
unmodified over the medium or can be used to modulate a carrier for transmission
over the medium. Putting the original voice, video, or digital signals directly into the
medium is referred to as baseband transmission. For example, in many telephone and
intercom systems, it is the voice itself that is placed on the wires and transmitted over
some dis- tance to the receiver. In most computer networks, the digital signals are
applied directly to coaxial or twisted-pair cables for transmission to another computer.
In many instances, baseband signals are incompatible with the medium. Although
it is theoretically possible to transmit voice signals directly by radio, realistically it is
impractical. As a result, the baseband information signal, be it audio, video, or data, is
normally used to modulate a high-frequency signal called a carrier. The higher-
frequency carriers radiate into space more efficiently than the baseband signals
themselves. Such wireless signals consist of both electric and magnetic fields. These
electromagnetic sig- nals, which are able to travel through space for long distances, are
also referred to as radio-frequency (RF) waves, or just radio waves.
Baseband
transmission
Carrie
r
Radio-frequency (RF)
wave
Broadband Transmission
Modulation is the process of having a baseband voice, video, or digital signal modify
another, higher-frequency signal, the carrier. The process is illustrated in Fig. 1-7. The
information or intelligence to be sent is said to be impressed upon the carrier. The
carrier is usually a sine wave generated by an oscillator. The carrier is fed to a circuit
called a modulator along with the baseband intelligence signal. The intelligence signal
changes the carrier in a unique way. The modulated carrier is amplified and sent to
the antenna for transmission. This process is called broadband transmission.
Consider the common mathematical expression for a sine wave:
Broadband
transmission
υ5 Vp sin (2πft 1 θ) or υ5 Vp sin (ωt 1 θ)
where υ5 instantaneous value of sine wave voltage
Vp 5 peak value of sine wave
f 5 frequency, Hz
ω 5 angular velocity 52πf t
5 time, s
ωt 52πft 5angle, rad (360° 5 2π rad)
θ 5 phase angle
Figure 1-7 Modulation at the transmitter.
Antenna
or
9
Introduction to Electronic Communication
Baseband signal
Microphone Amplifier Modulated carrier
Voice Modulator
other
intelligence
High-frequency carrier
oscillator
Power amplifier

26. Modes of communication:
• Broadcast

27. Broadcast
•Broadcasting, which involves the use of a single
powerful transmitter and numerous receivers
that are relatively inexpensive to build. In this
class of communication systems, information-
bearing signals flow only in one direction,
from the transmitter to each of the receivers
out there in the field.

28. Point to point communication
•Point-to-point communications, in which the
communication process takes place over a link
between a single transmitter and a single receiver.
In this second class of communication systems, there
is usually a bidirectional flow of information-bearing
signals, which, in effect, requires the use of a
transmitter and receiver (i.e., trans-receiver) at each
end of the link.

30. Modulation
It is the process of varying the characteristics
of high frequency carrier in accordance
with instantaneous values of modulating or
message or baseband signal.
(Or)
It is a frequency translation technique
which converts baseband or low frequency
signal to bandpass or high frequency signal.
Modulation is used in the transmitter.

31. Types of Modulation

32. Types of Modulation
•Amplitude Modulation: Amplitude of
the carrier is varied in accordance
with the instantaneous values of
modulating signal.
• Frequency Modulation: Frequency of the carrier
is varied in accordance with the
instantaneous values of modulating signal.
• Phase Modulation: Phase of the carrier is varied
in accordance with the instantaneous values
of modulating signal.

33. Need of Modulation
• To reduce the antenna height
• To overcome hardware system limitations
• To reduce the interference, noise & distortions made when we transmit
the signals with nearly same frequency in the audio frequency range (20-
20k) Hz.
• To multiplex the more number of signals
• To the assignment of channel frequency
• To narrow banding the signal
• To reduce the complexity of the transmission system
• To increase the bandwidth of the signal

34. Benefits of Modulation
• For multiplexing
• To reduce the length or height of antenna
• For narrow banding or to use antenna with
single or same length
• To reduce noise effect
• To avoid equipment limitation or to reduce the
size of the equipment.

35. Amplitude Modulation
The amplitude of the carrier signal varies
in accordance with the instantaneous amplitude
of the modulating signal.

36. Amplitude Modulation
The carrier signal is given by,
C(t) = Ac Coswct
Where, Ac= Maximum amplitude of the carrier
signal.
W= 2πfc= Frequency of the carrier signal.
Modulating or baseband signal is given by,
X(t) = Am Coswmt
Where, Am = Amplitude of the baseband signal.

37. Amplitude Modulation
The standard equation for amplitude modulated
signal is expressed as,
S(t)= Ac Cos2πfct[1+ma(Cos2πfmt)]
Where, ma = Am/Ac = Modulation Index
Time Domain representation of AM:
S(t)=AcCos2πfct+μAc/2Cos[2πfc+2πfm]t+μAc/2Cos[2πfc-
2πfm]t I term: Carrier signal with amplitude Ac and
frequency fc.
II. term: Amplitude= μAc/2, frequency= fc+fm , Upper
sideband
frequency

38. Amplitude Modulation
Frequency Domain representation of AM:
The time domain representation of AM
waveis given by,
S(t)= Ac Cos2πfct[1+ma(Cos2πfmt)] Taking Fourier
transform on both sides,
S(f) = Ac/2[δ(f-fc)+ δ(f+fc)] + Acma/2[M(f-fc)+ M(f+fc)]

39. Modulation Index
Modulation index or depth of modulation is given
by,
ma = [Amax-Amin/ Amax+Amin]= Am/Ac
Percentage of modulation index is,
%ma = [Amax-Amin/ Amax+Amin]X100= [Am/Ac ]X100
Types of AM with respect to modulation index:
• Under Modulation (ma <1)
• Critical Modulation (ma =1)
• Over Modulation (ma >1)

40. 40

41. 41
Pulse Amplitude Modulation(PAM)

42. 42
Introduction
- An analog signal:
amplitude can take any value over a continuous
range.
- Digital signals:
amplitude can take only discrete and finite values.
- Note:
can we convert an analog signal to a digital signal.

43. 43
Introduction
- One can convert an analog signal to a digital signal
by sampling and quantizing (collectively called
analog-to-digital conversion, or ADC).
- The processed signals are then converted back into
analog signals using a reconstruction or
interpolation operation (called digital-to-analog
conversion, or DAC).

44. 44
Sampling Process
- The sampling process is a basic operation in the
digital communication.
- In this process, the continuous-time analog signal
is sampled by measuring its amplitude at a discrete
instants.
- So, the continuous-time analog signal is converted
into a corresponding sequence of samples that are
usually spaced uniformly in time.
- It is necessary to choose the sampling rate
properly, so the sequence of samples uniquely
defines the original analog signal.

45. 45

46. 46
Sampling
- To sample a continuous-time signal x(t) is to
represent x(t) at a discrete number of points, t =
nTs , where Ts is the sampling period.

47. 47
Sampling
- The sampling theorem states that a band-limited
signal x(t) with a bandwidth W ( W is the highest
frequency) can be reconstructed from its sample
values if the sampling rate (frequency)
fs =1/Ts is greater than or equal to twice the
bandwidth W of x(t)
- The minimum sampling rate of fs for an analog
band-limited signal is called the Nyquist rate.

48. 48
Sampling
- There are 3 sampling methods:
• Ideal - an impulse at each sampling instant.
• Natural - a pulse of short width with varying
amplitude.
• Flattop - sample and hold, like natural but with
single amplitude value.

49. 49
Sampling
- As long as the sampling of the analog signal is taken
with a sufficiently high frequency (higher than the
minimum Nyquist rate of twice the signal largest
frequency), it can be shown that there is no loss in
information as a result of taking discrete samples.

50. 50
PAM
Pulse Amplitude Modulation

51. 51

52. 52
Introduction
- CW modulation: a parameter of a sinusoidal
carrier wave is varied continuously in accordance
with the message signal.
Amplitude, frequency and phase.
- Pulse Modulation: signal is transmitted at discrete
intervals of time.
- Pulse modulation can be analog pulse modulation
or digital pulse modulation.

53. 53

54. 54
Pulse Amplitude Modulation (PAM)
- In the PAM, the amplitude of periodic pulse train is
varied with a amplitude of the corresponding sample
value of a continuous message signal.
- In PAM: width and position are fixed but amplitude
varies.

55. 55
Pulse Amplitude Modulation (PAM)

56. 56
Pulse Amplitude Modulation (PAM)
- Natural PAM
top portion of the pulses are subjected to follow the
modulating wave.

57. 57
Pulse Amplitude Modulation (PAM)
- Pulse width modulation is also called pulse
duration modulation (PDM).
- Pulse width modulation: position and amplitude
are fixed but width varies.
- PWM is more often used for control than for
communication.
LEDs: output luminosity is proportional to average
current.

58. 58
Pulse Amplitude Modulation (PAM)
- Pulse position modulation:
width and amplitude are fixed but position
varies.
- The value of the signal determines the delay of the
pulse from the clock.

59. 59

60. Multiplexing

61. Frequency Division Multiplexing
• 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

62. Frequency Division Multiplexing
Diagram

63. FDM System

64. FDM of Three Voiceband Signals

65. Multiplexing
Multiplexing is a modulation method which improves channel bandwidth utilisation.
For example, a co-axial cable has a bandwidth of 100's of Mhz. Baseband speech
is a only a few kHz

66. 1) Frequency Division Multiplexing FDM
This allows several 'messages' to be translated from baseband, where they are all
in the same frequency band, to adjacent but non overlapping parts of the spectrum.
An example of FDM is broadcast radio (long wave LW, medium wave MW, etc.)

67. Time Division Multiplexing

68. TDM System

69. TDM Link Control
• No headers and trailers
• Data link control protocols not needed
• Flow control
—Data rate of multiplexed line is fixed
—If one channel receiver can not receive data, the others must carry on
—The corresponding source must be quenched
—This leaves empty slots
• Error control
—Errors are detected and handled by individual channel systems

70. Data Link Control on TDM

71. 2) Time Division Multiplexing TDM
TDM is another form of multiplexing based on sampling which is a modulation
technique. In TDM, samples of several analogue message symbols, each one
sampled in turn, are transmitted in a sequence, i.e. the samples occupy adjacent
time slots.

72. 72
Time Division Multiplexing (TDM)
- In many cases, bandwidth of communication link is
much greater than signal bandwidth.
- All three methods can be used with time-division
multiplexing (TDM) to carry multiple signals over a
single channel.

73. 73

74. Analog Carrier Systems
• Hierarchy of FDM schemes
• Group
— 12 voice channels (4kHz each) = 48kHz
— Range 60kHz to 108kHz
• Supergroup
— 60 channel
— FDM of 5 group signals on carriers between 420kHz and 612 kHz
• Mastergroup
— 10 supergroups

75. Framing
• No flag or SYNC characters bracketing TDM frames
• Must provide synchronizing mechanism
• Added digit framing
—One control bit added to each TDM frame
• Looks like another channel - “control channel”
—Identifiable bit pattern used on control channel
—e.g. alternating 01010101…unlikely on a data channel
—Can compare incoming bit patterns on each channel with sync pattern

76. Pulse Stuffing
• Problem - Synchronizing data sources
• Clocks in different sources drifting
• Data rates from different sources not related by simple rational
number
• Solution - Pulse Stuffing
—Outgoing data rate (excluding framing bits) higher than sum of incoming
rates
—Stuff extra dummy bits or pulses into each incoming signal until it
matches local clock
—Stuffed pulses inserted at fixed locations in frame and removed at
demultiplexer

77. TDM of Analog and Digital Sources