ECEG-6302 Digital Communication:

Introduction

Graduate Program in Telecommunications Engineering

School of Electrical and Computer Engineering
Introduction
• The purpose of a communication system is to transport an
information bearing signal from a source to a user
destination via a communication channel

• Three basic elements of every communication systems:

1. Transmitter
2. Receiver and
3. Channel

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Introduction …
• The message produced by a source, normally, is not
electrical
• Hence, an input transducer is used for converting the message to a
time-varying electrical quantity called message signal
• Similarly, at the destination point, another transducer
converts the electrical waveform to the appropriate
message

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Introduction …
• Transmitter
• Transforms the message signal produced by the source of
information into a form suitable for transmission over the channel

• Received signal is normally corrupted version of the

transmitted signal due to:
• Channel imperfections
• Noise and
• Interference from other sources

• Receiver
• Operates on the received signal so as to reconstruct a
recognizable form of the original message signal and to deliver it to
the user destination

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Introduction …
• Communication Systems are divided into 3 categories:
1. Analog Communication Systems
• Designed to transmit analog information using analog modulation
methods
2. Digital Communication Systems
• Designed for transmitting digital information using digital
modulation schemes, and
3. Hybrid Systems
• Use digital modulation schemes for transmitting sampled and
quantized values of an analog message signal

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Introduction - Basic Elements of Digital Comm. System

Information
Source and Source Channel Digital
Input Encoder Encoder Modulator
Transducer

Channel

Output Source Channel Digital

Transducer Decoder Decoder demodulator

Functional block diagram of a typical digital communication system

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Introduction - Basic Elements of Digital Comm. System

Information
Source and Source Channel Digital
Input Encoder Encoder Modulator
Transducer

Binary streams

Output Source Channel Digital

Transducer Decoder Decoder demodulator

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Introduction
• Source output (information source)
• The output from an information source could be
• Analog information source
• Voice - Microphone actuated by a speech
• Video - TV Camera scanning a scene, continuous amplitude signals
• Digital information source
• Signal that is discrete in time and having a finite number of characters
• These are teletype or the numerical output of computer which consists of
a sequence of discrete symbols or letters

• An analog information is transformed into a discrete

information through the process of sampling and quantizing

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Introduction
• Source encoder
• Efficiently converts source outputs into a sequence of binary
digits (0’s and 1’s), called information sequence
• Done by assigning codewords to the symbols in the sequence
• The representation of the source output in binary form should
have as little or no redundancy (data compression)
• Ideally, the source message should be represented by as few as possible
binary digits
• E.g., If a source set is having hundred symbols, then the number of
bits used to represent each symbol will be 7 because 27=128
unique combinations are available.
• The important parameters of a source encoder are
• Code word lengths
• Average data rate and
• Efficiency of the coder (i.e. actual output data rate compared to the
minimum achievable rate)

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Introduction …
• Channel encoding
• Introduce, in a controlled manner, some redundancy in the binary
information sequence
• The redundancy is used at the receiver to overcome the effects of
noise and other interferences on the transmission channel, i.e.,
error control

• Trivial example: Repeat each binary digit n times

• Non-trivial example: Taking k information bits at a time and
mapping each k-bit sequence into a unique n-bit sequence,
called the codeword
• (n > k) measure of redundancy is the ratio k/n ( or n/k), called the
code rate

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Introduction …
• Digital Modulator
• It maps/converts the coded information sequence into signal
waveforms that can be transmitted over the channel
• An interface between the channel encoder and the
communication channel

• Consider the coded sequence is to be transmitted one bit at a

time at some uniform rate R bits/s
• The modulator may simply map the binary digits as follows

0 ↔ s 0 ( t ) = + cos 2πft 1 ↔ s1( t ) = − cos 2πft

• This is an example of binary modulation in which each bit from
the encoder is transmitted separately
• Called binary phase-shift keying – BPSK

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Introduction …
• Alternatively, modulator may transmit b coded information
bits at a time using distinct waveforms si(t), i= 0,1,…M-1,
• Called M-ary modulation
• Note: M = 2
b

• Note here that a new b-bit coded sequence enters the

modulator every b/R seconds
• If the channel bit rate is fixed at R, the amount of time
available to transmit one of the M waveforms
(corresponding to the b-bit sequence) is b times the time
period in a system that uses binary modulation

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Introduction …
• Modulator can be effectively used to
• Minimize the effects of channel noise
• Match the frequency spectrum of transmitted signal with channel
characteristics,
• Provide the capability to multiplex many signals

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Introduction …
• Communication Channel: Physical medium that is used
to send the signal from the transmitter to the receiver
• Examples include
• Wireless transmission- the atmosphere or free space
• Wireline, optical fiber, coaxial cables

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Introduction …
• Transmitted signals are corrupted, in a random manner,
by a variety of additive noise such as thermal noise,
atmospheric noise, man made noise, etc and also
attenuated in amplitude
• Channels can be modeled in a variety of ways that take
into account the particular properties of the channel
• Examples are additive noise channel, linear filter channel, etc

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Introduction …
• Digital demodulator
• Processes the channel-corrupted transmitted waveforms and
reduces them to a sequence of numbers (digits)
• Extraction of the message from the information bearing
waveform
• The digits represent estimates of the transmitted coded data
symbols ( binary or M-ary)

• Channel decoder
• Attempts to reconstruct the original information sequence from
the knowledge of the code used and the redundancy contained
in the received data estimate
• Converts the binary output of the channel decoder into a symbol
sequence

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Introduction …
• A measure of how accurately the demodulator and decoder
recover the original sequence is the average probability of
bit-error at the output of the decoder for a given power level
(signal-to-noise-ratio)

• The probability of error is in general a function of

• Code characteristics
• Type of waveforms used
• Transmitter power
• Channel characteristics and
• Method of demodulation and decoding

• We will explore most of these factors that affect the

reliability of digital communication

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 Introduction - Modified Block Diagram
From other
source

Source Encryption Channel MUX Modulator

Source
Encoder Encoder

Channel

Source Decryption Channel DEMUX Demodulator

Destination
Decoder decoder

To other
destinations

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Introduction …
• Encryptor
• Prevents unauthorized users from understanding the messages
and from injecting false messages into the system
• MUX
• Multiplexer is used for combining signals from different sources so
that they share a portion of the communication system
• DeMUX
• DeMultiplexer is used for separating the different signals so that
they reach their respective destinations
• Decryptor
• It does the reverse operation of that of the Encryptor

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Introduction …
• Synchronization
• Involves the estimation of both time and frequency coherent
systems
• Need to synchronize their frequency reference with carrier in both
frequency and phase

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Advantages of Digital Communication
1. Less effect by distortion, noise and interference is less in
a digital communication system
• This is because the disturbance must be large enough to change
the pulse from one state to the other
2. Regenerative repeaters can be used at fixed distance
along the link, to identify and regenerate a pulse before it
is degraded to an ambiguous state
3. Digital circuits are more reliable and cheaper compared to
analog circuits
4. The Hardware implementation is more flexible than
analog hardware because of the use of microprocessors,
VLSI chips etc.

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Advantages of Digital Communication
5. Signal processing functions like encryption, compression
can be employed to maintain the secrecy of the
information
6. Error detecting and Error correcting codes improve the
system performance by reducing the probability of error
7. Combining digital signals using TDM is simpler than
combining analog signals using FDM. The different types
of signals such as data, telephone, TV can be treated as
identical signals in transmission and switching in a digital
communication system
8. Signal jamming can be avoided using spread spectrum
technique

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Disadvantages of Digital Communication
1. Large System Bandwidth:- Digital transmission requires a
large system bandwidth to communicate the same
information in a digital format as compared to analog
format
2. System Synchronization:- Digital detection requires
system synchronization whereas the analog signals
generally have no such requirement

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Factors for the Growth of Digital Communication
1. Impact of the Computer
• Computers are processors and sources of data as well as tools
for communication
2. Flexibility and compatibility
• The adoption of a common digital format makes it possible for a
transmission system to handle many different sources of
information in a flexible manner
3. Improved reliability due to improved theory,
microelectronics and system design
4. Availability of wide-band channels such as optical fibers,
coaxial cables and geo-stationary satellites
5. Availability of integrated solid-state electronics
technology makes possible increased system complexity
by orders of magnitude in a cost effective manner

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