Chapter 1: Introduction to Comm.
Eng’g
Undergraduate Program
School of Electrical and Computer Engineering
Overview
• Elements of communication system
• Channel characteristics
• Mathematical models of a channel
• Signals and systems – Review
Sem. I, 2015 Introduction to Communications – Introduction 2
Course objective
• To introduce basic principles and techniques for the
analysis and design of modern communication systems
Sem. I, 2015 Introduction to Communications – Introduction 3
Introduction to Communication Systems
• What is a communication system?
• Any means for transmission of information from one point to
another using electrical systems
• Types of communication systems
• Wireline and wireless
• Digital and analog
• Point-to-point and broadcast
• Low frequency and high frequency
• ….
• Examples
• Telephone, cell phone, TV, Internet, …
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Elements of Communication System
• High-level block diagrams
Information
source and input Transmitter
transducer
Channel
Output
transducer Receiver
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Elements of Communication System
• Source
• Generates information (speech, video, text, data, etc. )
• Transducer
• Transforms source output into electrical signal (e.g., microphone)
and back (at Rx end)
• Three major parts
• Transmitter (Tx)
• Channel
• Receiver (Rx)
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Transmitter
• Converts electrical signal into a form suitable for
transmission through the channel (physical medium)
• The transducer output signal cannot, in most cases, be transmitted
directly (does not match the channel)
• Transmitter converts message to a suitable form
• Conversion is made through modulation
• Amplitude (AM), frequency (FM) and phase (PM)
• Examples: AM and FM radio broadcast
• Other functions: Filtering, amplification, radiation
• Intended recipient of the message signal
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Channel
• Physical medium over which the information will be
transmitted from the transmitter to the receiver
• Can be wired (telephone line) or wireless (radio)
• Whatever the medium, the signal is corrupted by noise and
interference
• E.g., thermal noise, lightning discharge, automobile ignition noise,
interference from other users etc.
• Channel may be highly non-stationary (i.e., fading)
• Significant signal attenuation may be introduced
• E.g., 100 – 200dB
• Other types of signal distortions (i.e., spectrum distortion)
Sem. I, 2015 Introduction to Communications – Introduction 8
Receiver
• Main function: to recover the message from the received
signal
• Somewhat inverse of the transmitter function
• Demodulation: Inverse of the modulation
• Operates in the presence of noise and interference
• Hence, some distortions are unavoidable
• Some other functions: filtering, suppression of noise and
interference
Sem. I, 2015 Introduction to Communications – Introduction 9
Goal of Communications Engineer
• To design transmitters and receivers that are
• Cost efficient
• Bandwidth efficient
• Maximum information transfer (message at sink is a faithful
representation of the source message)
• Power efficient (uses as little power as necessary)
• Many of the above goals are contrary to one another
• For example, one way to improve message fidelity at the receiver
is to increase transmit power
• Therefore, tradeoffs are required
Sem. I, 2015 Introduction to Communications – Introduction 10
Overview
• Elements of communication system
• Channel characteristics
• Mathematical models of a channel
• Signals and systems – Review
Sem. I, 2015 Introduction to Communications – Introduction 11
Channel Characteristics
• Wireline channels
• Twisted-wire pair
• Coaxial cable
• Waveguides
• Optical fiber
• Signals are distorted in amplitude
and phase
• Some measures are required to
reduce the effect of distortions
• Bandwidth of
• Twisted pair: several hundred kHz
• Coaxial cable: Several hundred MHz
• Waveguides: few GHz
• Optical fiber : very wide
Sem. I, 2015 Introduction to Communications – Introduction 12
Radio (Wireless) Communication Channels
• Unguided electromagnetic
wave, radiated by the Tx
antenna, is a carrier of the
signal
• Strong signal attenuation
• Up to 200 dB
• Hence, high Tx power is
required
• Susceptible to external
interference
• Antennas are required
• Size of antenna:
Comparable with
wavelength
Sem. I, 2015 Introduction to Communications – Introduction 13
Overview
• Elements of communication system
• Channel characteristics
• Mathematical models of a channel
• Signals and systems – Review
Sem. I, 2015 Introduction to Communications – Introduction 14
Mathematical Models of Channels
• System-level model: linear time-invariant system
• Detailed model: based on Electromagnetics (i.e., radio
wave propagation)
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Overview
• Elements of communication system
• Channel characteristics
• Mathematical models of a channel
• Signals and systems – Review
Sem. I, 2015 Introduction to Communications – Introduction 16
Common Signals
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Common Signals
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Common Signals
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Common Signals
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Review of Linear Time Invariant Systems
• A system performs a transformation on an input x(t) to
produce an output y(t)
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Review of Linear Time Invariant Systems
• Linear Systems
• A linear system is a system for which the superposition property
applies
Sem. I, 2015 Introduction to Communications – Introduction 22
Time Invariant Systems
• A system is time invariant if a time shift to the input results
in no changes other than the same time shift being applied
to the output
• If y1(t) is the output of the system when x1 (t) is the input let
x2 (t) = x2 (t-τ) be the input that produces output y2 (t)
• The system is time invariant if y2 (t) = y1(t- τ)
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Linear time Invariant Systems
• A system is LTI if it is both linear and time invariant
• An LTI system is described by its impulse response
• The system’s impulse response is h(t) and it is the output
of the system when the input is x(t) = δ(t)
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Output of LTI system
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Causality
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Stability
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