Chapter 1
Introduction to Communication
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Outline
• What is communication system?
• Types of communication
• Digital and Analog Sources and Systems
• Encoding and Decoding for Digital Communication
• Deterministic and Random Waveforms
• Block Diagram of A Communication System
• What makes a Communication System GOOD
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What is a communication system?.
• Communication is transferring data reliably from one point to another
• Data could be voice, video, codes etc
• It is important to receive the same information that was sent from the transmitter.
• It is required that sender and receiver should understand the same language.
• It has been improving the quality of communication on behalf of growing demand for speed and
complexity of information.
• Communication system
• A system that allows transfer of information reliably
• Every communication has three essential elements : transmitter, channel and receiver
• Here the transmitter is placed at one place and receiver is placed another place and the channel is
the physical medium that connect them.
• The purpose of transmitter is to convert the data/signal into suitable form of signal that can
transmitted through the channel.
• If the o/p of the information source is a non electric signal then a transducer convert it into
electric form before it pass through the channel. Moreover, noise is introduced in channel so
receiver reconstruct it and send the information to user for.
• There are two basic type of communication namely point to point and broadcast. The former take
place over a link between a single transmitter and receiver, while second one have large number
of receiver corresponding to a single transmitter.
• Radio and TV comes under broadcast
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Communication systems, cont..
• Communication Channel
• The channel is central to operation of a communication System.
• The information -carrying capacity of a communication system is proportional to the
channel bandwidth.
• Needed wider bandwidth
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Copper wire: 1 MHz
Coaxial cable: 100 MHz
Microwave: GHz
Optical fiber: THz
• Uses light as the signal carrier.
• Highest capacity among all practical signals.
• Communication systems design concerns about:
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Selection of the information type–bearing waveform
Bandwidth and power of the waveform
Effect of system noise on the received information
Cost of the system
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Communication
• Main purpose of communication is to transfer information from a source to a
recipient via a channel or medium.
• Basic block diagram of a communication system:
• Brief Description
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Source Transmitter
Source: analog or digital
Transmitter: transducer, amplifier, modulator, oscillator, power amp., antenna
Channel: e.g. cable, optical fiber, free space
Receiver: antenna, amplifier, demodulator, oscillator, power amplifier, transducer
Recipient: e.g. person, (loud) speaker, computer
• Types of information
Voice, data, video, music, email etc.
• Types of communication systems
• Public Switched Telephone Network (voice, fax, modem)
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Satellite systems
Radio, TV broadcasting
Cellular phones
Computer networks (LANs, WANs, WLANs)
Channel
Receiver
Recipient
Types of communication
• Simplex
• One direction
• e.g. Television, FM and AM radios
• Half duplex
• Either direction, but only one way at a time
• e.g. police radio (walky-talky)
• Full duplex
• Both directions at the same time
• e.g. telephone
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Information Representation
• Communication system converts information into electrical electromagnetic
/optical signals appropriate for the transmission medium.
• Analog systems convert analog message into signals that can propagate through
the channel.
• Digital systems convert bits(digits, symbols) into signals
• Computers naturally generate information as characters/bits
• Most information can be converted into bits
• Analog signals converted to bits by sampling and quantizing (A/D
conversion)
Digital and Analog Sources and Systems
Basic Definitions:
• Analog Information Source:
An analog information source produces messages which are defined on a continuum. (E.g. :Microphone)
• Digital Information Source:
A digital information source produces a finite set of possible messages. (E.g. :Typewriter)
x(t)
x(t)
t
t
Analog
Digital
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Digital and Analog Communication Systems
 A digital communication system
 In digital communication digital signal is used rather than analog signal for communication in between the
source and destination. The digital signal consists of discrete values rather than continuous values. In digital
communication physical transfer of data occurs in the form of digital bit stream i.e 0 or 1 over a point-topoint or point-to-multipoint transmission medium. In digital communication the digital transmission data can
be broken into packets as discrete messages which is not allowed in analog communication.
 An analog communication system
 In analog communication the data is transferred with the help of analog signal in between transmitter and
receiver.
 Any type of data is transferred in analog signal.
 Any data is converted into electric form first and after that it is passed through communication channel.
 Analog communication uses a continuous signal which varies in amplitude, phase, or some other property
with time in proportion to that of a variable.
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Why digital?

Digital techniques need to distinguish between discrete symbols allowing regeneration versus
amplification
Good processing techniques are available for digital signals, such as medium.

Data compression (or source coding)
 Error Correction (or channel coding)(A/D conversion)
 Equalization
 Security
Easy to mix signals and data using digital techniques


• Basic Digital Communication Transformations
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Formatting/Source Coding
Transforms source info into digital symbols (digitization)
Selects compatible waveforms (matching function)
Introduces redundancy which facilitates accurate decoding despite errors
• It is essential for reliable communication
• Modulation/Demodulation
• Modulation is the process of modifying the info signal to facilitate transmission
• Demodulation reverses the process of modulation. It involves the detection and retrieval of the info signal
• Types
• Coherent: Requires a reference info for detection
• Noncoherent: Does not require reference phase information
• Coding/Decoding
• Translating info bits to transmitter data symbols
• Techniques used to enhance info signal so that they are less vulnerable to channel impairment (e.g. noise, fading,
jamming, interference)
• Two Categories
• Waveform Coding
• Produces new waveforms with better performance
• Structured Sequences
• Involves the use of redundant bits to determine the occurrence of error (and sometimes correct it)
• Multiplexing/Multiple Access Is synonymous with resource sharing with other users
Performance Metrics
• Analog Communication Systems
• Metric is fidelity: want m
ˆ (t )  m(t )
• SNR typically used as performance metric
• Digital Communication Systems
• Metrics are data rate (R bps) and probability of bit error
 P  p( bˆ  b) 
b
• Without noise/distortion/sync. problem, we will never make bit errors
• Main Points
• Transmitters modulate analog messages or bits.
• Receivers recreate signals or bits from received signal (mitigate channel effects)
• Performance metric for analog systems is fidelity, for digital it is the bit rate and error probability.
Why Digital Communications?
• Easy to regenerate the distorted signal
• Regenerative repeaters along the transmission path can detect a
digital signal and retransmit a new, clean (noise free) signal
• These repeaters prevent accumulation of noise along the path
• This is not possible with analog communication systems
• Two-state signal representation
• The input to a digital system is in the form of a sequence of bits
(binary or M_ary)
• Immunity to distortion and interference
• Digital communication is rugged in the sense that it is more
immune to channel noise and distortion
Why Digital Communications?, cont.
• Hardware is more flexible
• Digital hardware implementation is flexible and permits the use of microprocessors, mini-processors,
digital switching and VLSI
• Shorter design and production cycle
• Low cost
• The use of LSI and VLSI in the design of components and systems have resulted in lower cost
• Easier and more efficient to multiplex several digital signals
• Digital multiplexing techniques – Time & Code Division Multiple Access - are easier to implement than
analog techniques such as Frequency Division Multiple Access
• Can combine different signal types – data, voice, text, etc.
• Data communication in computers is digital in nature whereas voice communication between people is analog in
nature
• The two types of communication are difficult to combine over the same medium in the analog domain.
• Using digital techniques, it is possible to combine both format for transmission through a
common medium
• Encryption and privacy techniques are easier to implement
• Better overall performance
• Digital communication is inherently more efficient than analog in realizing the exchange of SNR for
bandwidth
• Digital signals can be coded to yield extremely low rates and high fidelity as well as privacy
Digital and Analog Sources and Systems, cont.
 A digital waveform is defined as a function of time that can have a
discrete set of amplitude values.
 An Analog waveform is a function that has a continuous range of values.
 Advantages and disadvantages of analog communication
 Advantages
• Transmitters and Receivers are simple
• Low bandwidth requirement
• FDM can be used
 Disadvantages
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Noise affects the signal quality
It is not possible to separate noise and signal
Repeaters can‘t be used between transmitters and receivers
Coding is not possible
It is not suitable for the transmission of secret information
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Digital and Analog Sources and Systems, cont.
Digital Communication
 Advantages
• Relatively inexpensive digital circuits may be used;
• Privacy is preserved by using data encryption;
• Data from voice, video, and data sources may be merged and transmitted over a
common digital transmission system;
• In long-distance systems, noise dose not accumulate from repeater to repeater.
• Data regeneration is possible
• Errors in detected data may be small, even when there is a large amount of noise on
the received signal; Errors may often be corrected by the use of coding.
 Disadvantages
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Generally, more bandwidth is required than that for analog systems;
Synchronization is required.
Requires A/D conversions at high rate
Nongraceful degradation
Performance Criteria
Probability of error or Bit Error Rate
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Encoding and Decoding for Digital Communication

Coding involves adding extra (redundant) bits to data to reduce or correct errors at the
output of the receiver.
 The disadvantage of these extra bits is to increase the data rate and the bandwidth of the
encoded signal.
General Digital Communication System
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Deterministic and Random Waveforms
 A Deterministic waveform can be modeled as a completely
specified function of time.
w(t )  A cos(0t  0 )
 A Random Waveform (or stochastic waveform) cannot be
modeled as a completely specified function of time and must be
modeled probabilistically.
 In this course we will focus mainly on deterministic waveforms.
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Block Diagram of A Communication System
 All communication systems contain three main subsystems:
 Transmitter
 Channel
 Receiver
Transmitter
Receiver
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Block Diagram of A Communication System
TRANSMITTER:
 The signal-processing block is used for more efficient transmission.
Examples:
•
•
In an analog system, the signal processor may be an analog low-pass filter to restrict the bandwidth of
m(t).
In a hybrid system, the signal processor may be an analog-to-digital converter (ADC) to produce
digital signals that represent samples of the analog input signal.
 The transmitter carrier circuit converts the processed base band signal into a frequency band that is
appropriate for the transmission medium of the channel.
Example:
•
An amplitude –modulated (AM) broadcasting station with an assigned frequency of 850 kHz has a
carrier frequency fc=850kHz. The mapping of the base band input information waveform m(t) into the
band pass signal s(t) is called modulation. It will be shown that any band pass signal has the form
s(t )  R(t ) cos(ct   (t ))
c  2 f
If R(t)=1 and θ(t) = 0, s(t) would be a pure sinusoid of frequency f=fc with zero bandwidth.
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Block Diagram of A Communication System
Channel:
 Channels represents the path in which signals travel from transmitter to receiver. Very general
classification of channels are:
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Wired: Twisted-pair telephone line, coaxial cable, waveguide, and fiber-optic cables.
Wireless: Air and space vacuum.
 In general, the channel medium attenuates the signal so that the delivered information 𝑚(𝑡)
deteriorated from that of the source m(t). The channel noise may arise from natural electrical
disturbances or from artificial sources.
Transmitter
Receiver
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Block Diagram of A Communication System
Receiver:
 The receiver takes the corrupted signal at the channel output and converts it to be a base band signal that can be
handled by the receiver’s base band processor.
 The base band processor cleans up this signal and delivers an estimate 𝑚(𝑡) of the source information m(t) to the
communication system output.
 In digital systems, the measure of signal deterioration is usually taken to be the probability of bit error P(e) – also
called Bit Error Rate (BER) of the delivered data m(t).
 In analog systems, the performance measure is usually taken to be the Signal-to-noise Ratio (SNR) at the
receiver output.
Transmitter
Receiver
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What makes a Communication System GOOD

We can measure the “GOODNESS” of a communication system in many ways:

How close is the estimate 𝑚(𝑡)
to the original signal m(t)
• Better estimate = higher quality transmission
• High Signal to Noise Ratio (SNR) for analog m(t)
• Small Bit Error Rate (BER) for digital m(t)

How much power is required to transmit s(t)?
• Lower power = longer battery life, less interference

How much bandwidth B is required to transmit s(t)?
• Less B means more users can share the channel
• Exception: Spread Spectrum -- users use same B.

How much information is transmitted?
• In analog systems information is related to B of m(t).
• In digital systems information is expressed in bits/sec.
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