WHAT IS COMMUNICATION SYSTEM?
 a communication system is to transmit intelligence
signal from a source to a destination at some point
away from the source. ... This system consists
three basic components: transmitter, channel, and
receiver
 Digital communication systems, by definition,
are communication systems that use such a digital
sequence as an interface between the source and the
channel input
INTRODUCTION
CHAPTER 1
Figure 1.1 Block diagram of Communication System
Figure 1.1 shows the general block diagram of a
communication system whose purpose is to
transport an information bearing signal from a
source to a user destination through a
communication channel.
The three basic elements of every communication system are:
Transmitter - The transmitter processes the input signal to produce a
signal suits to the characteristics of the transmission channel.
Receiver - is a collection of electronic components and circuits that
accepts the transmitted information from the channel and
converts it back into a form understandable by humans.
Channel - Communication channel is the medium by which the
electronic signal is sent from one place to another. The channel
can have different forms: The atmosphere (or free space), coaxial
cable, fiber optic etc.
The general classification of communication systems are:
1. Analog Communication Systems – to transmit analog
information using analog modulation methods.
2. Digital Communication Systems – to transmit digital
information using digital modulation methods.
Figure 1.2 Block diagram of a digital communication
system
Sources of Information:
Analog Information Sources
Speech signal (microphone) / Audio
Picture signal (TV, camera) / Video
Continuous amplitude signals
Digital Information Sources
Numerical output of computer (sequence of discrete symbols or
letters)
Generally analog information is transformed to discrete/digital
information through the process of sampling and quantizing.
Input Transducer
This is a transducer which takes a physical input and converts it to an
electrical signal (Example: microphone).
This block also consists of an analog to digital converter where a digital
signal is needed for further processes.
A digital signal is generally represented by a binary sequence.
Source Encoder & Source Decoder:
The source encoder or source coder converts the input which is
a symbol or character into a binary sequence of ‘0’s and ‘1’s (bit
stream) by assigning code words to the symbols in the input
sequence.
The important parameters of the source encoder are block size,
code word lengths, average data rate and the efficiency of the coder.
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At the receiver, the source decoder converts the binary output of
the channel decoder into a symbol sequence.
The decoder using fixed-length code word is quite simple, but
the one which is used when variable-length code word is very
complex.
The main purpose of the source coding is to remove the
redundancy in the transmitting information so as to reduce the
transmission bandwidth.
The code word is assigned based on the probability of the symbol.
Higher the probability, the shorter is the code word.
Example of source coding is the Huffman coding.
Channel Encoder & Decoder:
Channel encoding or channel coding refers to just a
signal transformation done to improve the communications
performance by enabling the transmitted signals to better
withstand the effects of various channel impairments such as
noise, interference and fading.
Thus error control is accomplished by the channel coding
operation that consists of symptomatically adding extra bits to
the output of the source coder.
These extra bits do not convey any information but helps the
receiver to detect and/or correct some of the errors in the
information bearing bits.
There are two methods of channel coding:
1. Block Coding
2. Convolution Coding
Block Coding:
The encoder takes a block of ‘k’ information bits from the
source encoder and adds ‘r’ error control bits, where ‘r’ is
dependent on ‘k’ and error control capabilities desired.
Convolution Coding:
The information bearing message stream is encoded in a
continuous fashion by continuously interleaving information bits
and error control bits.
The Channel decoder recovers the information bearing bits
from the coded binary stream. Error detection and possible
correction is also performed by the channel decoder. The
important parameters of coder / decoder are :
Method of coding, efficiency, error control capabilities and
complexity of the circuit.
Modulator:
The Modulator converts the input bit stream into an electrical
waveform suitable for transmission over the communication
channel.
• Modulator can be effectively used to minimize the effects of
channel noise, to match the frequency spectrum of transmitted
signal with channel characteristics, to provide the capability to
multiplex many signals.
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Demodulator:
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Demodulator is used to extract the message from the
information bearing waveform produced by the modulator.
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The output of the demodulator is a bit stream. The important
parameter is the method of demodulation.
Channel:
The Channel provides the electrical connection between the source
and destination.
• The different communication channels are:
• telephone channels coaxial cables, optical fibers, microwave radio ,
satellite channels
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The communication channels have only finite Bandwidth, non-ideal
frequency response, the signal often suffers amplitude and phase
distortion as it travels over the channel.
• Also the signal power decreases due to the attenuation of the
channel.
• The signal is corrupted by unwanted, unpredictable electrical
signals referred to as noise.
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The important parameters of the channel are :
• Signal to Noise power Ratio (SNR), usable bandwidth, amplitude
and phase response and the statistical properties of noise.
Refer: Handout
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Constraints of the communication channel:
The modulation and coding used in a digital communication system
depend on the characteristics of the channel. The two main characteristics
of the channel are BANDWIDTH and POWER.
 Bandwidth constraint:
 Data transmission systems work best in the frequency
range over which the medium behaves linearly;
 Over this passband we can rely on the fact that a signal
will be received with only phase and amplitude
distortions, and these are “good” types of distortion
since they amount to a linear filter.
 Further limitations on the available bandwidth can be
imposed by law or by technical requirements and the
transmitter must limit its spectral occupancy to the
prescribed frequency region
 Power constraint:
 The power of a transmitted signal is inherently limited
by various factors, including the range over which the
medium and the transmission circuitry behaves linearly.
 In many other cases, such as in telephone or radio
communications, the maximum power is strictly
regulated by law.
 Also, power could be limited by the effort to maximize
the operating time of battery-powered mobile devices.
 At the same time, all analog media are affected by noise,
which can come in the form of interference from
neighboring transmission bands (as in the case of radio
channels) or of parasitic noise due to electrical
interference (as in the case of AC hum over audio lines)
TYPES OF CHANNELS
 Telephone channel: (Refer Hand out)
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It is designed to provide voice grade communication.
 Also good for data communication over long distances.
 The channel has a band-pass characteristic occupying the
frequency range 300 Hz to 3400 Hz, a high SNR of about 30
dB, and approximately linear response.
 Coaxial Cable:
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The coaxial cable consists of a single wire conductor
centered inside an outer conductor, which is insulated from
each other by a dielectric.
 The main advantages of the coaxial cable are wide
bandwidth and low external interference.
 But closely spaced repeaters are required. With repeaters
spaced at 1km intervals the data rate of 274 Mbps have
been achieved.
Optical Fibers:
An optical fiber consists of a very fine inner core made of silica
glass, surrounded by a concentric layer called cladding that is also made
of glass.
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Compared to coaxial cables, optical fibers are smaller in size and
they offer higher transmission bandwidths and longer repeater
separations.
 The fastest fiber-optic cable speed offered in a business gigabit network
service today is 10 Gbps
Microwave radio:
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A microwave radio, operating on the line-of-sight link, consists of a
transmitter and a receiver that are equipped with antennas.
The antennas are placed on towers at sufficient height to have the
transmitter and receiver in line-of-sight of each other.
The operating frequencies range from 1 to 30 GHz.
Under normal atmospheric conditions, a microwave radio channel is very
reliable and provides path for high-speed digital transmission.
But during meteorological variations, a severe degradation occurs in the
system performance.
 Satellite Channel:
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A Satellite channel consists of a satellite in
geostationary orbit, an uplink from ground station, and a
down link to another ground station.
 Both link operate at microwave frequencies, with the
uplink frequency higher than the down link frequency.
 In general, Satellite can be viewed as repeater in the
sky.
 It permits communication over long distances at higher
bandwidths and relatively low cost.
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Advantages of Digital Communication:
1. Digital circuits are less subject to distortion, noise and
interference than the analog circuits since the digital
circuits operate in one of the two states ON or OFF.
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 chip etc.
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
COMMUNICATION (CONTINUOUS)
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.
(Due to analog to digital conversion, the data rate becomes high.
therefore more transmission bandwidth is required for digital
communication)
2.System Synchronization: Digital detection requires system
synchronization whereas the analog signals generally have no such
requirement.
REVIEW OF SIGNALS
Classification of Signals:
• Deterministic and random signals
• Periodic and aperiodic signals
• Analog and discrete signals
• Energy and power signals
DETERMINISTIC SIGNALS:
• A deterministic signal has no uncertainty in its amplitude at
any instant of time.
• Or, signals which can be defined exactly by a mathematical
formula are known as deterministic signals.
• It can be mathematically modeled as
• Eg:
A RANDOM SIGNAL (NON-DETERMINISTIC)
 A random signal always has some degree of uncertainty
before it occurs. It cannot be mathematically modeled.
 However, a random single exhibits certain regularities which
are described in terms of probabilities and statistical
averages, which is useful when dealing with noise in
communication system
Periodic and Aperiodic signals:
A signal is said to be periodic if it satisfies the condition x(t) = x(t + T) or x(n) =
x(n + N). Where
T = fundamental time period,
1/T = f = fundamental frequency.
A signal is said to be periodic if
X (t)=
;
--------- (1)
where To is the fundamental time period
A signal that does not satisfy equation (1) is called Aperiodic signal.
Analog (Continuous time) and Discrete time signals:
• Analog signal
is a continuous function of time if its amplitude
exists for all values of ‘t ’
• Discrete signal has amplitude only for discrete times given by KT
where k is an integer and T is a fixed time interval.
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Note : Analog signal is continuous time continuous amplitude
signal and
a digital signal is discrete time discrete amplitude signal
(digital signal is an amplitude quantization of discrete time
signal)
Energy and Power Signals (Refer Handout for Problems)
NOTE:A signal cannot be both, energy and power
simultaneously. Also, a signal may be neither energy nor power
signal.
Power of energy signal = 0
Energy of power signal = ∞