Communication System

Module-I
 Systems
System: A system is the combination or arrangement of different types
of objects (or physical components), which works together to acquires
the specific/certain objectives.
Example of System
 Communication Systems
Definition: The communication system is a system which describes the
information exchange between two stations/points, transmitter & receiver. The
process of transmission and reception of information is called communication.
The major elements of communication are the Transmitter, Channel or medium
and Receiver.

 The main objective of any

communication system is
to transmit a signal which
is generated by a source
to a destination through a
media/channel.
 Examples of Communication Systems
 The following are a few examples of communication systems:

 Radio is a very common mode of communication system

 Source: Micro-Phone
 Destination: Speaker in the radio
 Media/Channel: Space
 Television
 Source: Video camera and microphone
 Destination: Picture tube and speakers
 Media/Channel: Space.
 Telephone
 Source: Microphone in the phone set
 Destination: Speaker in the phone set
 Media/Channel: Wire line.
 Cellular Mobile
 Source: Micro-phone in the phone set
 Destination: Speaker in the phone set
 Media: Space
 Types of Communication Systems
 Depending on signal specification or technology, the communication
system is classified as follows:

Communication Systems

Analog Digital
 These signals are continuous in both  Unlike analog signals, digital signals are
values and time. not continuous, but signals are discrete
 Analog technology communicates data in value and time. These signals are
as electronic signals of varying represented by binary numbers.
frequency or amplitude.  In digital technology, the data are
 All signals that are natural or come generated, stored and processed in two
naturally are analog signals. states: High (represented as 1) and low
 Ex: Broadcast, telephone transmission (represented as 0).
and radar etc.  Ex: email, sms, web chat etc.
 Difference between Analog and Digital Signal
Analog Signals Digital Signals
Continuous signals Discrete signals
Generally represented by sine waves Generally represented by square waves
Human voice, natural sound, analog Computers, optical drives, and other
electronic devices are a few examples electronic devices

Continuous range of values Discontinuous values

Records sound waves as they are Converts into a binary waveform
Only used in analog devices Suited for digital electronics like
computers, mobiles and more
 Types of Communication Systems
 Depending on the communication channel, the communication
system is categorised as follows:
Communication Systems

Wired (Line communication) Wireless (Space Communication

It refers to the transmission of data It is used for long distance
over a wire-based communication communication. Antenna is used to
technology. convert electrical signal into
Exemples: Telephone networks, cable elctromagnetic (EM) signal.
television or internet access, and fiber- Exemples: Cell phone, radar, sonar, wifi,
optic communication. bluetooth, radio, TV, etc.
 Parallel wire communication  Ground wave communication
 Twisted wire communication  Sky wave communication
 Coaxial cable communication  Space wave communication
 Optical fibre communication  Satellite communication
Parallel Wire Twisted Wire

Coaxial Cable
Optical Fiber Cable
 Basic blocks of a communication system
 The block diagram given below represents the flow of the signal from the
source to the destination.
(High frequency & low energy
 Voice signal: Range is (300 Hz to 3400 Hz) +noise) EM wave
(High frequency
 Audio signal: Range is (20 Hz to 20 KHz)
& high energy) (low frequency & high energy)
 Video signal: Range is (0 to 5 MHz)
EM wave electrical signal
 Etc. (Low frequency & low
energy) electrical signal

It converts
electrical signal
It converts non-electrical into non-
signal into electrical signal. Wireless & wired channel electrical signal
Ex: For voice….microphone
will be amplifier, will be amplifier, Received sound,
modulator, filter, de modulator, filter, picture, speech data
mixer, antenna antenna etc. at Destination
Block diagram of communication system
 Elements of communication system
Information: Message or information is the entity that is to be transmitted. It
can be in the form of audio, video, temperature, picture, pressure, text, etc.

 The source generates some electrical signal, which is possibly captured

from real life audio, video or image, then a signal is generated by the
transducers, which need to be transmitted up to destination through
media channel

Types of Sources

Binary File

Non-Binary File
e.g.: Audio signal captured in micro-phone.
Video signal captured in camera.
 Elements of communication system
Signal: The Variation in any kind of physical parameter with respect to two
independent parameters, time and space.

 The single-valued function of time

carries the information.
 variation of temperature with respect to
time and space.
 Audio and video signals.
 The information is converted into an
electrical form for transmission.
 Picture, where intensity of color varies
with respect to space is an example of
Representation of Signal signal.
 Elements of communication system

Transducer: It is a device or an arrangement that converts one form of energy

to the other. An electrical transducer converts physical variables such as pressure,
force, and temperature into corresponding electrical signal variations.
For example:
 A microphone converts audio signals into electrical signals.
 The photodetector converts light signals into electrical signals.
Amplifier: The electronic circuit or device that increases the amplitude or the
strength of the transmitted signal is called an amplifier.
 When the signal strength becomes less than the required value,
amplification can be done anywhere between the transmitter and
receiver.
 A DC power source will be provided for the amplification.
 Elements of communication system
Modulator: As the original message signal cannot be transmitted over a large
distance because of their low frequency and amplitude, they are superimposed with high
frequency and amplitude waves called carrier waves. This phenomenon of superimposing of
message signals with a carrier wave is called modulation, and the resultant wave is a
modulated wave which is to be transmitted.
 There are different types of modulation as follows:
Modulation

Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

 The process of changing the  Frequency modulation is a  The phase of the carrier
amplitude of the signal wave technique in which the wave changes the phase of
by superimposing it on a high- frequency of the message the signal wave. The phase
frequency carrier wave, signal is varied by modulating shift after modulation is
keeping its frequency with a carrier wave. It is better dependent on the frequency
constant, is called amplitude than amplitude modulation of the carrier wave as well.
modulation. because it eliminates noise Phase modulated waves are
from various sources. immune to noise to a greater
extent.
 Elements of communication system
Transmitter: It is the arrangement that processes the message signal into a
suitable form for transmission and, subsequently, reception.
Antenna: An antenna is a structure or a device that will radiate and receive
electromagnetic waves. So, they are used in both transmitters and receivers. An
antenna is basically a metallic object, often a collection of wires. The
electromagnetic waves are polarised according to the position of the antenna.
Channel: Channel is the link between the source and destination. i.e., a
channel refers to a physical medium such as wire, cables, or space through which
the signal is passed from the transmitter to the receiver.

 There are many channel impairments that affect channel

performance to a pronounced level. The major channel impairments
are mention as follows:
 Noise
 Attenuation
 Distortion
 Elements of communication system
Noise: Noise is one of the channel imperfections or impairments in the
received signal at the destination. There are external and internal sources
that cause noise.

 External sources include interference, i.e. interference from nearby

transmitted signals (cross talk), interference generated by a natural
source such as lightning, solar or cosmic radiation, automobile-
generated radiation, etc.
 The external noise can be minimised and eliminated by the
appropriate design of the channel and shielding of cables. Also,
by digital transmission, external noise can be minimised.

 Internal sources include noise due to random motion and collision

of electrons in the conductors and thermal noise due to diffusion
and recombination of charge carriers in other electronic devices.
 Internal noise can be minimised by cooling and using digital
technology for transmission.
Note: The formula for calculating dBm
measurement is:
10 x log (signal strength per milliwatts).
Equivalent circuit of a resistor as a noise generator
T = 273+17 = 290 K
 Signal to Noise power Ratio (SNR)

 The quality of communication systems

varies with the Signal-to-Noise ratio (SNR)
𝑆 𝑃𝑠  A certain minimum SNR at the receiver is
SNR= = necessary for successful communication.
𝑁 𝑃𝑛

where, 𝑃𝑠 = signal power in watt

𝑃𝑛 = Noise power in watt
Note:
 Higher the SNR that means Lower Noise Power and Higher Signal Power is better for
Communication.
 Our Expectation is Zero Noise Power that means infinite SNR.
𝑃𝑠
SNR can also be expressed in dB unit: SNR(dB)= 10 log 𝑃𝑛

SNR in voltage:
𝑃𝑠 𝑉𝑠 2 /𝑅𝐿 𝑉 𝑉
(𝑆𝑁𝑅)𝑑𝐵 = 10 log = 10 log = 10 log (𝑉𝑠 )^2 = 20 log (𝑉𝑠 ) dB
𝑃𝑛 𝑉𝑛 2 /𝑅𝐿 𝑛 𝑛

where, 𝑉𝑠 = signal voltage in volt

𝑉𝑛 = 𝑁𝑜𝑖𝑠𝑒 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑖𝑛 𝑣𝑜𝑙𝑡
Home Work
 Elements of communication system
Attenuation: Means loss of energy -> weaker signal
 Attenuation is a problem caused by the medium. When the signal is propagating
for a longer distance through a medium, depending on the length of the
medium, the initial power decreases.
 The loss in initial power is directly proportional to the length of the medium.
 Using amplifiers, the signal power is strengthened or amplified so as to reduce
attenuation.
 Also, digital signals are comparatively less prone to attenuation than analogue
signals.
 Measurement of Attenuation

• To show the loss or gain of energy the unit “decibel” is used.

dB = 10 log10 (P2/P1)
P1 - input signal
P2 - output signal
Ex: Suppose a signal travels through a transmission medium and its power is
reduced to one-half. Calculate the attenuation.
Solution:
Since power is reduced to one-half, this means that P2 = (1/2)P1.
In this case, the attenuation (loss of power) can be calculated as:

i.e., A loss of 3 dB (–3 dB) is equivalent to losing one-half the power.

Ex: A signal travels through an amplifier, and its power is increased 10 times.
Calculate the amplification.
Solution:
Since power is increased 10 times, this means that P2 = 10 P1.
In this case, the amplification (gain of power) can be calculated as:
 Ex: One reason that engineers use the decibel to measure the changes in the
strength of a signal is that decibel numbers can be added (or subtracted)
when we are measuring several points (cascading) instead of just two.
 In Figure below, a signal travels from point 1 to point 4. In this case, the
decibel value can be calculated as:
 Ex: The loss in a cable is usually defined in decibels per kilometer (dB/km). If
the signal at the beginning of a cable with −0.3 dB/km has a power of 2 mW,
what is the power of the signal at 5 km?
Solution:
 The loss in the cable in decibels is:
5 × (−0.3) = −1.5 dB.
We can calculate the power as:
 Ex-5: At the transmitter, the signal power is 23 mW. The input SNR is 40 dB.
The channel offers 3 dB attenuation to the signal and the output noise is
thrice the input noise level. Determine the SNR at the output.
Solution:

Calculation of Output Power Level

 An attenuation of 3 dB equals halving the input transmission power.
 If the ratio of two quantities on the linear scale is 1/2, it translates to -3 dB on
the dB scale which is indicated as attenuation.
 So, the output signal power is 23mW/2 =11.5 mW.

Calculation of Input Noise Level

The input SNR is 40 dB. This means that the input power level is 10000 times
greater than the input noise level.

So, The input noise level is 2.3 μW.

 In the question, it is given that the output noise is thrice the input noise.
Thus, the output noise level is:
2.3μW x 3 = 6.9 μW.

Calculation Of Output SNR

Output signal power = 11.5 mW

Output noise power = 6.9 μW.
The ratio of the output signal power to the output noise power gives the output
SNR at the receiver. Inference:
 The input SNR is 40 dB while the
output SNR is 32.22 dB.
 Due to the channel noise, the output
SNR has decreased by 8 dB.
 However, the signal power is still
large enough than the noise power to
have a faithful detection and
decoding at the receiver.
 Ex-6: The initial SNR measured at the transmitter was 20 dB. In order to
combat the channel conditions, the signal power was doubled prior to
transmission. What is the new SNR at the transmitter?
Solution:
Initial SNR = 20 dB.

Therefore, the new SNR is 23 dB.

Distortion: It is also another type of channel problem. It is usually a degradation
of the signal.
When the signal is distorted, the distorted signal may have a frequency and
bandwidth different from the transmitted signal. The variation in the signal
frequency can be linear or non-linear.
 Elements of communication system
Receiver: An arrangement that extracts the message or information from the
transmitted signal at the output end of the channel and reproduces it in a suitable
form as the original message signal is a receiver.
Demodulator: It is the inverse phenomenon of modulation, i.e., the process
of separation of the message signal from the carrier wave takes place in the
demodulator. The information is retrieved from the modulated wave.
Repeaters: Repeaters are placed at different locations in between the
transmitter and receiver. A repeater receives the transmitted signal, amplifies it
and sends it to the next repeater without distorting the original signal.

Communication only
possible by bouncing
the signal through
the repeater
 Representation of Signals
 Electrical signal may be represented in two equivalent forms. i.e., as a
voltage signal or a current signal.

Representation of electrical signals as a

Thevenin’s form & Norton’s form

Further, electrical signal may be represented in two forms:

 Time domain representation, and
 Frequency domain representation
 When Fourier series is applied on the given time domain representation of
square wave, then we get

A symmetrical square-wave signal

(time-domain representation) Frequency spectrum of square-wave signal
(frequency domain representation)
 Here, it is clear that the frequency spectrum of a periodic signal is discrete.
Time domain representation of an
arbitrary signal waveform Frequency domain representation
of time-domain signal
How to Plot Line Spectrum?
 Double Side Line Spectrum
The double side line spectra can be obtained from the single sided spectrum as shown below:
Conclusion:
 In single sided spectrum there is
only one frequency component
present at f=f0 with an amplitude
A.
 Whereas, in double sided line
spectra, two frequency
components f0 & -f0 are present
with amplitude (A/2) but no
change in polarity.
 The single sided spectrum
contains only one component of
frequency at f=f0 with phase 𝜑.
Fig.: Equivalence between single and
 But double sided spectrum
double sided spectrums
contains components at f0 & -f0
Note: The double sided line spectrum with phase 𝜑 and −𝜑 ,
representation is very useful in mathematical respectively. Thus, phase shift
analysis. The negative frequency components remains unchanged but they have
present in the double sided spectrums are not opposite sign.
practically present.
 Example:

So, all the terms can be written as follows:

Thus, the negative amplitude has been made positive by adding a phase angle of 180 deg.

Thus, the sine term has been converted to the cosine term by a phase angle of -90 deg.
The amplitudes, frequencies and phase angles of the three terms are listed in the Table below:
With the help of the previous table, we can plot the line spectra as shown below:
Q: Draw the line spectrum of given signal.
 Q: Write the equation for given double sided spectrum.

Ans: w(t)=7 cos 2π0t + 10 cos (2π20t + 1200) + 4 cos (2π60t - 900)
 Line spectrum shown in Fig (b) is the representation of the same
signal of Fig (a) in frequency domain.
 It can be obtained by using either Fourier series or Fourier transform.
 It consists of amplitude and phase spectrum of the signal.
 The line spectrum indicates the amplitude and phase of various
frequency components present in the given signal.
Introducing Fourier
 Frequency Domain Representation of a Signal (Line Spectra)
 The frequency represented in the frequency domain is called the line
spectrum.
 It consists of two graph namely:

 The time domain signal gives the following information:

 But, we can not know
anything about what
frequency components
are present and in
what proportion they
have been mixed in
order to obtain the
particular shape of the
signal.
 Fourier Series
 Sine waves and cosine waves are the basic building functions for any periodic
signal.
 Any periodic signal basically consists of sine waves having different amplitude
of different frequencies and having different relative phase shifts.
 Fourier series represents a periodic waveform in the form of sum of infinite
number of sine and cosine terms. It is a representation of the signal in a time
domain series form. Fourier series is a tool used to analyse any periodic
signal. After the analysis, the following information about the signal can be
obtained:
 A periodic signal x(t) with a period of T0 may be represented by the
trigonometric Fourier series as under:

(1)
 Equation (1) can be expanded as:

(2)
  The polar Fourier series is derived from the trigonometric Fourier series by
combining the sine and cosine terms of same frequency. The polar Fourier
series representation of x(t) is as under:

(3)

 Equation (3) can be expanded as :

(4)
 Line Spectrum
 The line spectrum of x(t) can be plotted by using equation (4).
 A line spectrum of x(t) with arbitrary values of amplitude and phases has
been shown in figure below:

 As observed from figure (b), the frequency spectrum of a continuous signal is

discrete in nature. The frequency components f0, 2f0, 3f0…etc. are called as
the spectral components.
 The adjacent spectral components are spaced by ‘f0’ from each other. As the
spectrum is consisting of vertical lines, (C1,C2,…) this spectrum is called as the
line spectrum.
 EXPONENTIAL FOURIER SERIES
[OR COMPLEX EXPONENTIAL FORUIER SERIES]
 The sine and cosine term may be expressed in terms of the exponential terms
using Euler’s equations given by:

(5)

(6)
 Substituting the sine and cosine functions in terms of exponential function in
the expression for the trigonometric or quadrature Fourier series (equation 1),
we can obtain another type of Fourier series called the exponential Fourier
series.

(7)

(8)
 Concept of Negative Frequency
 In eq (7), it may be observed that ‘n’ is extending from -∞ to +∞ instead of 0 to
+∞. Due to this, the frequencies in the frequency spectrum will extend from -∞ to +∞.
 Thus, if we express the signal x(t) using exponential Fourier series, then, we
obtain a double sided frequency spectrum.
 However, the negative frequency signals do not exist physically. They are used
as an important mathematical concept and for mathematical convenience.
Amplitude and Phase Spectrum

(9)

(10)

 The phase spectrum is a graph of Φn on the y-axis versus frequency f = nf0 on

the x-axis.
 The amplitude spectrum is a symmetric or even function. This means that
|Cn|=|C-n|. But, the phase spectrum is an asymmetric or odd function. This
means that arg (Cn)=-arg (C-n).
 Ex: Obtain the exponential Fourier series for the rectangular pulse train shown
below and sketch the spectrum.

Solution: From eq (7) & (8), the exponential Fourier series is given as:
 -

(iii)

Multiplying and dividing the RHS of eq(iii) by

(iv)
Substituting the value of Cn into eq(i), we obtain the exponential Fourier series as:

(v)

From the value of Cn in eq (iv), it is obvious that Cn does not have any imaginary
part. Therefore, the amplitude spectrum of x(t) is given as:

(vi)

Thus, the amplitude

spectrum of a
rectangular pulse of
duration 𝜏 is a sinc
function. The
spectrum has been
shown in figure (b).
The imaginary part of Cn is zero. Hence the phase spectrum is zero for all the
values of f. The amplitude and phase spectrum of a periodic pulse train has
been shown in figure (c).
Baseband and Passband or Bandpass Signal
Baseband Signal:
 Baseband, as the name suggests, refers to the original or natural
transmission signal without modulation (without frequency shifting).
Baseband is also called the low pass signal or message signal.
 E.g. voice, image, or video signal.
 But to carry this baseband signal over a long distance as it has a low
frequency, modulation needs to take place.

 It generally occupies the frequency Spectrum of a baseband Signal

spectrum right from 0 Hz.
Passband or Band pass Signal:
 If the baseband or original message is used to modify a carrier wave (i.e.
modulation process) then after modulation, the information that is transferred
is the passband signal.
 Passband signal refers to filtered signal or modulated signal in which the
frequency or phase of the carrier signal is modulated to transmit.
 Passband transmission shifts the signal to be transmitted in frequency to a
higher frequency and then transmits it, where at the receiver the signal is
shifted back to its original frequency.
 It may be defined as a signal which has a non-zero lowest frequency in its
spectrum. This means that the frequency spectrum of a bandpass signal
extends from f1 to f2 Hz. The modulated signal is called as the bandpass signal.
 It is obtained by shifting the baseband signal in frequency domain.
Note: Bandpass signals are not Spectrum of a bandpass Signal
necessarily modulated signals.
They can be available naturally
as well.
 Examples of bandpass signals
are the ultrasound waves,
visible light, radio waves etc.
Baseband Transmission:
1. Basic Aspect:
 In some systems, called the baseband transmission systems, the
baseband signals (original information signals) are directly
transmitted.
 Example of these type of system are telephone networks where the
sound signal converted into electrical signal is placed directly on the
telephone lines for transmission.
 Another examples of baseband transmission is computer data
transmission over the coaxial cables in the computer networks.
 Thus, the baseband transmission is the transmission of the original
information signal as it is.
2. Limitations of Baseband Transmission:
 The baseband transmission cannot be used with certain mediums.
 E.g., it can not be used for the radio transmission where the medium
is free space. This is because the voice signal (in the electrical form)
cannot be travel long distance in air. It get supressed after a short
distance. Therefore, for the radio communication of baseband
signals, a technique called modulation is used.
Advantages of Baseband Transmission:
 Simple in implementation.
 Installation costs are low.
 Overall maintenance costs are lesser.

Disadvantages of Baseband Transmission:

 It works good only for short distance.
 Coverage of the signal is limited.
 It has low capacity.
 It has low data rate.
 It has lesser Bandwidth.
 Difference Between Baseband and Passband Signal
Baseband Signals Passband/Band pass Signals
All source of information generates baseband Passband signals are the transmitted
signal. modulated signal.
E.g.: voice, audio, image etc. E.g. AM, FM, PM etc.
Signals are transmitted without modulating. It is high frequency modulated carrier signal.
E.g.: Landline E.g.: Satellite signals
Baseband signal has low frequency around (0 Passband has high frequency around (550 -
-20khz) for audio or (0-5Mhz) for video signal. 1700khz) for AM or (88Mhz – 108Mhz) for FM
signal.
Baseband signal can be transmitted directly to Passband signal has limited band or (band
a channel. pass) at which the signal can be transmitted.
Baseband signal can travel for only short Passband can travel for long distance hence, it
distance. is used for long distance communication.
Baseband signal is more susceptible to Passband is less susceptible to interference as
interference. compared to baseband signal.
In baseband there is poor reception of the In Passband there is better reception of the
message signal. message signal.