Introduction to

Communication System and

Signal Analysis

Dr. Khawaja Bilal MahmoodDr. Khawaja Bilal Mahmood

Course: Communication Systems

(EL-322)

Communication System� A Communication system in the most simplest form can be defined

as any system which can help with the transmission of usefulinformation from one point to another.

Components of Communication

System

Information Source Transmitter

ChannelChannel

Information User Receiver

Typical Block Diagram of a Communication

System

Telecommunication� Telegraph

� Fixed line telephone

� Cable

�Wired networks�Wired networks

� Internet

� Fiber communications

� Communication bus inside computers to communicate between CPU and memory

Wireless Communications� Satellite

� TV � (Pictures transmission)

� Cordless phone

� Cellular phone

� Wireless LAN, WiFi and Wireless MAN, WiMAX

� Bluetooth� Bluetooth

� Ultra Wide Band

� Wireless Laser

� Microwave

� GPS

� Ad hoc/Sensor Networks

Analog or Digital� Common Misunderstanding: Any transmitted signals are

(ANALOG. NO DIGITAL SIGNAL CAN BE TRANSMITTED)

� The channel we transmit information through is not digital in nature

� It looks at the signal as voltage waveform as a function of time.

� Analog Message: continuous in amplitude and over time� Analog Message: continuous in amplitude and over time

� AM, FM for voice sound

� Traditional TV for analog video

� First generation cellular phone (analog mode)

� Record player

� Digital message: 0 or 1, or discrete value

� VCD, DVD

� 2G/3G cellular phone

� Data on your disk

Power, Distortion, Noise

Transmitter Characteristics

� A carrier signal is required to carry information which can then be transmitted over the channel.

� Typically, a carrier signal would be a pure sine wave � a high frequency signal.� a high frequency signal.

� This process is called Modulation

� Could modify the Amplitude of the carrier to get AM

� Also FM or PM can be achieved by modifying the frequency and Phase of the carrier signal

� The mathematical expression for the carrier signal will be given on the next slide as �

Transmitter Characteristics

( ) ( )cos 2 cccamv t tfA π θ= +

Change parameters of a carrier

Information signal: Ac(t)

f (t)fc(t)

θ(t)

Ac(t) : amplitude modulation AM ASK

fc(t) : frequency modulation FM FSK

θ(t) : phase modulation PM PSK

Ac(t) and θ(t) ⇒ QAM (Digital)

DigitalAnalog

Communication Channel

� Physical medium

� Free space

� Cables

� Optical fibres

� Easier to work with

� Relatively cleaner

� Less prone to undesired effects as we face

in free space

� A communication channel block also models

� Attenuation

� Noise

� Distortion

in free space

� Pair of copper wires / coaxial cables

� offer larger bandwidths

Channel

Noise in Communication Channel

� Channel is the main source of noise in communication systems

� Transmitter or Receiver may also induce noise in the system

� Noise in Communication Systems

� There are mainly 2-types of noise sources

� Internal noise source (� are mainly internal to

the communication system)

� External noise source

� External Noise Sources

� Natural

� Man-made

Noise in Communication Channel

� Lightening Discharges

� Biggest natural source which causes large amounts of EM-radiation

� It’s a very large magnitude waveform / impulse or A narrow burst of large energy.

Very important because they have the potential to � Very important because they have the potential to interfere over a large frequency range.

� Since actually it’s a pulse of finite duration

� The spectrum of a pulse of finite duration is defined by Sinc function

If the lightening discharge is of ‘Ƭ’ seconds, the spectrum can be given by

Sinc (f Ƭ) = Sin π f Ƭ

π f Ƭ

This is always b/w +1 to -1

Noise in Communication Channel

� Since this is the function of frequency, we will have �

α 1 / f

� This noise have spectrum which decays with frequency

� Also this noise affect more at lower frequency bands then at higher frequency bands

In time domain

Also sometimes called atmospheric noise

� In time domain �

� This noise is characterised by large amplitude narrow pulses

� Also called Impulsive noise

� AM Broadcast Radio (550KHz to 1.6MHz) � more affected by

this noise

� FM Broadcast Radio (>50MHz) � Not much affected by this

noise

Noise in Communication Channel� Man-made Noise Sources �

� High voltage power-line discharges

� Electrical motor noise generated by armature and switching taking place in the motor

� Ignition noise in automobiles and aircraft

� At Telephone exchanges where switching (electrical) takes place is a source of Impulsive Noise.place is a source of Impulsive Noise.

� Radio Frequency Interference (RFI)� Many users communicate at the same time

� High density transmission environment particularly in the context of mobile communication

� A lot of wireless systems are working in parallel �Interference

� RADAR communication taking place

� Satellite communications / Wireless and mobile communication etc

Noise in Communication Channel

� Radio Frequency Interference (RFI) � Natural Source

� Due to extra-terrestrial sources

� Sun and stars are the sources of this noise

� Internal Noise Sources

� Fading effects due to multi-paths propagation b/w transmitter � Fading effects due to multi-paths propagation b/w transmitter and receiver.

Tx Rx

� Thermal Noise � Occurs due to

random motion of free electrons in a conductor or a semi-conductor.

� Even when the voltage is not applied �

the electrons stays in random motion.

� Thermal noise is present in almost all electrical component like diodes, resistors, transistors etc. Multi-path Fading

effect

Constructive or Destructive

interference occurs at the receiver

Noise in Communication Channel� Since there are thousands of these components used � the overall effect of the thermal noise is quite significant.

� Shot Noise � Random arrival of charged carriers in semi-conductor devices i.e. transistor / diodes

� All active devices have charged carriers

� The move between junction (PN junctions)

� This random motion generates Shot Noise

� Collectively Thermal and Shot Noise can significantly degrade the performance of a communication system

Signal Analysis� Signal analysis is very important incommunication theory and system and circuitdesign.

� In order to predict and understand electronicsystem and circuit behavior, we use the results ofsystem and circuit behavior, we use the results ofmathematical analysis.

� The most common representation of signals andwaveforms is in the time domain. However,most signal analysis techniques work only in thefrequency domain.

Time & Frequency Domains…� In a digital communications link design, a goodgrounding is needed in the relationship betweenthe shape of a digital waveform in the timedomain and its corresponding spectral content inthe frequency domain.

� Time domain � signal as a function of time.

� Analog signal � signal’s amplitude varies

continuously over time, i.e. no discontinuities.

� Digital signal � data represented by sequence of

0’s and 1’s (e.g., square wave).

Time / Frequency Domains

� The performance of a digital communications link is constrained by two primary factors:

� Channel Bandwidth

� how much of the frequency spectrum do we give � how much of the frequency spectrum do we give each user?

� System Noise

� both thermal (kTB) and man made!

� Both of these effects are more evident in frequency domain

Time / Frequency Domains

� A grasp of the frequency content of various types of time domain data signals is key to understand the interaction between:

� System data / Symbol rate

Modulation type� Modulation type

� Pulse shape

and

� Channel bandwidth

� It is difficult to extract the above information from the time domain waveform but frequency domain waveform gives all this information.

Time domain – Sine Wave

amplitude

(volts)

zero crossing

time

(seconds)

period t

frequency = 1/t

if t = 1 ms, f= 1 kHz

Frequency Domain

� Signal consists of components of different frequencies.

� Spectrum of signal: Range of frequencies a signal contains.a signal contains.

� Absolute bandwidth: Width of signal’s spectrum or spectrum occupied by the signal

� Bandwidth also refers to the information transmission capability

Frequency Domain – Sine Wave

amplitude

(volts)

frequency

(hertz)

1 kHz

Frequency Domains� The frequency domain is simply another way of representing a signal. For example, consider a simple sinusoid �

Frequency Domain� The time - amplitude axes on which thesinusoid is shown define the time plane.

� If an extra axis is added to representfrequency, then the sinusoid would be �

Frequency Domain Analysis� The frequency - amplitude axes define the frequencyplane in a manner similar to the way the time plane isdefined by the time - amplitude axes.

� The frequency plane is orthogonal to the time plane,and intersects with it on a line which is the amplitudeaxis.axis.

� The actual sinusoid can be considered to be asexisting some distance along the frequency axis awayfrom the time plane.

� This distance along the frequency axis is thefrequency of the sinusoid, equal to the inverse of theperiod of the sinusoid.

Frequency Analysis• Fast & efficient insight on signal’s building blocks.

• Simplifies original problem –

• Powerful & complementary to time domain analysis techniques.

• Several transforms in DSPing: Fourier, Laplace, z, etc.

• Based primarily on Fourier series & Transform• Based primarily on Fourier series & Transform

time, t frequency, fF

s(t) S(f) = F[s(t)]

analysisanalysis

synthesissynthesis

s(t), S(f) : Transform Pair

General Transform as General Transform as problemproblem--solving toolsolving tool

Time Domain Representation Can Only

Seldom Reveal Small Signal Impairments

Frequency Domain Representation of the

Same Signal Reveals More!

Spectrum ExamplesTime Domain Frequency Domain

The Phasor: Definition

θθθ sincos jej ±=±

The Phasor is a complex number that carries the amplitude

and phase angle information of a sinusoidal function.

Euler’s

identity ���� [ ]

[ ]

θ θθ

1

2

1cos ee

jj += −

}{cos θθ jeℜ=

}{sin θθ jeℑ=

}{}{)cos( )( φωφωφω jtj

m

tj

mm eeVeVtVv ℜ=ℜ=+= +

Real

Imaginary

[ ]

θθ

θθ

θ

θ

θ

θθ

sincos

sincos

2

1sin

je

je

eej

j

j

jj

−=

+=

−−=

−

−

The Phasor}{ tjj

m eeVvωφℜ=

)}cos({ φωφ +Ρ== tVeV m

j

mV

Complex number that carries the amplitude and

phase angle of the given sinusoidal function.Phasor Transform

(polar form))}cos({ φω +Ρ== tVeV mmV

φφ sincos mm jVVV +=

Phasor transform of Vmcos(ωωωωt+φφφφ)

The Phasor transform transfers the sinusoidal function from the

time domain to the complex-number domain (the frequency

domain), since the response depends on ωωωω.

(rectangular form)

Complex Exponential

Phasor Signals and Spectra (cont.)

�� A sinusoid is usually represented by a complex A sinusoid is usually represented by a complex exponential or Phasor formexponential or Phasor form

�� Euler’s TheoremEuler’s Theorem::

where and where and θθ is an arbitrary angleis an arbitrary angle1j −�

c o s s i njje θ θ θ± = ±

LetLet ,, thenthen anyany sinusoidsinusoid cancan bebe writtenwrittenasas thethe realreal partpart ofof aa complexcomplex exponentialexponential::

0t φθ ω +=

0( )

0cos( ) Re j teA t A ω φω φ + + =

0Re j tjAe e

ωφ =

Phasor Signals and Spectra (cont.)

�� TheThe diagramdiagram showsshows aa PhasorPhasor representationrepresentation ofof aa signalsignalbecausebecause thethe termterm insideinside thethe bracketsbrackets maymay bebe viewedviewed asas aarotatingrotating vectorvector inin aa complexcomplex planeplane whosewhose axesaxes areare thethe realrealandand imaginaryimaginary partsparts..

TheThe phasorphasor hashas lengthlength A,A, rotaterotatecountercounter--clockwiseclockwise atat aa raterate ff00 revolutionrevolutioncountercounter--clockwiseclockwise atat aa raterate ff00 revolutionrevolutionperper second,second, andand atat timetime tt == 00 makesmakes ananangleangle φφφφφφφφ withwith respectrespect toto thethe positivepositiverealreal axisaxis..

TheThe threethree parametersparameters thatthat completelycompletelyspecifiesspecifies aa phasorphasor::1)1) AmplitudeAmplitude;;2)2) PhasePhase angleangle;; andand3)3) RotationalRotational frequencyfrequency

Phasor representation

Phasor Signals and Spectra (cont)

�� ToTo describedescribe thethe samesame phasorphasor inin thethe frequencyfrequency domaindomain,, thethecorrespondingcorresponding amplitudeamplitude andand phasephase mustmust bebe associatedassociatedwithwith thethe particularparticular frequency,frequency, ff00,, givinggiving usus thethe LINELINESPECTRASPECTRA.. (Line(Line spectraspectra havehave greatgreat conceptualconceptual valuevalue whenwhenextendedextended toto moremore complicatedcomplicated signals)signals)

Amplitude Spectrum Phase Spectrum

Line Spectra

Basic Identities

Fourier Series and Fourier

Transform�� Fourier seriesFourier series representation for periodic representation for periodic signalssignals

�� Fourier transformFourier transform for general periodic and for general periodic and nonnon--periodic signalsperiodic signalsnonnon--periodic signalsperiodic signals

Fourier Series and Fourier Transform

� Defined for periodic signals.

� Periodic signals repeats � Periodic signals repeats

themselves over time and

given by property x (t+To) = x(t)

for all values of T0

Reading Assignment

� Go through Time and frequency domain concepts

� Fourier Transforms and FFTs in your own time.time.

� Check Bruce Carlson or Haykin’s books for further reading