Upload
talhawaqar
View
267
Download
0
Tags:
Embed Size (px)
Citation preview
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