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Shujaat Ali Khan Tanoli,Ph.D.
EEE655EEE655
Wireless CommunicationWireless Communication
TechniquesTechniques
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Electrical communication systems are designed to send
messages from a source to one or more destinations.
I n f o S o u r c e
&
I n p u t T r a n s d .
T r a n s m . C h a n n e l R xO u p t p u t
T r a n s d .
Basic Communication System
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Info source: speech, video, data.
Transducer: microphone, video camera - convertinformation into electrical signal.
Transmitter: converts electrical signal into form
suitable for communication. Translates freuencies to appropriate range.
!erforms modulation - use of the information to varyattributes of a carrier. "odulation enables many users
to use same physical channel. !erforms filtering and amplification.
!rovides the radiation for #ireless transmission.
Basic Communication System
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Channel: the physical medium causes signal corruptionby a variety of mechanisms. Some are additions, someaffect o#n signal. $dditive noise - thermal noise.
"ultipath fading attenuates and distorts the signal. %eceiver: its function is to recover the message. It
performs: Carrier demodulation.
Signal filtering.
&oise suppression.
Symbol detection.
Basic Communication System
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Signal fidelity better controlled through digitaltransmission.
'iscrete-time signals -- digital communications.
(ur communication system needs to be e)panded to
include message signal discreti*ation at thetransmitter and message signal synthesis at thereceiver.
Source - outputs are bits.
Source encoder - represents the source in as fe#digits as possible. Converts the output of the sourceinto an efficient representation.
'igital Communication System
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C h a n n e l
I n f o S o u r c eI n p u t
T r a n s d .
S o u r c e
E n c o d e r
C h a n n e l
E n c o d e r
D i g i t a l
M o d u l a t i o n
O u t p u t
T r a n s d .
S o u r c e
D e c o d e r
C h a n n e l
D e c o d e r
D i g i t a l
D e m o d u l a t i
o n
'igital Communication System
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'emodulator - processes the noise corrupted received
information and decides on the transmitted bit. Binary decision - decide , or 1.
Ternary decision - ,1, neither.
Several performance measures used in #ireless: 3reuency of errors in the decoded seuence: average
probability of error at output of the decoder .
!robability of S&% falling belo# a specified threshold -
outage. 'igital communication stems from the #or of &yuist.
'igital Communication System
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Classifications of Transmission Media
# Transmission "edium
$ !hysical path bet#een transmitter and receiver
# 4uided "edia
$ 5aves are guided along a solid medium
$ E.g., copper t#isted pair, copper coa)ial cable,optical fiber
# 6nguided "edia
$ !rovides means of transmission but does not guideelectromagnetic signals
$ 6sually referred to as #ireless transmission
$ E.g., atmosphere, outer space
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Unguided Media - ireless Transmission
# Transmitting information /voice and data0 using
electromagnetic /E"0 #aves in open space.# Electromagnetic #aves
$ Travel at speed of light /c + 7)18 ms0
$ Characteri*ed by a freuency /f0 and a #avelength/l0
%c + f l
# Transmission and reception are achieved by means of
an antenna. E" energy is coupled to medium byantenna.
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!ignals
# physical representation of data
# function of time and space /location0
# Can also be e)pressed as a function of freuency
$ Signal consists of components of different
freuencies# classification
$ continuous timediscrete time
$ continuous valuesdiscrete values
$ analog signal + continuous time and continuousvalues
$ digital signal + discrete time and discrete values
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!ignals" Time-#omain Conce$ts
# $nalog signal - signal intensity varies in a smooth
fashion over time$ &o breas or discontinuities in the signal
# 'igital signal - signal intensity maintains a constantlevel for some period of time and then changes toanother constant level
# !eriodic signal - analog or digital signal pattern thatrepeats over time
s/t 9T 0 + s/t 0 - t 9#here T is the period of the signal
# $periodic signal - analog or digital signal pattern thatdoesn;t repeat over time
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$ signal parameters: parameters representing thevalue of data
$ signal parameters of periodic signals:
$ !ea amplitude /$0 - ma)imum value or strength of
the signal over time< typically measured in volts$ 3reuency /f 0 - %ate, in cycles per second, or =ert*
/=*0 at #hich the signal repeats
$ !eriod /T 0- amount of time it taes for one
repetition of the signal --- > T + 1f$ !hase /0 - measure of the relative position in time
#ithin a single period of a signal
!ignals" Time-#omain Conce$ts
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!ine a%e &arameters
# 'eneral sine (a%e as s$ecial $eriodic signal for acarrier
$ s)t * +Asin)2ft, *
#ffect of %ar.ing eac/ of t/e t/ree$arameters of a sine (a%e is s/o(n in ig
on net slide
)a*A+ 1 f+ 1 + 0 t/us T+ 1s
)* educed $ea am$litudeA+05
)c* :ncreased fre;uenc.f
+ 2 t/usT
+ <
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!ignal re$resentation 'ifferent representations of signals
amplitude /amplitude domain0 freuency spectrum /freuency domain0 phase state diagram /amplitude " and phase in polar
coordinates0
Composed signals transferred into freuency domain using 3ourier
transformation 'igital signals need
infinite freuencies for perfect transmission modulation #ith a carrier freuency for transmission /analog
signal@0
f &'()
* &+)
I, M cos
- , M sin * &+)
t&s)
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!ignals" re;uenc.-#omain Conce$ts
# undamental fre;uenc.
$(/en all fre;uenc. com$onents of a signal are integer multi$les ofone fre;uenc. it?s referred to as t/e fundamental fre;uenc.
# !$ectrum
$ range of fre;uencies t/at a signal contains
# @solute and(idt/
$ (idt/ of t/e s$ectrum of a signal# ffecti%e and(idt/ )or Aust and(idt/*
$ narro( and of fre;uencies (/ere most of t/e signal?s energ. is
contained in
# @n. electromagnetic signal can e s/o(n to consist of a collection of
$eriodic analog signals )sine (a%es* at different am$litudes
fre;uencies and $/ases
# T/e $eriod of t/e total signal is e;ual to t/e $eriod of t/e fundamental
fre;uenc.
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#igital !ignaling=Transmission
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@nalog Transmission
# Transmit analog signals #ithout regard to content# $ttenuation limits length of transmission lin
# Cascaded amplifiers boost signalAs energy forlonger distances but cause distortion
$ $nalog data can tolerate distortion
$ Introduces errors in digital data
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#igital Transmission
# Concerned #ith the content of the signal
# $ttenuation endangers integrity of data
# 'igital Signal
$ %epeaters achieve greater distance
$ %epeaters recover the signal and retransmit# $nalog signal carrying digital data
$ %etransmission device recovers the digital datafrom analog signal
$ 4enerates ne#, clean analog signal
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Transmission :m$airments
# Channel Impairments
a. &oise
b. $ttenuation and attenuation distortion
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&oise is the un#anted signal insertedadded to the mainsignal #hile it travels from the transmitter to the receiver
&oise is a maor limiting factor in communications system
performance
&oise is divided into four categories:
Thermal &oise
Intermodulation &oise
Crosstal
Impulse &oise
&oise
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Boise# :ntermodulation noiseH occurs if signals (it/ different
fre;uencies s/are t/e same medium
$ :nterference caused . a signal $roduced at a
fre;uenc. t/at is t/e sum or difference of
original fre;uencies
# CrosstalH un(anted cou$ling et(een signal $at/s
# :m$ulse noiseH irregular $ulses or noise s$ies
$ !/ort duration and of relati%el. /ig/ am$litude
$ Caused . eternal electromagnetic
disturances or faults and fla(s in t/e
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S&% is e)pressed in decibels /dB0
N
SSNRdB 10log10=
Typical S&% reuired: telephone 2 dB, hi-fi audio dB, Cellular #ireless 1D - 18 dB
$ll electrical circuits generate noise: S&% at the output is al#ays smaller than S&% at
the input.
Recei/erinSNR
outSNR
1>=
out
in
SNR
SNRF
&oise
&oise 3igure,
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@ttenuation
# !trengt/ of signal falls off (it/ distance o%er transmissionmedium
# @ttenuation factors for t/e transmission media"
$ ecei%ed signal must /a%e sufficient strengt/ so t/at circuitr.
in t/e recei%er can inter$ret t/e signal
$ !ignal must maintain a le%el sufficientl. /ig/er t/an noise to
e recei%ed (it/out error
$ @ttenuation is greater at /ig/er fre;uencies causing distortion
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/at is and(idt/I
# and(idt/ is t/e range of fre;uencies t/at can e transmitted
o%er a c/annel )a general name for communication medium*(it/out significant loss of $o(er
# :f a c/annel /as a and(idt/ of 1 M does t/at mean .ou can?t
transmit a signal of /ig/er fre;uenc.I
# /at is t/e relations/i$ et(een s$ectrum fre;uenc. and
and(idt/I
$ !$ectrum"a range of fre;uencies )eg :
re;uested a 25 M of s$ectrum from CC
does it tell .ou (/at is m. fre;uenc. ofo$erationI*
$ and(idt/"an in/erent $ro$ert. of a c/annel
)eg ier /as 10 T of and(idt/*
$ re;uenc. of o$eration)eg -J@B using 24
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o( man. its can (e transmitI# T/e actual numer of its transmitted in a medium is a function
of signal $o(er noise source and c/annel coding and
and(idt/
# !o 1 M does not mean 1 M$sKK
# and(idt/ is limited . egulator. allocation or t/e sie of
unlicensed ands )suc/ as :!M ands*
# !ignal $o(er can not e raised e.ond certain limit
# :nterference mostl. is e.ond .our control
# @fter a limit /o( (ell .ou code .our signal ecomes useless
and t/en t/e u$$er limit of numer of its=second is determined. signal $o(er noise and and(idt/ )t/is relations/i$ is also
no(n as !/annon?s t/eorem*
# and(idt/ can e re-used in different $laces as in cellular
s.stems
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C/annel Ca$acit.
# :m$airments suc/ as noise limit data
rate t/at can e ac/ie%ed
# or digital data to (/at etent doim$airments limit data rateI
# C/annel Ca$acit.t/e maimum rate at
(/ic/ data can e transmitted o%er a
gi%en communication $at/ or c/annel
under gi%en conditions
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elations/i$ et(een #ata ate and and(idt/
# T/e greater t/e and(idt/ t/e /ig/er t/e information-carr.ingca$acit.
# Conclusions
$ @n. digital (a%eform (ill /a%e infinite and(idt/
$ UT t/e transmission s.stem (ill limit t/e and(idt/ t/at
can e transmitted
$ @B# for an. gi%en medium t/e greater t/e and(idt/
transmitted t/e greater t/e cost
$ L limiting t/e and(idt/ creates distortions