WCT Lec2 Spring 2013

8/13/2019 WCT Lec2 Spring 2013

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Shujaat Ali Khan Tanoli,Ph.D.

[email protected]

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.



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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.



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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



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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.



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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

WCT Lec2 Spring 2013

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