answers class test

8/9/2019 Answers Class Test

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Institution of Engineers Sri Lanka

COMMUNICATION ENGINEERING 1 EC 3002

Cass Test

August 2!" 2013

Ti#e Ao$e% & 3 'rs(((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((

Answer all

questions.

1.(a) Draw a block diagram of a PCM communication system and give a short

description of each block.

Anti-aliasing flter

n anti!aliasing "lter is a "lter used before a signal sampler# to restrict the

bandwidth of a signal to appro$imately satisfy the sampling theorem. %o be sure

that the fre&uency content of the input signal is limited# a low pass "lter (a "lter

that passes low fre&uencies but attenuates the high fre&uencies) is added before

the sampler and the DC. %his "lter is an anti!alias "lter because by attenuating the

higher fre&uencies (greater than the 'y&uist fre&uency)# it prevents the aliasing

components from being sampled.

CODECs

t its simplest a transceiver CDC (coder*decoder) consists of an DC (analogue

to digital converter) in the transmitter# which converts an analogue signal into

digital pulses# and a DC (digital to analogue converter) in the receiver# which

converts these digital pulses back into an analogue signal.

DCs will generally consist of a sampling circuit# a &uantiser and a pulse code

modulator. %he sampling circuit provides discrete voltage samples taken# at regular


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intervals of time# from the analogue signal. %he &uantiser appro$imates these

voltages to the nearest one of an allowed set of voltage levels. +ndeed# it is the

&uantisation process that converts the analogue signal to a digital one. %he PCM

encoder converts each &uantised level to a binary codeword# i.e.# digital ones and

,eros.

+n the receiver-s DC received binary voltages are converted to &uantised voltage

levels by a PCM decoder which is then smoothed by a low pass "lter to reconstruct

the original# analogue# signal.

+n addition to PCM encoding and decoding a CDC may have up to additional

functions/ 0ource# 0ecurity and rror Control Coding.

2irstly# in the transmitter it may reduce the number of digital pulses (bits) re&uired

to convey a message. %his is called source codingand can be thought of as

removing redundant or surplus bits.

0econdly# it may encryptthe source coded digits using a cipher for security. %his

ensures security when passing private information.

2inally# the CDC may add e$tra digits to the (possibly source coded and

encrypted) PCM signal which can be used at the receiver to detect# and possibly

correct# errors made during signal detection. %his is known as channel coding.

Multiplexers

+n digital communications# multiple$ing# to accommodate several simultaneous

transmissions# usually means time division multiple$ing (%DM). %ime division

multiple$ers interleave either PCM codewords# or individual PCM bits# to allow more

than one information link to share the same physical transmission medium.

Demultiple$ers split the received composite bit stream back into its component PCM

signals.

Line Coding

%ransmission of serial data over any distance# be it a twisted pair# "ber optic link#

coa$ial cable# etc.# re&uires maintenance of the data as it is transmitted through

repeaters# echo chancellors and other electronic e&uipment. %he data integrity must

be maintained through data reconstruction# with proper timing# and retransmitted.

3ine codes were created to facilitate this maintenance.

4D5# M+ and '67 are the e$amples for line coding techni&ues

MODEMs

MDMs (modulators*demodulators) change digital pulse streams so that they can

be transmitted over a given physical medium# at a given rate# in a speci"ed or

allocated fre&uency

band. %ypically the modulator shapes# or "lters# the pulses to restrict their

bandwidth. %he input to a modulator is thus a baseband signal# while the output is

often a bandpass waveform.

Multiple Accessing


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Multiple accessing refers to those techni&ues# and*or rules# which allow more than

one transceiver pair to share a common transmission medium

Signal Transmission

%he communications path from transmitter to receiver may use lines or free space.

$amples of the former are wire pairs# coa$ial cables and optical "bres.

Pulse Shaping Matched !iltering

%he "rst is concerned with "ltering for transmission in order to minimise signal

bandwidth and is called Pulse 0haping. %he second is concerned with "ltering at the

receiver in order to ma$imise the 0'6 at the decision instant (and conse&uently

minimise the probability of symbol error) and is called Matched 2iltering.

"andpass ModulationDemodulation

1. 0ignals to be matched to the characteristics of transmission lines or channels.

8. 0ignals to be combined using fre&uency division multiple$ing and subse&uently

transmittedusing a common physical transmission medium.

. 9cient antennas of reasonable physical si,e to be constructed for radiocommunication

systems.

:. 6adio spectrum to be allocated to services on a rational basis and regulated sothat

interference between systems is kept to acceptable levels.

E#ualisation

&ualising "lter in the receiver has the inverse fre&uency response# to the raw line

or channel characteristic. Cascading the e;ect of the line with the e&ualiser

provides a


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(c) Describe the features of 3'# M'# and ='. =hich type of network would be

suitable to interconnect users within a >niversity e$tending over around ?km@

A local area net$or% &LA'(is designed to connect a group of computers in close

pro$imity to each other such as in an o9ce building# a school# or a home. 3' is

useful for sharing resources like "les# printers# games# or other applications.

A $ide area net$or% &)A'(spans a large geographic area# such as a state#

province# or country. ='s often connect multiple smaller networks# such as local

area networks (3's) or metropolitan area networks (M's). %he +nternet is the

largest ='# spanning the arth. =' is a geographically!dispersed collection of

3's.

A metropolitan area net$or% &MA'(is a large computer network usually

spanning a city. M' is a geographically small ='# typically less than AB miles

wide.

M' suitable to interconnect users within a >niversity e$tending over around

?km.

(d) =hat are the pros and cons of nalog and Digital 0ignals in communication@

Analog


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nalog systems are less tolerant to noise# make good use of bandwidth# and are

easy to manipulate mathematically. 4owever# analog signals re&uire hardware

receivers and transmitters that are designed to perfectly "t the particular

transmission. +f you are working on a new system# and you decide to change your

analog signal# you need to completely change your transmitters and receivers.

Digital

Digital signals are more tolerant to noise# but digital signals can be completelycorrupted in the presence of e$cess noise. +n digital signals# noise could cause a 1 to

be interpreted as a B and vice versa# which makes the received data di;erent than

the original data. %here are systems in place to prevent this sort of scenario# such

as checksums and C6Cs# which tell the receiver when a bit has been corrupted and

ask the transmitter to resend the data. %he primary bene"t of digital signals is that

they can be handled by simple# standardi,ed receivers and transmitters# and the

signal can be then dealt with in software (which is comparatively cheap to change).

8.(a) $plain Multiple$ing and write short notes on any three signal multiple$ing

techni&ues.

Multiple$ing is a method by which multiple analogue message signals or digital data

streams are combined into one signal and transmit over a shared medium

(Communication Channnel) in the form of a single# comple$ signal t the receiving end#

individual signals are recovered by the process called Demultiple$ing. %he aim is to share

an e$pensive resource. 2or e$ample# in telecommunications# several telephone calls may

be carried

using one

wire.


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Time division multiplexing (TDM)) In %igita trans#ission" signas are

*o##on+ #uti,e-e% using ti#e.%i/ision #uti,e-ing TM" in

$'i*' t'e #uti,e signas are *arrie% o/er t'e sa#e *'anne in

aternating ti#e sots)

Frequency division multiplexing (FDM)In anaog trans#ission" signas

are *o##on+ #uti,e-e% using freuen*+.%i/ision #uti,e-ing

4M" in $'i*' t'e *arrier 5an%$i%t' is %i/i%e% into su5*'annes of

%ifferent freuen*+ $i%t's" ea*' *arr+ing a signa at t'e sa#e ti#e in

,arae)

Statisti*a #uti,e-ing

Wavelength division multiplexing (WDM) In so#e o,ti*a fi5er

net$orks" #uti,e signas are *arrie% toget'er as se,arate $a/eengt's of

ig't in a #uti,e-e% signa using $a/eengt' %i/ision

#uti,e-ing6M)

(b) Calculate the data rate for a communication system that employs 1?!arysignaling if the signal transmission rate is 8BBB symbols per second.

1?! ary signal needs : bits per symbol.

8BBB sym*sec :$ 8BBB bits*sec (D 6 log8M)

kbps

(c) Describe 0E# 20E# 5P0E# FP0E.

AS*Gstrength of carrier signal is varied to represent binary 1 or B both

fre&uency H phase remain constant while amplitude changes commonly# one ofthe

amplitudes is ,ero

vd(t)

vc(t)

v0E(t)


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!S*G fre&uency of carrier signal is varied to represent binary 1 or B peak

amplitude H phase remain constant during each bit interval

PS*G phase of carrier signal is varied to represent binary 1 or B. peak amplitude H

fre&. remain constant during each bit interval

"PS*! +f the phase shift between the two states is 1B degrees# the modulation is

called 5P0E# or biphase shift keying

vd(t)

vc(t)

vP0E(t)

+PS* ! FP0E/ &uadrature phase shift keying

Fuadrature means the signal shifts among phase states that are separated by IB

degrees


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%he signal shifts in increments of IB degrees from :AJ to 1AJ# !:AJ (1AJ)# or

!1AJ (88AJ)

Data into the modulator is separated into two channels called + and F

%wo bits are transmitted simultaneously# one per channel

7 ach channel modulates a carrier. %he two carrier fre&uencies are the same# but

their phase is o;set by IB degrees (that is# they are Kin

&uadratureL)

%he two carriers are combined and transmitted

(d) ive an overview of ccess 'etwork %echnologies G D03# 2%%N and =iMN

Asymmetric digital su,scri,er line &ADSL(is a type of digital subscriber line

(D03) technology# a data communications technology that enables faster data

transmission over copper telephone lines (broadband communications technology)

than a conventional voiceband modem can provide. +t does this by utili,ing

fre&uencies that are not used by a voice telephone call. splitter# or D03 "lter#

allows a single telephone connection to be used for both D03 service and voice

calls at the same time. D03 can generally only be distributed over short distances

from the telephone e$change (the last mile)# typically less than : kilometres (8 mi)#

but has been known to e$ceed kilometres (A mi) if the originally laid wire gauge

allows for further distribution.

D03 di;ers from the less common symmetric digital subscriber line (0D03).

5andwidth (and bit rate) is greater toward the customer premises (known as

downstream) than the reverse (known as upstream). %his is why it is called

asymmetric. D03 is capable of providing up to AB Mbps# and supports voice# video

and data. %here are two competing and incompatible standards for modulating the

D03 signal/


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Carrierless mplitude Phase (CP)

Discrete Multi!%one (DM%)

t the telephone e$change the line generally terminates at a digital subscriber line

access multiple$er (D03M) where another fre&uency splitter separates the voiceband signal for the conventional phone network. Data carried by the D03 are

typically routed over the telephone companyOs data network and eventually reach a

conventional +nternet Protocol network.

>nlike regular dialup phone service# D03 provides continously!available# always

on connection.

!i,er to the x &!TT(is a generic term for any broadband network architecture

using optical "ber to provide all or part of the local loop used for last mile

telecommunications. %he term is a generali,ation for several con"gurations of "ber

deployment# ranging from 2%%' ("ber to the neighborhood) to 2%%D ("ber to thedesktop).

%he telecommunications industry di;erentiates between several distinct 2%%N

con"gurations. %he terms in most widespread use today are/

2%%' * 2%%3 ("ber!to!the!node# !neighborhood# or !last!ampli"er)/ 2iber is

terminated in a street cabinet# possibly miles away from the customer premises#

with the "nal connections being copper. 2%%' is often an interim step toward full


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2%%4 and is typically used to deliver advanced triple!play telecommunications

services.

2%%C * 2%%E ("ber!to!the!curb*kerb# !closet# or !cabinet)/ %his is very similar to 2%%'#

but the street cabinet or pole is closer to the userOs premises# typically within 1#BBB

feet (BB m)# within range for high!bandwidth copper technologies such as wired

ethernet or + 1IB1 power line networking and wireless =i!2i technology. 2%%C is

occasionally ambiguously called 2%%P ("ber!to!the!pole)# leading to confusion with

the distinct "ber!to!the!premises system.

2%%P ("ber!to!the!premises)/ %his term is used either as a blanket term for both

2%%4 and 2%%5# or where the "ber network includes both homes and small

businesses.

2%%5 ("ber!to!the!building# !business# or !basement)/ 2iber reaches the boundary of

the building# such as the basement in a multi!dwelling unit# with the "nal connection

to the individual living space being made via alternative means# similar to the curb

or pole technologies.

2%%4 ("ber!to!the!home)/ 2iber reaches the boundary of the living space# such as abo$ on the outside wall of a home. Passive optical networks and point!to!point

ethernet are architectures that deliver triple!play services over 2%%4 networks

directly from an operatorOs central o9ce.Q1RQ8R

2%%D ("ber!to!the!desktop)/ 2iber connection is installed from the main computer

room to a terminal or "ber media converter near the userOs desk.

2%% * 2%%7 ("ber!to!the!telecom!enclosure or "ber!to!the!,one) is a form of

structured cabling typically used in enterprise local area networks# where "ber is

used to link the main computer e&uipment room to an enclosure close to the desk

or workstation. 2%% and 2%%7 are not considered part of the 2%%N group oftechnologies# despite the similarity in name.

)iMA &)orld$ide .nteropera,ility /or Micro$a0e Access(is a wireless

communications standard designed to provide B to :B megabit!per!second data

rates# with the 8B11 update providing up to 1 bit*s for "$ed stations. =iMN is

described as a standards!based technology enabling the delivery of last mile

wireless broadband access as an alternative to cable and D03.

=iMN can provide at!home or mobile +nternet access across whole cities or

countries. +n many cases this has resulted in competition in markets which typically

only had access through an e$isting incumbent D03 (or similar) operator.dditionally# given the relatively low costs associated with the deployment of a

=iMN network (in comparison with # 40DP# $D03# 42C or 2%%$)# it is now

economically viable to provide last!mile broadband +nternet access in remote

locations.

+t is a point to multi!point technology enabling the delivery of last mile wireless

broadband access as an alternative to cable and D03. +t can provide a ma$imum

bandwidth of 1BMbps per sector# which is shared among users in that sector

.


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(a) ny periodic waveform may be analy,ed into or synthesi,ed from a number of

harmonically related sine and cosine functions as shown below.

f( t)=a0+

n=1

(ancos 2ntT +bnsin2nt

T )% is the period. 0how how

a0 #an and

bn are determined.

Consider the integration of both sides of the 2ourier series as follows.

t0

t0+T

f( t)dt=t0

t0+T

a0dt+t0

t0+T

n=1

(an cos 2ntT +bnsin2ntT )dt

t0

t0+T

a0dt0+0 (all 0ine and Cosine terms become

7ero)%herefore#

t0

t0+T

f( t)dt=a0T

a0=1

T

t0

t0+T

f( t)dt

Consider the integration of both sides of the 2ourier series# after multiplying

each term by

cos2ntT as follows.

t0

t0+T

f( t) cos2nt

T dt=

t0

t0+T

a0 cos2 nt

T dt

+t0

t0+T

n=1

(ancos2 ntT +bnsin2 ntT )cos2 ntT dt

nly(cos2

2nt

T )

term on the right hand side of the e&uation can give a non ,ero

integral.

t0

t0+T

f( t) cos2nt

T dt=0+

t0

t0+T

n=1

(an )cos22 nt

T dt

an=2

T

t0

t0+T

f( t) cos2 nt

T dt


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0imilarly integration of both sides of the 2ourier series# after multiplying by

sin2nt

T gives

bn=2

T

t0

t0+T

f( t) sin2nt

T dt

(b) $plain three types of symmetries of wave forms drawing e$amples for each

symmetry

E0en Symmetry

=hen even symmetry is present# the waveform from B to %*8 also corresponds to

the mirror image of the waveform from G%*8 to B. f(t) f(!t)

Odd Symmetry

=hen odd symmetry is present# the waveform from B to %*8 also corresponds to thenegated mirror image of the waveform from G%*8 to B. f(t) S f(!t)

1al/-$a0e Symmetry

=hen half!wave symmetry is present# the waveform from (toT%*8) to (toT%) also

corresponds to the negated value of the previous half cycle waveform from to to

(toT%*8).


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(c) 2ind the 2ourier 0eries of the periodic wave form given below.

Period of waveform 8%

Mean value of waveform B. %herefore o*8 B

=aveform has even symmetry. %herefore 5n B for all n

=aveform has half!wave symmetry. %herefore n# 5n B for even n

:.(a) $plain how to calculate the Mean# Mean!0&uare and Uariance of a random

variable# giving an e&uation for each.

Mean

%he Mean is found by multiplying each allowed value of the se&uence by the

probability with which it occurs# followed by summation. +t is sometimes referred

to as the dc value


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Mean-s#uare

Mean!s&uare is sometimes called the average power. +t is calculated by s&uaring

each se&uence value# multiplying by the relevant probability# and summing overall possible values

2ariance

Uariance refers to


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(8) mplitude Distortion

() Phase and roup Delay

Loss

%he attenuation or insertion loss is de"ned as the reduction or loss in signal power

as it is transferred across the transmission medium.

Amplitude Distortion

Distortion that occurs when the output signal does not have a linear relation to the

input signal.

Phase delay

Phase delay is a measure of the time delay of the phase# nor the delay of the

amplitude envelope# of each sinusoidal component.

3roup Delay

%he group delay tg(f) is the time delay of the amplitude envelopes# of the

various sinusoidal components of a signal# as it passes through the 3%+ system# and

is a function of fre&uency for each component.

A.(a) $plain what is meant by additive white aussian noise

'oise is dditive if it adds to the information bearing signal. 'oise is called =hite

'oise if the noise is having a


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(d) De"ne 'oise 2igure and 'oise %emperature. =hat is the relationship between

them@

&'oise !igure4 '!(d5 1B log1B('oise 2actor# 2) where 'oise factor 2

Input Signal

Out put Signal

Noise Ratio

Noise Ratio

'oise Temperature

0peci"es noise in terms of an e&uivalent temperature. %he temperature of a

passive system having an available noise power per unit bandwidth at a

speci"ed fre&uency e&ual to that of the actual terminals of a network

Alternati0e Defnition o/ 'oist Temperature5

%he e;ective noise temperature# %e # of a linear two port device is the

temperature of the input terminals at which the available noise power at the

output is twice which could be available if the input terminations were atabsolute ,ero


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Fn= GNIN+NSYS

GN

= 1 +

NSYS

GN

= 1 +GKTeB

GKT0B

n

= 1+Te

T0

!e = "

n#1 $!

%

(e) $plain why high!gain# low!noise ampli"ers are used only at the "rst stage or

two in a cascaded chain.verall 'oise 2actor of Cascaded 0ystems is given byV

+t can be seen from above &uation# the noise factor of the entire cascade chain is

dominated by the noise contribution of the "rst stage or two. %herefore high!gain#

low!noise ampli"ers typically use a low!noise ampli"er circuit for only the "rst stage

or two in the cascade chain

?.(a) +f you were asked to design a transmission link# mention four main factors you

would consider in doing so#

Design !actors /or Transmission Media

"and$idth5ll other factors remaining constant# the greater the band!width of

the medium# the higher the data rate that can be achieved.


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Transmission impairments6&Attenuation(3imit the distance a signal can

travel.

.nter/erence5Competing signals in overlapping fre&uency bands can distort or

wipe out a signal. +ssue especially in case of unguided medium

'um,er o/ recei0ers5ach attachment introduces some attenuation and

distortion# limiting distance and*or data rate.

(b) $plain why an 0%P cable is more immune to noise than an unshielded parallel

cable pair.

0%P (shielded twisted pair) cable is a twisted wire pair wrapped with metallic foil

or braid. =hen the wire pair is wrapped with Metallic foil or braid# it insulates

the pair from electromagnetic interference coming from outside of the pair.

=hen the wire pair is twisted# it cancels of the interference from closed by noise

sources as depicted by below diagram.

(c) =hy ptical 2iber is a better transmission media compared to any metallic

guided media# give "ve reasons. 0tate three disadvantages of using ptical2iber.

Ad0antages

i. greater capacity (bandwidth bps)ii. smaller si,e and lighter weightiii. lower attenuationiv. immunity to environmental interferencev. highly secure due to tap di9culty and lack of signal radiation

Dis0antages

i. e$pensive over short distanceii. re&uires highly skilled installersiii. adding additional nodes is di9cult

(d) receiver has sensitivity of G :A d5m and a 56 of 1BGI. =hat is the minimum

power that must be incident on the detector@

3et the minimum power P m=


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P (dBm )=10logP (mW)

1 (mW )

G :A d5m 1B log 1B P*1 m=

so that

P (1 m=) W 1BG:.A .1? W 1BGA m= 1.? nanowatts

for a probability of error of 1 in 1BI.

(e) communication system uses 1B km of "ber that has a 8.A!d5*km loss

characteristic. 2ind the output power if the input power is :BB m=.

LossdB=10log Pout

P

LossdB

10 =log

Pout

P

which becomes# then#Pout

P=10

LossdB10

0o# "nally# we have

2or 1B km of "ber with 8.A!d5*km loss characteristic# the lossd5becomes

3ossd5 1B km W (G8.A d5*km) G8A d5

Pout

400=102.5

%herefore the output power Pout :BBX 102.5

1.8?m=

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