chapter 5
DESCRIPTION
ChapterTRANSCRIPT
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Chapter
4FundamentalsofCellularCommunication
s
Topicstostudy:
C
ellularconceptand
frequencyreuse
C
o-channelandadja
centchannelinterference
T
runkingandgrade
ofservice
M
echanismsforcap
acityincrease
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4-1CellularConcept
Givenapropagation
environment,increasingtransmitted
pow
erwillincreasetheservicecoveragearea
Thecoverag
eareacanbecontrolledbyusinga
propertransmittedpowerlevel
In
cellularsystems,thetotalserviceareaisdividedintoa
num
berofsmallerareas,eachofwhichis
aradiocell
Advantages:
Lowtransmittedpower
Frequency
reusepossible
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Hexagonalcellsarepopul
arbecause
closesttoacircle
tig
htcellularpacking
pe
rfectpartitioningoftheservicearea
Freque
ncyreuseislimited
byco-channelinte
rference.Cells
whichusethesamefrequen
cychannelsarecalledco-channelcells
Freque
ncyisreusedfrom
cellclustertocellc
luster.Nofrequency
channelisreusedamongcellsinthesamecell
cluster
Cellsineachcellclusteruseuniquefrequ
encychannels
Regularpolygonsmaybeusedtorepresentthecellcoverage
(a
)
(b)
(c
)
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Let
K-totalnumberofcha
nnelsinthesystem
withoutfrequency
reuse
N-thenumberofcells
ineachcellcluster
J-totalnumberofchannelsineachcell
then
K=JN
or
J=K/N
Let
M-totalnumberofcellclustersinthesystem
C-totalnumberofcha
nnelsinthesystem
withfrequencyreu
se
then
C=MK=MJ
N
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clu
stersizeN
M,tocoverthesamearea
CforagivenKvalue
Thatis,givenK,
Cisma
ximizedwhenNis
minimized.Howev
er,
them
inimumNdepends
ontherequirementontheco-channel
interferencelevel
Thec
ellclustersizeNis
alsocalledthefreq
uencyreusefactor
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4-2FrequencyReuseFact
or
Considerhexagonalcells
Ahexagonalcellshasex
actly6equidistant
neighbours
Thelinesjoiningthecen
tersofanycelland
eachofitsneighbours
aresep
aratedbymultiples
of60degrees
Nearestco-
channelneighbours
Tofindthe
nearestco-channel
neighbourofaparticularcell,execute
thefolllowingtwosteps:
move
icellsalonganychainofhexagons
turn6
0degreescounter-clockwiseandmov
ejcells
wheretheintegersiandjarep
arametersfordeter
miningco-channel
cellsandfo
rdeterminingthesizeofthecellcluster(N)
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Figure:Locatingco-channelcellsinacellularsystem
j
i
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(a
)i=2andj=0
(c)i=2andj=2
(b)i=1andj=2
(d)i=2andj=3
1 2
3 4
4
2
1
3
1
3 4
21
3 4
2
4
3
1
3
12
3 4
5
67 12
3 4
5
67
12
3 4
5
67
12
3 4
5
6743
6
5
2
12
3
12
3
5
8
11
8 12
3
8
11 5
7
7 6 10
9
12
4
67
9
4
5
67
9
106
4
1
8
9
10
11
2
3 4
5
67
12
13
14
15
16
17
18
19
8
19
1
8
9
10
11
2
3 4
5
67
12
13
14
15
16
17
18
19
1
8
9 2 5
67 16
17
18
19
19
10
11
2
3 4
5
67
12
13
14
15
16
17
18
19
10
11
2
3 4
5
12
13
14
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Geometry
ofhexagonalcells
Let
R
-radiusofthecell(fromcentertovertex)
D
-distancefromth
ecenterofthecandidatecelltothecellof
thenearestco-chann
elcell
Theactualdistancebetweenthecentersoftwoa
djacentcellsis
0
32
2
cos30
2
3
R
R
R
=
=
2
0
2
0
2
2
2
2
(
3
cos30)(
3
3sin30)
(
)3
D
j
R
i
R
j
R
i
j
ijR
=
+
+
=
++
or,
2
2
2
0
2
2
2
(
3)
(
3)
2(
3)(
3)cos120
(
)3
D
i
R
j
R
i
R
j
R
i
j
ijR
=
+
=
+
+
Itcanbe
shownthatthecell
clustersizeisgivenby
2
2
N
i
j
ij
=
+
+
2
2
3
D
NR
=
or
3
D
NR
=
N
RD
Dnorm
=
3
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30
o
120o
0
o
30
Figure:Distance
betweennearestco
-channelcells
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Frequencyreuseratioq
Thefreque
ncyreuseratioqandthefrequencyreusefactorNcarrythe
sameinform
ation
q(orN)
co-channelinterference
frequencyreuselessoftenandthesystemcapacity
Weshould
choosetheminimumq(orN)subje
cttotheconstraint
on
thesignalto
co-channelinterferenceratiorequirem
ent
N
RRN
RD
q
3
3
=
=
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10.54
37
(4,3)
9.17
28
(4,2)
9.00
27
(3,3)
7.94
21
(4,1)
7.55
19
(3,2)
6.24
13
(3,1)
6.00
12
(2,2)
5.20
9
(3,0)
4.58
7
(2,1)
3.00
3
(1,1)
FrequencyReuseRa
tio
q
ClusterSize
N
FrequencyReusePattern
(i,j)
Table:Frequencyreusefactorandcellclustersize
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4
-3Co-channelandAdjacentChann
elInterference
Co-channel
interference
Let
IN
-thenumberofco-channelinterfering
cells
kI
-co-channelinterferencefromthekth
co-channelcell
S-
thereceivedpowe
rofthedesiredsignal
Thesignal-to
-co-channelinterfe
renceratio(S/I),alsoreferredtoas
carrier-to-co-channelinterferenceratio(CIR),is
1IN
k
k
S
S
I
I=
=
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Considero
nlydistance-dependentpathloss.From
chapter2,wehav
e
0
0
()
(/
)
rPd
Pdd
=
WhereP0-thereceivedpow
eratdistanced0
Pr(
d)thereceivedpoweratdistanced
(d)
thepathlossexponent
dthedistancebetw
eenthetransmitterandreceiver
Conside
rtheforwardlinkandassumethatthe
transmittedpower
levelsfromalltheBSsare
thesame,then
k
k
I
D
WhereD
kisthedistancefromthekthco-channelcellBStothem
obile
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Whenthem
obileisatthecellboundary(thewors
tcase),
S
R
1IN
k
k
S
R
I
D
=
=
whichisn
otafunctionofthe
transmittedpower!Ifweusethe
approximationDk
Dforallk,then
(
/)
(
3
)
I
I
I
I
S
R
D
R
q
N
I
N
D
N
N
N
=
=
=
=
1/
(
)
I
S
qN
I
=
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Ifweneg
lectinterferencefromco-channelcellsinthesecondand
highertie
rs,thenNI=6
1/
(6
)S
q
I
=
FromtherequiredS/Ivalue,
qcanbedetermined.Forexample,if
the
requiredS
/Iis20dBand=
4,then
1/4
2
(6
100)
4.9
492
/3
8.165
9
q
N
q
=
=
=
=
A9-cellclusterisneeded
forS/Iofatleast2
0dB.Ontheother
hand,withN=9and=4,
3
39
5.1962
q
N
=
=
=
4
1
1(5.1962)
121.5
6
6
S
q
I
=
=
=
or2
0.845dB
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Considera
betterapproximati
onofthedistances:
1
2(
)
2
2(
)
2(1
)
2
2(
1)
S
R
I
D
R
D
D
R
q
q
q
=
+
+
+
+
+
+
For=4
andN=7,q=(3N
)0.5=4.6
S/I=1
7.3dB
For=4
andN=9,q=(3N
)0.5=5.2
S/I=19.8dB
Designtradeoff:N=7or9
ifitrequiresS/I=18dB?
RD+R
D
D
D-R
D-R
DD+R
Mobile
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Adjacentch
annelinterference
Near-fareffect:d1P
r2attheB
S
BS
MS1
MS3
MS2
d2
d1
fc1
fc2
fc1
fc2
Pr1
Pr2
Pr1
Pr2
rece
iver
BPF
psd:
=
f f
psd:
>>
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ForthesignalfromMS2,the
adjacentchannelinterferencedue
to
thenear-fareffect.
Toreduceadjacentchannelinterference
usemod
ulationschemeswhichhavesmallout-of-bandradiation
(e.g.,MS
KisbetterthanQP
SK)
carefullydesignthereceiverBPF
useproperchannelinterlea
vingbyassigninga
djacentchannelsto
differentcells,e.g.,forN=7
furtherm
ore,donotuseadjacentchannelsina
djacentcells,whichis
possibleonlywhenNisverylarge.Forexample,ifN=7,adjacent
channels
mustbeusedinadjacentcells
useFDD
orTDDtosepara
tetheforwardlink
andreverselink
ch.
cell
1
2
3
4
5
6
7
8
9
10111213
14
A
B
C
D
E
F
G
A
B
C
D
E
F
G
f
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4-4Trunking
andGradeofService
Incellularsystems,arelativ
elysmallnumbero
fradiochannelsareused
toservea
largepopulationof
mobileusers,whic
hispossibleby
frequency
reuseandbytrunk
ing
Trunkingexploitsthestatisticalbehaviourofm
obileusers,sothata
largenumberofmobileusers
cansharethefixed
radiochannelsineach
cellondemand
Basedontrafficload,thenumberofradiochan
nelsineachcellsh
ould
bedeterminedinsuchaway
that
-allthechannelsareutilizedefficienctly
-call
blockingrateisbelowapredeterminedthreshold
Givenatrafficload,numberofchannels
utilizationefficiency
andcallblockingrate
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Themeasureoftrafficefficiency:1Erlangrepr
esentstheamountof
trafficintensitycarriedbyachannelthatiscomp
letelyoccupied,e.g
.,
aradiocha
nnelthatisoccupiedfor30minutesduringanhourcarries
0.5Erlangsoftraffic
Thegradeofservice(GoS)isameasureoftheabilityofauserto
accessatrunkedsystemdurin
gthebusiesthour.GoSistypically
givenasothel
ikelihoodthatacallisblocked(forErlangBsystems),or
othel
ikelihoodthatacallexperiencesadelaylargerthana
certain
queueingdelay(forErlangCsystems)
Definition
s
set-uptim
e:thetimerequire
dtoallocateatrunkedradiochanneltoa
requesting
user
blockedc
all(lostcall):callwhichcannotbecompletedatthetime
ofrequest,
duetocongestion
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holdingtime(H):average
durationofatypicalcall
trafficin
tensity(A):measu
reofchanneltime
utilization,whichisthe
averagechanneloccupancym
easuredinErlangs
load:trafficintensityacross
theentiretrunkedradiosystem,measured
inErlangs
requestr
ate():theaverage
numberofcallreq
uestsperunittime
per
user
Relations
Thetrafficintensityoffered
byeachuseris(in
Erlangs)
uA
H
=
Forasystemwithuusersandanunspecifiedn
umberofchannels,thetotal
offeredtrafficintensityis(in
Erlangs)
u
A
uA
u
H
=
=
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InaCchanneltrunkedsystem,ifthetrafficis
equallydistributed
amongthechannels,thenthetrafficintensityperchannelis
(Erlangs)
Differenc
ebetweenofferedtrafficandcarriedtraffic
C
H
u
C
Au
A
u
c
/
/
=
=
-channel
trunkedsystem
offeredtraffic
blockedcalls
blocking
carriedtraffic
Erlangs
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Typesoftr
unkedsystems
(a)Blocke
dcallscleared
Ifno
channelsareavailable,therequesting
userisblockedwithout
accessandisfreetotrya
gainlater
Call
arrivalsfollowsaP
oissondistribution
.LetXdenotethe
numberofcallsarrivalsp
erunittime,then
(
)
,
0,1,2,...
!i
PX
i
e
ii
=
=
=
whereistherequestrate(averagenumber
ofcallarrivalsper
unit
time),andwehaveE(X)=,V(X)=
Thereareaninfinitenu
mberofusers
Therearememorylessarrivalsofcalls,implyingthatallusers,
includingblockedusers,
mayrequestachan
nelatanytime
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Channelholdingtime,Y,followsanexponentia
ldistribution
withE(Y)
=H=1/andV(Y)=1/2
Thereare
afinitenumberof
channelsavailable
inthetrunkedpool
,
0
()
{
0,
0
x
Y
e
x
fx
x
=
=
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Ifnocha
nnelsareimmediatelyavailabletheca
llisdelayed,andthe
probabilitythatthedelayed
callisforcedtowa
itmorethantsecondsis
(delay
)
(delay
0)
(delay
|delay
0)
(
(delay
0)exp[
(
)/
]
P
t
P
P
t
P
C
AtH
>
=
>
>
>
=
>
TheaveragedelayDforallcallsinaqueuedsy
stemis
(delay
0)
H
D
P
C
A
=
>
Theaveragedelayforthose
callswhichareque
uedisH/(C-A)
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4-5CapacityE
nhancementinCe
llularSystems
Thecapacitycanbeimpro
vedby
cellsplitting
antennasectoring
dynamicchannelassignment
CellSp
litting
Su
bdivideacongestedcellintosmallerc
ells,eachwithitso
wn
basestationandacor
respondingreductioninantennaheigh
tand
tra
nsmittedpower
Ce
llsplittingincrease
sthecapacityofacellularsystemsinc
eit
increasesthenumber
oftimesthatchann
elsarereused(M)
Re
ducingcellsizeincreaseshandoffs,th
enumberofbase
sta
tionsneeded,andm
ayresultinadifficultyinfindingaproper
siteforthebasestatio
n
O
ldbasestationshou
ldbekeptinsome
splittingcells
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Figure:Illustrationofcell
splittingfromradiusRtoR/2andtoR
/4
R/2
R
largecells
mediumcells
smallcells
R/4
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Let
P
t1thetransmitted
poweroflargecellBS
P
t2-thetransmitted
powerofsmallcellBS
P
r-thereceivedpoweratcellboundar
y
ThenwhenR
R/2,wehav
e
P
r(largecell)Pt1R-
P
r(smallcell)Pt2(R/2)-
Onthebasisofequalreceivedpower,wehave
P
t1R-
=Pt2(R/2)-
P
t1/Pt2
=2
10log10(
Pt1
/Pt2)=1
0
log1023dB
For
=4,
Pt1
/Pt2=12dB.Ingeneral
R
R/2incellsplitting
Ce
llarea(1/4)cell
area
Ca
pacityisincreased
by3times(or4tim
esintotal)
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Assum
ing7-cellreusepat
tern,forthe3-secto
rcase,thenumber
of
interferersinthefirsttierisreducedfrom6to2
TheCI
Risgivenby
21
1IN
k
k
k
k
S
R
R
I
D
D
=
=
=
=
Which
islargerthantheomnidirectionalcase
whereNI=6.
CellSectoring
1
2
3
1
2
3
1
2
3 4
5
6
1
2
3 4
5
6
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Worst-c
asescenarioin120
0sectoring:
For=
4andN=7(q=4.6),wehave(S/I
)1200
=24.5dB>18dB
The
3-sectorworstcasefor7-cellreuseis
acceptablefor(S/I)
requirementof18dB
0
omni
120
1
1
()
and
()
6
2
S
S
q
q
I
I
=
=
ForDk=
Dforallk,
D
D+0.7
R
Mobile
+
+
=
+
+
=
)7.0
(1
)7.0
(
)(
0
120
q
q
R
D
D
R
IS
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ChannelAssignmentStrategies
1.
Fixedchannelassignm
ent(FCA)
Eachcellisallocateda
predeterminedsetofvoicechannels
Any
callattemptwithin
thecellcanonlyb
eservedbytheunused
channelsinthatparticul
arcell
Channelallocation
cannotadapttotrafficloaddynamics
Borrowingoption:
Acellisallowedtoborrowchannelsfromaneighbouringcell
ifall
ofitschannelsarea
lreadyoccupied
Borrowingissupervised
bytheMSCtosatisfyconstraintson
co-
channelandadjacentchannelinterference
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2.
Dynamicchannelassig
nment(DCA)
Voic
echannelsarenotallocatedtodifferen
tcellsonaperman
ent
basis
Each
timeacallrequest
ismade,thehome
BSrequestsachan
nel
from
theMSC
TheMSCdetermines(d
ynamically)theavailabilityofachann
el
andexecutesitsallocationprocedureaccordingly
TheMSConlyallocatesagivenchannelif
thechannelisnot
prese
ntlyinuseinthecelloranyothercellwhichfallswithin
the
minimumrestricteddistanceofthefrequen
cyreusetoavoid
severeco-channelinterf
erence(co-channel
reuselocking)
DCA
reducesthelikelih
oodofcallblockin
g,whichincreases
the
trunk
ingcapacityofthe
system,sinceallavailablechannelsunder
thecontroloftheMSCareaccessibletoallthecalls
DCA
strategiesrequiretheMSCtocollect
real-timedataon
channeloccupancy,trafficloaddistribution
andradiosignal
stren
gthindicationsofa
llthechannelsona
continuousbasis