MobileCommunication related document.pdf
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About This Presentation
Mobile Communication
Slide Content
1
By,
Dr.S.Selvakumar
Subject Name: Mobile Communication
Unit name: Cellular Concepts
By,
Dr.S.Selvakumar
Subject Name: Mobile Communication
Unit name: Cellular Concepts
2
Aim and Objectives
•To make students familiar with fundamentals of
mobile communication systems
•To understand the concept of frequency reuse
•To identify the limitations of 2G and 2.5G wireless
mobile communication and use design of 3G and
beyond mobile communication systems
•understanding the basic principles of mobile
communication systems
Aim and Objectives
•To make students familiar with fundamentals of
mobile communication systems
•To understand the concept of frequency reuse
•To identify the limitations of 2G and 2.5G wireless
mobile communication and use design of 3G and
beyond mobile communication systems
•understanding the basic principles of mobile
communication systems
3
Pre Test -MCQ type
1.The advantage of using frequency reuse is
a. Increased capacity
b. Limited spectrum is required
c. Same spectrum may be allocated to other network
d. All of the above
ANSWER: All of the above
2. Co-channel reuse ratio depends upon
a. Radius of the cell
b. Distance between the centers of the co channel cells
c. Frequency allocation of nearest cells
d. Both a and b
e. Both b and c
ANSWER: Both a and b
Pre Test -MCQ type
1.The advantage of using frequency reuse is
a. Increased capacity
b. Limited spectrum is required
c. Same spectrum may be allocated to other network
d. All of the above
ANSWER: All of the above
2. Co-channel reuse ratio depends upon
a. Radius of the cell
b. Distance between the centers of the co channel cells
c. Frequency allocation of nearest cells
d. Both a and b
e. Both b and c
ANSWER: Both a and b
4
3.Increase in Co-channel reuse ratio indicates
a. Better transmission quality
b. Larger capacity
c. Low co-channel interference
d. Both a and c
e. Both a and b
ANSWER: Both a and c
4. The strategies acquired for channel assignment are
a. Fixed
b. Dynamic
c. Regular
d. Both a and b
e. Both b and c
ANSWER: Both a and b
Pre Test -MCQ type
3.Increase in Co-channel reuse ratio indicates
a. Better transmission quality
b. Larger capacity
c. Low co-channel interference
d. Both a and c
e. Both a and b
ANSWER: Both a and c
4. The strategies acquired for channel assignment are
a. Fixed
b. Dynamic
c. Regular
d. Both a and b
e. Both b and c
ANSWER: Both a and b
Pre Test -MCQ type
5
5. In a fixed channel assignment strategy, if all the assigned channels are
occupied, the call
a. Gets transferred to another cell
b. Gets blocked
c. Is kept on waiting
d. All of the above
ANSWER: b.Gets blocked
6.In a fixed channel assignment strategy
a. Each cell is assigned a predetermined set of
frequencies
b. The call is served by unused channels of the cell
c. The call gets blocked if all the channels of the cell are
occupied
d. All of the above
ANSWER: All of the above
Pre Test -MCQ type
5. In a fixed channel assignment strategy, if all the assigned channels are
occupied, the call
a. Gets transferred to another cell
b. Gets blocked
c. Is kept on waiting
d. All of the above
ANSWER: b.Gets blocked
6.In a fixed channel assignment strategy
a. Each cell is assigned a predetermined set of
frequencies
b. The call is served by unused channels of the cell
c. The call gets blocked if all the channels of the cell are
occupied
d. All of the above
ANSWER: All of the above
Pre Test -MCQ type
6
7. In a dynamic channel assignment strategy,
a. Voice channels are not permanently assigned
b. The serving base station requests for a channel from
MSC
c. MSC allocates the channel according to the
predetermined algorithm
d. All of the above
ANSWER: All of the above
8. Advantage of using Dynamic channel assignment is
a. Blocking is reduced
b. Capacity of the system is increased
c. Both a & b
d. None of the above
ANSWER: Both a & b
Pre Test -MCQ type
7. In a dynamic channel assignment strategy,
a. Voice channels are not permanently assigned
b. The serving base station requests for a channel from
MSC
c. MSC allocates the channel according to the
predetermined algorithm
d. All of the above
ANSWER: All of the above
8. Advantage of using Dynamic channel assignment is
a. Blocking is reduced
b. Capacity of the system is increased
c. Both a & b
d. None of the above
ANSWER: Both a & b
Pre Test -MCQ type
7
9. Disadvantage of using Dynamic channel assignment is
a. More storage required
b. Calculations and analysis is increased
c. Both a & b
d. None of the above
ANSWER: Both a & b
10.In Dynamic channel assignment, any channelwhich is being used in one
cell can be reassignedsimultaneously to another cell in the system at a
reasonable distance.
a. True
b. False
ANSWER: True
Pre Test -MCQ type
9. Disadvantage of using Dynamic channel assignment is
a. More storage required
b. Calculations and analysis is increased
c. Both a & b
d. None of the above
ANSWER: Both a & b
10.In Dynamic channel assignment, any channelwhich is being used in one
cell can be reassignedsimultaneously to another cell in the system at a
reasonable distance.
a. True
b. False
ANSWER: True
Pre Test -MCQ type
8
Pre-Requisite:
•Basic knowledge of digital communication
•Antenna and wave propagation
•Applied random process
Pre-Requisite:
•Basic knowledge of digital communication
•Antenna and wave propagation
•Applied random process
Subject Name: Mobile Communication
Unit –V / Cellular Concepts
9
Unit –V / Cellular Concepts
9
Syllabus
FrequencyReuse–ChannelAssignment
Strategies–HandoffStrategies–Interference&
systemcapacity-Co-ChannelInterference-
AdjacentChannelInterference–Trunkingand
Gradeofservice–Improvingcoverage&
capacityincellularsystems-CellSplitting-
Sectoring-RepeatersforRangeExtension-
MicrocellZoneConcept.
10
FrequencyReuse–ChannelAssignment
Strategies–HandoffStrategies–Interference&
systemcapacity-Co-ChannelInterference-
AdjacentChannelInterference–Trunkingand
Gradeofservice–Improvingcoverage&
capacityincellularsystems-CellSplitting-
Sectoring-RepeatersforRangeExtension-
MicrocellZoneConcept.
10
Introduction to Cellular System
11
11
Cellular Systems-Basic Concepts
•Cellularsystemsolvestheproblemofspectralcongestion.
•Offershighcapacityinlimitedspectrum.
•HighcapacityisachievedbylimitingthecoverageareaofeachBS
toasmallgeographicalareacalledcell.
•Replaceshighpoweredtransmitterwithseverallowpower
transmitters.
•EachBSisallocatedaportionoftotalchannelsandnearbycells
areallocatedcompletelydifferentchannels.
•AllavailablechannelsareallocatedtosmallnoofneighboringBS.
•InterferencebetweenneighboringBS’sisminimizedbyallocating
differentchannels.
12
•Cellularsystemsolvestheproblemofspectralcongestion.
•Offershighcapacityinlimitedspectrum.
•HighcapacityisachievedbylimitingthecoverageareaofeachBS
toasmallgeographicalareacalledcell.
•Replaceshighpoweredtransmitterwithseverallowpower
transmitters.
•EachBSisallocatedaportionoftotalchannelsandnearbycells
areallocatedcompletelydifferentchannels.
•AllavailablechannelsareallocatedtosmallnoofneighboringBS.
•InterferencebetweenneighboringBS’sisminimizedbyallocating
differentchannels.
12
Cellular Systems-Basic Concepts
•Samefrequenciesarereusedbyspatiallyseparated
BS’s.
•Interferencebetweenco-channelsstationsiskept
belowacceptablelevel.
•Additionalradiocapacityisachieved.
•FrequencyReuse-Fixnoofchannelsservean
arbitrarilylargenoofsubscribers
13
•Samefrequenciesarereusedbyspatiallyseparated
BS’s.
•Interferencebetweenco-channelsstationsiskept
belowacceptablelevel.
•Additionalradiocapacityisachieved.
•FrequencyReuse-Fixnoofchannelsservean
arbitrarilylargenoofsubscribers
13
Topic 1
Frequency Re-use
14
Frequency Re-use
14
Frequency Reuse
•Usedbyserviceproviderstoimprovetheefficiencyofacellular
networkandtoservemillionsofsubscribersusingalimited
radiospectrum.
•Aftercoveringacertaindistancearadiowavegetsattenuated
andthesignalfallsbelowapointwhereitcanbenolonger
used.
•Atransmittertransmittinginaspecificfrequencyrangewill
haveonlyalimitedcoveragearea
•Beyondthiscoveragearea,thatfrequencycanbereusedby
anothertransmitter.
•Theentirenetworkcoverageareaisdividedintocellsbasedon
theprincipleoffrequencyreuse
15
•Usedbyserviceproviderstoimprovetheefficiencyofacellular
networkandtoservemillionsofsubscribersusingalimited
radiospectrum.
•Aftercoveringacertaindistancearadiowavegetsattenuated
andthesignalfallsbelowapointwhereitcanbenolonger
used.
•Atransmittertransmittinginaspecificfrequencyrangewill
haveonlyalimitedcoveragearea
•Beyondthiscoveragearea,thatfrequencycanbereusedby
anothertransmitter.
•Theentirenetworkcoverageareaisdividedintocellsbasedon
theprincipleoffrequencyreuse
15
Frequency Reuse
•Acell=basicgeographicalunitofacellularnetwork&thearea
aroundanantennawhereaspecificfrequencyrangeisused.
•Whenasubscribermovestoanothercell,theantennaofthe
newcelltakesoverthesignaltransmission
•Aclusterisagroupofadjacentcells,usually7cells;no
frequencyreuseisdonewithinacluster
•Thefrequencyspectrumisdividedintosub-bands&eachsub-
bandisusedwithinonecellofthecluster
•Inheavytrafficzonescellsaresmaller,whileinisolatedzones
cellsarelarger
16
•Acell=basicgeographicalunitofacellularnetwork&thearea
aroundanantennawhereaspecificfrequencyrangeisused.
•Whenasubscribermovestoanothercell,theantennaofthe
newcelltakesoverthesignaltransmission
•Aclusterisagroupofadjacentcells,usually7cells;no
frequencyreuseisdonewithinacluster
•Thefrequencyspectrumisdividedintosub-bands&eachsub-
bandisusedwithinonecellofthecluster
•Inheavytrafficzonescellsaresmaller,whileinisolatedzones
cellsarelarger
16
Frequency Reuse
•Thedesignprocessofselectingandallocatingchannel
groupsforallofthecellularbasestationswithina
systemiscalledfrequencyreuseorfrequency
planning.
•Celllabeledwithsameletterusethesamesetof
frequencies.
•CellShapes-Circle,Square,TriangleandHexagon.
•Hexagonalcellshapeisconceptual,inrealityitis
irregularinshape
17
•Thedesignprocessofselectingandallocatingchannel
groupsforallofthecellularbasestationswithina
systemiscalledfrequencyreuseorfrequency
planning.
•Celllabeledwithsameletterusethesamesetof
frequencies.
•CellShapes-Circle,Square,TriangleandHexagon.
•Hexagonalcellshapeisconceptual,inrealityitis
irregularinshape
17
Frequency Reuse
18
18
Frequency Reuse
•Inhexagonalcellmodel,BStransmittercanbein
centreofcelloronits3vertices.
•Centeredexcitedcellsuseomnidirectionalwhereas
edgeexcitedcellsusedirectionalantennas.
•Acellularsystemhaving‘S’duplexchannels,eachcellis
allocated‘k’channels(k<S).
•IfSchannelsareallocatedtoNcellsintouniqueand
disjointchannels,thetotalnoofavailablechannelis
•S=kN.
19
•Inhexagonalcellmodel,BStransmittercanbein
centreofcelloronits3vertices.
•Centeredexcitedcellsuseomnidirectionalwhereas
edgeexcitedcellsusedirectionalantennas.
•Acellularsystemhaving‘S’duplexchannels,eachcellis
allocated‘k’channels(k<S).
•IfSchannelsareallocatedtoNcellsintouniqueand
disjointchannels,thetotalnoofavailablechannelis
•S=kN.
19
Frequency Reuse
•Ncellscollectivelyusingallthechannelsiscalledacluster,-a
groupofadjacentcells.
•IfclusterrepeatedMtimes,thecapacityCofsystemisgivenas
C=MkN=MS
•Capacityofsystemisdirectlyproportionaltothenooftimes
theclusterrepeated.
•ReducingtheclustersizeNwhilekeepingthecellsizeconstant,
moreclustersarerequiredtocoverthegivenareaandhence
morecapacity.
•Co-channelinterferenceisdependentonclustersize,large
clustersizelessinterferenceandviceversa.
20
•Ncellscollectivelyusingallthechannelsiscalledacluster,-a
groupofadjacentcells.
•IfclusterrepeatedMtimes,thecapacityCofsystemisgivenas
C=MkN=MS
•Capacityofsystemisdirectlyproportionaltothenooftimes
theclusterrepeated.
•ReducingtheclustersizeNwhilekeepingthecellsizeconstant,
moreclustersarerequiredtocoverthegivenareaandhence
morecapacity.
•Co-channelinterferenceisdependentonclustersize,large
clustersizelessinterferenceandviceversa.
20
Frequency Reuse
•TheFrequencyReusefactorisgivenas1/N&eachcellis
assigned1/Noftotalchannels.
•Linesjoiningacellandeachofitsneighborareseparatedby
multipleof60
0
,certainclustersizesandcelllayoutpossible
•Geometeryofhexagonissuchthatnoofcellsperclusteri.e
N,canonlyhavevalueswhichsatisfytheequation
N = i
2
+ ij+ j
2
N,theclustersizeistypically4,7or12.
InGSMnormallyN=7isused.
•iandjareintegers,fori=3andj=2N=19.
21
•TheFrequencyReusefactorisgivenas1/N&eachcellis
assigned1/Noftotalchannels.
•Linesjoiningacellandeachofitsneighborareseparatedby
multipleof60
0
,certainclustersizesandcelllayoutpossible
•Geometeryofhexagonissuchthatnoofcellsperclusteri.e
N,canonlyhavevalueswhichsatisfytheequation
N = i
2
+ ij+ j
2
N,theclustersizeistypically4,7or12.
InGSMnormallyN=7isused.
•iandjareintegers,fori=3andj=2N=19.
21
Locating co-channel Cell
22
22
Topic 2
Channel Assignment Strategies
23
Channel Assignment Strategies
23
Fixed Assignment Strategy
•Aschemeforincreasingcapacityandminimizinginterferenceis
requiredforeffectiveutilizationofradiospectrum.
•CAScanbeclassifiedaseitherfixedordynamic
•ChoiceofCASimpactstheperformanceofsystemforcall
management.
•InFixedCAeachcellisassignedapredeterminedsetofvoice
channels
•Anycallattemptwithinthecellcanonlybeservedbythe
unusedchannelinthatparticularcell
•Ifallthechannelsinthecellareoccupied,thecallisblocked&
theuserdoesnotgetservice.
•InvariationofFCA–calledborrowingstrategy,acellcanborrow
channelsfromitsneighboringcellifitsownchannelsarefull.
24
•Aschemeforincreasingcapacityandminimizinginterferenceis
requiredforeffectiveutilizationofradiospectrum.
•CAScanbeclassifiedaseitherfixedordynamic
•ChoiceofCASimpactstheperformanceofsystemforcall
management.
•InFixedCAeachcellisassignedapredeterminedsetofvoice
channels
•Anycallattemptwithinthecellcanonlybeservedbythe
unusedchannelinthatparticularcell
•Ifallthechannelsinthecellareoccupied,thecallisblocked&
theuserdoesnotgetservice.
•InvariationofFCA–calledborrowingstrategy,acellcanborrow
channelsfromitsneighboringcellifitsownchannelsarefull.
24
Dynamic Channel Assignment
•Voicechannelsarenotallocatedtodifferentcellspermanently.
•Eachtimeacallrequestismade,theBSrequestachannelfrom
theMSC.
•MSCallocatesachanneltotherequestingcellusingan
algorithmthattakesintoaccount-
–likelihoodoffutureblocking
–Thereusedistanceofthechannel(shouldnotcause
interference)
–Otherparameterslikecost
•ToensureminQoS,MSConlyallocatesagivenfrequency,not
currentlyinuseinthecelloranyothercellwhichfallswithinthe
limitingreusedistance.
•DCAreducethelikelihoodofblockingandincreasescapacity
•RequirestheMSCtocollectrealtimedataonchanneloccupancy
andtrafficdistributiononcontinousbasis.
25
•Voicechannelsarenotallocatedtodifferentcellspermanently.
•Eachtimeacallrequestismade,theBSrequestachannelfrom
theMSC.
•MSCallocatesachanneltotherequestingcellusingan
algorithmthattakesintoaccount-
–likelihoodoffutureblocking
–Thereusedistanceofthechannel(shouldnotcause
interference)
–Otherparameterslikecost
•ToensureminQoS,MSConlyallocatesagivenfrequency,not
currentlyinuseinthecelloranyothercellwhichfallswithinthe
limitingreusedistance.
•DCAreducethelikelihoodofblockingandincreasescapacity
•RequirestheMSCtocollectrealtimedataonchanneloccupancy
andtrafficdistributiononcontinousbasis.
25
Dynamic Channel Assignment (contd)
•Thisincreasesthestorageandcomputationalloadon
thesystem
•Butprovidestheadvantageofincreasedchannel
utilizationanddecreasedprobabilityofablocked
call.
26
•Thisincreasesthestorageandcomputationalloadon
thesystem
•Butprovidestheadvantageofincreasedchannel
utilizationanddecreasedprobabilityofablocked
call.
26
Topic 3
Hand off Strategies
27
Hand off Strategies
27
Introduction to Hand-off
•Mobilemovesintoadifferentcellduringaconversation,MSC
transfersthecalltonewchannelbelongingtonewBS
•HandoffoperationinvolvesidentifyingthenewBSand
allocationofvoiceandcontrolsignaltochannelsassociated
withnewBS
•Mustbeperformedsuccessfully,infrequentlyandimpercitble
touser
•Tomeettheserequirementsanoptimumsignallevelmustbe
definedtoinitiateahandoff.
•Minimumusuablesignalforacceptablevoicequaltiy90to-
100dBm
•Aslighthighervalueisusedasthreshold
28
•Mobilemovesintoadifferentcellduringaconversation,MSC
transfersthecalltonewchannelbelongingtonewBS
•HandoffoperationinvolvesidentifyingthenewBSand
allocationofvoiceandcontrolsignaltochannelsassociated
withnewBS
•Mustbeperformedsuccessfully,infrequentlyandimpercitble
touser
•Tomeettheserequirementsanoptimumsignallevelmustbe
definedtoinitiateahandoff.
•Minimumusuablesignalforacceptablevoicequaltiy90to-
100dBm
•Aslighthighervalueisusedasthreshold
28
Introduction to Hand-off (contd)
By looking at the variations of signal strength from either BS it is
possible to decide on the optimum area where handoff can take
place
29
By looking at the variations of signal strength from either BS it is
possible to decide on the optimum area where handoff can take
place
29
30
Hand-off strategies
•HandoffismadewhenreceivedsignalattheBSfallsbelowa
certainthreshold
•Duringhandoff:toavoidcalltermination,safetymarginshould
existandshouldnotbetoolargeorsmall
=Power
handoff–Power
minusable
•Largeresultsinunecesarryhandoffandforsmall
unsufficienttimetocompletehandoff,socarefullychosento
meettherequirements.
•Figure2a,handoffnotmadeandsignalfallsbelowmin
acceptableleveltokeepthechannelactive.
•CanhappenduetoexcessivedelaybyMSCinassigninghandoff,
orwhenthresholdissettosmall.
•Excessivedelaymayoccurduringhightrafficconditionsdueto
computionalloadingornonavialabliltyofchannelsinnearby
cells.
31
•HandoffismadewhenreceivedsignalattheBSfallsbelowa
certainthreshold
•Duringhandoff:toavoidcalltermination,safetymarginshould
existandshouldnotbetoolargeorsmall
=Power
handoff–Power
minusable
•Largeresultsinunecesarryhandoffandforsmall
unsufficienttimetocompletehandoff,socarefullychosento
meettherequirements.
•Figure2a,handoffnotmadeandsignalfallsbelowmin
acceptableleveltokeepthechannelactive.
•CanhappenduetoexcessivedelaybyMSCinassigninghandoff,
orwhenthresholdissettosmall.
•Excessivedelaymayoccurduringhightrafficconditionsdueto
computionalloadingornonavialabliltyofchannelsinnearby
cells.
31
Hand-off Strategies (contd)
•Indecidingwhentohandoff,itisimportanttoensure
thatthedropinsignallevelisnotduetomomentary
fading.
•Inordertoensure,theBSmonitorsthesignalfora
certainperiodoftimebeforeinitiatingahandoff
•Thelengthoftimeneededtodecideifhandoffis
necessarydependsonthespeedatwhichthemobile
ismoving.
32
•Indecidingwhentohandoff,itisimportanttoensure
thatthedropinsignallevelisnotduetomomentary
fading.
•Inordertoensure,theBSmonitorsthesignalfora
certainperiodoftimebeforeinitiatingahandoff
•Thelengthoftimeneededtodecideifhandoffis
necessarydependsonthespeedatwhichthemobile
ismoving.
32
Hand-off strategies –First Generation
•In1
st
generationanalogcellularsystems,thesignalstrength
measurementsaremadebytheBSandaresupervisedbythe
MSC.
•AspareRxinbasestationmonitorsRSSofRVC'sinneighboring
cells
•TellsMobileSwitchingCenteraboutthesemobilesand
theirchannels
•LocatorRxcanseeifsignaltothisbasestationis
significantlybetterthantohostbasestation
–MSCmonitorsRSSfromallbasestations&decideson
handoff
33
•In1
st
generationanalogcellularsystems,thesignalstrength
measurementsaremadebytheBSandaresupervisedbythe
MSC.
•AspareRxinbasestationmonitorsRSSofRVC'sinneighboring
cells
•TellsMobileSwitchingCenteraboutthesemobilesand
theirchannels
•LocatorRxcanseeifsignaltothisbasestationis
significantlybetterthantohostbasestation
–MSCmonitorsRSSfromallbasestations&decideson
handoff
33
Hand-off strategies –2
nd
Generation
•In2
nd
generationsystems,MobileAssistedHandoffs(MAHO)are
used
•InMAHO,everyMSmeasuresthereceivedpowerfromthe
surroundingBS&continuallyreportsthesevaluestothe
correspondingBS.
•HandoffisinitiatedifthesignalstrengthofaneighboringBS
exceedsthatofcurrentBS
•MSCnolongermonitorsRSSofallchannels
–reducescomputationalloadconsiderably
–enablesmuchmorerapidandefficienthandoffs
–imperceptibletouser
34
nd
Generation
•In2
nd
generationsystems,MobileAssistedHandoffs(MAHO)are
used
•InMAHO,everyMSmeasuresthereceivedpowerfromthe
surroundingBS&continuallyreportsthesevaluestothe
correspondingBS.
•HandoffisinitiatedifthesignalstrengthofaneighboringBS
exceedsthatofcurrentBS
•MSCnolongermonitorsRSSofallchannels
–reducescomputationalloadconsiderably
–enablesmuchmorerapidandefficienthandoffs
–imperceptibletouser
34
Soft Handoff Vs Inter-system Handoff
•CDMAspreadspectrumcellularsystemsprovidesaunique
handoffcapability
•Unlikechannelizedwirelesssystemsthatassignsdifferentradio
channelduringhandoff(calledhardhandoff),thespread
spectrumMSsharethesamechannelineverycell
•ThetermhandoffhereimpliesthatadifferentBShandlesthe
radiocommunicationtask
•Theabilitytoselectbetweentheinstantaneousreceivedsignals
fromdifferentBSsiscalledsofthandoff.
•Ifamobilemovesfromonecellularsystemtoanothersystem
controlledbyadifferentMSC,aninter-systemhandoffis
required.
•MSCengagesinintersystemhandoffwhensignalbecomesweak
inagivencell.
35
•CDMAspreadspectrumcellularsystemsprovidesaunique
handoffcapability
•Unlikechannelizedwirelesssystemsthatassignsdifferentradio
channelduringhandoff(calledhardhandoff),thespread
spectrumMSsharethesamechannelineverycell
•ThetermhandoffhereimpliesthatadifferentBShandlesthe
radiocommunicationtask
•Theabilitytoselectbetweentheinstantaneousreceivedsignals
fromdifferentBSsiscalledsofthandoff.
•Ifamobilemovesfromonecellularsystemtoanothersystem
controlledbyadifferentMSC,aninter-systemhandoffis
required.
•MSCengagesinintersystemhandoffwhensignalbecomesweak
inagivencell.
35
Prioritizing Handoffs
PerceivedGradeofService(GOS)–servicequalityasviewed
byusers
•“quality”intermsofdroppedorblockedcalls
•assignhigherprioritytohandoffVsnewcallrequest
•adroppedcallmoreaggravatingthananoccasionalblockedcall
GuardChannels
–%oftotalavailablecellchannelssetasideforhandoffrequests
–makesfewerchannelsavailablefornewcallrequests
–agoodstrategyfordynamicchannelallocation
•Queuing of Handoff Requests
–usetimedelaybetweenhandoffthreshold&minimumuseablesignallevel
–ahandoffrequestduringthattimeperiod,insteadofhavingasingle
block/noblockdecision
–prioritizerequests&handoffasrequired
–callsstillbedroppediftimeperiodexpires
36
PerceivedGradeofService(GOS)–servicequalityasviewed
byusers
•“quality”intermsofdroppedorblockedcalls
•assignhigherprioritytohandoffVsnewcallrequest
•adroppedcallmoreaggravatingthananoccasionalblockedcall
GuardChannels
–%oftotalavailablecellchannelssetasideforhandoffrequests
–makesfewerchannelsavailablefornewcallrequests
–agoodstrategyfordynamicchannelallocation
•Queuing of Handoff Requests
–usetimedelaybetweenhandoffthreshold&minimumuseablesignallevel
–ahandoffrequestduringthattimeperiod,insteadofhavingasingle
block/noblockdecision
–prioritizerequests&handoffasrequired
–callsstillbedroppediftimeperiodexpires
36
Practical Handoff Considerations
•Problemsoccurbecauseofalargerangeofmobilevelocities
–pedestrianVsvehicleuser
•Smallcellsizesormicro-cells→larger#handoffs
•MSCloadisheavywhenhighspeedusersarepassedbetween
verysmallcells
•UmbrellaCells
–usedifferentantennaheights&Txpowerlevelstoprovidelargeand
smallcellcoverage
–multipleantennas&Txcanbeco-locatedatsinglelocationifrequired
–largecell→highspeedtraffic→fewerhandoffs
–smallcell→lowspeedtraffic
–exampleareas:interstatehighwaypassingthroughurbancenter,office
park,ornearbyshoppingmall
37
•Problemsoccurbecauseofalargerangeofmobilevelocities
–pedestrianVsvehicleuser
•Smallcellsizesormicro-cells→larger#handoffs
•MSCloadisheavywhenhighspeedusersarepassedbetween
verysmallcells
•UmbrellaCells
–usedifferentantennaheights&Txpowerlevelstoprovidelargeand
smallcellcoverage
–multipleantennas&Txcanbeco-locatedatsinglelocationifrequired
–largecell→highspeedtraffic→fewerhandoffs
–smallcell→lowspeedtraffic
–exampleareas:interstatehighwaypassingthroughurbancenter,office
park,ornearbyshoppingmall
37
Umbrella Cells
38
38
Topic 4
Interference & System Capacity
39
Interference & System Capacity
39
Interference
•Interferenceisthemajorlimitingfactorintheperformanceof
cellularradiosystems.
•Sourcesofinterferenceinclude-
anothermobileinthesamecell
acallinprogressinaneighboringcell
otherbasestationsoperatinginsamefrequencyband
anynon-cellularsystemwhichleaksenergyintoband
•Interferenceonvoicechannelscausescrosstalkduetoan
undesiredtransmission.
•Interferenceoncontrolchannels,leadstomissed&blocked
callsduetoerrorsindigitalsignaling.
•Interferenceismoresevereinurbanareas,duetoRFnoisefloor
&largenumberofbasestations/mobiles.
•Interferenceisamajorbottleneckinincreasingcapacity&
oftenresponsiblefordroppedcalls
40
•Interferenceisthemajorlimitingfactorintheperformanceof
cellularradiosystems.
•Sourcesofinterferenceinclude-
anothermobileinthesamecell
acallinprogressinaneighboringcell
otherbasestationsoperatinginsamefrequencyband
anynon-cellularsystemwhichleaksenergyintoband
•Interferenceonvoicechannelscausescrosstalkduetoan
undesiredtransmission.
•Interferenceoncontrolchannels,leadstomissed&blocked
callsduetoerrorsindigitalsignaling.
•Interferenceismoresevereinurbanareas,duetoRFnoisefloor
&largenumberofbasestations/mobiles.
•Interferenceisamajorbottleneckinincreasingcapacity&
oftenresponsiblefordroppedcalls
40
Interference (contd)
•ThetwomajortypesofcellularinterferenceareCo-channel
interference(CCI)&Adjacentchannelinterference(ACI).
•Eventhoughinterferingsignalsareoftengeneratedwithinthe
cellularsystem,theyaredifficulttocontrolduetorandom
propagationeffects.
•Moredifficulttocontrolisinterferenceduetoout-of-band
usersduetofrontendoverloadofsubscriberequipments.
•CCI is caused due to the cells that reuse the same frequency set.
•ThesecellsusingthesamefrequencysetarecalledCo-channel
cells.
•ACIiscausedduetothesignalsthatareadjacentinfrequency.
41
•ThetwomajortypesofcellularinterferenceareCo-channel
interference(CCI)&Adjacentchannelinterference(ACI).
•Eventhoughinterferingsignalsareoftengeneratedwithinthe
cellularsystem,theyaredifficulttocontrolduetorandom
propagationeffects.
•Moredifficulttocontrolisinterferenceduetoout-of-band
usersduetofrontendoverloadofsubscriberequipments.
•CCI is caused due to the cells that reuse the same frequency set.
•ThesecellsusingthesamefrequencysetarecalledCo-channel
cells.
•ACIiscausedduetothesignalsthatareadjacentinfrequency.
41
Topic 5
Co-channel Interference
42
Co-channel Interference
42
Co-channel Interference
•Frequencyreuseindicatesthatinagivencoverageareathere
areseveralcellsusingthesamesetoffrequencies.
•Thesecellsarecalledco-channelcells&interferencebetween
signalsfromthesecellsiscalledco-channelinterference.
•Co-channelinterferencecannotbecombatedbysimply
increasingthecarrierpowerofatransmitter.
•Becauseanincreaseincarriertransmitpowerincreasesthe
interferencetoneighboringco-channelcells.
•Toreduceco-channelinterference,co-channelcellsmustbe
separatedbyaminimumdistancetoprovidesufficientisolation
duetopropagation.
•Co-channelinterferenceisindependentofthetransmitted
power&dependsupontheradiusofthecell(R)andthe
distancebetweencentersofthenearestco-channelcells(D).
43
•Frequencyreuseindicatesthatinagivencoverageareathere
areseveralcellsusingthesamesetoffrequencies.
•Thesecellsarecalledco-channelcells&interferencebetween
signalsfromthesecellsiscalledco-channelinterference.
•Co-channelinterferencecannotbecombatedbysimply
increasingthecarrierpowerofatransmitter.
•Becauseanincreaseincarriertransmitpowerincreasesthe
interferencetoneighboringco-channelcells.
•Toreduceco-channelinterference,co-channelcellsmustbe
separatedbyaminimumdistancetoprovidesufficientisolation
duetopropagation.
•Co-channelinterferenceisindependentofthetransmitted
power&dependsupontheradiusofthecell(R)andthe
distancebetweencentersofthenearestco-channelcells(D).
43
Co-channel Interference (contd)
•ByincreasingD/Rratio,thespatialseparationbetweenco-
channelcellsrelativetothecoveragedistanceofacellis
increased.
•Thus,interferenceisreducedfromimprovedisolationofRF
energyfromtheco-channelcell.
•TheparameterQ-calledtheco-channelreuseratioisrelated
totheclustersize.
•AsmallvalueofQprovideslargercapacitysincethecluster
sizeNissmall,whereasalargevalueofQimprovesthe
transmissionquality,duetoasmallerlevelofco-channel
interference.
44
•ByincreasingD/Rratio,thespatialseparationbetweenco-
channelcellsrelativetothecoveragedistanceofacellis
increased.
•Thus,interferenceisreducedfromimprovedisolationofRF
energyfromtheco-channelcell.
•TheparameterQ-calledtheco-channelreuseratioisrelated
totheclustersize.
•AsmallvalueofQprovideslargercapacitysincethecluster
sizeNissmall,whereasalargevalueofQimprovesthe
transmissionquality,duetoasmallerlevelofco-channel
interference.
44
Signal to Interference Ratio (S/I)
•Leti
0bethenumberofco-channelinterferingcells.
•Thesignal-to-interferenceratio(S/I)foramobilereceiverwhich
monitorsaforwardchannelgivenby-
•whereSisdesiredsignalpowerfromthedesiredbasestation&
I
iistheinterferencepowercausedbytheithinterferingco-
channelcellbasestation.
•TheaveragereceivedpowerP
ratadistancedfromthe
transmittingantennaisapproximatedby
•whereP
0isthepowerreceivedinfarfieldregionoftheantenna
atasmalldistanced
0fromthetransmittingantenna&nisthe
pathlossexponent.
45
•Leti
0bethenumberofco-channelinterferingcells.
•Thesignal-to-interferenceratio(S/I)foramobilereceiverwhich
monitorsaforwardchannelgivenby-
•whereSisdesiredsignalpowerfromthedesiredbasestation&
I
iistheinterferencepowercausedbytheithinterferingco-
channelcellbasestation.
•TheaveragereceivedpowerP
ratadistancedfromthe
transmittingantennaisapproximatedby
•whereP
0isthepowerreceivedinfarfieldregionoftheantenna
atasmalldistanced
0fromthetransmittingantenna&nisthe
pathlossexponent.
45
Signal to Interference Ratio (contd)
•Whenthetransmitpowerofeachbasestationisequal&path
lossexponentisthesamethroughout,S/Iforamobilecanbe
approximatedas-
•Ifalltheinterferingbasestationsareequidistantfromthedesired
basestation&equaltodistanceDbetweencellcenters,then
abovesimplifiesto-
•Foraseven-cellcluster,withmobileunitatthecellboundary,the
mobileisadistanceD–Rfromthetwonearestco-channel
interferingcells&exactlyD+R/2,D,D–R/2,andD+Rfromthe
otherinterferingcellsinthefirsttier,asshowninfigure1.
46
•Whenthetransmitpowerofeachbasestationisequal&path
lossexponentisthesamethroughout,S/Iforamobilecanbe
approximatedas-
•Ifalltheinterferingbasestationsareequidistantfromthedesired
basestation&equaltodistanceDbetweencellcenters,then
abovesimplifiesto-
•Foraseven-cellcluster,withmobileunitatthecellboundary,the
mobileisadistanceD–Rfromthetwonearestco-channel
interferingcells&exactlyD+R/2,D,D–R/2,andD+Rfromthe
otherinterferingcellsinthefirsttier,asshowninfigure1.
46
Figure 1. First tier of co-channel cells for cluster size ofN= 7
•UsingtheapproximategeometryshowninFigure1&assumingn
=4,thesignal-to-interferenceratiofortheworstcasecanbe
approximatedas-
47
•UsingtheapproximategeometryshowninFigure1&assumingn
=4,thesignal-to-interferenceratiofortheworstcasecanbe
approximatedas-
47
Topic 6
Adjacent Channel Interference
48
Adjacent Channel Interference
48
Near–Far Effect
49
•Interferenceresultingfromsignalsadjacentinfrequencytothe
desiredsignaliscalledadjacentchannelinterference.
•Adjacentchannelinterferenceresultsfromimperfectreceiver
filterswhichallownearbyfrequenciestoleakintothepass-
band.
•Theproblemcanbeseriousifanadjacentchanneluseris
transmittinginverycloserangetoasubscriber’sreceiver,while
thereceiverattemptstoreceiveabasestationonthedesired
channel.
•Thisiscalledasthenear–fareffect,whereanearbytransmitter
capturesthereceiverofthesubscriber.
•Italsooccurswhenamobileclosetoabasestationtransmitson
achannelclosetoonebeingusedbyaweakmobile.
49
•Interferenceresultingfromsignalsadjacentinfrequencytothe
desiredsignaliscalledadjacentchannelinterference.
•Adjacentchannelinterferenceresultsfromimperfectreceiver
filterswhichallownearbyfrequenciestoleakintothepass-
band.
•Theproblemcanbeseriousifanadjacentchanneluseris
transmittinginverycloserangetoasubscriber’sreceiver,while
thereceiverattemptstoreceiveabasestationonthedesired
channel.
•Thisiscalledasthenear–fareffect,whereanearbytransmitter
capturesthereceiverofthesubscriber.
•Italsooccurswhenamobileclosetoabasestationtransmitson
achannelclosetoonebeingusedbyaweakmobile.
Minimizing Interference
50
•Thebasestationmayhavedifficultyindiscriminatingthe
desiredmobileuserfromthe“bleed-over”causedbytheclose
adjacentchannelmobile.
•Adjacentchannelinterferencecanbeminimizedthrough
carefulfiltering&channelassignments.
•Sinceeachcellisgivenonlyafractionoftheavailable
channels,acellneednotbeassignedchannelsadjacentin
frequency.
•Bykeepingthefrequencyseparationbetweeneachchannelin
agivencelllarger,theadjacentchannelinterferencemaybe
reduced.
•Ifthefrequencyreusefactorislarge,theseparationbetween
adjacentchannelsatthebasestationnotsufficienttokeepthe
adjacentchannelinterferencelevelwithintolerablelimits.
50
•Thebasestationmayhavedifficultyindiscriminatingthe
desiredmobileuserfromthe“bleed-over”causedbytheclose
adjacentchannelmobile.
•Adjacentchannelinterferencecanbeminimizedthrough
carefulfiltering&channelassignments.
•Sinceeachcellisgivenonlyafractionoftheavailable
channels,acellneednotbeassignedchannelsadjacentin
frequency.
•Bykeepingthefrequencyseparationbetweeneachchannelin
agivencelllarger,theadjacentchannelinterferencemaybe
reduced.
•Ifthefrequencyreusefactorislarge,theseparationbetween
adjacentchannelsatthebasestationnotsufficienttokeepthe
adjacentchannelinterferencelevelwithintolerablelimits.
Minimizing Interference (contd)
51
•Forexample,ifaclose-inmobileis20timesasclosetothe
basestationasanothermobile&hasenergyspill-outofits
pass-band,thesignal-to-interferenceratioatthebasestation
fortheweakmobileisgivenby-
•Forapathlossexponentn=4,thisisequalto–52dB.
•Iftheintermediatefrequency(IF)filterofthebasestation
receiverhasaslopeof20dB/octave,thenanadjacentchannel
interferermustbedisplacedbyatleastsixtimesthepass-
bandbandwidthtoachieve-52dBattenuation.
•Thisindicatesmorethansixchannelseparationsrequiredto
bringtheadjacentchannelinterferencetoanacceptablelevel.
•Inpractice,basestationreceiversareprecededbyahighQ
cavityfiltertorejectadjacentchannelinterference.
51
•Forexample,ifaclose-inmobileis20timesasclosetothe
basestationasanothermobile&hasenergyspill-outofits
pass-band,thesignal-to-interferenceratioatthebasestation
fortheweakmobileisgivenby-
•Forapathlossexponentn=4,thisisequalto–52dB.
•Iftheintermediatefrequency(IF)filterofthebasestation
receiverhasaslopeof20dB/octave,thenanadjacentchannel
interferermustbedisplacedbyatleastsixtimesthepass-
bandbandwidthtoachieve-52dBattenuation.
•Thisindicatesmorethansixchannelseparationsrequiredto
bringtheadjacentchannelinterferencetoanacceptablelevel.
•Inpractice,basestationreceiversareprecededbyahighQ
cavityfiltertorejectadjacentchannelinterference.
Topic 7
Trunking & Grade of Service
52
Trunking & Grade of Service
52
Trunking
•Cellularradiosystemsdependontrunkingto
accommodatealargenumberofusersinalimited
radiospectrum.
•Trunkingallowsalargenoofuserstoshareasmall
numberofchannelsinacellbyprovidingaccessto
eachuser,ondemand,fromapoolofavailable
channels.
•Inatrunkedradiosystem(TRS)eachuseris
allocatedachannelonapercallbasis,upon
terminationofthecall,thepreviouslyoccupied
channelisimmediatelyreturnedtothepoolof
availablechannels. 53
•Cellularradiosystemsdependontrunkingto
accommodatealargenumberofusersinalimited
radiospectrum.
•Trunkingallowsalargenoofuserstoshareasmall
numberofchannelsinacellbyprovidingaccessto
eachuser,ondemand,fromapoolofavailable
channels.
•Inatrunkedradiosystem(TRS)eachuseris
allocatedachannelonapercallbasis,upon
terminationofthecall,thepreviouslyoccupied
channelisimmediatelyreturnedtothepoolof
availablechannels. 53
Key Definitions
•SetupTime:Timerequiredtoallocatearadiochanneltoa
requestinguser
•BlockedCall:Callwhichcannotbecompletedatthetime
ofrequest,duetocongestion
•HoldingTime:Averagedurationofatypicalcall.
•RequestRate:Theaveragenumberofcallsrequestsper
unittime(λ)
•TrafficIntensity:Measureofchanneltimeutilization
measuredinErlangs.Dimensionlessquantity.Denotedby
A
•Load:TrafficintensityacrosstheentireTRS(Erlangs)
54
•SetupTime:Timerequiredtoallocatearadiochanneltoa
requestinguser
•BlockedCall:Callwhichcannotbecompletedatthetime
ofrequest,duetocongestion
•HoldingTime:Averagedurationofatypicalcall.
•RequestRate:Theaveragenumberofcallsrequestsper
unittime(λ)
•TrafficIntensity:Measureofchanneltimeutilization
measuredinErlangs.Dimensionlessquantity.Denotedby
A
•Load:TrafficintensityacrosstheentireTRS(Erlangs)
54
Erlang-Unit of Traffic
•Thefundamentalsoftrunkingtheoryweredevelopedby
Erlang,aDanishmathematician,theunitbearshisname.
•AnErlangisaunitoftelecommunicationstraffic
measurement.
•Erlangrepresentsthecontinuoususeofonevoicepath.
•Usedtodescribethetotaltrafficvolumeofonehour
•Achannelkeptbusyforonehourisdefinedashavingaload
ofoneErlang
•Forexample,aradiochannelthatisoccupiedforthirty
minutesduringanhourcarries0.5Erlangsoftraffic
•For1channel
–Minload=0Erlang(0%timeutilization)
–Maxload=1Erlang(100%timeutilization)
55
•Thefundamentalsoftrunkingtheoryweredevelopedby
Erlang,aDanishmathematician,theunitbearshisname.
•AnErlangisaunitoftelecommunicationstraffic
measurement.
•Erlangrepresentsthecontinuoususeofonevoicepath.
•Usedtodescribethetotaltrafficvolumeofonehour
•Achannelkeptbusyforonehourisdefinedashavingaload
ofoneErlang
•Forexample,aradiochannelthatisoccupiedforthirty
minutesduringanhourcarries0.5Erlangsoftraffic
•For1channel
–Minload=0Erlang(0%timeutilization)
–Maxload=1Erlang(100%timeutilization)
55
Erlang-Unit of Traffic (contd)
•Forexample,ifagroupof100usersmade30callsin
onehour,andeachcallhadanaveragecall
duration(holdingtime)of5minutes,thenthe
numberofErlangsthisrepresentsisworkedoutas
follows:
•Minutesoftrafficinthehour=numberofcallsx
duration
•Minutesoftrafficinthehour=30x5=150
•Hoursoftrafficinthehour=150/60=2.5
•TrafficIntensity=2.5Erlangs
56
•Forexample,ifagroupof100usersmade30callsin
onehour,andeachcallhadanaveragecall
duration(holdingtime)of5minutes,thenthe
numberofErlangsthisrepresentsisworkedoutas
follows:
•Minutesoftrafficinthehour=numberofcallsx
duration
•Minutesoftrafficinthehour=30x5=150
•Hoursoftrafficinthehour=150/60=2.5
•TrafficIntensity=2.5Erlangs
56
Grade of Service
•InaTRS,whenaparticularuserrequestsservice&all
theavailableradiochannelsarealreadyinuse,the
userisblockedordeniedaccesstothesystem.
•Insomesystemsaqueuemaybeusedtoholdthe
requestingusersuntilachannelbecomesavailable.
•Trunkingsystemsmustbedesignedinordertoensure
thatthereisalowlikelihoodthatauserwillbeblocked
ordeniedaccess.
•Thelikelihoodthatacallisblocked,orthelikelihood
thatacallexperiencesadelaygreaterthanacertain
queuingtimeiscalled“GradeofService”(GOS)’’.
57
•InaTRS,whenaparticularuserrequestsservice&all
theavailableradiochannelsarealreadyinuse,the
userisblockedordeniedaccesstothesystem.
•Insomesystemsaqueuemaybeusedtoholdthe
requestingusersuntilachannelbecomesavailable.
•Trunkingsystemsmustbedesignedinordertoensure
thatthereisalowlikelihoodthatauserwillbeblocked
ordeniedaccess.
•Thelikelihoodthatacallisblocked,orthelikelihood
thatacallexperiencesadelaygreaterthanacertain
queuingtimeiscalled“GradeofService”(GOS)’’.
57
Grade of Service (contd)
•GradeofService(GOS):Measureofabilityofauserto
accessatrunkedsystemduringthebusiesthour.
•AlsoaMeasureofthecongestionwhichisspecifiedasa
probability.
•Itisabenchmarktodefinethedesireperformanceofa
particulartrunksystem.
•Theprobabilityofacallbeingblocked-
•Blockedcallscleared(BCC)orErlangBsystems.
•Theprobabilityofacallbeingdelayedbeyondacertain
amountoftimebeforebeinggrantedaccess-
•Blockedcalldelayed(BCD)orErlangCsystems.
58
•GradeofService(GOS):Measureofabilityofauserto
accessatrunkedsystemduringthebusiesthour.
•AlsoaMeasureofthecongestionwhichisspecifiedasa
probability.
•Itisabenchmarktodefinethedesireperformanceofa
particulartrunksystem.
•Theprobabilityofacallbeingblocked-
•Blockedcallscleared(BCC)orErlangBsystems.
•Theprobabilityofacallbeingdelayedbeyondacertain
amountoftimebeforebeinggrantedaccess-
•Blockedcalldelayed(BCD)orErlangCsystems.
58
Blocked Call Cleared Systems (BCC)
•Whenauserrequestsservice,thereisaminimalcall
set-uptime&userisgivenimmediateaccesstoa
channelifoneisavailable.
•Ifchannelsarealreadyinuseandnonewchannelsare
available,callisblockedwithoutaccesstothesystem.
•Theuserdoesnotreceiveservice,butisfreetotry
againlater.
•Allblockedcallsareinstantlyreturnedtotheuser
pool.
59
•Whenauserrequestsservice,thereisaminimalcall
set-uptime&userisgivenimmediateaccesstoa
channelifoneisavailable.
•Ifchannelsarealreadyinuseandnonewchannelsare
available,callisblockedwithoutaccesstothesystem.
•Theuserdoesnotreceiveservice,butisfreetotry
againlater.
•Allblockedcallsareinstantlyreturnedtotheuser
pool.
59
Modeling of BCC Systems
•TheErlangBmodelisbasedonfollowingassumptions:
–CallsareassumedtoarrivewithaPoissondistribution
–Nearlyaninfinitenumberofusers
–Callrequestsarememorylesssothatallusers,including
blockedusers,mayrequestachannelatanytime
–Allfreechannelsarefullyavailableforservicingcallsuntilall
channelsareoccupied
–Theprobabilityofauseroccupyingachannelisexponentially
distributed.
–Longercallslesslikelytohappen
–Finitenumberofchannelsavailableinthetrunkingpool.
–Inter-arrivaltimesofcallrequestsareindependentofeach
other
60
•TheErlangBmodelisbasedonfollowingassumptions:
–CallsareassumedtoarrivewithaPoissondistribution
–Nearlyaninfinitenumberofusers
–Callrequestsarememorylesssothatallusers,including
blockedusers,mayrequestachannelatanytime
–Allfreechannelsarefullyavailableforservicingcallsuntilall
channelsareoccupied
–Theprobabilityofauseroccupyingachannelisexponentially
distributed.
–Longercallslesslikelytohappen
–Finitenumberofchannelsavailableinthetrunkingpool.
–Inter-arrivaltimesofcallrequestsareindependentofeach
other
60
Modeling of BCC Systems (contd)
•ErlangBformulaisgivenby
•where‘C’isthenumberoftrunkedchannelsofferedby
atrunkedradiosystem.
•‘A’isthetotalofferedtraffic.
61
Pr [blocking] = (A
C
/C ! )
•ErlangBformulaisgivenby
•where‘C’isthenumberoftrunkedchannelsofferedby
atrunkedradiosystem.
•‘A’isthetotalofferedtraffic.
61
Pr [blocking] = (A
C
/C ! )
Traffic Intensity in Erlang B (BCC)
62
62
Erlang B Trunking GOS
63
63
Blocked Call Delayed Systems (BCD)
•Queuesareusedtoholdcallrequeststhatareinitially
blocked.
•Whenauserattemptsacall&achannelisnot
immediatelyavailable,thecallrequestmaybedelayed
untilachannelbecomesavailable
•MathematicalmodelingofsuchsystemsisdonebyErlang
Cformula.
•TheErlangCmodelisbasedonfollowingassumptions:
–SimilartothoseofErlangB
–Ifofferedcallcannotbeassignedachannel,itisplacedina
queueofinfinitelength
–Eachcallisthenservicedintheorderofitsarrival
64
•Queuesareusedtoholdcallrequeststhatareinitially
blocked.
•Whenauserattemptsacall&achannelisnot
immediatelyavailable,thecallrequestmaybedelayed
untilachannelbecomesavailable
•MathematicalmodelingofsuchsystemsisdonebyErlang
Cformula.
•TheErlangCmodelisbasedonfollowingassumptions:
–SimilartothoseofErlangB
–Ifofferedcallcannotbeassignedachannel,itisplacedina
queueofinfinitelength
–Eachcallisthenservicedintheorderofitsarrival
64
Blocked Call Delayed Systems (contd)
65
•ErlangCformulagiveslikelihoodofacallnothaving
immediateaccesstoachannel(allchannelsarealready
inuse)
65
•ErlangCformulagiveslikelihoodofacallnothaving
immediateaccesstoachannel(allchannelsarealready
inuse)
Modeling of BCD Systems
•Probabilitythatanycallerisdelayedinqueueforawaittime
greaterthantsecondsisgivenasGOSofaBCDSystem
•Theprobabilityofacallgettingdelayedforanyperiodoftime
greaterthanzerois-
P[delayedcallisforcedtowait>tsec]=P[delayed]xConditional
P[delayis>tsec]
•Mathematically;
Pr[delay>t]=Pr[delay>0]Pr[delay>t|delay>0]
•Where P[delay>t|delay>0]=e
(-(C-A)t/H)
Pr[delay>t]=Pr[delay>0]e(-(C-A)t/H)
–whereC=totalnumberofchannels,t=delaytimeof
interest,H=averagedurationofcall
66
•Probabilitythatanycallerisdelayedinqueueforawaittime
greaterthantsecondsisgivenasGOSofaBCDSystem
•Theprobabilityofacallgettingdelayedforanyperiodoftime
greaterthanzerois-
P[delayedcallisforcedtowait>tsec]=P[delayed]xConditional
P[delayis>tsec]
•Mathematically;
Pr[delay>t]=Pr[delay>0]Pr[delay>t|delay>0]
•Where P[delay>t|delay>0]=e
(-(C-A)t/H)
Pr[delay>t]=Pr[delay>0]e(-(C-A)t/H)
–whereC=totalnumberofchannels,t=delaytimeof
interest,H=averagedurationofcall
66
Traffic Intensity in Erlang C (BCD)
67
=
67
=
Topic 8
Improving Coverage & Capacity in
Cellular Systems
68
Improving Coverage & Capacity in
Cellular Systems
68
Introduction
69
•Asthedemandforwirelessserviceincreases,thenumber
ofchannelsassignedtoacellbecomesinsufficientto
supporttherequirednumberofusers.
•Hencecellulardesigntechniquesareneededtoprovide
morechannelsperunitcoveragearea.
•Techniquessuchascellsplitting,sectoring&coveragezone
approachesareusedtoexpandthecapacityofcellular
systems.
•Cellsplittingallowsanorderlygrowthofthecellular
system.
•Sectoringusesdirectionalantennastocontrolthe
interferenceandfrequencyreuseofchannels.
69
•Asthedemandforwirelessserviceincreases,thenumber
ofchannelsassignedtoacellbecomesinsufficientto
supporttherequirednumberofusers.
•Hencecellulardesigntechniquesareneededtoprovide
morechannelsperunitcoveragearea.
•Techniquessuchascellsplitting,sectoring&coveragezone
approachesareusedtoexpandthecapacityofcellular
systems.
•Cellsplittingallowsanorderlygrowthofthecellular
system.
•Sectoringusesdirectionalantennastocontrolthe
interferenceandfrequencyreuseofchannels.
Introduction (contd)
70
•Thezonemicrocellconceptdistributesthecoverageofa
cell&extendsthecellboundarytohard-to-reachplaces.
•Whilecellsplittingincreasesthenumberofbasestationsto
increasecapacity,sectoring&zonemicrocellsdependon
basestationantennaplacementstoimprovecapacityby
reducingco-channelinterference.
•Cellsplitting&zonemicrocelltechniquesdonotsuffer
trunkinginefficienciesexperiencedbysectoredcells.
•Theyenablebasestationtooverseeallhandoffchores
relatedtothemicrocells,thusreducingthecomputational
loadattheMSC.
•Thesethreecapacityimprovementtechniqueswillbe
explainedindetail.
70
•Thezonemicrocellconceptdistributesthecoverageofa
cell&extendsthecellboundarytohard-to-reachplaces.
•Whilecellsplittingincreasesthenumberofbasestationsto
increasecapacity,sectoring&zonemicrocellsdependon
basestationantennaplacementstoimprovecapacityby
reducingco-channelinterference.
•Cellsplitting&zonemicrocelltechniquesdonotsuffer
trunkinginefficienciesexperiencedbysectoredcells.
•Theyenablebasestationtooverseeallhandoffchores
relatedtothemicrocells,thusreducingthecomputational
loadattheMSC.
•Thesethreecapacityimprovementtechniqueswillbe
explainedindetail.
Topic 9
Cell Splitting
71
Cell Splitting
71
Introduction to Cell Splitting
•Cellsplittingistheprocessofsubdividingacongestedcell
intosmallercellswith
–theirownBS
–acorrespondingreductioninantennaheight
–acorrespondingreductionintransmitpower
•Splittingthecellreducesthecellsize&thusmorenumberof
cellstobeused
•Forthenewcellstobesmallerinsizethetransmitpowerof
thesecellsmustbereduced.
•IdeaistokeepQ=D/RconstantwhiledecreasingR
•Morenumberofcells►morenumberofclusters►more
channels►highcapacity
72
•Cellsplittingistheprocessofsubdividingacongestedcell
intosmallercellswith
–theirownBS
–acorrespondingreductioninantennaheight
–acorrespondingreductionintransmitpower
•Splittingthecellreducesthecellsize&thusmorenumberof
cellstobeused
•Forthenewcellstobesmallerinsizethetransmitpowerof
thesecellsmustbereduced.
•IdeaistokeepQ=D/RconstantwhiledecreasingR
•Morenumberofcells►morenumberofclusters►more
channels►highcapacity
72
Example for Cell Splitting
73
•AnexampleofcellsplittingisshowninFigure1.
•Thebasestationsareplacedatcornersofthecells&areaserved
bybasestationAisassumedtobesaturatedwithtraffic.
•Newbasestationsareneededintheregiontoincreasethe
numberofchannelsinthearea&toreducetheareaservedby
thesinglebasestation.
•Infigure1,theoriginalbasestationAhasbeensurroundedby
sixnewmicrocells.
•Thesmallercellswereaddedinsuchawaytopreservethe
frequencyreuseplanofthesystem.
•Forexample,themicrocellbasestationlabeledGwasplaced
halfwaybetweentwolargerstationsutilizingthesamechannel
setG.
73
•AnexampleofcellsplittingisshowninFigure1.
•Thebasestationsareplacedatcornersofthecells&areaserved
bybasestationAisassumedtobesaturatedwithtraffic.
•Newbasestationsareneededintheregiontoincreasethe
numberofchannelsinthearea&toreducetheareaservedby
thesinglebasestation.
•Infigure1,theoriginalbasestationAhasbeensurroundedby
sixnewmicrocells.
•Thesmallercellswereaddedinsuchawaytopreservethe
frequencyreuseplanofthesystem.
•Forexample,themicrocellbasestationlabeledGwasplaced
halfwaybetweentwolargerstationsutilizingthesamechannel
setG.
Figure 1 –Cell Splitting
74
•Thisisalsothecasefortheothermicrocellsinthefigure.
•Hencecellsplittingmerelyscalesthegeometryofthecluster.
•Inthiscase,theradiusofeachnewmicrocellishalfthatofthe
originalcell.
74
•Thisisalsothecasefortheothermicrocellsinthefigure.
•Hencecellsplittingmerelyscalesthegeometryofthecluster.
•Inthiscase,theradiusofeachnewmicrocellishalfthatofthe
originalcell.
Cell Splitting-Power Issues
•Supposethecellradiusofnewcellsisreducedbyhalf
•Whatistherequiredtransmitpowerforthesenewcells?
Pr[atoldcellboundary]=Pt1(R)
-n
Pr[atnewcellboundary]=Pt2(R/2)
–n
•wherePt1andPt2arethetransmitpowersofthelargerand
smallercellbasestations&nisthepathlossexponent.
•So, Pt2=Pt1/2
n
•Ifwetaken=3andthereceivedpowersequaltoeachother,then
Pt2=Pt1/8
•Inotherwords,thetransmitpowermustbereducedby9dBin
ordertofillintheoriginalcoverageareawhilemaintainingthe
S/Irequirement.
75
•Supposethecellradiusofnewcellsisreducedbyhalf
•Whatistherequiredtransmitpowerforthesenewcells?
Pr[atoldcellboundary]=Pt1(R)
-n
Pr[atnewcellboundary]=Pt2(R/2)
–n
•wherePt1andPt2arethetransmitpowersofthelargerand
smallercellbasestations&nisthepathlossexponent.
•So, Pt2=Pt1/2
n
•Ifwetaken=3andthereceivedpowersequaltoeachother,then
Pt2=Pt1/8
•Inotherwords,thetransmitpowermustbereducedby9dBin
ordertofillintheoriginalcoverageareawhilemaintainingthe
S/Irequirement.
75
Illustration of cell splitting in 3x3 square centered
around base station A
76
around base station A
76
Overcome –Handoff
77
•Inpracticenotallthecellsaresplitatthesametimehence
differentsizecellswillexistsimultaneously.
•Insuchsituations,specialcareneedstobetakentokeepthe
distancebetweenco-channelcellsatminimum&hencechannel
assignmentsbecomemorecomplicated.
•Toovercomehandoffproblem:
–Channelsintheoldcellmustbebrokendownintotwo
channelgroups-oneforsmallercell&otherforlargercell
–Thelargercellisdedicatedtohighspeedtrafficsothat
handoffsoccurlessfrequently
–Initially,smallpowergrouphaslesschannels&largepower
grouphaslargenoofchannelsbutatmaturityofthesystem
largepowergroupdoesnothaveanychannel
77
•Inpracticenotallthecellsaresplitatthesametimehence
differentsizecellswillexistsimultaneously.
•Insuchsituations,specialcareneedstobetakentokeepthe
distancebetweenco-channelcellsatminimum&hencechannel
assignmentsbecomemorecomplicated.
•Toovercomehandoffproblem:
–Channelsintheoldcellmustbebrokendownintotwo
channelgroups-oneforsmallercell&otherforlargercell
–Thelargercellisdedicatedtohighspeedtrafficsothat
handoffsoccurlessfrequently
–Initially,smallpowergrouphaslesschannels&largepower
grouphaslargenoofchannelsbutatmaturityofthesystem
largepowergroupdoesnothaveanychannel
Topic 10
Sectoring
78
Sectoring
78
Introduction
79
•Cellsplittingachievescapacityimprovementbyrescalingthe
system.
•BydecreasingthecellradiusR&keepingtheco-channelreuse
ratioD/Runchanged,cellsplittingincreasesthenumberof
channelsperunitarea.
•Anotherwaytoincreasecapacityistokeepthecellradius
unchanged&todecreasetheD/Rratio.
•SectoringincreasesSIRsothatclustersizemaybereduced.
•FirsttheSIRisimprovedusingdirectionalantennas&then
capacityimprovementbyreducingthenumberofcellsina
cluster,thusincreasingthefrequencyreuse.
•However,itisnecessarytoreducetherelativeinterference
withoutdecreasingthetransmitpower.
79
•Cellsplittingachievescapacityimprovementbyrescalingthe
system.
•BydecreasingthecellradiusR&keepingtheco-channelreuse
ratioD/Runchanged,cellsplittingincreasesthenumberof
channelsperunitarea.
•Anotherwaytoincreasecapacityistokeepthecellradius
unchanged&todecreasetheD/Rratio.
•SectoringincreasesSIRsothatclustersizemaybereduced.
•FirsttheSIRisimprovedusingdirectionalantennas&then
capacityimprovementbyreducingthenumberofcellsina
cluster,thusincreasingthefrequencyreuse.
•However,itisnecessarytoreducetherelativeinterference
withoutdecreasingthetransmitpower.
Reducing Co-channel Interference
80
•Theco-channelinterferencemaybedecreasedbyreplacinga
singleomnidirectionalantennaatthebasestationbyseveral
directionalantennas.
•Byusingdirectionalantennas,agivencellwillreceive
interference&transmitwithonlyafractionoftheavailableco-
channelcells.
•Thetechniquefordecreasingco-channelinterference&
increasingsystemperformancebyusingdirectionalantennasis
calledsectoring.
•Thefactorbywhichco-channelinterferenceisreduceddepends
ontheamountofsectoringused.
•Acellisnormallypartitionedintothree120°sectorsorsix60°
sectorsasshowninFigure1(a)and1(b).
80
•Theco-channelinterferencemaybedecreasedbyreplacinga
singleomnidirectionalantennaatthebasestationbyseveral
directionalantennas.
•Byusingdirectionalantennas,agivencellwillreceive
interference&transmitwithonlyafractionoftheavailableco-
channelcells.
•Thetechniquefordecreasingco-channelinterference&
increasingsystemperformancebyusingdirectionalantennasis
calledsectoring.
•Thefactorbywhichco-channelinterferenceisreduceddepends
ontheamountofsectoringused.
•Acellisnormallypartitionedintothree120°sectorsorsix60°
sectorsasshowninFigure1(a)and1(b).
Figure 1 -120 & 60 degrees Sectoring
81
81
120 degree Sectoring reducing Interference
82
•InFigure2,considertheinterferenceexperiencedbya
mobilelocatedintheright-mostsectorinthecentercell
labeled“5”.
•Therearethreeco-channelcellsectorslabeled“5”tothe
rightofthecentercell&threetotheleftofthecentercell.
•Outofthesesixco-channelcells,onlytwocellshavesectors
withantennapatternswhichradiateintothecentercell.
•Henceamobileinthecentercellwillexperience
interferenceontheforwardlinkfromonlythesetwo
sectors.
•ThisS/IimprovementallowstodecreasetheclustersizeN
inordertoimprovethefrequencyreuse&thesystem
capacity
82
•InFigure2,considertheinterferenceexperiencedbya
mobilelocatedintheright-mostsectorinthecentercell
labeled“5”.
•Therearethreeco-channelcellsectorslabeled“5”tothe
rightofthecentercell&threetotheleftofthecentercell.
•Outofthesesixco-channelcells,onlytwocellshavesectors
withantennapatternswhichradiateintothecentercell.
•Henceamobileinthecentercellwillexperience
interferenceontheforwardlinkfromonlythesetwo
sectors.
•ThisS/IimprovementallowstodecreasetheclustersizeN
inordertoimprovethefrequencyreuse&thesystem
capacity
Figure 2 -120 degree Sectoring reducing Interference
83
83
Penalty for Improved S/I
84
•ThepenaltyforimprovedS/I&theresultingcapacity
improvementisanincreasednumberofantennasateach
basestation&decreaseintrunkingefficiencydueto
channelsectoringatthebasestation.
•Sincesectoringreducesthecoverageareaofaparticular
groupofchannels,thenumberofhandoffsalsoincreases.
•Becausesectoringusesmorethanoneantennaperbase
station,theavailablechannelsinthecellmustbe
subdivided&dedicatedtoaspecificantenna.
84
•ThepenaltyforimprovedS/I&theresultingcapacity
improvementisanincreasednumberofantennasateach
basestation&decreaseintrunkingefficiencydueto
channelsectoringatthebasestation.
•Sincesectoringreducesthecoverageareaofaparticular
groupofchannels,thenumberofhandoffsalsoincreases.
•Becausesectoringusesmorethanoneantennaperbase
station,theavailablechannelsinthecellmustbe
subdivided&dedicatedtoaspecificantenna.
Topic 11
Repeaters for Range Extension
85
Repeaters for Range Extension
85
Role of Repeaters
86
•Awirelessoperatorneedstoprovidededicatedcoveragefor
hard-to-reachareas,suchaswithinbuildings,orinvalleysor
tunnels.
•Radiore-transmitters,knownasrepeaters,areusedtoprovide
suchrangeextensioncapabilities.
•Repeatersarebidirectionalinnature&simultaneouslysend
signalstoandreceivesignalsfromaservingbasestation.
•Repeatersmaybeinstalledanywhere&capableofrepeatingan
entirecellularband.
•Uponreceivingsignalsfromabasestation,therepeater
amplifiesandreradiatesthebasestationsignalstothespecific
coverageregion.
•Unfortunately,thereceivednoise&interferenceisalso
reradiatedbytherepeateronbothforwardandreverselink.
86
•Awirelessoperatorneedstoprovidededicatedcoveragefor
hard-to-reachareas,suchaswithinbuildings,orinvalleysor
tunnels.
•Radiore-transmitters,knownasrepeaters,areusedtoprovide
suchrangeextensioncapabilities.
•Repeatersarebidirectionalinnature&simultaneouslysend
signalstoandreceivesignalsfromaservingbasestation.
•Repeatersmaybeinstalledanywhere&capableofrepeatingan
entirecellularband.
•Uponreceivingsignalsfromabasestation,therepeater
amplifiesandreradiatesthebasestationsignalstothespecific
coverageregion.
•Unfortunately,thereceivednoise&interferenceisalso
reradiatedbytherepeateronbothforwardandreverselink.
Repeaters Vs System Capacity
87
•Caremustbetakentoproperlyplacetherepeaters&to
adjustthevariousforwardandreverselinkamplifierlevels
andantennapatterns.
•Inpractice,directionalantennasordistributedantenna
systems(DAS)areconnectedtotheinputsoroutputsof
repeatersforspotcoverage,particularlyintunnelsor
buildings.
•Bymodifyingthecoverageofaservingcell,anoperatorcan
dedicateacertainamountofthebasestation’strafficfor
theareascoveredbytherepeater.
•However,therepeaterdoesnotaddcapacitytothesystem.
•Repeatersarebeingusedtoprovidecoverageintoand
aroundbuildings,wherecoverageisweak.
87
•Caremustbetakentoproperlyplacetherepeaters&to
adjustthevariousforwardandreverselinkamplifierlevels
andantennapatterns.
•Inpractice,directionalantennasordistributedantenna
systems(DAS)areconnectedtotheinputsoroutputsof
repeatersforspotcoverage,particularlyintunnelsor
buildings.
•Bymodifyingthecoverageofaservingcell,anoperatorcan
dedicateacertainamountofthebasestation’strafficfor
theareascoveredbytherepeater.
•However,therepeaterdoesnotaddcapacitytothesystem.
•Repeatersarebeingusedtoprovidecoverageintoand
aroundbuildings,wherecoverageisweak.
Location of Repeaters
88
•Determiningtheproperlocationforrepeaters&distributed
antennasystemswithinbuildingsrequirescarefulplanning,
toavoidre-radiationofinterferenceintothebuildingfrom
thebasestation.
•Also,repeatersmustbeprovisionedtomatchtheavailable
capacityfromtheservingbasestation.
•Softwareproducts,suchasSite-Plannerallowengineersto
determinethebestplacementsforrepeaters&requiredDAS
networkwhilesimultaneouslycomputingthetrafficand
installationcost.
88
•Determiningtheproperlocationforrepeaters&distributed
antennasystemswithinbuildingsrequirescarefulplanning,
toavoidre-radiationofinterferenceintothebuildingfrom
thebasestation.
•Also,repeatersmustbeprovisionedtomatchtheavailable
capacityfromtheservingbasestation.
•Softwareproducts,suchasSite-Plannerallowengineersto
determinethebestplacementsforrepeaters&requiredDAS
networkwhilesimultaneouslycomputingthetrafficand
installationcost.
Topic 12
Microcell Zone Concept
89
Microcell Zone Concept
89
Introduction
90
•Theincreasednumberofhandoffsrequiredduringsectoring
resultsinanincreasedloadontheswitchingandcontrollink
elementsofthemobilesystem.
•Asolutiontothisproblemisbasedonamicrocellconceptfor
sevencellreuse,asillustratedinFigure1.
•Inthisscheme,eachofthethreezonesitesrepresentedasTx/Rx
inFigure1areconnectedtoasinglebasestation&sharethe
sameradioequipment.
•Thezonesareconnectedbycoaxialcable,fiber-opticcable,or
microwavelinktothebasestation.
•Multiplezones&asinglebasestationmakeupacell.
•Asamobiletravelswithinthecell,itisservedbythezonewith
thestrongestsignal.
90
•Theincreasednumberofhandoffsrequiredduringsectoring
resultsinanincreasedloadontheswitchingandcontrollink
elementsofthemobilesystem.
•Asolutiontothisproblemisbasedonamicrocellconceptfor
sevencellreuse,asillustratedinFigure1.
•Inthisscheme,eachofthethreezonesitesrepresentedasTx/Rx
inFigure1areconnectedtoasinglebasestation&sharethe
sameradioequipment.
•Thezonesareconnectedbycoaxialcable,fiber-opticcable,or
microwavelinktothebasestation.
•Multiplezones&asinglebasestationmakeupacell.
•Asamobiletravelswithinthecell,itisservedbythezonewith
thestrongestsignal.
Time & Space Distribution
91
•Asamobiletravelsfromonezonetoanotherwithinthecell,it
retainsthesamechannel.
•Unlikeinsectoring,ahandoffisnotrequiredattheMSCwhen
themobiletravelsbetweenzoneswithinthecell.
•Thebasestationsimplyswitchesthechanneltoadifferentzone
site.
•Thusagivenchannelisactiveonlyintheparticularzonein
whichthemobiletraveling&hencethebasestationradiationis
localized&interferenceisreduced.
•Thechannelsaredistributedintime&spacebyallthreezones
&reusedinco-channelcellsinthenormalfashion.
•Thistechniqueisparticularlyusefulalonghighwaysoralong
urbantrafficcorridors.
91
•Asamobiletravelsfromonezonetoanotherwithinthecell,it
retainsthesamechannel.
•Unlikeinsectoring,ahandoffisnotrequiredattheMSCwhen
themobiletravelsbetweenzoneswithinthecell.
•Thebasestationsimplyswitchesthechanneltoadifferentzone
site.
•Thusagivenchannelisactiveonlyintheparticularzonein
whichthemobiletraveling&hencethebasestationradiationis
localized&interferenceisreduced.
•Thechannelsaredistributedintime&spacebyallthreezones
&reusedinco-channelcellsinthenormalfashion.
•Thistechniqueisparticularlyusefulalonghighwaysoralong
urbantrafficcorridors.
Advantage of Zone Cell Technique
•Theadvantageofthiszonecelltechnique-whilethecell
maintainsaparticularcoverageradius,theco-channel
interferenceisreduced.
•Andalargecentralbasestationisreplacedbyseveral
lowerpoweredtransmitters.
•Decreasedco-channelinterferenceimprovesthesignal
quality&increaseincapacitywithoutdegradationin
trunkingefficiencycausedbysectoring.
92
•Theadvantageofthiszonecelltechnique-whilethecell
maintainsaparticularcoverageradius,theco-channel
interferenceisreduced.
•Andalargecentralbasestationisreplacedbyseveral
lowerpoweredtransmitters.
•Decreasedco-channelinterferenceimprovesthesignal
quality&increaseincapacitywithoutdegradationin
trunkingefficiencycausedbysectoring.
92
Figure 1 –The Micro-cell Concept
93
93
Figure 2 –Microcell Architecture with N = 7
94
94
Reduction in Cluster size
•InFigure2,leteachindividualhexagonrepresentsazone,while
eachgroupofthreehexagonsrepresentsacell.
•ThezoneradiusR
zisapproximatelyequaltoonehexagonradius.
•Thecapacityofthezonemicrocellsystemisrelatedtothe
distancebetweenco-channelcells¬zones.Thisdistanceis
representedasD.
•ForaD
z/R
zvalue=4.6,thevalueofco-channelratio-D/R,is
equaltothree,whereRistheradiusofthecell&equaltotwice
thelengthofthehexagonradius.
•D/R=3correspondstoaclustersizeofN=3.
•ThisreductionintheclustersizefromN=7toN=3amountstoa
2.33timesincreaseincapacityforasystembasedonthezone
microcellconcept.
95
•InFigure2,leteachindividualhexagonrepresentsazone,while
eachgroupofthreehexagonsrepresentsacell.
•ThezoneradiusR
zisapproximatelyequaltoonehexagonradius.
•Thecapacityofthezonemicrocellsystemisrelatedtothe
distancebetweenco-channelcells¬zones.Thisdistanceis
representedasD.
•ForaD
z/R
zvalue=4.6,thevalueofco-channelratio-D/R,is
equaltothree,whereRistheradiusofthecell&equaltotwice
thelengthofthehexagonradius.
•D/R=3correspondstoaclustersizeofN=3.
•ThisreductionintheclustersizefromN=7toN=3amountstoa
2.33timesincreaseincapacityforasystembasedonthezone
microcellconcept.
95
Applications
•Intheworstcase,thesystemprovidesamarginof2dB
overtherequiredsignal-to-interferenceratiowhile
increasingthecapacityby2.33timesovera
conventionalseven-cellsystemusingomni-directional
antennas.
•Nolossintrunkingefficiencyisexperienced.
•Zonecellarchitecturesarebeingadoptedinmany
cellular&personalcommunicationsystems.
96
•Intheworstcase,thesystemprovidesamarginof2dB
overtherequiredsignal-to-interferenceratiowhile
increasingthecapacityby2.33timesovera
conventionalseven-cellsystemusingomni-directional
antennas.
•Nolossintrunkingefficiencyisexperienced.
•Zonecellarchitecturesarebeingadoptedinmany
cellular&personalcommunicationsystems.
96
SHORT QUESTION ANSWER
97
97
•1)Whatisfrequencyre-use?
Thedesignprocessofselecting&allocatingchannel
groupsforallthecellularbasestationswithinasystem
iscalledfrequencyreuseorfrequencyplanning.
2)Definecellinawirelessnetwork.
Acellisabasicgeographicalunitofacellularnetwork&
theareaaroundanantennawhereaspecificfrequency
rangeisused.Whenasubscribermovestoanothercell,
theantennaofthenewcelltakesoverthesignal
transmission.
98
Thedesignprocessofselecting&allocatingchannel
groupsforallthecellularbasestationswithinasystem
iscalledfrequencyreuseorfrequencyplanning.
2)Definecellinawirelessnetwork.
Acellisabasicgeographicalunitofacellularnetwork&
theareaaroundanantennawhereaspecificfrequency
rangeisused.Whenasubscribermovestoanothercell,
theantennaofthenewcelltakesoverthesignal
transmission.
98
•3)Mentionthefactorswhichinfluencetheco-channel
interferenceincellularnetworks.
Co-channelinterferenceisdependentonclustersize,
largeclustersizelessinterferenceandviceversa.
4)Whatischannelassignmentstrategy?
Aschemeforincreasingcapacityandminimizing
interferenceisrequiredforeffectiveutilizationofradio
spectrum.CAScanbeclassifiedaseitherfixedor
dynamic.ChoiceofCASimpactstheperformanceof
systemforcallmanagement.
99
interferenceincellularnetworks.
Co-channelinterferenceisdependentonclustersize,
largeclustersizelessinterferenceandviceversa.
4)Whatischannelassignmentstrategy?
Aschemeforincreasingcapacityandminimizing
interferenceisrequiredforeffectiveutilizationofradio
spectrum.CAScanbeclassifiedaseitherfixedor
dynamic.ChoiceofCASimpactstheperformanceof
systemforcallmanagement.
99
•5)Whatiscalledfixedchannelassignmentstrategy?
InFixedCAeachcellisassignedapredeterminedsetof
voicechannels.Anycallattemptwithinthecellcan
onlybeservedbytheunusedchannelinthatparticular
cell.
6)Whatiscalleddynamicchannelassignmentstrategy?
Voicechannelsarenotallocatedtodifferentcells
permanently.MSConlyallocatesagivenfrequency,not
currentlyinuseinthecelloranyothercellwhichfalls
withinthelimitingreusedistance.
100
InFixedCAeachcellisassignedapredeterminedsetof
voicechannels.Anycallattemptwithinthecellcan
onlybeservedbytheunusedchannelinthatparticular
cell.
6)Whatiscalleddynamicchannelassignmentstrategy?
Voicechannelsarenotallocatedtodifferentcells
permanently.MSConlyallocatesagivenfrequency,not
currentlyinuseinthecelloranyothercellwhichfalls
withinthelimitingreusedistance.
100
•7)Mentiontheadvantages&disadvantagesofdynamic
channelassignmentstrategy.
Advantage:Increasedchannelutilizationanddecreased
probabilityofablockedcall.
Disadvantage:Increasesthestorageandcomputational
loadonthesystem.
•8)Whatishand-offoperation?
HandoffoperationinvolvesidentifyingthenewBSand
allocationofvoiceandcontrolsignaltochannels
associatedwithnewBS.Itmustbeperformed
successfully,infrequentlyandimpercitbletouser.
101
channelassignmentstrategy.
Advantage:Increasedchannelutilizationanddecreased
probabilityofablockedcall.
Disadvantage:Increasesthestorageandcomputational
loadonthesystem.
•8)Whatishand-offoperation?
HandoffoperationinvolvesidentifyingthenewBSand
allocationofvoiceandcontrolsignaltochannels
associatedwithnewBS.Itmustbeperformed
successfully,infrequentlyandimpercitbletouser.
101
•9)Whatisthesafetymarginrequiredforhand-off
operation?
•Duringhandoff:toavoidcalltermination,safetymargin
shouldexistandshouldnotbetoolargeorsmall
=Power
handoff–Power
minusable
•Largeresultsinunecesarryhandoffandforsmall
insufficienttimetocompletehandoff,socarefully
chosentomeettherequirements.
102
operation?
•Duringhandoff:toavoidcalltermination,safetymargin
shouldexistandshouldnotbetoolargeorsmall
=Power
handoff–Power
minusable
•Largeresultsinunecesarryhandoffandforsmall
insufficienttimetocompletehandoff,socarefully
chosentomeettherequirements.
102
•10)WhatistheroleofaMSCinacellularnetwork?
•MSCnolongermonitorsRSSofallchannels
–reducescomputationalloadconsiderably
–enablesmuchmorerapidandefficienthandoffs
–imperceptibletouser
•11)Whatissoft-handoffoperation?
Theabilitytoselectbetweentheinstantaneous
receivedsignalsfromdifferentBaseStationsiscalled
softhandoff.
103
•MSCnolongermonitorsRSSofallchannels
–reducescomputationalloadconsiderably
–enablesmuchmorerapidandefficienthandoffs
–imperceptibletouser
•11)Whatissoft-handoffoperation?
Theabilitytoselectbetweentheinstantaneous
receivedsignalsfromdifferentBaseStationsiscalled
softhandoff.
103
•12)Whatisintersystemhand-off?
Ifamobilemovesfromonecellularsystemtoanother
systemcontrolledbyadifferentMSC,aninter-system
handoffisrequired.MSCengagesinintersystemhandoff
whensignalbecomesweakinagivencell.
13)Mentionthepracticalconsiderationsforhand-off
operation.
Problemsoccurbecauseofalargerangeofmobilevelocities
Smallcellsizesormicro-cells→largerhandoffs
MSCloadisheavywhenhighspeedusersarepassed
betweenverysmallcells
UmbrellaCells
104
Ifamobilemovesfromonecellularsystemtoanother
systemcontrolledbyadifferentMSC,aninter-system
handoffisrequired.MSCengagesinintersystemhandoff
whensignalbecomesweakinagivencell.
13)Mentionthepracticalconsiderationsforhand-off
operation.
Problemsoccurbecauseofalargerangeofmobilevelocities
Smallcellsizesormicro-cells→largerhandoffs
MSCloadisheavywhenhighspeedusersarepassed
betweenverysmallcells
UmbrellaCells
104
•14)WhatarecalledUmbrellaCells?
Cellswhicharemeanttousedifferentantennaheights&
Transmitterpowerlevelstoprovidelargeandsmallarea
coveragearecalledUmbrellacells.
•15)Mentionthesourcesofinterferenceinacellular
network.
Sourcesofinterferenceinclude-
anothermobileinthesamecell
acallinprogressinaneighboringcell
otherbasestationsoperatinginsamefrequencyband
anynon-cellularsystemwhichleaksenergyintoband
105
Cellswhicharemeanttousedifferentantennaheights&
Transmitterpowerlevelstoprovidelargeandsmallarea
coveragearecalledUmbrellacells.
•15)Mentionthesourcesofinterferenceinacellular
network.
Sourcesofinterferenceinclude-
anothermobileinthesamecell
acallinprogressinaneighboringcell
otherbasestationsoperatinginsamefrequencyband
anynon-cellularsystemwhichleaksenergyintoband
105
•16)Mentionthemajortypesofcellularinterferences.
•ThetwomajortypesofcellularinterferenceareCo-
channelinterference(CCI)&Adjacentchannel
interference(ACI).
•17)DifferentiatebetweenCCIandACI.
•CCIiscausedduetothecellsthatreusethesame
frequencyset.Thesecellsusingthesamefrequencyset
arecalledCo-channelcells.
•ACIiscausedduetothesignalsthatareadjacentin
frequency.
106
•ThetwomajortypesofcellularinterferenceareCo-
channelinterference(CCI)&Adjacentchannel
interference(ACI).
•17)DifferentiatebetweenCCIandACI.
•CCIiscausedduetothecellsthatreusethesame
frequencyset.Thesecellsusingthesamefrequencyset
arecalledCo-channelcells.
•ACIiscausedduetothesignalsthatareadjacentin
frequency.
106
•18)Whatareco-channelcells?
•Frequencyreuseindicatesthatinagivencoveragearea
thereareseveralcellsusingthesamesetof
frequencies.Thesecellsarecalledco-channelcells
•19)Whatiscallednear-fareffect?
•Ifanadjacentchanneluseristransmittinginveryclose
rangetoasubscriber’sreceiver,whilethereceiver
attemptstoreceiveabasestationonthedesired
channel,thenitsiscalledasthenear–fareffect
107
•Frequencyreuseindicatesthatinagivencoveragearea
thereareseveralcellsusingthesamesetof
frequencies.Thesecellsarecalledco-channelcells
•19)Whatiscallednear-fareffect?
•Ifanadjacentchanneluseristransmittinginveryclose
rangetoasubscriber’sreceiver,whilethereceiver
attemptstoreceiveabasestationonthedesired
channel,thenitsiscalledasthenear–fareffect
107
•20)Whatistrunking?
Atechniquewhichallowsalargenoofuserstosharea
smallnumberofchannelsinacellbyprovidingaccess
toeachuser,ondemand,fromapoolofavailable
channels.
•21)DefineErlang.
AnErlangisaunitoftelecommunicationstraffic
measurement.Erlangrepresentsthecontinuoususeof
onevoicepath.Itisusedtodescribethetotaltraffic
volumeofonehour
108
Atechniquewhichallowsalargenoofuserstosharea
smallnumberofchannelsinacellbyprovidingaccess
toeachuser,ondemand,fromapoolofavailable
channels.
•21)DefineErlang.
AnErlangisaunitoftelecommunicationstraffic
measurement.Erlangrepresentsthecontinuoususeof
onevoicepath.Itisusedtodescribethetotaltraffic
volumeofonehour
108
•22)Whatisgradeofservice(GOS)?
•Thelikelihoodthatacallisblocked,orthelikelihood
thatacallexperiencesadelaygreaterthanacertain
queuingtimeiscalled“GradeofService”(GOS)’’.
•23)WhatisCellsplitting?
•Cellsplittingistheprocessofsubdividingacongested
cellintosmallercellswith-
•theirownBS
•acorrespondingreductioninantennaheight
•acorrespondingreductionintransmitpower
109
•Thelikelihoodthatacallisblocked,orthelikelihood
thatacallexperiencesadelaygreaterthanacertain
queuingtimeiscalled“GradeofService”(GOS)’’.
•23)WhatisCellsplitting?
•Cellsplittingistheprocessofsubdividingacongested
cellintosmallercellswith-
•theirownBS
•acorrespondingreductioninantennaheight
•acorrespondingreductionintransmitpower
109
•24)WhatiscalledSectoring?
•Thetechniquefordecreasingco-channelinterference&
increasingsystemperformancebyusingdirectional
antennasinacellularnetworkiscalledsectoring.
•25)Whatismicrocellconcept?
•Basestationsaredecomposedintothreeormorezones
connectedbycoaxialcableorfiber-opticcabletothe
basestation.
•Multiplezones&asinglebasestationmakeupacell.
•Themobileisservedbythezonewiththestrongest
signalwithinthecell.
110
•Thetechniquefordecreasingco-channelinterference&
increasingsystemperformancebyusingdirectional
antennasinacellularnetworkiscalledsectoring.
•25)Whatismicrocellconcept?
•Basestationsaredecomposedintothreeormorezones
connectedbycoaxialcableorfiber-opticcabletothe
basestation.
•Multiplezones&asinglebasestationmakeupacell.
•Themobileisservedbythezonewiththestrongest
signalwithinthecell.
110
111
Applications:
•Transmission of music, news, road conditions, weather reports, and
otherbroadcast information are received via digital audio
broadcasting (DAB) with 1.5Mbit/s.
•Forpersonal communication, a universal mobile telecommunications
system (UMTS) phone might be available offering voice and data
connectivity with 384kbit/s.
•Forremote areas,satellite communication can be used, while the
current position of the car is determined via the GPS (Global Positioning
System).
•A local ad-hoc network for the fastexchange of
information(information such as distance between two vehicles, traffic
information, road conditions) in emergency situations or to help each
other keep a safe distance. Local ad-hoc network with vehicles close by
to prevent guidance system, accidents, redundancy.
Vehicles -Applications
Applications:
•Transmission of music, news, road conditions, weather reports, and
otherbroadcast information are received via digital audio
broadcasting (DAB) with 1.5Mbit/s.
•Forpersonal communication, a universal mobile telecommunications
system (UMTS) phone might be available offering voice and data
connectivity with 384kbit/s.
•Forremote areas,satellite communication can be used, while the
current position of the car is determined via the GPS (Global Positioning
System).
•A local ad-hoc network for the fastexchange of
information(information such as distance between two vehicles, traffic
information, road conditions) in emergency situations or to help each
other keep a safe distance. Local ad-hoc network with vehicles close by
to prevent guidance system, accidents, redundancy.
Vehicles -Applications
112
•Vehicle data from buses, trucks, trains and high speed train can be
transmitted in advance formaintenance.
•In ad-hoc network, car can comprise personal digital assistants
(PDA), laptops, or mobile phones connected with each other using
the Bluetooth technology.
Applications:
•Vehicle data from buses, trucks, trains and high speed train can be
transmitted in advance formaintenance.
•In ad-hoc network, car can comprise personal digital assistants
(PDA), laptops, or mobile phones connected with each other using
the Bluetooth technology.
Applications:
113
Emergency-Applications
Applications:
•Video communication: Responders often need to share vital
information. The transmission of real time situations of video could be
necessary. A typical scenario includes the transmission of live video footage
from a disaster area to the nearest fire department, to the police station or
to the near NGOs etc.
•Push To Talk (PTT):PTT is a technology which allows half duplex
communication between two users where switching from voice reception
mode to the transmit mode takes place with the use of a dedicated
momentary button. It is similar to walkie-talkie.
•Audio/Voice Communication: This communication service provides full
duplex audio channels unlike PTT. Public safety communication requires
novel full duplex speech transmission services for emergency response.
•Real Time Text Messaging (RTT): Text messaging (RTT) is an effective
and quick solution for sending alerts in case of emergencies. Types of text
messaging can be email, SMS and instant message.
Emergency-Applications
Applications:
•Video communication: Responders often need to share vital
information. The transmission of real time situations of video could be
necessary. A typical scenario includes the transmission of live video footage
from a disaster area to the nearest fire department, to the police station or
to the near NGOs etc.
•Push To Talk (PTT):PTT is a technology which allows half duplex
communication between two users where switching from voice reception
mode to the transmit mode takes place with the use of a dedicated
momentary button. It is similar to walkie-talkie.
•Audio/Voice Communication: This communication service provides full
duplex audio channels unlike PTT. Public safety communication requires
novel full duplex speech transmission services for emergency response.
•Real Time Text Messaging (RTT): Text messaging (RTT) is an effective
and quick solution for sending alerts in case of emergencies. Types of text
messaging can be email, SMS and instant message.
114
Business-Applications:
Travelling Salesman
•Directly access to customer files stored in a central location.
•Consistent databases for all agents
•Mobile office
•To enable the company to keep track of all the activities of their
travelling employees.
In Office
Wi-Fiwireless technology saves businesses or companies a considerable
amount of money on installations costs.
There is no need to physically setup wires throughout an office building,
warehouse or store.
Bluetoothis also a wireless technology especially used for short range that acts
as a complement to Wi-Fi. It is used to transfer data between computers or
cellphones.
Transportation Industries
In transportation industries, GPS technology is used to find efficient routes and
tracking vehicles.
Applications:
Business-Applications:
Travelling Salesman
•Directly access to customer files stored in a central location.
•Consistent databases for all agents
•Mobile office
•To enable the company to keep track of all the activities of their
travelling employees.
In Office
Wi-Fiwireless technology saves businesses or companies a considerable
amount of money on installations costs.
There is no need to physically setup wires throughout an office building,
warehouse or store.
Bluetoothis also a wireless technology especially used for short range that acts
as a complement to Wi-Fi. It is used to transfer data between computers or
cellphones.
Transportation Industries
In transportation industries, GPS technology is used to find efficient routes and
tracking vehicles.
Applications:
115
MCQ-post-test
1.The process of transferring a mobile station fromone base station to another is
a. MSC
b. Roamer
c. Hand off
d. Forward channel
ANSWER: Hand off
2. Interference in cellular systems is caused by
a. Two base stations operating in same frequency band
b. Two calls in progress in nearby mobile stations
c. Leakage of energy signals by non cellular systems
into cellular frequency band
d. All of the above
ANSWER: All of the above
MCQ-post-test
1.The process of transferring a mobile station fromone base station to another is
a. MSC
b. Roamer
c. Hand off
d. Forward channel
ANSWER: Hand off
2. Interference in cellular systems is caused by
a. Two base stations operating in same frequency band
b. Two calls in progress in nearby mobile stations
c. Leakage of energy signals by non cellular systems
into cellular frequency band
d. All of the above
ANSWER: All of the above
116
3. Interference in frequency bands may lead to
a. Cross talk
b. Missed calls
c. Blocked calls
d. All of the above
ANSWER: All of the above
4. Advantage of using Spread Spectrum modulationis/are
1. Interference rejection capability
2. Frequency planning is not required
3. Resistance to multipath fading
4. ISI is lesser
a. 1 and 2 are correct
b. 1, 2 and 3 are correct
c. 2 and 3 are correct
d. All the four are correct
ANSWER: All the four are correct
MCQ-post-test
3. Interference in frequency bands may lead to
a. Cross talk
b. Missed calls
c. Blocked calls
d. All of the above
ANSWER: All of the above
4. Advantage of using Spread Spectrum modulationis/are
1. Interference rejection capability
2. Frequency planning is not required
3. Resistance to multipath fading
4. ISI is lesser
a. 1 and 2 are correct
b. 1, 2 and 3 are correct
c. 2 and 3 are correct
d. All the four are correct
ANSWER: All the four are correct
MCQ-post-test
117
5. Guard band is
a. The small unused bandwidth between the frequencychannels to avoid
interference
b. The bandwidth allotted to the signal
c. The channel spectrum
d. The spectrum acquired by the noise between theSignal
ANSWER: The small unused bandwidth between thefrequency channels to avoid
interference
6. Radio capacity may be increased in cellularconcept by
a. Increase in radio spectrum
b. Increasing the number of base stations & reusing the
channels
c. Both a & b
d. None of the above
ANSWER: Increasing the number of base stations &reusing the channels
MCQ-post-test
5. Guard band is
a. The small unused bandwidth between the frequencychannels to avoid
interference
b. The bandwidth allotted to the signal
c. The channel spectrum
d. The spectrum acquired by the noise between theSignal
ANSWER: The small unused bandwidth between thefrequency channels to avoid
interference
6. Radio capacity may be increased in cellularconcept by
a. Increase in radio spectrum
b. Increasing the number of base stations & reusing the
channels
c. Both a & b
d. None of the above
ANSWER: Increasing the number of base stations &reusing the channels
MCQ-post-test
118
7. The techniques used to improve the capacity ofcellular systems are
a. Splitting
b. Sectoring
c. Coverage zone approach
d. All of the above
ANSWER: All of the above
8. Trunking in a cellular network refers to
a. Termination of a call
b. Spectrum unavailability
c. Accommodating large number of users in limitedspectrum
d. All of the above
ANSWER: Accommodating large number of users inlimited spectrum
MCQ-post-test
7. The techniques used to improve the capacity ofcellular systems are
a. Splitting
b. Sectoring
c. Coverage zone approach
d. All of the above
ANSWER: All of the above
8. Trunking in a cellular network refers to
a. Termination of a call
b. Spectrum unavailability
c. Accommodating large number of users in limitedspectrum
d. All of the above
ANSWER: Accommodating large number of users inlimited spectrum
MCQ-post-test
119
9. When all of the radio channels are in use in atrunking system
a. The user is blocked
b. The access to the system is denied
c. The queue may be provided
d. All of the above
ANSWER: All of the above
10.Grade of service refers to
a. Accommodating large number of users in limitedspectrum
b. Ability of a user to access trunked system duringbusy hour
c. Two calls in progress in nearby mobile stations
d. High speed users with large coverage area
ANSWER: Ability of a user to access trunked systemduring busy hour
MCQ-post-test
9. When all of the radio channels are in use in atrunking system
a. The user is blocked
b. The access to the system is denied
c. The queue may be provided
d. All of the above
ANSWER: All of the above
10.Grade of service refers to
a. Accommodating large number of users in limitedspectrum
b. Ability of a user to access trunked system duringbusy hour
c. Two calls in progress in nearby mobile stations
d. High speed users with large coverage area
ANSWER: Ability of a user to access trunked systemduring busy hour
MCQ-post-test
120
Conclusion
•To make students familiar with various
generations of mobile communications
•To understand the concept of cellular
communication
•To understand the basics of wireless
communication
Conclusion
•To make students familiar with various
generations of mobile communications
•To understand the concept of cellular
communication
•To understand the basics of wireless
communication
121
References
1.“Mobile Communications” by Jochen Schiler
2.“Mobile Communications Engineering: Theory and
Applications” by Willim C Y Lee
3.“Introduction to Wireless and Mobile Systems” by Dharma
Prakash Agarwal and Qing An Zeng
4.“Electronic Communications Systems Fundamentals Through
Advanced” by Wayne Tomasi
5.“Mobile Communication Design Fundamentals ” by William C
Y Lee
6.“Principles of Mobile Communication” by B Stuber Gordon L
References
1.“Mobile Communications” by Jochen Schiler
2.“Mobile Communications Engineering: Theory and
Applications” by Willim C Y Lee
3.“Introduction to Wireless and Mobile Systems” by Dharma
Prakash Agarwal and Qing An Zeng
4.“Electronic Communications Systems Fundamentals Through
Advanced” by Wayne Tomasi
5.“Mobile Communication Design Fundamentals ” by William C
Y Lee
6.“Principles of Mobile Communication” by B Stuber Gordon L
Assignment:
•1)Explaintheconceptoffrequencyre-useandthe
channelassignmentstrategiesindetail.
•2)Discussindetail,thehand-offstrategiesimplemented
inacellularnetwork.
•3)Explaintheco-channelinterferenceandadjacent
channelinterferenceindetail.
•4)Describeabouttrunkingandgradeofserviceina
cellularnetwork.
•5)Writeshortnoteson–(i)CellSplitting(ii)Sectoring.
•6)Whatistheconceptofmicrocellzone?Explainitwith
anillustrationindetail.
122
•1)Explaintheconceptoffrequencyre-useandthe
channelassignmentstrategiesindetail.
•2)Discussindetail,thehand-offstrategiesimplemented
inacellularnetwork.
•3)Explaintheco-channelinterferenceandadjacent
channelinterferenceindetail.
•4)Describeabouttrunkingandgradeofserviceina
cellularnetwork.
•5)Writeshortnoteson–(i)CellSplitting(ii)Sectoring.
•6)Whatistheconceptofmicrocellzone?Explainitwith
anillustrationindetail.
122
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