Achieving optimum system capacity with a limited frequency spectrum is one of the main research issues in cellular communications

Introduction
Achieving optimum system capacity with a limited frequency
spectrum is one of the main research issues in cellular
communications.
In a cellular system, frequency management and channel
assignment are essential in order to achieve the basic objectives
of spectrum utilization as well as adaptability to traffic density.
In this unit, the various aspects leading to an efficient and
effective frequency planning of cellular systems are discussed in
detail.
Depending upon the system parameters, the allocated frequency
spectrum is divided into a number of frequency channels.
These available frequency channels are then divided into the
subsets that can be assigned to each cell.
• Different strategies are followed for the assignment of these
channel sets to cells.
• Fixed channel assignment (FCA) technique and dynamic
channel allocation techniques are covered in detail.
• Frequency management includes operations such as
designation of set-up and voice channels, numbering the
channels, and grouping voice channels into subsets.
• The main objective of channel-assignment is to stabilize the
fluctuations in the probability of call blockage over the entire
coverage area of a cellular network over a period of time.
• Thechannel assignment does the allocation of specific channels
to cell sites and mobile units. It can be done in two ways:
• Short-term assignment, where one channel assignment per call
is handled by mobile telephone switching office (MTSO).
• Long-term assignment, where a fixed channel set consisting of
one or more subsets are assigned to cell site on a long-term
basis.
This unit introduces numbering of the radio channels, traffic and
channel assignment, non-FCA.

1. Unit-5
Frequency Management and Channel
Assignment

2. Achievingoptimumsystemcapacitywithalimitedfrequency
spectrumisoneofthemainresearchissuesincellular
communications.
Inacellularsystem,frequencymanagementandchannel
assignmentareessentialinordertoachievethebasicobjectives
ofspectrumutilizationaswellasadaptabilitytotrafficdensity.
Inthisunit,thevariousaspectsleadingtoanefficientand
effectivefrequencyplanningofcellularsystemsarediscussedin
detail.
Dependinguponthesystemparameters,theallocatedfrequency
spectrumisdividedintoanumberoffrequencychannels.
Theseavailablefrequencychannelsarethendividedintothe
subsetsthatcanbeassignedtoeachcell.
Introduction

3. •Differentstrategiesarefollowedfortheassignmentofthese
channelsetstocells.
•Fixedchannelassignment(FCA)techniqueanddynamic
channelallocationtechniquesarecoveredindetail.
•Frequencymanagementincludesoperationssuchas
designationofset-upandvoicechannels,numberingthe
channels,andgroupingvoicechannelsintosubsets.
•Themainobjectiveofchannel-assignmentistostabilizethe
fluctuationsintheprobabilityofcallblockageovertheentire
coverageareaofacellularnetworkoveraperiodoftime.
•Thechannelassignmentdoestheallocationofspecificchannels
tocellsitesandmobileunits.Itcanbedoneintwoways:

4. •Short-termassignment,whereonechannelassignmentpercall
ishandledbymobiletelephoneswitchingoffice(MTSO).
•Long-termassignment,whereafixedchannelsetconsistingof
oneormoresubsetsareassignedtocellsiteonalong-term
basis.
Thisunitintroducesnumberingoftheradiochannels,trafficand
channelassignment,non-FCA.
Numberingtheradiochannels
•Manycellularmobilesystemsoperateon666channels.Each
channelconsistsoftwofrequencychannelbandwidths(mobile
transmit/uplinkorreversechannelandcell-sitetransmit/downlink
orforwardchannel)toallowduplexoperation.
Thesetwochannelbandwidthsmustbeseparatedinfrequencyin
ordertoavoidinterference.

5. •Thefrequencyseparationbetweentheuplinkanddownlink
channelsistermedaschannelspacing(or)duplexspacing.
•Inthepresent800MHzbandcellularsystem,theseparation
betweenthemobiletransmitandthecell-sitetransmitisspecified
as45MHz.
•Thesub-sectionsthatfollowdescribethenumberingofradio
channelsbydiscussingthefrequencymanagementchartand
groupingofchannelsintosubsets.
(1)Frequencymanagementchart
•Thetotalchannelsavailableare832innumber.However,most
mobileunitsandsystemsarestilloperatingon666channels.
•Figureshowsthearrangementof666frequencychannelsin
blockAandblockBsystems,eachcontaining333channels.

7. •Outofthese333availablechannelsineachsystem,312
channelsareusedforvoicecommunicationand21channelsare
usedforcontrollingthesystem.
•These21channelsarecalledascontrolchannelsorset-up
channels.Therefore,atotalof42channelsareusedfor
controllingthesystem.
•Inchannel1,thetwofrequenciesavailableformobileandcell-
sitetransmitare
1.825.030MHz(mobiletransmit)
2.870.030MHz(cell-sitetransmit)
•Inchannel666,thetwofrequenciesavailableformobileandcell-
sitetransmitare
1.844.98MHz(mobiletransmit)
2.889.98MHz(cell-sitetransmit)

8. •Eachmarket(i.e.eachcity)hastwosystemsforaduopoly
marketpolicywitheachblockhaving333channels.
•The42set-upchannelsalsocalledascontrolchannelsetsare
assignedasfollows:
1.Channels313–333inblockA
2.Channels334–354inblockB
•Thevoicechannelsareassignedasfollows:
1.Channels1–312(312voicechannels)inblockA
2.Channels355–666(312voicechannels)inblockB
•The42set-upchannels(controlchannelsets)areassignedin
themiddleofalltheassignedchannelstofacilitatescanningof
thosechannelsbyfrequencysynthesizers.
•Inthenewadditionalspectrumallocationof10MHz,an
additional166channelsareassigned.

9. •Sinceafirstchannelisassignedbelow825MHz(or870
MHz),inthefuture,additionalchannelswillbenumberedup
to849MHz(or894MHz)andwillthencircleback.
•Thelastchannelnumberis1023.Therearenochannels
between799and991channels.Newadditionalspectrum
allocationsareshowninFigure

11. (2)Groupingintosubsets
•Sincethereare21set-upchannelsforeachblock,itislogical
togroup312voicechannelsinto21subsetsof15voice
channelseach(thelastsubgrouphas12voicechannels
only).
•Eachsubsetthenconsistsof16channels––15voice
channelsandonecontrolchannel.
•Ineachset,theclosestadjacentchannelis21channels
awayasshowninFigure.
•Thechannelseparationisprovidedinsuchawaythatitis
sufficienttomeettheadjacentchannelisolationrequirement.
•The16channelsineachsubsetcanbemountedonaframe
andconnectedtoachannelcombiner.

12. •Wideseparationbetweenadjacentchannelsisrequiredformeeting
therequirementofminimumisolation.
Set-upchannels
•Theset-upchannelsarealsocalledas controlchannels.
•Theyaredesignatedtoset-upcallsinthesystem.Butevenwithout
set-upchannels,asystemcouldworkwhereallthechannelsarein
eitherblockAorblockBwhichwillbeusedasvoicechannels.
•Ifafrequencyreusetechniqueisappliedtoacellularsystem,the
set-upchannelsactascontrolchannels.
•Theset-upchannelsareclassifiedwithrespecttotheirapplication.
Theyare
•accesschannels
•pagingchannels

13. Access channels
•Accesschannelsareusedforcallsoriginatingfrommobile.Whena
mobilesetscansallthe21set-upchannels(inblockA),two
conditionsareconsidered:
•Ifnoset-upchannelsareoperationalinblockA,thenthe
mobileunitswitchesautomaticallytoblockB.
•Ifthereisastrongset-upchannelwithnomessagedetected
thenwithinthesecondsetup,itwillbeselectedbythescanner.Paging channels
•Pagingchannelsareusedforcallsoriginatingfromland.Everycell
siteisassigneditsowncontrolorset-upchannels.Forexample,
FOCCistheforwardset-upchannelinwhicheverycellsiteare
mainlyusedtopagethemobileunitwithcontrolmessageofsame
mobilestation.
•Thesamemessageistransmittedbydifferentset-upchannels
andthereisnosimulcastinterference.
•Abetteralgorithmisusedtopagefromallthecellsites.

14. Traffic and channel assignment
•Thevehiculartrafficdensityofacoverageareaisacriticalelement
andmustbedeterminedbeforeasystemisdesigned.
•Thistrafficpatterninbusyhourscanbeconfinedtodifferentzones
withintheservicearea.
•Ifthetrafficpatternpredominatesoverthesimplesignalcoverage
pattern,thenthecell-siteselectionwillbebasedonthetraffic
pattern.
•Choiceoftheinitialcellsitesshouldbebasedonthesignalcovered
inzonesofheavyvehiculartraffic.
•Thismeansthatthecellsitewouldmostlikelybelocatedatthe
centreofthosezones.
•Ifcalltrafficdataarecollectedwhilethesystemisoperating,then
wecanupdatethecalltrafficdataateachcellsitetocorrelatewith
thevehiculartrafficdata.

15. •Thisinformationwillbeusefulfordeterminingwhethernewcell
splittingisneeded.Ifitis,thenwemustdeterminehowmanyradios
shouldbeinstalledatthenewsiteandwhereitistobelocated.
•These decisions are all related to frequency channel assignment
(1)Fixedchannelassignment
•InFCA,eachcellassignsitsownfrequencychanneltothemobile
subscriberswithinitscell.
•Channelassignmentisprimarilybasedoncausingleastco-channel
andadjacentchannelinterferenceinthecellularsystem.
•ThechannelassignmentforeachvoicecallisdeterminedbyMTSO
onashort-termbasis.
•InaFCA,theset-upandvoicechannelsareusuallyassignedtothe
cellsiteforrelativelylongperiods.Channelsinachannelsetare
usually21channelsapartandmustmeetminimumfrequency
spacingrequirementsofamulti-channeltransmittercombiner.

16. •Channelsareusuallynumberedinorderofincreasing
frequency.Regardlessofthenumberofchannelsinachannel
set,thehighestchannelsetisfrequencyadjacenttothelowest
channelset.
ThefollowingaretheadvantagesofFCA:
•Fixedparameters(power,frequency)fortransceivers.
•Goodperformanceunderuniform-and/orhigh-trafficloadsas
cellsindependentlydecidetheirchannelallocationdecisions.
•Ifeachcellisallocatedtoapre-determinedsetofvoice
channelsthenthecallisblockedandallthechannelsare
occupied.
•Borrowingstrategy:Acellisallowedtoborrowchannelsfroma
neighbouringcellifallofitsownchannelsareoccupied.
•Mobileswitchingcentre(MSC)supervisestheborrowing
proceduretoensurenodisruptingcallsorinterferencewithany
ofthecallsinprogressinthedonorcell.

17. (2)Dynamicchannelassignment
•Indynamicchannelassignment(DCA),thecentralcommon
poolmaintainsalltheavailablechannels.
•Channelsareassigneddynamicallyasnewrequestsforradio
resource(forafreshoriginatingcallorhandoffofexistingcall)
arriveinthesystem.
•Thisalsoimpliesthatwhentheuseofassignedchannelis
completed,thechannelcurrentlyinuseisreturnedtothe
centralpool.
•Inordertoachieveoptimumsystemcapacitywithlimited
frequencyspectrum,manyDCAschemeshavebeen
proposedtoallocatethechannelsmoreefficiently.
•Inacellularsystem,amobilesubscribermovesfromonecell
toanotherandcontinuationofcommunicationlinkisensured
withsuitablehandoffmechanism.

18. •Thisdemandsforadditionalandflexibleradioresourcesutilization.
•Onewaytoascertainminimumblockingprobabilityistoincrease
thenumberofchannelspercell.
•Theneverycellwouldexpecttohavealargenumberofchannels.
•However,becausealimitedfrequencybandisallocatedforcellular
communication,thereisanupperlimittothemaximumnumberof
channels,therebyrestrictingthenumberofavailablechannelsthat
canbeassignedtoeachcell.
•Anotherwayisnon-uniformFCAbasedontheamountoftraffic
expectedtobeservedindifferentcellsasperthestatisticaltraffic
data.
•Anotheralternativearrangementcouldbedynamicassignmentof
channelstodifferentcells,asperthe
currentdemand.

19. Thismaybedonefromacentralpoolofchannels,oracombinationof
bothFCAandDCA.
ThefollowingaretheadvantagesofDCA:
•Nofixedchannelsareassignedtoeachcell.
•Outoftheavailablechannels,anychannelcanbeassignedtoany
cellonneedbasis.
•Theservingbasestation(BS)requestsachannelfromtheMSC
wheneveracallrequestismade.
•Considerationofthelikelihoodoffutureblockinginthecell,the
frequencyuseofthecandidatecell,thereusedistanceofthe
channel,andothercostfunctions.
•MSCneedstocollectreal-timedataonchanneloccupancy,traffic
distribution,andreceivedsignalstrengthindicator(RSSI)ofall
channelsonacontinuousbasis,whichincreasesstorageand
computationalloadonthesystem.

20. (3)Channelsharingscheme
•Whenaparticularcellneedsmorechannelsinordertomeetthe
increasedtrafficdemand,thechannelsofanothersectoratthe
samecellsitecanbesharedtomeettheshort-termoverloadtraffic.
•Channelsharingcanbedonefromoneofthetwoadjacentsectors
oftheneighbouringcellsinasectoredcellularsystemconfiguration.
•Sharedchannelscanbereturnedbackwhenthechannelsbecome
availableinthesharedsector.
•Thisschemeiscalledthe orderedchannelassignmentscheme
withrearrangement.
•Analternateschemeischannelassignmentwithsharingand
reassignment.Thisschememakessurethatchannel-sharing
arrangementcausesminimumimpactoncall-blockingprobabilityin
neighbouringcells.

21. •Reassignmentofsharedchannelsisdoneinordertoprovidemaximum
assistancetotheneighbouringcellstomeetthetemporaryincreasedtraffic
demand.
•Thechannelcanalsobeorderedbasedonwhichchannelsprovidebetter
performance.
•Channelsharingalwaysincreasesthetrunkingefficiencyofthechannels.
Channel-borrowingscheme
•Thechannel-borrowingschemeisusedforslowgrowingsystemsonalong-
termbasisasanalternatetothecostlycell-splittingtechniquetohandle
increasedtraffic.
•Oneapproachtoaddressincreasedtrafficofeithermobileoriginatingcallsor
handoffcallsinacellistoborrowfreeavailablechannelsfromneighbouring
cells.
•Asimplechannel-borrowingschemeimpliesthatifallchannelsassignedtoa
cellhavealreadybeenused,thenadditionalchannelsasperthecurrentneed
canbeborrowedfromanyothercell(preferablyadjacentcells)thathassome
freeunusedchannels.
•Inaddition,thecentralcellsitecanalsoborrowchannelsfromneighbouring
cells.Theextentofborrowingchannelsdependsonthetrafficdensityinthe
area.

23. Channel assignment algorithms
•Thereareseveralalgorithmsavailablefornon-FCA.Theyarelisted
below:
•Fixedchannelalgorithm(FCA):Thisalgorithmisthemostcommonly
adoptedinmanycellularsystems.Here,eachcellassignsitsownradio
channelstothevehicleswithinitscell.
•Dynamicchannelalgorithm(DCA):Herenofixedchannelsare
assignedtoeachcell.Therefore,anychannelinacompositeof312
radiochannelscanbeassignedtothemobileunit.
Thismeansthatachannelisassigneddirectlytoamobileunit.Onthe
basisofoverallsystemperformance,theDCAcanalsobeusedduringa
call.
•Hybridchannelalgorithm(HCA):ThisisacombinationofFCAand
DCA.AportionofthetotalfrequencychannelswilluseFCAandtherest
willuseDCA.
•Borrowingchannelalgorithm(BCA):ItusesFCAasanormal
assignmentcondition.Whenallfixedchannelsareoccupied,thenthe
cellborrowschannelsfromtheneighbouringcells.

24. •
Forcible-borrowingchannelalgorithm(FBCA):Inthiscase,ifa
channelisinoperationandthesituationwarrantsit,then
channelsmustbeborrowedfromtheneighbouringcellsandat
thesametimeanothervoicechannelwillbeassignedto
continuethecallintheneighbouringcell.
“Channelcannotbeborrowedfrequentlyfromadjacent
cells”.
There are two types of blocking possible in FBCA algorithm.
Average blocking: It happens mostly in non-uniform traffic.
Handoff blocking: It happens mostly in uniform traffic.
• Queuing of handoff calls can increase traffic capacity.

25. Channel assignment to travelling mobile units
Thissituationalwaysoccursinthemorning,whencarstravel
intothecity,andatnight,whenthetrafficpatternreverses.
Ifthetrafficdensityisuniform,theunsymmetricalmobile-unit
antennapattern(assuminglargebackwardenergyfromthe
motionofthevehicle)doesnotaffectthesystemoperation
much.
However,whenthetrafficbecomesheavierasmorecars
approachthecity,thetrafficpatternbecomesnonuniformand
thesitesclosesttothecity,orinthecity,cannotreceivethe
expectednumberofcallsorhandoffsinthemorningbecauseof
themobileunitantennapatterns.
Atnight,asthecarsmoveoutofthecity,thecellsitesclosestto
thecitywouldhaveahardtimehandingoffcallstothesites
awayfromthecity.

26. To solve these problems, we have to use less transmitted power
for both set-up and voice channels for certain cell sites.
Wealsohavetoraisethethresholdlevelforreverseset-up
channelsandvoicechannelsatcertaincellsitesinorderto
controltheacceptanceofincomingcallsandhandoffcalls.
Threemethodscanbeused.
•Underlay-overlay
•Frequency assignment
•Tilted antenna

28. Underlay-overlay
•Thetrafficcapacityatanomnidirectionalcelloradirectionalcell
(seeFigure)canbeincreasedbyusingthe underlay-overlay
arrangement.
•Theunderlayistheinnercircle,andtheoverlayistheouterring.
•Thetransmittedpowersofthevoicechannelsatthesiteare
adjustedforthesetwoareas.Thendifferentvoicefrequenciesare
assignedtoeacharea.
•Inanomnidirectionalcell,thefrequency-reusedistanceofa
seven-cellreusepatternisD=4.6R,whereRistheradiusofthe
cell.OneoverlayandoneunderlayareshowninFigure(a).

29. Reuse partition scheme

30. •Becauseofthesectorizationinadirectionalcell,thechannel
assignmenthasadifferentalgorithminsixregions(Figureb),
i.e.,threeoverlayregionsandthreeunderlayregions.
Frequencyassignment
Weassignthefrequenciesbyasetofchannelsoranypartofa
setormorethanonesetofthetotal21sets.
Borrowed-frequencysetsareusedwhenneeded.
Onthebasisofcoveragepre-diction,wecanassignfrequencies
intelligentlyatonesiteoratonesectorwithoutinterferingwith
adjacentcochannelsectorsorcochannelcells.
TiltedantennaThetilteddirectionalantennaarrangementcan
eliminateinterference.

32. Sometimesantennatiltingismoreeffectivethandecreasing
antennaheight,especiallyinareasoftalltreesorathighsites.
Whenthetiltinganglesbecome22°orgreater,thehorizontal
patterncreatesanotchinthefrontoftheantenna,whichcan
furtherreducetheinterference(seeFigureinnextslide).