J.S. Blogh, L. Hanzo
Dept. of Electronics and Computer Science, Univ. of Southampton, SO17 1BJ, UK.
Tel: +44-2380-593125, Fax: +44-2380-593045
Email: [email protected]; http://www-mobile.ecs.soto n.a c.uk
ABSTRACT
Theperformanceofamulti-ratemobilecellular
net-work using the FrequencyDivision Duplex (FDD)
modeoftheCodeDivisionMultipleAccess(CDMA)
basedUniversalMobileTelecommunicationSystem
(UMTS)isinvestigated. Thenewcallblockingand
call dropping probabilities and the tolerable
net-workloadarestudiedinthecontextofamulti-rate
FDD-modeUMTS network incorporating dynamic
threshold assisted soft handovers and shadow
fad-ing.
1. INTRODUCTIONANDSOFTHANDOVERS
Thestandardisation ofthethird-generation(3G) systems,
suchasUMTS,isreachingastateofmaturity[1{ 5]. Hence
itisbenecialtostudytheassociatednetworkperformance,
whichistheobjectiveofthiscontribution.
Theprocessofsofthandoversisbasedona
make-before-breakapproach,whereanewcommunicationslinkis
estab-lishedbeforetheexistinglinkisrelinquishedduetothe
as-sociatedlinkqualitydegradation. Themobilestation(MS)
continuously monitors the power level of the received
PI-lot CHannels (PICH) transmitted from the neighbouring
basestations(BSs). Thepowerlevelsof thesebasestations
arecomparedagainsttwothresholds,TaccandTdrop.Ifthe
powerlevelisabovethebasestation'sacceptancethreshold,
T
acc
, then assuming the basestation is not already inthe
Active Basestation Set (ABS),it is addedto theABS. If,
however,thePICHofabasestationintheABSisfoundto
bebelowthe droppingthreshold,Tdrop,thenthe
basesta-tionisremovedfromtheABS.IfthethresholdT
acc issetto
toolowavalue,thenbasestationsareaddedunnecessarily
tothe ABS,whichresults inextraneous networkresource
utilisation. Conversely,if T
acc
is excessively high, thenit
ispossiblethat nobasestationsmay existwithinthe ABS
at the cell extremities. A mobile station is in
simultane-ouscommunication withtwo or morebasestations during
thesofthandover,henceoptimalcombiningofthedownlink
signalsofseveralBSsisperformedattheMS.
By contrast, the network invokes selective combining
of the MSs'signals decoded at each basestation. Since a
droppedcallislessdesirablefromtheuser'spointofview,
Thenancialsupportofthefollowingorganisationsis
grate-fullyacknowledged: EngineeringandPhysicalSciencesResearch
Council,Swindon,UK.
than a blocked call, two resource allocation queues were
invoked,one for newcalls and the other - higherpriority
-queue, for handovers. By forming a queue of the
han-dover requests, which have a higher priority during
con-tentionfor networkresourcesthannewcalls,it ispossible
toreducethenumberofdroppedcallsattheexpenseofan
increasedblockedcallprobability. A furtheradvantageof
the HandoverQueueingSystem(HQS) is that duringthe
time,whileahandoverisinthequeue,previouslyallocated
resourcesmaybecomeavailable,henceincreasingthe
prob-ability ofasuccessfulhandover.
A disadvantage of using xed handover thresholds is
that insome locations all the pilotsignals may be weak,
whereasinotherlocations theymayall bestrong. Hence,
dynamicthresholdsareadvantageous. Anadditional
ben-etofusingdynamic thresholdsisexperiencedinafading
environment, where the received pilotstrength may drop
momentarilybelow a xedthreshold and thus may cause
anABS removaland addition. However, this basestation
maybetheonlybasestationintheABS,whichwouldresult
inadroppedcall. Usingdynamic thresholdsthisscenario
wouldnothaveoccurred,sincethepilotstrengthwouldnot
havedroppedbelowthatofanyoftheotherpilotsignals.
2. POWERCONTROL
Accurate powercontrol is essential inCDMA inorderto
mitigatethenear-farproblem,whichaectsthenetwork
ca-pacityandcoverage. Closed-looppowercontrolisemployed
onboththeuplinkanddownlink. Themobilesand
basesta-tionsestimatetheSignal-to-InterferenceRatio(SIR)every
0.667ms, or ineachtimeslot, and comparethis estimated
SIRtoatargetSIR.IftheestimatedSIRishigherthanthe
target SIR,then the relevant transmitteris instructedto
reduceitstransmitpower. Likewise,iftheestimatedSIRis
lowerthanthetargetSIR,theassociatedtransmitteris
in-structedtoincreaseitstransmitpower. Transmittingatan
unnecessarilyhighpowerincreasesthepowerconsumption
anddegradestheotherusers'signalqualitybyinicting
ex-cessiveco-channelinterference. Hence,theotherusersmay
requestapowerincreaseinaneorttomaintaintheirtarget
linkquality,potentiallyleadingtoanunstablesystem.
Ifthemobileisinsofthandover,andtherefore
basestation-diversity combining is performed, then the basestations'
transmitpowers arecontrolled independently. Hence, the
mobile station may receive dierent power control
instructittodoso. However,ifanyoneofthebasestations
intheABSinstructsthemobiletodecreaseitspower,then
themobilewillreduceitstransmitpower. Thismethod
en-suresthatthemulti-userinterferenceiskepttoaminimum,
sinceatleastonebasestationhasasuÆcientlyhighquality
link.
3. CODEALLOCATION
Again, anoverview of the UTRA standard can be found
for example in [1,2,4,10]. The downlink is assumed to
be synchronous under the control of a basestation,
how-ever,thebasestationsareasynchronouswithrespecttothe
otherbasestations. The UMTS channelization codes are
known as Orthogonal Variable Spreading Factor (OVSF)
codes,whichprovidetotalisolationbetweendierentusers
on the synchronous downlink under perfect channel
con-ditions, thus perfectly eliminating intra-cell Multiple
Ac-cessInterference(MAI).However,theOVSFcodesexhibit
poor asynchronous cross-correlation properties and hence
theinter-cellMAImaybehigh,unlessthesamecodeisonly
allocated to BSs exhibiting a suÆciently high geographic
separation.Bycontrast,othercodessuchasGoldcodes,
ex-hibitalowasynchronouscross-correlation. Therefore,
cell-speciclongcodesareusedforreducingtheinter-cell
inter-ferenceonthedownlink. These so-calledscramblingcodes
areGoldcodesof2 18
1chip-durationandeachuserserved
by agiven basestation has the samedownlinkscrambling
code.Thereareatotalof512scramblingcodes,potentially
allowing thesystemtoassignadierentcell-specic
scam-bling code to 512 cell sites, whicheases the task of code
planningandallocation.
ThedownlinkOVSFcodesareallocatedbythe
basesta-tion,again,facilitating perfectly interference-free isolation
betweendierentusersonthesynchronousdownlink,ifthe
channelcoditionsareperfect. Thuseachusersupportedby
agiven basestation has a dierent downlink OVSF code,
while MSsservedby a dierent basestationmaybe using
thesameOVSFcode.
OntheasynchronousuplinktheMAIisreducedby
as-signing dierent scrambling codes to dierent users,
em-phasizingagain that the employmentof scrambling codes
exhibitinglowasynchronouscross-correlationisimportant.
Theprimary scrambling code is constructed from the
so-calledextendedVL-Kasamicodeset[2]oflength256,where
the shortlengthenables low complexity multi-user
detec-tion[6]tobeimplemented.
Forsingle-userdetectorassistedbasestations,along
sec-ondaryscramblingcodeisused[1,2,4,10],whichisa
Gold-codehavingalengthof2 41
1chips. Sincetheuplink
trans-missionsareasynchronousandhenceeachuserhasaunique
Goldcodeexhibitingahighasynchronouscross-correlation,
everyusercanemploythesamesetofchannelizationcodes.
4. SIMULATINGAN INFINITE PLANE
NETWORK
Atessellatingrhombicsimulationarea of7cells by7cells
wasused,asshowninFigure1,thusallowingthesimulation
Image
of user 1
User 1
User 2
Image
of user 2
Figure1: The7x7rhombicsimulationarea showingauser
andits\wrapped"image.
areatobereplicatedarounditself. Thisapproachavoidsthe
borderoredgeeectsassociatedwithsimulatinga
\desert-island-like" cellular network, which results in the central
cellsexperiencingsignicantlyhigherinterference,thanthe
edgecells. Afurtheradvantageisthatallcellsencountera
somewhathigherlevelofinterference,thanthatexperienced
by an \island-type" system. Thus a highercall dropping
rateoccursacross thenetwork, allowing statistically valid
resultstobeobtainedwithinareduced-durationsimulation
timeframe,henceexpeditingoursimulations. Theinnite
plane network replicates itself, such that when a mobile
leaves the central network, it enters an adjacent network
whichismappedontothecentralnetwork. Moreexplicitly,
asamobileleavesthenetwork,itis\wrappedaround"and
entersthenetworkattheoppositeedge,whilstinicting
co-channel interferencetoall users ofthe network, who may
belocatedateitheredgeofthenetwork. Inordertoenable
a callto be maintained underthese conditions, both the
signalsandtheinterferenceare\wrappedaround"theedges
ofthenetwork.
5. SYSTEMPARAMETERS
New call channel allocation requests were placed ina
re-sourceallocationqueuefor up to5s. Ifduring thisperiod
acallwas notnotserviced,itwasclassed asblocked. The
mobiles movedfreely, inrandom directions, at aspeedof
30mphwithin the simulation area, which consisted of 49
cells,asdescribedinSection4. Thecell-radius was218m.
Thecallduration andinter-call periods werePoisson
dis-tributedwith themean valuesshown inTable 1. For our
initialinvestigationswehaveassumedthatthebasestations
up-andthedown-link.
Furthermore,thebasestationsareassumedtobeequipped
with the Minimum Mean Squared Error Block Decision
FeedbackEqualiser (MMSE-BDFE)based Multi-User
De-tector(MUD)[6]. ThepostdespreadingSINRsrequiredby
thisMUDforobtainingthetarget BERsweredetermined
withthe aid of physical-layer simulationsusing a4-QAM
modulationscheme,inconjunctionwith1=2rateturbo
cod-ing and MUD over a COST 207 seven-path Bad Urban
channel [8]. Using this turbo-codedMUD-assisted
trans-ceiveranda spreadingfactorof 16, the post-de-spreading
SINRrequiredformaintainingthetargetBERof110 3
was 8.0 dB.TheBER correspondingtolow-qualityaccess
was stipulated at 510 3
. This BER was exceeded for
SINRsbelow7.0dB.Furthermore,alow-qualityoutagewas
declared,whentheBERof110 2
wasexceeded,namely
for SINRs below 6.6 dB. These values can be seen along
withtheothersystemparametersinTable1.
6. PERFORMANCEMETRICS
Thereareseveralperformancemetricsthatcanbeusedfor
quantifyingtheperformance orquality ofserviceprovided
by a mobile cellular network. Thefollowing performance
metricshave been widelyused in the literature and were
alsoadvocatedbyChuang[7]:
Newcallblockingprobability,P
B .
Calldroppingorforcedterminationprobability,PFT.
A callis dropped whenthelower ofthe uplinkand
downlinkSINRsdipsconsecutivelybelowtheoutage
SINR(5%BER)agivennumberoftimes.
Probability of a low quality access, Plow, quanties
the chances of either the uplink or downlink signal
qualitybeingsuÆcientlypoor,resultinginalow
qual-ityaccess(1%BER).
Probability ofoutage,Pout, isdenedas the
proba-bilitythattheSINRisbelowthevalue atwhichthe
callisdeemedtobeinoutage.
Grade-Of-Service(GOS) was dened by Cheng and
Chuang[7]as:
GOS = Pfunsuccessfulorlow-qualitycallaccessesg
= Pfcallisblockedg+Pfcallisadmittedg
Pflowsignalqualityandcallisadmittedg
= PB+(1 PB)P
low
: (1)
7. SIMULATION RESULTS
In order to determine the number of users that may be
supported with adequate call quality by the network, we
have dened a conservative and a lenient scenario which
areformedfromacombinationoftheperformancemetrics,
asfollows [9]:
Conservativescenario :
P
B
3%,P
FT
1%,P
low
1%andGOS4%.
Lenientscenario :
P
B
5%,P
FT
1%,P
low
2%andGOS6%.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Mean Carried Teletraffic (Erlangs/km
2
/MHz)
10
-3
2
5
10
-2
2
5
10
-1
2
5
Call
Dropping
Probability
,
P
FT
1%
-113, -115
-112, -114
-111, -114
-111, -113
-111, -112
T
acc
(dBm), T
drop
(dBm)
Figure 2: Call dropping probability versus mean carried
traÆcof aCDMAbasedcellularnetworkusingxed
re-ceivedpilotpowerbasedsofthandoverthresholdsin
con-junctionwith 0.5 Hz shadowing having a standard
deviationof3dBforSF=16.
7.1. FixedReceived Pilot PowerBasedHandover
Oursimplest handoveralgorithm studiedinthis
contribu-tion is based on the acceptance and dropping
thresh-olds,Tacc and T
drop
, respectively. In simpleterms,a call
is accepted and serviced, when the received pilot power
is inexcess of the threshold Tacc and dropped, when the
corresponding pilot power falls below T
drop
. In this
sec-tion we examine the achievable performance, upon using
xedreceivedpilotpowerbasedsofthandoverthresholdsof
TaccandTdrop,whentheMSsare subjectedtolog-normal
shadowfading havingastandard deviationof3dB and a
maximumshadow-fadingfrequencyof0.5Hz.
The call dropping results of Figure 2 suggested that
the network's performance was poor whenusing xed
re-ceived pilot power based soft handoverthresholds in the
above mentioned shadow fading environment. The root
causeoftheproblemisthatthexedthresholdsmustbeset
suchthatthereceivedpilotsignals,evenwhensubjectedto
shadowfading,areretainedintheactiveset. Therefore,
set-tingthethresholdstoohighresultsinthebasestations
be-ingremovedfromtheactiveset,thusleadingtoanexcessive
numberofdroppedcalls. However,ifthethresholdsareset
toolow,inordertocounteract thisphenomenon,thenthe
basestationscanbeinsofthandoverfortoohigha
propor-tionoftime,andthusanunacceptableleveloflowquality
accessesisgeneratedduetotheadditionalco-channel
inter-ferenceinictedbythehighnumberofactivebasestations.
Figure2showsthat reducingthesofthandoverthresholds
improvedthenetwork'scalldroppingprobability,but
Fig-ure3illustratesthatreducingthesofthandoverthresholds
engenderedanincreaseinthe probabilityof alow quality
access.
The network cannot satisfy the quality requirements
oftheconservativescenario, namelythat ofmaintaininga
calldroppingprobabilityof1%combinedwithamaximum
probability of low quality access below1%. However, the
le-Noiseoor -100dBm Pilotpower 1dBm
Framelength 10ms Cellradius 218m
Multipleaccess FDD/CDMA Numberofbasestations 49
Modulationscheme 4QAM/QPSK Spreadingfactor 16
MinimumBS transmitpower -44dBm MinimumMStransmitpower -44dBm
MaximumBStransmitpower 21dBm MaximumMStransmitpower 21dBm
Powercontrolstepsize 1dB Powercontrolhysteresis 1dB
Lowqualityaccess(0.5%BER)SINR 7.0dB Outage(1%BER)SINR 6.6dB
Pathloss exponent -3.5 SizeofActiveBasestation Set(ABS) 2
Averageinter-call-time 300s Max. new-callqueue-time 5s
Averagecalllength 60s MSspeed 30mph
Maximumconsecutiveoutages 5 Signalbandwidth 5MHz
Spreadingfactor 16 TargetSINR(atBER=0.1%) 8.0dB
Table1: Simulationparameters.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Mean Carried Teletraffic (Erlangs/km
2
/MHz)
10
-3
2
5
10
-2
2
Probability
of
low
quality
access,
P
low
1%
2%
-113, -115
-112, -114
-111, -114
-111, -113
-111, -112
T
acc
(dBm), T
drop
(dBm)
Figure3: Probabilityoflowqualityaccessversusmean
car-riedtraÆcofaCDMAbasedcellularnetworkusingxed
received pilot powerbased softhandoverthresholdsin
conjunctionwith0.5Hzshadowinghavingastandard
deviationof3dBforSF=16.
nientscenario'ssetofcriteria,whichconsistsofamaximum
call dropping probability of 1% and a probability of low
qualityaccess ofbelow2%, usingthe thresholdsofT
acc
=-113 dBm and T
drop
=-115 dBm, respectively. A range of
furthersystemperformance guresare summarisedin
Ta-ble2.
7.2. RelativeReceivedPilotPowerBasedHandovers
Basedon the relatively poor performance of the xed
pi-lotthresholdbasedscenariosoftheprevioussubsectionthe
importance ofemployingrelative received pilot power
thresholdscannotbeover-emphasizedinrealistic
propa-gation environmentsexposed to shadow fading. More
ex-plicitly,incontrasttothepreviouslyusedxedthresholds,
whichwereexpressedintermsofdBm,i.e. withrespectto
1mW,whenusingrelativethresholdsthevaluesofT
acc and
T
drop
areexpressed intermsofdBrelativetothereceived
pilotstrengthofthebasestationsintheABS.The
employ-mentofrelativethresholdsalsocatersforsituations,where
the absolute pilot power may be too low for use in
con-junctionwithxedthresholds, butnonetheless suÆciently
high for reliable communications. Similarly to the
xed-threshold based scenario, a range of system performance
resultsaresummarisedinTable2.
7.3. E
c =I
o
PowerBasedSoftHandovers
Analternativesofthandovermetricusedtodetermine\cell
ownership" is the pilot to downlink interference
ra-tio of a cell, whichis denotedby Ec=Io. This handover
metricwasproposedforemploymentinthe3rdgeneration
systems[10]. Thepilot to downlink interference ratio, or
Ec=Io,maybecalculatedthusas[11]:
E
c
Io =
P
pilot
Ppilot+N0+ P
N
cells
k =1 PkTk
; (2)
where Pk is the total transmit power of cell k, Tk is the
transmissiongainwhichincludesantennagainandpathloss
aswellasshadowing,N
0
isthethermalnoiseandN
cells is
thenumberofcellsinthenetwork. Theadvantageofusing
such a schemeis that it is not an absolute measurement
thatis used,buttheratio ofthe pilotpowertothe
inter-ferencepower. Thus, ifxedthresholdswereuseda form
ofadmission control maybeemployedfornew callsifthe
interferencelevelbecametoohigh. Afurtheradvantageof
thistechniqueisthatittakesintoaccountthetime-varying
natureoftheinterferencelevelinashadowedenvironment.
Thevarioussystemperformanceguresaresummarisedin
Table 2.
7.4. Relative Ec=Io ThresholdBasedHandovers
Finally,wealsoattemptedtocombinethebenetsofusing
thereceivedEc=Ioratioandrelativesofthandover
thresh-olds,thusensuringthat variationsinboththereceived
pi-lotsignal strength and interference levels weremonitored
inthesofthandoverprocess. Inthenextsubsectionwewill
comparetheperformanceofanetworkemployingtheabove
PFT=1%,Plow=1% PF
T
=1%,Plow=2%
Softhandover Power(dBm) Power(dBm)
algorithm Shadowing Users ABS MS BS Users ABS MS BS
Fixedpilotpwr. No 290 1.7 5.1 5.1 290 1.7 5.1 5.1
Fixedpilotpwr. 0.5Hz,3dB - - - - 127 1.83 -2.0 6.5
Fixedpilotpwr. 1.0Hz,3dB - - -
-Relativepilotpwr. No 288 1.7 4.1 4.7 288 1.7 4.1 4.1
Relativepilotpwr. 0.5Hz,3dB 144 1.77 -1.5 0.6 146 1.78 -1.5 1.3
Relativepilotpwr. 1.0Hz,3dB 127 1.5 -2.4 -1.9 144 1.72 -1.5 0.8
FixedE
c =I
o
No 223 1.83 2.0 10.0 231 1.86 2.0 10.3
FixedEc=Io 0.5Hz,3dB 129 1.88 -2.4 7.0 140 1.91 -2.4 8.7
FixedE
c =I
o
1.0Hz,3dB 107 1.86 -3.0 4.5 128 1.91 -3.0 9.5
RelativeE
c =I
o
No 256 1.68 3.1 2.7 256 1.68 3.1 2.7
RelativeEc=Io 0.5Hz,3dB 150 1.65 -1.2 -1.7 150 1.65 -1.2 -1.7
RelativeEc=Io 1.0Hz,3dB 144 1.65 -1.1 -1.6 144 1.65 -1.1 -1.6
Table 2: Maximum number of mobile users that can be supported by the network, for dierent soft handover
met-rics/algorithmswhilstmeetingthepresetqualityconstraints. ThemeannumberofbasestationsintheActiveBasestation
Set(ABS)isalsopresented,alongwiththemeanmobileandmeanbasestationtransmitpowers.
7.5. Performance OverviewandConclusions
Duetolackofspacewecannotpresentthecorresponding
re-sultsindiagrammaticformforallthepreviouslymentioned
performancemetricsandforallinvestigatedsystem
scenar-ios. Instead, Table 2 summarises the results obtained for
thevarioussofthandoveralgorithmsoverthethreedierent
propagation environments considered. The xed receiver
pilot power based algorithm performed the least
impres-sively overall, as expecteddue to itsinherent inability to
copewithshadowfading. However,itdidoerahigh
net-workcapacityina non-shadowedenvironment. Usingthe
relativereceivedpilotpowerbasedsofthandoveralgorithm
improvedtheperformanceundershadowfading,but
dier-ent fading rates requireddierent thresholds to meet the
conservativeandlenientqualitycriteria. Theperformance
ofthexedEc=Iobasedsofthandoveralgorithmalsovaried
signicantly,whenusingthesamethresholdsforthetwo
dif-ferentfadingratesconsidered. However,themaximum
net-workcapacity achieved underthe dierent shadowfading
conditions was signicantlyhigher, thanthat of the xed
receivedpilotpowerbasedalgorithm. Thisbenetresulted
fromtheinclusionoftheinterferencelevelsinthehandover
process, whichthus took into account the fading of both
thesignaland theco-channelinterference. Combiningthe
relative threshold based scheme with usingEc=Io
thresh-oldsallowedustosupportthehighestnumberofusers
un-derthe shadow fading conditions investigated. Whilst its
performancewas notthehighest inthenon-shadowed
en-vironment,this non-shadowedpropagationenvironmentis
often unrealistic. Hence, due to its best performance in
shadowed environments,the relativereceivedE
c =I
o based
softhandoveralgorithmwasourbestcandidatein
conjunc-tionwiththesofthandoverthresholdsofT
acc
=-10dBand
Tdrop=-18dB. Theadvantagesofthis handoveralgorithm
wereitsreducedfractionoftimespentinsofthandover,and
itsabilitytoperformwellunderbothshadowfading
condi-tions,whilstutilisingthesamesofthandoverthresholds.
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