Performance and Implementation of Dynamic Frequency Hopping in Limited-Bandwidth Cellular Systems

9 Pages·2002·0.243 MB·English

by  Kostic Z.

28 IEEETRANSACTIONSONWIRELESSCOMMUNICATIONS,VOL.1,NO.1,JANUARY2002
Performance and Implementation of Dynamic
Frequency Hopping in Limited-Bandwidth
Cellular Systems
ZoranKostic´,SeniorMember,IEEEandNelsonSollenberger,Fellow,IEEE
Abstract—We evaluate the performance of recently proposed the benefits of frequency diversity and interference averaging.
dynamic frequency hopping(DFH) when applied tocellular sys- Capacity improvements obtained by RFH in GSM are in the
temswithalimitedtotalbandwidth.Wealsoillustrateapractical
rangeof30%–100%.Recenttheoreticalandsimulationstudies
implementation for DFH deployment using network-assisted re-
indicate that significant performance improvements can be
sourceallocation(NARA).Theperformanceevaluationisaccom-
plishedbysystem-levelsimulationsofasystemwith12carriersand obtainedbytakingadvantageofthecombinationoffrequency
1/1 frequency reuse, based on the EDGE-Compact specification. hoppingandinterferenceavoidancetechniquescalleddynamic
Voice-onlycircuit-switchedoperationisassumed.Fadingchannel, frequencyhopping(DFH)[4].ThekeyconceptofDFHistoad-
multicellinterference,voiceactivity,andantennasectorizationare
just orbuildfrequencyhopping patternsbasedoninterference
modeled.Wepresenttheperformanceofdynamicfrequencyhop-
measurementsandcalculations,inordertoavoiddominantin-
pingcomparedtorandomfrequencyhoppingandfixedchannelas-
signmentbyshowingdistributionsofworderrorrates.Sensitivity terferers. Direct implementation of DFH (measurement-based
to occupancy, Rayleigh fading assumptions, number of carriers, DFH)requiresrapidsignalqualityandpathlossmeasurements,
voiceactivity,andmeasurementerrorsarestudied.Wealsocom- significant signaling overhead, and synchronization between
pare the uplink and downlink performance. The results indicate
base stations. However, network-assisted resource allocation
thatDFHcansignificantlyimprovetheperformancecomparedto
simplifiesDFHdeploymentsignificantlyandtherebymakesit
randomfrequencyhopping.Forexample,ata2%frameerrorrate
with 90% coverage, the capacity improvement of DFH is almost feasible.
100%whencomparedwithfixedchannelassignment,andabout
Recent developments in the evolution of time division mul-
50%whencomparedtorandomfrequencyhopping.Theamount
tiple access (TDMA) systems for providing high-speed data,
of improvement for the uplink direction is smaller than the im-
provementforthedownlinkdirection,especiallyforhigheroccu- calledEDGE[5],[6],takeintoaccountthefactthatsystemoper-
pancies. atorsmayinitiallydeployEDGEinsmallportionsofbandwidth,
IndexTerms—Adaptive,CDMA,cellular,dynamicchannelas- borrowedfromfrequenciescurrentlyusedforcellularvoiceser-
signment(DCA),EDGE,frequencyhopping,resourceallocation, vices.Theactivitiespursuedinthestandardizationbodies3G.IP,
spectralefficiency,wireless. 3GPP,and3GPP2relatedtoEDGEareworkingtodefinevoice-
over-IPoverEDGE.Thisismotivatedbythedesireofservice
I. INTRODUCTION providers to deploy an all-IP network infrastructure. Initially,
voice-over-IP over EDGE will probably be deployed within a
CELLULAR systems require large quantities of gener-
limited amount of spectrum. For example, EDGE-Compact is
ally scarce and increasingly expensive spectrum, so
envisionedasa2 1-MHz-widedatasystematinitialdeploy-
achieving high spectral efficiency is crucial to their economic
ment to support best-effort packet data services, and the typ-
viability. Recent advances in signal processing and digital
icalbandwidthrequiredfortheadditionaldeploymentofvoice-
communications offer new directions for pursuing higher
over-IPoverEDGEmaybeabout2 2.4MHz.
spectral efficiency—such as spatial diversity by exploiting
The developments described above provide motivation to
multiple antennas and statistical multiplexing through packet
study the use of dynamic frequency hopping in future cellular
switching. Frequency hopping [1], which is the focus of this
systems. This paper is an extension of the recently published
paper, is another important and not fully exploited technique
fundamental work on dynamic frequency hopping [4] and it
forimprovingsystemcapacity.
focusesonpracticalissuessurroundingtheDFHconcept.Two
FrequencyhoppinghasbeenutilizedinGSMforimproving
practical aspects are of particular interest: the performance
system capacity [2], [3]. GSM uses random or cyclical fre-
of DFH when deployed in limited system bandwidth and
quency hopping (FH). Random frequency hopping (RFH), in
the feasibility of DFH deployment. Changes required for
combination with channel coding and interleaving, provides
evolving GSM and GPRS into EDGE are significant, and for
the EDGE-Compact include system-wide base station frame
ManuscriptreceivedMay22,2000;revisedNovember5,2000;acceptedJan-
synchronization.Coincidentally,thesechangesarealsocrucial
uary9,2001.Theeditorcoordinatingthereviewofthispaperandapprovingit
forpublicationisM.Zorzi. for DFH implementation. With additions to the system that
Z.Kostic´ iswiththeAT&TLabs—Research,Middletown,NJ07748USA would support intercell communications, it will be possible to
(e-mail:[email protected]).
incorporate DFH into evolving standards and thereby provide
N.SollenbergeriswithMobilinkTelecomInc.,Middletown,NJ07748USA.
PublisherItemIdentifierS1536-1276(02)00184-8. higherspectralefficienciestofuturesystems.
1536–1276/02$17.00©2002IEEE
KOSTIC´ ANDSOLLENBERGER:DYNAMICFREQUENCYHOPPINGINLIMITED-BANDWIDTHCELLULARSYSTEMS 29
The work presented in this paper presentsresults indicating usedintheensuingtimeperiod.Reducedcomplexityschemes
that DFH can be successfully implemented in limited-band- have been also proposed, which reduce measurement require-
width cellular systems. A cellular system simulation was ments and signaling overhead.For example, the system might
implemented that provides the ability to compare the per- restricthoppingpatternchangestochangingonlyonehopata
formance of fixed channel assignment, random frequency time, or the system might predefine a set of hopping patterns
hopping, and dynamic frequency hopping cellular systems in andrestrictchangestooneofthosehoppingpatterns[7].
limited portions of bandwidth.The comparison is done on the For a cellular system downlink, an ideal DFH scheme that
basisofcumulativedistributionfunctions(CDFs)indicatingthe reliesonthemeasurement-basedapproach,consistsofthefol-
achievable word error rates (WERs) as a function of channel lowingprocesses.
conditions, loading, total number of frequencies, and other (cid:127) Each mobile continuously measures the quality of all
factors. The use of network-assisted resource allocation for channelsavailableinthesystem(signalstrength,interfer-
practical DFH deployment is presented through the architec- ence,SIR,orothercriteria).
turaldescriptionofthesystem. (cid:127) The measurements are communicated over the air to the
In Section II, we review the concept of DFH. Section III controllingbasestation.
describes the system and the simulation model. Section IV (cid:127) Thebasestationcollectsmeasurementsfromallofitsmo-
presentstheperformanceresults.SectionVaddressestheissues biles.
of DFH implementation based on network-assisted resource (cid:127) Basedonthemeasurements,thebasestationperiodically
allocation,andSectionVIistheconclusion. updatesfrequencyhoppatternssuchthattheoverallper-
formanceisoptimized.
(cid:127) Thebasestationsendsinformationtoeachindividualmo-
II. DYNAMICFREQUENCYHOPPING
bileinformingitofthefrequencyhoppatternthatwillbe
Conventional frequency hopping [2], [3] includes random usedintheensuingtimeperiod.
andcyclicalfrequencyhoppingandisameansofimplementing Similarprocessesrunforthecellularuplink.
frequencydiversityandinterferenceaveraging.Itissuitablefor ThispaperstudiesfullreplacementDFH,whereaslesscom-
providing robust communications links in frequency selective pleximplementationsofDFHhavebeenstudiedin[7].
andinterferencelimitedchannels.RandomFHisfairlysimple
to implement, and it is the most prevalent frequency hopping III. WIRELESSSYSTEMMODEL
technique in commercial communications systems. Recent
We have modeled a cellular system that is similar to the
studies indicate that the implementation of interference avoid-
EDGE-compact system, which is currently being defined in
ance can bring significant capacity improvements in generic
standardization bodies, including ETSI, 3GPP, and UWCC.
cellular systems with frequency reuse one. We have proposed
Thesimulationspresentedinthispaperassumecircuit-switched
a means of implementing interference avoidance by dynamic
voice.Somedetailsofthesimulatedsystemaretakenfrom[10].
frequency hopping [4], [7]–[9]. The objective is to achieve
Thesimulationsaredonebothfortheuplinkanddownlink.
capacity gains through interference avoidance that are larger
Mostof the simulations for frequencyhopping are done for
thanthoseprovidedbyconventionalfrequencyhopping,while
a systemwith 12carriers and frequency reuse1/1, with three-
retaininginterferenceaveragingcharacteristicsofconventional
sectorantennasectorization1.Baselinestudiesaredonewitha
frequency hopping to provide robustness to rapid changes in
system that supports fixed channel assignment with 4/12 fre-
interference.
quencyreuse.Thereare16basestationsinasystem,andawrap-
Dynamic frequency hopping is a combination of dynamic
aroundtechniqueisusedtoemulatealargesystem.Hexagonal
channel assignment (DCA), where a channel is one frequency
cells are used, and an irregularly shaped reuse simulation pat-
ina frequencyhoppattern,and traditional frequencyhopping.
tern has been devised to properly model reuse 4/12, as shown
InDFH,frequencyhoppatternsarecontinuouslymodifiedfor
in Fig. 1. Frequency hop patterns for all users within a single
everylinkinacellularsystem.Modificationsaredrivenbyrapid
base station (all three sectors) are orthogonal. In our simula-
measurementsofthequalityoffrequenciesusedinasystemby
tions,basestationsaresynchronizedtosupportstaggeredframe
allmobilesandbasestations,withthegoaloftrackingthedy-
frequencyhoppatternassignment.Thestaggeredframeconcept
namicbehaviorofthechannelqualityandofinterference,and
preventsbasestationsfromassigningatthesametimethesame
of avoiding strong interferers. Frequency hop pattern modifi-
goodfrequenciesforfutureFHpatternsoftheirusers.Suchsi-
cationscanalsobebasedonthefixedwirelessnetworkcalcu-
multaneous assignment could worsen the performance of the
latinginterferencelevelsbasedonknowledgeofpathlossesand
system,ratherthanenhancingit.Thereexistsnointracellinter-
channel occupancy, which we refer to as network-assisted re-
ference,and intercell interference appears randomized for this
sourcemanagementoftheDFH-basedsystem.
model.
TheformofDFHthatachievesthebestperformanceisfullre-
In a cellular system, the staggered frame concept would
placementDFH,sinceitallowsforchangesofallfrequenciesin
work in the following fashion: 1) base stations are frame and
apatterntofrequenciesofbetterquality,providedthattheyare
superframesynchronized;2)controlreusepatternisdefinedin
available.ThisisthemostcomplexformofDFHthat,whenre-
time, according to which only one base station is allowed to
lyingonthemeasurement-basedapproach,requiressubstantial
amountsofsignalingoverheadforcommunicatingfromabase
1All12carrierscanbeusedinanyofthesectors,butoneFHpatternnever
station to its mobiles what frequency hop patterns need to be usesall12frequencies.
30 IEEETRANSACTIONSONWIRELESSCOMMUNICATIONS,VOL.1,NO.1,JANUARY2002
TABLE I
SIMULATIONSYSTEMPARAMETERS
Fig. 1. Layout of the wraparound simulation for reuse 4/12. Thick line
indicatestheboundaryofthecellularpatternusedforsimulations.
change frequency hop patterns for the duration of one frame;
and3)allbasestationsgetanopportunitytomodifyfrequency
hop patterns of their mobiles once per superframe. The term
“staggered”ischosenbecauseforindividualbasestations, the
opportunitytoexecutemodificationisstaggeredintime.Token
passing would be an equally appropriate term. The staggered
frame concept is emulated in the simulations, the results of
whichweshowinSectionIV.
The framing and physical layer structure assumed in our
work is similar to the one presented in the work by Ericsson
[10],withsomechanges:1)channelcodingisReed–Solomon,
where one speech frame is interleaved over six bursts; 2) one
Reed–Solomon codeword contains 12 symbols; 3) frequency
hoppingisperformedonaslotbyslotbasis,sotwocodesym- shadowingwith8-dBstandarddeviation,andRayleighfading.
bols are carried over the same slot and frequency; and 4) the We simulate two cases: one in which the Rayleigh fading is
channeldecodercancorrectnomorethantwopairsofsymbols fullycorrelatedfromslottoslot,andtheotherwherefadingis
in outage—three or more bursts with poor qualities result in fully uncorrelated from slot to slot. For the diversity benefit,
a speech frame in error. Simulations are single-slot, thereby full decorrelation is assumed at the two antennas. The full
no advantage is taken of the time domain for interference correlation case is useful for cellular systems with small total
avoidance.Weassumedual-antennadiversity. spectrum.Thecoherencebandwidthforrealisticurbanmultipath
In a contemporary cellular system, it is a common prac- channels may be around 1 or 2 MHz, which may be the total
tice to use convolutional codes, rather than block codes that bandwidth allocated to EDGE-Compact, and which would
Reed–Solomoncodesbelongto.Thoughsomeauthorsconsider cause partially correlated Rayleigh fading on all frequencies.
block codes for actual system use [10], their decoding com- Power control is not used in the presented simulations. Voice
plexity is often cited as being a problem for the deployment. activity is simulated, with 50% activity and with uncorrelated
Here, we use Reed–Solomon codes for system evaluation positioning of slots that carry speech (continuous voice spurts
purposes—namely for the fact that when one only wants to are not modeled). Loading/occupancy for all base stations is
identify if a codeword is in error or not (rather than decode assumedtobethesameonaverageandfixedforanyparticular
thedata),itisenoughtocountthenumberofcodesymbolsin simulation scenario. Loading (occupancy) is expressed as the
error(inacodeword)andtoknowthecorrectingabilityofthe numberofusersonacallataparticulartime.Inthepresenceof
code. In our work, we use Reed–Solomon codes that encode voice activity, only 50% of users may actually transmit at any
one frame of data with one codeword that consists of 12 code instantoftime,therebycontributingtotheoverallinterference
symbols. Reference [10] explains how this arrangement, after in the system.
interleaving,fitsintothecurrentGSMframeandslotstructure. Inthesimulations,itisassumedthatineverysectorofabase
Thecodecancorrectforfourorfewersymbolsinerror.Since station, frequency hopping can be done overall carriers avail-
we have six FH hops per frame, two symbols are carried by a able to a base station, regardless of the fact that antenna sec-
single hop (frequency). When the quality of one frequency is torizationisused.Ontheotherhand,thecalculationofinterfer-
belowagiventhreshold,twosymbolsareinerror.Theframeis enceseenatreceiverstakessectorizationintoaccount.Although
in error if more than four symbols are in error. Toaccumulate thismethodologyrequiresmorehardwarefornarrow-bandim-
theWERvalue,weneedtocountthenumberofcodewordsin plementationsofbasestationradiosthanalternatives,itbrings
error.Bydoingso,weevadetheneedforlinklayerperformance someperformanceadvantages.Performanceimprovement,cou-
curvesmostoftenusedinsystem-levelsimulations. pledwiththefactthatwide-bandradiosarethewayofthefuture,
Wemodelpathattenuationwithanexponentialfactorof3.7 supportsargumentsforpursuingthismethodology.
andmedianpathgainof 129dBatonekilometer,lognormal SystemsimulationparametersaresummarizedinTableI.
KOSTIC´ ANDSOLLENBERGER:DYNAMICFREQUENCYHOPPINGINLIMITED-BANDWIDTHCELLULARSYSTEMS 31
Thefirststepinasimulationconsistsofgeographicallyposi-
tioningeverymobilewithinthecellularsystem.Propagationat-
tenuationandshadowingarecalculatedforallpossiblebase-sta-
tion/mobile links. Next, frequency-hop allocations are made.
Initially, they are random. For random hopping, they remain
randomthroughoutthesimulation.Fordynamicfrequencyhop-
ping, FH patterns are reallocated independently by each base
station.Thesimulationisstatic,fromthepointofviewoffre-
quency pattern allocation: the next frequency hop pattern al-
location does not depend on the previous adjustment, but is
donestartingfromrandomlyassignedpatterns.Next,manyin-
dependentRayleighfadingexperimentsarerunforeachmobile
andthenumberofoutagesthatoccurperframeforoneuseris
counted.Theresultofonesimulationisacumulativedistribu-
tionfunctionofthelinkWER,obtainedacrossallbasestations
forallmobiles.
Aconceptualrepresentationofthemethodologyforassigning
Fig.2. FHversusfixedchannelassignment(12carriers,sectorized1/1reuse
newfrequencyhoppatternstomobilestations,accordingtothe forFH,4/12forFXD).
idealfull-replacementdynamicfrequencyhopping,isillustrated
bythepseudocodebelow.SimulationresultsinSectionIVas-
The signal to interference ratio (SIR) threshold to consider a
sumethatFHpatternmodificationcanbedoneonceperframe.
symbolasaffectedbyerrorissetto4dB,anumberappropriate
Lower complexityDFHmethods putfurther constraints inthe
when Rayleigh fading is included in simulations2 [10]. No
flow of assignment, two examples being that in a single in-
more than 1000 Rayleigh-varied experiments are run for each
stant(frame),onlyonefrequencycouldbemodifiedorthatno
mobile, which implies that a reliable estimate of the WER at
changesinthefrequencyhoppatternaredoneifthequalityof
around0.01canbemade.Sincethesimulationshavebeenrun
currentlyusedfrequenciesisacceptable.
toestablishtheperformanceachievablewithsingleslotopera-
tion, a conservativeestimate of the effective bits/second/Hertz
PSEUDOCODE ILLUSTRATING IDEAL FULL REPLACEMENT DFH
performancecan be obtained by multiplication with the factor
ASSIGNMENT
of8(eightslotsperframe).
/* EXECUTED FOR EACH BASE STATION INDEPENDENTLY
COLLECT MEASUREMENTS FOR ALL FREQUENCIES FOR ALL
IV. SIMULATIONRESULTS
HOPS
RANK FREQUENCIES ACCORDING TO THE QUALITY (PER HOP) Theprimarygoalofthesimulationstudyistoestablishrela-
FOR MOBILE_INDEX =1, LAST_MOBILE_IN_BASE tiveperformanceimprovementsofdynamicfrequencyhopping
{ FOR HOP =1 LAST_HOP_IN_FH_PATTERN whencomparedtorandomfrequencyhoppingandfixedchannel
{ ASSIGN THE BEST AVAILABLE FREQUENCY TO THE CUR- assignment.Thetechniquesarecomparedbyobservingcumu-
RECT HOP lativedistributionfunctions(CDFs)ofWERobtainedforalarge
/* CONSTRAINTS: number ofusers in a cellular system. WER is the most appro-
-TWO HOPS IN THE SAME FH PATTERN CAN NOT USE THE priate measure of the link performance in speech cellular sys-
SAME FREQUENCY tems, since the performance of voice coders is best described
-ALL MOBILES IN THE SAME BASE HAVE TO HAVE by frame error rates (in this case the word and the frame are
ORTHOGONAL FH PATTERNS thesame).Whereasfixedchannelassignmentisstudiedforfre-
*/ quencyreuse4/12,FHtechniquesarestudiedforreuse1/1with
} three-sectorantennasectorization.Asacriterionofsatisfactory
serviceforacellularsystem,wechooseapointforwhich90%of
mobileusershaveWERssmallerthan0.02[11].InFig.3below,
The simulation study whose results are illustrated in Sec-
thispointisthelowerrightedgeofarectangle.AllCDFcurves
tion IV disregards latencies in measurements and pattern
withanyportionpassingthroughthisrectanglecorrespondtoa
assignments as well as signaling overhead. However, we have
cellularsystemthatmeetstheprescribedperformancerequire-
also built a full-fledged time-driven multislot EDGE system
ments.
simulator and experimented with DFH when latencies and
overhead are taken into account [15]. There, the measure-
A. ComparisonofChannelAssignmentTechniquesforFixed
ments are replaced by interbase station communication and
PerformancePoint
interference computation. The idealized performance of that,
practically more realizable DFH method, is well represented Fig.2illustratesseveralpoints.Onecurveshowstheperfor-
bythecurvesinthispaper.TheDFHmethodusedinthatstudy mance of a 12-carrier system that uses fixed channel alloca-
isfurtherclarifiedinSectionV.
2Thesystemlevelsimulationexplicitlyincludesfadingeffects.Inthiscase,it
The frequency quality criterion used in the simulations is
isappropriatetousedetectionthresholdsfortheGaussianchannel,and10e
signal to interference ratio (SIR). Thermal noise is ignored. bit-errorrateisattainableataround4dBforsimplemodulationssuchasBPSK.
32 IEEETRANSACTIONSONWIRELESSCOMMUNICATIONS,VOL.1,NO.1,JANUARY2002
Fig.3. PerformanceofDFHasafunctionofoccupancy(12carriers,sectorized Fig.4. Performanceofrandomfrequencyhoppingasafunctionofoccupancy
1/1reuse). (12carriers,sectorized1/1reuse).
tionwithfrequencyreuse4/12,forafullyloadedsystem(three
usersperslotperbasestation3). Thecurvepassesthrough the
satisfactory-performancerectangle.ExistingGSMsystemsde-
ployed with 12 carriers would have similar performance. An-
other curve illustrates the performance of random frequency
hoppingforfrequencyreuse1/1andforfourusersperbasesta-
tion(loadingof33%).Weobservethatrandomfrequencyhop-
pingimprovesthecapacityabout30%.Asystemusingdynamic
frequencyhoppingcanserveasmanyassixusersperbasesta-
tion(50%loading)andachievesimilarperformance.Capacity
improvementofDFHistherebyabout100%whencomparedto
fixedchannelassignment.
B. PerformanceasaFunctionofOccupancy
Figs. 3 and 4 show the performance of dynamic frequency
hoppingandrandomfrequencyhoppingasafunctionofoccu-
pancy.Onecanobservethattheinterferenceavoidancebenefit
Fig.5. SensitivityofDFHtothetotalnumberofcarriersforloadingof50%.
of DFH brings improved performance over random frequency
hoppingconsistentlyforeachoccupancylevel.
C. SensitivitytotheNumberofCarriers
Tobenefitfromtheinterferenceavoidancefeatureofthedy-
namicfrequencyhopping,theremustbeasufficientlylargeset
ofunusedchannelsavailableforfrequencyreassignmentswhile
executingfrequencyhoppatternmodifications.Thisisequiva-
lent to saying that the DFH requires that the burst occupancy
shouldneverbetoohigh.Intuitively,oneexpectsthatthelarger
the number of nonused frequencies is, the better the perfor-
mance will be. Consequently, it is expected that systems with
smallertotalbandwidth(totalnumberofcarriers)willachieve
lowerspectralefficiency.Fig.5showstheperformanceofthree
dynamicfrequencyhoppingsystemswithequalloading(occu-
pancy) of 50%, but with different total number of carriers. A
negligible difference in performance is observed. This is im-
portant since, in certain deployment scenarios, cellular opera-
Fig.6. Voiceactivityeffects.
3Inourterminology,onebasestationcanservethreesectorsobtainedbyan-
tennasectorization.Inreuse4/12withasystembandwidthof12carriers,each
torshavesignificantinterestinusing onlya limitedportionof
basestationcanusethreecarriersandeachofitsthreesectorscanuseonecar-
rier. theirfullspectrum.Fromothersimulations,wehaveobserved
KOSTIC´ ANDSOLLENBERGER:DYNAMICFREQUENCYHOPPINGINLIMITED-BANDWIDTHCELLULARSYSTEMS 33
that the sensitivity to the number ofcarriers is morevisible in
random FH cases, where the performance for four carriers is
a bit worse than the performance with six and 12 carriers for
loadingof25%.
D. EffectsofVoiceActivity
For interference dominated systems, such as systems with
frequency hopping and with aggressive frequency reuse, the
role ofvoice activityis important.Theimplementation of dis-
continuoustransmission(DTX)toexploitvoiceactivityresults
in direct interference reduction, and thereby in capacity im-
provement.Figs.6and7illustratethebenefitsofvoiceactivity
for different systems. We observe that all considered systems
display nonnegligible performance degradation in the absence
ofvoice-activity-drivendiscontinuoustransmission(DTX).The
capacity of the DFH system for satisfactory performance falls
from six to four users, while the capacity of the fixed assign- Fig.7. EffectsofvoiceactivityonDFH.
mentsystemfallsfromthreetotwo.Inbothcases,DFHoffers
100%improvementincapacity.
E. EffectsofCorrelatedRayleighFading
Deploymentofnewtechniqueswillinitiallybedoneinsmall
portions of bandwidth. A significant portion of the frequency
hoppingadvantagecomesfromfrequencydiversity.Frequency
diversitycanonlybeachievedifthecoherencebandwidthofthe
channelunderconsiderationissmallerthanthetotalbandwidth
assignedforhopping.Fig.8showsthesensitivityofFHtothe
correlationbetweenfrequenciesusedinasinglefrequencyhop
pattern. The performance of all frequency hopping techniques
degrades under correlated fading conditions. For random FH,
threeuserscanbeservedwhenthefadingbetweenfrequencies
is fully correlated. Under the same conditions of full correla-
tion between carriers, DFH can no longer provide satisfactory
servicetosixusersperbasestation,butitprovidessatisfactory
servicetofourusersperbasestation.
Fig.8. Effectsoffrequencycorrelation.
F. SensitivitytoMeasurementErrors
The operation of dynamic frequency hopping relies on fre-
quencyqualitymeasurements.Measurementprecisionisdriven
bycomponentsusedforsignalstrengthmeasurement,filtering
performedonmeasurements,andisheavilyinfluencedbyshad-
owingandRayleighfadingvariation.Mobileunitsatthebound-
ariesbetweenbasestationsareoftenexposedtoconditionsthat
result insignificanterrorsinsignalstrength measurement.We
presentthesensitivityofDFHperformancetothestandardde-
viationofthemeasurementerrorinFig.9.Signalstrengthmea-
surement errors with standard deviation of as much as 6 dB
cause only minor performance degradation. In [12], we con-
cludedthat4-dBstandarddeviationisachievablewithwell-de-
signed filtering for averaging over Rayleigh fading, with the
measurement rate of once every two seconds per carrier. This
measurement rate is actually a relaxation of the requirements
inIS-136andGSM,whicharespecifiedtobehigherthantwo
measurementspersecond.Theconclusionofthecurrentpaper
Fig.9. EffectsofmeasurementerroronDFH(12carriers,sixmobilesperbase
isthatDFHisveryrobusttomeasurementerrors. station).
34 IEEETRANSACTIONSONWIRELESSCOMMUNICATIONS,VOL.1,NO.1,JANUARY2002
G. UplinkPerformanceVersusDownlinkPerformance
Two of the main factors that contribute to the performance
differencebetweenthedownlinkanduplinkare:1)differentge-
ographicaldistributionandmobilityofinterferersand2)pres-
enceofreceiverantennadiversityontheuplink.Ononehand,
onecanarguethatstrongbasestationinterfererswillbeeasyto
avoidbymeansofDFH,andtherebyimprovetheperformance
onthedownlink.Ontheotherhand,theantennadiversityhelps
theuplinkperformance.Wehavegeneratedsimulationstocom-
pareuplinkanddownlinkunderthesameuserdistributioncon-
dition, the results of which are shown in Fig. 10. One can ob-
servethatwithlowerloading,theperformanceisaboutequalon
bothlinks,andattheoccupancyofsixusersperbasestationthe
downlink is much better than the uplink. Related simulations,
not shown in the figures in this paper, show the following: 1)
DFHhelpstheperformanceofdownlinkmorethantheperfor-
manceofuplinkand2)DFHislesssuccessfulinimprovingthe
Fig. 10. Comparative performance of uplink and downlink for DFH as a
performancewhenthereissignificantcorrelationbetweenhop- functionofoccupancyforuncorrelatedRayleighfading(12carriers,sectorized
pingfrequencies. 1/1reuse).
V. DFH IMPLEMENTATION VIA NETWORK–ASSISTED
RESOURCEALLOCATION
Two aspects of measurement-based DFH are demanding on
cellularsystems:1)theneedtoperformrapidinterferencemea-
surementsatallrelevantfrequencies,bothatthemobilesandat
thebaseand2)thesignalingoverheadrequiredtocommunicate
themeasurementresultstobases.Aswillbeclearshortly,these
requirements can be significantly reduced by using network-
assisted resource allocation (NARA) [12], [13]. NARA uses
real-timeinterbasesignalingforintercellinterferencemanage-
ment.Ittakesadvantageofframesynchronizationonasystem
levelandprovidesfunctionalityidenticaltothatofthemeasure-
ment-basedDFH.
Fig.11illustratesthestructureofasystemthatutilizesNARA
fordownlinkDFHmanagement.Base-station-autonomousfre-
quency hop pattern management is assumed in this paper al-
thoughcentralizedmanagementisalsopossible.Theblockdia-
gramshowsthateachmobilecontinuouslymeasurespathlosses
(only)toallbasestationsinitsneighborhood.Thesemeasure-
mentsaresentthroughalow-passfiltertoaverageoutinstanta-
neousRayleighfadingeffects.Then,themobilesendsthemea-
surementstoitsservingbasestationoverthewirelesslink.The
measurementreportingrateneednotbeveryhigh,e.g.,therate
usedforMobileAssistedHandoffwouldsuffice.
Regarding the logic used to generate the hopping pattern,
there exist several possibilities. We briefly outline one such
procedure appropriate for a TDMA system: least-interfer-
ence-based DFH with NARA (LI-DFH with NARA). In this
description, a resource consists of a time slot and a frequency
hoppingpatternusedinthatslot. Fig.11. BlockdiagramofacellularsystemthatsupportsDFHwithNARA
Eachterminalmeasurespathlossestoitsneighboringbases fordownlink.
and transmits this information to its serving base on a regular
basis. use.Notethatforthisparticularalgorithm,itisnotnecessaryfor
Eachbasecommunicatestoseveraltiersofitsneighborsthe thebasestoexchangeanypath-lossinformationofactivelinks.
information about its own resource utilization: time slots, fre- Obviously, more complex algorithms could take advantage of
quencyhoppingpatterns,andpowerlevelsthatarecurrentlyin thistoimprovetheperformance.
KOSTIC´ ANDSOLLENBERGER:DYNAMICFREQUENCYHOPPINGINLIMITED-BANDWIDTHCELLULARSYSTEMS 35
Based on the resource utilization information exchanged EDGEplatform[15]showthatresource(frequencyhoporslot)
among bases and the path loss measurements reported by its assignment (deassignment) occurs on average on the order of
terminals, the serving base calculates the interference level at oncepersecond,withoutlossofperformanceforDFH.There-
each available resource, determines the least-interfered time fore,therequiredinformationrateis bits/s/ter-
slot and FH pattern pair, and assigns this to the terminal. minal.Weconcludethattheover-the-airsignalingnecessaryfor
Note that the hopping pattern used in this discussion is not a LI-DFHwithNARAisquiteminimal.
predefined sequence, but a sequence generated based on the Interbase Signaling: For implementing LI-DFH with
interferenceleveloneachfrequencyateachhop. NARA, the resource assignment information needs to be
Thisprocedureappliestonewandcurrentlyactiveusers.The broadcasttoneighboringbases.Notingthat4/12isthenominal
basecontinuouslymonitorseachuser’sperformance.Iftheper- reusepatternusedforGSM,andthata1/3reusepatternhas12
formancedegradesbelowathreshold,theuserisreassignedan- cochannelsectorscloserthananearest4/12cochannelsector,it
otherresource. isreasonabletoassumethatthisassignmentinformationshould
Effectively, the data is used by each base station to manage be transmitted to 12 neighboring sectors. Thus, the effective
dynamic frequency hopping patterns and execute interference interbase signaling rate is bits/s/terminal.
avoidance.Rapidmeasurements ofallavailablefrequenciesat Assumingthesystemcansupport150users/sector(seeFig.3),
all available time slots are replaced by the computation of in- thisimpliesthatforeachthree-sectoredsite,theaveragebitrate
terference conditions by base stations. Variations of the above neededforintersitesignalingis kb/s/site.
basicalgorithmarepossible,butarenotdetailedforbrevity.A This may be compared to the effective traffic capacity of the
staggered-frameresourcemanagementcanbeusedtoavoidthe samesite,whichis Mb/s/site(assuming
problemoftwonearbybasessimultaneouslychoosingthesame a 7.4-kb/s vocoder). Thus, the ratio of the extra signaling
resource. Another variation, which permits an optimization of bandwidth to the traffic bandwidth is around 5%, which even
theoverallsystemperformance,ispossibleifthebasestations afterincludingsomemarginformessageandprotocolheaders
exchangepathlossinformationofactiveusersinadditiontothe isstillreasonable.
information on resource utilization [13]. Clearly, a centralized
implementation of this is also possible, where a central con- VI. CONCLUSION
troller has the access to all the measurement and assignment
We have presented the results of a simulation study into
information, and can use them to optimize the overall perfor-
the performance of dynamic frequency hopping for a cellular
mance.
system with a limited total bandwidth supporting 12 carriers
Study of the performance of DFH with NARA, for a
and with frequency reuse 1/1 and three-sector antenna sector-
voice-over-IP over EDGE system in 7.2-MHz bandwidth and
ization. For the assumptions in the paper, DFH brings about
with both frequency and multi-time-slot resource availability,
100% increase in capacity over fixed channel allocation and
hasbeenpresentedin[15].TheresultsindicatethatDFHbrings
50% increase in capacity over random frequency hopping,
100% capacity improvement compared to random frequency
when satisfactory performance is defined as 0.02 WER for
hopping.
90% of users. Although the performance of various systems
is sensitive to the number of available carriers, voice activity,
A. SignalingOverhead
and channel coherence bandwidth, the system using DFH
We discuss the signaling, both over the air, and among the
offers the greatest overall robustness. DFH performs equally
bases, required for implementing LI-DFH with NARA for the
well for uplink and downlink. We have described DFH with
systemdiscussedinSectionIV.
networkassistedresourceallocationwhichallowsforpractical
Signaling Over the Air: There are three types of messages
implementationofDFHincellularsystems.
thatneedtobetransmittedforimplementingdownlinkLI-DFH:
a) path-loss measurements from each terminal to its serving
ACKNOWLEDGMENT
base; b) resource (time-slot and FH pattern) assignment mes-
sagesfromthebasetotheterminal;andc)signalqualityreports The authors are thankful to L. F. Chang, K. Chawla, J.
fromtheterminaltothebase.Wefocusonthefirsttwotypesof Chuang,andX.Qiuforhelpfuldiscussions.
messages,assignalqualityreportsarenecessarywhetherornot
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SystemsResearchDepartmentatAT&T.Hisdepartmentperformedresearchon
nextgenerationwirelesssystemsconcepts.From1979through1986,hewasa
memberofthecellularradiodevelopmentorganizationatBellLaboratories.At
BellLaboratories,heinvestigatedspectrallyefficientanaloganddigitaltech-
nologiesforsecond-generationcellularradiosystems.In1987,hejoinedthe
radioresearchdepartmentatBellcore,andhewastheheadofthatdepartment
from1993to1995.In1995,hejoinedAT&T.
Dr.SollenbergerisanAT&TFellow.