基于GEP变频和流量控制系统 外文翻译.docx

基于GEP变频和流量控制系统 外文翻译.docx

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基于GEP变频和流量控制系统外文翻译

EfficientschedulingforPLCnetworks

T.Chiras,P.KoutsakisandM.Paterakis

Powerlinecommunications（PLC）arecurrentlybeingconsideredasan

alternativeforhigh-speeddatacommunicationsandInternetaccess.

Presentedisanefficientbandwidthallocationschemewhichsignifi-

cantlyexcelsincomparisontotheExtendedAlohaMediumAccess

Control（MAC）protocolforthe‘lastmile’accessPLCnetworks.

Introduction:

TheunparalleledgrowthoftheInternet,combinedwith

significanttechnologicaladvancementsofVLSIanddigitalsignal

processing,andwiththetelecommunicationsmarketderegulation

aroundtheworld,havemadepowerlinecommunications（PLC）a

viabletechnologyfornextgenerationtelecommunications.With

multipleoutletsinalmosteveryroom,everywhere,powerlinesare

alreadythemostpervasivenetworkinthehomeorsmalloffice;

therefore,theywouldbethepreferredmediumforprovidingbroad-

bandconnectiontoruralorremoteareaswheretelephoneandcable

connectionsmaynotexist.ThemarketforPLCistwofold:

tothe

home,or‘lastmile’access,andinthehome,or‘lastinch’access[1].

RelevantresearchontheMAClayerforPLChasfocusedmoreon

in-homenetworking[2,3].TheworkpresentedinthisLetterfocuses

onthe‘lastmile’problem,andintroducesschedulingideaswhich

leadtosignificantimprovementsinnetworkperformanceanduser

QualityofService（QoS）comparedtotheExtendedALOHA[4,5]

protocol,forpowerlinecommunicationnetworks.

Proposedschedulingscheme:

Orthogonalfrequencydivisionmodula-

tion（OFDM）hasbeenoutlinedasoneofthebestcandidatesfor

applicationinPLCsystemswithhigherdatarates,becauseofits

excellentbandwidthefficiency[4–6].WeconsideranOFDMtransmis-

sionsystemwhichusesanumberofsubcarriersdistributedina

frequencyspectrum.Theworkpresentedin[4,5]proposedthree

extensionsofthebasicALOHAprotocolinordertoimproveits

performanceonthePLCnetwork:

（a）piggybacking,whichleadstoa

decreaseinthesignallingdelay;thisisdefinedasthetimeneededfor

therealisationoftherequestingprocedureforthetransmissionofa

packetandincludesthetransmissionofarequestmessagetothebase

stationandthereceptionofitsresponseregardingtheaccessrights;

（b）useofdatachannelsforsignalling（from[7]）;（c）applicationofan

adaptivebackoffmechanismforuseraccesstothesignallingchannel,

aswellasforuseraccesstothedatachannelsforsignallingpurposes.

Inourwork,wealsoadopttheideasofpiggybackingandusingdata

channelsforsignalling.However,wedonotusetheadaptivebackoff

mechanismproposedin[4,5]foruserstoselecttheslotinwhichthey

willtransmit=retransmittheirrequests;instead,weproposethreenew

ideas,tworegardingtheslotselectionmechanismandoneregardingthe

channelselectionmechanismforaPLCaccessnetwork.

A.Channelselection:

Weuseandcomparetwomechanismsfor

channelselectioninourstudy.Thefirstmechanismissimilarto

thatusedin[4,5]andisnameduniformchannelselectioninthis

Letter.Withtheuseofthismechanism,eachterminalwhichneeds

toaccessthemediumselectsuniformlyoneofthe15channels（one

forsignallingand14fordatatransmissions）;theonlyconstraintis

thatselectionismadeamongchannelswhichhaveatleastoneidle

slotinthecurrentchannelframe（notransmissionisscheduledin

thatslotfrompreviouschannelframes）.Ifthechanneliscongested,

itisnottakenunderconsiderationinthechannelselectionprocess

forthecurrentframe.

Ourproposalforasecondchannelselectionmechanismisnamed

weightedchannelselection.Atthebeginningofeachchannelframethe

basestationhasfullknowledgeofthetotalnumberofidleslotsinall

thedatachannelsandthesignallingchannel.Letthistotalnumberof

idleslotsbeS.TheprobabilityforaterminaltochoosechannelY,

whichhasthreeidleslotsinthecurrentchannelframe,inordertosend

itsrequestis3=S.Therespectiveprobabilityforthesignallingchannel

isequaltothetotalnumberofslotsofthesignallingchannel（theslots

ofthesignallingchannelarebynaturealwaysidleatthebeginningofa

channelframe,asnoinformationtransmissiontakesplaceinthem）

dividedbyS.Theweightedchannelselectionmechanismisdesignedin

awayasto‘push’requestinguserstochoose,ineverychannelframe,

withgreaterprobabilitythechannelswiththelargernumberofidle

slots,inordertodecreasetheprobabilityofcollisionsinthesystem.

B.Slotselection:

Afterselectingachannel,aterminalneedsto

choosetheslotinwhichitwilltransmititsrequest.Weproposetwo

differentmechanismsforslotselectioninourstudy.Thefirst

mechanismisnameduniformslotselection:

afterselectinga

channelwithMidleslots（thisinformationisgiventotheterminals

bythebasestationafterthechannelisselected）,theterminal

attemptstotransmitinthefirstoftheseslotswithaprobability

P¼1=M（forM¼1,Pisbydefault0.5,otherwiseacollision

wouldbeunavoidable）.Inthecaseofasuccessfultransmission,a

terminalacquiresthespecificslotfortransmissioninsubsequent

channelframes,whileinthecaseofacollisiontheterminal

continuestotransmitinidleslotswiththeabove-defined

probability.Ifthechannelframeendswithouttheterminalhaving

succeededinitsrequesttransmission,theterminalrepeatsthe

processesofchannelandslotselectionforeverynewchannel

frame,foraslongasitneedstogainaccesstomedium.Thesecond

proposedmechanismforslotselectionisnamedweightedslot

selectionandworksasfollows.AfterselectingachannelwithM

idleslots,theterminalcreatesthefollowinggroupofM

probabilities:

{1=M,1=M,2=M,3=M,...,（M1）=M},and

randomlyassociateseachoneoftheidlechannelslotswithoneof

theprobabilitiesinthegroup.IfM¼1,theprobabilityisagain

chosenbydefaulttobeequalto50%.Theweightedslotselection

mechanismaimsatofferingthechancetorequestingterminalsto

transmittheirrequestssooner,byusingmuchhighertransmission

probabilitiesthantheuniformslotselectionmechanism（atthecost

ofapossiblylargernumberofcollisions）.

Withtheuseoftheaboveideas,fourversionsofourMACprotocol

wereexamined:

theuniform-weightedselection（U-W）,referringtoa

uniformchannelandweightedslotselection,theuniform-uniform

selection（U-U）,theweighted-uniformselection（W-U）andthe

weighted-weighted（W-W）.

Resultsanddiscussion:

Thesystemparametersusedinourworkare

takenfrom[4,5],inordertomakeadirectcomparisonwiththat

work,whichfocusedondata（Internet）traffic.Sincepackettransmis-

sioninPLCshouldbemadeinveryshortframessothatthereceiver

canadapttotherapid（<1ms）changesinthePLCchannelcondi-

tions,wechosetoconsideronlypacketswithaveragesizeequalto

300bytesandwithmeaninterarrivaltime0.96s;thisisthecase

definedin[4,5]asthe‘frequentrequestcase’,andenablesustotest

ourschemeunderheavytrafficconditions.Theofferedtrafficloadper

networkstationis2.5kbit=s.Thepacketsizesandinterarrivaltimes

aregeometricallydistributedrandomvariables.Theframedurationis

47ms,theslotdurationisequalto4ms,theslotcapacityis32bytes

andthepayloadineachslotis28bytes.Inoursimulationsweassume

thatatransmissionchanneloffersafixeddatarateof64kbit=s,and

thatthenetworkconsistsof15bidirectionaltransmissionchannels,

oneofwhichisreservedforsignalling.Wesimulatedonehourof

networkperformance.Eachsimulationpointistheresultofan

averageof10independentruns（Monte-Carlomethod）.

Fig.1showscomparisonofourresultswiththeExtendedALOHA

protocolof[4,5].ItisclearfromtheFigurethatatlowtrafficloadsthe

signallingdelayachievedbyallversionsofourprotocolisremarkably

smallerthanthatachievedbytheExtendedALOHAprotocol.Asthe

trafficloadincreases,signallingdelaynaturallyincreasesalso,dueto

theincreaseinthenumberofcollisionsinthenetwork.Still,asshown

inFig.1,signallingdelayachievedbyallversionsofourprotocol

remainsmuchsmallerthanthatoftheExtendedALOHAprotocol,by

severalhundredsofms.Whencomparingtheresultsofthefour

versionsofourprotocols,W-Uselectionachievesthelowestsignalling

delayforlow-to-mediumtrafficloadsandU-Wselectionachievesthe

lowestsignallingdelayformedium-to-hightrafficloads.

AsshowninFig.2,forupto150userstheuseoftheExtended

ALOHAprotocolprovidesalmostidenticalperformanceinnetwork

utilisationwiththefourversionsofourprotocol.However,astraffic

loadincreases,thenetworkutilisationachievedbytheExtended

ALOHAprotocolissignificantlysmallerthanthoseachievedbythe

fourversionsofourprotocol,thedifferencebetweenthemexceeding

20%whenthenumberofusersrangesbetween300and400and

remaininglargeevenforhighertrafficloads.

SincethedifferencebetweentheExtendedALOHAprotocolandour

protocolexistsinthetransmission=retransmissionalgorithmsused,itis

clearthatourproposedalgorithmsarethereasonforwhichourschemes

excel.Morespecifically,theadaptivebackoffmechanismusedin[4,5]

hastheinherentdisadvantagesthat:

（a）afterthecalculationofthe

retransmissioninterval,theterminalwillattempttoretransmitinthe

newlycalculatedslot,disregardinganyidleslotswhichmayexist

beforethecalculatedone;onthecontrary,inallversionsofour

scheme,aterminalwhichfailstotransmitits