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1、High-SpeedDownlinkPacketAccessFromWikipedia,thefreeencyclopediaHigh-SpeedDownlinkPacketAccess(HSDPA)isanenhanced3G[1](thirdgeneration)mobiletelephony[2]communicationsprotocol[3]intheHigh-SpeedPacketAccess[4](HSPA)family,alsodubbed3.5G,3G+orturbo3G,whichallowsnetworksb
2、asedonUniversalMobileTelecommunicationsSystem[5](UMTS)tohavehigherdatatransferspeedsandcapacity.CurrentHSDPAdeploymentssupportdown-linkspeedsofupto42Mbit[6]/s.FurtherspeedincreasesareavailablewithHSPA+[7],whichprovidesspeedsofupto337Mbit/swithRelease11ofthe3GPP[8]stan
3、dards.[1]Technology[editsource[9]
4、editbeta[10]]High-SpeedDownlinkSharedChannel[editsource[11]
5、editbeta[12]]ForHSDPA,anewtransportlayer[13]channel,High-SpeedDownlinkSharedChannel[14](HS-DSCH),hasbeenaddedtoUMTSrelease5andfurtherspecification.Itisimplementedbyintroducin
6、gthreenewphysicallayer[15]channels:HS-SCCH,HS-DPCCHandHS-PDSCH.TheHighSpeed-SharedControlChannel(HS-SCCH)informstheuserthatdatawillbesentontheHS-DSCH,2slotsahead.TheUplinkHighSpeed-DedicatedPhysicalControlChannel(HS-DPCCH)carriesacknowledgmentinformationandcurrentchan
7、nelqualityindicator(CQI)oftheuser.Thisvalueisthenusedbythebasestationtocalculatehowmuchdatatosendtotheuserdevicesonthenexttransmission.TheHighSpeed-PhysicalDownlinkSharedChannel(HS-PDSCH)isthechanneltowhichtheaboveHS-DSCHtransportchannelismappedthatcarriesactualuserda
8、ta.Hybridautomaticrepeat-request(HARQ)[editsource[16]
9、editbeta[17]]Dataistransmittedtogetherwitherrorcorrectionbits.Minorerrorscanthusbecorrectedwithoutretransmission;seeforwarderrorcorrection[18].Ifretransmissionisneeded,theuserdevicesavesthepacketandlatercombinesitw
10、ithretransmittedpackettorecovertheerror-freepacketasefficientlyaspossible.Eveniftheretransmittedpacketsarecorrupted,theircombinationcanyieldanerror-freepacket.Retransmittedpacketmaybeeitheridentical(chasecombining[19])ordifferentfromthefirsttransmission(incrementalred
11、undancy[20]).SinceHARQretransmissionsareprocessedatthephysicallayer,their12msround-triptimeismuchlowercomparedtohigherlayerr
