【PPT】展频通讯(Spread解读.ppt

PhysicalLayerOverview,2020/6/11,2,展頻通訊（SpreadSpectrum）,原始目的是用於軍事上的保密通訊，並可抵制敵方的刻意干擾802.11的實體層所採用的即為展頻通訊技術中的跳頻展頻（FrequencyHoppingSpreadSpectrum，簡稱FHSS或FH）技術、802.11b與802.11g的實體層所採用直接序列展頻（DirectSequenceSpreadSpectrum，簡稱DSSS或DS）技術,2020/6/11,3,展頻通訊:延展通訊時所用到的頻帶寬度，使得所占用的頻寬遠大於要傳送原始資料所需的最小速率,展頻通訊系統模型,2020/6/11,4,展頻通訊的優點,抑制故意的干擾竊聽的困難度較高可用於定位系統可提供多人同時共用無線頻道,2020/6/11,5,抑制故意的干擾,2020/6/11,6,竊聽的困難度較高,W大P小,2020/6/11,7,可用於定位系統,在定位系統中，通常以脈衝在空間中傳遞所發生的延遲作為距離長短計算的依據而在延遲大小的量測中，其不確定的部分則與單一脈衝所占用的頻寬成反比,2020/6/11,8,可提供多人同時共用無線頻道,展頻通訊中所使用延展信號為一特定之代碼串列，只要每個頻道的使用者所使用的代碼串列彼此獨立（也就是線性代數理論上所謂的正交（Orthogonal）可以形成多人公用頻道的狀況,2020/6/11,9,直接序列展頻,直接序列展頻的原理為將原始資料速率為的資料串列轉換成一資料速率為偽隨機代碼串列，以所得之偽隨機代碼串列進行調變才傳遞去。,2020/6/11,10,2020/6/11,11,2020/6/11,12,BPSK直接序列展頻系統之信號PSD,PowerSpectralDensity,2020/6/11,13,偽隨機碼（也就是片序列）的選取原則,偽雜訊序列（Pseudo-Noise（PN）Sequence）若N為序列的週期，則,2020/6/11,14,二進位的週期性序列是否具有類似雜訊的特性?,平衡（balance）：在一個週期的序列中，二進位1（+1）的個數與二進位0（-1）的個數最多只相差一個。同號連續（Run）：長度為一的同號連續的個數要佔該序列所有同號連續總數的一半，長度為二的同號連續要占總數的1/4，長度為四的同號連續要占總數的1/8，相關性運算（correlation）：一個序列若要具有類雜訊的特性，其比較後的結果最好是相同數值的總位元數與相異數值的總位元數最多只差1。,2020/6/11,15,1.平衡屬性來說，+1的個數為8個，-1的個數為7個,2.,3.,Example:,2020/6/11,16,2020/6/11,17,2020/6/11,18,如果因第二路徑效應而產生之信號,的時間延遲,大於,，也就是,內含的偽隨機碼信號被位移超過一個位元，則在進行解調變時，,其對於正常信號的干擾就會變得很小。,2020/6/11,19,Anygirlcanbeglamorous.Allyouhavetodoisstandstillandlookstupid.-HedyLamarr,2020/6/11,20,2020/6/11,21,TheISMPHYs:FH,DS,andHR/DS,802.11FHPHYTheelectronicsusedtosupportfrequency-hoppingmodulationarerelativelycheapanddonothavehighpowerrequirements.Themainadvantagetousingfrequency-hoppingnetworkswasthatagreaternumberofnetworkscouldcoexistandtheaggregatethroughputofallthenetworksinagivenareawashigh.,2020/6/11,22,Frequency-HoppingTransmission,Timingthehopsaccuratelyisthekeytosuccess;boththetransmitterandreceivermustbesynchronizedsothereceiverisalwayslisteningonthetransmittersfrequency.,2,8,4,7,2020/6/11,23,Eachfrequencyisusedforasmallamountoftime,calleddwelltime.(max:390timeunits)1TU=1024micro-sec.Usually,beaconinterval=100TU=0.1sec.19TUissuggestedby802.11.Thehoppingprocesscantakenolongerthan224micro-sec.,2020/6/11,24,Hoppingsequencesthatdonotoverlaparecalledorthogonal.,2,8,4,76,3,7,2,2020/6/11,25,802.11FHdetailsChannelsaredefinedbytheircenterfrequencies,whichbeginat2.400GHzforchannel0.99%oftheradioenergyisconfinedtothechannel.Successivechannelsarederivedbyadding1-MHzsteps:Channel1hasacenterfrequencyof2.401GHzChannel2hasacenterfrequencyof2.402GHzChannel95hasacenterfrequencyof2.495GHz,2020/6/11,26,2020/6/11,27,802.11hopsequencesAsanexample,hoppingsequence1forNorthAmericaandmostofEuropebeginswiththesequence3,26,65,11,46,19,74,50,22,2020/6/11,28,2020/6/11,29,Joiningan802.11frequency-hoppingnetworkTheFHParameterSetelementincludesthehoppatternnumberandahopindex.(e.g,(1,2)ByreceivingaBeaconframe,astationknowseverythingitneedstosynchronizeitshoppingpattern.Example:nextchannel65,timestamp/dwelltimewhentoswitchtochannel.,2020/6/11,30,ISMemissionrulesandmaximumthroughput(USFCCsrules)Theremustbeatleast75hoppingchannelsintheband,whichis83.5-MHzwide.Hoppingchannelscanbenowiderthan1MHz.1MHz1Mbps(2-levelencoding)Devicesmustuseallavailablechannelsequally.Ina30-secperiod,nomorethan0.4secmaybespentusinganyonechannel.,2020/6/11,31,EffectofinterferenceWithapproximately80usablechannelsintheU.S.andEurope,interferenceononechannelreducestherawbitrateofthemediumbyapproximately1.25%,2020/6/11,32,GaussianFrequencyShiftKeying(GFSK),2-levelGFSKNoisechangestheamplitudeofsignal;(advantageofusingfrequencyformodulation),2020/6/11,33,Therateatwhichdataissentthroughthesystemiscalledthesymbolrate.Althoughthecarrierfrequencyisroughly2.4GHz,thesymbolrateisonly1or2millionsymbolspersecond.FrequencychangeswithGFSKarenotsharpchanges.(sharphighcost),2020/6/11,34,4-levelGFSKThefoursymbols(00,01,10,and11)eachcorrespondtoadiscretefrequency,andtherefore4GFSKtransmitstwiceasmuchdataatthesamesymbolrate.Distinguishingbetweentwolevelsisfairlyeasy.(fourisharder.)Eachdoublingofthebitraterequiresthattwiceasmanylevelsbepresent,andtheRFcomponentsdistinguishbetweeneversmallerfrequencychanges.,2020/6/11,35,2020/6/11,36,2Mbps.,2020/6/11,37,2020/6/11,38,FHPHYConvergenceProcedure(PLCP),FramingandwhiteningThePLCPfortheFHPHYaddsafive-fieldsheadertotheframeitreceivesfromtheMAC.,2020/6/11,39,ThePLCPframing:PreambleSync(0101010101010)StartFrameDelimiter(SFD)(0000110010111101)HeaderPSDULengthWord(PLW)(ThelengthofMACframe)PLCPSignaling(PSF)(bit0issetto0)HeaderErrorCheck(HEC),2020/6/11,40,2020/6/11,41,WhitenArbitrarydatamaycontainlongstringsofconsecutive0sor1s,whichmakesthedatamuchlessrandom.Tomakethetransmitteddatamorelikerandomwhitenoise,theFHPHYsapplyawhiteningalgorithmtotheMACframe.Thisalgorithmscramblesthedatabeforeradiotransmission.Receiversinverttheprocesstorecoverthedata.,2020/6/11,42,2020/6/11,43,2020/6/11,44,Frequency-HoppingPMDSublayer,Thegeneraldesignofthetransceiverusedin802.11frequency-hoppingnetworks,2020/6/11,45,PMDfor1.0-MbpsFHPHYInkeepingwiththecommonregulatoryrestrictionofa1-MHzbandwidth,1millionsymbolsaretransmittedpersecond.2GFSKisusedasthemodulationscheme,soeachsymbolcanbeusedtoencodeasinglebit.,2020/6/11,46,PMDfor2.0-MbpsFHPHYInthePLCPheader,thePSFfieldindicatesthespeedatwhichtheframebodyistransmitted.Atthehigherdatarate,theframebodyistransmittedusingadifferentencodingmethodthanthephysical-layerheader.RegulatoryrequirementsrestrictallPMDstoasymbolrateof1MHz,so4GFSKmustbeusedfortheframebody.Twobitspersymbolyieldsarateof2.0Mbpsat1millionsymbolspersecond.,2020/6/11,47,CharacteristicsoftheFHPHY,2020/6/11,48,802.11DSPHY,Direct-SequenceTransmission,2020/6/11,49,Chipping,Achipisabinarydigitusedbythespreadingprocess.Chippingstream(pseudorandomnoisecodes(PNcodes)mustrunatamuchhigherratethanunderlyingdata.,2020/6/11,50,Barkercode,2020/6/11,51,一個巴克碼的數字序列循環位移，其結果仍為巴克碼一個巴克碼的所有數字之正負符號顛倒(也就是+1變1，1變+1)，其結果也仍是巴克碼一個巴克碼的相反順序也仍是一個巴克碼,2020/6/11,52,802.11DSdetails802.11adoptedan11-bitbarkerwordforPNcode.,2020/6/11,53,OperatingchannelsTheDSPHYhas14channelsinthe2.4-GHzband,each5MHzwide.Channel1isplacedat2.412GHz.,2020/6/11,54,Channelenergyspread,2020/6/11,55,MaxtheoreticalthroughputISMband80MHzSpreadingfactor11Maxbitrateslightlymorethan7MbpsInterferenceresponse,2020/6/11,56,DifferentialPhaseShiftKeying(DPSK),2020/6/11,57,2020/6/11,58,2020/6/11,59,DSPLCP,FramingTheFHPHYusesadatawhitenertorandomizethedatabeforetransmission,butthedatawhitenerappliesonlytotheMACframe.TheDSPHYusesascramblerandappliestoPLCPheader+preamble+MACframe.,2020/6/11,60,SynSFD0000010111001111Signal000010101Mbps000101002MbpsService(reserved)LengthmicrosecondCRC,2020/6/11,61,DSMDP,Transmissionat1.0Mbps,2.0MbpsCS/CCAforthePHYMode1:Whentheenergytheenergydetection(ED)thresholdbusyMode2:AnactualDSSSsignalbusyMode3,2020/6/11,62,DSPHYParameters,2020/6/11,63,802.11b:HR/DSSSPHY,DSPHYs11MchipspersecChipstream11-bitBarkerwords1MwpsEachwordencodes1or2bits1or2MbpsHighratePhase-shiftencoding?Complementarycodekeying(CCK)dividesthechipstreamintoaseriesof8-bitcodesymbols,sotheunderlyingtransmis