IEEEStd802.ppt

IEEEStd802.11Part11:WirelessLAN,MediumAccessControl(MAC)andPhysicalLayer(PHY)Specifications,-Edgar_Huang/201308,Part.1背景介紹,1.1背景介紹1.2802.11networkstandards1.3CSMA/CDvs.CSMA/CA1.4DSSSvs.FHSS1.5CCK补码键控1.6DFSandTPC1.7Channellist1.8IBSS/BSS/ESS,1.1.1背景介紹,1997年美國電機電子協會IEEE(instituteofElectricalandElectronicsEngineer)公告IEEE802.11無線區域網路標準，在這個規範中，共定義了三種不同的實體屆介面(MediumAddressControl;MAC):直接序列展頻(DirectSequenceSpreadSpectrum;DSSS)跳頻展頻(FrequencyHoppingSpreadSpectrum;FHSS)紅外線(lnfrared)技術1999年更進一步提出IEEE802.11的延伸規格:IEEE802.11a(5GHz，OFDM，最高速率54Mbps，因5G頻道數量稀少，被採用的進程緩慢)IEEE802.11b(2.4GHz，使用補碼鍵控CCK對DSSS增強，最高速率11Mbit/s，國際上普遍允許2.4G用於商用，因而獲得巨大的市場成功，而IR和FHSSPHY的市場則沒有成長起來）,1.1.2背景介紹,2001年FCC允許在2.4G頻帶上使用OFDM。2003年IEEE制定802.11g標準修訂，將OFDM包含到2.4G頻帶，此外兼容802.11b設備。802.11g設備獲得了巨大的市場成功2003年IEEE制定802.11h，之後FCC和ETSI在5G頻帶上允許額外的400MHz無需授權的頻譜用於商用，為802.11n廣泛運行于40MHz模式鋪平了道路。2009年IEEE制定802.11n標準修訂，改進無綫網絡速率的不足。802.11n增加了對於MIMO的標準，使用多個發射和接收天線來允許更高的資料傳輸率，並使用了Alamouti於1998年提出的空時分組碼（Space-timeblockcode，STBC）來增加傳輸範圍。802.11n標準頻寬（20MHz），使用4xMIMO時速度最高可達300Mbit/s。802.11n雙倍頻寬（40MHz），使用4xMIMO時速度最高可達600Mbit/s。,1.1.3背景介紹,IEEE802.11ac，是一個正在發展中的802.11無線區域網路（WLAN）通訊標準，它透過5GHz頻帶進行通訊。理論上，它能夠提供最少1Gbps頻寬進行多站式無線區域網通訊，或是最少500Mbps的單一連線傳輸頻寬。Extendedchannelbinding,Mandatory80MHzchannelbandwidthforSTAs(vs.40MHzmaximumin802.11n),160MHzavailableoptionallyMoreMIMOspatialstreams,Supportforuptoeightspatialstreams(vs.fourin802.11n)Multi-userMIMO(MU-MIMO),MultipleSTAs,eachwithoneormoreantennas,transmitorreceiveindependentdatastreamssimultaneously,“SpaceDivisionMultipleAccess”(SDMA):streamsnotseparatedbyfrequency,butinsteadresolvedspatially,analogousto11n-styleMIMODownlinkMU-MIMO(onetransmittingdevice,multiplereceivingdevices)includedasanoptionalmodeModulation,256-QAM,rate3/4and5/6,addedasoptionalmodes(vs.64-QAM,rate5/6maximumin802.11n)Otherelements/featuresBeamformingwithstandardizedsoundingandfeedbackforcompatibilitybetweenvendors(non-standardin802.11nmadeithardforbeamformingtoworkeffectivelybetweendifferentvendorproducts)MACmodifications(mostlytosupportabovechanges)Coexistencemechanismsfor20/40/80/160MHzchannels,11acand11a/ndevices,1.2802.11networkstandards,1.3CSMA/CDvs.CSMA/CA,CSMA/CD(CarrierSenseMultipleAccesswithCollisionDetection)CSMA/CA(CarrierSenseMultipleAccess、withCollisionAvoidance)有綫局域網上的碰撞可以由電信號檢測出來，因此近乎實時。無限局域網上的碰撞是在整個幀發送完以後沒有得到遠端的確認，或其他形式響應的情況下才能被推斷出來，因而所帶來的性能損失要大很多。,SIFS(ShortInterframeSpace)PIFS(PointCoordinationlnterframeSpace)DlFS(DistributionCoordinationlnterframeSpace),1.4.1DSSSvs.FHSSDirectSequenceSpreadSpectrum,DSSS提供一個可靠的無線連結，其基本原理是將要發送的基頻(BaseBand)訊號轉換成能量降低，但是頻寬更寬的展頻(SpreadingSignal)訊號再傅送出去。IEEE802.11的直接序列展頻是利用11-chip的展頻碼(SpreadingCode)，又稱為虛擬噪音碼PseudoNoiseCode:PNCode)將原始訊號展開，這個展頻碼有一個特性，當原始訊號經過二次展頻碼的處理後，會還原為原始訊號。抗干扰能力强,防竊聽較強的抗多径干扰能力系统带宽太大，进入接收机前端的干扰信号增多,1.4.2DSSSvs.FHSSFrequencyHoppingSpreadSpectrum,跳頻展頻(FrequencyHoppingSpreadSpectrum)的技術是在同步且同時的狀況下，無線連結的兩端利用相互協議的無線電波形式來傳遞訊號，這種形式為載波在不同頻道間跳躍的模式。跳頻技術是利用一個很寬的頻段來傅遞資料，雖然細分成數十個小頻道，但由於訊號在頻道上跳躍的速度很快，感覺上，好像是使用整個2.4GHz的頻段。因為這樣的架構，當受到環境的干擾，只會造成某個頻帶可能無法傳輸，發射端會針對無法傅遞的部分重送資料，如此雖會減低部分的傳輸速率，但是不太容易造成完全的斷訊。IEEE802.11同時規範了全球跳頻順序的形式(HoppingSequences)，這主要目的是在建立一個FHSS的無線網路時，溝通的二方必須使用相同的跳躍模式(Pattern)才能夠互通。規範的跳頻模式是為了降低在傳輸時無線電波問的相互干擾(Interference)。,1.5CCK补码键控ComplementaryCodeKeying,802.11b仍使用DSSS但採用CCK(ComplementaryCodeKeying)調變技術，CCK並非使用固定的展頻碼，是根據所要傳送的訊號，使用不同的展頻碼，因此能提升資料傳輸速率。,1.6DynamicFrequencySelection(DFS)andTransmitPowerControl(TPC),DFSdynamicallyinstructsatransmittertoswitchtoanotherchannelwheneveraparticularcondition(suchasthepresenceofaradarsignal)ismet.Priortotransmitting,adevicesDFSmechanismmonitorsitsavailableoperatingspectrum,listeningforaradarsignal.Ifasignalisdetected,thechannelassociatedwiththeradarsignalwillbevacatedorflaggedasunavailableforusebythetransmitter.Thetransmittingdevicewillcontinuouslymonitortheenvironmentforthepresenceofradar,bothpriortoandduringoperation.Portionsofthe5GHzbandareallocatedtoradarsystems;thisallowsWLANstoavoidinterferencewithincumbentradarusersininstanceswheretheyarecolocated.Suchfeaturescansimplifyenterpriseinstallations,becausethedevicesthemselvescanautomaticallyoptimizetheirchannelreusepatterns.TPCtechnologyhasbeenusedinthecellulartelephoneindustryformanyyears.Settingthetransmitpoweroftheaccesspointandtheclientadaptercanbeusefultoallowfordifferentcoverageareasizesand,inthecaseoftheclient,toconservebatterylife.Indevicesthathavetheabilitytosetpowerlevels,thesettingsareusuallystaticandindependentofeachother(accesspointandclients).Forexample,anaccesspointcanbesettoalow5mWtransmitpowertominimizecellsize,whichisusefulinareaswithhighuserdensity.Theclientswill,however,betransmittingattheirpreviouslyassignedtransmitpowersettings,whichislikelymoretransmitpowerthanisrequiredtomaintainassociationwiththeaccesspoint.ThisresultsinunnecessaryRFenergytransmittingfromtheclients,creatingahigherlevelthanisnecessaryofRFenergyoutsidetheaccesspointsintendedcoveragearea.WithTPC,theclientandaccesspointexchangeinformation,thentheclientdevicedynamicallyadjustsitstransmitpowersuchthatitusesonlyenoughenergytomaintainassociationtotheaccesspointatagivendatarate.Theendresultisthattheclientcontributeslesstoadjacentcellinterference,allowingformoredenselydeployedhigh-performanceWLANs.Asasecondarybenefit,thelowerpowerontheclientprovideslongerbatterylife-lesspowerisusedbytheradio.,Gchannellist,1.7.25Gchannellist,1.8.1IBSS/BSS/ESSIndependentBasicServiceSet,IndependentBasicServiceSet(IBSS)allowstwoormoredevicestocommunicatedirectlywitheachotherwithoutaneedforacentraldevice.ThisisknownasAdhocmodewhereapeertopeernetworkbetweenstationsisformedwithouttheneedforanAccessPoint.,1.8.2IBSS/BSS/ESSBasicServiceSet,BasicServiceSet(BSS)WirelessLANisestablishedusingacentraldevicecalledanAccessPointthatcentralizesaccessandcontroloveragroupofwirelessdevices.AllwirelessdevicesdonotcommunicatedirectlywitheachotherbutinsteadtheycommunicatewiththeAP,andtheAPforwardstheframestothedestinationstations.TheAccessPointmanagesthewirelessnetwork,advertisesitsownexistencebybroadcastingtheServiceSetIdentifier(SSID)andanydevicethatneedstousethewirelessnetworkmustfirstsendanassociationrequesttotheAccessPoint.TheAccessPointcanrequireanyofthefollowingcriteriabeforeallowingaclienttojoin.ThewirelesscoverageareaofanAPiscalledtheBasicServiceArea(BSA),sometimesalsoreferredasWirelessCell.AnAPcanalsobeconnectedtoawiredEthernetLocalAreaNetworkthroughanuplinkportconnectionunliketheIndependentBasicServiceSetinwhichthewirelessnetworkcannotbeconnectedtothewirednetwork.,TheBSSisuniquelyidentifiedbytheBasicServiceSetIdentifier(BSSID)whichistheLayer2MacaddressoftheBSSaccesspoint.ThewirelessnetworkalthoughisadvertisedusinganSSIDwhichannouncestheavailabilityofthewirelessnetworktodevices.,1.8.3IBSS/BSS/ESSExtendedServiceSet,ESSiscreatedbyconnectingmultipleBasicServiceSet(BSS)viaadistributionsystem.TwoormoreAccessPointsareconnectedtothesameLocalAreaNetworktoprovidealargercoverageareawhichallowstheclienttomovefromoneAPtoanotherAPandstillbethepartoftheLAN.ThisprocessisknownasroaminginwhichaclientcanphysicallychangelocationsandstillbeconnectedtotheLAN.WhenaclientsensesthatradiosignalfromthecurrentAParegettingweaker,itfindsanewAPwithstrongersignalsstartsusingthatAP.AnESSgenerallyincludesacommonSSIDtoallowroamingfromaccesspointtoaccesspointwithoutrequiringclientre-conguration.ThewirelesscoverageareacreatedbyjoiningtwoormoreAccessPointsviadistributionsystemiscalledanExtendedServiceArea(ESA).StationswithinthesameESAmaycommunicatewitheachother,eventhoughthesestationsmaybeindifferentbasicserviceareasandmayevenbemovingbetweenbasicserviceareas,thisrequiresthatthewirelessmediumandthebackboneoftheESSmustbelayer2link.,ADistributionSystemconnectsmultipleAccessPointstoformanESS,whiledoingsoitprovidesthewirelessstationswiththeoptionofmobility.ItisthemeansbywhichanaccesspointcommunicateswithanotheraccesspointtoexchangeframesforstationsintheirrespectiveBSSs,forwardframestofollowmobilestationsastheymovefromoneBSStoanother,andexchangeframeswithawirednetwork.NetworkequipmentoutsideoftheextendedservicesetviewstheentireESSandallofitsmobilestationsasasinglelayer2networkwhereallstationsarephysicallystationary.ThustheESShidesthemobilityofthemobilestationsfromeverythingoutsidetheESS.Thisallowscorrectinter-operationwithothernetworkprotocolsthatdonotunderstandtheconceptofmobility.,Part.2PHY,2.1OFDM2.2MIMOandSDM2.3802.11a2.4802.11b2.5802.11g2.6802.11n2.7802.11ac,2.1OFDM無綫信號傳輸特點Multi-Path,OFDM徑損失(pathLoss),自由空間(FreeSpace)所謂自由空間是指傳送與發射端天線之間的區域是沒有任何物體會阻礙電磁波的傳遞或是吸收、反射電磁波的能。遮蔽效應(shadowing)造成的慢速衰(slowfading)遮蔽效應主要是接收機周圍十公尺到百公尺的範圍內有大型障礙物，如大、山丘、樹及樹木等，對訊號所造成的遮蔽效應而產生的衰現象多普勒效应(Dopplereffect)與多重徑延遲(Multi-pathdelay)所造成的快速衰(fastfading),OFDM徑損失(pathLoss),2.1.1OFDMOrthogonalfrequency-divisionmultiplexing,正交频分复用之基本观念为将一高速数据流，分割成数个低速数据流，并将这数个低速数据流同时调制在数个彼此相互正交载波上传送。由于每个子载波带宽较小，故可以有效对抗频率选择性衰弱，因此现今以大量采用于无线通信。OFDM使用大量紧邻的正交子载波（Orthogonalsub-carrier），每个子载波采用传统的调制方案，进行低符号率调制。可以视为一调制技术与复用技术的结合。,2.1.2OFDM的特點,传送与接收端需要精确的同步各子信道上的正交调制和解调可以采用IDFT和DFT实现，运算量小，实现简单。OFDM系统可以通过使用不同数量的子信道，实现上下行链路的非对称传输。所有的子信道不会同时处于频率选择性深衰落，可以通过动态子信道分配充分利用信噪比高的子信道，提升系统性能。对频率偏差敏感：传输过程中出现的频率偏移，如多普勒频移(由于波源和接收者之间存在着相互运动而造成接收者接收到的频率与波源发出的频率之间发生变化)，或者发射机载波频率与接收机本地振荡器之间的频率偏差，会造成子载波之间正交性破坏。存在较高的峰均比：OFDM调制的输出是多个子信道的叠加，如果多个信号相位一致，叠加信号的瞬间功率会远远大于信号的平均功率，导致较大的峰均比，这对发射机PA的线性提出了更高的要求。每个子载波的功率谱密度的尖峰发生在其他子载波功率的零点.子载波间隔(f)等于1/Ts(符号传输周期)频谱效率上升,OFDMvsSingleCarrierModulationFrequencyDomainView,OFDMvsSingleCarrierModulationTimeDomainView,OFDM循环前缀和保护间隔,传送信号在通过具有多重路径干扰的信道后，会造成前一个符元的后端部份干扰到下一个符元的前端，此称之为ISI（inter-symbolinterference符号间干扰）为了克服ISI的问题，在OFDMsymbol前端加入一保护区间（GuardInterval）。为了对抗信号因信道延迟的影响，Guardinterval(Tgi)长度要大于最大的Delayspread，即Tgidelayspreadtime。为了避免多径传播造成子载波间的正交性破坏，复制OFDMsymbol后半段信号Tcp并摆放于保护区间内，称之为循环前缀(cyclicprefix)；循环前缀会造成带宽效益下降，故必须小于OFDMsymbol长度的1/4。,OFDMinter-carrierinterference,在保护区间未放信号的OFDM系统称ZP-OFDM(zeropadding)。ZP-OFDM有比较低的传输功率，但在接收端接收于zeropadding区域信号时，由于多径传播等因素的影响，会破坏载波的正交性造成ICI（inter-carrierinterference）:在FFT运算时间长度内，第一子载波与带有时延的第二子载波之间的周期个数之差不再是整数，所以当接收机试图对第一子载波进行解调时，第二子载波会对此造成干扰。同样，当接收机对第二子载波进行解调时，也会存在来自第一子载波的干扰。,2.1.4OFDM信道估计及均衡器,由于在信号传输时，接收端收到的信号是传送信号和信道响应作用过的结果，所以为了解出传送信号势必要得到信道响应，所以要作信道估计。在高速移动环境时变信道估计更是重要，不好的信道估计会造成会造成误码率上升；信道估计常见的方法就是加入测试信号，由测试信号得到测试信号那些点的信道响应对信道其它点作估计，进而求出整个信道响应。均衡器由信道估计的结果对接收信号作信道补偿，降低错误率。由于OFDM将频宽切割成数个小频带，故更接近信道的相干带宽，所以信号受到信道失真变小，故可以用简单的一阶均衡器补偿。,2.2MultipleAntennaSystems,2.2.1MIMOspatialdiversity,空间分集能改善信号质量，并在接收端达到更高的信噪比。分集原理依赖于结构化冗余的传输。这种冗余可以在任何时间，从任何天线、通过任何频率。Tx分集，一个信号的副本从另外一个天线发送(例如2x1)；好比单声道和立体声信号，如果是立体声信号，人耳可以感受到更好的声音效果。Rx分集，接收到的信号进行多次评估(例如2x1)；好比两只耳朵所听到的效果比单只耳朵更好。,MIMO-Rxdiversity,对于来自发射端的同一个信号，由于在接收端使用多天线接收，那么这个信号将经过多条路径（多个天线）被接收端所接收。多个路径质量同时差的几率非常小，一般地，总有一条路径的信号较好。那么在接收端可以使用某种算法，对这些各接收路径上的信号进行加权汇总（显然，信号最好的路径分配最高的权重），实现接收端的信号改善。当多条路径上信号都不太好时，仍然通过MRC技术获得较好的接收信号。分集的支路数M越大，分集的效果越好。但当M较大时（如M3），分集的复杂性增加，分集增益的增加随着M的增大而变得缓慢。,MIMO-Rxdiversity分集信号的合并技术-SC,选择式合并（SelectiveCombining）M个接收机的输出信号送入选择逻辑，选择逻辑从M个接收信号中选择具有最高基带信噪比（SNR）的基带信号作为输出。,选择分集提供了一种通过减少硬件复杂度得到适量的分集合并增益的方法。每增加一条分集支路，它对输出信噪比的贡献仅为总分集支路数的倒数倍。,MIMO-Rxdiversity分集信号的合并技术-MRC,最大比合并（MaximumRatioCombing）M个分集支路经过相位调整后，按适当的增益系数同相相加（检测前合并），再送入检测器.,合并后信号的振幅与各支路信噪比相联系，信噪比愈大的支路对合并后的信号贡献愈大。在具体实现时，需要实时测量出每个支路的信噪比，以便及时对增益系数进行调整。,MIMO-TxdiversityCyclicdelaydiversity(CDD),发射分集就是在发射端使用多幅发射天线发射相同的信息，接收端获得比单天线高的信噪比一種發射分集為Cyclicdelaydiversity(CDD)CDDintroducesvirtualechoesintoOFDM-basedsystems.Thisincreasesthefrequencyselectivityatthereceiver.InthecaseofCDD,thesignalsaretransmittedbytheindividualantennaswithatimedelay.BecauseCDDintroducesadditionaldiversitycomponents,itisparticularlyusefulasanadditiontospatialmultiplexing.在不同的发射天线上发送具有不同相对延时的同一个信号,人为地制造时间弥散，能够获得分集增益。且循环延时分集采用的是循环延时而不是线性延时，延迟是通过固定步长的移相（CyclicShift，循环移相）来等效实现延迟。,MIMOmultiple-inputandmultiple-output,MIMOtechnologyhasattractedattentioninwirelesscommunications,becauseitofferssignificantincreasesindatathroughputandlinkrangewithoutadditionalbandwidthorincreasedtransmitpower.Itachievesthisgoalbyspreadingthesametotaltransmitpowerovertheantennastoachieveanarraygainthatimprovesthespectralefficiency(morebitspersecondperhertzofbandwidth)and/ortoachieveadiversitygainthatimprovesthelinkreliability(reducedfading).Becauseoftheseproperties,MIMOisanimportantpartofmodernwirelesscommunicationstandardssuchasIEEE802.11n(Wi-Fi),4G,3GPPLongTermEvolution,WiMAXandHSPA+.InMIMOsystems,atransmittermaysendsmultiplestreamsbymultipletransmitantennas.Thetransmitstreamsgothroughamatrixchannelwhichconsistsofallpathsbetweenthetransmitantennasatthetransmitterandreceiveantennasatthereceiver.Then,thereceivergetsthereceivedsignalvectorsbythemultiplereceiveantennasanddecodesthereceivedsignalvectorsintotheoriginalinformation.,MIMOSpatialMultiplexing,SpatialmultiplexingrequiresMIMOantennaconfiguration.Inspatialmultiplexing,ahighratesignalissplitintomultiplelowerratestreamsandeachstreamistransmittedfromadifferenttransmitantennainthesamefrequencychannel.IfthesesignalsarriveatthereceiverantennaarraywithsufficientlydifferentspatialsignaturesandthereceiverhasaccurateCSI(channelstateinformation),itcanseparatethesestreamsinto(almost)parallelchannels.Spatialmultiplexingisaverypowerfultechniqueforincreasingchannelcapacityathighersignal-to-noiseratios(SNR).Themaximumnumberofspatialstreamsislimitedbythelesserofthenumberofantennasatthetransmitterorreceiver.SpatialmultiplexingcanbeusedwithoutCSIatthetransmitter,butcanbecombinedwithpre-codingifCSIisavailable.Spatialmultiplexingcanalsobeusedforsimultaneoustransmissiontomultiplereceivers,knownasSDMA(space-divisionmultipleaccess)ormulti-userMIMO,inwhichcaseCSIisrequiredatthetransmitter.Theschedulingofreceiverswithdifferentspatialsignaturesallowsgoodseparability.,MIMOSpatialMultiplexing基本原理,接收端的信號可以表示為：信道矩陣H通過合適的信道估計算法估算出來，那麽接收端的信號就可以通過矩陣W=H-1獲得。,MIMOAmuchsimplified2*2MIMOoperation.,Amuchsimplified2*2MIMOchannelexample,intendedtohelpremovesomeofthemysteryaboutMIMOoperation,MIMOBeamforming,Beamformingisadigitalsignalprocessingtechniqueaimingtoenhancetransmissionorreceptionatadesireddirection.Thisistypicallyachievedbycombiningmultiplephasedarrayelementsissuchawaythatsignalsinthedesiredparticularangleexperienceconstructiveinterference.波束成形技术是发送方在获取一定的当前时刻当前位置发送方和接收方之间的信道信息，调整信号发送的参数，使得射频能量向接收方所处位置集中，从而使得接收方接收到的信号质量较好，最终能保持较高的throughput。,2个弱波谷和1个强波峰叠加出较差结果,调节第二根天线上的相位，使得3个信号同步，叠加出较好结果,TxBF形成的某种辐射图,MIMObeamforminggain,Beamformingmandatesmultipletransmitchains,anditmayintroduceatheoreticaladditionalgaintothelinkbudgetintheorderof:10*log10(NTX/SSTX),whereNTXisthenumberoftransmitchain,andSSTXisthenumberoftransmittedspatialstreams.Thefollowingtablesummarizesthetheoreticaldigitalcodinggainaddedbybeamformingfortransmissionsystemsofupto3x3:3:,MIMObeamforminggain,Infact,theeffectiveSNRacrossallsub-carriersisnotasimplearithmeticaverageofsubcarriersSNR,andisgreatlydeterminedbythelowestSNRsubcarriers.Thustheeffectivegainofthebeamformingoperationismuchmorethanthebeamforminggainof10log(Ntx).ThisisespeciallyimportantforMCSshavinghighcoderate.ForexamplethehighestratesMCS7,MCS15andMCS23whichusesa5/6codecantypicallycorrect1/6ofthesub-carriers.Thismeansthatthecodecannotcopewithascenariowhereinmorethan1/6ofthesubcarriershavelowSNRswhichiscommontoseveremulti-pathchannels(highvariabilityacrosssubcarriers),reducingtheprobabilityofoperatinginthesehighMCSratesastheyaresensitivetonon-flatSNRvectors.Weseethatafterbeamforming,thefirstEigenmodesarenotonlystrongonaverage,butalsoflat,thusenablingtheuseofhighcoderatesinmultipathchannels.,SVD(SingularValueDecomposition)奇异值分解是线性代数中一种重要的矩阵分解，在信号处理、统计学等领域有重要应用。,Atypicalplotofanon-BFtransmissionofa4spatialstreamtransmissioninamultipathchannel:,AndthecorrespondingplotofaBFtransmission:,MIMOBeamforming,Beamforemer-Adevicetransmittingwithbeam-formingBeamformeeAdevicetowhichbeamformedsignalistransmittedExplicitbeamformingisbasedonfeedbackfromthebeamformeetothebeamformerthusmandatingsupportbybothdevices(e.g.APandSTA)priortodata-transmissionthebeamformeesendssoundingsignal,basedonwhichthebeamformercalculatesthechanneltransferfunction,andfeedbackittothebeamformertobeusedintheactualdatatransmission.AdedicatedNDP(NonDataPacket)frameissentbytheBeamformertosoundthewirelesschannelandenabletheBeamformeetoestimatealldimensionsofthechannel.TheNDPissentwithanumberofspatialstreamsthatisequaltothenumberoftheBeamformersantennas,regardlessofthenumberofsupportedspatialstreamsoftheBeamformee.ThustheBeamformeecanestimatethefullchannelmatrixofdimension(NtxxNrx),whereinNtxisthenumberofBeamformersantennasandNrxisthenumberofBeamformeesantennas.AsaresponsetotheNDP,theBeamformeecalculatesacandidateBeamformingtransmitmatrixandsendsitbacktotheBeamformerintheCompressedBeamformingReportframe,immediately(afteraSIFSinterval)asaresponseframetotheNDP.Implicitbeamformingtypicallydoesnotmandatesupportonthebeamformeeandmaybewithoutitsawarenesstothefactthatbeamformingisused.InsteadofsendinganNDPsoundingpacketinthedownlinktoestimatethedownlinkchannel,asisdoneinExplicitBFmode,thedownlinkchannelinImplicitBFisestimatedfromuplinkreceptions.,Beamforming(BCM43605G)LDPCX14for2Mb/s;X37for5.5Mb/s;X6Efor11Mb/s.,2.4.2ShortPPDUformat802.11bHR/DSSSFramestructure,ShortPLCPsynchronization(shortSYNC)TheshortSYNCfieldshallconsistof56bitsofscrambled0bits.Thisfieldisprovidedsothereceivercanperformthenecessarysynchronizationoperations.Theinitialstateofthescrambler(seed)shallbe0011011ShortPLCPSFDfield(shortSFD)TheshortSFDshallbea16-bitfield.Thefieldisthebitpattern0000010111001111.Therightendbitshallbetransmittedfirstintime.AreceivernotconfiguredtousetheshortheaderoptiondoesnotdetectthisSFD.,2.4.3TransmitSpecifications802.11bHR/DSSSPMD,Transmitcenterfrequencytolerance:shallbe25ppmmaximum.Chipclockfrequencytolerance:shallbebetterthan25ppmmaximumTransmitpower-onandpower-downramp:Thetransmitpower-onrampfor10%to90%ofmaximumpowershallbenogreaterthan2s.Thetransmitpower-downrampfor90%to10%maximumpowershallbenogreaterthan2s.RFcarriersuppression:TheRFcarriersuppression,measuredatthechannelcenterfrequency,shallbeatleast15dBbelowthepeakSIN(x)/xpowerspect