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wifi 802.11n 协议标准

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wifi 802.11n 协议标准Contents2. Normative references ........................................................................................................................... 13. Definitions ...........................................................................................................EEE Std802.11n"-2009(Amendment to ieEE Std 802.11-2007as amended by ieee std 802. 11k-2008EEE Std80211r-2008, EE Std802.11y-2008,and IEEE Std 802. 11w-2009)EEE Standard forInformation technology-Telecommunications and informationexchange between systems-Local and metropolitan area networks-Specific requirementsPart 11: Wireless LAN Medium Access Control (MAC)and Physical Layer(PHY SpecificationsAmendment 5: Enhancements for HigherThroughputSponsorLANIMAN Standards committeeof theIEEE Computer SocietyApproved 11 September 2009IEEE-SA Standards boardAbstract: This amendment defines modifications to both the IEEE 802 1 1 physical layer(PHY)andthe IEEE 802 11 medium access control (MAC) sublayer so that modes of operation can be enabledthat are capable of much higher throughputs, with a maximum throughput of at least 100 Mb/s, asmeasured at the Mac data service access point (SAP)Keywords: high throughput, MAC, medium access control, MIMO, MIMO-OFDM, multiple inputmultiple output, PHY, physical layer, radio, wireless local area network, WLANThe Institute of Electrical and Electronics Engineers, Inc.3 Park Avenue New york NY 10016-5997 USACopyright@ 2009 by the Institute of Electrical and Electronics Engineers, IncAll rights reserved. Published 30 October 2009. Printed in the United States of AmericaIEEE is a registered trademark in the U.S. Patent Trademark Office, owned by the Institute of Electrical and ElectronicsEngineers, IncorporatedPDFSBN978-0-7381-60467STD95961Print:|SBN9780-7381-6047-4STDPD95961No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the priorwritten permission of the publisherIEEE Standards documents are developed within the ieee Societies and the standards Coordinating Committees ofthe IEee Standards Association (IEEE-SA) Standards Board. The ieee develops its standards through a consensusdevelopment process, approved by the American National Standards Institute, which brings together volunteersrepresenting varied viewpoints and interests to achieve the final product. Volunteers are not necessarily members of theInstitute and serve without compensation. While the ieee administers the process and establishes rules to promotefairness in the consensus development process, the ieee does not independently evalof any of the information or the soundness of any judgments contained in its standar uate, test, or verify the accuracyUse of an IEEE Standard is wholly voluntary. The ieee disclaims liability for any personal injury, property or otherdamage, of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectlyresulting from the publication, use of, or reliance upon this, or any other iEFF Standard documentThe ieee does not warrant or represent the accuracy or content of the material contained herein, and expresslydisclaims any express or implied warranty, including any implied warranty of merchantability or fitness for a specificpurpose, or that the use of the material contained herein is free from patent infringement. IEEE Standards docunentsare supplied‘ASIS.The existence of an IEEE Standard does not imply that there are no other ways to produce, test, measure, purchasemarket, or provide other goods and services related to the scope of the ieee Standard. Furthermore, the vintexpressed at the time a standard is approved and issued is subject to change brought about through developments in thetate of the art and comments received from users of the standard. Every ieee standard is subjected to review at leastevery five years for revision or reaffirmation, or every ten years for stabilization. When a document is more than fiverears old and has not been reaffirmed, or more than ten years old and has not been stabilized, it is reasonable toconclude that its contents, although still of some value, do not wholly reflect the present state of the art. Users arecautioned to check to determine that they have the latest edition of any IEEE StandardIn publishing and making this document available, the IEee is not suggesting or rendering professional or othcrservices for, or on behalf of, any person or entity. Nor is the IEEE undertaking to perform any duty owed by any otherperson or entity to another. Any person utilizing this, and any other IEEE Standards document, should rely upon his orher independent judgment in the exercise of reasonable care in any given circumstances or, as appropriate, seek theadvice of a competent professional in deternining the appropriateness of a given IEEE standardInterpretations: Occasionally questions may arise regarding the meaning of portions of standards as they relate tospecific applications. When the need for interpretations is brought to the attention of IEEE, the Institute will initiateaction to prepare appropriate responses. Since IEFF Standards represent a consensus of concerned interests, itimportant to ensure that any interpretation has also received the concurrence of a balance of interests. For this reason,IEEE and the members of its societies and Standards Coordinating Committees are not able to provide an instantresponse to interpretation requests except in those cases where the matter has previously received formal considerationA statement, written or oral, that is not processed in accordance with the IEEE-SA Standards Board Operations Manualshall not be considered the official position of ieee or any of its committees and shall not be considered to be, nor berelied upon as, a formal interprctation of the IEEE. At lectures, symposia, seminars, or educational courses, anindividual presenting information on IEEE standards shall make it clear that his or her views should be considered thepersonal views of that individual rather than the formal position, explanation, or interpretation of the IeEeComments for revision of IEEE Standards are welcome from any interested party, regardless of membership affiliationwith IEEE. Suggestions for changes in documents should be in the form of a proposed change of text, together withappropriate supporting comments. Recommendations to change the status of a stabilized standard should include atonale as to why a revision or withdrawal is required. Comments on standards and requests for interpretations shouldbe submitted to the following addressSecretary, IEEE-SA Standards board445 Hoes lanePiscataway, NJ 08854USAAuthorization to photocopy portions of any individual standard for internal or personal use is granted by The Instituteof Electrical and Electronics Engineers, Inc, provided that the appropriate fee is paid to Copyright Clearance CenterTo arrange for payment of licensing fee, please contact Copyright Clearance Center, Customer Service, 222 RosewoodDrive, Danvers, MA 01923 USA; +1978750 8400 PerImission lo photocopy portions of any individual standard foreducational classroom use can also be obtained through the Copyright Clearance CenterIntroductionThis introduction is not part of IEee Std 802. 11n-2009, IEEE Standard for Information technology-Telecommunications and information exchange between systems--Local and metropolitan networks--Specifirequirements-Part 11: Wireless LAN Medium Access Control(MAC) and Physical Layer(PHY) SpecificationsAmendment 5: Enhancements for Higher ThroughputThis amendment specifies enhancements to IEEE 802. 11 physical layer(PHY)and medium access control(MAC) sublayer to provide modes of operation with useful data rates substantially higher than thosepreviously available. Significantly higher IEEE 802. 11 wireless local area network(WLAN) throughput isexpected to improve user experiences for current applications and to enable new applications and marketsegmentsNotice to usersLaws and requlationsUsers of these documents should consult all applicable laws and regulations. Compliance with theprovisions of this standard does not imply compliance to any applicable regulatory requirementsImplementers of the standard are responsible for observing or referring to the applicable regulatoryrequirements. iEEE does not, by the publication of its standards, intend to urge action that is not incompliance with applicable laws, and these documents may not be construed as doing so on that is not inCopyrightsThis document is copyrighted by the Ieee. It is made available for a wide variety of both public and privateuses.These include both use, by reference, in laws and regulations, and use in private self-regulationstandardization, and the promotion of engineering practices and methods. By making this documentavailable for use and adoption by public authorities and private users, the ieee does not waive any rights incopyright to this documentUpdating of IEEE documentsUsers of ieFF standards should be aware that these documents may be superseded at any time by theissuance of new editions or may be amended from time to time through the issuance of amendments,corrigenda, or errata. An official ieee document at any point in time consists of the current edition of thedocument together with any amendments corrigenda or errata then in effect In order to determine whethera given document is the current edition and whether it has been amended through the issuance ofamendmentscorrigendaorcrrata,visittheIeeEStandardsAssociationwebsiteathttp://ieeexplore. ieee. org/xpl/standards. jsp or contact the iEee at the address listed previously.For more information about the IEEE Standards Association or the Ieee standards development process,visittheIeeE-sawebsiteathttp://standards.ieeeorgErrataErrata,ifanyforthisandallotherstandardscanbeaccessedatthefollowingUrl:http://standards. ieee. org/reading/ieee/updates/errata/index. html. Users are encouraged to check this URL forta periodicallyInterpretationsCurrentinterpretationscanbeaccessedatthefollowingUrl:http://standards.ieeeorg/reading/ieee/interp/index htmlCopyright C 2009 IEEE. All rights reservedPatentsAttention is called to the possibility that implementation of this amendment may require use of subjectmatter covered by patent rights. By publication of this standard, no position is taken with respect to theexistence or validity of any patent rights in connection therewith. a patent holder or patent applicant hasfiled a statement of assurance that it will grant licenses under these rights without compensation or underreasonable rates, with reasonable terms and conditions that are demonstrably free of any unfairdiscrimination to applicants desiring to obtain such licenses. Other Essential Patent Claims may exist forwhich a statement of assurance has not been received. The Ieee is not responsible for identifying EssentialPatent Claims for which a license may be required, for conducting inquiries into the legal validity or scopeof Patents Claims, or determining whether any licensing terms or conditions are reasonable or nondiscriminatory. Users of this standard are expressly advised that determination of the validity of any patentrights, and the risk of infringement of such rights, is entirely their own responsibility. Further informationmay be obtained from the ieee Standards associationParticipantsWhen this amendment was sent to sponsor ballot, the IEEe 802. 11 Working Group had the followingofficersBruce P. Kraemer, ChairJon Walter Rosdahl, Vice Chair; Treasurer; and Chair, Task Group mbAdrian P. Stephens Vice chairStephen McCann, Secretary and Chair, Publicity standing Committee and Task group uTerry L. Cole, Technical Editor and Assigned Number AuthorityTeik-Kheong Tan, Chair, Wireless Next Generation Standing CommitteeDavid Bagby, Chair, Architecture Standing CommitteeBruce p. kraemer Co-Chair IMT-Advanced ad hoc commiteeLee armstrong, Chair, Task Group pDonald E Eastlake Ill, Chair;, Task Group sNeeraj Sharna, Chair, Task group lDorothy V. Stanley, Chair, Task Group v and IETF Ad hoc CommitteeJesse Walker, Chair. cTl Ad hoc committeePeter Ecclesine, Chair, Task group yMenzo Wentink, Chair, Task group zGancsh Venkatesan, Chair, Task Group aaEldad Perahia, Chair, Very High Throughput Study groupDarwin Enger, Co-Chair. IMT-Advanced ad hoc committeeWhen this amendment was sent to sponsor ballot, Task Group n had the following officersBruce P Kraemer. chairSILi. vice chairGarth Hillman and Jon Walter Rosdahl, SecretaryAdrian P. Stephens, Technical editorWhen this amendment was sent to sponsor ballot, the IEeE 802. 1l Working Group had the followingmembershiOsama S. aboulmagdMalik audchAmit bansalTomoko adachiCcrt A, awatoJohn. barrAlok aggarwalDavid bagbGal bassonCarlAldanaMichael bahrussa BavafaThomas AlexanderFan baiTuncer BaykasLee r. ArmstrongGabor BaikoJohn L. BenkoAlex ashleyDennis j. bakerMathilde benvenisteCopyright C 2009 IEEE. All rights reservedBiorn A. berkeRobert Y huangChiu ngoDaniel borgesDavid hunterEero nikulaAnthony braskichYasuhiko inoueRichardH. noensJoseph brennanAkio isoHideaki odagiriDavid britzJunghoon JeeJisung OhG. BumillerHongseok JeonChandra s. olsonNancy Cam-WingetYeonkwon JeongYouko omorNecati CanpolatJorjeta G JetchevaSatoshi oyamaJavier cardonaLusheng jiRichard H. PaineDouglas s ChanDaniel JiangAuld. palaniveluClint F. ChaplinPadam AflChangmin ParkCarl W. KainJungsoo parkMinho CheongNaveen K KakanMinyoung ParkMasato katoⅤ ijaykumar PatelNakjungDouglas KanBcminih. peirisLiwen chuRichard H. KennedyEldad perahiaTerry L. ColeJohn KenneJames e. petranovichRyon K. ColemanStuart 3. KerryAl PetrickCharles IJoonsuk kinFahd pirzadaTodor CooklevKyeongpyo KimJames d. portaroXavier p CostaSeong S. kimHenry s. PtasinskiDavid E CypherYongsan KimRene purnadiMarc de courvilleJarkko kneckChang w. PyoRolf J de vegtMark m. kobayashiEmily H QiTheodorus denteneerFumihide KojimaLuke QianJeremy de vriesTom KolzeHuyu QuSusan DickeyBruccp KJim e raabZhiming DingJohannes P KruysⅤ nuth raiYoshiharu doThomas KuehnelAli RaissiniaJohn dorseyThomas m. KuriharaHarish ramamurthyRoger P durandStephen G raymentSrinivasa duvvuriEdwin kwonLeonid razoumovDonald e. eastlakeZhou lanIvan reePeter Ecclesiaejeremy A landtEdward reussMichael ellisJoseph P. lauerAlex reznikStephen P. EmeottTae H. LeeRandal roebuckMarc emmelmannWooyoung leeJon walter rosdahlDarwin EngwerJoseph levyRichard royJoseph epsteinSheung liAlexander safonovⅤ inko ercegPaul linKazuyuki sakodaLars p. falkIlang liuHemanth sampathRobert fanfcllMichael livshitzKatsuyoshi SatoStefan fechterPeter LocHirokazu sawadaPaul h. feinberDaniel lubarDon schultzMatthew FischerAnthony f. maidaYongho SeokWayne k. FisherJakub majkowskiHuairong shaoRoberta fracchiAlastair MalarkyNeeraj sharmaRyuhei FundaJouni K. malinenStephen J ShellhammerMatthew gastAlexander maltsevlan SherlockJames p gilbBill marshallKai shiJeffrey gilbertRoman m. maslennikovShusaku shimadaReinhard GlogerSudheer mattaFrancois simonDavid goodallStephen Mc cannHarkirat SinghTugrul guenerJustin p mcnewGraham K. SmithJianlin gven meseckeMatt smithMark hamiltonRobert r. millerYoo-Seung SongC.J. HanseMichael monteKapil seBrian d. hartRajendra T. moortiVinay SridharaAmer A haHitoshi moriokaDorothy stanleⅤ egard hasselPeter MurrayAdrian P. StephensRobert f heileAndrew milesDavid S Stephenson〔 undo r. HertzRohit nabarCarl R. StevensonJunling hiJohn stineWendong huKengo nagataGuenaelt strutCopyright C 2009 IEEE. All rights reservedChin s SumGanesh VenkateHarry R. worstellEiji TakagiDalton t.Ⅴ ictorPengfei xiaMineo takaiGeorge A. vlantisAkira yamadaTcik-Khcong TanCssc r. walkerTakeshi yamamotoAllan thomsonunlWangmoya yamauraJerry thrasherngEric okuboCraig d. WarrenSu K. YongAlexander TolpinFujio WatanabeSeiji YoshioJason trachewskyPatrick WayeChristopher YoungSolomon b. traininMenzo m. wentinkArtur zaksRichard D. Van NeeFrank whettenHongyuan zhangAllert van zelstKyle williamsHuimin zhangMathieuⅤ arlet-AndmJames worshamJing zhiPrabodh varshneyJuan ZunigaContributions to this amendment was received from the following individualsBill ablⅵ inko ErcegPatrick labbeSantosh abrahamMustafa rozJoseph lauerTomoko adachiStefan fechterDongjun leeDmitry akhmetovPaul feinbergLin-Nan leeCarlos aldanaMatthew fischereeDave andrusGuido frederiksSok-kyu LeeMicha anholtTakashi FukagawaZhongding leiTsuguhide AokiPatrick FungJoseph levyYusuke asarEdoardo gallizioScott leyonhjelGeert awaterJames gardnerPen liDavid BaDevis gattSheung LiRaja banerjeaDavid HedbergYuan liKaberi banerieeGarth hillmanKevin liaoAmit bansalJin-Meng hoIsaac limGal bassonDale hocevarAlfred linAnui batraLouise hooAlbert liuJohn benkoMuhammad akramDer-Zheng liuMathilde benvenisteYasuhiko inoueMichael livshitzBjorn BjerkeKaz ishidaYufei BlakenshipTakashi ishidoshiroPeter LojkoDaniel borgesLakshmi lyerHui-Ling LouDouglas chanEric JacobsonAdina matacheJerry ChangYuh-Ren jauhLaurent mazetJohn ChangTaehyun JeonWilliam mcfarlandJeng-Hong ChenJari JokelaDarren mcnamaraStephen ChenVKJonesIrina medvedevYi-Ming ChenPadam AflArnaud meylanPei-ju ChiangNaveen KakaniMorgan mikFrancois chinSriniavs kandalaSeungwook minWoon Hau ChinAteet KapurPatrick mo上 mily chouAssaf KasherAndy molischLiwen chuMasato katoR. Tushar moortiKeith ChuggJohn KetchumMike moretonBrian ClassonJoonsuk KYuichi moriokaSean CoffeyPansop KimMarkus muckGabriella convertinoYoungsoo KimSyed Aon mujtabaMarc de courvilleGuenter KleindlAndrew mylesRolf de vegtKiyotaka KobayashiRohit nabarFranz dielacherCenk Koselhasa NagaiYoshiharu doiBruce p KraemerKengo NagataJohn deThomas KuehnelSeigo NakaodouglasTakushi KunihiroSanjiv nandaPeter ecclesiaeJoe KwakChiu nDarwin EngwerJoseph kwakAjit NimbalkerLeonid epsteinEdwin kwonGunnar NitscheCopyright C 2009 IEEE. All rights reservedHuaning NiuErik SchylanderRichard van neeIvan oakesMichael sealsNico van waesYoshihiro ohtanilual-Rong ShaoAllert van zelstEric OjardSuman sharmaAndres vila casadoTakeshi onizawaSteve shellhammerGeorgeⅵ lantisJob oostveenIan SherlockTimothy WakeleyFabio osnatoMasaaki shidaJesse walkerKafle padamTakashi shonoBrad wallacePratima paiSebastien SimoensMark wallaceArul palaniveluMassimiliano sitiRod waltonSubra parameswaraDoug SmithQi WThomas pareMatt smithXiaowen WangJean-Noel PatillonAmjadDeric WatersWei-Chung PengRobert staceyMark WebsterEldad perahiaAdrian P. StephensMenzo WentinkJim petranovichRichard WeseKrishna madhavan pillaiVictor StolpmanMike WilhoyteMadhavan pillaiSumei sunTimothy WongAngelo PoloniXiantao sunJames WoodyattNeeraj poojaryShravan surineniHenry ptasinskiIlan sutskoverAriton XhafaAleksandar PurkovicEiji Takagio xiaLuke oianMasahiro takasTomoya YamauraAli RaissiniaSeiichiro TakahashiEric YangSthanunathan ramakrishnanDaisuke takedaWen-Chang YehHarish RamamurthyTeik-Kheong TanChristopher YoungSridhar rameshYasuhiro TanakaHeejung YuRaffaele rivaStephan ten brinkArtur ZaksJon walter rosdahlGanesan ThiagarajanEldad zeiraStephanie Rouquette-LeveilEric OkuboHongyuan zhaMike rudeTim TowellJin ZhangJohn sadowskiJason trachewskyNing ZhangAtul salhotraSolomon traininQuan ZhouVincenzo scarpaDavid tungJohnny ZweigDonald schultzStefano valleJim syrenThe following members of the individual balloting committee voted on this amendment. Balloters may havevoted for approval, disapproval, or abstentionOsama aboulmagdBill cannonSouray duttaTomoko adachiJing CaoRichard eckardAlok AggarwalEdward CarlesCarl EklundThomas alexandeJames CarloStephen emeottRichard AlvinJuan CarreonMarc emmelmannEdward ChalfDarwin EngwcButch AntonDouglas s ChanJoseph epsteinDanilo antonelliKuor-Hsin ChangRobert fanfelleClint ChaplinShahin FarahaniDavid BagbyWei-Peng ChenShulan FengGabor BaikoYung-Mu ChenOddgeir FikstvedtMatthew ballHong ChengMatthew fischerRaja banerjeaAik ChindapoPrince francisVered BKeith ChowAvraham FreedmanJohn barrLiwen ChuDevon gaylePhilip beecherRyon ColemanMichael GeipelMathilde benvenisteCharles cookTheodore Georgantasharry bimsTodor CookiePieter -Paul giesbertsBjorn BjerkeHelge CowardJames gilbMonique brownJoseph decuirReinhard glogerRussell dietzMariana goldhamerWilliam byrdThomas dineenScrgiu gomaPctcr. calderonJohn dorseyDavid goodallCopyright C 2009 IEEE. All rights reserved

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nikolov团队本身也将其推广到了句子和文档(),因此觉得确实有必要对word2vec里的算法原理做个了解,以便对他们的后续研究进行追踪.于是,沉下心来,仔细读了一回代码,算是基本搞明臼里面的做法了.筼一个感觉就是,“明明是个很简单的浅层结构,为什么被那么多人沸沸扬扬地说成是Decp Learning呢?”解剖word2vec溟代码的过程中,除了算法层面的收获,其实编程技巧方面的收获乜颇多.既然花了功夫来读代码,还是把理解到的东西整理成文,给有需要的朋友提供点参考吧在整理本文的过程中,和深度学习群的群友北流浪子(15,16)进行了多次有益的讨论在比表示感谢另外,也参考了其他人的一些资料,鄱列在参考文献了,在此对他们的工作也并表示感谢2预备知识本节介绍word2vee中将用到的些重要知识点,包括 sigmoid函数、 Beyes公式和Huffman编码等821 sigmoid函数sigmoid函数是神经网络中常用的激活函数之一,其定义为1+e该函数的定义域为(-x,+x),值域为(0,1).图1给出了 sigmoid函数的图像0.5图1 sigmoid函数的图像sigmoid数的导函数具有以下形式)=0(x)1-0(x)由此易得,函数logo(a)和log(1-0(x)的导函数分别为log o(a)(21)公式(2.1)在后面的推寻中将用到822逻辑回归生活中经常会碰到二分类问题,例如,某封电子邮件是否为垃圾邮件,某个客户是否为在客户,某次在线交易是舌仔在诈行为,等等.设{(x,)}1为一个二分类问题的样本数据,其中x∈R",∈{0,1},当1=1时称相应的样本为正例,当v=0时称相应的样本为负例利用 sigmoid函数,对于任意样木x=(x1,x2,…,xn),可将二分类问题的 hypothesis函数写成h(x)=0(o+61x1+622+…+nxn),其中0=(0o,01,…,O)为待定参数.为了符号上简化起见,引入x0=1将x扩展为(x0,x1,x2,…,xrn)},且在不引起混淆的情况下仍将其记为ⅹ.于是,he可简写为取阀值T-0.5,则二分类的判别公式为1,b(x)≥0.5y(x0.5那参数θ如何求呢?通常的做法是,先确定一个形如下式的整体损失函数∑co(x,v)然后对其进行优化,从而得到最优的參数θ实际应用中,单个样本的损失函数cost(x,)常取为对数似然函数cosl(xi, yi)),v-1;(1-(x),v=0注意,上式是一个分段函数,也可将其写成如下的整体表达式cost(x2,3)=·log(ho(x)(1y1)·log(1h(x)323 Baves公式贝叶斯公式是英国数学家贝叶斯( Thomas Bayes)提出来的,用来描述两个条件概率之间的关系.若记P(A),P(B)分别表示事件A和事件B发生的概率,P(AB)我示事件B发生的情况下事件4发生的慨率P(A,B)表示事A.B同时发生的概率.则有P(AB)P(B), P(BLA)=P(A, B)P(A, B利用上式,进一步可得P(B AP(AB)-P(A)P(B)这就是 Bayes公式g2.4 Huffman编码本节简单介绍Humn编码(具体内容主要来白百度百F的词条.[10),为此,首先介绍Huffman树的定义及其构造算法§24.1 Huffman树在计算机科学中,树是一种重要的非线性数据结构,它是数据元素(在树中称为结点)按分支关系组织起来的结构.若干棵互不相交的树所构成的集合称为森林.下面给出几个与树相关的常用概念·路径和路径长度在一棵树中,从一个结点往下可以达到的孩子或孙子结点之间的通路,称为路径.通路中分支的数目称为路径长度.若规定根结点的层号为1,则从根结点到第L层结虑的路径长度为L-1●结点的权和带权路径长度若为树中结点赋予一个具有某种含义的(非负)数值,则这个数值称为该结点的权结点的带权路径长度是指,从根结点到该结点之间的路径长度亐该结点的杈的乘矾·树的带权路径长度树的带权路径长度规定为所有叶子结点的带权路径长度之和二叉树是每个结点最多有两个子树的有序树.两个子树通常被称为“左子树”和“右子树”,定义中的“有序”是指两个子树有左石之分,顺序不能颠倒给定n个权值作为n个叶子结点,树造一棵二叉树,若它的带权路径长度达到最小,则称这样的二叉树为最优二叉树,也称为 Huffman树82.4.2 Huffman树的构造给定m个权值{mn,m2;…,mn}作为二叉树的m个叶子结点,可通过以下算法来构造颗 Huffman树算法2.Ⅰ(Hu「man树构造算法)(1)将{1,2,……,wn}看成是有n棵树的表林(每树仅有一个结点)2)在森林中选出两个根结,的权值最小的树合并,作为-棵新树的左、右子树,且新树的根结点权值为其左、右子树根结点权值之和〔3)从森林中燜除选取的两樑树,并将新树加入森林(4)重复(2)、(3)步,直到森林中只剩一棵树为止,该树即为所求的 luffman树接下来,给出算法2.1的一个具体实例例2.1假设2114年世界杯期间,从新浪毀博中抓取了若干条与足球相关的微博,经统计,“我”、“喜欢”、“观看”、“巴西”、“足球”、“世界杯”这六个词岀现的次薮分别为15,8,6,5,3,1.请以这6个词为叶子结点,以相应词频当权值,构造一棵Hu∥n树.⊙Q⑨Q⊙只66如→只只③⊙图2 Huffman树的构造过程利用算法.,易知其枃造过程如国g所示,团中第六步给出了最终的 Hutman树,由囚可见词频越大的词离根结点越近构造过程中,通过合并新増的结点被标记为黄色.由于每两个结点邡要进行一次合并,因此,若叶子结点的个数为η,刘枃造的H們πω树中新増结点的个数为π-1.本例中n6,因此新增结,的个数为5注意,前面有捉到,二叉树的丙个子树是分左右的,对于某个非叶子结点来说,就是其两个孩子结点是分左右的,在本例中,统一将词频大的结点作为左孩子结点,词频小的作为右孩子结点当然,这只昃一个约定:你要将词頻大的结点作为右孩子结点也浸有问题§24.3 Huffman编码在数据通倍中,需要将传送的文宁转换成二进制的字符串,用0,1码的不同排列米表示字符.例如,需传送的报文为“A上 TER DATA EAR ARE ART AREA”,这里用到的字符集为“A,E,R,T,F,D”,各字母出现的次数为84,5,3,1,1,现要求为这些字母设计编码要区别6个字母,最简单的二进制编码方式是等长编码,固定采用3位二进制(23=8>6),可分别用000.001、010、011、100、101对“A,E,R,T,F,D”进行编码发送,当对方接收报文时再按照三位一分进行译码显然编码的长度取决报文中不同字符的个数,若报文中可能出现26个不同字符,则固定编码长度为5(2=32>26).然而,传送报文时总是希望总长度尽可能短.在实际应用中,各个字符的出现频度或使用次数是不相同的,如A、B、C的使用频率远远高于X、Y、7,自然会想到设计编码时,让使用频率高的用短码,使用频率低的用长码,以优化整个报文编码.为使不等长编码为前缀编码(即要求一个字符的编码不能是另一个字符編码的前缀),可用字符集中的每个宇符作为叶子结点生成一棵编码二叉树,为了获得传送报文的最短长度,可将每个字符的岀现频率作为字符结烹的权值赋予该结点上,显然字使用频率越小权值越小,权值越小叶子就越靠下,于是颎率小编码长,频率高编码短,这样就保证了此树的最小带权路径长度,效果上就是传送报文的最短长度.因此,求传送报文的最短长度问题转化为求由字符集中的所有字符作为叶子结点,由字符出现频率作为其权值所产生的Hman树的问题.利用 Hultman树设计的二进制前缀編码,称为 LuminaL编码,它既能满足前缀编码的条件,又能保证报文编码总长最短本文将介绍的word2ve工具中也将用到 Huffman编码,它把训练语料中的词当成叶子缩点,其在语料中出现的次数当作权值,通过构造相应的 Huttman树来对每一个词进行Huffman编码图3给岀了例2.1中六个词的 Huffman编码,其中约定(词频较大的)左孩子结点编码为1,(词频较小的)石孩子编码为θ.这惮一米,“我”、“喜欢”、“观看”、“巴西”、“足球”、“世界杯”这六个词的 Huffman编码分别为0.111,110,101,1001和10000我告欢巴匹0足球图3 Huffman编码示意图注意,到目前为止,关于 Huttman树和 Huttman編码,有两个约定:(1)将权值大的结点作为左孩子结点,权值小的作为右孩子结点(2)左孩子结点编码为1,右孩子结点编码为0.在word2vec源码中将权值较大的孩子结点编码为1,较小的孩子结点编码为0.为与上述约定统一起见,下文中提到的“左孩了结点"都是指权值较大的孩了结点83背景知识word2vec是用来生成词向量的工具,而词向量与语言模型有着密切的关系,为此,不妨先了解一些语言模型方面的知识83.1统计语言模型当今的互联网迅猛发展,每天都在产生大量的文本、图片、语音和视频数据,要对这些数据进行处理并从中挖掘岀有价值的信息,离不开自然语言处理( Nature Language processing,NP)技术,其中统计语言模型( Statistical language model)就是很重要的一环,它是所有NLP的基础,被广泛应用于语音识别、机器翻译、分词、词性标注和信息检索等任务.例.1在语音识别糸统中,对于给定的语音段Vire,霄要找到一个使概率p( TertVoice最大的文本段Tert.利用 Bayes公式,有P(Teat voice)p(VoiceText). p(Textp(Voice)其中p( CicetE.c)为声学模型,而 elEct)为语言模型(18])简单地说统计语言模型是用来计算一个句子的概率的概率模驷,它通常基于一个语料库来构建.那什么叫做一个句子的概率呢?假设W=m1:=(m1,2,…,mr)表示由T个词,2,……,按顺序构成的一个句子,则1,c2…,w的联合慨率p()=p(x1)=p(01,t2,…,r)就是这个句子的概率利用 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