Systemidentificationisacommonmethodforbuildingthemathematicalmodelofaphysicalplant,whichiswidelyutilizedinpracticalengineeringsituations.Ingeneral,thesystemidentificationconsistsofthreekeyelements,i.e.,thedata,themodel,andthecriterion.Thegoalofidentificationisthentochooseonefromasetofcandidatemodelstofitthedatabestaccordingtoacertaincriterion.Thecriterionfunctionisakeyfactorinsystemidentification,whichevaluatesthecon-sistencyofthemodeltotheactualplantandis,ingeneral,anobjectivefunctionfordevelopingtheidentificationalgorithms.Theidentificationperformances,suchastheconvergencespeed,steady-stateaccuracy,robustness,andthecomputationalcomplexity,aredirectlyrelatedtothecriterionfunction.
Well-knownidentificationcriteriamainlyincludetheleastsquares(LS)crite-rion,minimummeansquareerror(MMSE)criterion,andthemaximumlikelihood(ML)criterion.Thesecriteriaprovidesuccessfulengineeringsolutionstomostpracticalproblems,andarestillprevalenttodayinsystemidentification.However,theyhavesomeshortcomingsthatlimittheirgeneraluse.Forexample,theLSandMMSEonlyconsiderthesecond-ordermomentoftheerror,andtheidentificationperformancewouldbecomeworsewhendataarenon-Gaussiandistributed(e.g.,withmultimodal,heavy-tail,orfiniterange).TheMLcriterionrequirestheknowledgeoftheconditionalprobabilitydensityfunctionoftheobservedsamples,whichisnotavailableinmanypracticalsituations.Inaddition,thecomputationalcomplexityoftheMLestimationisusuallyhigh.Thus,selectinganewcriterionbeyondsecond-orderstatisticsandlikelihoodfunctionisattractiveinproblemsofsystemidentification.
Inrecentyears,criteriabasedoninformationtheoreticdescriptorsofentropyanddissimilarity(divergence,mutualinformation)haveattractedlotsofattentionsandbecomeanemergingareaofstudyinsignalprocessingandmachinelearningdomains.Informationtheoreticcriteria(orbriefly,informationcriteria)cancapturehigherorderstatisticsandinformationcontentofsignalsratherthansimplytheirenergy.Manystudiessuggestthatinformationcriteriadonotsufferfromthelimita-tionofGaussianassumptionandcanimproveperformanceinmanyrealisticsce-narios.Combinedwithnonparametricestimatorsofentropyanddivergence,manyadaptiveidentificationalgorithmshavebeendeveloped,includingthepracticalgradient-basedbatchorrecursivealgorithms,fixed-pointalgorithms(nostep-size),orotheradvancedsearchalgorithms.Althoughmanyelegantresultsandtechniqueshavebeendevelopedoverthepastfewyears,tillnowthereisnobookdevotedtoasystematicstudyofsystemidentificationunderinformationtheoreticcriteria.The
Preface
primaryfocusofthisbookistoprovideanoverviewofthesedevelopments,withemphasisonthenonparametricestimatorsofinformationcriteriaandgradient-basedidentificationalgorithms.Mostofthecontentsofthisbookoriginallyappearedintherecentpapersoftheauthors.
Thebookisdividedintosixchapters:thefirstchapteristheintroductiontotheinformationtheoreticcriteriaandthestate-of-the-arttechniques;thesecondchapterpresentsthedefinitionsandpropertiesofseveralimportantinformationmeasures;thethirdchaptergivesanoverviewofinformationtheoreticapproachestoparameterestimation;thefourthchapterdiscussessystemidentificationunderminimumerrorentropycriterion;thefifthchapterfocusesontheminimuminfor-mationdivergencecriteria;andthesixthchapterchangesthefocustothemutualinformation-basedcriteria.
Itisworthnotingthattheinformationcriteriacanbeusednotonlyforsystemparameteridentificationbutalsoforsystemstructureidentification(e.g.,modelselection).TheAkaike’sinformationcriterion(AIC)andtheminimumdescriptionlength(MDL)aretwofamousinformationcriteriaformodelselection.TherehavebeenseveralbooksonAICandMDL,andinthisbookwedon’tdiscussthemindetail.Althoughmostofthemethodsinthisbookaredevelopedparticularlyforsystemparameteridentification,thebasicprinciplesbehindthemareuniversal.Someofthemethodswithlittlemodificationcanbeappliedtoblindsourcesepa-ration,independentcomponentanalysis,timeseriesprediction,classificationandpatternrecognition.
Thisbookwillbeofinteresttograduates,professionals,andresearcherswhoareinterestedinimprovingtheperformanceoftraditionalidentificationalgorithmsandinexploringnewapproachestosystemidentification,andalsotothosewhoareinterestedinadaptivefiltering,neuralnetworks,kernelmethods,andonlinemachinelearning.
TheauthorsaregratefultotheNationalNaturalScienceFoundationofChinaandtheNationalBasicResearchProgramofChina(973Program),whichhavefundedthisbook.WearealsogratefultotheElsevierfortheirpatiencewithusoverthepastyearweworkedonthisbook.Wealsoacknowledgethesupportandencouragementfromourcolleaguesandfriends.
Xi’an
P.R.ChinaMarch2013