Joint Source-Channel Decoding. A Cross-Layer Perspective with Applications in Video Broadcasting over Mobile and Wireless Networks

324 Pages·3.192 MB·English

by  Pierre Duhamel and Michel Kieffer (Auth.)

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Preface and Acknowledgements
Ilnefautpastoujourstellementépuiserunsujetqu’onnelaisserienàfaire
aulecteur.Ilnes’agitpasdefaireliremaisdefairepenser.
Montesquieu(L’espritdeslois)
(approximate)Englishtranslation(andinshort):
Oneshouldnotalwaysexhaustasubjectinsuchawaythatthisleaves
nothingtodotothereader.Thisisnottoreadbuttothink.
Jointsource-channelcoding/decodingisaratherspecializedtermfordesignating
techniquesallowingtotransmitefficientlyaudiovisualcontentsthroughwireless
channelsbymeansofpossiblyunconventionalmethods.Suchasimplegoalturns
out to be quite complex since it involves many different techniques (namely,
source coding, networking, and transmission techniques) usually mastered by
differentteams.
Therefore,thisbookdoesnotintendtoaddresswithfullaccuracyallthese
aspects,butrathertriestogiveapreciseideaofthebasicsofeachandtopro-
vide a state of the art of the “joint source/network/channel decoding” results
obtainedonvideosignals.Manyexamplesillustratethecorrespondingconcepts
andalgorithms.Itisessentiallybasedonatutorialbythesameauthors,which
waspresentedattheIEEEInternationalConferenceonAcoustics,Speech,and
Signal Processing in 2007. The joint source-channel coding (JSCC) aspect is
outlinedonlyattheendofthebook.
Beingatopicwithsuchawidescope,wewereobviouslyhelpedandstimu-
latedbymanypeople.Ouronlyhopeisthatwewillnotforgettoomanyofthem
inthelistbelow.
Veryspecialthanksareduetoseveralcolleagues:OlivierRioul,whointro-
ducedustothetopicofJSCC,beforeweeventhoughtofJSCD,PierreSiohan,
whoalwaysshowedastronginterestinthistopic(andledoneofthecorrespond-
ingprojects),andChristineGuillemotandherteam,withwhomtheneologism
“coopetition”seemstobethemostappropriateforcharacterizingourstimulat-
ingcooperation/competition.CatherineLamy-BergotandKaveSalamatianalso
tookpartinmanyfruitfuldiscussionsonthetopic.Partsofthisbookwereproof-
readbycolleagueswhodidaverydetailedjob,manythankstoAureliaFraysse,
Jean-BenoistLeger,PabloPiantanida,PhilSchniter,andClaudioWeidmann.
Obviously,whenworkingonthistopic,wewerehelpedbymanyPhDstu-
dents,namely,BarhamZahirazami,AviGabay,HangNguyen,RachelChiang,
Chang-MingLee,GalinaSabeva,SalmaBen-Jamaa,CedricMarin,UsmanAli,
vii
viii PrefaceandAcknowledgements
andZiedJaouawhoworkeddirectlyonthetopicscoveredbythisbook.Allof
themwerefundedbyeitherindustrygrantsorcollaborativecontractsfundedby
theFrenchgovernment.OtherPhDstudents,namely,JoséSaavedraandOlivier
Derrien,werealsoofgreathelpforsomepartsofthebook.
Infact,wewereveryluckytoobtainfundingforsuchalong-termresearch
topic, almost continuously since 1997. Special thanks are thus due to Gerard
Eude, from France Telecom R&D, who initiated these fundings, followed by
CNES(theFrenchSpaceAgency,withCatherineLambert-Neboutasaninter-
face), andAlcatel-Lucent, a long-term contributor to this work, with an initial
strongcommitmentofDenisRouffet.CNES(withGaëlScotascontactperson,
andJerômeLacanascoworker)alsofundedacross-layerstudyonJSCD,which
demonstratedtousthatbothaspectsweretwosidesofthesamecoin.
Part of the material described in this book was funded by the French Min-
istry of Research and Industry by means of collaborative contracts named
COSOCATI,DIVINE,VIP,DITEMOI,andTVMSL.
Finally, most of this work benefited from contacts established by means
of two Networks of Excellence of the European Commission, NEWCOM and
NEWCOM++, scientifically led in a friendly and efficient manner by Sergio
Benedetto. We also benefited from fundings through these NoEs and helpful
collaborativeworkwithLajosHanzo,MichaelNg,andMortenHolmLarsen.
Thefinalthankswillgotothepeoplewhoshowedthehighestpatienceduring
thewritingofthisbook:ourfamilies,whodidnotsigntowritethebook,buthad
to undergo the consequences... and our editor, Tim Pitts, whose patience and
senseofhumorwerecombinedwithsufficientpressuresothatthebookfinally
cametoanendwithouttoomuchdelay.
More materials (Appendices X, Y and Z) can be found at website:
http://www.elsevierdirect.com/companions/9780123744494.
Chapter 1
Introduction: Context
1.1 MultimediaWireless:TheNeedforNewTools 3
1.2 ExampleApplications 5
1.3 JointSource-ChannelCodingandDecoding 9
1.4 Outline 12
Thisbookaddressesaquestionofgrowingimportance:Howcanoneimprove
theperceivedqualityofvideocommunicatedoverawirelessnetwork?Forsuc-
cessfulcommunicationofmultimediadata,especiallyvideo,thenetworkmust
behave as if it is predictable and stable. Wireless networks are not known for
predictabilityandstability,butrathervariabilityandscarcityofresources.Over-
sizing, the usual way of coping with variability, is not practical in wireless
systems due to the scarcity of resources. Yet, the number of applications for
video broadcasting over wireless channels keeps increasing. Therefore, there
isagrowingneedfortechniquesthatprovidesufficientreceivedvideoquality
withoutoverspendingthescarceresourcesofthewirelessnetwork.
In the classical approach to network design, the networking task is parti-
tioned into distinct layers, where each layer, assuming that the lower layers
behaveperfectly,attemptstoprovideperfectinformationtotheupperlayers.In
recent years, the so-called joint approaches have been proposed for improved
performanceandbetteruseofresources.Injointapproaches,thenetworklayers
become more transparent so that the information previously available at only
onelayerisaccessibletootherlayers.Thedownsidetothisambitiousapproach
isalossofthearchitecturalcoherencethatwastheprimarydrivingforcebehind
theuseofdecoupledlayers.
Wirelessnetworkdesigns,thus,fallbetweentwoextremepoints:ageneric
layeredarchitecture,whichwouldbesafebutnotveryefficientatresourceuti-
lization,andanoptimizedarchitecture(i.e.,optimalinbothresourceutilization
andperformance),whichwouldbededicatedtoasingleapplication.Findingjoint
1
2 CHAPTER|1 Introduction: Context
mechanismsthatarecompatiblewiththeuseoflayered,genericarchitecturesis
aseriouschallenge.
Given the complexity of the task, the joint mechanisms usually proposed
arequitelimited.Mosttechniquesadapttheparametersusedinsourcecoding,
channel coding, and other procedures based on a limited set of information
providedbyotherlayersandthus,donotreachtheirfullpotential.Whilethese
so-called cross-layer approaches have made some impact on wireless network
performance,theyarenottrulyjointschemesandconstituteafirststeptowarda
globaloptimizationofthenetwork.Infact,thereseemstobealargedifficultyin
obtainingfullyjointtechniquescompatiblewithgenericnetworkarchitectures.
Thisbookintendstofillthegapbydeveloping–insomedetail–toolsthat
allowtheimplementationoffullyjointtechniquesforrobustreceptionofmul-
timedia transmitted over wireless/Internet Protocol (IP) networks. Since these
techniques require the cooperation of many layers, each with its own peculiar
characteristics;thisbookcoversawidevarietyoftopicsfromsourcecodingto
channelcodingtonetworking.
Whiletherearemanybooksthatfocusonanyoneofthesetopics(e.g.,source
coding,channelcoding,wirelesscommunications,orwirelessnetworking),the
readershipsofthesebooksareoftenverydifferent.Forexample,thecommunity
thatworksonsourcecodingdoesnotoverlaptoomuchwiththeonethatworks
on the so-called physical layer. So, years ago, a tool known as joint source-
channelcoding(JSCC)wasproposedtoaddressoptimalsourcetransmissionfor
applications in which Shannon’s separation principle does not hold. However,
theproblemwasconsideredasacuriositybymanypractitioners,andevenifa
(relativelysmall)numberofarticleshaveappearedregularlyonthistopic,the
impactshavenotbeenwidespread.ThemaindrawbackofJSCCisthat,sinceitis
notcompatiblewithanyexistingstandards,widespreadimplementationwould
requiresignificanttimeandeffort,eveniftheperformanceimprovementswere
known to be potentially worthwhile. Motivated by the practical challenges of
JSCC, an interest in joint source-channel decoding (JSCD) emerged, whereby
the goal is to extract the best performance out of the received signal without
consideringchangestothetransmittedsignal.Clearly,theabilitytouseJSCD
withexistingstandardsmakesitpotentiallyverypractical.Therefore,thereisa
goodpossibilitythatJSCDtoolscouldbecomeusefulinthenearfuture,provided
thatallthedifferentlayerscanbeincorporated.Forexample,manyJSCDarticles
treatthenetworklayersastransparent,whichwouldclearlybeadrawbackfor
practicalimplementation.Atthesametime,cross-layercommunicationprotocols
havebecomeagreatinteresttosomewho,motivatedbymultimediaapplications,
haveworkedonjointlyadaptingthecharacteristicsofeachlayertothetypeof
signalthatwastransmitted.Theaimofthisbookispreciselytoprovideacross-
layerperspectiveonJSCC/JSCDtoolssothattheirapplicabilityisincreased;on
onehand,JSCDtoolsbecomemorepracticalwhennetworklayersarenottreated
asfullytransparent,andontheotherhand,thecross-layerapproachbenefitsfrom
additionalefficienttools.
Section|1.1 MultimediaWireless:TheNeedforNewTools 3
Though this book necessarily treats topics from a wide range of fields
(e.g., source coding, channel coding, and networking), it attempts to be as
self-containedaspossible.
1.1 MULTIMEDIAWIRELESS:THE NEED FOR NEWTOOLS
1.1.1 The ClassicalApproach to MultimediaWireless
Many years ago, when communication systems were first designed, they were
targeted toward a specific application. For example, the wired phone system
wasprincipallydesignedtocarryspeechsignals,aswerethefirstgenerationsof
wirelessphonesystems,whiletheInternetwasdesignedtocarrydatafiles.From
thatstartingpoint,thesesystemshaveevolvedgreatlyandnowsupportamuch
wider variety of applications, using (almost) the same channels and protocols.
Forexample,faxwasaddedtothewiredphonesystem,shortmessageservices
(SMS)andmultimediaservices(MMS)wereaddedtowirelessphonesystems,
andVoice over IP(VoIP) was added to the Internet. Furthermore, the systems
havebeenintegratedtothepointthat,now,manyphonecallsgo(atleastpartially)
throughtheInternetwithoutusersbeingnotifiedorevenabletotellthedifference,
andmanye-mailsaretypedand/orreadonwirelessmobilephones.
Moreover,thebitrateavailabletomostsystemshasgrownsohighthatthey
now support very demanding signals. For example, high-definition television
(HDTV)goesthroughwiredphonelines,viaasymmetricdigitalsubscriberline
(ADSL), and video is routinely transmitted through wireless channels.As dis-
cussedbelow,thesechangeshaveledtopronouncedshiftsinboththeusageand
thedesignofthesesystems.
ChangesinSystemDesign
Thedominantapproachtothedesignofwirelessmultimediasystemsisthelay-
eredapproach,wherethenetworklayersaredesignedtofunctionindependently
fromeachother.Eachlayerisdedicatedtoaspecificnetworkingtask,andthe
layersaredesignedregardlessofthesignaltypescarriedbythenetwork.
ThislayeredapproachismotivatedtosomeextentbyShannon’sseparation
principleforsource-channelcoding,which(looselyspeaking)establishedthat,
under certain conditions, the best joint source/channel coder is the concatena-
tionofthebestsourcecoderwiththebestchannelcoder.Insuchsituations,the
source coder generates the lowest-rate bitstream given an allowed distortion –
freeofredundancy,whereasthechannelcodergeneratesthelowest-ratebitstream
enabling the correction of “almost” all channel-induced errors – via judicious
use of redundancy. Thus, the optimal source coding is invariant to the chan-
nel characteristics, whereas optimal channel coding is invariant to the source
characteristics. However, this separation principle (described in more detail in
Chapter 2) holds only under certain not-so-practical conditions, such as in the
limitofinfinitecode-wordlength.Implementationsthatattempttosatisfythese
4 CHAPTER|1 Introduction: Context
conditionsdirectlywouldinvolveveryhighcomplexityandpossiblyverylarge
transmissiondelays.
ChangesinSystemUsage
Predicting the way that wireless multimedia systems will be used turns out
to be very difficult. In the early 1970s, during the time of analog telephones,
there were already proposals for videophones in various parts of the world.
At that time, cost was a significant problem. Several years later, in the early
1990s,digitaltechniqueshadevolved,andthefirststudiesofvideocompression
(againtargetedatvideophones,i.e.,visiophony)resultedinthewidelyaccepted
H.263 standard, published in 1996. It took some time for this technology to
be viable on widely deployed terminals, but at that time, visiophony did not
seem to be a driving need. Only recently did some “killer applications” for
mobile networks emerge: video database consultation and video broadcasting.
Whiletheseusesofwirelessnetworksappeartobelinkedprimarilytotheways
that mobile phones are now being used (e.g., more like a PC for surfing the
Internetanddownloadingfiles,images,andvideos),theyshowaclearshiftin
thewaythatnetworksarebeingused:theresourcesrequiredfortransmittinga
3-minute video clip are far beyond those required to support voice communi-
cation for the same duration. Even downloading audio files is about 10 times
morecostly(intermsofbitrate)thancommunicatingspeech.Sincemultimedia
trafficisincreasingatamuchfasterratethanspeechtraffic,oneiswelljustified
toconcentrateonthemultimediaaspectsofmobiledevices.
As a result of these trends, it may well happen that demands for increased
bandwidth,inconjunctionwiththeuseoflimited-efficiencylayerednetworks,
will overload the global network. While many actions have been taken to
prevent this occurrence (e.g., the addition of broadcast mode in some stan-
dards), new tools as JSCC/JSCD and cross-layer optimization could be of
greathelp.
1.1.2 A NewApproach to MultimediaWireless
Theclaimthatcurrentnetworksareimplementedusingfullyindependentlayers
is really an oversimplification. For example, the second generation European
mobiletelephonesystem(GSM)residualbiterrorrate(BER)istunedsothatit
is compatible with the BER tolerated by GSM’s speech coder.This, in fact, is
alreadyaformofcross-layeroptimizationinwhichthebehaviorofthephysical
(PHY)layerismatchedtotheneedsoftheapplicationlayer.Clearly,thetuning
wouldbedifferentforotherapplicationsandsignaltypes(e.g.,video).
The primary factors motivating the shift from fully independent layers to
closelycoupledlayersarethehighvariabilityofwirelesschannelsandtheneed
forincreasedrobustnessandgracefuldegradation.Ifanetworkmustsupporta
widevarietyofchannelsandusers,thendesigningthenetworktoworkperfectly
on all of them amounts (for the most part) to designing the network to satisfy
Section|1.2 ExampleApplications 5
theworstchannel/user.Inthiscase,ifoneuserexperiencesaverybadchannel,
thenthatuserwouldconsumeadisproportionateamountofsystemresources.
Inaddition,itshouldberememberedthatthewirelessmultimedianetworks
are composed of many links, some wireless (e.g., to the terminals) and some
wired.Themeansofprotectingthesignalissomewhatdifferentoverthesetwo
links due to the differing natures of their impairments. For the wired portion,
thenetworkaddsheaders(containing,e.g.,theMACaddressoftheuserandthe
numberassignedtoeachpacket)andcyclicredundancychecks(CRCs),which
areusedtochecktheintegrityofthereceivedpacket.Becauseofthereliability
of the wired channel, it is feasible to implement protection using an acknowl-
edgment (ACK/NACK)-based procedure known asAutomatic Repeat reQuest
(ARQ): a packet is retransmitted if, upon reception, the CRC does not match
thepartthatitprotects(e.g.,thepayload,header,orboth).Clearly,ARQ-based
errorcorrectionrequiresfeedbackandinduceslargedelays(thoughhopefullyon
onlyafewpackets).Iffeedbackwassomehowunavailable(e.g.,inthebroadcast
scenario),thentheCRCwouldatleastindicatewhichpacketswerenotcorrectly
received.
The amount of delay that is tolerable, as well as the channel conditions
themselves,canvarywidelyacrossdifferentapplications.
Finally,practicalcommunicationlinksarerarelydedicatedentirelytoasingle
user,andagivenuserrarelycommunicatesthroughonlyasinglelink.Rather,
each user’s signal is segmented into packets which are scheduled, along with
thepacketsofmanyotherusers,throughavailablelinks.Thissegmentationof
packets has often been overlooked in the design of robust reception schemes,
even though it can have a large (positive or negative) impact on those strate-
gies.Thistopicwillbeaddressedinthesecondpartofthebook,i.e.,Chapters
7–9,describingthecorrespondingtoolsthataresimilartotheonesusedinmore
traditionalJSCD(explainedinthefirstpart).
To make these notions more concrete, a few examples of applications that
might benefit from the tools discussed in this book are described, and their
essentialcharacteristicsareanalyzed.
1.2 EXAMPLEAPPLICATIONS
Over the past several years, the authors have worked with engineers, network
operators, and academic colleagues (all of them gratefully acknowledged) on
variouswirelessmultimediasystemdesignprojects.Someoftheseprojectsare
describedhereandlinkedtolaterchaptersofthebook.Theseexamplesareuseful
inbridgingtheabstractmodelsusedlaterinthebookwithpracticalreal-world
problems.
1.2.1 Example 1:Visiophony Over IP (RNRTVIP)
Thisprojectwasconcernedwiththeclassicalscenarioofavideocommunication
through wireless terminal links that are connected by a wired central link that
6 CHAPTER|1 Introduction: Context
Wired
network
GPRS GPRS
UMTS UMTS
FIGURE1.1 WirelessvisiophonyoverIP.
uses the IP, see Figure 1.1.Admittedly, the model assumed in the project was
abstract,andmanyimplementationdetailswereneverconsidered.However,the
essentialconstraintsresultingfromthevisiophonyapplicationwere(1)thatthe
maximumdelayinducedbycodingandtransmissionwasatmost≈0.5sand(2)
thatthereconstructedspeech/videosignalwasofsufficientlyhighquality.
The proposed solution leveraged the fact that the error mechanism on the
wiredlinkisdifferentthanthatonthewirelesslink.Forexample,thewiredlink
canintroducepacketlosses,which–intermsoftransmission–areequivalent
toerasures.Whilethedelayconstraintmakestheretransmissionoflostpackets
difficult,theknowledgeofwhichpacketsarelostcanbeusedforerrorconceal-
ment;portionsofthestreamreceivedwithouterrorcanbeinterpolatedto“fill
theholes.”Incontrast,thewirelesslinkmustcontendwithintersymbolinterfer-
ence(ISI)andnoise,themitigationofwhichcanbeaccomplishedusingJSCD.
Sinceitwasassumedthatthewirelesslinkdoesnotsupportretransmission,the
decodermustbeabletodisplayanimageonthescreen,whatevertheoutputof
thechannel.Buteveninthissituation,CRCcheckscanbeexploitedbythePHY
layer:thepacketsthatcanbecorrectlyreceivedusingasimple“harddecision”
procedure(asindicatedbytheCRC)donotneedtheapplicationofamorecom-
plexJSCDprocedure.Packetsthatdonotpassthischeckcanbeprocessedbya
robustdecoder,afterwhichCRC-basedcheckswouldbeinfeasible.
While the mechanisms described above are somewhat simple and ignore
manypracticalaspectsofimplementation,theyalreadygobeyondwhathasbeen
proposedinmanyJSCDstudies,where,e.g.,erasureswerenottakenintoaccount.
The primary aim of this study was to quantify the potential improvements of
“classical”JSCD.(SeeFigure1.2.)
Example1:Summary
● Mixedwired/wirelesslinksfacilitateerrorconcealment.
● StrongdelayconstraintslimitthepossibilityforARQ.
● CRCscanbeusedtodeterminewhichpacketsrequirecomplexJSCD.
1.2.2 Example 2:Video Broadcasting (CNES SDMB,Alcatel
TVMSL)
ThesecondexamplepertainstoaprojectlaunchedbyAlcatel-Lucentin2006,
partiallyfundedbythe“Agencedel’InnovationIndustrielle”(AII)oftheFrench
Section|1.2 ExampleApplications 7
Satellite component Global geographical coverage
+
Ground component Cellular-type coverage
FIGURE1.2 Videobroadcastingusingamixtureofsatelliteandterrestriallinks.
government, where the goal was to provide (relatively) low-cost video broad-
casting to mobiles.The project was motivated by the observation that the first
users of 3G were using video consultation much more than anticipated. In the
proposed solution, mobiles were addressed by satellites when possible or ter-
restrial relays when satellite reception was poor, see Figure 1.2.While, in this
project,mostoftheeffortwasdevotedtoimplementation,someeffortwasmade
toevaluatetheimpactofJSCD.
Atfirstglance,thisapplicationdoesnotseemtoimposeadelayconstraint;
sincethereisnoneedtosupportacomfortableconversationbetweentwousers,
theredoesnotseemtobeadrawbacktoaseveral-seconddelaybetweensource
anddestination.However,itturnsoutthatusersdowanttheabilitytoswitchfrom
onechanneltoanotherwithoutwaitingtoolong.Thus,theapplicationdoesinfact
imposeadelayconstraint,thoughperhapsnotasstrictastheoneintheprevious
example.Forthisproject,theDVS-S/Hstandardwasassumed,whichallowedthe
introductionofMAC-layerredundancybutrequiredcross-layertuning.Sincea
broadcastscenariowasconsidered,feedbackwasnotfeasible,andthesystemwas
designedtomaximizerobustnessandcoverage.Finally,awiderangeofterminal
mobility(i.e.,slowtofast)wasallowed.Sincemobilitygreatlyimpactsthefading
characteristicsofthewirelesschannel,itcangreatlyimpactperformanceinterms
ofvideoquality.
Example2:Summary
● Primarily,wirelesslinks
● Milddelayconstraints,e.g.,uptoseveralseconds
● Nopossibilityoffeedback
● Widerangeofterminalmobility
1.2.3 Example 3: Multimedia Multicasting (RNRT DIVINE)
The application envisioned here is a museum in which supplemental informa-
tiononeachexhibit,intheformofmultimediafiles,isservedtovisitors’mobile