VIDEO BROADCASTING
C.S. Lee, S. Vlahoyiannatos, L. Hanzo
Dept. of Electr. and Comp. Sc., Univ. of Southampton, SO17 1BJ, UK.
Tel: +44-1703-593 125, Fax: +44-1703-594508
Email: [email protected]
http://www-mobile.ecs.sot on.a c.u k
ABSTRACT
The Pan-European Digital Satellite Video
Broad-casting (DVB-S) system's performance is
charac-terised. Various congurations of blind equalisers
andconvolutionalcodecsoperatingatdierentcode
rateswereinvestigated. Thestandardsystem's
per-formancewasimproveduponreplacingthe
conven-tionalconvolutionalcodecbyaturbocodec. Lastly,
thefeasibilityofemploying16-levelQuadrature
Am-plitude Modulation (16-QAM) within the DVB-S
system is demonstrated, potentially doubling the
availablebitratehenceimprovingtheassociatedvideo
qualityatthecostofahigherSignal-to-NoiseRatio
(SNR) requirement. This extratransmitted power
requirementcanbeeliminateduponinvokinga
sim-ilar complexity turbo codec, which requires lower
transmitted power for attaining the same
perfor-manceasthe standardconvolutional codes.
1. BACKGROUNDANDMOTIVATION
ThePan-EuropeanDigitalVideoBroadcasting(DVB)
stan-dardsdesignedforterrestrial[1 ],cable-based[2]and
satellite-oriented [3 ] transmission of DVB signals have been
har-monised with each other, in order to maintain seamless
transportof videosignal. These standardisationactivities
werefollowedbyavarietyofsystemperformancestudiesin
theopen literature[4, 5 , 6,7]. Against this backcloth,in
thistreatiseweproposedturbo-codingbasedimprovements
tothesatellite-based DVBsystem[3 ]andstudiedthe
per-formanceof theproposedsystemunderdispersivechannel
conditions in conjunction with a variety of blind channel
equalisation algorithms. The transmitted power
require-mentsofthestandardconvolutionalcodecscanbereduced
uponinvokingasimilar-complexityturbocodec. More
ex-plicitly,thecomplexityofthe constraint-length7
convolu-tionalcodeand thatoftheconstraint-length3turbocode
is similar, since there are two turbo decoders, which are
invokedeight timesduring theeightiterations. Hencethe
numberof trellis states encounteredby the concolutional
andturbodecodersissimilar. Alternatively,thestandard
system'sbit error rate(BER) versus signal-to-noise ratio
(SNR) performance can almost be matched by a
turbo-coded16-levelquadratureamplitudemodulation(16QAM)
ICT'2000,22-25MAY,ACAPULCO,MEXICO
AREA:MULTIMEDIASERVICESANDTERMINALS
Reed
Scrambler
Solomon
Encoder
Convolutio-nal
Interleaver
Convolutio-nal
Encoder
Baseband
Shaping
&
Modulator
Channel
Descrambler
Reed
Solomon
Decoder
Convolutio-nal
Dein-terleaver
Convolutio-Decoder
nal
Equali-zer
tor
Demodula-
&
Figure1: SchematicoftheDVBsatellitetransmitter
func-tions.
basedscheme,whilstdoublingtheachievablebitratewithin
the same bandwidth and hence improving the associated
video quality. Thisis achievedat thecostofanincreased
systemcomplexity.
Theremainderofthe paperis organisedas follows. A
terseoverviewoftheturbo-codedandstandardDVB
satel-lite schemeis presented inSection 2. Following this, the
performanceoftheimprovedDVBsatellitesystemwas
ex-aminedoveradispersivetwo-pathchannelinSection3,
be-foreour conclusionsand futureworkareaswerepresented
inSection4.
2. DVBSATELLITESCHEME
TheschematicoftheDVB satellite(DVB-S)system [3]is
portrayed in Figure 1. The MPEG-2 encoded video bit
stream is channel encoded and QPSK modulated [8 ]. In
ordertoprotecttheMPEG-2videocodedstream,powerful
concatenated channelcoding is invoked,constituted by a
shortenedReed-SolomonRS(204,188)outercode[9 ],which
corrects up to eightcorrupted8-bit symbols inablockof
204bytes,andahalf-rateinnerconvolutionalencoder
hav-ing aconstraintlengthof7[9, 10 ]. A variablepuncturer
-not showninthegure -canalso be invoked,which
sup-portscoderatesof1=2(nopuncturing)aswellas2=3,3=4,
5=6 and 7=8. Theconvolutionalencoder's parametersare
portrayedinTable1.
Thestandard DVB-Ssystem was invokedas a
bench-markerand we haveimprovedthestandard system's
per-formance by replacing theconvolutionalcodec by aturbo
codec[11 ][12 ]employingtheparameterssummarisedin
ConstraintLength 7
k 1
n 2
Polynomials(octal) 171,133
Table 1: ParametersoftheCC(n,k,K) convolutionalinner
encoderintheDVB-Smodem.
Rate 1=2
Inputblocklength 17952bits
Interleaver random
Numberofiterations 8
ConstraintLength 3
k 1
n 2
Polynomials(octal) 7,5
Table 2: Parameters of the inner turbo encoder used to
replacetheDVB-Ssystem'sconvolutionalcoder.
systemarereferred totheDVB-Sstandard[3 ].
Thepropertiesofthesatellitechannelhavebeen
charac-terisedforexamplebyVogelandhiscolleagues[13,14 ,15 ,
16 ]. Thechannelmodelemployedinthisstudywasthe
two-path(nT)-symbolspacedchannelimpulseresponse(CIR),
whereT isthesymbol-durationandinourstudiesweused
n = 1 and n = 2. This corresponds to a stationary
dis-persivetransmission channel. Ourchannelmodelassumed
thatthereceiverhadadirectline-of-sightwiththesatellite
aswellasasecondpathcausedbyasinglereector. Inour
work,westudiedtheabilityofarangeofblindequaliser
al-gorithmstoconverge undervariouspathdelayconditions.
Theblindequalisersusedinthesystemareasfollows:
TheModiedConstantModulusAlgorithm(or
Mod-iedCMA)[17 ]
TheBenveniste-GoursatAlgorithm(orB-G)[18 ]
TheStop-and-Goalgorithm[19 ]
ThePer-SurvivorProcessing (PSP)Algorithm[20]
A summary of the equalisers' parameters is given in
Table3.
Equal. Tap No. of Initial
Update Equal. Tap{
Step{Size Taps Vector
Benveniste-Goursat 5x10
4
10 (1:2;0;;0)
Modied-CMA 5x10
4
10 (1:2;0;;0)
Stop-and-Go 5x10
4
10 (1:2;0;;0)
PSP(1symdelay) 10
2
2 (1:2;0)
PSP(2symdelay) 10
2
3 (1:2;0;0)
Table 3: Summaryoftheequaliserparametersusedinthe
simulations. The tap{bector (1:2;0;;0) indicates that
therstequalisercoeÆcient is initialised tothe value 1:2,
whiletheothersto0
0.2
0.4
0.6
0.8
PATH DELAY
TIME DELAY
ONE-SYMBOL OR
TWO-SYMBOL DELAY
Amplitude
Figure2: Two-path satellite channelmodel witheither a
one-symbolortwo-symboldelay.
3. PERFORMANCE OFTHEDVBSATELLITE
SCHEME
Inthis section,theperformance oftheDVB-Ssystem was
evaluated bymeansofsimulations. Twomodulationtypes
were used, i.e. QPSKand 16-QAM [8], and the channel
modelof Figure2was employed. Therstchannelmodel
had a one-symbol second-path delay, while inthe second
onethepath-delaycorrespondedtotheperiodoftwo
sym-bols. TheaverageBERversusSNRperbitperformanceis
presentedaftertheequalisationanddemodulationprocess,
as wellas afterViterbi[9 ]orturbodecoding[12 ].
TheaverageBERcurvesafterViterbiorturbodecoding
areshowninFigure3forQPSK.Inthisgure,theaverage
BER over the non-dispersive AWGN channel after turbo
decodingconstitutes the bestcase performance,while the
averageBERoftheone-symboldelaytwo-pathunequalised
channelafterturbodecodingcorrespondstotheworstcase
performance. Again,inthisgureonlytheModied-CMA
was featured for simplicity. The performance of the
re-mainingequaliserswassimilarandhenceitisnotexplicitly
shown.
It isobservedinFigure 3that thecombinationofthe
Modied CMA basedblindequaliserwithturbodecoding
exhibited the best SNR performance in conjunction with
QPSK. Theonly comparable alternative was the PSP
al-gorithm. Althoughtheperformance ofthe PSPalgorithm
isbetteratlowSNRs,theassociatedcurvescrossoverand
thePSPalgorithm'sperformancebecomesinferiorafterthe
average BER becomesapproximately 10
3
. Althoughnot
showninFigure 3,the Reed-Solomondecoder,whichwas
concatenated to either the convolutional or the turbo
de-coder,yieldingtypicallyanerror-freeoutputafterthe
aver-age BERofitsinputreachedapproximately 10
4
. Inthis
case, thePSP algorithmperformed worseinchannelSNR
terms by atleast1 dBinthe area ofinterest,whichisat
anaverageBERof10
4
. Analobservationinthecontext
of QPSKinFigure3is that whenconvolutionaldecoding
is used,theassociatedEb=No performanceof therate1/2
schemeappearsinferiortothatoftherate3/4andtherate
7/8scenariosbeyondcertainE b
=Novaluesduetothehigh
0
2
4
6
8
10
12
14
SNR per bit (dB)
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
BER
R7/8
R3/4
R1/2
MCMA TURBO
NE TURBO
AWGN TURBO
MCMA CONV
PSP
Figure 3: Average BER versus SNR perbit performance
afterconvolutional orturbodecoding forQPSK
modula-tionandone-symbol delayCIR(NE:Non-Equalised
B-G:Benveniste-GoursatS-a-G:Stop-and-GoMCMA:
Mod-ied Constant Modulus Algorithm PSP:
Per-Survivor-Processing).
4
6
8
10
12
14
16
18
20
SNR per bit (dB)
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
BER
R7/8
R3/4
R1/2
MCMA TURBO
B-G TURBO
AWGN TURBO
MCMA CONV
B-G CONV
Figure4:AverageBERversusSNRperbitafterViterbior
turbodecodingfor 16-QAMandtheone-symbol delay
two-pathCIRofFigure2.
0
2
4
6
8
10
12
14
SNR per bit (dB)
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
BER
R7/8
R3/4
R1/2
MCMA TURBO
NE TURBO
AWGN TURBO
MMA
PSP
Figure5: AverageBERversusSNRperbitperformance
af-terconvolutionalorturbodecodingforQPSKmodulation
overthetwo-symbol delay two-pathCIRofFigure2.
InFigure4,thecorrespondingBERcurvesaregivenfor
16-QAM,underthesamechannelandequaliserconditions.
TheStop-and-Goalgorithmhasbeenexcludedfromthese
experiments,sinceit doesnotconverge for highSNR
val-ues. Thishappensbecausethisprocedureisonlyactivated,
whenthereis ahigh probability of correct equaliser
coef-cient update. Inour case, theequaliser is initialised far
fromitsconvergencepointandhencethedecision{directed
updatesareunlikelytoprovidecorrectequalisercoeÆcient
updates. In the absence of noise this leads to the
algo-rithm being permanentlyde-activated. If noise is present
though, then some random perturbations from the point
of the equaliser'sinitialization can activate the algorithm
andcanlead toconvergence. Thiswas ourobservationat
mediumSNR values. Butagain, for high SNRvaluesthe
algorithmdoesnotconverge.
Itisalsointerestingtocomparetheperformanceofthe
system for the QPSK and 16-QAM schemes investigated.
Whentheone-symboldelaytwo-pathchannelmodelof
Fig-ure2 was considered, the systemwas capable of
support-ingtheuseof16-QAMwiththeprovisionofanadditional
E b
=Noof5dB.AlthoughtheDVB-Satellitestandardsystem
only employs QPSK modulation, our simulations showed
that-especially withtheadventof turbo coding -16-QAM
canbe employed equally well. Thisallowsus todoublethe
video bitrate and hence improve its quality. The
compar-ison of Figures 3 and 4 also reveals that the extra SNR
requirementof 5dB for 16QAMoverQPSKcanbe
elim-inatedby employingturbocodingat the costof asimilar
implementationalcomplexity.
InFigure5thecorrespondingBERresultsforthe
two-symbol delay two-path channel of Figure 2 are given for
QPSK. This gure is similar to Figure 3 in terms of its
4
6
8
10 12 14 16 18 20 22
SNR per bit (dB)
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
BER
R7/8
R3/4
R1/2
MCMA TURBO
S-a-G TURBO
B-G TURBO
AWGN TURBO
MCMA CONV
B-G CONV
Figure6:AverageBERversusSNRperbitperformance(a)
afterequalisationanddemodulationand(b)afterViterbior
turbodecodingfor16-QAMoverthetwo-symboldelay
two-pathCIR ofFigure2.
performance of the PSP algorithm becomes worse, than
that of the Modied CMA inconjunction with turbo
de-codingis now at10 4
,which isinthe areawhere the RS
decoderprovidesanerror-freeoutput.
Finally,inFigure6the associated 16-QAMresultsare
presented.NoticethattheStop-and-Goalgorithmwasagain
excludedfromtheseresults. Wecanobservethehigh
per-formancedierencebetweentheB-GandtheModiedCMA.
Inthepreviouscaseswedidnotobservesucha
signi-cantdierence. Thedierenceinthiscaseisthatthe
chan-nelexhibitsanincreased delayspread. Thisillustratesthe
capability of the equalisersto cope withmore widespread
multipaths,keepingtheirlengthconstant. The
Benveniste-GoursatequaliserismoreeÆcientthantheModiedCMA
inthiscase.
Ifwecomparetheperformanceofthesystememploying
QPSKand16-QAMunderthetwo-symboldelaytwo-path
channelmodelofFigure2,weagainobservethat16-QAM
can be incorporated into the DVB system, if an extra 5
dB of SNR per bit is aordable in power budget terms.
However,onlytheB-Galgorithmisworthwhileconsidering
hereoutofthethreelinearequalisersusedinourwork.
Figure7portraysthecorrespondingreconstructedvideo
performance in terms of the average peak signal-to-noise
ratio(PSNR)versuschannelSNRfortheone-symboldelay
andtwo-symboldelaytwo-pathchannelmodelofFigure2.
TheaveragePSNRisdenedasfollows:
PSNR=10l og10 P
N
n=0 P
M
m=0 255
2
P N
n=0 P
M
m=0
2
(1)
av_psnr_vs_channelSNR_for_dvbS_one_symbol_delay_2-path_channel_model.gle
0
5
10
15
20
25
30
Channel SNR (dB)
10
15
20
25
30
A
verage
PSNR
(dB)
16QAM
QPSK
CONV R7/8 (Benveniste-Goursat)
CONV R3/4 (Modified Constant Modulus)
CONV R1/2 (Modified Constant Modulus)
CONV R1/2 (PSP)
TURBO R1/2 (Modified Constant Modulus)
error free
2dB degradation
QPSK
16-QAM
.
(a)One-symboldelaytwo-pathchannelmodel
av_psnr_vs_channelSNR_for_dvbS_two_symbol_delay_2-path_channel_model.gle
0
5
10
15
20
25
30
Channel SNR (dB)
10
15
20
25
30
A
verage
PSNR
(dB)
16QAM
QPSK
CONV R7/8 (Modified CMA)
CONV R7/8 (Benveniste-Goursat)
CONV R3/4 (Benveniste-Goursat)
CONV R1/2 (Benveniste-Goursat)
CONV R1/2 (PSP)
TURBO R1/2 (Benveniste-Goursat)
error free
2dB degradation
QPSK
16-QAM
.
(b)Two-symboldelaytwo-pathchannelmodel
Figure7: Average PSNRversus channel SNRfor(a)
one-symboldelay two-path channelmodeland(b)two-symbol
delaytwo-pathchannelmodelofFigure2.
where isthe dierencebetweenthe uncodedpixelvalue
andthereconstructedpixelvalue.
Table 4provides anapproximation ofthe convergence
speedofeachalgorithmforeachcase. ItisclearthatPSP
exhibitedthefastestconvergence,followedbythe
Benveniste-Goursatalgorithm. Inoursimulationstheconvergencewas
quantied by measuring the slope of the BER curve, as
this curve was reaching the associated residualBER,
im-plying that theBER hasreachedsteady-state. The
Stop-and-Go algorithm convergessignicantlyslower, thanthe
otheralgorithms,whichcanalsobeseenfromTable4. This
happens because during the startup the algorithm is
de-activated mostof thetime, aneectwhichbecomes more
QPSK1sym 210 5
4:410 5
2:610 5
380
QPSK2sym 210
5
3:910 5
2:110 5
380
16-QAM1sym 5:610
5
8:810 5
1910 5
16-QAM2sym 4:910
5
5:610 5
1810 5
Table4: Equaliserconvergencespeedmeasuredinthe
simu-lations,givenasthenumberofbitsrequiredforconvergence
(xsym: x-symboldelaytwo-pathchannel).
4. CONCLUSIONSANDFUTURE WORK
Inthiscontribution,wehaveinvestigatedtheperformance
ofaturbo-codedDVBsysteminasatellitebroadcast
envi-ronment. A rangeofsystem performance results was
pre-sentedbasedonthestandardDVB-Sscheme,aswellason
aturbo-coded schemeinconjunction withblind-equalised
QPSK/16QAM. The convolutional code specied in the
standard system was substituted by turbo coding, which
resulted in a substantial coding gain of around 4-5 dB.
Wehave alsoshownthat 16-QAMcan beutilisedinstead
ofQPSK, if anextra 5 dB SNR perbit gain is added to
thelinkbudget. Thisextratransmittedpowerrequirement
canbeeliminateduponinvokingasimilar-complexityturbo
codec,whichrequireslowertransmittedpowerforattaining
thesameperformanceasthestandardconvolutionalcodecs.
Our future work will be focused on extending the
DVB-Satellitesystemtosupportingmobileusersforthereception
ofsatellite broadcast signals. The use ofturboequalisers
willalsobeinvestigatedincomparison toblindequalisers.
Furtherworkwillalsobededicatedtotrelliscoded
modu-lation(TCM)andturbotrelliscodedmodulation(TTCM)
basedorthogonal frequencydivision multiplexed(OFDM)
andsingle-carrierequalisedmodems. Arangeoffurther
re-latedwirelessvideocommunicationsaspectsaretreatedin
[21 ].
5. ACKNOWLEDGEMENT
Thenancialsupportofthefollowingorganisationsis
grate-fullyacknowledged: EPSRC, UKinthe frameworkofthe
contractGR/K74043;theEuropeanCommission;the
Mo-bileVCE,UK.
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