B GIO DC V O TOTRNG I HC GIAO THNG VN TI TP.HCMKHOA: IN - IN T VIN THNGB MN: IN T VIN THNG

N MN HC 2Welcome to: www.hcmutrans.edu.vn

ti: ng dng cng ngh OFDM trong Truyn Hnh S Mt t, v thc trng s dng ti Vit Nam.

GVHD: TRN VN THSVTH: NGUYN TH TCHLp: DV11MSSV: 1151040060

TP.HCM, ngy 25 thng 12 nm 2014

LI NI U4DANH MC T VIT TT5CHNG 1: TNG QUAN TRUYN HNH S231.1 Lch s pht trin:231.2 Gii thiu chung v truyn hnh s241.3 S khi v chc nng ca h thng truyn hnh s271.4 nh s v c im ca truyn hnh s281.41nh s281.42 c im281.5 C s bin i tn hiu truyn hnh311.5.1 Bin i tn hiu Video311.52 Chn tn s ly mu311.53 Lng t ha tn hiu Video321.5.4 M ha tn hiu Video321.6 Cc h thng truyn hnh s qung b331.7.1 H thng qung b truyn hnh s hu tuyn DVB-C34c im chung:341.7.2 H thng qung b truyn hnh s v tinh DVBS351.7.3 H thng qung b truyn hnh s trn mt t DVB T36c im chung:361.8. Nn tn hiu truyn hnh361.82 Khi nim v phn loi MPEG401.83 Cc cu trc nh421.84 Cu trc phn lp MPEG2441.9 iu ch tn hiu s481.91 Kha dch chuyn bin (ASK)491.92Kha dch chuyn v pha PSK50+ Pha chia 8 (8.PSK)27+ Pha chia 16271.93 iu ch bin gc phn t (QAM)27Chng 2 TIU CHUN TRUYN HNH S MT T DVB T292.1 Gii thiu v truyn hnh s mt t DVB - T29CHNG 3: K THUT OFDM313.1 Gii thiu chng313.2 Nguyn tc c bn ca OFDM323.3 Tnh trc giao343.4 S dng FFT/IFFT trong OFDM363.5 Nhiu giao thoa k t v nhiu giao thoa sng mang373.5.1 Khi nim373.5.2 Phng php chng nhiu lin k hiu393.6 Cc vn k thut trong OFDM413.6.1 c lng tham s knh413.6.2 ng b trong OFDM423.6.2.1 ng b k t423.6.2.2 ng b tn s sng mang433.6.2.3 ng b tn s ly mu443.6.3 Gim PAPR (Peak to Average Power Ratio)443.7 H thng OFDM453.8 u im v khuyt im ca OFDM473.8.1 u im473.8.2 Khuyt im47Chng 3 TRIN KHAI TRUYN HNH S MT T TI VIT NAM484.1 Tnh hnh pht trin truyn hnh s mt t trn th gii484.2 L do Vit Nam chn h thng truyn hnh s DVB T514.3 Qu trnh th nghim524.4 Tnh hnh pht trin544.5 nh gi55

LI NI U

Trong nhng nm gn y, k thut thng tin v tuyn c nhng bc pht trin vt bc. S pht trin nhanh chng ca video, thoi v thng tin d liu trn Internet, in thoi di ng c mt khp mi ni cng nh nhu cu v truyn thng a phng tin trn di ng ang ngy mt tng cao. V truyn hnh s vi nhng u im vt tri hn so vi truyn hnh tng t cng ra i nh mt tt yu.Truyn hnh smt t theo tiu chun Chu u DVB - T hin ang c s dng ph bin nhiu nc trn th gii. Vi nhiu u im v k thut cng nh ph hp vi iu kin t nhin - x hi, h truyn hnh s mt t theo tiu chun ny ang c trin khai v s dng ti Vit Nam.Em nhn thy truyn hnh s mt t theo tiu chun Chu u DVB - T l cng ngh c nhiu tim tim nng hin nay vi kh nng pht trin vng chc v lu di. ti cho n mn hc 2 ca em l Cng Ngh OFDM v ng dng trong Truyn Hnh S Mt t, thc trng s dng ti Vit Nam. Trong qu trnh lm ti, em c gng rt nhiu song do lng kin thc vn ang con hn ch nn khng th trnh khi nhng thiu st, em rt mong nhn c s thng cm, ph bnh, hng dn v s gip tn tnh ca Thy.Cui cng em xin by t lng bit n su sc ti cc thy, c gio trong khoa in t vin thng gip em v mt kin thc hon thnh n ny. c bit em xin gi li cm n chn thnh ti thy hng dn Trn Vn Th, trc tip hng dn em hon thnh bn n ny.Em xin chn thnh cm n!

DANH MC T VIT TT

T vit ttTing Anh y Ting Vit

ATSCAdvanced Television System CommiteeHi ng v h thng truyn hnh ci bin

C/NCarrier/NoiseSng mang/tp m

CDCompact DiskCD

COFDMCoding Othogonality Fequency Dvision MltiplexingM ha ghp knh theo tn s trc giao

DiBEGDigital Broadcasting Expert GroupNhm chuyn gia truyn hnh s

DVBDigital Video BroadcastingTruyn hnh s

DVB-C/S/TDigital Video Broadcasting-Cable / Satellite / TerrestrialTruyn hnh s qua cp / v tinh / pht sng trn mt t

EDTVEnhanced Definition TelevisionTruyn hnh phn gii m rng

FECForward Error CorrectionSa li tin (thun)

HDTVHigh Definitiom TelevisiomTruyn hnh phn gii cao

ISDBIntegrated Services Digital BroadcasingTruyn hnh s cc dch v tch hp

LDTVLow Definitiom TelevisionTruyn hnh phn gii thp

MPEGMoving Pictures Experts GroupNhm chuyn gia nghin cu v nh ng

M-PSKM-ary Phase Shift KeyingKha dch pha M trng thi

M-QAMM-ary Quadrature Amplitude Modulationiu ch bin vung gc M trng thi

NTSCNational Television System CommitteeHi ng h thng truyn hnh quc gia M

OFDMOthogonality Fequency Dvision MltiplexingGhp knh phn chia theo tn s trc giao

PALPhase Alternating LinePha lun phin theo dng

QAMQuadrature Amplitude Modulationiu ch bin vung gc

QPSKQuadrature Phase Shift KeyingKha dch pha vung gc

RFRadio FrequenceCao tn

SDTVStandard Definition TelevisionTruyn hnh phn gii tiu chun

SFNSingle Frequence NetworkMng n tn

SMPTESociety of Motion Picture and Television EngineersHip hi nh ng v k s truyn hnh

VODVideo On DemandTruyn hnh theo yu cu

4

2

CHNG 1: TNG QUAN TRUYN HNH S

1.1 Lch s pht trin:

Lch s pht trin ca truyn hnh rt phc tp v n ng dng kt qu nghin cu ca nhiu lnh vc khoa hc k thut khc nhau.- Nm 1839 Bee Quell tm ra hin tng quang in.- Nm 1898 Volsske tm ra h thng truyn hnh khng dy dn (truyn hnh bng sng in t). - Nm 1945 tiu chun truyn hnh 525 dng v 625 dng cc th nghim truyn hnh mu bt u t sau i chin th gii ln th II v ngy nay tr thnh ph bin trn th gii. - Truyn hnh mu ra i khi h thng truyn hnh en trng hon thin.V vy khi xy dng h thng truyn hnh mu cn phi gii quyt sao cho my thu hnh, en trng c th thu c vi hnh nh en trng. Mt khc phi gii quyt c ngc li l dng my thu hnh mu c th thu c chng trnh truyn hnh en trng vi nh nhn c cng l en trng. l tnh kt hp ca h truyn hnh mu. Phng php to tn hiu hnh mu hon chnh (bao gm tn hiu chi v tn hiu mu) c thc hin u tin M. Tn hiu hnh mu hon chnh mang tin tc v mu sc v bo ho mu ca nh mu. Nng lng ph tn hiu chi phn b khng u trong c bng tn tn hiu truyn hnh. Tn hiu mu c ph nm trong gii tn hp hn v b tr min tn s cao ca bng tn tn hiu chi . T c im trn c th thc hin cc iu kin kt hp truyn hnh mu v truyn hnh en trng. Tn hiu hnh mu v tn hiu chi nm trong cng 1 bng tn c bn c thc hin trong h truyn hnh mu NTSC nm 1950 M do FCC (Feeal Communcation Commion - y ban thng tin lin bang).Nhng nm sau h truyn hnh mu pht trin nhanh chng, cht lng nh h NTSC ha phi l tt v tn hiu h NTSC rt nhy vi mo pha v mo bin .Ngi ta chuyn sang nghin cu tm ra cc h thng m ho tn hiu mu khc, sao cho mo pha v mo bin xut hin trong knh truyn hnh l nh hng nh nht.- Nm 1957 Php xut hin h truyn hnh mu SECAM do Henry De France nghin cu v thc hin. - Nm 1962 gio s Walter Bruce Ty c cng b H truyn hnh PAL. C hai h SECAM v PAL v nguyn l chung thng nht vi h NTSC.- Nm 1966 sl (Na Uy) tin hnh hi ngh CCIR chn h truyn hnh mu thng nht cho c Chu u, tin cho vic trao i chng trnh truyn hnh mu gia cc nc.Kt qu mt s nc chn h SECAM cn mt s nc dng h PAL, M v Nht s dng h NTSC. Vit Nam chn h PAL tiu chun OIRT (Organization International Radio and Television - t chc pht thanh truyn hnh quc t). - Nm 1994 M nghin cu v th nghim truyn hnh s, n thng 12 nm 1996 ban hnh tiu chun ATSC.- Nm 1997 Nht Bn ban hnh tiu chun ISDB - hay cn gi l tiu chun DIBEG. - Nm 1997 tiu chun DVB-T ca Chu u ra i. Nhiu nc Bc u, mt s nc Chu trong c Vit Nam v nhiu nc khc la chn tiu chun ny v d kin pht sng s hon ton vo nm 2010-2015. - Vit Nam t nm 1997 n nay c mt s n v k thut c nghin cu v tip cn vi cng ngh s, cho n nay nhiu cng on trong sn xut chng trnh, truyn dn c s ho, nhiu ti nghin cu truyn hnh s v ang c nghin cu th nghim chnh v vy m n mang tnh khoa hc v thc tin cao nhm cng ngy cng nng cao cht lng cho vic pht hnh s ti Vit Nam.Xu hng chung cho s pht trin truyn hnh l nhm t c thng nht, l h thng truyn hnh hon ton c k thut c cht lng cao v d dng phn phi trn knh thng tin, v vy truyn hnh k thut s v ang c pht trin mnh m.1.2 Gii thiu chung v truyn hnh s

Cc h thng truyn hnh ph bin hin nay nh: NTSC, PAL, SECAM l cc h thng truyn hnh tng t. Tn hiu Video l hm lin tc theo thi gian. Tn hiu truyn hnh tng t (t khu to dng, truyn dn, pht sng n khu thu tn hiu u chu nh hng ca nhiu yu t (nhiu v can nhiu t ni b h thng v t bn ngoi) lm gim cht lng hnh nh.Truyn hnh s l tn gi ca mt h truyn hnh m tt c cc thit b k thut t Studio (Camera truyn hnh,..) n my thu u lm vic theo nguyn l k thut s. Trong , mt hnh nh quang hc do camera thu c qua h thng ng knh, thay v c bin i thnh tn hiu in bin thin tng t nh hnh nh quang hc ni trn (c v chi v mu sc) s c bin i thnh mt dy tn hiu nh phn (dy cc s 0 v 1) nh qu trnh bin i tng t - s.

Trong nhng nm gn y, cng ngh truyn hnh ang chuyn sang mt bc ngot mi. Qu trnh chuyn i t cng ngh tng t sang cng ngh s. S dng phng php s to, lu tr v truyn tn hiu ca chng trnh truyn hnh trn knh thng tin m ra mt kh nng c bit rng ri cho cc thit b truyn hnh. Trong mt s ng dng, tn hiu s c thay th hon ton cho tn hiu tng t v n c kh nng thc hin c cc chc nng m tn hiu tng t hu nh khng th lm c hoc rt kh thc hin, nht l trong vic x l tn hiu v lu tr.So vi tn hiu tng t, tn hiu s cho php to, lu tr, ghi c nhiu ln m khng lm gim i cht lng nh. Tuy nhin, khng phi trong tt c cc trng hp, tn hiu s u t c hiu qu cao hn so vi tn hiu tng t. Mc d vy xu hng chung cho s pht trin cng nghip truyn hnh trn th gii, nhm t c mt s thng nht chung, l mt h thng hon ton k thut s c cht lng cao v d dng phn phi trn knh thng tin. H thng truyn hnh s v ang c pht trin trn ton th gii, to nn mt cuc cch mng thc s trong cng nghip truyn hnh.i vi cc nh pht sng truyn hnh, vic chuyn dch ln mi trng s s lm gim vic s dng bng tn/knh, lm tng kh nng cung cp cc ng dng Internet cho thu bao v m ra mt lnh vc mi, cc c hi mi v thng mi. Nhiu dch v mi trn c s truyn hnh s s c hnh thnh: Truy cp Internet tc cao Chi Game v gii tr trn mng Video theo yu cu VOD (video - on - demand) Cung cp cc dng video v audio Dch v thanh ton tin ti nh (home banking) Cc dch v thng mi in t1.3 S khi v chc nng ca h thng truyn hnh sHnh 1.2. S khi ca h thng truyn hnh sAudioCT nCT 1VideoGhp knh chng trnhMPEG-2A/DA/DGii iu chiu chTch knh chng trnhGii MPEG-2D/AVideoVideoAudioAudioDVB-TDVB-CDVB-S

MPEG-2Khi s ha tn hiu truynPha thuPha phtKhi nn vidieo s

- Theo hnh 1.2: Mi mt chng trnh truyn hnh cn mt b m ha MPEG-2 ring trc khi bin i tng t sang s.- Khi c nn gim ti d liu, cc chng trnh ny s ghp li vi nhau to thnh dng bt lin tip.- Lc ny chng trnh sn sng truyn i xa, cn c iu ch pht i theo cc phng thc:+ Truyn hnh s v tinh DVB-S (QPSK)+ Truyn hnh s cp DVB-C (QAM) + Truyn hnh s mt t ( COFDM).- Pha thu sau khi nhn c tn hiu s tin hnh gii iu ch ph hp vi phng php iu ch, sau tch knh ri gii nn MPEG-2, bin i ngc li s sang tng t, gm 2 ng hnh v ting ri n my thu hnh.

1.4 nh s v c im ca truyn hnh s

1.41 nh s- V gc k thut th nh c nhn bit thng qua h thng th gic hai chiu. nh ng l tp hp ca nhiu nh tnh lin tip. Khi mt nh c s ha th n tr thnh nh s v nh s ny l mt ca cc phn t nh nh c gi l im nh pixel. Mi im nh c biu din di dng mt s hu hn cc bit, da vo ta c th chia ra lm cc loi nh khc nhau: nh en trng: Mi im nh c biu din bi mt bit, cc nh ny cn c gi l Bi- level hay Bi- Tonal images. nh Gray-scale: Mi im nh c biu din bng cc mc chi khc nhau, thng th nh ny biu din bng 256 mc chi hay 8 bit cho mi im nh. nh mu: Mi nh tn hiu mu c chia ra gm 1 tn hiu chi v mt tn hiu mu.

1.42 c im

+ C kh nng pht hin li v sa sai.+ Thu di ng tt. Ngi xem d i trn t, tu ha vn xem c cc chng trnh truyn hnh. S d nh vy l do x l tt hin tng Doppler (tn s v bc sng ca cc sng m, sng in t hay cc sng ni chung b thay i khi m ngun pht sng chuyn ng tng i vi ngi quan st.).+ Truyn ti c nhiu loi thng tin.+ t nhy vi nhiu vi cc dng mo xy ra trn ng truyn. Bo ton cht lng hnh nh, thu s khng cn hin tng bng ma do cc tia sng phn x t nhiu hng n my thu. y l vn m h analog ang khng khc phc ni.+ Pht nhiu chng trnh trn mt knh truyn hnh: Tit kim ti nguyn tn s:+ Mt trong nhng u im ca truyn hnh s l tit kim ph tn s+ 1 transponder 36MHz truyn c 2 chng trnh truyn hnh tng t song c th truyn c 10 12 chng trnh truyn hnh s (gp 5 6 ln)+ Mt knh 8 MHz (trn mt t) ch truyn c 1 chng trnh truyn hnh tng t song c th truyn c 4 5 chng trnh truyn hnh s i vi h thng ATSC, 4 8 chng trnh i vi h DVB T (ty thuc M-QAM, khong bo v v FEC)Bo ton cht lng :Cht lngKhong cch gia my pht v my thuTn hiu sTn hiu tng t

Hnh 1.1 So snh cht lng tn hiu s v tng t+ Tit kim nng lng, chi ph khai thc thp: Cng sut pht khng cn qu ln v cng in trng cho thu s thp hn cho thu analog ( nhy my thu s thp hn -30 n -20 DB so vi my thu analog).+ Tn hiu s d x l, mi trng qun l iu khin v x l rt thn thin vi my tnh + C th tin hnh rt nhiu qu trnh x l trong Studio (trung tm truyn hnh) m t s S/N khng gim (bin i cht lng cao). Trong truyn hnh tng t th vic ny gy ra mo tch ly ( mi khu x l u gy mo ).+ Thun li cho qu trnh ghi c: c th ghi c v hn ln m cht lng khng b gim.+ C kh nng lu tn hiu s trong cc b nh c cu trc n gin v sau c n vi tc ty .+ Kh nng truyn trn c ly ln: tnh chng nhiu cao (do vic ci m sa li, chng li, bo v...).+ D to dng ly mu tn hiu, do d thc hin vic chuyn i h truyn hnh, ng b t nhiu ngun khc nhau, d thc hin nhng k xo trong truyn hnh.+ Cc thit b s lm vic n nh, vn hnh d dng v khng cn iu chnh cc thit b trong khi khai thc.+ C kh nng x l nhiu ln ng thi mt s tn hiu (nh ghp knh phn chia theo thi gian).+ Tit kim c ph tn nh s dng cc k thut nn bng tn, t l nn c th ln n 40 ln m hu nh ngi xem khng nhn bit c s suy gim cht lng. T c th truyn c nhiu chng trnh trn mt knh sng, trong khi truyn hnh tng t mi chng trnh phi dng mt knh sng ring.+ C kh nng truyn hnh a phng tin, to ra loi hnh thng tin 2 chiu, dch v tng tc, thng tin giao dch gia im v im. Do s pht trin ca cng ngh truyn hnh s, cc dch v tng tc ngy cng phong ph a dng v ngy cng m rng. Nhc im ng quan tm:+ Di thng ca tn hiu tng do rng bng tn ca thit b v h thng truyn ln hn nhiu so vi tn hiu tng t.+ Vic kim tra cht lng tn hiu s mi im ca knh truyn thng phc tp hn (phi dng mch chuyn i s - tng t).+ Cht lng phc v gim nhanh khi my thu khng nm trong vng phc v.+ kim tra tnh trng ca thit b truyn hnh s, s dng cc h thng o kim tra tng t nh i vi h thng truyn hnh tng t, thng qua o kim tra tn hiu chun.

1.5 C s bin i tn hiu truyn hnh 1.5.1 Bin i tn hiu Video Bin i tn hiu Video tng t thnh Video s l bin i thun, cn bin i tn hiu Video s thnh tng t l bin i ngc. Trong h thng truyn hnh s c rt nhiu b bin i thun v ngc.Khi bin i tn hiu Video mu tng t thnh tn hiu Video mu s ta c th dng 2 phng php sau: Phng php 1:Bin i trc tip tn hiu mu tng hp NTSC, PAL, SECAM ra tn hiu sPhng php 2:Bin i ring tng tn hiu thnh phn (tn hiu chi Y, tn hiu s R-Y v B-Y hoc cc tn hiu mu c bn R, G, B) ra tn hiu s v tryun ng thi theo thi gian hoc ghp knh theo thi gian.Phng php 2 Bin i ring cc tn hiu thnh phn (ca tn hiu mu) thnh tn hiu s s lm tc bit tng cao hn so vi vic bin i tn hiu mu Video tng hp. Cch ny c u im l khng ph thuc cc h thng truyn hnh tng t, thun tin cho vic trao i cc chng trnh truyn hnh. Cng c th gim tc bit nu s dng m thch hp. Do m ring cc thnh phn tn hiu mu, nn c th kh c nhiu qua li (nhiu ca tn hiu ly mu vi cc hi ca ti tn mu).V nhng nguyn nhn trn cho nn cch bin i s cc tn hiu thnh phn (ca tn hiu Video mu tng hp) u vit hn cch bin i trc tip tn hiu Video mu tng hp. Do , t chc truyn thanh truyn hnh quc t khuyn co nn dng loi ny cho trung tm truyn hnh (studio), truyn dn, pht sng v ghi hnh.1.52 Chn tn s ly muCng on u tin ca qu trnh bin i tn hiu tng t sang tn hiu s l ly mu (c ngha l ri rc tn hiu tng t theo thi gian). Do tn s ly mu l mt trong nhng thng s c bn ca h thng k thut s. C nhiu yu t quyt nh vic la chn tn s ly mu. Tn s ly mu cn c xc nh sao cho hnh nh nhn c c cht lng cao nht, tn hiu truyn i vi tc bit nh nht, rng bng tn nh nht v mch n gin.Ly mu tn hiu Video : cho vic ly mu khng gy mo, ta phi chn tn s ly mu tho mn cng thc Kachenhicop sa 2max (max = 5,5MHz i vi h PAL) ngha l sa 11MHz.Trng hp sa < 2max s xy ra hin thng chng ph lm xut hin cc thnh phn ph (alias components) v xut hin mo, v d nh hiu ng li trn mn hnh (do cc tn hiu v ch nm trong bng tn video), mo sn xung tn hiu, lm nho bin nh (do hiu ng bc thang), cc im sng ti nhp nhy trn mn hnh.Tr s sa ti u s khc nhau cho cc trng hp: tn hiu chi (trng en), tn hiu mu c bn (R, G, B). cc tn hiu s mu, tn hiu Video mu tng hp. Cui cng vic chn tn s ly mu ph thuc vo h thng truyn hnh mu.Trong trng hp ly mu tn hiu Video mu tng hp phi ch n tn s sng mang ph sc, khi chn sa c th xut hin cc trng hp sau y:+ sa gp nhiu ln sc, v d sa = 3sc hoc 4sc (h PAL, NTSC ch dng mt tn s sc). H SECAM dng hai sng mang ph mu nn khng dng c mt tn s sa cho cc tn hiu hiu s mu.1.53 Lng t ha tn hiu Video Qa trnh lng t ha tn hiu ri rc (sau khi ly mu) bao gm vic chia bin thnh nhiu mc (nhiu khong) v sp xp mi tr ca mu bng mt mc.Cc khong chia (khong lng t) c th u nhau v cng c th khng u nhau v ta gi l lng t tuyn tnh v lng t phi tuyn. Trong qu trnh lng t ha bin ca cc mu nm trong cng mt khong lng t (Q) s c bin bng nhau, bin ny c th l nm bc trn hay nm bc di ca mc lng t. Mi bc tng ng vi mt m s nht nh.Nu ta lm trn vi bc trn ca thang lng t th gi l lng t ha trn bc. Nu lm trn vi bc di th gi l lng t ha di bc. Hai phng php ny gi chung l lng t ha c thang na bc.Nu lm trn vi mc gia khong lng t th gi l lng t ha c thang na bc. Loi c thang na bc cho chnh xc cao hn (sai s lng t nh hn) so vi lng t ha khng c thang na bc. Tuy nhin n c nhc im l nhiu knh trng.1.5.4 M ha tn hiu VideoM ha tn hiu Video l bin i tn hiu lng t ha thnh tn hiu s bng cch sp xp s nh phn cho cc mc lng t ha v nh x ca cc mc ny thnh tn hiu c 2 mc logic 0 v 1.Theo l thuyt v thc nghim ta c th dng m 8 bit (tc 28 =256 mc lng t) m ha tn hiu Video. Nu s bit tng chnh xc ca b chuyn i tng nhng tc bit tng i hi knh truyn rng ng thi p ng ca b chuyn i thp. Cc m s dng trong truyn hnh s c th c chia thnh 4 nhm nh sau:+ Cc m m ho tn hiu truyn hnh+ Cc m truyn c hiu qu cao theo knh thng tin+ Cc m thun tin cho vic gii m v ng b bn thu+ Cc m x l s tn hiu trong cc b phn khc nhau ca h thng truyn hnh sM s cp to tn hiu s trung tm truyn hnh, c dng tn hiu nh phn lin tc, cc bit 0 v 1 c th c biu din bng cc phng php khc nhau, c phn bit bng thi gian tn ti, cc tnh, mc pha chng hn NRZ, RZ, Biphase (hai pha)1.6 Cc h thng truyn hnh s qung bTruyn hnh qung b l truyn hnh s kt hp vi cng ngh nn s cho u im ni bt l tit kim c b nh v tit kim knh truyn. Mt knh truyn hnh qung b truyn thng khi truyn tn hiu truyn hnh s c th truyn trn 6 chng trnh v mi chng trnh c th km theo 2 n 4 ng ting. ng dng k thut truyn hnh s c nn c th truyn mt chng trnh truyn hnh phn gii cao HDTV trn mt knh thng thng c bng thng (6-8)MHz, iu m k thut tng t khng th gii quyt c.Truyn hnh s c nn c s dng rng ri cho nhiu cp cht lng khc nhau. T SDTV c cht lng tiu chun n HDTV c cht lng cao vi tc bt t 5-24Mb/s, c truyn dn v pht sng qua cp, qua v tinh v trn mt t. C rt nhiu tiu chun nn dng cho truyn hnh s: MPEG-1, 2, 3, 4, 7(Moving Picture Experts Group). Vic pht chng trnh qung b truyn hnh s (digital video broadcasting DVB) ch yu s dng tiu chun nn MPEG 2, n c phng thc sa m sai; cn c vo cc chng trnh multimedia, s chn la cc phng thc iu ch tng ng v bin m ca cc ng thng tin.Hin nay c ba tiu chun truyn hnh s c nn dng trong truyn dn v pht sng l DVB (chu u), ATSC (M), ISDB-T (Nht), trong DVB t ra c nhiu u im v c khong 84% s nc trn th gii, trong c VN la chn s dng.M hnh h thng truyn dn DVB c m t nh hnh v di y:

Truyn a chng trnhM ho u cui cpiu ch QAMTruyn a chng trnhM ho knhiu ch QPSKTruyn a chng trnhM ho knh

iu ch COFDMGhp knh chng trnhDng chg trnh 1Dng chg trnh 2Dng chg trnh nTruy cp c iu kinn mng cpn v tinhn my pht sng trm mt tHnh 1.7 M hnh h thng truyn dn DVB

Sau khi xc nh cc tiu chun ca pht truyn hnh s DVB, do cc s truyn ti Multimedia khc nhau, lnh vc ng dng khc nhau nn DVB c t chc v phn chia thnh mt s h thng, c th l h thng qung b truyn hnh s v tinh DVBS (Satellite); h thng qung b truyn hnh s hu tuyn DVBC (Cable); h thng qung b truyn hnh s trn tri t DVBT (Terrestrial); h thng qung b truyn hnh s vi ba DVBM (Microwave); h thng qung b truyn hnh s theo mng tng tc DVBI (Interact); h thng truyn hnh s h thng cng ng DVBCS (Community System),v.v .1.7.1 H thng qung b truyn hnh s hu tuyn DVB-Cc im chung:DVB-C: H thng truyn dn qua cp s dng rng knh truyn 7-8MHz, iu ch QAM vi 64 trng thi (64-QAM), tc d liu cc i t lp truyn MPEG-2 l 38,1Mb/s.Trong mng truyn hnh hu tuyn do tn hiu hnh nh c truyn ti trn ng dy cp ng trc nn n t b can nhiu bn ngoi. Trong cc nguyn tc DVB qui nh s dng cc phng thc iu ch QAM, cn c vo trng thi mi trng truyn ti c th s dng cc tc iu ch khc nhau nh 16-QAM; 128-QAM; 256-QAM .My thu v tinh sMy thu v tinh sMy thu v tinh sB gii iu ch sB gii iu ch sB gii iu ch s

B trn

My phtMng hu tuynTn hiu t v tinhTn hiu t v tinhTn hiu t v tinhHnh 6.6 s khi h thng truyn hnh s hu tuyn

Hnh 6.6 l s ca h thng qung b truyn hnh s hu tuyn . Nu tn hiu truyn hnh ly ngun t v tinh th cn mt my thu v tinh s IRD (Integrated Receiver Coder) thu cc chng trnh khc nhau v chuyn i thnh dng d liu MPEG-2, i vi tn hiu th tn m tn AV th cn b gii nn bin m s gii m tn hiu, to ra dng d liu MPEG-2. Ngun tn hiu khc nhau s to ra dng d liu MPEG-2 b trn nhiu ng s tin hnh trn v thu c dng tn hiu c tc cao hn . Sau tn hiu ny a vo b iu ch QAM, b bin tn t c di tn cn thit cho mng truyn hnh hu tuyn.1.7.2 H thng qung b truyn hnh s v tinh DVBSc im chung:DVB-S: H thng truyn dn qua v tinh DVB-S c cc c trng nh sau: S dng bng tn bng C v KU, iu ch s QPSK, ti u ho cho tng ti ring cho tng b pht p (Transponder: thit b thu pht trn v tinh) v cng sut hiu dng, tc d liu cc i t lp truyn MPEG-2 l 38,1Mb/s.B m ha MPEGB m ha MPEGB m ha MPEGB trn nhiu ngB iu ch QPSKB i tn lnPht ln v tinhHnh 6.7 S khi h thng qung b truyn hnh s v tinh

Nguyn l qung b truyn hnh s v tinh trnh by hnh 6.7. Thng tin m tn v th tn v cc tn hiu s trc tin s i qua b nn bin m s MPEG 2 (ENC) tin hnh vic nn bin m , tn hiu truyn hnh s vi tc trn 200Mb/s c nn xung cn 6Mb/s, dng s liu MPEG-2 b nn nhiu ng s c a vo b trn nhiu ng s tin hnh vic trn, ng ra s nhn c dng m MPEG-2 c tc cng cao hn. Cn c vo yu cu, cc chng trnh truyn hnh cn truyn ti s c thc hin vic m ha, sau dng s liu MPEG-2 c a vo b iu ch s QPSK. Cui cng tin hnh bin tn, tn hiu QPSK c iu ch ti trung tn IF, t ti tn s vi ba cn thit ca di sng C hoc KU, thng qua anten pht tin hnh pht x ln truyn hnh v tinh. S khi ca h thng thu truyn hnh s v tinh nh hnh 6.8. Tn hiu v tinh qua b bin tn LNB, my thu v tinh s IRD (integrated receiver coder ) s tin hnh vic gii iu ch QPSK, gii m a ra tn hiu m tn v th tn, nu dng u ni thu CATV trc th mng truyn hnh hu tuyn c th c chia thnh phng thc truyn ti tng t v phng thc truyn ti s.Trong phng thc truyn ti tng t th s ng truyn t v s lng my thu bng nhau, do tn hiu u ra ca my thu v tinh s IRD l AV cho nn cn phi dng cc b iu ch tng t vi cc knh tn khc nhau truyn ti tn hiu ti h dng. B bin tn

My thu v tinh s

Tivi thng thngAVTn hiu t v tinhHnh 6.8 S khi h thng thu truyn hnh s

1.7.3 H thng qung b truyn hnh s trn mt t DVB Tc im chung:DVB-T: H thng pht sng s trn mt t DVB-T s dng rng knh 7-8MHz, tc d liu cc i t lp truyn MPEG-2 l 24Mb/s. Ngi ta s dng phng php iu ch s m ho ghp knh theo tn s trc giao COFDM do s truyn ti ca h thng qung b truyn hnh s trn mt t tng i c bit, c hin tng phn x tn hiu nhiu ln, can nhiu rt nghim trng.

1.8. Nn tn hiu truyn hnhGii php nn cho php ngi s dng la chn mt trong cc phm vi thay i cc thng s ly mu v cc t s nn, cc lin kt thch hp nht cho mc ch s dng. X l tn hiu s ha hn thay th tt c cc phng php tng t (c) v tc dng, tc mnh, NTSC, PAL, SECAM, HDTV v cui cng tp trung vo HDTV s bng rng.K thut tng t: Nn thng tin video bng cch gim rng bng tn mu < 1,2MHz.K thut gim (nn) d liu video: (c 2 nhm) nn c tn tht v nn khng tn tht.

JPEG, MPEG-1/2,DV

Nn video

video

Khng tn tht

Tn thtDCTVLCRLCTch vng xa

fs thp (bng con)

DPCMLng t ha, VLC -Huffman-M ha entropy

Ch cc gi tr ca sample 0 l c m ha theo s chy (RUN): cn cc gi tr = 0 dc theo dng qut (to li bng tch tng quan DCT)

Cho cc h s DCT

Dng cho tn hiu mu C

+Phng php m ha lot di ( Run Length Coding )Nguyn tc ca phng php l pht hin mt lot cc bt lp li, th d nh mt lot cc bt 0 nm gia hai bt 1, hay ngc li, mt lot bt 1 nm gia hai bt 0. Phng php ny ch c hiu qu khi chiu di dy lp ln hn mt ngng no . Dy cc bt lp gi l lot hay mch (run). Tip theo, thay th chui bi mt chui mi gm 2 thng tin: chiu di chui v bt lp k t lp. Nh vy, chui thay th s c chiu di ngn hn chui cn thay.+ Phng php m ha Huffman: Da vo d liu gc, ngi ta tnh tn sut xut hin ca cc k t. Vic tnh tn sut c thc hin bi cch duyt tun t tp gc t u n cui. Vic x l y tnh theo bit. Trong phng php ny ngi ta gn cho cc k t c tn sut cao mt t m ngn, cc k t c tn sut thp t m di. Ni mt cch khc, cc k t c tn sut cng cao c gn m cng ngn v ngc li. R rng vi cch thc ny, ta lm gim chiu di trung bnh ca t m ha bng cch dng chiu di bin i. Tuy nhin, trong mt s tnh hung khi tn sut l rt thp, ta c th khng c li mt cht no, thm ch cn b thit mt t bit.+ Cc phng php nn da trn cc bin iM ha dng bin i Cosine: Cosine mt chiu, Cosine nhanh, Cosine ngc. Php bin i Cosine mt chiu: c ng dng rt rng ri trong nhiu phng php m ha nh khc nhau nh hiu sut gn nh ti u ca n i vi cc nh c tng quan cao gia cc im nh ln cn. Bin i Cosine ri rc c s dng trong nh dng phim MPEG. Php bin i Cosin ri rc mt chiu c nh ngha bi:

Khi dy u vo x(n) l thc th dy cc h s X(k) cng l s thc. Tnh ton trn trng s thc gim i mt na thi gian so vi bin i Fourier. t c tc bin i tha mn yu cu ca cc ng dng thc t, ngi ta ci tin k thut tnh ton v a ra nhiu thut ton bin i nhanh Cosine.

+ Nn video tn tht DPCM-u xung m vi sai:- y l mt phng php nn quan trng v hiu qu. Nguyn l c bn ca n l: ch truyn ti tn hiu vi sai gia mu cho v tr d bo (c to ra t cc mu trc )- Cng ngh DPCM thc hin loi b tnh c nh v cc thng tin d tha ca ngun tn hiu bng mt b lc c bit c p ng u ra l tn hiu s gia mu u vo v gi tr d bo ca chnh n. Rt nhiu gi tr vi sai ny gn bng 0 nu cc im nh bin i ng u. Cn vi nh c nhiu chi tit gi tr sai s d bo c th ln. Khi c th lng t ha chng bng mc lng t cao hn do c im ca mt ngi khng nhy cm vi nhng chi tit c tng phn cao, thay i nhanh. S gim tc bit y thu c t qu trnh lng t ha v m ha. - Hu ht cc cch thc nn nh u s dng vng lp DPCM.

Hnh 1.5. M ha, gii m DPCM

1.3.1. Nn video theo tiu chun MPEGCc tiu chun nn vi ng dng ca chng c khi qut trong bng sau:Bng 1.1. Khi qut cc tiu chun nn

ChunPhm vi ng dng

CCITT T.4CCITT T.6JPEGCC ITT H.261MPEG -1MPEG - 2MPEG - 4Fax, nh d liu Fax, nh d liu nh Fax, nh d liu in thoi hnh nh, HDTV, DSM Truyn thanh thng thng, qung b, cm nhn t xa.

Trong s , c s dng ph bin v c phm vi ng dng rng ri l MPEG.1.82 Khi nim v phn loi MPEGNn tn hiu video theo chun MPEG ( Moving Picture Experts Group ) l phng php nn nh ng khng nhng lm gim d tha khng gian m cn lm gim d tha thi gian gia cc khung nh.Chun MPEG nh ngha mt khi nim mi l nhm cc khung nh (GOP) gii quyt d tha thi gian v cho php truy xut ngu nhin khi m ha MPEG dng lu tr. Trong chun MPEG, ngi ta quy nh ba loi khung nh ph thuc vo phng php nn: nn trong khung nh, nn c on v ni suy hai chiu theo thi gian.

Chun nn MPEG bao gm cc tiu chun nn video c tc lung bit khc nhau.

Hnh 1.4. H thng cc chun MPEG

MPEG - 1: Cn c gi l tiu chun ISO/IEC 11172 l chun nnaudio v video vi tc khong 1,5 Mb/s. MPEG - 2: Nn tn hiu audio v video vi mt di tc t 1,5 ti 60 Mb/s. Tiu chun ny cn gi l tiu chun quc t ISO/IEC 13818, l chun nn nh ng v m thanh. N cung cp mt di cc ng dng nh: lu tr s liu, truyn hnh qung b v truyn thng. MPEG - 3: Tiu chun nn tn hiu s xung cn < 50 Mbps truyn tn hiu truyn hnh c phn gii cao (HDTV). Sau nhp chung vo MPEG - 2 v tr thnh tiu chun quc t MPEG - 2 vo thng 11- 1994 (ISO/IEC 1381). MPEG - 4: L s hp nht cung cp cho rt nhiu ng dng truyn thng, truy cp, iu khin d liu m thanh s nh: in thoi hnh, thit b u cui a phng tin (Multimedia), th in t v cm nhn t xa. MPEG- 4 cho kh nng truy cp rng ri v hiu sut nn cao. MPEG - 7: Chun ny c ngh vo thng 7 - 1998 v thnh chun quc t vo 9 - 2001. L chun m t thng tin ca rt nhiu loi a phng tin. M t ny s kt hp vi chnh ni dung ca n cho php kh nng tm kim nhanh v hiu qu theo yu cu ngi dng. Chnh v vy MPEG - 7 c gi l giao thc m t ni dung a phng tin.

1.83 Cc cu trc nhMPEG nh ngha cc loi nh khc nhau cho php s linh hot cn nhc gia hiu qu m ha v truy cp ngu nhin. Cc loi nh nh sau: nh loi I (Intra-picture)L nh c m ha ring, tng t nh vic m ha nh tnh trong JPEG. nh I cha ng d liu ti to li ton b hnh nh v chng c to thnh bng thng tin ca ch mt nh v d bo cho nh B,P. nh I cho php truy cp ngu nhin, tuy nhin cho t l nn thp nht. nh loi P (Predicted-picture)L nh c m ha c b chuyn ng t nh I hoc nh P pha trc. nh p cung cp cho h s nn cao hn nh I v c th s dng lm mt nh so snh cho vic b chuyn ng cho cc nh P v B khc. nh loi B (Bi-directional predicted picture)L nh c m ha s dng b chuyn ng t cc nh I hoc P pha trc v pha sau. nh B cho t l nn cao nht. nh loi D (Dc-coded picture)L nh c s dng trong MPEG-1 v MPEG-4 nhng khng c s dng trong MPEG-2. N ging nh nh I, tuy nhin ch c thnh phn mt chiu u ra DCT c th hin. N cho php d tm nhanh nhng cht lng nh thp. Nhm nh (GOP)i vi chun MPEG, cht lng nh khng nhng ph thuc vo t l nn trong tng khun hnh m cn ph thuc vo di ca nhm nh. Nhm nh (GOP-Group of picture) l khi nim c bn ca MPEG. Nhm nh l n v mang thng tin c lp ca MPEG.MPEG s dng ba loi nh I, B, P. Trong , nh P, B khng phi l mt nh hon chnh m ch cha s khc bit gia nh v nh xut hin trc n (i vi nh P) hay s khc bit i vi c khun hnh xut hin trc v sau n (i vi nh B). c mt khun hnh hon chnh nh P v B cn c d liu t cc nh ln cn, chnh v vy i vi MPEG mt khi nim mi l GOP (nhm nh) c s dng. Mi GOP bt buc phi bt u bng mt nh hon chnh I v tip sau n l mt loi cc nh P v B. Nhm nh c th m (Open) hoc ng (Closed).Nhm nh m lun bt u t mt nh I v kt thc mt nh trc nh trc nh I tip theo, tc l nh cui cng ca GOP dng nh u tin ca GOP tip theo lm nh chun.

Hnh 1.5. Cu trc GOP mTrong Hnh 1.5, nh P(nh 4) c d bo trc trn c s nh I (nh 1). nh B c d on t hai hng, nh B (nh 2) v nh B (nh 3) c d on t hai nh I (nh 1) v nh P (nh 4). Anh B (nh 5,6) c d on t nh P (nh 4) v nh I tip theo (nh 6). Mt iu ch l th t truyn nh v hin nh trn mn hnh l khng ging nhau.i vi cu trc khp kn (ng), vic d on nh khng s dng thng tin ca GOP khc. Trong trng hp ny, theo quy nh, nh cui cng ca mt GOP bao gi cng l nh P

Hnh 1.6. Cu trc GOP ngNhm nh c xc nh bi hai thng s m v n. Thng s m xc nh s khung hnh p v khung hnh B xut hin gia hai khung hnh I gn nhau nht. S n xc nh s khung hnh B gia hai khung hnh p.T l nn video ca MPEG ph thuc rt nhiu vo di ca GOP. Tuy nhin, GOP di thng gy kh khn cho qu trnh tua, nh v, sa li... Do ty thuc vo tng khu (sn xut, dng hnh, truyn dn, pht sng v..v) m ta chn di GOP thch hp. Trong sn xut hu k, nu c yu cu truy cp ngu nhin vo bt c nh no, iu cng c ngha l yu cu dng chnh xc n tng nh, GOP ng nhin s phi ch c duy nht nh I. Trong trng hp ny, t l nn s t rt thp. tng t l nn cho truyn dn v pht sng, trong GOP s lng nh p, B s phi tng ln. Lc ny khng cho php vic dng hnh cng nh lm cc k xo trn chui hnh nh . Trong trng hp ny ta c th c GOP gm 12 nh.

1.84 Cu trc phn lp MPEG2 Tn hiu audio, video d liu

Lp nn

Lp h thng

ESPSTSTo dng ADC

Tch dng ADC

M ha nn

Gii m nn

ng gi

M gi

Ghp knhPS

Tch knhPS

Ghp knh TS

Tch knh TS

Hnh 1-10 Cu trc MPEG-2 phn lp+ Lp nn biu din c php ( syntax) ca cc dng audio v video trn c s cu trc dng d liu video v audio. Cc chui audio v video hoc d liu c lp c m ha MPEG-2 cc dng d liu c lp, gi l dng c bn ES (elemantary strems).+ Lp h thng xc nh vic kt hp cc dng audio v video c lp thnh mt dng lu tr (dng chng trnh PS program stream ) hoc truyn dn ( dng truyn TS transmission stream).- Dng chng trnh PS:

SC (m khi u): 3 bytesSI (nhn dng dng):1 bytePL (chiu di gi): 2 bytesBS (kch thc b nh): 2 bytes

header gi

PES headerKt thc gi

header gi

Gi n 8 KB (max)

Gi n

Gi n+1

Gi d liu PES

Video1Audio 1Audio 2 Data Video2Audio 2Data ES SC M bt u

SCRChun ng h

MRTc ghp knh

Hnh 1-11: Dng cc hnh PS

Cc gi PS c th c chiu di bt k . S lng v chui cc gi / gi khng c xc nh, nhng cc gi t cc dng ring c chuyn t 1 bc thi gian. Mt PS c th ti n 32 dng audio, 16 dng video, 16 dng d liu. Tt c u c n v thi gian c bn c ghp knh ng b. Dng truyn ti TS.Nu chia cc gi PES c di khc nhau thnh cc gi TS c di khng i (mi gi TS c bt u bng TS header) v truyn cc gi ny i, ta s c dng truyn ti TS ( transport Stream).Cc gi TS c di khng i l 188 byte. Dng TS c kh nng chng li cao , c thit k truyn trn cc knh truyn c nhiu nh: knh truyn hnh thng thng ( thng qua mt t ) cng nh cc knh truyn hnh cp.

Hnh 1.12 nh dng dng truyn ti MPEG-2

Cc gi PES xut pht t mt hoc nhiu dng ES c cng hoc khc n v thi gian c bn ( nh audio, video, d liu) c ghp knh thnh 1 dng TS qua vic bin i trong cc gi PES. Kh nng ghp knh cc chng trnh vi nhiu tc bit khc nhau thnh 1 dng TS c dng trong h truyn hnh c phn gii cao HDTV

Dng truyn ti TS Hnh 1.13 mi ch ra qu trnh ghp cc gi PES audio, video, data, to thnh gi truyn ti TS. tng tnh hiu qu, cc dng truyn ti c th ghp li vi nhau to thnh dng truyn ti ghp knh cp h thng ( System Level Multiplex).

Hnh 1.14 Ghp knh dng bit truyn ti cp h thngSau khi cc bc trn hon thnh, cc b lc ti b tch knh c th thit lp cc bt dng truyn ti ti bn thu ph hp cho tng chng trnh cn quan tm.

1.9 iu ch tn hiu sTrong nhiu h thng dng dy dn hoc v tuyn (chng hn modem truyn d liu) tn hiu d liu iu ch sng mang hnh Sin trc khi c truyn i. C mt s loi iu ch thng dng l:1. Kha dch chuyn bin ASK.1. Kha dch chuyn tn s FSK.1. Kha dch chuyn v pha PSK. C nhiu cch chuyn dch pha khc nhau:+ Pha chia 2 hay c s 2 (2.psk hay BPSK).+ Pha chia 4 hay gc phn t (4.PSK hay QPSK).+ Pha chia 8 hay pha chia 16 (8.PSK hay 16.PSK) Tuyt i hoc vi phn.+ iu ch bin gc phn t QAM.1.91 Kha dch chuyn bin (ASK)+ iu chTrong loi iu ch ny, sng mang hnh sin ly hai gi tr bin , xc nh bi tn hiu d liu c s 2. Thng thng, b iu ch truyn i sng mang khi bit d liu l 1 v hon ton kh tn hiu khi d liu la 0.Cng c loi ASK gi l a mc, trong bin ca tn hiu iu ch ly nhng gi tr nhiu hn 2.+ Gii iu chTn hiu c iu ch thnh dng ASK ti b gii iu ch s c tch ly hnh bao bng b tch sng sau tn hiu qua b lc thng thp ly i cc thnh phn sng mang cn d. Tn hiu t li ra ca mch lc thng thp c a n mch to xung vung, to ra tn hiu d liu ban u.

DATA

To xung

Hnh: Gii iu ch ASK+ Cc tnh cht ca mch ASK1. Dng ch yu trong in tn v tuyn.1. Yu cu cc mch n gin.1. Kh nhy vi nhiu (xc sut sai s ln).1. Hiu sut truyn nh hn 1.1.92 Kha dch chuyn v pha PSK+ Kha dch chuyn v pha chia 2 (2.PSK)+ iu chTrong loi iu ch ny gi l pha chia 2 - pha c s 2 - (BPSK) - sng mang hnh sin ly hai gi tr pha, c xc nh bi d liu c s 2. K thut iu ch l dng b iu ch vng cn bng. Dng sng hnh sin li ra ca b iu ch l ging hay ngc (ngha l lch pha 1800) ca tn hiu li vo l mt hm s ca tn hiu d liu.S Hnh. Tn hiu iu ch 2.PSK

Hiu sut truyn bng 2.+ Pha chia 8 (8.PSK)Sng mang hnh sin ly 8 gi tr pha, cch nhau 450 v xc nh bi t hp ca cc nhm 3 bit tn hiu d liu s. Cc d liu c m ha theo kiu Tribit bng cc mch in pht ra.Nhng c trng chnh ca 8.PSK:1. p dng cho cc Modem truyn d liu v v tuyn s.1. Thit b i hi c phc tp cao.1. t li hn iu ch 4.PSK1. Hiu sut truyn bng 3.+ Pha chia16Sng mang hnh sin ly 16 gi tr pha, cch nhau 22.50 v xc nh bi t hp ca cc nhm 4 bit ca tn hiu d liu c s 2.Nhng c trng chnh ca 16.PSK:1. ng dng trong truyn v tuyn s.1. Thit b i hi phc tp cao.1. t li hn 8.PSK.1. Hiu sut truyn bng 4.1.93 iu ch bin gc phn t (QAM)QAM l iu ch s, trong thng tin s s c cha trong c pha v bin ca sng mang c pht i.Trong 8-QAM: d liu c chia lm cc nhm 3 bit (bit s 1 trong lm thay i bin ca sng mang, 2 bit cn li lm thay i v pha).Tn hiu c iu ch c th ly 4 pha khc nhau v c 2 bin khc nhau, tng cng ta c 8 trng thi khc nhau.Trong 16-QAM: d liu c chia lm cc nhm 4 bit, 16 t hp c th lm thay i bin v pha ca sng mang, chng c th ly 16 trng thi khc nhau ca sng mang.c trng ch yu ca QAM l:1. Dng trong cc Modem truyn d liu v trong pht v tuyn s.1. Cc mch i hi phc tp cao.1. t li hn PSK.1. Hiu sut truyn bng 4.

Hnh: 8-QAM trn min thi gian.

Chng 2 TIU CHUN TRUYN HNH S MT T DVB T

Chun truyn dn truyn hnh s (DTV - Digital television) s dng qu trnh nn v x l s c kh nng truyn dn ng thi nhiu chng trnh TV trong mt dng d liu, cung cp cht lng nh khi phc tu theo mc phc tp ca my thu.DTV l mt s thay i ng k trong nn cng nghip sn sut v qung b cc sn phm truyn hnh. N mang li tnh mm do tuyt vi trong s dng do c nhiu dng thc nh khc nhau trong nn s.Hin nay trn th gii tn ti song song ba tiu chun truyn hnh s. Trong chng ny em xin trnh by v tiu chun truyn hnh s mt t theo tiu chun chu u DVB - T.2.1 Gii thiu v truyn hnh s mt t DVB - TVic pht trin cc tiu chun DVB khi u vo nm 1993 v tiu chun DVB-T c tiu chun ho vo nm 1997 do Vin tiu chun truyn thng chu u (ESTI: European Telecommunication Standards Institute). Hin nay tiu chun ny c cc nc Chu u v nhiu nc khc trn th gii tha nhn. Nm 2001 i truyn hnh Vit Nam quyt nh chn n lm tiu chun pht sng cho truyn hnh mt t trong nhng nm ti. DVB l s truyn da trn tiu chun MPEG-2,l mt phng php phn phi t mt im ti nhiu im video v audio s cht lng cao c nn. N l s thay th c tng cng tiu chun truyn hnh qung b tng t v DVB cung cp phng thc truyn dn linh hot phi hp video, audio v cc dch v d liu. Trong truyn hnh s mt t khng th s dng phng php iu ch n sng mang c v multipath s lm nh hng nghim trng n ch tiu k thut ca truyn sng mang n tc cao v l do ny OFDM c s dng cho tiu chun truyn hnh mt t DVB_T. DVB-T cho php hai mode truyn ph thuc vo s sng mang c s dng.H thng trm mt t DVB- T: Cc knh VHF/UHF ca trm mt t l nhng phng tin quan trng nht vi vic truyn dn tn hiu s tc cao v cc th tc truyn li a ng to ra s di vang v s gim m thanh ca tn s la chn. Tr ca vic m rng cc tn hiu trong vic truyn lp l do s phn x ca ni, i hay dy nh cao ... c th ln ti hng chc |1S. Trong trng hp pha thu c th di chuyn, tn hiu tn hiu trc tip t pha pht c th b mt (knh Rayleigh) do bn pha thu buc phi khai thc nhng m my tn hiu phn hi xung quanh vt th.Bng 2.1. M t cc thng s cc mode lm vic trong DVB - TTham sMode 2KMode 8K

S lng sng mang con17056817

Khong cch sng mang4464hz1116Hz

Bng thngT/4, T/8, T/12774, 778

Khong bo v AQPSK, 16-QPSK, 16-

Phng thc iu ch64QAM64QAM

Kiu 2K ph hp cho hot ng b truyn n l v cho cc mng SFN loi nh c khong cch b truyn gii hn, n s dng 1705 sng mang con. Kiu 8K c th c s dng cho hot ng b truyn om l cng nh cho cc mng SFN loi nh v ln; n s dng 6817 sng mang con. gim nh nh hng khng bng phng ca knh th dng nhiu sng mang cng tt. Tuy nhin khi s sng mang nhiu, mch s phc tp hom, trong giai on u khi cng ngh ch to chip cha hon thin cc chip iu ch cn t ngi ta thng dng mode 2K v cng ngh ch to chip om gin v r hom.Trong mng n tn s (SFN), s la chn tn s knh c th rt quan trng khi tt c cc my pht tn hiu ging nhau cng thi im v c th pht cc tn hiu lp li nhn to trong khu Yc dch v (tr ln n vi trm ns). khc phc vn ny, cc b tng thch DVB- T c thit k da trn vic iu ch a sng mang trc giao COFDM.Phn b nng lng

Chn m ViterbiOFDM

Reed Solomon

2k, 8k

Hnh 2.1. Tiu chun DVB - T

C th chia dng bt truyn ti thnh hng ngn sng mang ph tc thp, trong ghp knh FDM. H thng c th hot ng hai mode chnh: mode 2k cho cc mng chuyn i (tng voi 1705 sng mang ph trong di thng 7,61 MHz v khong thi gian Symbol hiu dng Tu = 224 is) v mode 8k cho SFNMi sng mang c iu ch theo lc AM - QAM (4,16 hay 32 QAM).iu ch COFDM bn cht l phaing tn s chn, khi mi sng mang c iu ch tc bt trung bnh (tc Symbol vo khong 1 hay 4 Kbaud tng ng Yi moode 2k hay 8k) v khong thi gian rt di so vi thi gian p ng thay i knh.

CHNG 3: K THUT OFDM

3.1 Gii thiu chng

Trong h thng tin v tuyn cn thit phi c sng mang cao tn truyn thng tin. Cc k thut iu ch cho php b tr d liu trn sng mang. Cc h thng thng tin mt tn s hn ch tc d liu v hn ch v dung lng. gim nhiu th cc phng php pht tn hiu tng t trc y nh AM, FM cn thit phi tng cng sut my pht, bng tn rng hiu qu s dng bng tn thp. a phn chia chia s rng bng tn vi nhiu knh d liu c lp khc nhau. Nm 1980 trung tm nghin cu ca Php (tp on Telecom) CCETT (Centre Commun d'tude en Ddiufftion t Tlcommunication), a ra phng php mi truyn tn hiu s m vn tit kim c bng tn l OFDM. OFDM l k thut ghp knh phn chia theo tn s trc giao, chia ton b bng tn ra thnh nhiu sng mang nhnh m cc sng mang ny phi trc giao.Ngy nay, k thut OFDM c ng dng trong cc h thng truyn dn bng rng ADSL/HDSL/VDSL, cc h thng pht thanh v truyn hnh s qung b DAB (Digital Audio Broadcasting) v DVBT (Digital Video Boadcasting Terrestrial). OFDM cn l gii php k thut c c cho cc chun LAN khng dy (Wireless Local Area Network). Do vy, trong chng ny chng ta s i su vo tm hiu tng c im ca OFDM: Nguyn tc c bn ca OFDM, tnh cht trc giao trong OFDM, nhiu ISI v ICI, thut ton FFT/IFFT, cc vn k thut v xy dng m hnh h thng OFDM. 3.2 Nguyn tc c bn ca OFDM

Trong OFDM chui d liu u vo ni tip c tc cao (R) c chia thnh N chui con song song (t chui d liu 1 n chui d liu N) c tc thp hn (R/N). N chui con ny c iu ch bi N sng mang ph trc giao, sau cc sng mang ny c cng vi nhau v c pht ln knh truyn ng thi, c m t nh hnh 3.1. pha qu trnh thu tin th ngc li.

D liuTc RD liu 1D liu ND liu 2D liutngB iu chB phn chuyn ini tip / song song Chn chui bo v

TinTc R/NHnh 3.1: S qu trnh pht tin

Bn cht trc giao ca cc sng mang ph OFDM cho php ph ca cc chui con sau iu ch chng ln ln nhau m vn m bo vic tch ring bit tng thnh phn ti pha thu. Nh vy m hiu qu s dng bng tn tng ng k v trnh c nhiu gia cc sng mang ln cn ICI (Inter-carrier Interference). Ta c th thy c iu ny qua ph ca tn hiu OFDM v tn hiu FDM trn hnh 3.2

Hnh 3.2: Ph ca tn hiu FDM v OFDMMt khc, do chui d liu ni tip tc cao c chia thnh cc chui con c tc thp nn tc k hiu ca cc chui con nh hn rt nhiu so vi tc ca chui ban u, v vy cc nh hng ca nhiu lin k t ISI, ca hiu ng tr tri u c gim bt. Nh vy c th gim phc tp ca cc b cn bng pha thu.

Hnh 3.3: a.Tc ng ca nhiu i vi h thng n sng mang b.Tc ng ca nhiu n h thng a sng mangMt u im na ca k thut OFDM l kh nng chng li fading chn lc tn s v nhiu bng hp. h thng n sng mang, ch mt tc ng nh ca nhiu cng c th gy nh hng ln n ton b tn hiu (Hnh 3.3a). Nhng i vi h thng a sng mang, khi c nhiu th ch mt phn trm nh ca nhng sng mang con b nh hng (Hnh 3.3b), v v vy ta c th khc phc bng cc phng php m ho sa sai.3.3 Tnh trc giao Cc tn hiu l trc giao nhau nu chng c lp vi nhau. Tnh trc giao l mt tnh cht cho php nhiu tn hiu thng tin c truyn v thu tt trn mt knh truyn chung v khng c xuyn nhiu gia cc tn hiu ny. Mt i tnh trc giao s lm cho cc tn hiu thng tin ny b xuyn nhiu ln nhau v u thu kh khi phc li c hon ton thng tin ban u. Trong OFDM, cc sng mang con c chng lp vi nhau nhng tn hiu vn c th c khi phc m khng c xuyn nhiu gia cc sng mang k cn bi v gia cc sng mang con c tnh trc giao. Mt tp cc tn hiu c gi l trc giao tng i mt khi hai tn hiu bt k trong tp tha iu kin.

(3.1) vi S*(t) l k hiu ca lin hp phc S(t). Ts l chu k k hiu. K l hng s.Tp N sng mang ph trong k thut OFDM c biu thc:

(3.2)vi k = 0, 1, , N-1

Cc sng mang ny c tn s cch u nhau mt khong v trc giao tng i mt do tha iu kin (3.1).

Ta xt hai sng mang v

(3.3)

Hnh 3.4: Ph ca cc sng mang trc giaoNh vy, cc sng mang thuc tp (3.2) l trc giao tng i mt hay cn gi l c lp tuyn tnh. Trong min tn s, ph ca mi sng mang ph c dng hm sincx do mi k hiu trong min thi gian c gii hn bng mt xung ch nht. Mi sng mang ph c mt nh tn s trung tm v cc v tr null ti cc im cch tn s trung tm mt khong bng bi s ca FS. V vy, v tr nh ca sng mang ny s l v tr null ca cc sng mang cn li (Hnh 3.4). V do cc sng mang khng gy nhiu cho nhau. 3.4 S dng FFT/IFFT trong OFDM

Nh bit, OFDM l mt k thut iu ch a sng mang, trong d liu c truyn song song nh v s sng mang ph mang cc bit thng tin. Bng cch ny ta c th tn dng bng thng tn hiu, chng li nhiu gia cc k t,....Tuy nhin, iu bt li l mt s sng mang cn c mt my pht sng sin, mt b iu ch v gii iu ch ca ring n, iu ny l khng th chp nhn c khi s sng mang ph rt ln i vi vic thi cng h thng. Nhm gii quyt vn ny, thut ton IDFT/DFT (Discrete Fourier transform) c vai tr ging nh hng lot cc b iu ch v gii iu ch. Gi s tn hiu x(n) c chiu di l N (n = 0,1, 2, , N-1). Cng thc ca php bin i DFT l[10]

, k = 0, 1, , N-1(3.4) - Cng thc ca php bin i IDFT l

, k = 0, 1, , N-1(3.5) - Chuyn i Fourier nhanh (FFT) l thut ton gip cho vic tnh ton DFT nhanh v gn hn.T cng thc (3.4), (3.5) ta thy thi gian tnh DFT bao gm: Thi gian thc hin php nhn phc. Thi gian thc hin php cng phc. Thi gian c cc h s . Thi gian truyn s liu.

Trong ch yu l thi gian thc hin php nhn phc. V vy, mun gim thi gian tnh ton DFT th ngi ta tp trung ch yu vo vic gim thi gian thc hin php nhn phc. M thi gian thc hin php nhn phc t l vi s php nhn. Do gim thi gian tnh DFT th ngi ta phi gim c s lng php tnh nhanh bng cch s dng thut ton FFT. tnh trc tip cn php nhn. Khi tnh bng FFT s php nhn ch cn . V vy tc tnh bng FFT nhanh hn tnh trc tip l . Ngoi ra FFT cn c u im gip tit kim b nh bng cch tnh ti ch.3.5 Nhiu giao thoa k t v nhiu giao thoa sng mang 3.5.1 Khi nim Trong mi trng a ng, k t pht n u vo my thu vi cc khong thi gian khc nhau thng qua nhiu ng khc nhau. S m rng ca chu k k t gy ra s chng ln gia k t hin thi vi k t trc v kt qu l c nhiu lin k t (ISI). Trong OFDM, ISI thng cp n nhiu ca mt k t OFDM vi k t trc .

Hnh 3.5: Ph ca bn sng mang trc giao

Trong OFDM, ph ca cc sng mang chng ln nhng vn trc giao vi sng mang khc. iu ny c ngha l ti tn s cc i ca ph mi sng mang th ph ca cc sng mang khc bng zero. My thu ly mu cc k t data trn cc sng mang ring l ti im cc i v iu ch chng trnh nhiu t cc sng mang khc. Nhiu gy ra bi k t trn sng mang k cn c xem l nhiu xuyn knh (ICI).Tnh cht trc giao ca sng mang c th c nhn thy trn gin trong min thi gian hoc trong min tn s. T gin min thi gian, mi sng mang c dng sin vi s nguyn ln lp vi khong FFT. T gin min tn s, iu ny tng ng vi mi sng mang c gi tr cc i tn s trung tm ca chnh n v bng khng ti tn s trung tm ca sng mang khc. Hnh 3.5 biu din ph ca bn sng mang trong min tn s cho trng hp trc giao. Tnh trc giao ca mt sng mang vi sng mang khc b mt nu gi tr ca sng mang khng bng khng ti tn s trung tm ca sng mang khc. T gin min thi gian, tng ng hnh sin khng di hn s nguyn ln lp khong FFT. Hnh 3.6 biu din ph ca bn sng mang khng trc giao.

Bin tn sHnh 3.6: Ph ca bn sng mang khng trc giao

ICI xy ra khi knh a ng khc nhau trn thi gian k t OFDM. Dch Doppler trn mi thnh phn a ng gy ra b tn s trn mi sng mang, kt qu l mt tnh trc giao gia chng. ICI cng xy ra khi mt k t OFDM tri qua ISI. S b tn s sng mang ca my pht v my thu cng gy ra ICI n mt k t OFDM.3.5.2 Phng php chng nhiu lin k hiu

Hnh 3.7: nh hng ca ISIHnh 3.7 cho ta thy mt k hiu v phin bn tr ca n. Chnh thnh phn tr ny gy ra nhiu nh hng n phn u ca k hiu tip theo. y chnh l nhiu lin k hiu ISI.

Hnh 3.8: Chn khong bo v l khong trng loi b s nh hng ca ISI, chng ta di k hiu th i ra xa k hiu trc (k hiu i 1) mt khong bng khong tr tri (max). Mt khong rng do s c chn vo gia hai k hiu (Hnh 3.8), nhng nh vy tn hiu s b thay i t ngt v mt tnh lin tc. V vy, trong thc t ngi ta chn khong bo v G c copy t phn cui ca k hiu v dn vo phn u k hiu nh hnh 3.9. Khong bo v ny c gi l cyclic prefix. Chiu di ca khong bo v cn c hn ch m bo hiu sut s dng bng tn, nhng n vn phi di hn khong tr tri ca knh truyn nhm loi b c nhiu ISI. my thu, khong bo v ny c loi b trc khi thc hin gii iu ch

Hnh 3.9: Chn khong bo v Cyclic prefix

3.6 Cc vn k thut trong OFDM - OFDM l gii php k thut rt thch hp cho truyn dn v tuyn tc cao. Tuy nhin, c th em p dng vo cc h thng, c ba vn cn phi gii quyt khi thc hin h thng s dng OFDM:+ c lng tham s knh.+ ng b sng mang+ Gim t s cng sut tng i cc i PAPR(Peak to Average Power Ratio) - Vn th nht lin quan trc tip n ch tiu cht lng h thng OFDM nu dng phng php gii iu ch lin kt, cn hai vn sau lin quan n vic x l cc nhc im ca OFDM. Ngoi ra, nng cao ch tiu cht lng h thng, ngi ta s dng m ha tn hiu OFDM.3.6.1 c lng tham s knhc lng knh (Channel estimation) trong h thng OFDM l xc nh hm truyn t ca cc knh con v thi gian thc hin gii iu ch bn thu khi bn pht s dng kiu iu ch kt hp (coherent modulation). c lng knh, phng php ph bin hin nay l dng tn hiu dn ng (PSAM-Pilot signal assisted Modulation). Trong phng php ny, tn hiu pilot bn pht s dng l tn hiu c bn thu bit trc v pha v bin . Ti bn thu, so snh tn hiu thu c vi tn hiu pilot nguyn thy s cho bit nh hng ca cc knh truyn dn n tn hiu pht. c lng knh c th c phn tch trong min thi gian v trong min tn s. Trong min thi gian th cc p ng xung h(n) ca cc knh con c c lng. Trong min tn s th cc p ng tn s H(k) ca cc knh con c c lng. C hai vn chnh c quan tm khi s dng PSAM : Vn th nht l la chn tn hiu pilot : Phi m bo yu cu chng nhiu, hn ch tn hao v nng lng v bng thng khi s dng tn hiu ny. Vi h thng OFDM, vic la chn tn hiu pilot c th c thc hin trn gin thi gian-tn s, v vy k thut OFDM cho kh nng la chn cao hn so vi h thng n sng mang. Vic la chn tn hiu pilot nh hng rt ln n cc ch tiu h thng. Vn th hai l vic thit k b c lng knh: Phi gim c phc tp ca thit b trong khi vn m bo c chnh xc yu cu. Yu cu v tc thng tin cao (tc l thi gian x l gim) v cc ch tiu h thng l hai yu cu ngc nhau. V vy, khi thit k cn phi dung ha hai yu cu trn. 3.6.2 ng b trong OFDMng b l mt trong nhng vn ang rt c quan tm trong k thut OFDM bi n c ngha quyt nh n kh nng ci thin cc nhc im ca OFDM. Chng hn, nu khng m bo s ng b v tn s sng mang th s dn n nguy c mt tnh trc giao gia cc sng mang nhnh, khin h thng OFDM mt i cc u im c trng nh s trc giao ny. Trong h thng OFDM, ngi ta xt n ba loi ng b khc nhau l : ng b k t (symbol synchronization), ng b tn s sng mang (carrier frequency synchronization), v ng b tn s ly mu (sampling frequency synchronization). 3.6.2.1 ng b k t ng b k t nhm xc nh chnh xc thi im bt u mt k t OFDM. Hin nay, vi k thut s dng tin t lp (CP) th ng b k t c thc hin mt cch d dng hn. Hai yu t cn c ch khi thc hin ng b k t l li thi gian (timing error) v nhiu pha sng mang (carrier phase noise). Li thi gianLi thi gian gy ra s sai lch thi im bt u mt k t OFDM. Nu li thi gian nh sao cho p ng xung ca knh vn cn nm trong chiu di khong tin t lp (CP) th h thng vn m bo s trc giao gia cc sng mang. Trong trng hp ny th thi gian tr ca mt k t c xem nh l dch pha ca knh truyn v dch pha ny c xc nh nh k thut c lng knh. Trong trng hp ngc li, nu chiu di ca CP nh hn li thi gian th h thng s xut hin li ISI. C hai phng php thc hin ng b thi gian, l: ng b thi gian da vo tn hiu pilot v ng b thi gian da vo tin t lp. Phng php ng b thi gian da vo tn hiu pilot c p dng cho cc h thng OFDM m tn hiu c truyn i bng k thut iu tn. Trong phng php ny, bn pht s m ha mt s tn hiu bit trc thng tin v pha v bin trn mt s sng mang ph. Phng php ny sau c iu chnh s dng cho c h thng OFDM m tn hiu truyn i c truyn theo k thut iu bin. Thut ton ng b thi gian s dng tn hiu pilot gm 3 bc l : nhn bit cng sut (power detection), ng b th (coarse synchronization)v ng b tinh (fine synchronization). Nhiu pha sng mang Nhiu pha sng mang l hin tng khng n nh v pha ca cc sng mang do s khng n nh ca b to dao ng bn pht v bn thu. 3.6.2.2 ng b tn s sng mangTrong ng b tn s sng mang, hai vn chnh c quan tm n l: Li tn s (frequency error) v thc hin c lng tn s. Li tn sLi tn s c to ra do s khc bit v tn s gia hai b tao dao ng bn pht v bn thu, do dch tn Doppler, hoc do nhiu pha xut hin khi knh truyn khng tuyn tnh. Hai nh hng do li tn s gy ra l: suy gim bin tn hiu thu c (v tn hiu khng c ly mu ti nh ca mi sng mang hnh sin) v to ra nhiu xuyn knh ICI (v cc sng mang b mt tnh trc giao). c lng tn s Tng t nh k thut ng b k t, thc hin ng b tn s, c th s dng tn hiu pilot hoc s dng tin t lp. Trong k thut s dng tn hiu pilot, mt s sng mang c s dng truyn nhng tn hiu pilot (thng l cc chui gi nhiu). S dng nhng k t bit trc v pha v bin s gip ta c lng c quay pha do li tn s gy ra. tng chnh xc cho b c lng, ngi ta s dng thm cc vng kha pha (Phase Lock Loop-PLL). Nhn xt: Mt vn cn c quan tm n l mi quan h gia ng b k t v ng b tn s sng mang. gim nh hng ca s mt ng b tn s sng mang th c th gim s lng sng mang, tng khong cch gia hai sng mang cnh nhau. Nhng khi gim s sng mang th phi gim chu k ca mi k t trn mi sng mang, dn n vic ng b k t rt kh khn v phi cht ch hn. iu chng t hai vn ng b trn c quan h cht ch ln nhau, cn phi c s dung ha hp l h thng t c cc ch tiu k thut ra. 3.6.2.3 ng b tn s ly mu Ti bn thu, tn hiu lin tc theo thi gian thu c ly mu theo ng h bn thu, v vy s xut hin s bt ng b gia ng h bn pht v bn thu. Ngi ta a ra hai phng php khc phc s bt ng b ny. Phng php th nht l s dng b dao ng iu khin bng in p (Voltage Controlled Oscillator-VCO). Phng php th hai c gi l: Ly mu khng ng b. Trong phng php ny, cc tn s ly mu vn c gi nguyn nhng tn hiu c x l s sau khi ly mu m bo s ng b. 3.6.3 Gim PAPR (Peak to Average Power Ratio) - T s cng sut nh trn cng sut trung bnh l mt trong nhng hn ch c bn ca tn hiu OFDM. Khi t s ny cao, vic s dng b khuych i cng sut s khng t hiu sut cao v phi dnh d tr cng sut trnh nhiu phi tuyn. Nh vy, gim PAPR l yu cu quan trng ca h thng s dng OFDM. - PAPR ca mt k t OFDM l t s gia gi tr ln nht ca bnh phng mt mu n l trn min thi gian vi gi tr trung bnh bnh phng ca mu ny:

(3.6) - PAPR biu din di bin ca cc mu to ra bn my pht tn hiu OFDM. Ni cch khc, PAPR biu din khong cch n gc ca k t trong khng gian tn hiu - H thng iu ch pha M mc (M-PSK) : Do cc k t trong khng gian tn hiu ch khc nhau v pha trong khi ln bng nhau nn PAPR=1. H thng dng 16QAM PAPR=1.8 - C hai phng php gim PAPR chnh : a thm mt s thng tin h tr (data, m) vo k t OFDM. S dng cc x l khng gian tn hiu (QAM, DPSK) sao cho tn hiu min thi gian sau b IDFT c PAPR thp.3.7 H thng OFDMNhiu

Hnh 3.10 S mt h thng OFDMS h thng OFDM c cho nh hnh 3.10. my pht, chui d liu ni tip qua b S/P c bin i thnh N chui con song song, mi chui ny qua mt b iu ch. ng ra cc b iu ch, ta thu c mt chui s phc D0, D1, , DN-1, trong Dk = Ak + jBk. Chui s phc ny i vo b IFFT:

(3.7)

(do vi Ts l chu k k hiu, fk l tn s cc sng mang)Ng ra b IFFT l cc mu ri rc ca k hiu OFDM trong min thi gian.

(3.8)

Cc mu y(n) ny c chn thm khong bo v, cho qua b bin i D/A tr thnh tn hiu lin tc y(t), c khuch i, a ln tn s cao ri pht ln knh truyn.

(3.9)Trong qu trnh truyn, trn cc knh s c cc ngun nhiu gy nh hng nh nhiu Gausian trng cng AWGN. my thu, ta lm qu trnh ngc li: Tn hiu OFDM c i tn xung, bin i A/D, loi b khong bo v, ri c a vo b FFT. Sau gii iu ch, bin i t song song sang ni tip thu li chui d liu ban u.

(3.10)3.8 u im v khuyt im ca OFDM3.8.1 u im+ Tng hiu qu s dng bng thng.+ Bn vng vi fading chn lc tn s do cc k hiu c bng thng hp nn mi sng mang ph ch chu fading phng.+ Chng c nhiu lin k hiu ISI do chu k k hiu di hn cng vi vic chn thm khong bo v cho mi k hiu OFDM.+ S phc tp ca my pht v my thu gim ng k nh s dng FFT v IFFT.+ C th truyn d liu tc cao.3.8.2 Khuyt im+ Nhy vi offset tn s- Ch cn mt sai lch nh cng c th lm mt tnh trc giao ca cc sng mang ph. V vy OFDM rt nhy vi hiu ng dch tn Dopler.- Cc sng mang ph ch tht s trc giao khi my pht v my thu s dng cng tp tn s. V vy, my thu phi c lng v hiu chnh offset tn s sng mang ca tn hiu thu c.+ Ti my thu, s rt kh khn trong vic quyt nh v tr nh thi ti u gim nh hng ca ICI v ISI.+ T s cng sut nh trn cng sut trung bnh PAPR (Peak to Average Power Ratio) l ln v tn hiu OFDM l tng ca N thnh phn c iu ch bi cc tn s khc nhau. Khi cc thnh phn ny ng pha, chng to ra ng ra mt tn hiu c bin rt ln. Ngc li, khi chng ngc pha, chng li trit tiu nhau lm ng ra bng 0. Chnh v vy, PAPR trong h thng OFDM l rt ln.

Chng 3 TRIN KHAI TRUYN HNH S MT T TI VIT NAM Truyn hnh s mt t theo tiu chun DVB - T vi nhiu u im vt tri v ang pht trin mnh trn th gii ni chung. Ti Vit Nam, chun ny c th nghim thnh cng v c pht sng bi mt s i truyn hnh v cng ty truyn thng a phng tin. Trong chng ny, em xin trnh by v trin khai truyn hnh s mt t ti Vit Nam.4.1 Tnh hnh pht trin truyn hnh s mt t trn th giiRa i vo cui thp k 90, truyn hnh s mt t ang ngy cng c chp nhn, pht trin rng ri v tr thnh xu th khng th thay i ca truyn hnh th gii trong tng lai.Theo nh gi ca cc chuyn gia, truyn hnh s c nhiu u im hn hn so vi cng ngh truyn hnh thng thng m ni bt trc ht l kh nng chng nhiu cao, t nhy cm vi nhiu, c kh nng pht hin sa li v thu tt trong truyn sng a ng. Ngoi ra, truyn hnh s cn cho php tit kim ph tn, truyn c nhiu chng trnh trn cng mt knh sng trong khi truyn hnh tng t phi dng mt knh cho mi chng trnh. Hn th na, truyn hnh s cn c kh nng kho m, qun l chng trnh theo yu cu ng thi cn cho php truyn hnh a phng tin. iu c ngha l truyn hnh s c th truyn nhiu loi d liu khc nhau, nhiu ng ting cho mt knh truyn hnh v truyn hnh km theo ph a ngn ng, thm ch cn cho php nhn tin v t mua hng ho ngay qua tivi.Hin nay, ang thnh hnh 3 tiu chun cho truyn dn truyn hnh s mt t l DVB-T ca chu u, ATSC ca M v ISDB-T ca Nht Bn. s liu thng k cho thy cho ti nay, trong tng s 38 nc chn la tiu chun pht hnh s mt t, c 32 nc chn tiu chun DVB-T ca chu u (chim 84%), 5 nc chn tiu chun ATSC ca M (chim 13%) v duy nht Nht Bn s dng cng ngh ISDB-T. Trong cc h pht hnh s mt t, tiu chun chu u DVB-T t ra c nhiu u im v c hu ht cc nc trn th gii chp nhn.Anh l nc tin phong trin khai pht hnh s mt t theo tiu chun DVB-T (t 15/11/1998). Sau mt thi gian ngn, mt lot quc gia chu u nh Anh, Thu in, Australia, Ty Ban Nha, Singapore, Na Uy, H Lan, cng Nam Phi, Australia, Singapore trin khai pht s theo h DVB-T trn din rng. n nay, hu ht chu u, chu i dng, chu Phi v nhiu nc khc cng trin khai truyn hnh s. c bit, Berlin (c) tuyn b chm dt pht sng truyn hnh mt t bng k thut analog t 4/8/2003. Nhiu nc khc cng c k hoch chm dt pht analog t 2006 n 2010. Xung quanh ta c Thi Lan, Hng Kng, i Loan, Singapore... cng th nghim truyn hnh s mt t theo tiu chun DVB-T. Nhiu nc khc cng ang c k hoch pht hnh s mt t.

Hnh 3.1 Phn b truyn hnh s mt t trn th g [7]

3.1. Truyn hnh s mt t ti Vit Nam [5]THAI LANCAM PU CHIATWliifri

Hnh 3.2. Phm v ph sng DVB - T t Vt NamTi Vit Nam, xu hng chuyn i pht hnh s DVB-T ang din ra rt nhanh t nm 2000 cc thnh ph ln nh H Ni, TP HCM, Bnh Dng, Tin Giang, Hi Phng, Qung Ninh, Thi Nguyn, Thi Bnh...V mng thit b truyn hnh, hin nay B Thng tin v Truyn thng mi bt tay vo vic xy dng cc tiu chun/quy chun k thut cho lnh vc ny. y l mng i hi phi xy dng c h thng cc tiu chun phc v cho cng tc qun l chuyn ngnh cng nh gii quyt vn can nhiu gia chng loi thit b ny vi nhau v gia cc h thng truyn hnh vi cc h thng thng tin khc.Bn cnh , B B Thng tin v Truyn thng cng ban hnh cc quyt nh phc v cho vic qun l mng truyn hnh. V d nh quyt nh s 192/2003/Q-BBCVT ph duyt Qui hoch phn b knh tn s cho truyn hnh tng t mt t bng tn VHF/UHF n nm 2010. Quyt nh ny hin c s dng qun l v mt tn s cho truyn hnh qung b mt t s dng cng ngh tng t (qui hoch v phn b knh tn s). Tuy nhin, quyt nh ny mi ch p dng cho vic qun l v mt tn s, cng sut v chiu cao anten, m cha cp n vn bc x, pht x trnh can nhiu ln nhau cng nh can nhiu ti cc h thng khc. c bit l quyt nh ny ch lin quan n truyn hnh tng t, m khng cp n truyn hnh s. Trong khi , mng truyn hnh s c th nghim v trin khai, v i hi c nhng tiu chun, vn bn php qui phc v cho vic qun l, khai thc mng.4.2 L do Vit Nam chn h thng truyn hnh s DVB T C rt nhiu l do h thng truyn hnh s mt t theo tiu chun DVB - T c chn trin khai Vit Nam. H thng truyn hnh s mt t theo tiu chun DVB - T ph hp vi a hnh c nhiu i ni ti Vit Nam. Trong ng k l vic trin khai cc mng DVB - H v cc mng n tn SFN. h thng truyn hnh s mt t theo tiu chun DVB - T thch hp Yi truyn hnh tng t h PAL hin ang c s dng rng ri Vit Nam. iu ny gip tit kim chi ph, c th pht trin da trn nn tng truyn hnh tng t. Ngoi ra n cn ph hp Yi mng in li 220v, 50 Hz ti Vit Nam. Vit Nam l nc ang pht trin v ang trong xu th hi nhp quc t. Vic trin khai h thng truyn hnh s mt t theo tiu chun DVB - T gip Vit Nam c th thun li trao i, tip thu nhng tin b khoa hc k thut trong vic trin khai v s dng truyn hnh s DVB - T t rt nhiu nc ang s dng h truyn hnh ny. DVB-T ph hp vi di bng tn 8 MHz ang s dng ti VIT NAM v ph hp vi tiu chun pht s qua v tinh ca chu u ang s dng ti VT NAM (DVB-S).4.3 Qu trnh th nghimNhn thc c xu th tt yu ca cch mng k thut s, ngay t nm 1998, Cng ty u t v Pht trin cng ngh truyn hnh Vit Nam (VTC) bt u nghin cu, tip cn vi cng ngh ny. VTC quyt nh chn cng ngh DVB-T cho mc ch th nghim trn c s nh gi tng hp cc tiu ch v k thut, kinh t v xu th pht trin ca cng ngh trn th gii. VTC khng nh: gii php a ra gii quyt c bi ton ang tn ti hng trm my pht hnh tng t cng sut ln v hng nghn my pht hnh tng t cng sut nh trong thi k qu . Kh nng sn xut hng lot b bin i cho php cung cp gii php ng b cho cc i truyn hnh a phng khi c nhu cu chuyn sang pht hnh s mt t trong mt tng lai khng xa.Vi tiu chun la chn l DVB-T, VTC tin hnh th nghim chuyn i my pht hnh Analog sang pht hnh s cng sut va, trin khai pht th nghim trn din hp t thng 12/2000. Vic th nghim ny cho kt qu tt. chun b p dng k thut pht hnh s trn ton quc, VTC c i Truyn hnh Vit Nam giao nhim v pht th nghim pht hnh s trn din rng ti a bn H Ni v cc vng ln cn. Sau , VTC phi hp vi i PTTH Bnh Dng v i TH TPHCM th nghim ti hai a phng ny.Sau hn 3 nm th nghim, ngy 22/9/2004 VTC t chc Hi tho nhm nh gi kt qu th nghim DVB-T trn din rng. Hi tho tp hp c kin ca cc nh khoa hc, cc chuyn gia, cc nh qun l v ng dng cng ngh ny. Bo co ti Hi tho, VTC cho bit, n nay c th khng nh VTC thnh cng trong vic chuyn i my pht hnh analog sang pht hnh s... VTC cng lm ch c cng ngh ch ng sn xut c my pht hnh s, ch to c h thng anten p ng nhu cu khi trin khai trn ton quc. c th a truyn hnh s tr nn ph bin v i tr trong x hi, mt yu cu quan trng l khi trin khai pht hnh s phi c c cc thit b h tr ngi dn c th thu tn hiu. Vi cc nc c nn kinh t pht trin, gi thnh mt chic ti vi s t 1.000 USD tr ln. Bn cnh vic hon thnh ti "Nghin cu ng dng cng ngh chuyn i my pht hnh analog sang my pht hnh digital", VTC cng a ra gii php k thut sn xut hng lot cc b bin i digital/analog bo m cho vic thu, xem tn hiu truyn hnh s mt t v tn hiu v tinh s bng cc my thu hnh thng thng hin nay. T cui nm 2000 VTC a ra gii php thu tn hiu truyn hnh s bng b chuyn i tn hiu s/tng t, cho php thu truyn hnh s bng tivi analog thng thng. T n nay VTC khng ngng hon thin thit k cc chng loi u thu s. Qua 3 nm th nghim cho thy u thu do VTC sn xut hon ton c th tho mn cc chng trnh qung b cc ch pht khc nhau. Gi thnh ca u thu s cng gim xung ng k nh cng ngh sn xut hng lot, vi s lng ln.Thi gian u th nghim VTC mi ch pht 4 chng trnh, n nay tng ln 16 chng trnh, l cc knh thi s, th thao, gii tr hp dn. c bit, ngi dn xem truyn hnh k thut s khng phi tr ph thu bao hng thng nh truyn hnh cp, m ch cn u t ban u mt b u thu s l c th xem c.Bn cnh , nhn thy Vit Nam l nc c mt xe t lu hnh kh ln, VTC cng ang trin khai th nghim dch v pht thu di ng trn cc phng tin giao thng ti H Ni, nhm mc ch ngi ang di chuyn trn t cng c th xem truyn hnh s. Nh vy truyn hnh s s c pht trin v s dng rng ri hn.Vic nghin cu ti Chuyn i my pht hnh analog sang my pht hnh s v trin khai th nghim mng pht hnh s thnh cng hon ton c th khng nh l Vit Nam c kh nng chuyn i my pht hnh tng t thnh my pht hnh s mt t theo tiu chun chu u cng nh c th sn xut c anten v cc b chia cng sut cho my pht hnh s mt t. Vic chuyn i s gip tn dng c c s h tng ang c, tit kim kinh ph u t khi chuyn sang pht truyn hnh s v to kh nng cho vic ch to tng phn thit b trong nc, gim kinh ph nhp ngoi. Vic ch to b bin i STB cng mang li li ch kinh t ln, ph hp vi iu kin kinh t ca Vit Nam do gi thnh ca STB r hn rt nhiu so vi vic mua mi mt tivi s trong khi Vit Nam hin cn ang s dng ti 8 triu tivi analog. Tuy nhin, khuyn khch pht trin cho vic pht v thu tn hiu truyn hnh s trong nm nay v cc nm ti, cn c cc chnh sch khuyn khch v h tr sn xut b STB trong nc, cn c cc chnh sch qun l minh bch, r rng cho mng cng ngh truyn hnh s ang ngy mt ti gn.4.4 Tnh hnh pht trinSau bui hi tho nh gi kt qu th nghim ca cng ty VTC, B Bu chnh vin thng bo co ln Th tng Chnh ph kt thc thi gian th nghim, ng thi B s ch o VTC xy dng l trnh chnh thc trin khai trn ton quc sau khi c Chnh ph cho php.VTC c php pht sng cc chng trnh truyn hnh v pht thanh qung b phc v nhim v cng ch trong cng tc thng tin, tuyn truyn ca ng v Nh nc; cc chng trnh phc v nhu cu thng tin, gii tr ca nhn dn.Cc h chng trnh VTC c php cung cp l cc chng trnh qung b c pht thng, trc tip (cc chng trnh ca i pht thanh, truyn hnh quc gia, i pht thanh, truyn hnh cc tnh, thnh ph thuc Trung ng). H cc chng trnh truyn hnh nc ngoi c bin tp, bin dch ph hp vi cc quy nh v h cc chng trnh do Ban bin tp truyn hnh k thut s VTC thuc cng ty VTC sn xut. Nhng h chng trnh ny u phi tun th theo quy nh ca Lut bo ch.Mng truyn hnh k thut s mt t s s dng cng ngh truyn dn digital, thit b dn l cp quang, sng v tuyn v tn s s dng hai knh 26 v 34. Giy php hot ng truyn hnh k thut s mt t ca VTC s c gi tr trong 5 nm, ti ht thng 10/2010.Cui qu 4 nm 2004 u qu 1 nm 2005 VTC tin hnh xy dng mng dch v truyn hnh s ton quc giai on I ti cc tnh Lng Sn, Ninh Bnh, Thanh Ho, Ngh An, H Tnh, Qung Tr, Nng, Hu, TPHCM, Kin Giang, Vnh Long, Nha Trang, k Lk. Gii php s dng b thu gii m tn hiu truyn hnh (settop-box) cho php thu hnh s mt t bng cc my thu hnh analog hin ti l ph hp vi iu kin thc t ca nc ta v c ng o ngi dn quan tm ng h.4.5 nh giNhng kt qu ca vic xy dng mng dch v truyn hnh s giai on0 cho thy:v mt chnh tr, d n p ng nhu cu pht trin kinh t - x hi, thc y cng cuc i mi, tng cng nng lc cnh tranh ca cc doanh nghip, to kh nng i tt n u thc hin thng li s nghip cng nghip ho, hin i ho, tng cng tuyn truyn ng li chnh sch ca ng v Nh nc n ng bo c nc qua cc chng trnh truyn hnh Quc gia.v mt vn ho, x hi d n gp phn qung b, tuyn truyn vn ho Vit Nam qua cc chng trnh truyn hnh Quc gia. D n mang thng tin n cho ngi dn, c th ni, mt cch gin tip d n gp phn nng cao i sng vt cht tinh thn ca nhn dn, thc y kinh t pht trin. D n gp phn to cng n vic lm cho cn b cng nhn vin VTC thng qua vic sn xut settop box.v mt k thut, cht lng hnh nh tt hn truyn hnh tng t rt nhiu, cht lng hnh nh v m thanh ca cc chng trnh khng b bng, nhiu v . Cht lng cc chng trnh ng u. s lng chng trnh pht trn mng truyn hnh s nhiu, cc chng trnh ny u l nhng chng trnh c ni dung lnh mnh hp dn nn p ng c nhu cu xem truyn hnh ngy cng cao ca ngi dn, cnh tranh c vi cc dch v truyn hnh khc nh: Truyn hnh v tinh, truyn hnh cp... Gp phn y li cc dch v truyn hnh lu c ni dung ko lnh mnh v khng c kim duyt. Dch v truyn hnh k thut s cung cp ti ngi dn nhanh chng (thi gian lp t chm nht ch mt 30 pht k c chnh hng anten thu), lp t d dng, c ng, d s dng.Vi kt qu nh vy, m rng v nng cao cht lng mng truyn hnh k thut s nhm mc ch phc v nhn dn c tt hn v trn phm vi rng hn, VTC xy dng d n u t xy dng mng pht hnh s giai on 2, d n mang tnh x hi v kinh t cao, ng thi to ra c hi tin kp v hi nhp vi cc nc tin tin trn th gii.Mc tiu ca d n ny l u t xy dng 10 i pht s, v cc thit b phc v cho vic sn xut chng trnh ti trung tm H ni cng vi 13 i pht xy dng giai on I to nn mt mng pht hnh s DVB-T c phm vi ph sng rng hn nhm mc ch: M rng phm vi ph sng ca mng dch v truyn hnh k thut s, p ng nhu cu xem truyn hnh ca mt s lng ln ngi dn ti 10 tnh. Truyn ti cc chng trnh truyn hnh Quc gia, cc chng trnh th thao, ca nhc, gii tr phc v nhu cu nghe xem ca ngi dn.

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