HOANG dvb-t

7/26/2019 HOANG dvb-t

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B GIO DC V O TO TP ON BU CHNH VIN THNG VIT NAM

HC VIN CNG NGH BU CHNH VIN THNG VIT NAM----------------------------------------------------

NGUYN HUY HONG

NGHIN CU, XUT LA CHN CNG NGH CHO

CC H THNG TRUYN HNH S MT T VIT NAM

CHUYN NGNH: K THUT IN T

M S: 60.52.70

TM TT LUN VN THC S K THUT

H NI NM 2011


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Lun vn c hon thnh ti:

Hc vin Cng ngh Bu chnh Vin thng

Tp on Bu chnh Vin thng Vit Nam

Ngi hng dn khoa hc:

PGS TS Trn Hng Qun

Phn bin 1: .......................................................................................................

............................................................................................................................

Phn bin 2: ......................................................................................................

..........................................................................................................................

Lun vn s c bo v trc hi ng chm lun vn ti Hc vin Cng ngh Bu chnh

Vin thng

Vo lc: ........gi.......ngy........thng........nm..........

C th tm hiu lun vn ti:

- Th vin Hc vin Cng ngh Bu chnh Vin thng


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GII THIU V TI

Vi nhng u im vt tri ca truyn hnh s so vi truyn hnh analog,

trong nhng nm qua, truyn hnh s mt t pht trin mnh m trn th giiv Vit Nam, vic thay th hon ton truyn hnh mt t analog bng cng nghtruyn hnh s mt t trn ton th gii s din ra trong tng lai khng xa.

n nay c nhiu Quc giatrn th gii la chntiu chun DVB T pht sng mt t. Tuy nhin, trc cc nhu cu i hi v dung lng, v khngli ng truyn, nngcao tin cy vi cc loi hnh dch v, v gim t s cngsut nh / cng sut trung bnh, nhu cu v phn chia ph tn ca cc dch v vinthng khc, cng vi xu th hi t trong cc mi trng truyn dn, s pht trinmnh m ca truyn hnh phn gii cao HDTV vi dung lng bit ln m DVB

T cha p ng c.

T cc yu cu thc t t ra , nhm DVB Project pht trin chuntruyn hnh s th h th 2 l DVB T2. Tiu chun ny c xut bn ln u tinvo 6/2008 v c ETSI chun ha t thng 9/2009. Hin nay, Anh, Phn Lan thng bo trin khai cc dch HDTV trn ng truyn mt t dng chun DVBT2; c, Ty Ban Nha, Thu in cng ang tin hnh th nghim pht sngDVB - T2.

T nm 2001, Vit Nam tin hnh th nghim pht sng truyn hnh mtt. Th tng Chnh ph ph duyt quy hoch truyn dn, pht sng truynhnh, theo n nm 2020 c bn chm dt truyn dn, pht sng truyn hnh mtt s dng cng ngh analog trn ton quc. Trcsc p v l trnh chuyn i

hon ton sang pht sng s mt t trc nm 2020, xu hng pht trin catruyn hnh s mt t DVB T2 trn th gii,lun vntp trung phn tch cc gii

php k thut ca h thng truyn hnh s mt t DVB T, nhng bt cp, tn tikhi trin khai ti Vit Nam, ng thi nghin cucu trc v nhng gii php kthut c bn, nhng tnh nng u vitca tiu chun DVB T2. T , xut lachon gii php cng ngh hp l nht cho h thng truyn hnh s mt t tiVit

Nam trong nhngnm ti.

Chng 1TNG QUAN V

H THNG TRUYN HNH S MT T,NHU CU V XU HNG PHT TRIN

1.1 Gii thiu cc tiu chun truyn hnh s mt tVi truyn hnh s mt t, hin ti hu ht cc nc ang s dng th h th

nht truyn hnh s gm 3 tiu chun:1.1.1 Chun ATSC


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Nm 1996 FCC chp nhn tiu chun truyn hnh s ca M da trn tiuchun gi d liu quc t 188 byte Mpeg 2. Cc ch tiu k thut c th c quynh bi ATSC. ATSC cho php 36 chun Video t HDTV n cc dng thcVideo tiu chun SDTV khc vi cc phng thc qut (xen k, lin tc) v cc tl khun hnh khc nhau.

ATSC c cu trc dng lp, tng thch vi m hnh lin kt h thng m(OSI) 7 lp ca cc mng d liu. Mi lp ATSC c th tng thch vi cc ngdng khc cng lp. ATSC s dng dng thc gi MPEG-2 cho c Video, Audio vd liu ph. Cc n v d liu c di c nh ph hp vi sa li, ghp dngchng trnh, chuyn mch, ng b, nng cao tnh linh hot v tng thch vidng thc ATM. Hnh 1.1 di y m t khung d liu VSB.

ng b mnh s 1

D l iu

ng b mnh s 2

D l iu

8 2 8 b i u t r- n g

77,7 s

46,8 s312 o n d l iu

Hnh 1.1 Khung d liu VSB [3].

1.1.2 Chun ISDB-TISDB T cn gi l tiu chun DiBEG ca Nht Bn ban hnh nm 1997, s

dng k thut ghp knh on di tn BTS (Band Segmened) OFDM v cho phps dng cc phng thc iu ch tn hiu s khc nhau i vi tng on d liunh QPSK, DQPSK, 16 QAM hoc 64 QAM

ISDB-T s dng tiu chun m ho MPEG-2 trong qu trnh nn v ghpknh. H thng s dng phng php ghp a tn trc giao OFDM cho php truyna chng trnh phc tp vi cc iu kin thu khc nhau, truyn dn phn cp, thudi ng v.v... c th s dng cho cc knh truyn 6, 7 v 8MHz.

Cc tham s c bn ca ISDB-T nh bng 1.1 sau:

Bng 1.1: m t cc thng s truyn dn cho ISDB-T knh cao tn 8MHz

[12].

Kiu Kiu 1 Kiu 2 Kiu 3

S on d liu Ns 13


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rng bng tn (MHz) 7,433 7,431 7,426

Khong cch sng mang

(Khz)

5,291 2,645 1,322

S sng mang 1405 2809 5617

Kiu iu ch sng mang QPSK, 16QAM, 64QAM, DQPSKS biu trng trong mt

khung

204

Khong thi gian tch cc

trong mt biu trng (S)

189 378 765

Khong

phng v

(S)

1/4 47,25 94,5 189

1/8 23,625 47,25 94,5

1/16 11,8125 23,625 47,251/32 5,90625 11,8125 23,625

M ho ni M ha cun (1/2, 2/3, 3/4, 5/6, 7/8)

M ho ngoi M Reed Solomon (204, 188)

1.1.3 Tiu chun Chu u (DVB T)DVB dng iu ch ghpknh phn chia tn s trc giao c m (COFDM),

tc bit ti a 24 Mbps(di thng 8Mhz).

im ni tri nht ca COFDM l ch dng d liu cn truyn ti cphn phi cho nhiu sng mang ring bit. Mi sng mang c x l ti mt thiim thch hp v c gi l mt COFDM Symbol.

Do s lng sng mang ln, mi sng mang li ch truyn ti mt phn cadng bt nn chu k ca mt biu trng kh ln so vi chu k ca mt bt thng tin.Cc sng mang ring bit c iu ch QPSK, 16QAM hoc 64QAM. T l mho thch hp ca m sa sai cng gp phn ci thin cht lng h thng.1.1.4 nh gi 3 tiu chun truyn hnh s mt t

im ging nhau ca ba tiu chun trn l s dng chun nn MPEG -2 chotn hiu video. im khc nhau c bn l phng php iu ch.

H thng ATSC s dng k thut iu ch iu bin ct ca nhng nm1980. iu bin ct 8VSB (Vestigal Side Band) vn b cc khuyt tt vn c can, khng c kh nng cho thu di ng, khng khc phc hin tng phn x vkhng thit lp c mng n tn nh gii php ca h ISDB-T v DVB-T.

ISDB-T c ba im khc bit so vi DVB-T. Mt l, s dng ghp xen thigian, trong khi DVB-T khng s dng k thut ny. Hai l,phn on (vng) tn scho cc dch v khc nhau. Trong ISDB-T, ngi ta dnh 10 khong tn s cho dch


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v hnh nh v 3 khong cho dch v m thanh trong khi DVB T ch s dngphng php iu ch phn cp. Ba l, ISDB-T khng tng thch vi cc h phts qua v tinh DVB-S v pht s qua cp DVB-C1.2 Thc trng truyn hnh s mt t ti Vit Nam1.2.1 Truyn hnh tng t mt t

Theo s liu kho st mi y, hin c 63.69% s h gia nh (22 triu h)ang xem truyn hnh tng t t cc i pht khc nhau:i Truyn hnh Vit Nam (VTV) hin ang c mng ph sng analog trn

phm vi ton quc vi nhiu knh chng trnh. i truyn hnh ca 63 tnh thnhtrong c nc, i truyn hnh k thut s (VTC), i Ting ni Vit nam (VOV)hin ang pht sng analog.

Mng pht hnh s mt tVi tng s 25% s h gia nh c nc ang xem truyn hnh s mt t t

cc i:Mng truyn hnh s VTC pht sng ton quc theo chun DVB-T, nn

MPEG2, l mng truyn hnh c din ph sng trn 44 tnh, thnh ph vi trn 80im pht sng. Ngoi ra, i TH TP.HCM ang pht sng DVB-T trn knh 25.i PT-TH Bnh Dng cng trin khai pht sng DVB-T trn knh 53, AVGang trin khai mng DVB T n tn.

Hin nay VTC pht sng 28 knh truyn hnh v nhiu knh pht thanhc iu ch trn hai knh sng mang lin k.

Vic truyn dn tn hiu n cc my pht c thc hin qua mng cp quangv qua vtinh. Tn hiu Video v Audio sau khi c nn v ghp thnh 02 lungs liu ASI 1 v ASI 2 s c ghp v truyn qua cp quang hoc qua v tinh ncc a im pht sng, pht 02 knh RF lin k pht sng mt t.

Nh vy, hin nay Vit Nam, vicpht sng k thut s s dng cng nghDVB-T c mt s n v v cc i truyn hnh tng bc trin khai. Cc iv n v ny u s dng cng ngh nn MPEG-2, pht sng theo m hnh mnga tn (tr An Vin Group) v ghp kh nhiu chng trnh trn mt knh sng.1.3 Nhu cu v xu hng pht trin

Ngi xem lun c nhu cu thu c nhiu knh t sng truyn hnh mt t.p ng nhu cu ny vi truyn hnh Analog gp nhiu kh khn v thit b pht vknh sng cao tn, v mi knh cn mt my pht v mt knh sng.

Cc bng tn VHF v UHF lu nay dng cho truyn hnh mt t n nay khng cho yu cu pht nhiu knh, trong khi cc mng thng tin chuyn ngnh,thng tin di ng cng c nhu cu cc di tn ny.

Cht lng ca cc knh truyn hnh i hi ngy cng cao, c bit l nhucu thng thc truyn hnh nt cao (HDTV) ngy cng ln. p ng cc nhu cu trn ch c gii php l chuyn sang truyn hnh s

mt t vi nhng u im vt tri so vi truyn h nh analog. Vi truynhnh sDVB-T c th thit lp mng pht hnh gm nhiu my pht trn cng mt knhsng. iu ny gip cho vic nng cao hiu qu s dng ph tn v tit kim kinhph u t, chi ph vn hnh...

pht huy c cc u im trn, Th tng Chnh ph ph duyt quyhoch pht sng truyn dn, theo s chm dt truyn dn, pht sng truyn hnh


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bng cng ngh Analog, chuyn sang pht sng s mt t trc nm 2020. Vicpht trin nhanh cc h thng pht hnh s mt t vi s knh ln, c kh nngpht HDTV l mt nhu cu cp thit nc ta.

Mng DVB T c nhiu Quc gia la chn trin khai cng nh khng nh c u th r rt trn th gii. Tuy nhin, qua cc h thng truyn hnhs ang khai thc v nhu cu thc t t ra v i hi ngy cng tng v dung

lng, v khng li ng truyn, nng cao tin cy vi cc loi hnh dch v, vgim t s cng sut nh / cng sut trung bnh, nhu cu v phn chia ph tn cacc dch v vin thng khc, cng vi xu th hi t trong lnh vc truyn dn,truyn hnh di ng, s pht trin mnh m ca truyn hnh phn gii cao HDTVvi dung lng bit ln m DVB T cha p ng c.

Trc nhng nhu cu t ra trn, t thng 6/2007 n thng 3/2008, trn 40 tchc tp trung nghin cu tiu chun mi ca DVB - T. n cui nm 2008, nhngni dung c bn ca tiu chun truyn hnh s mt t th h th hai DVB-T2 c

ban hnh. Cng vi vic ra i cc chun nn mi nh MPEG 4, chun H264 kt hpvi cc u im vt tri ca DVB T2, nht l khi s dng vi mng n tn vnhiu knh lin k s l xu hng pht trin cho truyn hnh s mt t trn th gii.

Trc hon cnh , truyn hnh s mt t Vit Nam s la chn cng ngh no? yl mt bi ton cn c li gii p ng c nhu cu pht trin ca kinh t - x hiVit Nam. Lun vn ny s tp trung nghin cu cng ngh truyn hnh mt t angv s c Vit Nam, th gii t xut khuyn ngh la chn cng ngh hp lcho truyn hnh s Vit Nam.

Chng 2TRUYN HNH S MT T THEO TIU CHUN DVB T

2.1 S khi ca h thng DVB-TH thng c nh ngha l mt thit b gm nhng khi chc nng, tn hiu

u vo l dng truyn ti MPEG-2 nhn c ti u ra ca b ghp(Multiplexer), u ra l tn hiu RF iti anten.

H thng tng thch vi chun nn tn hiu video MPEG-2 ISO/IEC 13818.Do h thng c thit k cho truyn hnh s mt t hot ng trong bng tn

UHF hin c, nn i hi h thng phi c kh nng chng nhiu tt t cc mypht tng t hot ng cng knh hoc knh lin k, i hi h thng phi c hiusut s dng ph tn cao bng tn UHF, iu ny c th m bo bng vic s dngmng n tn (SFN).

S khi chc nng ca h thng pht theo tiu chun DVB T c thhin hnh 2.1 di y.


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Hnh 2.1: S khi chc nng hthng pht hnh smt t DVB-T [6]y l s khi ca mt h thng pht hnh s mt t y . Cc tn hiu

hnh nh, m thanh s qua mt lot qu trnh x l cui cng, ti u ra antencng vn l tn hiu cao tn pht i.

Nhng tnh nng u vit ca truyn hnh s mt t li hon ton c thhin trong cc qu trnh x l ny.

Cc khi nt t trn hnh 2.1 s c s dng khi h thng dng theo cu

hnh iu ch phn cp. Khi t khi ghp knh m ha ngun d liu MPEG 2phn chia dng d liu thnh 2 lung vi nhng mc u tin khc nhau, tc bitv t l m ha khc nhau, c ngha l kh nng chng li ca tng dng bit l khcnhau. S khi cho ta thy mt h thng my pht ch yu s gm phn iu chOFDM v phn m ho sa li.2.2 Gii phpc bntng khi chc nng ca DVB-T2.2.1 Ghp knh v m ha ngun d liu MPEG-2

Cc tn hiu u vo gm hnh nh, m thanh v cc d liu ph s c sha nh khi ghp knh, m ha v nn MPEG-2. u ra ca khi ny s l dngtruyn truyn ti MPEG-2 vi mt tc bit nht nh a vo my pht. y l

qu trnh s ha tn hiu.2.2.2 Khi ghp knh truyn ti v ngu nhin ha s liu

hn ch ti a cc li trongtruyn dn, dng d liu TS n t khi nn sc ngu nhin ho. Cc gi d liu ny u tin c nhn dng bi chui gingu nhin PRBS. Mc ch ca qu trnh ny l phn tn nng lng trong ph tnhiu s v xc nh s nh phn thch hp (loi b cc chui di 0 v 1), ngthi y cng c xem l qu trnh phi hp ghp knh truyn ti.2.2.3 Khi m ngoi v ghp xen ngoi


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Dng d liu sau khi c ngu nhin ha s tip tc c x l ti khim ngoi v ghp xen ngoi. S d gi l ngoi v vic x l y l theo byte,cn m ni v ghp xen ni l x l theo bit. B m ngoi s dng m Reed-Solomon RS (204, 188, t=8) m ho d liu c ngu nhin ho nhm to racc gi d liu c bo v li.

Do c m ho theo m RS (204,188, t=8) nn mi gi d liu s cthm 16 bytes sa li v n c kh nng sa ti 8 li trong mt gi.

Vic ghp ngoi chnh l ghp cc byte vi mt chu k ghp qui nh trc,thng su ghp l l=12. y cng l vic nhm gim tnh thng k ca li.2.2.4 Khi m ni (inner coder)

y l qu trnh m ho tip theo nhng vic m s chi tit n tng bit.Thng s m ha y chnh l t l m ha n/m (1/2, 2/3, 3/4, 5/6, 7/8). Ngha lc m bt truyn i th ch c nbit mang thng tin, cc bit cn li l sa li.2.2.5 Khi ghp ni(inner interleaver)

D liu n y s c tro hon ton theo tng bit, thng tin s rt khc so

vi ban u.B ghp kiu btQu trnh ny gim thiu li n mc ti a.

B ghp xen symbol.Mc ch ca b ghp xen symbol l t nhng k t c v bit ln 1512 (mode

2k) hoc 6048 (mode 8k) sng mang. B ghp xen symbol uc thc hin trn cckhi c 1512 (mode 2k) hoc 6048 (mode 8k) k t d liu.2.2.6 Chm sao tn hiu v qu trnh nh x(mapper)

H thng pht sng truyn hnh s mt t OFDM s dng k thut truyn dnghp knh phn chia tn s trc giao OFDM. Mi khung OFDM gm cc sng

mang c iu ch theo mt trong cc phng php QPSK, 16QAM, 64QAM,16QAM khng ng nht, 64QAM khng ng nht. COFDM cho php tri d liu truyn i trn c min thi gian v min tn s.

Do c hin tng fading tn sgia cc di tn cn k, nn COFDM c sdng xen tn s, ngha l cc bit dliu lin tip nhau sc tri ra trn cc sngmang cch bit nhau Hnh 2.6

Hnh 2.6 Thc hin nh x (mapping) d liu ln cc symbol

Trong DVB-T vic nh x (mapping) d liu ln cc symbol OFDM thc ra liu ch tng sng mang ring r, v c th theo mt trong ba chm sao to


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phc 4QAM, 16QAM, 64QAM. Ti mi symbol, mi sng mang s c iu chbi mt s phc ly t tp chm sao.2.2.7 Cu trc khung OFDM

Tn hiu truyn dn c sp xp thnh cc khung. Mi khung c chu k TFgm c 68 k t OFDM. C 4 khung lp thnh mt siu khung. Mi k t cthit lp bi k = 6817 sng mang mode 8k hoc k = 1705 trong mode 2k, n c

chu k Ts.COFDM thc hin vic phn chia knh truyn dn c trong min thi gian

v min tn s, t chc knh RF thnh tp cc "di tn ph"hp v tp cc "onthi gian" lin tip nhau hnh 2.8.

t i m e

fre

quen

cy

C h a n n e l

B a n d w i d t h

f r e q u e n c y

s u b - b a n d

t i m e

s e g m e n t

Hnh 2.8 Phn chia knh

Mi ci c mt dch tn Doppler khc nhau, cng vi ng tn hiuchnh. Trc z miu t bin p ng knh.

Ngoi d liu truyn i, 1 khung OFDM cn cha:Cc ti ph TPS (transmission parameter signalling).Tn hiu dn ng dng cho ng b khung, ng b tn s, thi gian. Cc sng mang trong 1 k t OFDM c nh s t kminn kmax.kmin= 0, kmax=1704 trong mode 2k.kmin= 0, kmax= 6816 trong mode 8k.

sng mang kminv kmaxl (k-1)/TU.Khong cch gia cc sng mang lin k nhau l 1/TU. Khong cch gia cctham s ca 1 k t OFDM theo tng mode c cho trong bng 2.4.

Bng 2.4: Cc tham s ca 1 k t OFDM [7]Tham s Mode 8k Mode 2kS cc sng mang k 6817 1705Ch s ca sng mang kmin 0 0Ch s ca sng mang kmax 6816 1704Chu k TU 896s 224s

Khong cch gia cc sng mang 1116 Hz 4464 hzKhong cch gia sng mang kminv kmax 7,61 MHz 7,61 MHz.

Chn cc sng mang phTrong mi on thi gian, gi l mi symbol OFDM, mi di tn ph c

trang b mt sng mang ph m t trn hnh 2.9. trnh nhiu gia cc sngmang, chng c b tr vung gc vi nhau, ngha l khong cch gia cc sngmang c t bng vi nghch o ca mt chu k symbol

.


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Hnh 2.9. Chn cc sng mang ph[1]Chn khong bo vDo cc "echo" c to ra bi cc bn sao ca tn hiu gc khi b tr, n n ti

phn cui ca mi symbol OFDM s c nhiu lin symbol vi phn u ca symboltip theo. trnh iu ny, mt khong bo v c chn vo mi symbol. Nhtrn hnh 2.10.

. Hnh 2.10: Chn khong bo v[1]ng b knh gii iu ch tn hiu mt cch chnh xc, cc my thu phi ly mu chnh

xc tn hiu trong sut khong hu ch ca symbol OFDM (b qua khong bo v).Do , H thng DVB-T s dng cc sng mang "pilot", tri u n trong knhtruyn dn, ng vai tr lm cc im nh du ng b.2.2.8Cc tn hiuchun

Ngoi d liu truyn i trong khung OFDM cn c cc thng tin ph gipcho thit b thu nhn dng v gii iu ch c lung d liu.

Cc ti ph TPS (transmission parameter signalling) c truyn song song vmang thng tin v:iu ch, gm gi tr ca gin chm sao QAM; Thng tin mho theo lp; Khong bo v; T l m ni; Model truyn (mode 2k hoc 8k); Skhung trong mt siu khung ( khi truyn trong mng SFN); Cc tn hiu dn ng(pilot) dng cho ng b khung, ng b tn s, ngb thi gian.2.2.9 Khi D/A


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Thc ra y khng phi l khi bin i Digital/Analog thng thng. M l qu trnh hon chnh hng ngn sng mang m bo vic pht tn hiu lnAnten.2.2.10 Khi thit b u cui

L qu trnh cui cng nh khuch i, lc thng di.. pht ra khng gian.

2.3 DVB-T trong mi trng b phn x- Mng n tn (SFN).H pht s DVB-T s dng ghp a tn trc giao OFDM c nhiu u im.Trong c mt u im rt ni tri v quan trng (hn hn h ATSC ca M), l cho kh nng thit lp mng n tn. Khi thit lp mng n tn, tt c cc my

pht thuc mng n tn u pht cng knh sng, rt thun li cho quy hochv tit kim ti nguyn tn s. Mng n tn tun th 3 iu kin:

*. Mt l, cc my pht cng mt dng truyn ti TS;*. Hai l, pht cng tn s;*. Ba l, pht cng thi im.V vy, mng n tn hot ng hiu qu cn thc hin tt vic thit lp v

hiu chnh ng b gia cc my pht.2.3.1. DVB-T trong mi trng b phn x nh l mng n tn t nhin

Phn x l hin tng chung v ph bin ca truyn sng in t.Trong mi trng thc t, chng ta ang chu hu qu ca hin tng phn x

sng khi thu cc chng trnh truyn hnh hnh 2.14. i vi cng ngh analog,nhiu sng n anten thu v gy ra nhiu hnh trn ti vi, to nn bng ma lem nhem,thm ch cc hnh ph nhau lm mt ng b v khng th xem c.

Hnh 2.14 Thu trong mi trng thc tSng ca my pht hnh s cng chu quy lut phn x, nhng do k thutghp a tn trc giao v nh c thng s khong thi gian bo v ca DVB-T,nn cc thit b thu s DVB T, khc phc hiu qu hin tng phn x.2.2.2. iu kin thu tt trong mi trng c phn x

My pht DVB-T c mt thng s lin quan n vic chng hin tng phnx, l khong thi gian bo v Tbv. Khong thi gian bo v Tbvc cc gi trkhc nhau theo quy nh ca DVB (xem bng 2.7) v ngi pht sng c th lachn mt trong cc gi tr . Bng nu khong thi gian bo v cho thy, mt


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ch pht 8k hoc 2k, c th chn mt trong 4 gi tr cho khong thi gian bo v.Ch pht 8k cho khong thi gian bo v ln gp 4 ln ch 2k (nu c ng t lTbv/Tsymbol).

Bng 2.7 Khong thi gian bo v cc ch pht khc nhau

Tbv

/Tsymbol

Khong thi gian bo v Tbv[s]

v chnh lch v qung ng i tng ng D=c.Tbv

Ch 8KT(symbol)= 896s

Ch 2KT(symbol)= 224s

1/4 224s; 67,2km 56s; 16,8km1/8 112s; 33,6km 28s; 8,4km1/16 56s; 16,8 km 14s, 4,2km1/32 28s; 8,4km 7s; 2,1km

Khi chnh lch thi gian ca cc tia sng n u thu khng vt qu khongthi gian bo v Tbv, th my thu hon ton khc phc tt hin tng phn x.

2.2.3. Mng n tn (Single Frequency Network - SFN)Tr li 3 iu kin cho cc my pht thuc mng n tn: mt l, pht cngmt dng truyn ti TS; hai l, pht cng tn s; ba l, pht cng thi im. Sauy xin trnh by chi tit v 3 iu kinny.

iu kin th nht bt buc cc my pht thuc mng n tn ch pht ngmt dng truyn ti duy nht (c v ni dung, c v thi gian). V ni dung, cngha l ti bt k my pht no cng khng c lm mt tnh thng nht ca dngTS .

i vi iu kin th hai l cc my pht phi pht cng tn s, nh bit cc my pht s DVB-T khng c b dao ng hnh SIN to cc sng mang nh cch thng truyn hnh tng t. Hng ngn sng mang trong b iu ch s c

to ra l do tn hiu ca dng TS sau khi chia nh ra c bin i Fourier ngcto nn. chnh xc ca tn s lin quan cht ch ti chnh xc ca dng TS.Nh vy, vic ng b cc dng TS cng ng ngha vi vic thc hin ng b tns khu iu ch ca cc my pht.

i vi iu kin th ba l cc my pht pht cng thi im (at the sametime) mc ch nhn mnh tnh kht khe ca ng b: cng thi im pht giu tin ca cng mt Mega-frame ra khng trung tt c cc my pht trongmt mng n tn, khng c s nhanh chm hn nhau, ni cch khc s chnh lchthi gian pht gi ny ti tt c cc my pht trong mt mng n tn phi bngkhng. y chnh l vn ct li ca qu trnh ng b.

Trong DVB T cn p ng yu cu v khong phng v, mbo n cn

phi gi khong cch gia cc my pht phi m bo cc iu kin khong bo v,tc l khng c vt khong cch cc i cho php vi nhau.Chng 3.

DVB T2 V GII PHP CNG NGH CHOTRUYN HNH S MT TVIT NAM

3.1 Ni dung c bn ca tiu chun DVB T23.1.1. Cu trc DVB-T2


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H thng DVB-T2 c chia thnh 3 khi chnh pha pht v 2 khi chnh pha thu nh trnh by trong hnh 3.1:

Hnh 3.1. M hnh cu trc DVB-T2 [9]

M ho v ghp knh.Khi m ho v nghp knh c chc nng m ho tn hiu video, audio cng

cc tn hiu ph tr km theo nh thng tin v chng trnh/ thng tin dch vPSI/SI hoc tn hiu bo hiu lp 2 (L2 Signalling) vi cng c iu khin chungnhm m bo tc bit khng i i vi tt c cc dng bit. Khi ny c chcnng hon ton ging nhau i vi tt c cc tiu chun ca DVB. u ra ca khil dng truyn ti MPEG-2TS

u vo ca Baisic T2 - Gateway c nh ngha trong [9], u ra l dngT2 - MI. Mi gi T2-MI bao gm Baseband Frame, IQ Vector hoc thng tin bohiu (LI hoc SFN

B iu ch DVB-T2 s dng khung c s (Baseband Frame) v T2- Framemang trong dng T2-MI u vo to ra DVB-T2 Frame.

B gii iu ch DVB-T2 nhn tn hiu cao tn (RF Signal) t mt hocnhiu my pht (SFN Network) v cho mt dng truyn ti (MPEG-TS) duy nhtti u ra.

B gii m dng truyn tinhn dng truyn ti (MPEG-TS) ti u vo v

cho tn hiu video/audio ti u ra.3.1.2. Lp vt l DVB T2

M hnh lp vt l ca DVB-T2 c trnh by trong hnh 3.2. u vo hthng c th bao gm mt hoc nhiu dng truyn ti MPEG-TS hoc dng GS(Generic Stream).

u ra ca lp vt l l tn hiu cao tn RF. Tn hiu u ra cng c th cchia thnh hai ng cung cp cho antenth 2, thng l mt my pht khc


15/21

Hnh 3.2. Lp vt l DVB-T2Cc thng s COFDM ca DVB-T2 cng c m rng so vi DVB-T, trong

bao gm:- FFT: 1K, 2K, 4K, 8K, 16K, 32K- Khong bo v:1/128, 1/32, 1/16, 19/256, 1/8, 19/128, 1/4- Pilot phn tn:8 bin th khc nhau ph hp vi cc khong bo v khc

nhau.- Pilot lin tc:Tng t nh DVB-T, tuy nhin ti u hn.

- Tro: Tro bit, tro t bo, tro thi gian v tro tn s.3.1.3. Nhng gii php k thut c bn3.1.3.1 ng lp vt l (Physical Layer Pipes - PLPs)

i hi ca th trng i vi tin cy ca cc dch v v s cn thit phic cc loi dng d liu khc nhau dn ti khi nim ng lp vt l hon tontrong sut c kh nng truyn ti d liu c lp vi cu trc v cc thng s PLPkhc nhau.

DVB-T2 cn cho php gn cc gi tr: th chm sao, t l m v tro thigian cho tng PLP, ngoi ra cn dng thc ho ni dung theo cng mt cu trckhung nh c p dng trong DVB-S2.

3.1.3.2 Bng tn ph (1,7 Mhz v 10 Mhz) p ng cc dch v chuyn dng, v d truyn tn hiu t camera v mt

trng quay (Studio) lu ng, DVB-T2 cn bao gm tu chn bng tn 10Mhz.Cc my thu dn dng khng h tr bng tn ny. DVB-T2 cn s dng c bng tn1,712 Mhz cho cc dch v thu di ng (trong bng III v bng L)3.1.3.3 Mode sng mang m rng (i vi 8K, 16K, 32K)

Do phn nh xung vung trong th ph cng sut suy gim nhanh hni vi kch thc FFT ln. im ngoi cng ca ph tn hiu OFDM c th trirng hn, iu ny cng ng ngha vi vic nhiu sng mang ph trn mt symbolc s dng truyn ti d liu. li (gain) t c gia 1,4% (8Kmode)

v 2,1% (32Kmode). Hnh 3.5 so snh ph ca 2K so vi 32K iu kin bnhthng v 32K trong mode sng mang m rng. Sng mang m rng l mt ctnh tu chn, bi l vi c tnh ny kh c c th t c mt n ph (spectrummask) cng nh t s bo v.


16/21

Hnh 3.5. Mt ph cng sut i vi 2K v 32K [9]

3.1.3.4 MISO da trn Alamouti (trn trc tn s)DVB-T2 c tu chn s dng k thut Alamouti: Vi mt cp my pht (hnh

3.6). Alamouti l mt v d ca MISO, trong mi im ca th chm sao ctruyn bi mt my,my pht th 2 truyn phin bn c chnh sa mt cht ca

tng cp ca chm sao vi th t ngc li trn trc tn s.

Hnh 3.6. M hnh MISO [5]3.1.3.5 Symbol khi u (P1 v P2)

Nhng symbol u tin ca khung DVB-T2 lp vt l l cc symbol khiu (preamble symbols). Cc symbol ny truyn mt s lng hn ch cc thng tin

bo hiu bng phng thc truyn c tin cy cao. Khung u tin c bt ubng symbol P1, iu ch BPSK vi tin cy cao. Vi khong bo v c haiu, symbol P1 mang 7 bit thng tin (bao gm kch thc FFT ca symbol d liu).Cc symbol P2, s lng c c nh cho mi kch thc FFT, cung cp thng tin

bo hiu lp 1 k c tnh, ng v kh nng cu trc.Cc bit u tin ca thng tin bo hiu (L1 - Pre-signalling) c phng thc

iu ch v m ho c nh, cc bit cn li (L1 - Post-signalling) t l m c xcnh l 1/2 nhng phng thc iu ch c th c la chn gia QPSK, 16QAMv 64QAM.3.1.3.6 Mu hnh tn hiu Pilot (Pilot Pattern)


17/21

Pilot phn tn (Scattered Pilots) c xc nh t trc c v bin v pha,v c cy vo tn hiu vi khong cch u nhau trn c hai trc thi gian vtn s. Pilot phn tn c s dng nh gi s thay i trn ng truyn.

Pilot phn tn cho php gim thiu vt mc (overhead) t 4 8% khi sdng mu hnh PP3 v khong bov 1/8. i vi Pilot lin tc

3.1.3.7 Phng thc iu ch 256-QAMTrong h thng DVB-T, phng thc iu ch cao nht l 64-QAM cho phptruyn ti 6bit/symbol/sng mang (c ngha l 6bit/t bo OFDM). DVB-T2,phng thc iu ch 256QAM cho php tng ln 8bit/t bo OFDM, tng 33%hiu xut s dng ph v dung lng d liu i vi mt t l m cho trc.3.1.3.8 Chm sao xoay (Rotated Constellation)

Mt trong s cc k thut mi c s dng trong DVB-T2 l chm sao xoay(Rotated Constellation) v tr Q (Q-delay).

Sau khi nh v, chm sao c xoay mt gc trn mt phng I-QCc thnh phn I v Q c tch bi qu trnh tro sao cho chng c truyn

trn min tn s v thi gian khc nhau. Nu c mt thnh phn b hu hoi trnknh truyn, thnh phn cn li c th c s dng ti to li thng tin.3.1.3.9 16K, 32K FFT v t l khong bo v 1/128

Tng kch thc FFT ng ngha vi vic lm hp khong cchgia cc sngmang v lm tng chu k symbol. Vic ny, mt mt lm tng can nhiu gia ccsymbol v lm gim gii hn tn s cho php i vi hiu ng Doppler. Mt khc,chu k symbol di hn, cng c ngha l t l khong bo v nh hn i vi cnggi tr tuyt i ca khong bo v trn trc thi gian. T l khong bo v bng1/128 trong DVB-T2, cho php 32K s dng khong bo v c cng gi tr tuyti nh 8K 1/323.1.3.10 M sa sai LDPC/BCH

Trong khi DVB-T s dng m sa sai ni v ngoi l m cun v m R-S(Convolutional and Reed-Solomon Codes), DVB-T2 v DVB-S2 s dng LDPC vBCH. Cc m ny cho php kh nng bo v tt hn, truyn nhiu d liu hn trncng mt knh thng tin.3.1.3.11 K thut gim thiu t s cng sut nh/cng sut trung bnh (Peak -to - Average Power Ratio - PAPR)

PAPR trong h thng OFDM cao c th lm gim hiu sut b khuch icng sut RF. C hai k thut lm gim PAPR c s dng trong h thng DVB-

T2: m rng chm sao tch cc (Active Constellation Extension - ACE) v hn chm sc (Tone Reservation - TR).3.1.3.12 Tro bit, nh x bit ln th chm sao

+ Tro bt trong phng thc iu ch 16 QAM, 64 QAM v 256 QAMD liu ti u ra ca b m ha LDPC c tro, tro bit bao gm tro chn

l(parity interleaving) v tro ct xon(column twist interleaving). Tro chn lc k hiu l U, tro ct xon c k hiu l V.

+ nh x bit ln th chm sao


18/21

Mi FECFRAME c nh x sang khi FEC m ha v iu ch bngcch trc tin tch knh dngbit u vo thnh cc t m song song v sau nhx cc t m ny thnh cc gi tr tng ng trn th chm sao.3.1.3.13 nh x ln th chm sao I/Q

Mi t m yo,q... yn mod -1,q t b tch knh c iu ch bi phng thc

QPSK, 16 QAM, 64 QAM hay 256 QAM c gn vi mt im Z q trn thchm sao trc khi chun ha.3.1.3.14 Tro t bo, tro thi gian

Nhm nng cao tin cy trong qu trnh truyn sng, khng ch s dng trobt, tro symbol nh th h u, truyn hnh s mt t th hth2(DVB-T2) cns dng k thut tro t bo (cell interleaving- CI) v tro thi gian (timeinterleaving- TI).

3.1.3.15 iu ch v m sa sai d liu lp 1

D liu bo hiu lp 1 (L1 - signalling) cung cp thng tin cho u thu c

th truy nhp vo lp vt l trong khung d liu DVB-T2. D liu bo hiu lp 1c bo v bi m ngoi BCH v m trong LDPC.

3.1.3.16 Cu trc khung tn hiu DVB-T2Mt phn quan trng ca h thng DVB-T2 l siu khung (Super Frame).

Trong siu khung cha nhng khung-T2 v phn m rng dnh cho tng lai (FEF:Future-Extension Frame) v c miu t chi tit Hnh 3.11.

S lng ti a ca khung T2 trong mt siu khung l 255 v di ln nhtca n l 250ms, cng nh vy, di ln nht ca mtphn m rng (FEF) ccha trong siu khung l 250ms. Nh vy nhng khung-T2 c cha trong mtsiu khung c th c di khc nhau tu thuc vo phn m rng (FEF) gn sau

n. Khung-T2 c chia ra thnh cc Symbol OFDM, mi khung-T2 u bt uvimt Symbol P1, tng t nh vy i vi phn m rng FEF. Khong thi giangia 2 Symbol P1 ny l 250ms.

Hnh 3.11. Cu trc khung DVB-T2[5].


19/21

Mc ch cc khung T2 ny l mang tn hiu PLPs v tn hiu L1 km theo l cc dch v DVB-T2.

Mc ch ca phn m rng FEF l kh nng kt hp linh hot cc dch v a ra trong chun DVB-T2 hin ti v phin bn dnh cho tng lai, cc phnm rng ny c th mang d liu hoc khng mang d liu, theo khi thu nhn tn hiu DVB-T2 cng phi c kh nng d v x l chnh xc cc phn FEF

ny trnh trng hp cc khung T2 b xo trn trong khi nhn c ti bt kthi gian no.3.2 Kt qung dng DVB T2 trn th gii

Hin nay, Anh v Phn Lan thng bo trin khai cc dch v HDTV trnng truyn mt t dng chun DVB-T2. Ngoi ra, mt s th nghim pht sngDVB-T2 ang c k hoch trin khai hoc trin khai th nghim xong mt snc khc nh : c, , Ty Ban Nha, Thy in...

Anh, nhm iu hnh truyn thng OFCOM quyt nh dng mt trong 6ghp knh DTT (Multiplex B) bng tn UHF cho vic cung cp cc dch v HDdng chun DVB-T2 kt hp vi k thut nn MPEG-4 AVC.

3.3 So snh, nh gi, la chn gii php cng ngh cho truyn hnh s VitNam3.3.1 So snh cc tham s c bn

Vi vic ng dng nhiu gii php k thut mi, DVB-T2 cho php pht trinti a hiu qu cc ng dng trong mng n tn. So vi chun DVB-T, cc modesng mang mi c cng thm ci thincc ch tiu k thut.

Cc thng s ca DVB T2 ang s dng ti Anh so vicc thng s catiu chun DVB T th hin trn bng 3.7.

Bng 3.7 DVB-T2 s dng ti UK so vi DVB-T [4]

DVB-T DVB-T2Phng thc /ch 64 - QAM 256 - QAMFFT 2K 32KKhong bo v 1/32 1/128FEC 2/3CC + RS 3/5 LDPC + BCHPilot phn tn 8,3% 1,0%Pilot lin tc 2,0% 0,53%Mc vt L1 1,0% 0,53%

Sng mang Tiu chun M rngDung lng 24,1Mbps 36,1 Mbps

Vi cc thng s c la chn trn, tiu chun DVB T2 th hin nhiuu imni tri so vi DVB T.Cc u im ca DVB T2

Dung lng cao


20/21

Kh nng khng nhiu Kh nng chng pha inh Gim thiu t s cng sut nh/cng sut trung bnh Tng hiu qu s dng ph tn Kh nng cung cp dch v

3.3.3 Khuyn ngh la chn cng nghT cc so snh trn, lun vn c mt s khuynngh:1.Khng pht trin thm cc h thng truyn hnh mt t theo cng ngh c

analog v DVB T, tng bctrin khai h thng truyn hnh s mt ttheo tiuchun DVB T2 bn cnh cc h thng DVB T ang c.

2.Khi trin khai mng DVB T2 cn phi qui hoch thng nht, khoa hctrong tng vng v ton quc, thit k hp l gia mng a tn v mng n tn.

3.Vi cc h thng ang trin khai lp t, dou thu DVB T2 cha phbin, gi thnh cao, trc mt gc kinh t cha ph hp th iu quan trng lvic xy dng h thng phi c tnh m vn pht DVB T nhngchncc thit

b tng thch vi tiu chun DVB T2, khi iu kin cho php d dng chuynsang hot ng theo tiu chun DVBT2.

4.Trong tng vng tng dung lng, tit kim di sng mang v phc vcho kh nng thu truyn hnh di ng nn thit k h thng mng n tn.

5. i vi cc khu vc min ni, a hnh phc tp,sng truyn hnh kh ixa, ng thicn a vo h thng cc chng trnh truyn hnh a phng th nns dng mng a tnvi cc knh lin k.

6. p ng nhu cu c th xem c nhiu knh chng trnh, tng cc loihnh dch v, c bit l cc knh HDTV, ngoi vic buc phi chn cng nghDVB T2 cn phi ng dng cc k thut mi v nn tn hiu nh nn MPEG 4AVC.


21/21

KT LUN

Lun vn thc hin c:- Tnh cp thit ca vic la chn c mt cng ngh tin -trincho cc h

thng truyn hnh s mt t ti Vit Nam.

- Nu c cc c im chnh ca 3 tiu chun truyn hnh hin c trn thgii, s khc nhau c bn gia tiu chun DVB T m Vit Nam la chon sovi hai tiu chun cn li. Nu c thc trng truyn hnh mt t Vit Nam v xuhng pht trin.

- Trnh by cu trc v nhng gii php k thut c bn ca hai tiu chunDVB T v DVB T2, qua so snh cc ch tiu k thut c bn ca hai tiuchun, ch ra nhng u im ni bt ca tiu chun DVB T2. Thng qua kt qutrin khai tiu chun mi DVB T2 mt s nc trn th gii, lun vn a ramt s khuyn ngh v la chn cng ngh cho cc h thng truyn h nh s mt tti Vit Nam.

Trong iu kin truyn hnh s cha ph bin ti Vit Nam, tiu chun DVBT2 cn rt mi m trn th gii cng nh Vit Nam, do trnh v thi giannghin cu ti liu lin quan n ti cn hn ch, lun vn khng trnh khi ccthiu st nht nh, hy vng c s thng cm ca cc thy, c gio v tt cnhng ai c lun vn ny.

Xin chn thnh cm n Thy gio hng dn, cc Thy trong hi ng bo v cng, cc thy, c trong Hc vin Cng ngh Bu chnh Vin thng, KhoaQuc t v o to sau i hc, bn b, ng nghip gip tc gi hon thnhlun vn ny./.

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