Truyen Hinh So Mat Dat DVB

7/31/2019 Truyen Hinh So Mat Dat DVB

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ti: Truyn hnh s mt t DVB-T v qu trnh chuyn i sang DVB-T2.

GVHD: TS.Nguyn Hong HV: ng Vn Anh

Trang 1

M U

Vi s pht trin ca kinh t v khoa hc k thut, cc nghnh cng ngh

trong c cng ngh in t vin thng c s pht trin vt bc trong ba thp

k va qua em li nhiu thnh tu pht minh ng dng trong sn xut, trong i

sng x hi. Cng ngh truyn hnh l mt b phn quan trng trong lnh vc in

t vin thng, n c nhng ng dng rng ri to ln trong pht trin vn ha i

sng tinh thn x hi. Trong hn mt thp k qua chng ta chng kin s chuyn

i mnh m ca cng ngh truyn hnh t phng thc tng t xang cng ngh

s. Vit Nam qu trnh chuyn i ny thc s ngon mc vi s ph cp tng

bc trong lnh vc truyn hnh qung b v truyn hnh tr tin. T u nhng

nm 90 cho n nay nghnh truyn hnh ng dng cc thnh tu v cng ngh

truyn hnh s trong truyn dn v tinh, pht trin mng truyn hnh cp v ph cp

h thng truyn hnh s mt t.

Truyn hnh s c p dng u tin cng ty VTC hn 10 nm qua, s

pht trin h thng truyn hnh s ca VTC gp phn quan trng a cc thng

tin v kinh t chnh tr, vn ha th thao gii tr phong ph n ng o cngchng cc a phng vi cht lng cao.

Cng vi s pht trin ca cng ngh truyn hnh, chun truyn hnh s

DVB-T l chun pht sng truyn hnh s mt t c trin khai thnh cng,

c nhiu nc chp nhn. Tuy nhin, t sau s ra i ca chun DVB-T th cc

nghin cu v k thut truyn dn vn tip tc c trin khai . Mt khc, nhu cu

v ph tn cao cng khin cho vic gia tng hiu qu s dng ph tn ln mc ti a

cng cp thit. T pht trin ln chun truyn hnh s mt t th h th 2 lDVB-T2.

Vic nghin cu tm hiu cc c tnh cng ngh ca tiu chun truyn s

DVB-T trong qu trnh pht trin ln th h mi DVB-T2 l nhim v cn thit i

vi cc c quan nghin cu ng dng truyn hnh cng nh cn b k thut nghin


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Trang 2

cu trong lnh vc ny. l l do em chn ti: Truyn hnh s mt t DVB-T

v qu trnh chuyn i sang DVB-T2

B cc lun vn bao gm bn chng, trong chng I: gii thiu tng qut

v truyn hnh s, u im ca truyn hnh s so vi truyn hnh tng t, phn tch

cc tiu chun truyn hnh s mt t trn th gii. Tip theo, trong chng II: phn

tch nhng u im ni bt ca truyn hnh s mt t DVB-T vi k thut ghp a

tn trc giao c m (COFDM), mt k thut iu ch c rt nhiu u im v s la

chn tiu chun truyn hnh s mt t DVB-T Vit Nam. Trong chng III: trnh

by mt s ni dung chnh ca tiu chun truyn hnh s mt t DVB-T2, nhng

u im vt tri ca DVB-T2 so vi DVB-T. Chng IV: phn tch qu trnh

chuyn i truyn hnh s mt t t DVB-T sang DVB-T2 Chu u v trnh by

qu trnh pht trin truyn hnh s mt t Vit Nam, nhng kin ngh khi trin

khai truyn hnh s mt t DVB-T2 ti Vit Nam.

Sau mt thi gian tm hiu v nghin cu, c s hng dn khoa hc tn

tnh ca Thy gio TS.Nguyn Hong, lun vn c hon thnh.Do thi gian c

hn, trnh bn thn cn hn ch, thm vo lun vn ca em l vn tng

i mi nn khng th trnh khi nhng sai st. Knh mong c s ng gp ca

cc thy, cc c cng cc bn.


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Trang 3

CHNG I : TNG QUAN V TRUYN HNH S

1.1. Gii thiu v truyn hnh s

Truyn hnh s l tn gi mt h thng truyn hnh m tt c cc thit b kthut t Studio cho n cc my thu u lm vic theo nguyn l k thut s. Trong

, mt hnh nh quang hc do cammera 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.

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 btruyn 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 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(b lc l mt v d nh th). 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 truyn hnh hon ton k thut s c cht lng cao v d dng

phn phi trn knh thng tin. H truyn hnh k thut s v ang c pht

trin trn ton th gii, to nn mt cuc cch mng tht s trong cng nghip

truyn hnh.

Nguyn l cu to ca h thng v cc thit b truyn hnh s c a ranh trn hnh 1-1:


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Trang 4

Hnh 1.1. S cu trc tng qut ca h thng truyn hnh s

u vo ca thit b truyn hnh s s tip nhn tn hiu truyn hnh tng t.

Trong thit b m ho (bin i A/D), tn hiu hnh s c bin i thnh tn hiu

truyn hnh s, cc tham s v c trng ca tn hiu ny c xc nh t h thng

truyn hnh c la chn. Tn hiu truyn hnh s c a ti thit b pht. Sau

tn hiu truyn hnh s c truyn ti bn thu qua knh thng tin. Ti bn thu,

tn hiu truyn hnh s c bin i ngc li vi qu trnh x l ti pha pht.

Gii m tn hiu truyn hnh thc hin bin i tn hiu truyn hnh s thnh tn

hiu truyn hnh tng t. H thng truyn hnh s s trc tip xc nh cu trc

m ho v gii m tn hiu truyn hnh.

Ngoi ra, trc khi truyn qua knh thng tin, tn hiu truyn hnh s c m

ho knh. M ho knh m bo chng cc sai st cho tn hiu khi truyn trong

knh thng tin. Thit b m ho knh phi hp c tnh ca tn hiu s vi knh

thng tin. Khi tn hiu truyn hnh s c truyn i theo knh thng tin, cc thit

b bin i trn c gi l b iu ch v b gii iu ch.

1.2. u im ca truyn hnh s.

- Mt my pht truyn hnh s c th pht c 4 n 5 chng trnh truyn

hnh trong khi mt my pht analog nh ta ang s dng ch pht c mt

chng trnh duy nht theo h PAL(Phase Alternating Line). Xt v mt ph ta thy

tn hiu

Tn hiu truynhnh tng t

Tn hiu truynhnh s

Tn hiu truynhnh tng t

Bin iA/D

M haknh

Bin i

Knhthng tin

Bin itn hiu

Gii mha knh

Tn hiu truynhnh sBin i

D/A

Thit b pht

Thit b thu


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Trang 5

tn hiu tng t ph ch tp trung nng lng vo cc sng mang hnh, ting v

burst mu. Trong khi tn hiu s bao gm hng ngn sng mang tp trung dy c

vo trong mt di ph c rng tng ng. S tn dng ti a hiu qu ph

nh ch ra hnh 1.1 cho php truyn hnh s c th truyn pht c nhiu chng

trnh ng thi. y l u im ng k so vi truyn hnh tng t .

Hnh 1.1: Ph ca tn hiu tng t v tn hiu s

Hnh 1.2: Ph ca tn hiu tng t v tn hiu s

Ngoi ra truyn hnh s cn mt s u im khc so vi truyn hnh tngt:

- Cng sut pht khng cn qu ln v cng in trng cho thu s thp

hn cho thu analog ( nhy my thu s cao hn -30dB n -20dB so vi my thu

analog).

- 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 pht analog ang khng khc phc ni.

Hnh

Ting

Ph tn hiu s

Ph tnhiutng t

Hnh Hnh

TingTingTing

Hnh


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Trang 6

Hnh 1.3: Pht hnh DVB-T.

- Thu bng anten c nh trong nh hay anten di chuyn (ca my thu xch

tay) u thc hin c.

- Thu di ng tt, ngi xem d i trn t, tu ho vn xem c cc

chng trnh truyn hnh. S d nh vy l v c th x l tt c hin tng do

hiu ng dch tn Doppler gy ra.

- Cho kh nng thit lp mng n tn SFN (Single Frequency Network),

ngha l nhiu my pht trn cng mt knh sng. iu ny cho hiu qu ln xt v

mt cng sut v tn s.

- Pht hnh s em li cho ta c hi xem cc chng trnh vi nt cao.

Vn d th tn hiu s c tnh chng nhiu cao.

- Tn hiu s d x l, mi trng qun l iu khin v x l rt thn thinvi my tnh.

Vi cc u im ca mnh, h thng truyn hnh s c thc hin hu

ht cc quc gia trn th gii. y l mt qu trnh tt yu, Truyn hnh Vit nam

cng ang giai on chuyn tip. Hin nay qu trnh s ho tn hiu truyn hnh

Vit nam l s thay th dn cc cng on, trang thit b t tng t sang s. l

qu trnh s ho tng phn. Ri y truyn hnh s s thay th hon ton truyn

Distanttransmitter

Nearest transmitter


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Trang 7

hnh tng t, to iu kin cho ngnh cng nghip ny pht trin mnh m hn,

kt hp vi cc mng truyn thng khc, to thnh mt th gii thng tin s, phc

v cho con ngi mt cch hu hiu.

1.3. C bn v video s.

S ha tn hiu video l qu trnh bin i tn hiu truyn hnh mu tng t

thnh tn hiu s. C hai phng php bin i l:

+ Bin i trc tip tn hiu video mu tng hp

+ Bin i ring tng tn hiu video mu thnh phn

1.3.1.Tiu chun s ha tn hiu video tng hp.

Xu hng pht trin cc studio hon ton k thut s yu cu mt tiuchun chung cho cc thit b video s. Cc tiu chun video s tng hp c

xy dng hng ti mc tiu . Ph hp vi yu cu cng ngh, hai h

thng tiu chun s ho tn hiu video tng hp c pht trin rng ri.

l tiu chun 4 fSC NTSC v tiu chun 4fSC PAL.

Tn hiu video tng hp tng t c ly mu ti tn s bng 4 ln tn

s sng mang ph (4fSC). S bit biu din mu ng vai tr quan trng trong vic

xc nh cht lng nh v tnh kinh t ca thit b. Thng thng cc thit b s

dng 8 hoc 10 bit biu din mu.

a.Tiu chun 4fSC NTSC

Tiu chun 4fSCNTSC c xc nh trong SMPTE 244M gm cc tham

s ca tn hiu video tng hp theo h NTSC cng nh tnh cht kt ni song

song ca h thng. Chun SMPTE c m t trong bng 1-1.

Tn s ly mu tn hiu bng 4 ln tn s sng mang ph: flm=14,3181 MHz

(thng vit 14,3 MHz). ng b qu trnh ly mu c ly t tn hiu ng b mu

ca tn hiu video tng hp. Hnh 1-4 minh ho ph tn hiu NTSC ly mu ti tn s

4fSC


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Trang 8

Bng 1-1. Tiu chun 4 fSCNTSC

Tn hiu vo NTSC

S mu trn mt dng 910S mu trn mt dng tch cc 768

Tn s ly mu 4fmp=14,32818 MHz

Cu trc ly mu Trc giao

Khong cch ly mu +330,+1230,+2130,+3030,

M ho Lng t ho u

Thang lng t 8 hoc 10 bit

Trong ph tn hiu, c mt khong cch h gia cc tn s 4,2 MHz - tn

s ln nht ca NTSC c bn, v tn s 7,16 MHz (tn s Nyquist). Vi khong

h ny, cc b lc chng nhiu v b lc ti to c c tuyn khng l tng

c php s dng. Tiu chun khng xc nh tnh cht ca cc b lc chng

nhiu v b lc ti to, v vy cc nh sn xut c th t do la chn cc b lc,

phc v cho mc ch kinh t v k thut.

fSC

3,58MHz

2fSC7,16MHz

Tn s Nyquist

3fSC10,7MHz

4fSC14,3MHz

5fSC

Tn s lymu

Di thng mc

danh nh

f

Di thng caoc th chp nhn

Hnh 1.4: Ph ca tn hiu ly mu chun 4fSCNTSC.

A


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Trang 9

b. Tiu chun 4FSCPAL.

Tn hiu tng t PAL v NTSC c nhiu im tng ng, song tn hiu

s theo chun 4fSC ca hai h thng ny rt khc nhau. Bng 1-2 tng kt cim ca tn hiu s h 4fSC PAL.

Tn s ly mu bng 4 ln tn s sng mang ph hay 17,734475 MHz

(thng vit 17,73 MHz). Tn hiu xung clock c ly t xung ng b mu ca

tn hiu video tng hp.

Bng 1-2. Tiu chun 4 fSCPAL

Tn hiu vo PAL

S mu trn mt dng 1135

S mu trn mt dng tch cc 948

Tn s ly mu 4fSC = 17,734475 MHz

Khong cch ly mu +450,+1350,+2250,+3150,

M ho Lng t ho u

Thang lng t 8 hoc 10 bit

fSC

4,43 MHz

2fSC8,86 MHz

Tn s Nyquist

3fSC13,29 MHz

4fSC17,7 MHz

5fSC

Tns ly mu

Di thng mc

danh nh

f

Di thng caoc th chp nhn

Hnh 1-5. Ph ca tn hiu ly mu chun 4fsc NTSC.

A


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Trang 10

Hnh 1-5 minh ho ph ly mu h 4fSC PAL. y c mt khong cch

gia cc tn s 5 MHz, tn s cao nht ca h PAL c bn, v tn s 8,86 MHz (tn

s Nyquist). Tiu chun khng xc nh tnh cht ca cc b lc chng nhiu v b

lc ti to, song cc nh sn xut thng s dng ti tn s cao. Cng nh vi tn

hiu 4fSCNTSC, cn ch n vn vt qu mc tn hiu trong qu trnh chuyn

i sang tn hiu s khi a trc tip vo cc thit b s 4fSC, theo to cc yu cu

nghim ngt v gii hn di thng ca b lc ti to. khc phc, ra v sn

trc xung xo ca tn hiu s cn ph hp vi tn hiu tng t.

1.3.2.Tiu chun ly mu tn hiu video thnh phn

C nhiu tiu chun ly mu tn hiu video s thnh phn, im khc nhauch yu t l gia tn s ly mu v phng php ly mu tn hiu chi v cc tn

hiu mu, trong bao gm: Tiu chun 4:4:4; 4:2:2; 4:2:0; 4:1:1. Cc nh dng s

video c nn ch ly mu cho cc dng tch cc ca video. nm c ngha

ca cc chun ly mu ta i tm hiu cc phng thc ca tng chun.

a) Tiu chun 4:4:4

Mu tn hiu ch c

ly i vi cc phn t tch ccca tn hiu video. Vi h PAL,

mn hnh c chia lm

625x720 im (pixel).

Cc tn hiu chi (UY), tn

hiu hiu mu (CR, CB) c ly

mu ti tt c cc im ly mu trn dng tch cc ca tn hiu video. Cu trc ly mu

l cu trc trc giao, v tr ly mu nh hnh 1-5.

Theo tiu chun 4:4:4 c kh nng khi cht phc lng hnh nh tt, thun

tin cho vic x l tn hiu. Cc t chc tiu chun quc t thng nht v ch tiu

tn s ly mu cho truyn hnh s theo tiu chun ny - vi tn gi l CCIR - 601.

Vi chun 4:4:4 tc dng d liu (v d cho h PAL) c tnh nh sau:

im ly mu tn hiu chi UY

im ly mu mu CR

im ly mu mu lam CB

Hnh 1-6: Tiu chun 4:4:4.


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Trang 11

Khi ly mu 8 bit: (720+720+720) x 576 x 8 x 25 = 249 Mbit/s.


b) Tiu chun 4:2:2Theo hnh 1-7, trn mt dng

tch cc: im u ly mu ton b

ba tn hiu: chi (UY) v hiu mu

(CR, CB). im k tip ch ly mu

tn hiu UY, cn hai tn hiu hiu

mu khng ly mu. Khi gii m

mu suy ra t mu ca im nh

trc. im sau na li ly mu

c ba tn hiu UY, CR, CB. Tun t nh th, c 4 ln ly mu UY, th c hai ln ly

mu CR, hai ln ly mu CBto nn c cu 4:2:2.

i vi h PAL tc dng d liu theo chun ny c tnh nh sau:


Khi ly mu 10 bit: (720+360+360) x 576 x 10 x 25 = 207 Mbit/s

c) Tiu chun 4:2:0Theo hnh 1-8, ly mu tn

hiu UY ti tt c cc im nh

ca dng, cn tn hiu mu th c

cch mt im s ly mu cho

mt tn hiu mu. Tn hiu mu

c ly xen k, nu hng chn

ly mu cho tn hiu mu CR thhng l s ly mu cho tn hiu CB.

i vi h PAL tc dng d liu theo chun ny c tnh nh sau:

Khi ly mu 8 bit: (720+360) x 576 x 8 x 25 = 124,4 Mbit/s.

Khi ly mu 10 bit: (720+360) x 576 x 10 x 25 = 155,5 Mbit/s.


im ly mu mu CR

im ly mu mu lam CB

Hnh 1-7:Tiu chun 4:2:2.


im ly mu mu CR

im ly mu mu lam CB



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Trang 12

d) Tiu chun 4:1:1

Chun 4:1:1 trnh t ly mu nh hnh 1-9. Trong im nh u ly mu

UY, CR, CB; ba im nh tip sau ch ly mu UY, khng ly mu ca tn hiu CR, CB.

Khi gii m, mu ca ba im nh sau phi suy t im nh u. Tun t nh th,

c bn ln ly mu UY, c mt ln ly mu CR, mt ln ly mu CB, y l c cu

4:1:1. i vi h PAL tc dng d liu theo chun ny c tnh nh sau:

Khi ly mu 8 bit: (720+180+180) x 576 x 8 x 25 = 124,4 Mbit/s.

Khi ly mu 10 bit: (720+180+180) x 576 x 10 x 25 = 155,5 Mbit/s.

S "4" u mi chun biu

th tn s ly mu tn hiu chi (flm

=

13,5 MHz), tuy khng cn bng bn

ln tn s sng mang nh trc

(4fSC). Cc con s khc biu th t l

gia tn s ly mu tn hiu hiu mu

so vi tn hiu chi. 13,5 MHz l tn

s duy nht trong khong t 12 MHz n 14 MHz c gi tr bng mt s nguyn ln

tn s dng cho c hai tiu chun (525 v 625) v do vy cho mt s nguyn ln s

mu i vi c hai h.

Vi tn s ly mu 13,5 MHz, tn hiu video s khng cn b ph thuc

vo cc tiu chun khc nhau ca video tng t. Thit b trong cc trung tm

truyn hnh s s hon ton ging nhau cho c hai h thng, iu ny s to thun

li cho vic hp tc sn xut, trao i chng trnh gia cc t chc truyn hnh .

1.4. Nn tn hiu trong truyn hnh s:

1.4.1 Mc ch ca nn:Vi cng ngh hin nay, cc thit b u c di thng nht nh. Cc dng s

tc cao yu cu di thng rt rng vt qu kh nng cho php ca thit b. Mt

cch s b, nn l qu trnh lm gim tc bit ca cc dng d liu tc cao m

vn m bo cht lng hnh nh hoc m thanh cn truyn ti.

im ly mu tn hiu chi Y

im ly mu mu CR

im ly mu mu lam CB



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Trang 13

1.4.2 Bn cht ca nn:

Khc vi ngun d liu mt chiu nh ngun m, c tuyn a chiu ca

ngun hnh nh cho thy : ngun nh cha nhiu s d tha hn cc ngun thngtin khc. l :

- S d tha v mt khng gian (Spatial redundancy): Cc im nh k nhau

trong mt mnh c ni dung gn ging nhau.

- S d tha v mt thi gian (temporal redundancy): Cc im nh c cng

v tr cc mnh k nhau rt ging nhau.

- S d tha v mt cm nhn ca con ngi: Mt ngi nhy cm hn vi

cc thnh phn tn s thp v t nhy cm vi thay i nhanh, tn s cao.

Do vy, c th coi ngun hnh nh l ngun c nh (memory source)

Nn nh thc cht l qu trnh s dng cc php bin i loi b i cc s

d tha v loi b tnh c nh ca ngun d liu, to ra ngun d liu mi c lng

thng tin nh hn. ng thi s dng cc dng m ho c kh nng tn dng xc

sut xut hin ca cc mu sao cho s lng bit s dng m ho mt lng thng

tin nht nh l nh nht m vn m bo cht lng theo yu cu .

Nhn chung qu trnh nn v gii nn mt cch n gin nh sau :

Bin i M hoD liu nn

Gii mBin i

ngc

D liuD liu nn

Qu trnh gii nn

Qu trnh nn

D liu

Hnh 1.10 : S khi qu trnh nn v gii nn


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Trang 14

* Bin i: Mt s php bin i v k thut c s dng loi b tnh

c nh ca ngun d liu ban u, to ra mt ngun d liu mi tng ng cha

lng thng tin t hn. V d nh k thut to sai s d bo trong cng ngh DPCM

hay php bin i cosin ri rc ca cng ngh m ho chuyn i. Cc php bin

i phi c tnh thun nghch c th khi phc tn hiu ban u nh php bin

i ngc.

* M ho:

Cc dng m ho c la chn sao cho c th tn dng c xc sut xut

hin ca mu. Thng thng s dng m RLC (run length coding: m ho lot di)

v m VLC ( variable length coding): gn cho mu c xc sut xut hin cao t m

c di ngn sao cho cha ng mt khi lng thng tin nhiu nht vi s bit

truyn ti t nht m vn m bo cht lng yu cu.

1.4.3 : Phn loi nn:

Cc thut ton nn c th phn lm hai loi: Nn khng tn tht (lossless

compression) v nn c tn tht (lossy compression).

* Thut ton nn khng tn tht khng lm suy gim, tn hao d liu. Do

vy nh khi phc hon ton chnh xc vi nh ngun.

* Cc thut ton nn c tn tht chp nhn loi b mt s thng tin khng

quan trng nh cc thng tin khng qu nhy cm vi cm nhn ca con ngi

t c hiu sut nn cao hn, do vy nh khi phc ch rt gn ch khng phi l

nh nguyn thu.

* i vi nn c tn tht, cht lng nh l mt yu t v cng quan trng,

tu theo yu cu ng dng m cc mc loi b khc nhau c s dng, cho

mc cht lng theo yu cu.

1.5. Truyn dn tn hiu truyn hnh s:Vic s dng k thut s truyn tn hiu Video i hi phi xc nh tiu

chun s ca tn hiu truyn hnh, phng php truyn hnh c cht lng nh

thu khng km hn cht lng nh trong truyn hnh tng t.

C th s dng cc phng thc truyn dn sau cho tn hiu truyn hnh s


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Trang 15

1.5.1. Truyn qua cp ng trc:

truyn tn hiu video s c th s dng cp ng trc cao tn. Knh c

th c nhiu lm nh hng n cht lng truyn v sai s truyn. V d nhiu

nhit.

Ngc li, nhiu tuyn tnh ca knh s khng xy ra trong trng hp

truyn s vi cc thng s ti hn.

t c cht lng truyn hnh cao, cp c chiu di 2500km cn m

bo mc li trn on trung chuyn 10-11 10-10 .

rng knh dng cho tn hiu video bng khong 3/5 tc bit ca tn

hiu. rng knh ph thuc vo phng php m ho v phng php ghp knh

theo thi gian cho cc tn hiu cn truyn v rng hn nhiu so vi rng knh

truyn tn hiu truyn hnh tng t.

1.5.2. Truyn tn hiu truyn hnh s bng cp quang:

Cp quang nhiu u im trong vic truyn dn tn hiu s so vi cp ng

trc :

+ Bng tn rng cho php truyn cc tn hiu s c tc cao

+ suy hao thp trn mt n v chiu di

+ Suy gim gia cc si quang dn cao (80dB)+ Thi gian tr qua cp quang thp

Mun truyn tn hiu video bng cp quang phi s dng m truyn thch

hp. pht hin c li truyn ngi ta s dng thm cc bt kim tra chn. M

sa sai thc t khng s dng trong cp quang v suy gim ng truyn nh

hn 20dB, li xut hin nh v c th b qua c.

1.5.3. Truyn tn hiu truyn hnh s qua v tinh :

Knh v tinh khc vi knh cp v knh pht sng trn mt t l c bngtn rng v s hn ch cng sut pht. Khuch i cng sut ca cc Transponder

lm vic gn nh bo ho trong cc iu kin phi tuyn. Do s dng iu ch

QPSK l ti u. Cc h thng truyn qua v tinh thng cng tc di tn s c

Ghz. V d : Bng Ku : ng ln : 14 15GHz


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ng xung : 11,7 12,5 GHz

1.5.4. Pht sng truyn hnh s trn mt t:

H thng pht sng truyn hnh s mt t s dng phng php iu ch

COFDM (ghp knh theo tn s m trc giao). COFDM l h thng c kh nng

chng nhiu cao v c th khc phc hiu ng bng ma, cho php bo v pht sng

s trc nh hng ca can nhiu v cc knh ln cn.

H thng COFDM hot ng theo nguyn tc iu ch dng d liu bng

nhiu sng mang trc giao vi nhau. Do mi sng mang iu ch vi mt dng

s liu.

Cc tn hiu s liu c iu ch M-QAM, c th dng 16-QAM hoc 64-

QAM. Ph cc sng mang iu ch c dng sinx/x trc giao. C ngha cc sng

mang k nhau c gi tr cc i ti cc im 0 ca sng mang trc v sau iu ch

v gii iu ch cc sng mang thc hin nh b bin i Fourier nhanh FFT di

dng FFT 2K v FFT 8K, Vi loi vi mch trn c th thit k cho hot ng ca

6785 sng mang. Cc hng RACE c thit b pht sng truyn hnh cho 896 sng

mang, hng NTL cho 2000 sng mang.

1.6. Cc tiu chun truyn hnh s mt t hin nay trn th gii.

u nm 1999, cc h thng ATSC v DVB c ITU chp nhn lm cc

tiu chun quc t v pht sng truyn hnh s trn mt t (DTTB) v pht hnh

cc khuyn co ITU-R.Rec ca nhm nghin cu SG10&11.

Mong mun l c mt tiu chun thng nht, tuy nhin do nhiu l do m hin

nay trn th gii c 3 tiu chun v truyn hnh s .

Chu u, Australia, New Zealand,.... chp nhn DVB-T, cn Hn Quc,

i loan, Canada v M... chn ATSC, Nht bn v mt s nc khc chn tiu

chun ISDB-T. C th tham kho s la chn cc tiu chun truyn hnh s trn th

gii. cng l yu t gip ta nh hng vic nghin cu, vic la chn tiu

chun ph hp cho ring mnh.


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Hnh 1.11: Bn phn b cc nc trn th gii la chn tiu chun DVB-T.

1.6.1 Chun ATSC

* c im chung :

H thng ATSC c cu trc dng lp, tng thch vi m hnh OSI 7 lp ca

cc mng d liu. Mi lp ATSC c th tng thch vi cc ng dng khc cng

lp. ATSC s dng dng thc gi MPEG-2 cho c Video, Audio v d liu ph.

Cc n v d liu c di c nh ph hp vi sa li, ghp dng chng trnh,

chuyn mch, ng b, nng cao tnh linh hot v tng thch vi dng thc ATM.

Tc bt truyn ti 20 Mbps cp cho mt knh n HDTV hoc mt knh

truyn hnh chun a chng trnh.

Chun ATSC cung cp cho c hai mc: truyn hnh phn gii cao (HDTV) vtruyn hnh tiu chun (SDTV). c tnh truyn ti v nn d liu ca ATSC l theo

MPEG-2.

ATSC c mt s c im nh sau:


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Phng php iu ch VSB ca tiu chun ATSC. Phng php iu ch VSB

bao gm hai loi chnh: Mt loi dnh cho pht sng mt t (8-VSB) v mt loi

dnh cho truyn d liu qua cp tc cao (16-VSB). C hai u s dng m Reed

- Solomon, tn hiu pilot v ng b tng on d liu. Tc biu trng (Symbol

Rate) cho c hai u bng 10,76Mb/s. N c gii hn t s tn hiu trn nhiu

(SNR) l 14,9dB v tc d liu bng 19,3 Mb/s.

D liu c truyn theo tng khung d liu. Khung d liu bt u bng

on d liu ng b mnh u tin v ni tip bi 312 on d liu khc. Sau

n on d liu ng b mnh th 2 v 312 on d liu ca mnh sau.

Mi on d liu bao gm 4 biu trng dnh cho ng b on d liu v 828biu trng d liu.

Mt gi truyn ti MPEG-2 cha 188 byte d liu v 20 byte tng suy cho

208 byte. Vi t l m ho 2/3, u ra ca m sa sai ta c:

Tham s c tnh

Video

Nhiu dng thc nh (nhiu phn gii khc nhau). Nn

nh theo MPEG-2, t MP@ML ti MP@HL.

Audio m thanh Surround ca h thng Dolby AC-3.

D liu phCho cc dch v m rng (v d hng dn chng trnh,

thng tin h thng, d liu truyn ti ti computer).

Truyn tiDng ng gi truyn ti a chng trnh. Th tc truyn ti

MPEG-2.

Truyn dn RF iu ch 8-VSB cho truyn dn truyn hnh s mt t

Bng 1.3: c im c bn ca ATSC


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208 x 3/2 = 312 bytes.

312 bytes x 8 bit = 2496 bit.

Tm li mt on d liu cha 2496 bit.

Cc biu trng s c iu ch theo phng thc nn sng mang v hu

ht di bin di iu bin ct. Tn hiu pilot c s dng phc hi sng mang

ti u thu, c cng thm ti v tr 350 KHz pha trn gii hn di di tn.

n g b m n h s 1

D l i u

n g b m n h s 2

D l i u

8 2 8 b i u t r n g

7 7 , 7 s

4 6 , 8 s

3 1 2 o n d l i u

1.6.2.Chun ISDB-T

H thng chuyn dng cho pht thanh truyn hnh s mt t c hip hi

ARIB a ra v c hi ng cng ngh vin thng ca B thng tin bu in

(MPT) thng qua nh mt bn d tho tiu chun cui cng Nht Bn.

Bn thng s k thut di m t chi tit h thng truyn hnh s mt t s

dng mng a dch v (ISDB-T). H thng ny c th truyn dn cc chng trnh

truyn hnh, m thanh hoc d liu tng hp.

ISDB-T s dng tiu chun m ho MPEG-2 trong qu trnh nn v ghp

knh. H thng s dng phng php ghp a tn trc giao OFDM cho php truyn

a chng trnh phc tp vi cc iu kin thu khc nhau, truyn dn phn cp, thu

Hnh 1.12: Khung d liu VSB


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di ng v.v... cc sng mang thnh phn c iu ch QPSK, DQPSK, 16QAM

hoc 64QAM. Chun ISDB-T c th s dng cho cc knh truyn c rng 6, 7

hay 8Mhz.

Kiu Kiu 1 Kiu 2 Kiu 3

S on d liu Ns. 13

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 sngmang.

QPSK, 16QAM, 64QAM, DQPSK

S biu trng trong mt

khung.204

Khong thi gian tch cc

trong mt biu trng

(S).

189 378 765

1/4 47.25 94.5 189

1/8 23.625 47.25 94.5

1/16 11.8125 23.625 47.25

Khongbo v

(S).1/32 5.90625 11.8125 23.625

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

M ho ngoi M Reed Solomon (204, 188)

Bng 1.4: Cc thng s truyn dn ISDB-T cho knh truyn 8 Mhz

1.6.3. Chun DVB

DVB (Digital Video Broadcasting) l mt t chc gm trn 200 thnh vin ca

hn 30 nc nhm pht trin k thut pht s trong ton Chu u v cho cc khu

vc khc. T chc DVB phn ra nhiu phn ban, trong c cc phn ban chnh:


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DVB-S - Pht trin k thut truyn s qua v tinh: H thng DVB -S s dng

phng php iu ch QPSK (Quadratue Phase - Shift Keying), mi sng mang cho

mt b pht p. Tc bit truyn ti ti a khong 38,1Mbps. B rng bng thng

mi b pht p t 36 n 54 Mhz.

DVB-C - Pht trin pht s qua cp: S dng cc knh cp c rng bng

thng t 7 n 8 MHz v phng php iu ch 64QAM (64 Quadratue Amplitude

Modulation). DVB-C c mc SNR (t s Signal/Noise) cao v iu bin k sinh

(Intermodulation) thp. Tc bit lp truyn ti MPEG-2 ti a l 38,1 Mbps.

DVB-T - Pht trin mng pht hnh s mt t: Vi vic pht minh ra iu ch

ghp a tn trc giao (COFDM) s dng cho pht thanh s (DAB) v pht hnh smt t (DVB), rt nhiu nc s dng phng thc ny. Tc bit ti a 27,14

Mbps (ng vi di thng cao tn 8Mhz).

1.7. Kt lun chng I.

Trong nhiu nm tr li y, truyn hnh s tr thnh i tng nghin

cu ca nhiu nh khoa hc v nhiu t chc trn th gii . Cng vi s tin b

vt bc ca cng ngh ch to cc vi mch t hp cao, cng ngh cao, tc cao ,

p ng yu cu lm vic vi thi gian thc, cng ngh truyn hnh c nhngtin b vt bc. Truyn hnh s mt t c nhng u im vt tri so vi truyn

hnh tng t nh s dng mt my pht c kh nng truyn ti c t 6 n 8

chng trnh ng thi; vi cng mt vng ph sng th cng sut pht yu cu ca

my pht s s nh hn t 5 n 10 ln so vi my pht tng t, iu ny gip cho

vic tit kim ng t v chi ph vn hnh; mt iu ng c quan tm na l

cht lng chng trnh trung thc, t b nh hng bi nhiu ng truyn , trnh

c hin tng bng hnh thng gp truyn hnh tng t.ng dng pht hnh s Vit Nam l nhm cc mc ch:

- Tin kp cc nc tin tin v cc nc xung quanh trn lnh vc thng tin i

chng ni chung v truyn hnh ni ring.


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CHNG II:

TRUYN HNH S MT T THEO TIU CHUN CHU U (DVB-T)

VI K THUT GHP A TN TRC GIAO C M (COFDM)2.1. Tiu chun truyn hnh s mt t ETSI EN 300744

Tiu chun pht thanh truyn hnh s mt t ETSI EN 300744 c U ban

k thut pht thanh truyn hnh Chu u JTC nghin cu v xut. Tiu chun

ny c D n truyn hnh s Chu u (DVB project) thng qua ngy 11 thng

6 nm 1999, cng b v ngy 30 thng 9 nm 1999. Thnh lp thng 9 nm 1993,

n nay DVB c hn 200 thnh vin thuc 30 nc trn th gii, nhim v ca

n l thit lp mi trng dch v truyn hnh s s dng tiu chun nn MPEG-2.

JTC c thnh lp nm 1990, l mt t chc kt hp ca U ban pht thanh

truyn hnh Chu u (EBU), U ban tiu chun k thut in t Chu u

(CENELEC) v Vin tiu chun vin thng Chu u (ETSI).

2.1.1 Phm vi ca tiu chun

Tiu chun ny m t h thng truyn dn cho truyn hnh s mt t. N

xc nh h thng iu ch, m ho knh dng cho cc dch v truyn hnh s mtt nhiu chng trnh nh: LDTV/SDTV/EDTV/HDTV.

- Tiu chun m t chung h thng c bn ca truyn hnh s mt t.

- Tiu chun xc nh cc yu cu ch tiu chung, v cc c im ca h

thng c bn, mc ch t c cht lng dch v.

- Tiu chun xc nh tn hiu c iu ch s cho php vic tng thch

gia cc phn thit b c sn xut bi cc nh sn xut khc nhau. t c iu

ny bng cch m t chi tit tn hiu x l pha cc module, trong khi th vic

x l cc my thu l m cho cc gii php thc hin khc nhau.


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2.1.2 Ni dung chnh ca tiu chun

- H 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 knh(Multiplexer), u ra l tn hiu RF i ti anten.

H thng tng thch trc tip 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 my

pht tng t hot ng cng knh hoc knh lin k, i hi h thng phi c hiu

sut s dng ph tn cao bng tn UHF, iu ny c th m bo bng vic s dngmng n tn (SFN).

Hnh 2.1 l s khi ca mt h thng pht hnh s mt t. Cc tn hiu

hnh nh, m thanh s qua mt lot qu trnh x l cui cng ti u ra anten

cng vn l tn hiu cao tn pht i nhng nhng tnh nng u vit ca truyn hnh

s mt t li hon ton c th hin trong cc qu trnh x l ny. Cc khi nt

t trn hnh s c khi cu hnh h thng dng cho iu ch phn cp. Khi Splitter

(b tch)phn chia dng d liu thnh 2 lung vi nhng mc u tin khc nhau,tc bit v t l m ha khc nhau, c ngha l kh nng chng li ca tng dng

bit l khc nhau. S chung ta thy mt h thng my pht ch yu s gm phn

iu ch OFDM v phn m ho sa li. C th chc nng ca cc khi nh sau:


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Hnh 2.1. S khi chc nng h thng pht hnh s mt t.

Ghp knhchng trnh

Ghp knhTruyn ti

M ha ngun v ghpKnh MPEEG-2

M ha video

M ha video

M ha video

MUX

MUX

Btch

Ghpknhtngthchv trinnglng

MhangoiR-S

Trongoi

Mtrong

Trotrong

nhxvdiuch

Tokhung

MhangoiR-S

OFDM

Chnkhongbo v

D/A ucui

Ghpknhtngthchv tri

nnglng

MhangoiR-S

Trongoi

Mtrong

Tokhung


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a. Phn 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 s

ha nh khi ghp knh v m ha nn MPEG-2. u ra ca khi ny s l dngtruyn truyn ti MPEG-2 (dng d liu s) vi mt tc bit nht nh a vo

my pht. y l qu trnh s ha tn hiu.

b. Khi m ha phn tn nng lng v phi hp ghp knh.

m bo cho vic truyn dn khng c li, dng d liu TS n t khi

nn s c ngu nhin ho. Cc gi d liu ny u tin c nhn dng bi chui

gi ngu nhin PRBS. Mc ch ca qu trnh ny l phn tn nng lng trong ph

tn hiu s v xc nh s nh phn thch hp (loi b cc chui di 0 v 1),ng thi y cng c xem l qu trnh phi hp ghp knh truyn ti.

c. Khi m ngoi v ghp xen ngoi (Outer encoder and interleaver)

Dng d liu sau khi c ngu nhin ha s tip tc c x l ti khi

m 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 ra

cc gi d liu c bo v li. Do c m ho theo m RS (204,188, t=8) nn

mi gi d liu s c thm 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 ph thuc

thng k ca li.

d. Khi m ni (inner encoder)

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...). Ngha l c m bt

truyn i th ch c n bit mang thng tin, cc bit cn li l sa li.


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e. Khi ghp xen ni (inner interleaver)

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

vi ban u. Qu trnh ny gim thiu li n mc ti a.f. Cc khi iu ch tn hiu (Mapper, Frame Adaptation, OFDM...)

y l qu trnh x l phc tp nht, nhng v nguyn l ta c th hiu nh

sau: d liu sau khi hon thnh m sa li s c nh x ln chm sao iu ch

(khi mapper), v sau khi thm cc pilot ng b, cc d liu s c a ln cc

sng mang. V tt nhin l c rt nhiu sng mang. Vic chn thm cc khong bo

v cng s c thc hin nhm ti u ho tnh u vit ca truyn hnh s.

g. Khi D/A:

Thc ra y khng phi l bin i Digital/Analog thun tu thng thng.

M l qu trnh hon chnh hng ngn sng mang m bo vic pht tn hiu

ln anten.

H thng DVB-T c th hot ng trong bng tn 8Mhz, 7Mhz v 6Mhz, ch yu

ch khc nhau tn s clock ca h thng v mt s thng s lin quan n tn s

clock s phi tnh li. S cu trc, cc nguyn tc m, s xp xp, ghp xen c

gi nguyn, ch c tc thng tin ca h thng s gim theo h s 7/8 hoc 6/8.

- Truyn hnh s mt t s dng nguyn l ghp a tn trc giao c m

(COFDM). Ghp a tn trc giao (OFDM) c thc hin tip ni theo sau qu

trnh m ho knh (Channel Coding).

- Ghp knh phn chia tn s (FDM) l c s ca ghp a tn trc giao

OFDM. Dng truyn ti ni tip MPEG-2 u vo c chuyn i thnh n dng

bt song song, vi n ph hp vi s lng sng mang. Nhng dng bt song songny s c nh x ln nhng sng mang ring r, nhng sng mang ring r c

ghp trc giao, k thut ny cho php truyn ng thi a sng mang trn knh

truyn m cc sng mang k cn khng gy can nhiu sang nhau. Nhng sng mang

ring r c iu ch QPSK, 16 QAM hoc 64 QAM.


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- M ho knh cn thit cho vic truyn ti d liu nhm chng li sai trn

ng truyn do tc ng ca nhiu. M ho knh gm hai phn chnh: khi m

ngoi (Outer Coder) nhm kim sot sa lot li sai xy ra c chiu di xc nh,

khi m ho trong (Inner Coder) nhm kim sot sa v bo li cho mt lot li sai

c chiu di ln hn chiu di li quy nh.

- M ngoi s dng m Reed-Solomon RS(188,204), ghp xen ngoi (Outer

Interleave) c chiu su l=12 bytes, ging nh truyn hnh v tinh v truyn hnh

cp. M trong s dng m vng xon ging nh truyn hnh v tinh vi cc t l m

ho khc nhau.

- Khong bo v mm do cho php thit k h thng vi nhiu cu hnhkhc nhau nh: mng n tn din rng hoc my pht n l, m bo vic s

dng ti a bng tn.

- thch ng vi cc tc truyn dn khc nhau, k thut OFDM c hai

chn la v s lng sng mang, ba s iu ch QPSK, 16 QAM v 64 QAM v

khong bo v khc nhau cho php lm vic vi mng n tn nh v ln.

- Trong mt iu kin xc nh, vic thu chng trnh truyn hnh t mt s

my pht hot ng trn cng mt tn s l rt c li, tt nhin, cc my pht nyu truyn ti mt chng trnh truyn hnh c ng b cht ch to nn mng

n tn.

- H thng cng cho php hai mc m knh v iu ch phn cp. Trong

trng hp ny s khi ca h thng c thm phn cc khi v t nt nh trn

hnh v 2.1. B chia chia dng tn hiu u vo thnh hai dng tn hiu MPEG c

lp: dng tn hiu c u tin cao v dng tn hiu c u tin thp. Hai dng

bt ny c phn b trn biu chm sao bi khi Mapper.

- m bo c th dng my thu n gin thu c tn hiu phn cp ny,

h thng ch s dng m knh v iu ch phn cp m khng dng m ngun phn

cp. Theo phng thc ny, mt chng trnh truyn hnh c truyn ng thi


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di hai dng: tc bt thp vi phn gii thp v dng th hai l tc bt cao

vi phn gii cao hn. Trong c hai trng hp, my thu ch cn mt b bao

gm cc khi vi chc nng ngc li vi my pht: khi gii ghp xen trong, gii

m ho trong, gii ghp xen ngoi, gii m ho ngoi v gii ghp knh. My thu

ch cn c thm chc nng phn tch dng bt c chn t s phn b trong biu

sao.

- iu ch phn cp cho php truyn song song cc chng trnh khc nhau

vi mc sa li khc nhau v vng ph sng khc nhau.

- trnh nhiu do sng phn x hoc do cc my pht lin k trong mng

n tn, khong bo v c a xen vo gia cc symbol lin tip ca OFDM.Nu khng, sng phn x s gy nhiu ln cc symbol nm pha sau v lm tng t

s li. Nh vy, di khong bo v s ph thuc vo ln ca vng ph sng.

Hay ni cch khc, khong cch gia cc i pht k cn s quyt nh di ca

khong bo v. V d, vi mng n tn ln, khong bo v phi t nht l 200s.

- C 2 phng n v s lng sng mang. Khong cch tt nht l 896s i

vi 8k-mode v 224s i vi 2k-mode. Tng ng vi 2 phng n v s lng

sng mang, khong cch gia cc sng mang s l 1116 Hz v 4464 Hz.

- i vi h thng DVB-T s dng rng bng tn 8MHz, iu ny quyt

nh s lng c th ca sng mang: 6817 sng mang cho OFDM symbol i vi

8k-mode (6048 sng mang dng truyn thng tin, cn li dng truyn ngb

v tn hiu khc) v 1705 sng mang cho OFDM symbol i vi 2k-mode (1512

sng mang dng truyn thng tin). Cc OFDM symbols c tnh ton bng

phng php bin i Fourier ngc (IDFT).

2.2 Thc hin bng cch s dng COFDM :

p ng cc yu cu cng nh tnh nng ca truyn hnh s mt t,

nhm nghin cu ca DVB-T a ra mt phng thc iu ch mi COFDM .


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Tnh u vit cng nh l do ti sao dng phng thc ny s c trnh by trong

phn sau, y l nguyn l ct li ca mt h thng DVB-T.

2.3 Ghp a tn trc giao OFDM2.3.1 Nguyn l OFDM:

- COFDM l mt phng thc ghp knh a sng mang trc giao trong

vn s dng cc hnh thc iu ch s c s ti mi sng mang. Tuy nhin ta c th

gi l phng thc iu ch COFDM. Phng thc ny rt ph hp cho nhng yu

cu ca pht hnh mt t.

- COFDM ph hp vi iu kin truyn sng nhiu ng, thm ch c khi

c tr ln gia cc tn hiu thu c. Chnh iu ny dn n khi nim mng

n tn (SFN), ni c nhiu my pht cng gi tn hiu ging nhau trn cng mt

tn s, m thc ra y chnh l hiu ng "nhiu ng nhn to". COFDM cng

gii quyt c vn nhiu ng knh di hp. y l hin tng thng thy

trong cc dch v tng t do cc sng mang gy ra.

- Chnh nh cc u im trn m COFDM c chn cho hai tiu chun

pht sng l DVB-T v DAB, v tu theo tng ng dng ca tng loi m c nhng

la chn cng nh yu cu khc nhau. Tuy nhin u th c bit ca COFDM v

hin tng nhiu ng v nhiu ch t c khi c s la chn tham s cn thn

v quan tm n cch thc s dng m sa li.

- tng u tin ca COFDM xut pht t khi xem xt s suy yu xy ra

trong pht sng cc knh mt t. p ng ca knh khng tng ng vi tng di

tn nh do c nhiu tn hiu nhn c (tn hiu chnh + tn hiu echo), ngha l s

khng cn nng lng thu hoc s thu c nhiu hn mt tn hiu. gii

quyt vn ny th c ch u tin l phi phn tch lung d liu truyn ti

trn mt s lng ln cc di tn s nh cch bit nhau, ngha l iu ch d liu

ln mt s lng ln sng mang da trn k thut FDM. V c th xy dng li

c nhng d liu mt bn thu th cn m ha d liu trc khi pht. Do c


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mt s c im ch cht sau y gip cho COFDM rt ph hp cho cc knh

mt t, l:

Cc sng mang trc giao - orthogonality (COFDM). Chn thm cc khong bo v - guard interval.

S dng m sa li (COFDM), xen bit - symbol v thng tin

trng thi knh..

Phn ny chng ta s cng gii thch cc c im ny cng nh ngha ca chng.

2.3.2 S lng sng mang

- Gi thit rng chng ta iu ch cc thng tin s cho mt sng mang. Trong

mi symbol, chng ta truyn sng mang vi bin v pha xc nh. Bin v

pha ny la chn theo chm sao iu ch. Mi symbol vn chuyn mt lng bt

thng tin nht nh, lng bit ny bng vi loga (c s 2) ca s trng thi khc

nhau trong chm sao.

- By gi hy tng tng l c hai ng tn hiu nhn c vi mt tr

tng i gia chng. Gi s ta xem xt symbol th n c pht i, th my thu s

c gng gii iu ch d liu bng cch kim tra tt c thng tin nhn c lin

quan n symbol th n k c thng tin thu trc tip ln thng tin thu c do tr.

- Khi khong tr ln hn mt chu k symbol (xem hnh 2.2- tri), th tn hiu

thu c t ng th hai s ch thun tu l nhiu, v n mang thng tin thuc v

cc symbol trc . Cn nhiu gia cc symbol(ISI) ng rng ch c mt cht t

tn hiu tr nh hng vo chu k symbol mong mun (mc chnh xc tu thuc

vo chm sao s dng v mc suy hao c th chp nhn).

- Khi khong tr nh hn mt chu k symbol (hnh 2.2- phi) th ch mt

phn tn hiu thu c t ng th hai oc xem nh l nhiu v n mang thng

tin ca symbol trc . Phn cn li s mang thng tin ca chnh symbol mong

mun, tuy nhin s ng gp ca n cng c th c ch hoc c th mang tnh tiu

cc i vi thng tin t ng thu chnh thc.


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Hnh 2.2: Hin tng tr gy xuyn nhiu gia cc symbol

- iu ny cho chng ta thy rng, nu chng ta mun gii quyt vi tt ccc mc tn hiu tr khc nhau th tc symbol phi c gim xung sao cho

tng khong tr (gia tn hiu thu c u tin vi tn hiu thu c cui cng)

cng ch l mt phn khim tn ca chu k symbol. Khi thng tin m mt sng

mang n vn chuyn s b gii hn khi c hiu ng nhiu ng. Vy th nu mt

sng mang khng th vn chuyn c tc thng tin theo yu cu th tt nhin s

dn n tng chia dng d liu tc cao thnh rt nhiu dng song song vi

tc thp hn, mi dng c vn chuyn bi mt sng mang, ngha l s c rtnhiu sng mang. y chnh l mt dng ca FDM - bc u tin tin ti

COFDM.

- Mc d vy th vn c th tn ti ISI vi cc symbol trc . kh hon

ton th phi ko di khong truyn ca mt symbol sao cho n ln hn khong

tng hp tn hiu m my thu thu c. Vy th vic chn thm khong bo v c

th l tng tt (chng ta s tr li vn ny sau).

2.3.3. c tnh trc giao v vic s dng DFT/FFT

a. Trc giao

Vic s dng mt s lng ln cc sng mang c v nh khng c trin

vng lm trong thc t: chc chn, chng ta s cn rt nhiu b iu ch, gii iu


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ch v b lc i km theo?V cng c v nh s cn mt di thng ln hn cha

cc sng mang ny. Nhng tht may c hai iu lo lng ny u c xua tan nu

chng ta thc hin mt vic n gin sau y: cc sng mang c t u n

cch nhau mt khong fU = 1/ TU, vi TU l khong symbol hu ch (u: useful) vi

iu kin l cc sng mang ny phi c t trc giao nhau.

V mt ton hc, vic trc giao s nh sau: sng mang th k

c biu din:

tjk

KUet = )(

vi U = 2/ TU, v iu kin trc giao m sng mang phi tho mn l:

lkT

Lkdttt

U

T

LK

U

==

=+

,

,0)()( *

V ngha vt l: khi gii iu ch tn hiu cao tn ny, b gii

iu ch khng nhn thy cc tn hiu cao tn kia, kt qu l khng b cc tn

hiu cao tn khc gy nhiu.

V phng din ph: im ph c nng lng cao nht ri voim bng khng ca sng mang kia. Hn na chng ta s khng b lng ph

v mt ph. Cc sng mang c t rt gn nhau v th tng cng di ph

cng ch nh iu ch sng mang n - nu chng c iu ch vi tt c

d liu v s dng b lc ct nh l tng.

b. Cng c tnh trc giao bng khong bo v

Thc t, cc sng mang c iu ch c th phn tch thnh cc s phc.

Nu khong t hp thu c tri di theo 2 symbol th khng ch c nhiu ca cng

sng mang (ISI) m cn c nhiu xuyn sng mang (ICI). trnh iu ny chng

ta chn thm khong bo v gip m bo cc thng tin tng hp l n t cng

mt symbol v xut hin c nh.


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Hnh 2.3: Chn thm khong bo v

Mi khong symbol c ko di thm v th n s vt qu khong t hp

ca my thu TU. V tt c cc sng mang u tun hon trong TU nn ton b tn

hiu c iu ch cng vy. V th on thm vo ti phn u ca symbol to

nn khong bo v s ging vi on c cng di ti cui symbol. Min l tr

khng vt qu on bo v, tt c thnh phn tn hiu trong khong t hp s n

t cng mt symbol v tiu chun trc giao c tho mn. ICI v ISI ch xy ra

khi tr vt qu khong bo v.

di khong bo v c la chn sao cho ph hp vi mc hin tng

nhiu ng. DAB s dng khong bo v xp x TU / 4; DVB-T c nhiu la chn

hn nhng ti a cng ch l TU / 4.

Cn nhiu th na c th gy ra s suy gim tnh trc giao v do s gy ra

ICI. Chng c th l cc li xy ra trong cc b to dao ng ni hoc trong vic ly

mu tn s ca my thu hay cc tn hiu tp pha (phase-noise) trong cc b to dao

ng ni. Tuy nhin trong thc t, nhng nh hng ny c th c gi mc

gii hn c th chp nhn c.

c. S dng FFT

Chng ta trnh c hng ngn b lc, nh tnh trc giao, vy th vic

thc hin gii iu ch cc sng mang, cc b ghp knh v cc b t hp th sao?


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Trang 35

Thc t, chng ta lm vic vi tn hiu thu c di dng ly mu (theo

nh l Nyquyst). Qu trnh t hp tr thnh qu trnh tng kt, v ton b qu trnh

gii iu ch da trn dng bin i Furier ri rc (DFT). Rt may l vic thc hin

bin i Furier nhanh c ri (cc mch t hp sn c), v vy chng ta c th

xy dng thit b COFDM phng th nghim rt d dng. Cc phin bn chung ca

FFT u hot ng trn c s cc mu thi gian 2M (tng ng vi cc mu c

ly trong khong t hp) v vn chuyn cng s lng cc h s tn (frequency

coefficient). Cc h s ny tng ng vi d liu c gii iu ch t nhiu sng

mang. Thc t v chng ta ly mu trn c s gii hn Nyquyst, nn khng phi tt

c cc h s c ly u tng ng vi cc sng mang tch cc m chng ta s

dng.

Bin i FFT ngc c s dng trong my pht to ra tn hiu OFDM

t dng d liu u vo.

d. La chn iu ch c s

Ti mi symbol, mi sng mang s c iu ch bi mt s phc ly t tp

chm sao. Nu cng c nhiu trng thi trong chm sao th mi sng mang cng vn

chuyn c nhiu bit trong mt symbol, tuy nhin khi cc im trong chm saocng cng gn nhau hn, trong khi cng sut pht th c nh nn s gim kh nng

chng li. Do vy cn c s cn i gia tc v mc li.

Ti my thu, gi tr gii iu ch tng ng (h s tn ly t FFT my thu)

c nhn vi mt s phc tu (p ng knh ti tn s sng mang). Chm sao s

c quay lun phin v thay i v kch c. Vy th lm th no chng ta xc nh

c im trong chm sao m chng ta gi i?

Cch n gin l gii iu ch vi sai (differential demodulation), kiu nh

DQPSK trong DAB. Thng tin c mang i chnh l s thay i v pha ca

symbol ny so vi symbol tip theo. Min l knh thay i chm th s khng c

vn g vi p ng knh ca n. S dng qu trnh gii iu ch visai (khc vi


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Hnh2.4: Chn thm cc scattered pilot

gii iu ch kt hp - coherent demodulation ) s gy ra suy gim v ch tiu canhiu tp m nhit (thermal noise) - tuy nhin DAB khng cn l h thng chng li

mnh. Khi i hi tc ln hn (nh trong DVB-T), s rt c li nu s dng

gii iu ch kt hp . phng php ny, p ng knh s c xc nh v chm

sao nhn c c cn bngchnh xc ri mi xc nh xem im no trn chm

sao c pht i (ngha l xc nh c bit no truyn i). lm c iu

ny DVB-T th mt s pilot phn tn phi c pht km theo (gi l scattered

pilots). Sau php ni suy s c thc hin, s dng b lc mt chiu hoc hai

chiu cn bng tt c chm sao mang d liu.

2.3.4 T chc knh trong OFDM

Cc c tnh ca knh truyn dn khng c nh trong min thi gian. Nhng

trong mt khong thi gian ngn th thng l n nh.

a - Phn chia knh

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

"on thi gian" lin tip nhau. Xem trn hnh 2.5 sau y:


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Trang 37

time

frequency

ChannelBandwidth

frequency

sub-band

time

segment

Hnh 2.5: Phn chia knh

Hnh 2.6: V d v p ng knh thay i theo thi gian vi hai ng tr,

mi ci c mt dch tn Doppler khc nhau, cng vi ng tn hiu chnh. Trc

z miu t bin p ng knh.


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b - Chn cc sng mang ph

Trong mi on thi gian, gi l mi symbol OFDM, mi di tn ph c

trang b mt sng mang ph. trnh nhiu gia cc sng mang, chng c b trvung gc vi nhau, ngha l khong cch gia cc sng mang c t bng vi

nghch o ca mt chu k symbol.

t ime

frequ

en

cy

Channe lBandwidth

sub-carr iers

O F D M s y m b o l

O F D M s y m b o l

Hnh 2.7: Chn cc sng mang ph

c - Chn khong bo v

Do cc "echo" c to ra bi cc bn sao ca tn hiu gc khi b tr, nn ti

phn cui ca mi symbol OFDM s c nhiu lin symbol vi phn u ca symbol

tip theo. trnh iu ny, mt khong bo v c chn vo mi symbol nh ta

thy hnh 2.8 sau:


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Trang 39

time

frequ

ency

ChannelBandwidth

Used

part

duration

Guard

Interval

duration

OFDM

symbol

duration

Hnh 2.8: Chn khong bo vTrong khong bo v ny, m thc ra tng ng vi mt nhiu giao thoa

gia cc symbol, cc my thu s b qua tn hiu thu c.

Hnh 2.9: Dng tn hiu minh ho khi c khong bo v

d - 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 , mt ca s thi gian s c n nh chnh xc ti khong thi gian mi chu

k symbol din ra.

Tn hiu chnh

Phn x 2

ng knh

Phn x 1

Tn hiu thu csau khi sakhuch i

Tg TwantTs

t

t

t

t

t


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H thng DVB-T s dng cc sng mang "pilot", tri u n trong knh

truyn dn, ng vai tr lm cc im nh du ng b, nh trn hnh 2.10:

time

freq

uency

ChannelBandwidth

DVB-T Transmission Frame

Boosted pilot

Cc tnh nng khc nhau ny (phn chia knh, m ho d liu, chn khong

bo v v cc sng mang ng b) to ra cc c tnh c s ca phng thc

iu ch COFDM.

Tht khng may, tt c tnh nng ny li lm gim tc hu ch ca ti

thng tin. Tuy nhin, ta cng c th cn bng gia kh nng chng li vi dunglng knh.

V gip cc nh pht hnh c th thoi mi s dng cc h thng truyn

dn ca mnh trong tng iu kin c th, DVB-T a ra nhiu tham s c th

la chn nh: kch c FFT (2K, 8K), t l m ha (1/2, 2/3, 3/4...), v khong bo

v (1/4 TS, 1/8 TS, 1/16 TS...).

2.3.5. Phng thc mang d liu trong COFDM

COFDM cho php tri d liu truyn i trn c min thi gian v min tn

s, sau khi s dng m ha vng bo v d liu.

Hnh 2.10: Cc sng mang ng b


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Do c hin tng fading tn s gia cc di tn cn k, nn COFDM c s

dng xen tn s, ngha l cc bit d liu lin tip nhau s c tri ra trn cc sng

mang cch bit nhau.

time

GUARD

intervall

USED PART

of the symbol

USEFUL DATA

PROTECTED DATA

GUARD INTERVALL INSERTION

OFDM

SYMBOL

DURATIONfrequency

01101

(PILOTs NOT REPRESENTED )

Hnh 2.11: Thc hin mapping d liu ln cc symbol

Trong DVB-T vic mapping d liu ln cc symbol OFDM thc ra l iu

ch tng sng mang ring r, v c th theo mt trong ba chm sao to phc

4QAM, 16QAM, 64QAM, nh sau:

00

0010

000010

4QAM16QAM

64QAM

Hnh 2.12: Chm sao c s ca DVB-T


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Tu theo dng iu ch c la chn, ti mt chu k symbol cho mi sng

mang s c 2 bit (4QAM), 4 bit (16QAM) hay 6 bit (64QAM) c truyn i. Mi

dng iu ch c mt kh nng chng li khc nhau. Thng th 4QAM c khong

dung sai chu nhiu ln gp 4 n 5 ln so vi 64QAM.

2.4. M ha knh trong DVB-T

Tn hiu a vo l lung s liu ni tip, lung s liu ny bao gm cc gi

c nn theo tiu chun MPEG-2, mi gi d liu c 188 byte (gm c 1 byte

ng b v 187 byte d liu).

Cc gi d liu ny u tin c nhn dng bi chui gi ngu nhin PRBS.

Mc ch ca qu trnh ny l phn tn nng lng trong ph tn hiu s v xc nhs nh phn thch hp (loi b cc chui di 0 v 1). Cc t m ng b khng

c a vo qu trnh phn tn ni trn.

Sau khi c nhn dng bi chui gi ngu nhin PRBS, cc gi d liu

c a vo b m ngoi (outer coding). Ti y cc gi s liu c ghp thm

cc m sa sai vo tng gi.

D liu ly ra khi b m ngoi (outer coding) c a n khi ghp xen

ngoi (outer interleaving) thc hin vic ghp chp. Ti y cc gi s liu c

ghp theo byte. Mc ch ca qu trnh ny l loi b tnh thng k ca nhiu.Ta s

xem xt c th cc loi m ha c dng nh sau:

2.4.1. M ha phn tn nng lng


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Trang 43

D liu ly ra khi b ghp d liu l lung s liu gm cc gi s liu c

nn theo tiu chun MPEG-2 c di 188 byte (gm 1Byte ng b v 187 Byte

d liu). Th t x l s lun c bt u t MSB (bit "0") ca byte ng b gi

(01000111).

m bo cho vic truyn dn khng c li, d liu s c ngu nhin

ho theo s trong hnh 2.13.

Thanh ghi dch to ra chui gi ngu nhin gm c 15 bit. a thc to chui

nh phn gi ngu nhin s l: 1 + x14 + x15

Vic khi to chui gi ngu nhin c thc hin bng cch np chui

1001010101000000 vo thanh ghi dch. Qu trnh khi to ny c thc hin

theo chu k c 8 gi MPEG-2 th np mt ln.

to tn hiu ban u cho b tch, byte ng b trong gi MPEG-2 u

tin trong 8 gi MPEG-2 s c o bit (t 47HEX thnh B8HEX). Qu trnh o

bit ny c gi l phi hp ghp truyn ti.

0 0 0 0 0 0 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1 0 0 1 0 1 0 1 0 0 0 0 0 0 0Chuikhi to

Tn hiu ra c ngunhin (gii ngu nhin)Tn hiu vo cha ()

c ngu nhin ho

Enable

D liu a vo (MSB u tin): 1 0 1 1 1 0 0 0 x x x x x x x x...Chui PRBS: 0 0 0 0 0 0 1 1

Hnh 2.13: S m t nguyn l ngu nhin, gii ngu nhin chui s liu.


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Vic thc hin ngu nhin ho ch p dng vi cc byte s liu do bit u

tin ly ra khi thanh ghi dch s c tch chp vi bit u tin ca byte u tin

theo sau byte ng b c o bit (B8HEX). tr gip cc chc nng ng

b khc, khi 7 byte ng b ca 7 gi tip sau c truyn, chui PRBS vn hot

ng nhng u ra ca thanh ghi dch b kho do cc byte ny khng c ngu

nhin ho. V vy chu k ca PRBS l 1503 bytes.

2.4.2. M ngoi (outer coding)

B m ngoi s dng m Reed-Solomon RS (204, 188, t=8) m ho d

liu c ngu nhin ho nhm to ra cc gi d liu c bo v li.

Do c m ho theo m RS (204,188, t=8) nn mi gi d liu s c

thm 20 bytes sa li v n c kh nng sa ti 8 li trong mt gi.

a thc to m l: G(x) = (x + 0)(x +1)(x +2).....(x+ 15)

Vi = 02 HEX.

M RS ngn c thc hin bng cch thm 51 bytes, tt c l 0, trc khi

byte d liu c a vo b m ho RS. Sau khi m ho RS, th cc byte rng s

c loi b v cc t m RS s cn s byte l N= 204 byte.

2.4.3. Ghp xen ngoi (outer interleaving)

S nguyn l chung thc hin vic ghp ngoi c cho trong hnh 2.14.

Theo s vic ghp chp kiu byte vi su ghp l=12 s c p dng

vi cc gi c ly ra khi b m ngoi. Cu trc d liu sau khi ghp c ch ra

trong hnh 2.15-d. Qu trnh ghp chp ny phi da trn tip cn tng hp vi

tip cn Ramsey kiu III, l=12 l tip cn Forney. Nhng byte d liu c ghp lcc byte s liu trong gi c bo v li v c gii hn bi byte ng b (o

hay khng o). Chu k chn l 204 byte.


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B ghp gm 12 nhnh, c kt ni theo kiu vng vi cc byte s liu

bng chuyn mch u vo. Mi nhnh j s l mt thanh ghi dch First in -First out,

vi j x M nh. Trong : M= 17 =N/I, N= 204 byte

`

Hnh 2.14: S nguyn l ca b ghp v tch ngoi


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Trang 46

SYNC1or

SYNCn

SYNC1 byte

D liu ghp truyn ti MPEG-2

187 byte.

a) Gi ghp truyn ti MPEG-2

SYNC2D liu

c ngunhin

(187 byte)SYNC1

D liuc ngu

nhin(187 byte)

SYNC8D liu

c ngunhin

(187 byte)SYNC1

D liuc ngu

nhin(187 byte)

8 gi ghp truyn ti

Chu k PRBS = 1503 bytes

b) Cc gi truyn ti c ngu nhin ho: byte SYNC v Byte d liu c ngu nhin.

SYNC1or

SYNCn

187 bytes d liu c ngu

nhin

16 byte chn l

204 bytes

c) Cc gi d liu c bo v li theo m Reed-Solomon (204,188, 8).

SYNC1or

SYNCn

203 bytes

d) Cu trc d liu sau khi c ghp ngoi

Hnh 2.15: Cc bc trong qu trnh ngu nhin, m ngoi, ghp ngoi. (n =2, 3,... 8)

SYNC1or

SYNCn


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Mi ca thanh ghi dch s c 1 byte. u vo v u ra ca b ghp phi

c ng b. to s ng b, byte SYNC v SYNC1 phi c truyn trong

nhnh 0 ca b ghp.

2.4.4.M ho ni (inner coding)

B m ni s dng m chp l. N cho php la chn cc tc m ho

khc nhau: 1/2,2/3, 3/4, 5/6, 7/8. Cc phng php m ho ny c da trn

phng php m chp vi tc m ho l 1/2 c 64 trng thi c gi l m m.

S nguyn l ca thc hin vic m chp vi tc 1/2 c cho trong hnh

2.16. a thc to m l G1 = 171 oct cho u ra X v G2=133 oct cho u ra Y.

Bng sau a ra chui bit truyn dn c to ra tng ng vi cc tc m ho

khc nhau. Trong X v Y tng ng vi hai u ra ca b m chp.

Tc m ho cng cao th dng s liu cng ln nhng t s C/N cng cng

ln. Tc m ho 1/2 to ra dng s liu ln nht nhng t s C/N cng cao nht,

tc m ny c dng cho cc knh b nhiu mnh. Tc m ho 7/8 to ra

dng s liu nh nht nhng t s C/N thp nht nn c dng cho cc knh t b

nhiu.

Tr1 bit

Tr1 bit

Tr1 bit

Tr1 bit

Tr1 bit

Tr1 bit

Cng modulo-2

Cng modulo-2

u ra X (G1 = 171 Octal)

u ra Y (G2 =1 33 Octal)

D liuvo

Hnh 2.16: S thc hin m chp tc 1/2


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Bng 2.1: S puncturing v dy c truyn sau khi bin i ni tip song song.

Tiu chun OFDM cho php la chn 3 phng thc iu ch QPSK,

16 QAM, 64 QAM.

2.4.5.Ghp xen ni

B ghp xen ni gm 2 khi: ghp kiu bit v ghp kiu k t.

a- B ghp kiu bit.

Lung s liu a vo b ghp ni kiu bit (c th ln ti 2 lung) c tch

thnh v lung con.

v Loi iu ch2 QPSK

4 16-QAM

6 64-QAM

Tc m r S puncturingDy c truyn sau khi bin i

song song - ni tip

1/2X: 1

Y: 1X1Y1

2/3X: 1 0

Y: 1 1X1Y1 Y2

3/4X: 1 0 1

Y: 1 1 0X1Y1 Y2 X3

5/6

X: 1 0 1 0 1

Y: 1 1 0 1 0 X1Y1 Y2 X3Y4 X5

7/8X: 1 0 0 0 1 0 1

Y: 1 1 1 1 0 1 0X1Y1 Y2 Y3Y4 X5 X6 X7


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Trong trng hp khng phn cp, d liu c tch thnh v lung s liu

con.

Trong trng hp phn cp d liu a vo gm c 2 lung: lung c mcu tin cao v lung c mc u tin thp. Lung c mc u tin cao s c tch

thnh 2 lung con, lung c mc u tin thp c tch thnh v-2 lung con.

Qu trnh tch cc lung con ny c xem nh vic chuyn cc bit xdi

thnh cc bit u ra be,do.

Trong kiu khng phn cp:

xdi = b[di(mod)v](div)(v/2)+2[di(mod)(v/2)],di(div)v

Trong kiu phn cp:

x'di = bdi(mod)2,di(div)2;

x''di = b[di(mod)(v-2)](div)((v-2)/2)+2[di(mod)((v-2)/2+2,di(div)(v-2)

Trong :

xdi Bit a vo trong mode khng phn cp

xdi L bit ca lung u tin cao trong mode phn cp.

xdi L bit ca lung u tin thp trong mode phn cp.

di Ch s bit u vo.

be,do Bit ly ra khi b tch.

e Ch s lung bit c tch (0 e v)

o Ch s bit ca lung bit ti u ra b ghp


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Sau khi tch cc bit c nh x nh sau:

QPSK: x0 b0,0

x1 b1,016 QAM khng phn cp 16 QAM phn cp

x0 b0,0 x0 b0,0

x1 b2,0 x1 b1,0

x2 b1,0 x0 b2,0

x3 b3,0 x1 b3,0

64 QAM khng phn cp 64 QAM phn cpx0 b0,0 x0 b0,0

x1 b1,0 x1 b1,0

x2 b2,0 x0 b,2,0

x3 b4,0 x1 b4,0

x4 b3,0 x2 b,3,0

x5 b5,0 x3 b5,0


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DEMUX

Ghp kiubit l0

Ghp kiubit l1

Mapping

b0,0, b0,1

b1,0, b1,1

a0,0, a0,1

a0,0, a0,1

Y0,Y1Ghp kiu

k t

Re(z)y0,0

Im(z)y1,0

QPSK

x0, x1, x2

DEMUX

Ghp kiubit l1

Ghp kiubit l2

Mapping

b1,0, b1,1

b2,0, b2,1

a1,0, a1,1

a2,0, a2,1

Y0,Y1Ghp kiuk t

Re(z)y0,0

Im(z)y1,0

16 QAM

Ghp kiubit l0

Ghp kiubit l3 a3,0, a3,1

a0,0, a0,1b0,0, b0,1

b3,0, b3,1

x0, x1, x2

Ghp kiubit l5b5,0, b5,1

DEMUX

Ghp kiubit l2

Ghp kiubit l3

Mapping

b2,0, b2,1

b3,0, b3,1

a2,0, a2,1

a3,0, a3,1

Y0,0,Y0,1Ghp kiu

k t

Re(z)y0,0

Im(z)y1,0

64 QAM

Ghp kiubit l1

Ghp kiubit l4 a4,0, a4,1

a1,0, a1,1b1,0, b1,1

b4,0, b4,1

Ghp kiubit l0

b0,0, b0,1a0,0, a0,1

x0, x1, x2

Hnh 2.17: S thc hin vic ghp ni v mapping theo m hnh khng phn cp


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Vi mi mt b ghp bit, u vo s l

B(e) = (be,0, be,1,be,2,be,3... be,125) (0 e v)

Vector ly ra khi mi b ghp bit l:

A(e) = (ae,0, ae,1, ae,2, ae,3,... ae,125)

Trong cc ae,w vi w = 0, 1, 2...125 c xc nh nh sau: ae,w = be

He(w) l hm hon v c xc nh nh sau:

DEMUX Ghp kiu bit

l1

Ghp kiu bitl2

Mapping

b1,0, b1,1

B2,0, b2,1

a1,0

, a1,1

a2,0, a2,1

Y0,Y1Ghp kiuk t

Re(z)y0,0

Im(z)y1,0

16 QAM

Ghp kiu bitl0

Ghp kiu bitl3

a3,0, a3,1

a0,0, a0,1b0,0, b0,1

b3,0, b3,1

x0,

DEMUX

x0, x1, x2

b5,0, b5,1 a5,0, a5,1Ghp kiu bit

l5

b0,0, b0,1 a0,0, a0,1Ghp kiu bitl0DE

MUX

Ghp kiu bitl2

Ghp kiu bitl3

Mapping

b2,0, b2,1

b3,0, b3,1

a2,0, a2,1

a3,0, a3,1

Y0,0,Y0,1Ghp kiuk t

Re(z)y0,0

Im(z)y1,0

64 QAM

Ghp kiu bitl1

Ghp kiu bitl4 a4,0, a4,1

a1,0, a1,1b1,0, b1,1

b4,0, b4,1

x0, x1, x2

DEMUX

x0, x1, x2

Hnh 2.18: S thc hin ghp ni v mapping theo m hnh phn cp


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B ghp bit Hm hon v H(w)

I0 H0(w) = wI1 H1(w) = (w + 63) mod 126

I2 H2(w) = (w + 105) mod 126

I3 H3(w) = (w + 42) mod 126

I4 H4(w) = (w + 21) mod 126

I5 H5(w) = (w + 84) mod 126

u ra ca b ghp bit c nhm vi nhau to thnh cc k t d liu.

Mi k t d liu s gm c v bit c ly t v b ghp bit. V vy u ra ca bghp bit l cc k t y c v bit:

yw = (a0,w, a1,w,... av-1,w)

b- B ghp k t.

Mc ch ca b ghp k t l t nhng k t c v bit ln 1512 (mode 2k)

hoc 6048 (mode 8k) sng mang. B ghp k t uc thc hin trn cc khi c

1512 (mode 2k) hoc 6048 (mode 8k) k t d liu.

V vy trong mode 2k, 12 nhm mi nhm c 126 k t d liu ly t b

ghp bit s c c mt cch tun t vo trong vect

Y =(y0, y1, y2,...,y1511).

Trong mode 8k, 48 nhm mi nhm c 126 k t d liu c c vo trong

vector:

Y =(y0, y1, y2,... y6047)

Vector Y thu uc sau khi ghp l:

Y=(y1, y2, y3,... yNmax-1)

Trong :


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yH(q) = yq vi nhng k t c ch s chn q=0,...Nmax-1

yq = yH(q) vi nhng k t c ch s l q=0,...Nmax-1

Nmax = 1512 trong mode 2k v Nmax = 6048 trong mode 8k.

H(q) l hm hon v c tnh nh sau:

Ri l t c (Nr-1) bit, trong Nr = log2Mmax.

Mmax = 2048 trong mode 2k.

Mmax = 8192 trong mode 8k.

Ri c xc nh nh sau:

i=0,1 Ri [Nr-2,Nr-3,..,1,0] =0,0...0

i=2 Ri [Nr-2,Nr-3,..,1,0] =0,0...,1

2


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Hm hon v H(q) c xc nh theo thut ton sau:

Tng t nh y, y gm c v bit:

Yq = (y0,q, y1,q,...,yv-1,q)

Trong q l ch s k t ti u ra ca b ghp k t.

2.5 .Mt s kh nng u vit ca DVB-T

2.5.1 iu ch phn cp

Tiu chun Chu u DVB-T gm rt nhiu kiu loi truyn dn, to iu

kin trin khai tn hiu COFDM trong nhiu dch v pht sng khc nhau. Trong s

, iu ch phn cp cho php pht sng ng thi 2 dng truyn ti MPEG c

lp vi kh nng chng li khc nhau trn cng mt knh RF.

V vi phng thc iu ch phc tp v kh l lm ny, chng ta c th

hnh dung ra mt s tnh nng dch v mi:

Mt my pht vi 2 vng ph sng ring bit.

Gii quyt c c thu di ng ln thu c nh.

Pht ng thi cc chng trnh s c dng tiu chun

(SDTV) ln dng c phn gii cao (HDTV).

Cng vi ng dng trong hot ng ca mng n tn SFN, iu ch phn

cp nu bt c cc kh nng to ln ca h thng DVB-T. V liu iu ch


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phn cp c phi l mt c hi cho nhiu dch v mi? Chng ta s cng nhau xem

xt vn ny.

a- Chm sao iu ch phn cp

Dng iu ch phn cp thc ra l s bin dch ca 16QAM v 64QAM

thng qua 4QAM, nh ta thy trn hnh v:

4 Q A M o v e r 4 Q A M 1 6 Q A M o v e r 4 Q A M

1 0 0 1

1 0H P

0 1

L P1 0

H P L P

1 1 0 1

1 0 1 1 0 1

Hnh 2.19: Chm sao phn cp DVB-T

iu ch phn cp c xem nh l s phn tch knh RF thnh 2 mch o,

mi mch c mt tc bit ring, mt mc li ring v theo s c vng ph

sng khc nhau mt cht. Chnh s kt hp gia dng chm sao v t l m ha s

quyt nh s khc nhau gia 2 knh o ny.

Trn thc t, vic mapping dng d liu u tin s s dng chm sao

4QAM. Mi cp 2 bt ca dng d liu ny s xc nh gc phn t m sng mang


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chim gi trong chm sao. Sau , dng d liu th hai s quyt nh v tr bn

trong gc phn t , bao gm c phn thc v phn o ca sng mang.

Nu dng d liu th hai c nh x bi cc cp 2 bt th chm sao phncp chnh l "4 QAM thng qua 4 QAM". V hnh dng n s ging nh 16 QAM.

Cn nu 4 bt c s dng th chnh l "16 QAM thng qua

4 QAM", to nn dng chm sao 64 QAM. Dng bt d liu u tin s lun lun

s dng dng iu ch 4 QAM, v c gi l dng bt u tin cao (HP). Dng th

hai, t li hn, c iu ch dng 4 QAM hoc 16 QAM, c gi l dng c

mc u tin thp.

Cc loi iu ch phn cp cng c mt h s thay i, l . y l t lgia khong cch ca hai im gn nht ca hai gc phn t lin k v khong cch

gia hai im gn nhau nht trong cng mt gc phn t ca chm sao. Thc ra h

s cng ln th cng c li cho iu ch HP 4 QAM nhng khi iu ch LP li

kh chng li hn.

b- Cc c tnh iu ch phn cp

Trong tiu chun DVB-T, iu ch phn cp c hai c tnh chnh l:

- Cho php pht sng hai dng truyn ti MPEG c lp trn

cng mt knh RF

- Mi dng truyn ti s c mt kh nng chng li ring, do

s c vng ph sng ring.

Thc ra s khc nhau gia mc li gia HP v LP s ph thuc c vo

dng iu ch (4 QAM hay 16 QAM) v t l m ha dng cho dng LP.

Dng d liu HP, lun c iu ch dng 4 QAM, c tc bitrate hu

ch ch ph thuc vo t l m ha. Trong khi dng LP c cc my thu xem

nh l nguyn nhn tng thm nhiu ca cc gc phn t trong chm sao. Do vy

t c lng C/N c th chp nhn c th dng HP phi chu thm mt lng

n b.


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c- Ti sao li dng iu ch phn cp ?

Ti nhiu nc, vic trin khai dch v s thng bng cch chia s di tn

UHF/VHF vi cc dch v analog hin hnh, nhng vn l s dng cc knh khngmong mun (taboo channel). ti u ha tc bit trong pht hnh DVB-T, cc

nh lp k hoch mng la chn cc cch iu ch mt cao nh: chm sao 64

QAM v t l m bo v 2/3.

Tu theo cu hnh mng v vng ph sng m ta c nhng s la chn sau:

+ 2K/8K: tu theo mng MFN hay SFN, v kch c ln nht ca cc

khong cch gia cc my pht.

+ Khong bo v: tu theo vng phc v (thnh th s khc nng thn,min ni).

Nhng trn tt c, cc tnh nng ca iu ch phn cp cn cho php nhng

iu chnh hn na trong vic lp mng nh mt s tnh hung ph sng nh trn

hnh sau y:

HPHP

LPLP

HPHP

LPLPREGREG

Hnh 2.20: S ph sng tng trng s dng iu ch phn cp.

iu ch phn cp cn cho ta s cn bng gia tc bit vi mc li, hay

chnh l gia tc bit vi vng phc v.

Trong thi k u ca DVB-T, iu ch phn cp ch c xem nh l cch

to ra hai vng ph sng cho mt my pht xc nh. iu ny thc s l ng,

tuy nhin ch cn thit khi thc thi hai loi hnh dch v v lc iu ch phn cp


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s th hin c u th v ph m vn d b cc dch v analog chim gi kh

nhiu.

Tm li iu ch phn cp th hin c kh nhiu tnh linh hot, tu theoquan im cng nh mi quan tm ca cc nh pht hnh m c nhng la chn

khc nhau. Mt s v d di y c ghi nhn:

Qung b cho c my thu c nh v my thu c th di chuyn

ng dng chnh ca iu ch phn cp l cho cc my thu c th dch

chuyn khi anten thu t trong nh bng vic thay i mt cht cc tham s iu

ch. Trong khi cc my thu c nh c u th v khuch i ca cc anten thu

t trn nc nh th thu dch chuyn li chu bt li do suy hao gy bi chnh cc

building. Trn hnh v ta thy so vi loi iu ch thng thng (REG) th HP v

LP bao ph hai vng ring bit.

H PH P

L PL P

H PH P

Fixed receiversPortab le rec eiv ers

R E GL PL P

Dng d liu th hn HP (cha cc chng trnh "li") s c c my c nh v

di chuyn thu. Tuy nhin khi thu di chuyn th vng ph sng HP s ln hn mt

cht so vi chnh n khi th loi iu ch thng thng.

Thc t th u l s cn bng nu loi iu ch thng thng 64 QAM 2/3

c chuyn i thnh phn cp vi HP l 4 QAM 1/2 v LP l 16 QAM 2/3.


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Bitrate REG: 24,13 Mbps HP: 6,03 Mbps

LP: 16,09 Mbps

C/N REG: 16,5 dB HP: 8,9 dB

(Gaussian) LP: 16,9 dB

V kha cnh bitrate hu ch th vic chuyn t tc 24,13 Mbps sang

22,12 Mbps ca phn cp s lm gim tc ton cc i 2,01 Mbps. V t l C/N,

vi knh Gaussian, th HP c bo v tt hn, trong khi LP gn nh ging vi loi

truyn dn thng thng.

Tm li vic gim tc tng th cng khng bng vic chng ta t c

dng HP c kh nng chng li cao. Cn vng LP th hu nh khng i thm ch

ngay c trong trng hp cc my thu di chuyn chu s suy gim.

C l trong nhng ngy u tin th nghim tiu chun DVB-T, iu ch

phn cp cha thu ht c s ch ca cc nh pht hnh. Nhng hin nay tnhhnh thay i khin iu ch phn cp ang trong hon cnh rt hp dn. Nh

mt phn quan trng trong u th ca DVB-T, iu ch phn cp cho php tn dng

thm hiu qu v ph khi s dng rt nhiu th loi my thu trong nhiu hon cnh

khc nhau.

iu ny gip cc nh pht hnh trin khai truyn hnh s vi nhiu loi hnh

dch v khc nhau m khng i hi gia tng v ph, y chnh l mt li th cho

h thng DVB-T.Thm ch nu ai cho rng nhng li th ny khng ng phn tch th

hin nhin "Hierarchical Modulation vn khng ch l iu tuyt diu ca cc k s:

n l ca tt c, mt u vit khng th chi ci ca DVB-T".


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2.5.2. Mng n tn SFN

a- Nguyn l hot ng ca SFN

Mng n tn (Single Frequency Network) hot ng da trn cc my

pht ng knh. Cc my pht ny pht cng mt tn hiu ti bt k thi im no

v ti bt k im no trong vng phc v.

C 3 lut l "vng" i vi mng n tn:

Mi my pht trong mng n tn s pht:

-Cng mt tn s,

-Ti cng mt thi im,

-Lng thng tin pht i ging nhau.

Nhng lut l ny to nn nhng yu cu cho SFN c s, v n c nh hng

trc tip trong qu trnh thit k mng pht hnh: l yu cu phi ng b cc

my pht c v mt thi gian ln tn s.

b- Yu cu trong min tn s ca SFN

Thc ra th mi my pht trong mng SFN cng s c qun l v iu

khin chnh xc v mt tn s lm vic nh trong cc mng tn s thng thng.

Nhng vi hot ng ca mng SFN COFDM th s n nh cng nh tnh chnh

xc ca tn s lm vic phi m bo sao cho mi sng mang ph c mt v tr tuyt

i trong "khng gian" m tn s knh RF tng s dng.


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Thc t, tn s chun ton cc ly t cc my thu GPS c s dng ng

b mng SFN, nh thy trn hnh 2.21.

P AUP

C O N V E R T E R

C O F D MP R O C E S S O R

G P S1 0 M H z(10 -9 )

S I G N A Lto

B R O A D C A S TP A

U PC O N V E R T E R

C O F D MP R O C E S S O R

S I G N A Lto

B R O A D C A S T

G P S1 0 M H z(10 -9 )

T X 1 T X 2

T X 1

T X 2

f r e q u e n c y

f r e q u e n c y

F 0 i n f ew H z

C O F D MR E C E I V E R

Hnh 2.21:ng b min tn s

c- Yu cu trong min thi gian i vi SFN

Gi tr khong bo v c chn s l im chnh trong cu hnh mng SFN:

v khong bo v s phn nh tr phn x ln nht m h thng c th chp nhn

c, v n cng phn nh khong cch ln nht gia 2 my pht trong mng.

C l yu cu v mt thi gian chnh l mt thch thc i vi cc nh pht

hnh: v n i hi mi my pht phi pht cng mt symbol ti cng mt thi

im, nn tt yu dn n ng b v thi gian. Vic ng b ny s m bo sao

cho cc echo (t nhin hay nhn to) u nm trong phm vi ca khong bo v.

Nh ta thy minh ha trn hnh 2.22.


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PACOFDMPROCESSOR

GPS

1 pps

PACOFDM

PROCESSOR

TX1 TX2

TX1

TX2

Time

Delivery Time in fews

GPS

1 pps

PRIMARY

DISTRIBUTION

NETWORK

PRIMARY

DISTRIBUTION

NETWORK

Time

COFDMRECEIVER

Hnh 2.22: ng b v mt thi gian

Ca s thi gian s cho php loi b khong bo v trong lc ly mu tn

hiu ti my thu. V th khong bo v phi c b tr nh l mt "qu thi gian":

n s c dng trong khng gian nhng khng s dng b li ng b thi gian

ca cc my pht.

Thc t cc nh iu hnh mng s dng ng h tham chiu ly t GPS vi

xung chun 1PPS. Xung chun ny cho php chn thm nhn thi gian ti lc ghp

knh, gip cho ti mi my pht b x l COFDM c th gy tr ghp knh u vo

cho n khi c s n nh thi gian chung.

d- SFN: ng dng thc t.

C ch ca COFDM trong mng SFN c thc hin trong rt nhiu quc

gia c h thng DAB ln DVB-T. Ngy nay cc mng c tnh thng mi s dng

kh nng ny ti u ha vng ph sng cng nh thc hin mng pht hnh

nh l cc cell RF Anh, Thy in, Ty Ban Nha v Php.

SFN da trn COFDM hon ton khng ch l tnh nng th v ch trong

phng th nghim m n hon ton c th ng dng trong thc t.

- SFN to ra hiu qu v ph ln


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- Trong khi MFN c th hot ng xen vi cc dch v analog th SFN i

hi mt knh RF trng hon ton cho vng phc v.

- Hot ng tt nht vi mng gm nhiu my pht cng sut thp. Ngha lSFN tn dng tt hiu qu v cng sut.

2.6. Kt lun chng II

T nhng phn tch trn ta c th thy c nhng u, nhc im khi s

dng k thut iu ch DVB-T:

- u im:

p ng c nhu cu truyn thng tc cao ( nht l vi

cng ngh truyn hnh khi ghp nhiu knh chng trnh) vi kh nngkhng nhiu tt trn knh phainh chn lc tn s.

Tnh phn tp tn s cao do thng tin c tri ra trn nhiu

sng mang con khc nhau, to nn kh nng chng c cc nh hng ca

knh pha inh chn lc tn s.

Hiu qu s dng ph cao do OFDM s dng nhiu sng mang

con sng mang ny trc giao ngha l cc sng mang con c mt phn

chng ln nhau trong min tn s m vn m bo chng ISI ti u thu. Rt n gin v hiu qu trong trin khai h thng.

- Nhc im:

T s cng sut cc i trn cng sut trung bnh cao do tn

hiu OFDM l tng ca nhiu thnh phn tn hiu nn bin ca n c nh

cao dn n t s PAPR l cao.

Qu trnh ng b gp nhiu kh khn hn so vi h thng

thng thng v h thng kh nhy vi nhiu tp m, li dch tn s sngmang, li nh thi tn s ly mu...

Vi nhng u im trn cng vi tnh tin ch khi xt n yu t trao i

chng trnh, vi s lng ln cc quc gia ly chun DVB lm tiu chun. i

truyn hnh Vit Nam quyt nh la chn DVB lm chun, ngy 26/3/2001 i


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Truyn Hnh Vit Nam quyt nh chn h DVB-T cho pht s mt t v sau

khong 5 nm th nghim cho kt lun v tnh ng n ca phng n la

chn. Hin nay s ha tn hiu truyn hnh Vit Nam l s thay th dn cc cng

on, trang thit b t tng t xang s. l qu trnh s ha tng phn. Ri y

truyn hnh s s thay th hon ton truyn hnh tng t to iu kin cho nghnh

cg nghip truyn hnh pht trin mnh m hn kt hp vi cc mng truyn thng

khc to thnh mt th gii thng tin s phc v cho con ngi mt cch hu hiu.


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CHNG III: TRUYN HNH S MT T THEO TIU CHUN

CHU U TH H TH 2 (DVB-T2).

3.1. Gii thiu

Truyn hnh k thut s ra i v nhanh chng khng nh c v th ca

n trn th trng. Chnh v nhng u im ni tri ca truyn hnh s m hu ht

cc nc pht trin trn th gii u a ra l trnh pht trin truyn hnh s v

ngng pht sng truyn hnh tng t: Anh (2012) , M (17/02/2009), Nht

(24/07/2011) c (2008), Php (30/11/2011) v rt nhiu quc gia khc s ngng

hon ton vic pht sng mt t tng t. Hin nay, mt s nc khc nhau c

la chn cho mnh nhng tiu chun truyn hnh s mt t khc nhau nhng DVB-

T vn l tiu chun c nhiu nc la chn nht. Vit Nam cng c quyt

nh chnh thc la chn DVB-T l tiu chun pht sng s mt t ca mnh. Vi

xu th hi t trong lnh vc Media, ng thi vi s pht trin mnh m ca truyn

hnh phn gii cao HDTV, tiu chun truyn hnh s mt t DVB-T nhanh

chng cn b sung thm cc tnh nng khc h tr c v mt k thut ln mt

thng mi. Rt nhiu yu cu mi v dch v c a ra v chng l tin

xy dng mt th h th hai cho DVB-T.

Chun DVB-T l chun pht sng truyn hnh s mt t c trin khaithnh cng, c nhiu nc chp nhn. Ngay t khi cng b ln u nm 1995,

chun ny c s ng h ca trn 50% cc quc gia trn th gii. Tuy nhin, t

sau s ra i ca chun DVB-T th cc nghin cu v k thut truyn dn vn tip

tc c trin khai v cc ty chn v iu ch, v khng li ng truyn vn tip

tc c pht trin. Mt khc, nhu cu v ph tn cng gia tng v vi p lc v

ph tn dng cho cc dch v phi qung b (cng cng chia x vng bng tn ca

cc dch v qung b) cng khin cho vic gia tng hiu qu ca ph tng ln mcti a cng cp thit.

T , nhm DVB Project pht trin chun truyn hnh s mt t th h

th 2 l DVB-T2. Tiu chun ny c xut bn ln u tin vo 6/2008 v c


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ETSI (European Telecommunication Standardisations Institute) chun ha t thng

9/2009. Vic trin khai v pht trin sn phm cho chun mi ny cng bt u.

Kh nng gia tng dung lng trong mt multiplex truyn hnh s mt t l

mt trong nhng u im chnh ca chun DVB-T2. So snh vi chun truyn hnh

s hin nay l DVB-T th chun th h th hai DVB-T2 cung cp s gia tng dung

lng ti thiu 30% trong cng iu kin thu sng v dng cc anten thu hin c.

Tuy nhin, mt s th nghim s b cho rng dung lng trong thc t c th gia

tng n gn 50%. iu ny cng thun li cho vic trin khai cc dch v qung b

mi i hi nhiu dung lng hn.

3.2. Nhng tiu ch c bn ca DVB-T2

- DVB-T2 phi tun th tiu ch u tin c tnh nguyn tc l tnh tng

quan gia cc chun trong h DVB. iu c ngha l s chuyn i gia cc tiu

chun DVB phi thun tin cao nht n mc c th, v d gia DVB-S2 (tiu

chun truyn hnh s qua v tinh th h th 2 vn ly) v DVB-T2.

- DVB-T2 phi k tha nhng gii php tn ti trong cc tiu chun DVB

khc. DVB-T2 phi chp nhn 2 gii php k thut c tnh then cht ca DVB-S2,

c th: + Cu trc phn cp trong DVB-S2, ng gi d liu trong khung BB

(Base Band Frame).

+ S dng m sa sai LDPC (Low Density Parity Check).

- Mc tiu ch yu ca DVB-T2 l dnh cho cc u thu c nh v di

chuyn c, do vy, DVB-T2 phi cho php s dng c cc anten thu hin ang

tn ti mi gia nh v s dng li cc c s anten pht hin c.

- Trong cng mt iu kin truyn sng, DVB-T2 phi t c dung lng

cao hn th h u (DVB-T) t nht 30%.- DVB-T2 phi t c hiu qu cao hn DVB-T trong mng n tn SFN

(Single Frequency Network)


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- DVB-T2 phi c c ch nng cao tin cy i vi tng loi hnh dch v

c th. iu c ngha l DVB-T2 phi c kh nng t c tin cy cao hn

i vi mt vi dch v so vi cc dch v khc.

- DVB-T2 phi c tnh linh hot i vi bng thng v tn s.

- Nu c th, phi gim t s cng sut nh/ cng sut trung bnh ca tn

hiu gim thiu gi thnh truyn sng.

Trn c s nhng tiu ch trn, t thng 6/2007 n thng 3/2008, trn 40 t

chc tp trung nghin cu tiu chun DVB-T2, thng qua nhiu bui hi tho,

hi ngh qua mng v Email. Cui cng cui nm 2008, nhng ni dung c bn ca

tiu chun DVB-T2 c ban hnh.

Vi nhng cng ngh s dng trong DVB-T2, dung lng d liu t c

ti Anh ln hn khong 50% so vi DVB-T, ngoi ra DVB-T2 cn c kh nng

chng li phn x nhiu ng (Multipaths) v can nhiu t bin tt hn nhiu so

vi DVB-T.

Bng 3.1: DVB-T2 s dng ti Anh so vi DVB-T


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DVB-T2 thm ch cn t c dung lng cao hn so vi DVB-T trong

mng n tn (SFN) vi cng gi tr tuyt i ca khong bo v (67%). DVB-T2

cn cho php s dng khong bo v ln hn 20% so vi DVB-T, iu ny cng

ng ngha vi vic m rng vng ph sng ca cc my pht trong mng SFN.

Bng 3.2: Dung lng d liu trong mang SFN

3.3. Mt s ni dung chnh trong tiu chun DVB-T2.

3.3.1. M hnh cu trc DVB-T2.

H thng DVB-T2 c chia thnh 3 khi chnh pha pht (SS1, SS2, SS3)

v 2 khi chnh pha thu (SS4, SS5) nh trnh by trong hnh 3.1


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Hnh 3.1: M hnh cu trc DVB-T2.

a. SS1: M ho v ghp knh.

Khi SS1 c chc nng m ho tn hiu video/audio cng cc tn hiu ph

tr km theo nh PSI/SI hoc tn hiu bo hiu lp 2 (L2 Signa

Truyen Hinh So Mat Dat DVB

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