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7/31/2019 Truyen Hinh So Mat Dat DVB
1/111
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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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 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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ti: Truyn hnh s mt t DVB-T v qu trnh chuyn i sang DVB-T2.
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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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ti: Truyn hnh s mt t DVB-T v qu trnh chuyn i sang DVB-T2.
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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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ti: Truyn hnh s mt t DVB-T v qu trnh chuyn i sang DVB-T2.
GVHD: TS.Nguyn Hong HV: ng Vn Anh
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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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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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ti: Truyn hnh s mt t DVB-T v qu trnh chuyn i sang DVB-T2.
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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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ti: Truyn hnh s mt t DVB-T v qu trnh chuyn i sang DVB-T2.
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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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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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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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Khi ly mu 8 bit: (720+720+720) x 576 x 8 x 25 = 249 Mbit/s.
Khi ly mu 10 bit: (720+720+720) x 576 x 10 x 25 = 311 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 8 bit: (720+360+360) x 576 x 8 x 25 = 166 Mbit/s.
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 tn hiu chi UY
im ly mu mu CR
im ly mu mu lam CB
Hnh 1-7:Tiu chun 4:2:2.
im ly mu tn hiu chi UY
im ly mu mu CR
im ly mu mu lam CB
Hnh 1-8: Tiu chun 4:2:0.
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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
Hnh 1-9: Tiu chun 4:1:1.
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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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* 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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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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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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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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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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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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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