Thông số kỹ thuật DVB S2

8/13/2019 Thng s k thut DVB S2

1/14

Thng s k thut cng ngh truyn d n v tinh DVB-S2

DVB-S2 l th h truyn dn th hai cho pht qung b v tinh. S ra i ca DVB-S2 l mtbc t ph v cng ngh so vi th h th nht DVB-S bi nhng ci tin trong m sa saimi (BCH & LPDC) v kt hp vi cc kiu iu ch v m ho cp cao. ng dng cng ngh

DVB-S2 s lm tng ng k hiu sut s dng bng thng trong truyn dn v tinh: t 30%n 131% so vi DVB-S ng thi gim gi thnh thu transponder v tinh.

Bi vit ny gii thiu v cng ngh DVB-S2, im khc bit gia hai h thng DVB-S2 vDVB-S, ti sao nn s dng DVB-S2, m sa sai BCH & LDPC, iu ch v m ho cp cao (VCM & ACM) ng thi phn tch hiu qu ca cng ngh DVB-S2 so vi cng ngh DVB-Shin ti.

1. Gii thiu.

K nguyn truyn dn thng tin bng v tinh thc s c hiu qu vo nhng nm 80. Khi , truyn dn

qua v tinh tit kim bng thng v gi thnh khi s dng cc kiu iu ch QPSK v BPSK. Nhngnm 90, cng ngh pht qung b qua v tinh pht trin rng ri sau khi ETSI cng b chun DVB-Su tin, kt hp iu ch QPSK vi m sa li hng truyn trong v ngoi (Viterbi v Reed-Solomon).Cuc cch mng v m sa li kt hp vi cc cu hnh iu ch mi v mt lot cc c tnh mi l nntng lm nn tiu chun DVB-S2. y l tiu chun mi nht trong cc tiu chun ca ETSI v truyndn thng tin v tinh. Kiu iu ch ny cng khp li con ng tim cn gii hn v mt l thuyt(gii hn shannon).DVB-S2 vi hiu sut s dng bng thng tng t 30% n 131% so vi DVB-S ang c k vng sem li hiu qu to ln khi c ng dng, vi kh nng truyn dn ng thi nhiu dch v c tc ln nh truyn hnh c phn gii cao HDTV, Internet tc cao, truyn s liu v cc ng dngchuyn nghip trn cng mt b pht p ca v tinh m h thng DVB-S trc kh c th thchin c.

2. Cng ngh DVB-S2.

Nhn thong qua (Hnh 1) th tiu chun DVB-S2 (ETSI TR 102 376) c t s thay i ng k no khi sosnh vi tiuchun DVB-S (EN-301210 v EN-300421): M sa sai trong Viterbi v ngoi Reed-Solomonc thay th bng m sa sai LDPC (Low-Density Parity Check) v BCH (Bose-Chaudhuri-Hocquenghem) tng ng.


2/14

Ngoi ra, iu ch phc tp hn v s lng bit hu ch/ hertz tng ln. Tiu chun mi cung cp cckiu iu ch QPSK (2 bit/hz), 8PSK (3 bit/Hz), 16APSK (4 bit/Hz) v thm ch l 32APSK (5 bit/Hz). So

snh vi kiu iu ch QAM, cc cu hnh iu ch APSK (Amplitude and Phase-Shift Keying ) cho phpvic b d dng vi b pht p-transponder phi tuyn.S khc nhau chnh v cng l hiu qu ca DVB-S2 khi so snh vi DVB-S l kh nng kt hp cc dngd liu vo mt sng mang, iu ch, m ho thay i v tng thch (VCM v ACM) v cp bn trongdng dliu khng phi MPEG (non-MPEG2).S kt hp cc dng d liu khc nhau s lm tng s lng tn hiu truyn ti trn mt sng mang.Trong thc t, iu ny c th xem nh vic s dng b ghp knh (MUX), nhng li khng phi chunhng bt li t vic nh li tham chiu thi gian chng trnh PCR v s thay i thng tin dch v -thng tin c trng chng trnh (SI-PSI).Chc nng iu ch v m ho thay i -VCM (Variable Coding and Modulation) cho php xc nh mtcu hnh iu ch khc nhau v mcsa li cho mi dng d liu trn cng mt sng mang.Chc nng iu ch v m ho tng thch -ACM (Adaptive Coding and Modulation) cho php thay ing cu hnh iu ch v mc bo v li cho mi khung d liu. Khi kt hp cc dng d liu vi ccu cui thu c c cu hi tip, tnh nng ACM c bit thch hp cho vic ti u bng thng cho mtmng tng tc: Cc thng s truyn dn c th c ti u cho mi b u cui v cc nh hng dothi tit nh l fading do ma c th c b d dng v an ton.DVB-S2 c v nh l mt b cng c cho cc dch v tng tc: iu ch v m ho cao cp, truynti bt k dng (format) d liu no. Mc tiu ca b cng c DVB-S2 l mt h thng n phc v chocc ng dng khc nhau.


3/14

2.1. iu ch v m sa sai cao cp.

a. iu ch: C 4 kiu iu ch QPSK, 8PSK, 16APSK v 32APSK (Hnh 2). Tt c c ti u hotng vi cc transponder phi tuyn. B iu ch s dng b lc bng gc (B.B filter) v iu ch vunggc (Quadrature Modulation) to ra cc dng ph tn hiu v tn hiu cao tn RF. y l b lc dng

cosin tng c 3 dng ph ng vi 3 h s roll-off (): 0,20; 0,25 v 0,35. Gim h s s lm tng suy gim bn ngoi knh phi tuyn (do nhiu ISI tng ln), ph tn hiu ra s vung hn.

b. M sa sai cao cp:DVB-S2 s dng phng php m ho khi BCH v LDPC.- Kch thc khi m ho: 64800 bit (hoc 16200).- T l m sa sai: 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 8/9, 9/10.

c. Qu trnh to khung (Frame):Truyn dn tn hiu c t chc thnh cc khung ni tip nhau. C th hnh dung cc khung ny nhnhng toa tu ch hng ni ui nhau, Trn mi khung (toa tu) gm phn mo u-Header (H), trongmi Header cha cc bit ng b v bo hiu (iu ch, m ho...) v khi m ho mang thng tin(Code block). Vi ACM qu trnh bo v c th thay i tng khung mt (16APSK, 8PSK, QPSK2/3) tutheo iu kin truyn dn (tri nng, c my m hay ma).

2.2. DVB-S2 truyn ti bt c nh dng d liu no.

- DVB-S2 c th tip nhn dng truyn ti n chng trnh hoc a chng trnh, dng MPEG-TS haydng generic (v d nh IP...).- DVB-S2 tng thch tt vi cc loi m ho MPEG-2, HDTV cng nh cc h thng m ho mi (nhH.264AVC, VC1).- Mi dng tn hiu u vo c th c bo v bng cc cch khc nhau. Khi qua h thng truyn dn


4/14

DVB-S2, tn hiu sau gii m ng nh ban u.

3. Ti sao nn s dng DVB-S2?

S dng tiu chun DVB-S2 hin l hng i hon thin trong thc t. Hon thin y mun ni n

s tit kim ng k v di thng, tng hiu qu cng sut ca qu trnh truyn dn, c thit k vicc tnh nng ti u v yu cu t s C/N thp khi thu.

3.1. Tng dung lng truyn dn trn cng mt bng thng.

So snh vi tiu chun DVB-S trc y vicng iu kin truyn dn: DVB-S2 c kh nng truyn dliu ln hn ti 30% so vi DVB-S trong cng iu kin bng thng [1]. Hay ni cch khc mt tn hiutruyn dn theo tiu chun DVB-S2 yu cu bng thng t hn 30% so vi khi s dng tiu chun DVB-S.c bit khi ng dng iu ch, m ho VCM v ACM hiu sut s dng bng thng tng tng ng66% v 131% [4]. iu ny c ngha l gi thnh thu transponder s thp hn.

3.2. Tng hiu qu cng sut ca qu trnh truyn dn.

S dng DVB-S2 cn lm tng hiu qu cng sut ca qu trnh truyn dn. Trong vng ph sng, mttn hiu DVB-S2 yu cu thu c mc thp hn khong 2.5 dB so vi mt tn hiu DVB-S trong cngiu kin bo v li. Ngoi ra, DVB-S2 cn c th tng thch c bt k c tnh transponder v tinhvi s khc nhau ln ca cc tn s ph (t 0.5 n 4.5 bit/s trn mt n v bng thng) v yu cu vt s C/N kt hp (t -2dB n 16dB).Khi DVB-S2 c ng dng vi cc ng dng im-im nh IP unicasting, gain ca tn hiu DVB-S2 cnln hn DVB-S. Chc nng iu ch v m ho thay i (VCM) cho php thc hin iu ch v mc bov li khc nhau s dng hoc thay i trn c s tng frame mt. Chc nng ny cn c th kt hpvi vic s dng knh ng v (return channel) t c iu ch m ho tng thch kho vng(closed-loop).

V vy cc thng s truyn dn c ti u cho mi thu bao ring bit ph thuc vo iu kin ngtruyn.ACM cho php s dng li t 4 n 8 dB phn cng sut thng dng d phng cho suy hao do ma(clear sky margins) trong cc truyn dn thng tin v tinh thng thng DVB-S. Do vng ph sng vtinh tng gp 2 hoc 3 ln dn ti gi thnh dch v gim t ngt.

3.3. DVB-S2 c thit k vi cc tnh nng ti u.

DVB-S2 cti u cho cc ng dng v tinh bng rng nh:

- Cc dch v qung b: Truyn dn cc chng trnh SDTV hoc HDTV.

- Cc dch v tng tc bao gm c truy nhp internet.- Cc ng dng chuyn nghip: phn phi tn hiu truyn hnh s ti cc trm pht hnh s mt t,truyn s liu v cc ng dng chuyn nghip khc (DSNG, Internet Truncking, Cable Feeds).

DVB-S2 khng b hn ch vi kiu m ho video v audio MPEG-2 m c th tng thch vi cc kiu mho MPEG-2, MPEG-4 v HDTV. Tiu chun ny cng mm do hn khi chp nhn bt k dng u vo,bao gm dng bit lin tc, dng truyn ti MPEG n hoc a chng trnh, IP hay ATM. c tnh nycho php cc dng d liu khc v cc cu hnh d liu trong tng lai c th s dng c vi DVB-S2m khng cn ti mt tiu chun mi.


5/14

3.4. Yu cu v t s C/N thp.

Cc mode truyn dn ca DVB-S2 i hi t s C/N khi thu thp. Kt qu t c bi m phng mytnh trn knh nhiu trng (Hnh 4).

Mi lin h gia dung lng knh v bng thng ca ccknh nhiu trng c xc nh bi cng thcni ting do Shannon tm ra, l:C = W log2(1 P/N)

Se = C/W

Trong :

Se : Hiu qu ph (bit/s/Hz).C : l dung lng knh (bit/giy).W: l bng thng ca knh (Hz).P : l cng sut my pht (w).N: l cng sut nhiu (w).So snh DVB-S2 vi DVB-S chng ta c th thy li (gain) dung lng tc bit ca tn hiu DVB-S2t ti 25-35 % so vi DVB-S (cng t s C/N v tc symbol) tu thuc vo mode truyn dn v ngdng. DVB-S2 c tnh linh hot rt ln, n c th tng thch vi bt k c tnh ca transponder no:Hiu sut ph c th t t 0.5 n 4.5 bit/s/Hz, C/N trong khong t -2 n 16 dB trong mi trng cnhiu trng.Nhn trn hnh 4, chng ta thy DVB-S2 ch cn cch ng gii hn Shannon t 0.7 n 1 dB, iu ny

c ngha rng vi DVB-S2T nay v sau, chng ta s khng bao gi phi thit k mt h thng khc chopht qung b v tinh [2].

4. M sa sai BCH & LDPC.

M sa sai kim tra u tin cng thp LDPC (low-density parity check code) l mt lp cc mkhi tuyn tnh tng ng l mt ma trn kim tra u tin H. Ma trn H ch gm cc s 0 v 1 nm rirc. iu c ngha rng cng ca cc s 1 trong ma trn ny rt thp. Vic m ho c thc


6/14

hin bng cc phng trnh bin i t ma trn H to ra cc bit kim tra u tin. Vic gii mc thit lp bng cch s dng cc u vo mm (soft-inputs) kt hp vi cc phng trnh ny to ra cc c lng mi cho cc gi tr thng tin c gi. M LDPC c th loi tr c cc tng li. xc nh c tng li, mt m pha ngoi (m ngoi) c thm vo trong cng ngh LDPC, lm BCH. M ngoi BCH c hiu qu vi tng li thp. V vy, t chc DVB chn phng php m

ngoi BCH v m ni LDPC l m sa sai ca tiu chun DVB-S2.

5. iu ch v m ho thay i VCM.

Mt c tnh ni bt ca cng ngh DVB-S2 l cc dch v khc nhau c th pht trn cng mt sngmang. Mi dch v vn gi cu hnh iu ch v t l m sa sai ring. y l mt kiu ghp knh trnlp vt l c gi l iu ch v m ho thay i VCM (Variable Coding and Modulation). VCM thc spht huy hiu qu khi cc dch v khc nhau khng cn t l m sa sai ging nhau (chng hn c thchp nhn mt mt knh th hai trongtrng hp fading do ma) hoc cc dch v khc nhau c chnh cho cc trm khc nhau trong nhng iu kin thu thng thng khc nhau.

6. iu ch v m ho tng thch ACM.

Hin nay cc dch v DVB IP/Unicast cung cp bi v tinh s dng tiu chun DVB-S cho ng truyndn. DVB-S c pht trin cho cc ng dng qung b qu trnh bo v tn hiu khi xuyn qua cc

lp vt l ca bu kh quyn i vi tt c cc dch v l hng s v n khng i sut thi gian truyndn (qu trnh ti u ng truyn dn ch thc hin cho trng hp xu nht: dch v ti nht, pht tinht v v tr ti nht).DVB-S2 s dng cng ngh ACM (Adaptive Coding and Modulation) cho php iu ch v m ho sa lithch nghi tu thuc vo cc iu kin truyn dn:bu tri trong xanh hay ma, cho thu nm trungtm hay mp ca bp sng (beam) pht t v tinh...

Mi u cui DVB-S2 c th hot ng vi mt ngng (margin) C/N rt thp v vy lm tng s lngchng trnh trn cng mt bng thng ca v tinh.


7/14

H s khuch i (gain) ca cc b pht p trn v tinh ng dng ACM s tng ln khi tn s tng(bng C, Ku v Ka). Do bng tn Ku v Ka thch hp hn c vi ACM. Bng Ka t cht tri hn v thnn s dng ACM.DVB-S2 thc hin qu trnh truyn dn thch nghi ny vi tng ngi s dng c lp v ch vi cc dch

v im-im (hnh . Tn hiu c a vo iu ch ACM pha pht, truyn dn qua v tinhbng DVB-S2.

Ti pha thu, sau gii iu ch c my o t s C/N I v truyn tn hiu o ny v trung tm iu khintc bit chuyn i kiu truyn dn ng xung vi mc bo v thp (8PSK, FEC 5/6) khi trinng khng c my hoc mc bo v cao (QPSK, FEC 1/2) khi tri nhiu my c ma.

7. Hiu qu ca cng ngh DVB-S2 so vi DVB-S.

thy c hiu qu ca cng ngh DVB-S2 so vi DVB-S, ta xt v d so snh v s lng cc chngtrnh SDTV v HDTV c truyn dn bng 2 cng ngh DVB-S v DVB-S2 trong cng iu kin cngsut v bng thng v tinh (transponder 36 MHz): Cho thu 60 cm; trng hp th nht s dng nnMPEG-2, tc nn video l 4.4 Mbit/s vi chng trnh SDTV v 18 Mbit/s vi chng trnh HDTV;trng hp th hai s dng nn video cao cp Advanced Video Coding (AVC), tc nn vi chngtrnh SDTV l 2.2 Mbit/s v 9 Mbit/s vi chng trnh HDTV.


8/14

Yu cu v t s C/N ca hai h thng DVB-S v DVB-S2 l bng nhau trong vic khai thc cc modetruyn dn khc nhau v trong qu trnh vi chnh h s roll-off, tc symbol ca h thng DVB-S2. Ktqu kim chng cho thy li v dung lng ca h thng DVB-S2 so vi DVB-S l khong 30 %. Hnna bng vic kt hp h thng DVB-S2 vi m ho AVC (H.264/AVC hoc VC1), mi transponder36Mhz ca v tinh c th truyn dn 21-26 knh SDTV, hoc 5-6 chng trnh HDTV trn mt

Transponder. iu ny s lm gim gi thnh thu trn mt knh ca v tinh.

8. Sa li tin (FEC) v iu ch:

FEC l k thut sa li ch cht cho php thc hin ch tiu k thut tuyt vi trong ng truyn v tinh, ni c

mc nhiu (noise)v can nhiu (interference) cao. Sau qu trnh m phng cc s FEC khc nhau, nhm d n

DVB-S2 chn m kim tra parity mt thp (Low Density Parity Check Code LDPC) lm m FEC cho DVB-S2.

Cc LDPC c chn s dng chiu di khi rt ln (64800 bit cho cc ng dng m tr l khng quan trng, v

16200 bit). T l m , 1/3, 2/5, , 3/5, 2/3, , 4/5, 5/6, 8/9 v 9/10, ph thuc vo vic chn mode iu ch v

yu cu h thng cng c chn, cc t l m , 1/3, v 2/5, kt hp vi iu ch QPSK, cho php h thng hot

ng trong iu kin ng truyn km, ni mc tn hiu thp hn mc nhiu. M xon ngoi BCH c a vo

trnh li ti ni c t l bit li thp (BER). 4 loi iu ch c th chn cho qu trnh truyn ti .

QPSK v 8PSKl loi c ngh s dng cho cc ng dng qung b, chng l loi iu ch c ng bao l

hng s v c th c s dng trong cc b pht p v tinh khng tuyn tnh c hot ng gn v tr bo ha

ca p tuyn hot ng. Mode 16APSK v 32 APSK, ch yu nhm ti cc ng dng chuyn nghip, c th c s

dng cho qung b, nhng i hi mc C/N cao v p dng phng php tin sa mo trong trm up -link gim

thiu tnh phi tuyn b pht p.


9/14

Trong khi cc mode ny khng hiu qu cng sut bng cc mode khc, hiu sut ph li ln hn nhiu. Cc chm

sao 16APSK v 32APSK c ti u ha hot ng trn b pht p phi tuyn bng cch t cc im trn

cc vng trn. Tuy nhin ch tiu k thut ca chng trn knh tuyn tnh c th so snh c vi ch tiu ca cc

loi 16QAM v 32QAM tng ng.Bng cch chn chm sao iu ch v cc t l m, c th t c hiu sut ph t 0.5 n 4.5 bit/symbol, ph

thuc vo kh nng v cc hn ch ca b pht p c s dng.DVB-S2 c 3 chn la h s roll-off xc nh hnh dng ph: 0.35 cho DVB-S, 0.25 v 0.2 cho bng thng hn

ch.

9. Cu trc Framing.

Hai mc cu trc framing c thit k:

u tin mc vt l (PL), mang vi bit tn hiu c bo v cao; Th hai mc c bn, mang cc bit tn hiukhc nhau, cho php linh hot cc i trong thch nghi vi tn

hiu vo.

Mc u tin ca cu trc framing c thit k cung cp s ng b tt v tn hiu cho lp vt l. V vy my

thu c th ng b (khi phc sng mang v pha, ng b frame) v pht hin cc thng s m ha, iu ch trc

khi gii iu ch v gii m FEC.Tham kho hnh 2, tn hiu lp vt l DVB-S2 gm c chui cc frame xp theo trnh t: "ging nh cc xe ti xp

ni ui nhau" trong mi frame lc m ha v iu ch phi ng nht, nhng c th chuyn i (ACM) trong

cc frame lin k. Mi frame l mt payload cha 64800 bit (hoc 16200 bit), tng ng vi mt m khi chp FEC

LDPC/BCH, v mt header (90 symbol iu ch nh phn), cha thng tin tn hiu v ng b. V header PL c b

thu gii m u tin, n phi khng c bo v bi mch FEC LDPC/BCH.

Mc th 2 ca cu trc framing, l frame c bn, thch hp cho cc ng dng: n hoc a dng truyn vo, dng

truyn ti, m ho v iu ch c nh CCM (Constant Coding & Modulation) hoc m ho v iu ch thch nghi

ACM (Adaptive Coding & Modulation), v nhiu cu hnh chi tit khc. Nh c bo him li LDPC/BCH v di ln

ca frame FEC, Baseband Header (BB Header) c th cha nhiu bit nh du (80) m khng lm gim hiu qu

truyn dn hoc tnh mnh kho chng nhiu.

10.Cc ch tng thch ngc.

Do tn ti mt s rt ln my thu v tinh theo tiu chun DVB-S trn th trng nn cc nh qung b kh c thchuyn t ngt vic thay i cng ngh sang DVB-S2, t bit nhng ni c tr gi my thu v cc dch v cng

cng min ph (Free To Air FTA). Do vy cn phi c s tng thch ngc trong thi k chuyn dch t DVB-S

sang DVB-S2, cho php cc my thu DVB-S tip tc hot ng, trong khi cung cp dung lng v dch v b sung

cho cc my thu mi, tin tin. cui thi k chuyn dch, khi ton b my thu chuyn sang DVB-S2, tn hiu

pht x c th c chuyn sang ch khng tng thch ngc, cho php khai thc y tim nng ca DVB-

S2.

Vic chn la ch tng thch ngc (BC) c xc nh trong DVB -S2, bng cch d kin gi 2 dng truyn

ti trn mt knh v tinh n. Dng truyn th nht (u tin cao HP) tng thch vi my thu DVB-S cng nh vi


10/14

my thu DVB-S2, trong khi dng truyn th 2 (u tin thp LP) ch tng thch vi b thu DVB-S2.

Kh nng tng thch ngc c th c thc hin bi iu ch theo lp, ni c hai dng truyn ti HP v LP ng

thi c kt hp ng b mc iu ch symbol trn mt chm sao khng u 8PSK. Tn hiu u tin thp (LP)

tng thch DVB-S2 l m ho BCH v LDPC, vi t l m LDPC , 1/3, hoc 3/5. K nh x theo lp to ra

chm sao khng u 8PSK. Hai bit u tin cao (HP) DVB-S xc nh mt im trong chm sao QPSK, trong khi mt

bit n tb m ho LDPC DVB-S2 thit lp thm gc xoay

trc khi truyn i. V vy tn hiu truyn i cuicng c mt hnh bao hu nh bt bin, c truyn i trn mt b pht p n gn nh bo ha.

11. Ch tiu k thut ca h thng.

Ph thuc vo vic la chn t l m v chm sao iu ch, h thng c th hot ng trong iu kin t s sng

mang trn nhiu (C/N) t -2.4 dB (s dng QPSK ) n 16 dB (s dng 32APSK 9/10), vi gi thit knh nhiu

trng AWGN v b gii iu ch tiu chun (xem hnh 3). Kt qu thu c t qu trnh m phng trn my tnh cho

t l li gi (PER) 10-7 (mt gi dng truyn ti b li trn mt gi truyn i bng dch vu video 5Mbit/s). Khong

cch vi gii hn Shannon t 0,7 n 1,2dB. Trn knh nhiu trng AWGN, kt qu tiu biu cho thy dung lng

gia tng ln 20 35% so vi DVB-S v DVB-DSNG vi cng iu kin truyn dn, thu mnh hn 2 2,5dB vi cng

hiu qu ph.

H thng DVB-S2 c th c s dng trong cc cu hnh "mt sng mang trn mt b pht p" hoc "a sng

mang trn mt b pht p" FDM. Hnh 3 cng ch r dung lng tc bit c ch Ru c th thc hin c bi h

thng trong cc cu hnh iu ch /m ho khc nhau, gi thit tc symbol l Rs. Tc symbol Rs tng ngvi di thng -3dB ca tn hiu iu ch, trong khi Rs(1 ) tng ng vi ton b di thng tn hiu theo l

thuyt sau b iu ch, vil h s roll-off ca qu trnh iu ch . S dng h s roll-off hp hn= 0,25 v =

0,20 c th cho php gia tng dung lng truyn i, tuy nhin cng c th to ra suy gim phi tuyn ln hn bi v

tinh cho hot ng sng mang n.Khi v tinh truyn i tn hiu DVB-S2, cc tn hiu iu ch hnh bao gn nh bt bin: QPSK v 8PSK c hiu qu

cng sut trong cu hnh truyn n sng mang trn b tip sng, v chng c th hot ng trn b tip sng

ch gn bo ha. 16APSK v 32APSK vn nhy hn vi cc loi mo phi tuyn v yu cu cc b tip sng hotng ch gn tuyn tnh (c ngha vi Back-off u ra, OBO, ln hn), c thci thin trong hiu qu cng sut

bng cch dng k thut b phi tuyn trm pht ln v tinh (up -link). Trong cu hnh FDM, nhiu sng mang hot

ng trn cng mt b tip sng, nn phi gi cho vng hot ng gn nh tuyn tnh (OBO ln) trnh can

nhiu iu ch qu mc tng h gia cc tn hiu.

Bng 1 biu th, cho cu hnh truyn n sng mang trn b tip sng, m phng suy gim C/N s dng cc model

knh v tinh v mt n nhiu pha (TWTA phi tuyn, nhiu pha tng ng cc khi khuch i nhiu thp LNB) ti

im TWTA hot ng ti u nht (my tnh m phng bng ESA). CSAT l cng sut sng mang khng iu ch khi

tng khuch i cng sut cui HPA bo ha, OBO l t s cng sut (dB) c o gia cc sng mang khng iu

ch bo ha v sng mang iu ch (sau OMUX). Cc s liu ch ra u im ln do dng tin mo ng i vi 16

APSK v 32APSK. Suy gim nhiu do nhiu pha cho APSK v c bit 32APSK, c th c xem nh l mt s bi

quan, v chng lin quan n loi LNB dn dng trong khi i vi nhng ng dng chuyn nghip, cc thit b tthn c th c p dng trong khi gi thnh tng khng ng k.

12. S phn b nhiu ghp knh MPEG n cc my pht k thut s mt t DTT.

Truyn hnh k thut s mt t (DTT) c s dng ti nhiu quc gia trn th gii. Mt trong nhng gii php

phn b dng truyn MPEG n cc my pht k thut s l thng qua v tinh. Cc h thng hin hnh c t

trn c s DVB-S, cho php truyn mt ghp knh n MPEG trn tng tn hiu. Kt qu l phn b n ghp knh


11/14

MPEG, cn pht i n sng mang cho tng b tip sng, do vy yu cu OBO ca tng khuch i cng sut cui

(HPA) ca v tinh ln (nu khng cn dng n b tip sng). Vic p dng DVB-S2 cho php phn b nhiu ghp

knh MPEG khi s dng mt cu hnh mt sng mang trn mt b tip sng, v vy ti u ha hiu qu cng sut

bi s bo ho tng khuch i cng sut cui ca v tinh. iu ch v m thch nghi khng c quan tm v nhiu

trm s phi thu cng mt tn hiu.

V d, mt tn hiu DVB-S2 vi tc symbol 30Mbaud c th c truyn i trn mt b tip sng bng thng 36MHz, h s roll-off= 0,20. Nh vy truyn i 2 ghp knh truyn hnh s mt t DTT trn DVB -S2, tc bit

24Mbit/s, yu cu hiu qu ph 1,6 [bit/s/Hz], tng ng vi QPSK 5/6. C/N cn thit phi t 6dB trong bng thng

30MHz. Kh nng kt ni cho php (99,9%) phi c thc hin vi an ten up-link ng knh 3m (EIRP 64dBW), b

tip sng gn bo ha trong vng khng gian trong, anten thu 1,2m ti ni t my pht sng mt t. Khi s dng

DVB-DSNG vi 8PSK 2/3 v ch nh 2 sng mang FDM trong 36MHz vi tc symbol 13,3 Mbaud, C/N cn thit

ca my thu phi t 9dB trong bng thng. m bo 99,9% kh nng kt ni, cng sut (EIRP) pht up-link

phi t c 75dBW, iu chnh li ca b tip sng t c OBO = 5.5dB trn mt sng mang trong

khng gian sch v kch thc ca anten thu khng c nh hn 2m. V vy DVB -S2 cho php anten thu nh

(gim i gn mt na) v trm up-link r hn.

13. iu ch v m ho thch nghi ACM cho cc dch v "im im" (one-to-one)

Khi DVB-S2 c s dng cho cc ng dng tng tc im im ging giao thc truyn trn internet, kt qu l

tt hn so vi DVB-S, nu s dng iu ch v m ho thch nghi ACM. Thc t ACM cho php khi phc ci gil"d phng khng gian sch" (4 n 8dB cng sut), b ph trong kt ni v tinh"iu ch v m ho hng s", v

vy tng gp i thm ch gp 3 thng lng trung bnh ca v v gim ng k gi thnh dch v. ACM lc s cho

cc bng tn s cao (th d nh Ka) v cho vng kh hu nhit i.

Vic p dng mc vt l c thc hin nh sau:

Mi trm thu v tinh ST o tnh trng knh (C/N I c sn) v bo co thng qua knh phn hi n cng(GW).

Cc bo co ca trm thu v tinh (ST) c cng GW tnh ti khi chn mc bo v ch nh cho gi dliu a ch n ST;

trnh trn thng tin trong lc tn hiu yu, v nguyn l c cu iu khin tc bit ca ngi s dngphi c thc hin, tng thch lu lngc ngh vi dung lng knh c sn. iu ny c th c

thc hin theo nhiu cch, tu theo cc yu cu dch v c bit v cu trc mng. Cng GW p t bo

him li, p dng i vi phn d liu ngi s dng cho qua k thut giao din thch hp. Vi dch v

"one-to-one" (v d DSNG), cc kt ni unicast IP s dng ACM cho DVB-S2 phi p dng bo him li trn

c s tng cp ngi s dng: s ngi s dng c th rt ln (c th ln n hng trm, hng ngn

ngi). Hn na, vic iu khin tc ngun trc tip c th l iu khng th c, v cc ngun thng

tin (ngi cung cp thng tin IP) xa cng GW v tinh.

Mt quyt nh quan trng trong cc h thng ACM l tr vng p dng lp vt l, v n ni kt mt cch cht

ch vi h kh nng thng theo di cc thay i knh. Nu vic p dng vng nhanh, tnh lin tc ca dch v c

bo m ngay c trong thi gian tn hiu yu, ng thi gi d phng ng truyn C/N thp cc i ha

thng lng ton b h thng. V tc bin thin ti a C/N I bng tn Ka c tnh ton khong 0,5dB mi


12/14

giy trong khi c ma ln, v v khong cch C/N gia 2 mc bo him DVB-S2 gn st nhau l khong 1 dB, cc

tr vng iu khin nh hn 1 giy s cho php gim thiu ti a s mt mt gi d liu truyn i.

14. Cc dch v DSNG v gp tin (contribution) TV s dng ACM.

K thut ACM rt ha hn ci thin vic thc hin kt ni im im v im a im nhng kt ni gp tin TV

ni m dng truyn ti TS n c gi n mt hoc vi trm thu. Trong trng hp ny, bo him cc gi dngtruyn ti TS phi theo nhng bin thin C/N I trn knh v tinh ti im thu. Tc bit dng truyn ti l hng s,

thi gian tr thp, nh c yu cu bi MPEG, c th c bo m bng cch dng cng c thch nghi dng DVB-

S2. trnh dng truyn b trn, khi thng lng knh b gim, phi dng b m ho video tc bit thay i.

Trc tin chng ta xem xt cc dch v ca TV contribution s dng trm pht v thu ln truy nhp vo b tip

sng loi36MHz vi 4 tn hiu FDMA. S dng tiu chun DVB-DSNG v mode iu ch 16QAM , 4 tn hiu TV

contribution, tc bit 18.5Mbit/s c th c n nh trong b tip sng (cng sut bc x trung bnh tng ng

EIRP uplink 76dBW, 14,3 global IBO v ng knh anten thu/pht l 7m, 99,9% kt ni c kh nng thc hin

c). S dng DVB-S2, 16APSK 5/6 v h s roll-off = 0.2, tc ng truyn mi kt ni c th tng ln 24.75

Mbit/s, v vy tc bit ca DVB-S2 cao hn ca DVB-DSNG 30%. Mc khc, khi gitc bit nh DVB-DSNG, ch

tiu DVB-S2 tt hn c th c s dng gim ng k kch thc ng knh ca anten pht/thu xung cn

4,5m (8PSK 5/6, EIRP uplink 74dBW, 13dB global IBO). Vi cng kch thc anten 4,5m, nhng thm chc nng

ACM ca DVB-S2, tc bit c ch s c tng n 24.75 Mbit/s, vi iu kin bu tri trong (clear sky).Nhng u im ca DVB-S2 v ACM cng r rng cho dch v DSNG. V d, trong bng thng v tinh 9MHz, DSNG

s dng anten 1.2m (EIRP uplink 61dBW) c th truyn 19.8 Mbit/s trong iu kin khng gian sch (16APSK 2/3,

h s roll-off = 0.2) v chuyn sang 14.85 Mbit/s khi suy hao ng truyn ln (8PSK 2/3). Vi mc ch so snh,

DVB-DSNG vi QPSK 7/8 s ch cho php truyn 10.7 Mbit/s.Mt v d na, xem xt trm DSNG dng gn nh (fly-away) vi kch thc anten 90cm v cng sut HPA ch 12W

(99,9% kh nng thc hin c kt ni, EIRP up-link 49dBW, 12 dB global IBO, anten thu ng knh 4 m, 4 tn

hiu cho mt b tip sng). S dng DVB-S2 v ACM, 9.9 Mbit/s (QPSK 2/3, roll-off = 0.2) trong khng gian sch,

8.9 Mbit/s (QPSK 3/5) vi iu kin trung bnh v 3.68 Mbit/s (QPSK ) vi iu kin kt ni kht khe. iu ny s

cho cht lng hnh nh tt khi s dng m ho video MPEG-2, v cht lng tuyt vi nu s dng b m ho mi

AVC. DVB-S (QPSK ) cn mt trm cng sut ln hn 5dB (so vi DVB -S2) truyn tc bit c nh 6.1 Mbit/s.

15. Kt lun.

DVB-S2 l tiu chun mi nht trong h thng tiu chun DVB cho cc ng dng v tinh bng rng, vihiu sut sdng bng thng tng t 30% n 131% so vi cng ngh DVB-S hin nay. Cng ngh nythc s l b cng c hu hiu cho cc dch v tng tc qua v tinh.T chc DVB khng cho rng DVB-S2 s thay th DVB-S trong mt thi gian ngn trong lnh vc qungb truyn hnh thng thng. Hng triu b gii m DVB-S ang hot ng tin cy v ng gp vonhng thnh cng ca thng mi v tinh s trn ton cu. Cc ng dng mi c d tnh pht quav tinh nh truyn dn HDTV v phn phi cc dch v da trn nn IP s thc hin hiu qu da trnh thng DVB-S2. Vic kt hp DVB-S2 v cu hnh m ho video v audio mi (v d nh

H.264/AVC/VC-9) c th pht 21-26 chng trnh SDTV hoc 5-6 chng trnh HDTV trn mttransponder 36Mhz. Trong cc ng dng truyn dn chuyn nghip, DVB-S2 c kh nng cung cp iuch v m ho tng thch (ACM), tnh nng ny c hiu qu ln vi cc dch v im-im nh l cctrm DSNG nh. Trong cc ng dng mi ny, h thng DVB-S2 s lm c nhng iu m h thngDVB-S khng th lm c.

Cc ch vit tt:


13/14

ACM: Adaptive Coding and Modulation iu ch v m ha thch nghi

APSK: Amplitude Phase-Shift Keying iu ch dch pha, bin ATM: Asynchronous Transfer Mode Mode truyn bt ng b

AVC:(MPEG-4) Advance Video Coding M video tin tinAWGN:Additive White Gaussian Noise Nhiu trng

BCH:Bose-Chaudhuri-Hocquenghem (code) M BCHBER: Bit-Error Ratio T s bit li

C/N:Carrier-to-Noise ratio T s sng mang trn nhiuCCM:Constant Coding and Modulation iu ch v m ha c nh

DSNG:Digital Satellite News Gathering Thu thp tin tc k thut s v tinhDTT:Digital Terrestrial Television Truyn

hnh k thut s mt tDVB:Digital Video Broadcasting Truyn hnh k thut s

DVB-S:DVBSatellite Truyn hnh k thut s v tinh

DVB-S2:DVBSatellite, version 2 Truyn hnh k thut s v tinh, th h 2EIRP:Effective Isotropic Radiated

Power Cng sut bc x ng hng tng ng

FDM:Frequency Division Multiplex Ghp knh phn chia tn sFEC:Forward Error Correction Sa li tin

HP:HighPriority Mc u tin caoHPA:High Power Amplifier Tng khuch i cng sut cao

IBO:Input Back-Off Back-Off u voIP:Internet Protocol Giao thc truyn internet

LDPC:Low Density Parity Check M kim tra Parity mt thp LNB:Low-Noise Block Khi khuch i nhiu thp

LP:Low-Priority Mc u tin thpMUX:Multiplex/Multiplexer/Multiplexing B ghp knh

OBO:Output Back-Off Back-Off u ra

OMUX:Output Multiplexer B ghp knh u raPER:Packet Error Rate T l li giPSK:Phase-Shift Keying iu ch dch pha

QAM:Quadrature Amplitude Modulation iu bin trc phaQPSK:Quadrature (Quaternary) Phase-Shift Keying iu ch vung pha

VCM:Variable Coding and Modulation iu ch v m ha bin thin

Trung c (Tng Hp)

Ti liu tham kho.

[1]. ETSI TR 102 376 V1.1.1 (2005-02)Digital Video Broadcasting (DVB), User guidelines for the second

generation system for Broadcasting, Interactive Services, News Gathering and otherbroadband satellite

applications (DVB-S2).

[2]. Cutting down space segment costs for interactive services- Alberto Morello, Chairman DVB-S2.

[3]. DVB-S2 ready for lift off - Alberto Morello and Vittoria Mignone.

[4]. Analysis of the bandwidth eifficiency of DVB-S2 in a typical data distribution network, Dirk Breynaert,

Maximilien dOreye de Lantremange, Newtec CCBN2005, Beijing, March 21-23 2005.[5].Tp ch Khoa Hc K Thut Truyn Hnh.


14/14

[6] A. Morello and V. Mignone: New DVB standard for DSNG and contribution satellite linksEBU Technical Review No. 277, Autumn 1998

[7] ETSI: EN 301 210: DVB: Framing structure, channel coding and modulation for DSNG and other

contribution applications by satellite.

Documents

Thông số kỹ thuật DVB S2