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CAN NHIU GIA HAI H THNG ANALOG V DVB-T

KS. Anh TNgun ti nguyn tn s ngy cng hn hp trong lnh vc truyn dn thng tin, do

trong qu trnh quy hoch cho pht s, chng ta cn quan tm n vn pht s xen vo giacc khong trng ca cc knh analog.

V vy, bi vit ny xin bn v can nhiu gia hai h thng thu pht: analog/s v gia svi s khi pht xen k nhau.

Nhm nghin cu v nh gi cng ngh phthnh s DVB-T, cng ty VTC tin hnh thnghim pht s trn hai knh lin k v chen vocc khe trng ca cc knh analog. V d pht strn hai knh lin k 29+30 bn cnh knh 28analog ti Hi phng (hnh 1) v knh 31 (QungNinh). T tri sang, ph analog knh 28 ch c 3

khu vc: mang hnh, mang mu v mang mthanh, trong khi ph pht s (knh 29+30)ng u c hai di thng 8 MHz.

Hnh 1. Ph pht s knh hai knh lin k

T s bo v l t s cng tn hiu cn thu cht lng tt trn cng tn hiu cannhiu (dB). T l ny c th nh hn 1 (tnh sang dB mang gi tr m), tc l cng tn hiu

cn thu thp hn cng tn hiu can nhiu v c th ln hn 1 (tnh sang dB mang gi trdng).

i vi thu s, th l gi tr t s bo v, m ti gi tr ny BER (t l li bit) phi nhhn hoc bng 2.10-4 sau gii m Viterbi; gi tr 2.10-4 cn c tn l gi tr QEF (Quasi ErrorFree-gn nh khng li), tng ng vi BER nh hn hoc bng 1.10-11 ti u vo ca giiMPEG-2 [1; trang 66].

1. Can nhiu analog v s cng knhCng trng cn cho thu analog PAL D/K phi ln hn cng trng ca s l

34dB (i vi nhiu chim 1% thi gian-nhiu i lu-hay gi l nhiu ngn) v 40dB (i vinhiu nhiu lin tc xut hin 50% thi gian) (Bng 1).

H thng analog mong mun Nhiu i lu Nhiu lin tcPAL B, B1, G, D, K 34 40

PAL I 37 41SECAM L 37 42SECAM D, K 35 41

Bng 1. T s bo v cho thu analog vi can nhiu l tn hiu s DVB-T

Nh chng ta thy, t s bo v cho thu analog rt ln, mc d can nhiu ca tn hiuDVB-T c tnh cht nh nhiu Gaussian v ch th hin trn hnh nh c mui (nh b ht) chkhng lm mt ng b hnh. Tuy nhin, trong thc t ngi dn thu tt cc knh analog hinc, chng ta s khng b tr pht s trng knh vi analog.

Tp ch Bu chnh Vin thng & Cng ngh thng tin Thng 8/20041

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Bng 2 nu cng trng cho thu s 64QAM, FEC 3/4 phi ln hn cng trnganalog l 10 dB. Nu thu s vi nhm thng s 64QAM,FEC2/3 (ITU gi l mode M3), thcng trng cho thu s ch phi ln hn trng analog l 4dB. Bng 2 cng ni ln s khcbit rt r gia hai gi tr t l m sa sai FEC 3/4 v 2/3. V vy, nhiu n c chn m sa sai l2/3.

Bng 2. T s bo v cho thu s DVB-T vi can nhiu l tn hiu analog

T s bo vChm sao QPSK 16QAM 64QAMTc m 1/2 2/3 3/4 5/6 7/8 1/2 2/3 3/4 5/6 7/8 1/2 2/3 3/4 5/6 7/8ITU-Mode M1 M2 M3Tn hiu

PAL/SECAM-12 -8 -5 2 6 -8 -4 0 9 16 -3 4 10 17 24

Tm li, thu tt analog ng knh, t s bo v cho n l 40dB, cn thu s ngknh, t s bo v cho n ch l 10dB (thp hn ca analog 30dB). iu chng t kh nngchng can nhiu ca cng ngh analog km hn nhiu so vi cng ngh s.

2. Can nhiu knh s v analog lin k (knh s thp, knh analog cao)Knh s thp (v d knh 34), knh analog cao (v d knh 35), nh vy knh s s can

nhiu sang phn hnh ca knh analog.Bng 3. T s bo v cho thu hnh ca knh analog (knh n) khi b can nhiu do

pht s knh ln cn di (knh n-1)

H thng analog mong mun Nhiu i lu Nhiu lin tcPAL B1, G, D, K -7 -4PAL I -8 -4SECAM L -9 -7SECAM D, K -5 -1

Bng 3 cho thy, thu tt analog trong trng hp ny, cng trng cho thu analog thphn (th hin du tr) cng trng ca s l 7 dB (nhiu ngn) v 4 dB (nhiu lin tc).

Ni chung, s can nhiu sang thu analog khng ng l bao v khng cm nhn c, khi mcng trng ca pht s cao hn cng trng ca analog khong 3-4dB.

Bng 4. T s bo v cho thu s knh di, khi b can nhiu ca knh analog ln cn trn. [1;bng 21]

Tn hiu mong mun Tn hiu nhiuH thng BW Mode PAL B PAL B1, G PAL I PAL DK SECAM L SECAM D, K

M1 -46DVB-T 8MHz M2 -40

M3 -38M1 -43

DVB-T 7MHz M2 -38

M3 -36

y l trng hp thu s knh 34 ti Thi Bnh b can nhiu ca sng knh 35 analog ph.Bng 4 mi ch th hin cho h PAL I, cng trng cho thu s thp hn(du tr)

cng trng ca analog 38-40dB, m s vn khng b can nhiu. ITU gi: M1 l16QAM,FEC1/2; M2 l 64QAM,FEC1/2 v M3 l 64QAM,FEC2/3.

Thc t, chng ti thu s knh 34 tip pht li cho khu vc Thi Bnh, Nam nh.Ti v tr gin cng tc (cao 45m so vi mt t) ca ct pht sng trn khu t i PTTH ThiBnh, chng ti phi thu s knh 34 pht i t H Ni, trong iu kin trng gn ca knh 35cng sut 5kW (knh pht chng trnh ca a phng) rt mnh. Khong cch Thi Bnh-H


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Ni theo ng chim bay l 85km. Ct pht sng s ti H Ni ch cao 100m. V vy, sng sknh 34 n Thi Bnh khng phi sng truyn thng, b nhiu lm cy che chn, qu yu.Sng s truyn qua khong cch di b suy gim qu nhiu v b cc can nhiu khc trn ngtruyn tc ng, lm phm cht b gim. Trng gn ca my pht knh 35 ti ct pht sng liqu mnh v cha nhiu thnh phn ph xung quanh knh 35, thm ch trng c vo knh 34.Phi ni y l trng hp khc nghit.

Kim chng thc t, chng ti nhn thy ti Thi Bnh, nu s chnh lch cng trng s/analog vt qu 35dB, li thu s xy ra lin tc, thu khng thnh cng. T s li bit(BER) tng ln rt nhanh, vt gi tr gn nh khng li QEF (Quasi Error Free). S liu o thct [35dB] v s liu khuyn co ca DVB [38dB], chnh lch 3dB ny c th l gii bngba l do: mt l v tr thu s ti ct pht sng ca Thi Bnh qu xa (85km) khng phi sngthng, hai l pht analog knh 35 li qu gn, cc thnh phn ph xuyn iu ch qu nhiu,thm ch c thnh phn ph trng knh 34, ba l chng ti thu vi thng s 64QAM 3/4 chkhng phi 64QAM 2/3 nh s liu khuyn co ca DVB.

Cc s liu o ti hin trng Thi Bnh trong trng hp khc nghit ny cho chng timt kt lun nh lng, nu t s bo v cao hn hoc bng 35dB, th c th khng gy ra licho thu s knh 34. Cu trn c cp cm t c th, v cn mt thng s quan trng na khngth b qua l t s S/N ca thu s l bao nhiu, tc l phm cht ca tn hiu s thu c. Can

nhiu ca cc thnh phn ph ca sng mang hnh vo ph thu s th hin khng ch trong ph,m cn cng vi cc yu t ng truyn, phch tn s v cc yu t khc, lm gim t s S/Nca thu s v cht lng sng pht ra ca chnh bn thn my pht s DVB-T.

Thc t, gii quyt vn thu s khi b can nhiu ca analog lin k trn, do mc trngca s knh 34 n Thi Bnh qu thp, nn cc chng ti phi s dng khuch i anten c hs tp m thp gii quyt ci thin cht lng thu s knh 34. Tuy nhin, kt qu cha thtm mn. Do sng ca pht s knh 34 phi truyn xa 85km b fading, i khi khng phi lfading su, m ch l fading chn lc (selective fading), phm cht ca sng thu c xu,khng th thng ni can nhiu do my pht knh 35 ln sng.

Qua vic o c v kim chng, chng ti nhn thy, nu thu sng s trong iu kinbnh thng, khng qu ti hn v khc nghit (xa v kh nh ti Thi Bnh), sng analog khngqu mnh, khng c thnh phn ph cng knh, th t s bo v cho thu s l m [38-40dB] doDVB cng b, s liu ng tin cy.

3. Can nhiu knh s v knh analog lin k (knh s cao, knh analog thp)y l trng hp ti ni thnh Hi Phng, analog pht trn knh 28 v s pht trn knh

29+30.Bng 5. T s bo v cho hnh ca thu analog PAL-I (knh n, c th knh 28) khi b can

nhiu ca pht s knh lin k trn (knh n+1, knh 29) [1; bng 29]

H thng analog mong mun Nhiu i lu Nhiu lin tcPAL B1, G -9 -7PAL I -10 -6SECAM L -1 -1SECAM D, K -8 -5

PAL D, KBng 5 cho thy, i vi PAL-I l m 6-10dB. Cng trng ca analog thp hn (th

hin du tr) cng trng ca s 6-10dB, m hnh khng h b s can nhiu. Knh analogthp, knh s cao, nn s can nhiu sang phn m thanh l ch yu. Ph ca s cch xa tn smang hnh, kh can nhiu sang hnh. Trng hp ny th nghim ti Hi Phng, my pht DVB-T knh 29+30 ti Ph Lin (Hi Phng) cng sut rt thp (100W-110W/knh 8MHz). V vy,cng trng ca s thp hn ca analog knh 28 l 30-35dB, nn s khng gy can nhiusang thu analog knh 28.


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Bng 6. T s bo v cho thu s (knh n=29) b can nhiu do pht analog knh lin k di(knh n-1=28) [1; bng 20].

Tn hiu mong mun Tn hiu nhiu

H thng BW Mode PAL B PAL B1, G PAL I PAL DK SECAM L SECAM D, KM1 -43

DVB-T 8MHz M2 -38M3 -34M1 -43

DVB-T 7MHz M2 -40M3 -37

i vi ch pht M3 (64QAM2/3) l m 34dB. Trn bng 4, DVB cng ch a cng bvi PAL D,K, m ch c PAL-I. i vi mode M3 (ITU gi: M3 l 64QAM2/3) th cng trng cho thu s thp hn (th hin du tr) cng trng ca analog 34dB. Trong trnghp ny, analog can nhiu sang thu s ch yu l do sng mang m thanh. Nh ng sng mang mthanh l iu tn, ph thp (thp hn sng mang hnh 10dB), tnh ngu nhin, nn t gy can

nhiu cho thu s.Kt qu thc nghim v o kim ti Hi Phng (analog knh 28, s knh 29), khi mccng trng (sng mang m thanh) ca knh 28 analog cao hn ca mc s 30-32dB, ccu thu s bnh thng cho hnh khng b li. Kt qu o ti Hi Phng v ti Thi Bnh in kt lun nh lng c th. Kt qu o ca cng ty VTC thc hin vi ch pht khngphi M3 (64QAM2/3), m l 64QAM3/4, chnh vi s liu ca 64QAM2/3 do DVB cng b,cng hon ton hp l.

4. Can nhiu s sang s cng knhy l vn cn c bit quan tm, khi thit lp mng pht s DVB-T. i vi mng

n tn tht s (cc my pht pht cng mt tn hiu, trn cng mt tn s v ti cng mt thiim), ngi ta phi tnh ton b tr cc my pht hp l v phi thc hin vic ng b cho ccmy pht ca mng n tn ny. Nhng cc my pht DVB-T cng knh, nhng khng thchin vic ng b, th do truyn sng tt (trong mi trng thun li, hoc truyn sng ven bin)sng s i xa v gy can cho qu trnh thu, gi l can nhiu ng knh DVB-T.

Thc t ti Vit Nam, hin tng sng i qu xa (v l do truyn sng bt thng), cannhiu sang nhau xy ra vi analog: sng Bc Ninh cng knh sng Nam nh-37 th hin cannhiu ti mt s v tr ng nam Nam nh, Ty Ninh-Tr Vinh: knh11; Sng knh 9 caHu, men theo bin vo n Tam K-Qung Nam nhiu knh 9 ca Nng pht n.

i vi pht s DVB-T, DVB gi y l hin tng can nhiu cng knh. Trong [2; trang66; bng 18] cp vn ny, tuy nhin, mi ch nu cho 2k, cha cp ti 8k.

Bng 7. T s bo v s sang s cng knh

ITU-Mode iu ch Tc m PR Gaussian PR Rice PR RayleighQPSK 1/2 5 7 8M1 16QAM 1/2 13 14

16QAM 3/4 14 16 20M2 64QAM 1/2 18 19M3 64QAM 2/3 19 20 22

Vi M3 (iu ch 64-QAM FEC 2/3) cho thy, t s bo v (PR) cho thu s khi b s cng knhcan nhiu ch cn 20dB (thu c nh-m hnh Rice) v 22dB (thu di ng-m hnh Rayleigh).


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Ti liu [3] cc tc gi o trong phng th nghim cho s liu cn thp hn khuyn co ca DVBkhong 1dB.

i vi cng ngh k thut analog, t s bo v khi b can nhiu analog cng knh khikhng s dng offset cao ti 50dB [5], cn khi offset tht chnh xc, th t s bo v thp nhtcng phi t 22dB. Nh vy, t s bo v cng knh vi cng ngh pht s DVB-T (20-22dB)thp hn nhiu so vi pht analog (22-50dB). Mt ln na, cho thy, kh nng chng can nhiuca DVB-Tu th hn hn cng ngh analog.

Do t s bo v cho thu s khi b can nhiu s cng knh ch cn 20-22dB, nn trongthc t, vi anten thu TV bnh thng c cc chn t dn ng v phn x, th bn thn t ltrc/sau ca anten thu gp phn khc phc c hiu qu hin tng can nhiu s/s cngknh. Ngha l, trong vng cng trng ca hai my pht ngang nhau, ch cn quay anten(c nhiu chn t) v hng i cn thu, th can nhiu ca i kia loi b c.

5. V fading chn lc i vi DVB-TNh cp trn v hin tng fading chn lc (Selective fading). i vi analog,

fading chn lc khng quan trng v khng tc ng nhiu. Nhng khi thu s ti cc khongcch xa (Ph Lin-Kin An-Hi Phng cch H Ni 88km; Thi Bnh cch H Ni 85km),chng ti cm nhn, kim chng tc hi ca fading chn lc i vi DVB-T.

Xem trn my o ph thy ph p v kh ng u, nhng c lc thu vn b li, rtkh l gii, nu nh khng quan tm n fading chn lc. V trn my o ph khng th hinph tc thi, nn kh pht hin ra fading chn lc (chn tn). Hin tng fading chn lc tc lfading ch xy ra vi vng tn s rt nh no ca ton di thng 8MHz. V th, nhiu ti liucn gi l fading chn tn. i vi pht hnh s mt t s dng k thut ghp a tn trc giaoOFDM (Orthogonal Frequency Division Multiplex) c rt nhiu sng mang (ch 2k l 1705sng mang; ch 8k l 6.817 sng mang), th fading chn lc (hay chn tn) ch xy ra vimt vi chc sng mang ca hng ngn sng mang . Nhng khng may, fading li xy ra vicc pilot lin tc, ri rc v cc sng mang thng s pht, m u thu tr c tin phi da vochng nh gi knh, ng b tn s, ng b pha, t ng tm ra cc thng s pht, th thttai hi v nghim trng. Khi cc pilot v sng mang b fading, suy gim di mt mc no, th u thu khng th lm vic, kt qu nh hon ton dng.

Khi i mt vi hin tng fading chn lc i vi DVB-T, chng ti mi hiu k c kin ca cc chuyn gia Braxin ph phn nhc im ca h pht s ISDB-T ca Nht [4].Trong ISDB-T (h Nht), ngi ta chia ra lm 10 khong tn s cho cc dch v hnh nh v 3khong cho cc dch v m thanh. Vic phn chia ny lm tng gi thnh c v phn cng vphn mm, v my thu phi lm vic vi cc bng tn khc nhau. Vic phn on tn s ny cnlm sai nguyn tc ca mt knh truyn s, l mt knh bng rng trong cc dch v c t cc mc khc nhau. Nu chia knh thnh cc on tn s khc nhau cho cc dch v khcnhau, nu mt on tn s b nh hng (do fading chn lc), th ton b dch v nm trongan tn s s b mt. l mt trong nhng l do ti sao cc nh thit k DVB-T khngs dng k thut ny. Vi kin nh gi , cng vi phn tch mt lot yu t khc na dn ti vic Braxin khng chn h Nht m chn DVB-T ca chu u.

6. Ti liu tham kho[1]. TR 101 190 V1.1.1 (1997-12) Digital Video Broadcasting; Implemention guidelines forDVB terrestrial services; Transmission aspects.[2]. ti Xy dng quy hoch phn b knh tn s cho truyn hnh tng tmt t bng tnVHF/UHF doCc Tn s V tuyn in, B Bu chnh, Vin thng thc hin nm 2003.[3].Th.S inh c Vnh, KS. Ng Vn Lo,Mt s vn cn quan tm khi quy hochmng truyn hnh s DVB-T; Tp ch KHKT Truyn hnh, s 4/2003.


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[4]. DVBS Comments to the Submission for the ANATEL CONSULTA PUBLICA; No 291-Brazilian Digital Terrestrial Television (DTV) System. July 2001.[5]. ITU-RECOMMENDATION 655-2: Radio-Frequency protection ratios for AM vestigialsideband television systems.


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