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THU THP D LIU QUAV TINH

( Digital Satellite News Gathering)

I. Trm mt t (Earth Station) :I.1Kinh tuyn v V tuyn

Tri t c chia ra thnh cc kinh tuyn v cc v tuyn xc nh v tra l ca cc quc gia, lnh th hay mt v tr a l c th no

Cc ng kinh tuyn u quy v hai cc Bc Nam v c chia thnh3600. Kinh gc c chn l ng i qua Greenwich (Anh), ni t i thinvn u tin ln nht. T kinh (00) v hng ng bao gm 1800ng v v

hng ty bao gm 1800Ty.Cc ng v tuyn u ct v ngang qua cc ng kinh tuyn. V l

ng xch o chia tri t lm hai phn: bn cu Bc v bn cu Nam. V 900N ly

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a cc Bc (Bc cc) v 900S ly a cc Nam (Nam cc). Cc ng kinhtuyn v v tuyn c m t nh hnh v sau:

Hnh 1.1 Kinh tuyn v V tuynVit Nam nm ta 80 240 v Bc v 10001100 kinh ng. H Ni l

21004 v bc v 105084 kinh ng. TP H Ch Minh 10046 v bc v 10604 kinhng.

I.2a cc v a t:

Cc ng kinh tuyn u tp trung v hai a cc nn gi l Bc v Nam al. Cn la bn s ch hng Bc t trng ca qu t nn gi l Bc a t. Docc trm thu u nm trn cc kinh tuyn v v tuyn a l nn c s khc bit vihng a t mt gc, gi l gc lch. Gc lch thay i theo v tuyn v kinhtuyn, hay c th hn l n tng t l vi v . V d vi v 50750th gclch 0,7708,330. Gc lch ca H Ni 3017 v TP HCM 1066.

I.3 Gc ngng (Elevation), Gc phng v (Azimuth)v Gc phn cc (Angleof polavisation)

i vi v tr anten ti mt t xc nh to v tinh cn ba thng ssau:

l gc ngng, gc phn cc v gc phng v. Thit b c lin quan ti 3thng s ny l Anten parabol v phn thu sng.I.3.1 Gc ngng (e)

Gc ngng l gc to thnh gia tip tuyn ti dim thu mt t v ngni im thu n v tinh. c m t nh hnh v

Gc ngng ti xch o ln nht bng 900v cng i v hai cc th n cnggim.

O

1600

1400

1200

1000

80

60

40

2020

40

60

80

1000

1200

1400

1600

A

20

60

40

OV tuyn200

Kinh tuyn1000

Xch o

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Trong phm vi hp ta c th tm coi mt t ni t Anten l ng tip tuyn.Cch tm hay dng gc ngng rt kh. Bi vy ta c th tm gc nghing ca n lp t d dng hn. Ta tnh c gc ngng nh sau:

e = 900 gc nghing (i)Gc ngng c tnh:

Hnh 1.2 Gc ngng v gc nghing

Gi tr gc lch v gc nghing c cho bng sau :

V

Gc nghing (i) Gc lch (d)

10 0 46 - TP.HCM 11,5 1,66210 04 TP.H NI 21,48 3,17

Khi bit v ni thu ta c th tm ra gc ngng.cos(Lon)xcos(Lat) 0.151263

e = arctg[1 cos2(Lon)xcos2(Lat)]1/2 (I.1.3.1)

Lat : V ni thu ; Lon : sai bit kinh gia ni thu v v tinh.

I.3.2 Gc phng v:(Azimuth) [a]Cc v tinh trn qu o a tnh nm trong mt phng xch o. Mi mt

trm thu mt t ch c th nhn thy v tinh na phn qu t, t kinh tuyn001800.

e = 900 [ gc lch (d) + gc nghing (i)]

ng ngang song song vi mt t

Gc ngng e

( 90- e)

Gc nghing: ( 90- e)o

Mt phng t

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Trong cc biu c ly chun kinh tuyn 00, r v hng Ty v hngng . Thc t do s che khut ca b mt tri t, ch cn 700v hng Ty-ng.

Gc phng v l gc dn hng cho anten quay tm v tinh trn qu o atnh theo hng t ng sang Ty. Gc phng v c xc nh bi ng thng

hng ti v tinh.Gc c tnh theo chiu kim ng h, theo cng thc:

a = 1800+ kinh Ty

Hoc a = 1800-kinh ng

Gc phng v ph thuc vo kinh ca im thu v kinh ca v tinh.tg(Lon)

a= 180o+ arctgsin(Lat) (I.1.3.2)

cos(a) - 0.151263tg(e)=

sin(a) (I.1.3.3)I.3.3 Gc phn cc :

Khi ng trc tm cho parabol thu hng thng n tm bp sng chnhca anten pht (Downlink) ca v tinh th mt cho anten thu s nhn c gnnh ton b nng lng ca chm sng chnh trong mt phng phn cc.

Nu nh Anten thu nm lch tm vi chm sng chnh ca tn hiu v tinh, hiusut thu nng lng gim v cn gy ra cc tc hi khc nh lm mo dng tnhiu, tng tp nhiu. V vy cn phi hiu chnh li gc phn cc bng u d

phn cc u thu. Gc phn cc cng thay i theo v tuyn v kinh tuyn giatm chm sng bc x vi im thu. Thng thng gi tr ca n c tnh sn

theo v v kinh .Khi dng c cu ng b d tm tn hiu cc v tinh trn qu o a tnh

nu t cc gc khng ng th anten s khng bm theo ng qu o a tnh.Trng hp ny s khng thu c tn hiu ca tt c v tinh trn qu o.

S phn cc sng in t m t s nh hng ca vector in trng trongkhng gian. S phn cc c quyt nh bi cch m tn hiu RF c bc x tanten vo khng gian. Chc nng ny c thc hin bi mt b phn cc n l mt phn ca h thng anten. Mt anten c kh nng pht v nhn tn hiu nu nc phn cc.

Mt sng phn cc tuyn tnh c hng ca vector in trng hp vi trcng hoc ngang mt gc khng i khi n lan truyn trong khng gian. V vy,khi vector in trng song song vi chiu ngang th sng c phn cc ngang,v khi vector in trng thng ng th n l phn cc ng. Hnh v sau cho tathy s phn cc ngang v phn cc ng ca mt sng i vo mt phng t giy.

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Hnh1.3 (a) phn cc ng (b) phn cc ngang (c) phn cc dng elipVector in trng ca cc sng phn cc dng trn v trn cc vng trn khi

sng lan truyn. Chiu quay ca n theo chiu kim ng h nu cc sng cphn cc trn tay phi( Right Hand Circularly Polarized Wave) v ngc li. Smo dng ca sng c phn cc dng ellip i vo mt phng t giy.S mo dng c o bi t s dc trc Arc cho bi

Ar= EMax/ EMinTrong :

EMaxv EMin l hai trc ln v nh ca elip.Mt thng s quan trng khc ca sng l gc nghing ca ellip vi trc thamchiu. S phn cc dng ellip c th xem l mt trng hp tng qut bi v loi

phn cc ny s t phn cc thng khi Ar n v phn cc trn khiAr=1.

Mt cch l thuyt, mt Anten c phn cc i c th ngn cch cc sngc truyn trong phn cc trc giao, cho php mi phn cc c nhn mt knhring bit. Mt h thng s dng tnh cht ny ca Anten c xem l h thng

phn cc i.

I.4 nh v v tinh :a) Cn chnh hng Bc Nam ca Anten: Trc tin s dng la bn chnh

cho trc xuyn tm ca Anten nm ngang hng Bc Nam. Vit Nam ta phicng thm 62E c c hng Bc Nam thc.

c th xc nh c ta V Tinh, phi iu chnh Anten Parabol theocc thng s gc ngng, gc phng v v gc phn cc .

b) Cn chnh gc ngng cho- El (Elevation): l gc to thnh gia tip tuyn

thu mt t v ng ni im thu n v tinh.

(b)(a)

i

Emin

Emax

(c)

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Cch 1:

Hnh 1.5 Gc ngng v gc nghingTrong phm vi hp c th tm coi mt t ni t Anten l ng tip tuyn.Ta tm gc nging ca Antenna Parabol lp t. Gc nghing to bi mt phngming cho v mt t nh hnh v.

Do cc trm thu u nm trn kinh tuyn a l (tp trung v 2 cc) nn c skhc bit vi hng a t (c xc nh bng la bn) mt gc, gi l gc lch

biu th s sai lch v kinh . Gc lch thay i theo kinh tuyn v v tuyn, hayc th hn l n tng t l vi v . V d v 575th gc lch 0,778,33;gc lch ca H Ni 317 v Tp HCM 166 .

Lc ny gc ngng c tnh bng :El = 90 - (gc lch + gcnghing)

Ti xch o, gc lch bng 0, gcnghing = v . Khi y ng tm trcParabol s song song mt phng xcho, v gc ngng = 90.

Xch oXch o

Tip tuynngang

Gc ngng

Gc ngng

Gc nghing

Gc lch

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Hnh 1.6 Gc lchCch 2 : p dng cng thc ton hc tnh gc ngng.

Trong :

cos(Lon)xcos(Lat) 0.151263e = arctg

[1 cos2(Lon)xcos2(Lat)]1/2

Lonl sai bit kinh gia ni thu v v tinhLatl v ni thu (ni t Anten)

V d:V tinh AsiaSat 1, 1055 E

Ni thu ti Tp HCM (1064 E; 1046 N) Lon = 1064 - 1055 = 09

Lat = 1046

Thay vo cng thc ta c :cos(0o9)cos(10o46) 0.151263

e = arctg[1 cos2(0o9)cos2(10o46)]1/2

Gc ngng e= 77,7

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c) Cn chnh gc phng v Az (Azimuth) :Gc phng v l gc dn hng cho Anten quay tm v tinh trn qu o a

tnh theo hng t ng sang Ty. Gc phng v c xc nh bi ng thnghng ti v tinh .

V tinh 1 V tinh 2

Hnh 1.7 nh v gc phng vtg(0o9)

a= 180o+ arctgsin(10o46)

a = 185

0

II.1 S lng chng trnh truyn hnh qua mt knh:Mt knh truyn c di thng c nh cho php truyn c cc chng trnh

truyn hnh .Vn ny ph thuc vo : rng knhDng iu chTc m sa li FECDch v truyn hnh

ln khong bo v (i vi pht sng trn mt t theo tiu chunDVB-T COFDM)S dung ha gi d liu s

* rng knh : xc nh rng knh c th truyn c tn hiu s c tc ln nht l

bao nhiu cn da vo nh l Shannon. nh l ny xc nh s ph thuc ca tc

Cc Bc

Nam

Gc Az ca v tinh 2180- 30= 150

Gc Az ca v tinh 1180+ 45= 225

45W30E

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bit cc i vo rng knh truyn t s S/N chun, truyn tn hiu s c tc bit C (Mbps) yu cu rng knh bng W 3/4 C (Hz) .

T vi rng W cho th c th truyn c tn hiu s c tc bitln nht bng 4/3 W. V d vi knh c rng 8MHz th tn hiu s c tc bitln nht l 8 4/3 10,7 Mbps c th truyn qua n .

* Dng iu chs:Khi dng iu ch s ta thng gp khi nim di thng hiu dng. Vi cc

dng iu ch s ta c cc di thng hiu dng thc t nh sau :2-PSK 0,75Mbps/Hz4-PSK 1,5 Mbps/Hz8-PSK 2,25 Mbps/Hz16-PSK 3,0 Mbps/Hz64-QAM 3,36 Mbps/Hz256-QAM 6,0 Mbps/Hz

V d vi knh c rng 8MHz khi dng iu ch 64-QAM th c thtruyn c dng bit c tc cc i l 36 Mbps .* Tc sa li FEC:

Tn hiu iu ch s thng c x l bng cc m c bit cho ccmy thu (IRD hoc Set-Top-Box) c th kim tra cc bit thng tin c gi i cc thu ng khng. K thut sa li tin (gi thng tin sa li trc n mythu cng vi cc d liu gc) to tn hiu c kh nng chng nhiu tt hn hn sovi truyn s khng c m c bit .

Cc thc nghim m ha trc y so snh ch tiu ca tin tc c mso vi khng m v thy rng s ci thin tn hiu ny l khong 3,3 dB. Ngoi ra,mt b m ha s dng c hai k thut m khc nhau s to ra li b sung.

Trong k thut FEC ngi ta dng cc symbols d b sung vo tin tc gc. Mcd iu ny tng tc truyn ton b v cc yu cu di thng, cc symbols dtng cng tnh thng nht ca tin tc, ngn nga khng cho tp knh che khutcc symbols ph b s thng nht ca tn hiu c ch. Cc b gii m dng ccsymbols FEC phc hi d liu sau khi tin tc c thu .

Mt dng m FEC, c gi l m Viterbi, c biu th bng t s, vd1/2, 3/4,hoc 7/8. T s biu th s symbols gc u vo b m ha, cn mus biu th s symbols c sa li u ra b m ha. Do vy, FEC 7/8 c ngharng c 7 symbols truyn d liu i vo b m ha v 1symbols sa li trong 8symbols i ra .

Mt dng m FEC khc l Reed-Solomon. M ny b sung cc symbols dvo cc chui hoc cc khi s nh phn. Reed-Solomon dng 188 bytes trong mikhi 204 bytes truyn thng tin gc. S cn li c dng gi cc bit kimtra n IRD tr gip vic sa li truyn .

B gii m Reed-Solomon dng thut ton gii quyt ng thi mt tphp cc chng trnh i s da trn biu hin kim tra chn t khi thu c .* Dch v truyn:

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Mt dng MPEG-2 c th cha nhiu chng trnh truyn hnh cng viAudio, cc audio ph, d liu truy cp c iu kin, cc d liu ph nh teletext,kt ni Internet. Yu cu tc i vi tng loi l khc nhau. V d phim chtlng VHS c th c truyn tc bit 1,5 Mbps; tin tc v chng trnh TV 3,4 - 4 Mbps; qung b cht lng cao (studio) tc hn 8 Mbps. Tc m

cn thit cho qung b MPEG-2 bt k thay i ph hp vi cc quyt nh phnphi bit c lm bi mi nh cung cp chng trnh .

Bng cc tc d liu MPEG-2 cho cc th loi chng trnh :Cc dch v Video Tc d liu

Truyn hnh c phn gii cao (HDTV) 14 20 MbpsTruyn hnh c cht lng Studio (CCIR 601) 8,064 MbpsTruyn hnh c t l khun hnh 16 : 9 5,760 MbpsChng trnh th thao trc tip 4,608 MbpsChng trnh phim/qung b 3,456 MbpsChng trnh phim pay-per-view 3,152 Mbps

Audio musicamMono 128 KbpsStereo 256 KbpsMt cp stereo 512 KbpsD liu s 96 KbpsD liu iu khin dch v 30,72 Kbps

* Khong bo v : Trong pht sng trn mt t theo tiu chun DVB-TCOFDM, khc phc hin tng phn x nhiu ng khong bo v s cdng u mi chu k symbols. Do vy di thng c ch s b gim tng ng vi di khong bo v.

* Vn dung ha gi tn hiu s: Tc symbols thng thay i t mtgi s ny ti gi s tip theo. Tc symbols cc i c th t c l mt hmca di thng knh. Thng thng c th tnh :

Tc symbols cc i = rng di thng / 1,2Con s 1,2 (hoc 1,3 ...) ch ra rng trong thc t di thng truyn l b

hn di thng knh do cc b lc truyn khng l tng .V d i vi ng truyn v tinh c di thng transponder 36 MHz th tc

symbols cc i = 48 Msym /s .Gi s dng tc FEC 3/4 th tc d bit cho php knh s l :48 Msym /s 2 ( 2 bit cho mt symbol, iu ch QPSK) = 96 Mb/s

96 Mb/s 3/4 (tc FEC inner code) = 72 Mb/s72Mb/s 188/204 (tc FEC Reed-Solomon outer code)= 66,3 Mb/sNu ch thay i tc FEC ca inner codet 3/4 xung 1/2 th tc bit c

ch c gi tr: 48 Msym /s 2 = 96 Mb/s96 Mb/s 1/2 = 48 Mb/s48 Mb/s 188/204 = 44,2 Mb/s

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Ta thy mc d tc FEC 1/2 s lm tng kh nng chng nhiu, n cnglm gim rt nhiu tc bit c ch, c ngha l gim s dch v chng trnh cth gi qua transponder .

Do vy ty theo tnh hnh c th m c s dung ha gia tham s FEC vtc bit c ch .

* Tc bit li (BER) v Eb/No:Tc bit li (BER) xc nh ch tiu ca ng truyn s. BER 110-3

biu th xc sut mt li xy ra trong mt khi 1000 bits. BER 5 10-5 l tt hn

BER 9,0 10-4 v l xc sut li xy ra s thp hn. BER cng c th c

hin th nh 5E-4 hoc 3E-3, tng ng vi BER 5 10-4 hoc 3 10-3

o nh lng v ng truyn s bng Eb/No(t s nng lng bit trn mt tp).

Eb/No(dB) biu din t s bit trn tp ca h thng thu. Khi Eb/Notng sbit li s gim.

BER

10-3

10-4 typical QPSK viterbi

10-5 FEC rate 1/2

10-6 IDEAL10-7

10-8

10-9

3 4 5 6 7 8 Eb/NoHnh 1.14 iu ch QPSK c kh nng thc hin BER Eb/No tng.i thp cho cc ng dng di rng nh thng tin v tinh .

S chng trnh c th truyn trn mt knh (di thng c nh) thay i trongmt phm vi rt ln, t mt chng trnh n hng chc chng trnh (phng

php thng k tc m thay i ).V d Band C trn mt v tinh (uplink 5927MHz 6403MHZ ,downlink

3702MHZ 4178MHZ) c 12 Transponders mi transponders cch 4MHZ lctruyn tn hiu analog c th truyn c 24 chng trnh truyn hnh (michng trnh truyn trn mt knh c rng khong 36 MHz), khi truyn tn

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hiu s c nn bng phng php ghp knh s chng trnh truyn hnh c thtruyn ln ti hng trm .

Hnh 1.15 Ghp knh

Tch knh thc hin qui trnh ngc li ca b ghp knh .

Khi phc tn hiu ban u m thng c nhng ch n vi nhau ti nithu ca kt ni truyn dn .d. S khi trm thu hnh v tinh (TVRO-Television Receiver Only)Trm thu hnh c nhn TVRO c nhim v thu tn hiu t cc v tinh a tnh, quax l tn hiu gii iu ch v hon tr li dng nguyn th.

Mt trm thu TVRO gm cc b phn chnh sau y:Thit b bn ngoi (Outdoor) gm:

* Cho anten parabol, ng knh cho t 0,66m, ty thuc vo bngtn thu v cng trng ti im thu.

* Phu thu sng, ng dn sng v que phn cc.* B khuych i dch tn nhiu thp, LNA hay LNB.* C cu iu khin cho (Positioner) quay theo gc ngng v phng

v.* C cu iu khin gc quay phn cc (Polarotor)

Thit b bn trong (Indoor) gm:* My thu TVRO* Mch in iu khin gc quay ca Polarotor v Positioner* B iu khin t xa v b nh.

Multiplexer

Info

rmati

on

signa

ls

M ultiplexer signal

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S khi ca trm thu TVRO biu th di tn s v chc nng ca tngkhi.

10.95 to 11.7 GHz | 0.95 to 1.7GHz | 134MHz |(11.7 to 12.1 GHz)

Outdoor Unit | IndoorUnit

Hnh 1.29 S khi trm thu TVRO

Khi bn ngoi gm anten parabol v b dch tn LNB. Trong :(2a) L mch i phn cc trc giao(2b) L mch chuyn tn sband C, Ku v UHFDi tn lm vic ca khi ny bng tn C (3,74 GHz) v bng tn Ku

(10,9512GHz), dch tn xung trung tn 1, IF1=0,951,7GHz.

(3) Gia khi bn ngoi v khi bn trong l mch khuych i cp tuyntnh , lm vic gii tn s 0,95 1,7GHz.

(4) Khi trong nh gm b khuych i trung tn (IF2 distributor) c t45 u phn nhnh ng ra , cho tn hiu RF Video v Audio.Khi (5) l b khuych i trung tn my thu (IF receiver). N bao gm:

(5a) Chuyn tn UHF xung VHF = 134MHz, gi l trung tn 2 (IF2)(5b) B gii iu ch FM.

SHF

IndUHF

VHF

2a

UHF

VHF

SHF

UHF

2b 3 4 5

5a 5b 5c

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(5c) Mch khuych i tn hiu ra Video + FM sound

Gain (dB) : 50 | Approx 50 | - 20

Level(dBm):-80 | -30 | -55 to 30 | -50 | -50 |1V

PP |

Hnh 1.30 Mc tn hiu ca trm thu TVRO

S khi m t mc tn hiu cung ti im thu mt t cho php vi trmthu c nhn TVRO vo khong 130dBw/m2. Sau khi anten khuych i 50dB,nng mc tn hiu n ng vo LNB l 80dB. li ca b khuych i dch tnkhong 50dB, nng mc tn hiu ng ra l 30dB. ng dy cp truyn tnhiu 0,951,7GHz t khi bn ngoi n khi bn trong, suy gim tn hiu n 20dB (mc cho php). Nu ng cp truyn di qu 100m th cn phi c mch

khuych i cp tuyn tnh. Ty theo di cp m b khuych i cp phi btn hao, thng thng li cho php t 1520dB. Do vy mc tn hiu sau bkhuych i cp c gi tr t -55-30dB. y ly mc chun 50dB. Tn hiuqua mch trung tn phn nhnh (distributor) khng b suy gim, n n mc a nng vo my thu t -55-30dB, ly chun 50dB. Mc tn hiu ng ra ctiu chun ha mc nh 1Vp-p. y l mc tn hiu tng hp Video + Soundca bng tn gc Baseband.

II. Cng sut tn hiu :II.1c trng:

Cng sut tn hiu thu c ti b mt hiu dng ca anten thu cch xa antenpht mt khong cch R c tnh bng mt thng lng nhn vi bmt ca anten v c tnh nh sau:

I. CableAmpl IF

Receiver

unit

Video,Sou(Base ban

- 130dB

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(II.2.1.1)

Trong : Pt : cng sut phtGt: : li hay h s khuch i ca anten

Aref: : b mthiu dng ca antenB mt ca anten c tnh nh sau:

(II.2.1.2)

Cng sut tn hiu thu l:

Trong :

Gr: li anten thu

Lfree : suy hao truyn sng trong truyn t do

Gr

Aref

Cng sut thu c ti my thu c tnh bng cng thc :

Pr=L

G).G.P( rTT (II.2.1.3)

Trong :PTl cng sut ca my pht .GT l li ca Anten pht .

Gr l li ca Anten thu .L l suy hao tng trn ng truyn

P=

4

2lt

refGA

Pt

RPr Gt

Hnh 2.4 biu din cng sut tn hiu

)(.Pr 2 WAR

GPA ref

ttref

=F=

p

2PR( ) ( ) [ ]W

LG

GPGGPGrRGP

Pfree

ttrtttt

r ..4

222 =

=

P

P= ll

22

P=

lR

Lfree

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Hnh 2.5 Khong cch ng truyn

suy hao tng trn ng truyn gm c : suy hao bi tng inly, suy hao do ma, suy hao trn ng truyn, suy hao do mt cng sut .

L=LAG. Lrain .LFS.LT .

Trong : LAG suy hao bi tng in ly .Lrain suy hao do ma .LFS suy hao ng truyn .LT suy hao do mt cng sut .

II.2 T s sng mang trn nhiu C/N (Carrier/Noise). l t s gia cng sut ca tn hiu nhn c vi cng sut N ca ting

n :C/N = E + G - L - 10 lgT-10 lgB - 10 lgK . hay tnh cch

khcC/NU = G/TS

C/ND = EIRPSG/TSLD(II.2.2.1)

Trong : E = EIRP (dBW): cng sut ng hng bc x tng ngEIRP c tnh theo cng sut ca my pht sng v li ca anten:

EIRP = 10log( )GPt Vi mt cng sut trn din tch pha bn trong hnh cu l:

2t R4/P (W/ )m

2 .

G: li anten (dB).L: s suy gim chung (dB).T. nhit ting n ca anten v u thu LNB( )K .B: rng di tn F ca my thu (MHz).

vi 10logK = 226.6 dB (hng s Bolzman). T s C/N = 6 : qu n cht lng xu.

8 :mc gii hn, mt vi vt nhiu.

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10 : thu tt, mu sc p.12 : thu rt tt, cht lng truyn hnh bng cp

m bo t s tn hiu trn nhiu ti im thu cn m bo cc thng ssau:

- tng ch ca anten: khong 48.5dB (i vi anten thu 3m).

- Nhit nhiu ca anten: khong 23 K .- Mt cng sut ti ng vo my thu: khong110dBw/ 2m .

II.3.H s nhiu nhit G/T ca trm thu:H s ny biu th nh hng ca nhit ln li ca v tinh. Nhit

cng tng th nhiu cng ln, thng ti gi tr nhit l 0 K th khngb nh hng ca nhiu. Gi tr ny cho php chn anten thu thch hp tytheo C/N. Chng ta c:

G/T = C/N - E+L+10logB+10logK hayG/T = ( G/L) .(1/T) .(1/K) II.2.3.1

Ty theo EIRPca v tinh, chng ta s khng nh cht lng h nh nhC/N. Sau chn gi tr ting n ca u LNB thm vo, s suy gim chungL, rng di tn B v 10logK s cho ra gi tr ca li anten tnh bngdB.

III. Tnh ton ng truyn tuyn ln:III.1. Tnh ton tuyn ln (Uplink) :

Cho mt trm mt t vi s liu ban u nh sau:-Anten c ng knh DES= 4 m , DS=2m- Cng sut pht Pt = 500 W = 10lg500 = 27 dBw

- Tn s tuyn ln : fu = 14 GHz- rng bp sng Anten mt t : 3dB= 2- Khong cch gia Anten mt t v v tinh : R = 36000 Km- Hiu sut Anten mt t : = 0,6- Hiu sut Anten v tinh : = 0,55 ,- Gi s mt t vng trung tm ca chm tia v tinh bao ph .

Cc thng s cn tnh ton trn ng truyn Uplink :

Mt thng lng cng sut ti u vo v tinh :

= PtGt [W/m2] ( II.3.1.1)

4R2H s khuch i ca Anten trm mt t :

Gmax = D 2

= D fu 2

u c (II.3.1.2)Thay cc thng s vo, ta c :

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Gmax = 0.6

3.14414109 2= 206131

3108Tnh theo Decibel :

Gmax = 10lg206131 = 53,1 [dB]Cng sut bc x ng hng hiu dng ca Anten trm mt t l :PtGmax = 53,1 + 27 = 80,1 [dBw]

Mt thng lng cng sut ti u vo v tinh :

= PtGt [W/m2]

4R2

thay vo ta c := 80,1 - 10lg[43,14(36106)2] = 80,1 10lg 16.277 .1012

= 80,1 - 10[lg 16.277 + 12lg10] = 80,1-10[4,21 +12] = - 82 [dBw/m2]H s khuch i Anten thu ti v tinh :

Gmax = 10lg D 2

= 10lg 47238 = 46,7 [dB]

uSuy hao do khong cch :

LFS= 10 lg4R 2

= 10lg4361061410

9 2

u 3108= 10lg 44524 .1014=10[lg 44524 + 14lg10] = 10[6,64 + 14] = 206,4

[dB]Cng sut tn hiu thu ti u vo v tinh :

Pr = EIRP LFS + GmaxThay vo ta c :

Pr = 80,1 206,4 + 46,7 = - 79,6 [dBw]

III.2. Tnh ton ng ln trong iu kin tri trong:S liu trm mt t :

- Cng sut pht : Pt = 500 W = 10lg500 = 27 dBw- Suy hao feeder gia Anten v my pht : LFTX= 0,5 dB

- Tn s cng tc tuyn ln : fu = 14 GHz-ng knh Anten : DES= 4 m , DS = 2m- Hiu sut Anten : = 0,6- Li bm v tinh : t = 0,1- Khong cch gia trm mt t v v tinh : R = 36000km- Suy hao kh quyn : La = 0,3 dB

S liu ln quan n v tinh :

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- rng bp sng Anten v tinh : 3dB= 2- Hiu sut Anten v tinh : = 0,55- H s tp m my thu : F = 3 dB- Nhit tp m feeder : T0= 290 K- Nhit tp m Anten : Ta = 290 K-1/K = 228,6 dB

EIRP ca trm mt t :

EIRP = PT GMAX

LTLFTX (II.3.2.1)

Vi :

GRMAX= D 2

u3,14 . 4. 14 .109 2

10lg 0,6 = 53,1 dB3 .108

Pt = 500 W = 27 dBwSuy hao do Anten trm mt t v anten v tinh lch nhau tuyn pht :

LT[dB] =12T

2= 12

T.D.fu 2

3dB 70 C (II.3.2.2)

{3dB = 70 (/D) = 70 (c. /f .D)]

= 12

0,1 .4.14.109 2

70 . 3.108LT (dB) = 0,85dBSuy hao feeder tuyn pht :

LFTX= 0,5 dBT ta c :

EIRPES= 27dB + 53,1dB 0,85dB 0,5dB = 78,7 [dBw]Suy hao truyn sng hng ln :

Lu = LF S + La (suy hao do khi quyn)Trong suy hao do khong cch :

LF S= 10lg4R 2

= 10lg4.3,14 .36.106.14 .10 9 2

= 206,4dB

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u 3 .108 Lu = 206,4 + 0,3 = 206,7 dB

H s phm cht trm thu v tinh tnh theo cng thc :

Suy hao lch hng Anten thu trn v tinh v pha Anten trm mt t

LR[dB] = 12 (r/3dB)2Trong trng hp trm mt t trn mp 3dB ca vng v tinh bao ph ta

c cc tham s sau :LR = 3 dBL

FRX= 3 dB

LPOL= 0 dBTA= 290 K ( nhit tp m ti anten)TF= 290 K (nhit tp m ti feeder )

TR= (F 1) TF= (100,3

1 ).290 = 288 KThay cc gi tr vo ta c h s phm cht ca trm thu trn v tinh:(G/T)sat = 46,7 3 3 -10 [lg(290/100,3+ 290 (1- 1/10 0,3) + 288)]

= 46,7 3 3 27,6= 13,1 dB/K

* Cng sut tn hiu thu ti u vo v tinh :

Pr = EIRP LU + GmaxThay vo ta c :Pr = 78,7 206,7 + 46,7 = - 81,3 [dBw]

T s cng sut sng mang trn mt cng sut ph tp m :(C/N)u = EIRP (1/Lu) (G/T)sat (1/k)

(C/N)u = 78,7 206,7 +13,1 + 228,6 = 113,7 dB( K = 1,38.10-23) : hng s Bolzman

Eb/N0 U= (C /N)U - 10lg( 12.1024.1024) =113,7 73 = 40,7 dB(Vi bit rate = 12Mbps)

III.3. Trng hp ng ln b ma:Khi ng truyn tuyn ln b ma th sng in t b hp th nng lng, bbin i phn cc do s bin dng ca ht ma, bn thn ma cng sinh ra bc xsiu cao tn lm nhiu tn hiu hu ch v nhit nc ma lm cng thm nhit tp m cho ng truyn .

(G/T)sat =G RmaxL RL FRX L POL

TaL FRX

+ TF 1_ 1

L FRX+ T R

(II.3.2.3):

(II.3.2.4)

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Suy hao do ma ti Vit Nam bng Ku trong iu kin gc ngng di 50 theo kt qu tnh trn c s s liu ma ca Vit Nam l 9 dB cng vi 0,3 dB dosai hng Anten pht, nh vy suy hao ng truyn s l :

Lu = 206,4 + 0,3 + 9 = 215,7 dB* Cng sut tn hiu thu ti u vo v tinh :

Pr = EIRP LU + GmaxThay vo ta c :

Pr = 78,7 215,7 + 53,1 = - 83,2 [dBw] (C/N)u = 78,7 215,7 + 13,1 + 228,6 = 104,7dB Eb/N0 U = 104,7 73 = 31,7 dB

IV. Tnh ton ng truyn tuyn xung:IV.1. Tnh ton tuyn xung Downlink :

Cho mt ng xung vi s liu ban u nh sau:

-ng knh Anten trm mt t : D ES = 2 m , DS = 2m- Cng sutpht ca v tinh : Pt = 100 W = 10lg100 = 20 dBw- Tn s tuyn xung : FD= 12 GHz- rng bp sng Anten v tinh : 3dB= 2- Khong cch gia Anten mt t v v tinh : R = 36000 Km- Hiu sut Anten mt t : = 0,6- Hiu sut Anten v tinh : = 0,55- Gi s mt t vng trung tm ca chm tia v tinh bao ph .

Cc thng s cn tnh ton trn ng truyn Downlink :

Hnh 2.6 Cc thng s tnh ton suy hao

Mt thng lng cng sut ti u vo trm mt t :

= PtGt [W/m2]

4R2

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H s khuch i ca Anten ti v tinh :

Gmax = D 2

= DfD

2

D c

3,14 . 2. 12 .109 210lg 0,55 = 10 lg 34705 = 45,4 dB

3 .108

Gmax = 45,4 dBEIRPsat = 45,4 +20 = 65,4 dBw

= 65,4 - 10lg(43,14(36106)2= 65,4 10 (lg 16277+12 lg10)

= 65,4 10(4,21 +12) = 65,4 162,1 = - 96,7 dBW/m2Suy hao t do khng gian tuyn xung :

LFS= 10lg 4R 2 = 10lg 43,14 361061210

9 2

d 3108

= 10(lg 3.271178 + 14lg 10) = 10 (6,51 + 14) = 205,1 dB

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H s khuch i Anten thu ti mt t :

Gmax = 10lg D 2

= 10lg 0,63,14 21210

9 2

d 3108

= 10 lg 37860 = 45,78 dB

Cng sut tn hiu thu ti uvo anten trm mt t :Pr = EIRPSAT LFS+ Gmax

Thay vo ta c :Pr = 65,4 205,1 + 45,78 = - 93,92 [dBw]

IV.2. Tnh ton ng truyn xung trong iu kin tri trong:S liu ln quan n v tinh :

- Cng sut pht : Pt = 100 W = 10lg100 = 20 dBw , DS =2m- Suy hao gia Anten v my pht : LFTX = 1 dB

- Suy hao do lch hng pht : LT= 3 dB- Tn s cng tc tuyn xung : fD= 12 GHz- Hiu sut Anten : = 0,55- rng bp sng Anten v tinh : 3dB= 2- Khongcch gia trm mt t v v tinh : R = 36000 km- Suy hao kh quyn : La = 0,3 dB

S liu trm mt t :-ng knh Anten trm mt t : D = 2 m- Hiu sut Anten mt t : = 0,6-Suy hao u ni gia anten v my thu : LFRX= 0,5 dB

- H s tp m my thu : F = 2,2 dB- Nhit tp m feeder : TFEED= 290 K- Nhit tp m mt t : TGROUND = 45 K- Lch hng Anten :T= 0,1- Suy hao kh quyn : La = 0,3 dB , Khi ma LR= 0,6 dB

Cc thng s cn tnh ton :EIRP ca trm v tinh :

Gmax = D 2

= 10lg 0,553,14 21210

9 2

d 3108

= 10 lg 34.705 = 45,4 dB

Suy hao do lch hng anten pht : LT = 3dBSuy hao anten v my pht : LFTX = 1dBTnh EIRP ca trm v tinh :

(II.4.1.1)

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EIRPsat = PtGmax

(W) EIRPsat = 45,4dB + 20dB 3 1 = 61,4dBw

LTLFTX:Suy hao truyn sng tuyn xung trong khng gian l : LD= LFS+ La

trong :

LFS = 10lg 4R 2d

10lg43,14 36.10 61210

9 2

3108=10 (lg3.271178 + 14 lg 10) = 10 (6,51 + 14) = 205,1

LD= 205,1 + 0,3 = 205,4 dBLa = 0,3 dB

H s phm cht trm thu mt t :

H s khuch iAnten thu trm mt t :

Gmax = 10lg D 2

= Dfd 2

d c

10lg 0,63,14 21210

9 2

3108= 10 lg 37860 = 45,7 dB

Suy hao lch hng Anten thu trm mt t v pha Anten v tinh :

LR[dB] = 12 (R /3dB)2

LR(dB) = 12

RD fD

70 . C

= 12 0,1. 2. 12 .10 9 2

70 .3.108

(G/T)sat =

G RmaxL RL FRX L POL

TaL FRX

+ TF 1_ 1

L FRX+ T R

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LR(dB) = 0,15 dB

Trong trng hp trm mt t trn mp 3 dB ca vng v tinh bao phta c cc tham s :

LFRX = 0,5 dB

LPOL = 0 dBNhit tp m anten : TA= TSKY+ TGOUND vi TSKY= 20K

-> TA= 20 + 450K = 65 0K

TF= 290 KTR= (F 1) TF= (10

0,22 1 ).290 = 191,3 K

Thay cc gi tr vo ta c h s phm cht :(G/T) = 45,7 0,15- 0,5 10 lg[ 65/100,05+290(1-1/10 0,05) + 191,3 ]

= 45,7 0,15 0,5 24,2 = 20,8 dB/KCng sut tn hiu thu ti u vo anten trm mt t :

Pr = EIRPSAT LD+ GmaxThay vo ta c :

Pr = 61,4 205,4 + 45,7 = - 98,3 [dBw]T s cng sut sng mang trn mt cng sut ph tp m :

(C/N )D= EIRPsat (1/LD) (G/T)ES(1/k)-> (C/N )D= 61,4 205,4 +20,8 + 228,6 = 105,4 dB-> Eb/N0 D = (C/N )D 10lg (12.1024.1024) = 105,4 73 = 32,4 dB

IV.3.Trng hp ng truyn xung b ma:Khi ng truyn tuyn ln b ma th sng in t b hp th nng lng, b

bin i phn cc do s bin dng ca ht ma, bn thn ma cngsinh ra bc xsiu cao tn lm nhiu tn hiu hu ch v nhit nc ma lm cng thm nhit tp m cho ng truyn .

Suy hao do ma bng Ku trong iu kin chun l 7 dB cng vi 0,3dB dosai hng anten pht nh vy suy hao ng truyn xungs l :

LD= 205,4 + 7,3 = 212,7 dBNhit tp m anten l:

Ly Tm = 2750K thay vo ta c :

T s cng sut sng mang tn mt cng sut ph tp m tuyn xung l :(C/N)D=EIRP.(1/LD).(G/T).(1/K)

groundrain

mrain

skyA T

AT

A

TT +

-+= 11

][183.19110

11290

10

265log105,06,07,45)/(

2654510

11275

10

20

05.005.0

7.07.0

dBKdBTG

KT

SES

A

(II.4.3.1)

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Cng sut tn hiu thu ti u vo anten trm mt t :Pr = EIRPSAT LD+ Gmax

Thay vo ta c :Pr = 61,4 212,7 + 45,4 = - 105,9 [dBw]

suy ra : (C/N)D = 61,4 212 ,7 +18 + 228,6 = 95,3 dB

(Eb/N0 )D = 95,3 - 73 = 22,3 dB

Gi tr C/N ton h thng :

(C/NUP). (C/N DOWN)C/N =

(C/N UP)+(C/N DOWN)

113,7 105,4C/N = = 54 ,6 dB

113,7 + 105,4

i vi ng truyn b ma :

104,7 95,3C/N = = 49 ,8 dB

104,7 + 95,3

Gi tr Eb/ N0 ton h thng l :

( Eb/N0 U) (Eb/N0 D)Eb/ N0 =

( Eb/N0 U) (Eb/N0 D)

40,7 . 32,4Eb/ N0 = = 18 ,04 dB

40,7 + 32,4

Khi ng truyn b ma

31,7 . 22,3Eb/ N0 = = 13,1 dB

31,7 + 22,3

V. Cc suy hao trong thc t:

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Khi thit k ng truyn v tinh ngi ta phi ch n nh hng ca ccloi suy hao sau :

1/ Suy hao do truyn sng trong mi trng kh quyn c tnh n nhhng ca hi nc. Cc loi kh, sng m, my ma ... gi tt l suy hao ngtruyn .

L = LF S+ LALF S: suy hao Free space ; LA: suy hao kh quyn

2/Suy hao trong bn thn cc thit b thu pht tn hiu nh u ni dy, dydn sng ....

+ Suy hao gia my pht v Anten (LFTX) c tnh bi quan h:PTX= PT. LFTX

PTX: cng sut u ra b khuch i ca my phtPT: cng sut pht ca Anten

+ T y c th suy ra cng sut bc x ng hng hiu dng:

EIRP = PTGT= (PTX . GT )/ LFTX [W]+ Suy hao LFRX gia my thu v Anten l suy hao to nn bi cc feeder

dn sng v cc u ni, cng sut thu l PR th ti u vo my thu c cngsut :

PRX=PR / LFRX

3/Suy hao do Anten thu v pht lch nhau .Khi Anten thu v pht lch nhau s to ra suy hao v bp sng chnh ca

Anten thu khng hng ng chm tia Pht x ca Anten pht.

LT[dB] = 12 (T/3dB)2

LR[dB] = 12 (R/3dB)2

4/Suy hao do khng ng phn cc .Loi suy hao ny cng khng th b qua khi Anten thu khng hng ng

hng pht cng vi phn cc ca sng mang thu, v d i vi sng in t phti c phn cc trn th ch trn trc bc x phn cc b bc x ca Anten sngmi c phn cc trn, ngoi trc bc x phn cc b bin dng thnh Elip, ngoi rakhi truyn trong mi trng phn cc b bin i do mi trng ...

+ Nu gi l gc gia hai mt sng th suy hao do lch phn cc c

biu din : L POL= 20lg(cos)v thng ly bng 3 dB trong trng hp phn cc trn .

Tnh n 4 loi suy hao k trn ta c th biu din cng sut tn hiu thu nhsau:

PRX =PTxGmax 1 1

[W]

LTLFTX LA LFS LRLFRXLPOL

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Kt lun :

Cht lng tn hiu trn ng truyn gia my pht v my thu c nhgi bng t s gia cng sut tn hiu so vi cng sut ph tp m C/N. y hms ca c tnh thit b s dng trn ng truyn, EIRP ca my pht, h s phm

cht trm thu v c tnh mi trng truyn sng.Cht lng ng ln c nh gi bng (C/N)u, cn cht lng ng

xung c nh gi bng t s (C/N)D , nh hng ca mi trng truyn sngn ng ln v ng xung rt khc nhau, v d ma lm gim t s (C/N)u

bng cch lm gim cng sut tn hiu thu tuyn ln ti u vo Anten v tinh, vituyn xung n li gim t s (C/N)Dtheo cch gim cng sut tn hiu thu tuynxung v tng nhit tp m tuyn xung .Qua chng tnh ton ng truyn v c kim nghim qua thc t trong kSeagames , v cuc ua xe p xuyn vit va qua ti i Truyn Hnh ThnhPh H Ch Minh vi cng sut pht v thu vn hp l v ngng cho php

ta c th iu chnh mc nng lng hp l trn v tinh thu tn hiu trongngng cho php-65dBw - 25dBw, i vi thi tit tt hay tri c ma, m bo tn hiukhng b dng hnh iu chnh BER (Bit Error Rate) cho hp l ,ng vi ngknh anten v cng sut pht iu chnh b phn HPA cho ti u.