thiet ke thso ve tinh

LI CM N hon thnh c ti ny th trc tin em xin chn thnh cm n Ban gim hiu Trng I HC VINH, cc thy c trong Ban ch nhim KHOA CNG NGH v cc thy c b mn to iu kin cho em c hc tp v truyn th nhiu kin thc cho em lm nn tng hc vn trn con ng cng danh s nghip ca mnh. Sau l em v cng bit n thy CAO THNH NGHA l ngi thy trc tip nh hng v hng dn cho em nghin cu v mt lnh vc kh l mi m so vi nhng kin thc m em hc c trng, v em t long bit n ti cn b, nhn vin nhn vin khch sn Sao Mai - Thnh ph Thanh Ha to iu kin tt nht em c th hon thnh n ny.

Vinh, thng 5 nm 2010 Sinh vin thc hin Nguyn Ngc Huy

TNH TON, THIT K TRUYN HNH S V TINH

LI NI UTrong qu trnh pht trin ca con ngi, nhng cuc cc mng v cng ngh ng mt vai tr rt quan trng, chng lm thay i tng ngy tng gi cuc sng ca con ngi, theo hng hin i hn. Dn s cng tng, nhu cu cng tng theo, cc dch v, cc tin ch t cng c hnh thnh v pht trin theo. Chng ta v ang p dng cng ngh ca cc ngnh in t, cng ngh thng tin v vin thng vo trong thc tin p ng nhu cu ngy cng tng ca con ngi nht l lnh vc gii tr. Hin nay, truyn hnh s v tinh khng cn xa l l g vi chng ta. Nhng hiu ht c n v khai thc trit nhng tnh nng u vit ca n th vn ang cn l mt kh khn v thch thc. c s nh hng v ch dn ca thy Cao Thnh Ngha, v s gip nhit tnh ca cn b, nhn vin khch sn Sao Mai - Thnh ph Thanh Ha, em mnh dn chn ti n: Tnh ton, thit k truyn hnh s v tinh. Vi mc ch tm hiu v h thng thng hu tuyn, c th l h thng thng tin v tinh v phng n lp t truyn hnh cp cho khch sn da trn nhng thit b c sn trn th trng. Ni dung ca n c th hin nh sau: Phn I: L thuyt chung. Chng I: Tng quan v thng tin v tinh. Chng II: H thng thu truyn hnh s qua v tinh. Phn II: H thng thu truyn hnh cp cho mt khch sn Sao Mai - Thanh Ha Do kin thc v kh nng ca em cn hn ch, nn n tt nghip ny khng trnh khi cc sai st. Mong c s gp ca cc thy, cc c v cc bn ni dung n c hon thin hn. Em xin chn thnh cm n thy Cao Thnh Ngha, khch sn Sao Mai, hng dn em v chuyn mn, phng php lm vic em c th xy dng v hon thnh ni dung n theo ng k hoch. Em cng xin gi li cm n chn thnh n cc thy, cc c, cc bn trong Khoa Cng Ngh trng i hc Vinh gip , to iu kin cho em hon thnh n ny. Vinh, thng 5 nm 2010 Sinh vin thc hin

SVTH : Nguyn Ngc Huy

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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH Nguyn Ngc Huy

MC LC


3

Lp 46K - TVT


DANH SCH HNH VHnh 1.1. Ba dng qu o c bn ca v tinh..........Error: Reference source not found1 Hnh 1.2. Cu hnh ca h thng thng tin v tinh....Error: Reference source not found4 Hnh 1.3. Quy hoch tn s v phn cc...................Error: Reference source not found5 Hnh 1.4. Cc knh ca b pht p v tinh..............Error: Reference source not found7 Hnh 1.5. My thu bng rng v tinh.........................Error: Reference source not found8 Hnh 1.6. B phn knh vo......................................Error: Reference source not found9 Hnh 1.7. B khuch i n sng chy (TWTA)..........................................................20 Hnh 1.8. B ghp knh u ra..................................Error: Reference source not found2 Hnh 1.9. Anten loa hnh ch nht.............................Error: Reference source not found3 Hnh 1.10. Anten phn x..........................................Error: Reference source not found3 Hnh 1.11. Sai lch do t anten cha ng..............Error: Reference source not found6 Hnh 1.12. Suy hao trong thit b pht v thu............Error: Reference source not found6 Hnh 1.13. Cc ngun tp m nh hng n thng tin v tinh...Error: Reference source not found8 Hnh 1.14. Can nhiu gia viba v trm mt t v v tinh...........................................31 Hnh 1.15. Can nhiu gia cc h thng thng tin v tinh.....Error: Reference source not found1 Hnh 1.16. c tnh vo ra ca TWT.........................Error: Reference source not found2 Hnh 1.17: S khi h thng thu truyn hnh s...Error: Reference source not found4 Hnh 1.18. S h thng Headend s.....................Error: Reference source not found5 Hnh 1.19. Thu tn hiu t v tinh..............................Error: Reference source not found6 Hnh 1.20. Thu tn hiu truyn hnh s mt t.........Error: Reference source not found7 Hnh 1.21. Thu tn hiu cc i a phng..............Error: Reference source not found7 Hnh 1.22. S chm sao ca 16-QAM..................Error: Reference source not found4 Hnh 1.23. a) Cu hnh b iu ch Q-PSK...............Error: Reference source not found6 b) Cu hnh b gii iu ch Q-PSK.......Error: Reference source not found6 c) Biu sao tn hiu.............................Error: Reference source not found6 Hnh 1.24. K thut a truy nhp FDMA, TDMA v CDMA.....Error: Reference source not found9 Hnh 1.25. So snh gia khi nim ghp knh v a truy nhp.....................................50 Hnh 1.26. FDMA..........................................................................................................50 Hnh 1.27. Cc cu hnh truyn dn FDMA..................................................................51 a) FDM/FM/FDMA; b) TDM/PSK/FDMA; c) SCPC/FDMA.......................................51 Hnh 1.28. a truy nhp phn chia theo thi gian.....Error: Reference source not found1 Hnh 1.29. S khi u cui thu thu DBS TV/FM gia nh. . .Error: Reference source not found6 Hnh 1.30. Cu trc h thng anten TV ch (MATV)Error: Reference source not found9 Hnh 1.31. Cu trc khi trong nh cho h thng TV anten tp th (CATV)..........Error: Reference source not found9 Hnh 1.32. Cc phn t cn bn ca mt trm mt t c d phng.............................60 Hnh 1.33. S chi tit ca mt trm pht thu............................................................61 Hnh 1.34. S thay i pha trong tn hiu iu ch pha vi sai ti tn......Error: Reference source not found5 SVTH : Nguyn Ngc Huy 4 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH Hnh 1.35. a) Bc nhy ti tn cc i...................Error: Reference source not found5 b) Bc nhy ti thi im ti tn c gi tr 0....Error: Reference source not found5 c) Mt ph ca bc nhy pha..........Error: Reference source not found5 Hnh 1.36. a) Tn hiu iu ch s...........................Error: Reference source not found6 b) Tn hiu iu ch theo loi A............Error: Reference source not found6 c) Tn hiu iu ch theo loi B.............Error: Reference source not found6 Hnh 1.37. Gc ngng v gc nghing......................Error: Reference source not found7 Hnh 1.38. a) phn cc ng b) phn cc ngang (c) phn cc dng elip.............Error: Reference source not found9 Hnh 2.1. S nguyn l khi thu...........................Error: Reference source not found4 Hnh 2.2. S khi thit k khi thu TVRO...........Error: Reference source not found5 Hnh 2.3. a) Cu trc hnh xng c..........................Error: Reference source not found5 b) Cu trc hnh cy...................................Error: Reference source not found5 Hnh 2.4. Mt ct ngang mt tng ca khch sn......Error: Reference source not found9 Hnh 2.5. S thit k h thng theo mt ct ng Error: Reference source not found7 Hnh 2.6. H thng cp ton b khch sn................Error: Reference source not found8


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Lp 46K - TVT


THUT NG VIT TTAM ASK ATM CATV CDMA DM DPCM DSB DTH DVB EIRP FDMA FFT FM FSK FSS HFC IDU IMUX LNA LNB MATV MPEG NTSC Amplitude Modulation iu ch bin Amplication Shift Key Kha dch chuyn v bin Asynchronous Transfer Mode Phng thc chuyn ti khng ng b Community Antenna Television Truyn hnh cp Code Division Multiple Access a truy nhp phn chia theo m Delta Modulation Dng nhiu trong truyn s liu Differential Pulse-Code Modulation iu ch xung m vi sai Double Side Band iu ch sng bin Direct-To-Home Dch v truyn hnh v tinh Digital Video Broadcasting Truyn hnh s qung b Effective Isotropic Radiated Power Cng sut bc x ng hng hiu dng Frequency Division Multiple Access a truy nhp phn chia theo tn s Fast Fourier Transform Bin i furie nhanh Frequency Modulation iu ch tn s Frequency Shift Key Kha dch chuyn v tn s Fixed Satellite Service Dch v v tinh c nh Hybrid Fiber/Coax Cp quang/ ng trc hn hp Indoor Unit Cu kin ca VSAT t trong nh Inverse Multiplexer B ghp knh nghch o Low noise Amplifiers Cc b khuch i c mc nhiu thp Low Noise Block Converter B chuyn i khi c mc nhiu thp Master Antenna Television Truyn hnh Anten chung Motion Picture Experts Group Nhm chuyn gia hnh nh ng National Television System y ban quc gia v cc h thng truyn hnh Cu kin ca VSAT t ngoi nh iu ch bin ca xung iu ch xung m iu ch tn s ca xung iu ch pha iu ch pha ca xung Kha dch chuyn v pha iu ch rng ca xung iu ch bin cu phng Tn s v tuyn Mi knh mt sng mang Phn cp mng s ng b Tn s siu cao Bng tn siu cao Giao thc qun l mng n gin Lp 46K - TVT

Commitee ODU Outdoor Unit PAM Pulse Amplitude Modulation PCM Pulse Code Modulation PFM Pulse Frequency Modulation PM Phare Modulation PPM Pulse Phare Modulation PSK Phare Shift Key PWM Pulse Width Modulation QAM Quadrature Amplitude Modulation RF Radio Frequency SCPC Single-Channel-Per-Carrier SDH Synchronous Digital Hierarchy SHF Super High Frequency SHF Super High Frequency SNMP Simple Network Management Protocol SVTH : Nguyn Ngc Huy 6

TNH TON, THIT K TRUYN HNH S V TINH SNR SSB TDM TDMA TS UT XMTR Signal-To-Noise Ratio Single Side Band Time Division Multiplex Time Division Multiple Access Transport Stream User Terminal Transmitter T s tn hiu trn tp m iu ch n bin Ghp knh phn chia theo thi gian a truy nhp phn chia theo thi gian Dng chuyn ti Trm v tinh thu bao My pht


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Lp 46K - TVT


PHN I L THUYT CHUNGCHNG 1: TNG QUAN V THNG TIN V TINH 1.1.1. LCH S PHT TRIN CA THNG TIN V TINH Vo cui th k 19, nh bc hc Nga Tsiolkovsky (1857-1035) a ra cc khi nim c bn v tn la y dng nhin liu lng. ng cng a ra tng v tn la y nhiu tng, cc tu v tr c ngi iu khin dng thm d v tr. Nm 1926 Robert Hutchinson Goddard th nghim thnh cng tn la y dng nhin liu lng. Thng 5 nm 1945 Arthur Clarke nh vt l ni ting ngi Anh ng thi l tc gi ca m hnh vin tng thng tin ton cu, a ra tng s dng mt h thng gm 3 v tinh a tnh dng pht thanh qung b trn ton th gii. Thng 10 / 1957 ln u tin trn th gii, Lin X phng thnh cng v tinh nhn to SPUTNIK - 1. nh du mt k nguyn v TTVT. Nm 1958 bc in u tin c pht qua v tinh SCORE ca M. Nm 1964 thnh lp t chc TTVT quc t INTELSAT. Nm 1965 ra i h thng TTVT thng mi u tin INTELSAT-1 vi tn gi Early Bird. Nm 1971 thnh lp t chc TTVT quc t INTERSPUTNIK gm Lin X v 9 nc XHCN. Nm 1972-1976 Canada, M, Lin X v Indonesia s dng v tinh cho thng tin ni a. Nm 1979 thnh lp t chc thng tin hng hi quc t qua v tinh INMARSAT. Nm 1984 Nht bn a vo s dng h thng truyn hnh trc tip qua v tinh. Nm 1987 Th nghim thnh cng v tinh phc v cho thng tin di ng qua v tinh.


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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH Thi k nhng nm 1999 n nay, ra i tng v hnh thnh nhng h thng thng tin di ng v thng tin bng rng ton cu s dng v tinh. Cc h thng in hnh nh GLOBAL STAR, IRIDIUM, ICO, SKYBRIGDE, TELEDESIC. 1.1.2. CC NH LUT KEPLER Cc v tinh quay quanh tri t tun theo cng cc nh lut iu khin s chuyn ng ca cc hnh tinh xung quanh mt tri. T lu da trn cc quan trc k lng ngi ta hiu c s chuyn ng ca cc hnh tinh. T cc quan trc ny, Johannes Kepler (1571-1630) rt ra bng thc nghim ba nh lut m t chuyn ng hnh tinh. Tng qut cc nh lut Kepler c th p dng cho hai vt th bt k trong khng gian tng tc vi nhau qua lc hp dn. Vt th c khi lng ln hn trong hai vt th c gi l s cp cn vt th th hai c gi l v tinh. 1.1.2.1. nh lut Kepler th nht V tinh chuyn ng vng quanh tri t theo mt qu o Ellip vi tm tri t nm mt trong hai tiu im ca Ellip. im xa nht ca qu o so vi tm tri t nm pha ca tiu im th hai, c gi l vin im cn im gn nht ca qu o c gi l cn im. 1.1.2.2. nh lut Kepler th hai V tinh chuyn ng theo mt qu o vi vn tc thay i sao cho ng ni gia tm tri t v v tinh s qut cc din tch bng nhau khi v tinh chuyn ng trong cng mt thi gian nh nhau. 1.1.2.3. nh lut Kepler th ba Bnh phng ca chu k quay t l thun vi lu tha bc ba ca bn trc ln ca qu o Ellip. 1.1.3. C IM CA THNG TIN V TINH 1.1.3.1 u im ca thng tin v tinh Trong thi i hin nay, thng tin v tinh c pht trin v ph bin nhanh chng v nhiu l do khc nhau. Cc u im chnh ca thng tin v tinh so vi cc phng tin thng tin di bin v trn mt t nh h thng cp quang v h thng chuyn tip viba s l: - C kh nng a truy nhp. SVTH : Nguyn Ngc Huy Lp 46K - TVT

9

TNH TON, THIT K TRUYN HNH S V TINH - Vng ph sng rng. - n nh cao, cht lng v kh nng p ng cao v thng tin bng rng. - C th ng dng cho thng tin di ng. - Hiu qu kinh t cao trong thng tin c ly ln. Sng v tuyn in pht i t mt v tinh qu o a tnh c th bao ph hn 1/3 ton b b mt tri t, nn nhng trm mt t t trong vng c th thng tin trc tip vi bt k mt trm mt t khc trong vng qua mt v tinh thng tin. K thut s dng mt v tinh chung cho nhiu trm mt t v vic tng hiu qa s dng ca n ti cc i c gi l a truy nhp. Ni cch khc a truy nhp l phng php dng mt b pht p trn mt v tinh chung cho nhiu trm mt t. 1.1.3.2. Cc qu o v tinhQu o elip nghing Qu o xch o Qu o cc trn

Hnh 1.1 Ba dng qu o c bn ca v tinh.

1.1.3.2.1. Qy o cc trn u im ca dng qy o ny l mi im trn mt t u nhn thy v tinh thng qua mt qy o nht nh, vic ph sng ton cu ca dng qy o ny t c v qy o bay ca v tinh s ln lt qut tt c cc v tr trn mt t. Dng qy o ny c s dng cho cc v tinh d bo thi tit, hng hi, thm d ti nguyn v cc v tinh do thm, khng thng dng cho truyn thng tin. 1.1.3.2.2. Qy o xch o trn i vi dng qy o ny, v tinh bay trn mt phng ng xch o v l dng qy o c dng cho v tinh a tnh, nu v tinh bay mt cao ng th dng qy o ny s l tng i vi cc v tinh thng tin. SVTH : Nguyn Ngc Huy Lp 46K - TVT

10

TNH TON, THIT K TRUYN HNH S V TINH Qu o a tnh GEO (Geostationalry Earth Orbit) Nu ba v tinh a tnh c t cch u nhau bn trn xch o th c th thit lp thng tin lin kt gia cc vng trn tri t bng cch chuyn tip qua mt hoc hai v tinh. iu ny cho php xy dng mt mng thng tin trn ton th gii. 1.1.3.2.3. Qy o elip nghing u im ca loi qy o ny l v tinh c th t n cc vng m cc v tinh a tnh khng th t ti. Tuy nhin qy o elip nghing c nhc im l hiu ng Doppler ln v vn iu khin bm ui v tinh phi mc cao. Ngoi ra ngi ta cn c 2 loai qy o khc: 1.1.3.2.4. Qy o thp LEO (Low Earth Orbit) cao in hnh ca dng qy o ny l 160 n 480 km, n c chu k 90pht. S gn k ca cc v tinh LEO c thun li l thi gian d liu pht i n v tinh v i v l rt ngn. Do kh nng thc hin nhanh ca n, tc dng tip sc tng h ton cu gia cc mng v loi hnh hi thoi v tuyn truyn hnh s c hiu qa v hp dn hn. Nhng h thng LEO i hi phi c khong 60 v tinh loi ny mi bao trm ht b mt a cu. 1.1.3.2.5. Qy o trung bnh MEO (Medium Earth Orbit) V tinh MEO cao t 10.000 km n 20.000 km, chu k ca qy o l 5 n 12 gi, thi gian quan st v tinh t 2 n 4 gi. ng dng cho thng tin di ng hay thng tin radio. H thng MEO cn khong 12 v tinh ph sng ton cu. 1.1.4. TN S LM VIC CA THNG TIN V TINH 1.1.4.1. Khi nim ca s v tuyn


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Lp 46K - TVT


100 Suy 50 hao (dB)10 5 1 0,1 0,5 1 Ca s tn s 5 10 Tn s (GHz) 50 100 Suy hao do tng in ly Suy hao do ma 25mm/h

Bng 1.1. th biu dn suy hao do ma v do tng in ly theo tn s.

Cc sng v tuyn in truyn n hay i t cc v tinh thng tin chu nh hng ca tng in ly v kh quyn. Tng in ly l mt lp kh long b ion ho bi cc tia v tr, c cao t 60km n 400km so vi mt t, lp mang in ny c tnh cht hp th v phn x sng. Do cc bin i trng thi ca tng in ly, lm gi tr hp th v phn x thay i gy ra s bin thin cng sng i vo, gi l s thng ging. Tuy nhin tnh cht ny nh hng ch yu i vi bng tn thp, khi tn s cng cao nh hng ca tng in ly cng t, cc tn s bng sng viba (1GHz) hu nh khng b nh hng ca tng in ly. Khi tn s >10GHz th cn tnh ton suy hao do ma nh bng 1.1. T hnh v ta thy cc tn s nm trong khong gia 1GHz v 10GHz th suy hao kt hp do tng in ly v ma nh l khng ng k, do vy bng tn ny c gi l "ca s tn s". Lc nu sng nm trong ca s v tuyn th suy hao truyn dn c th c xem gn ng l suy hao khng gian t do. V vy, cho php thit lp cc ng thng tin v tinh n nh, nhng phi lu n s can nhiu vi cc ng thng tin viba trn mt t v cc sng trong thng tin viba cng s dng tn s nm trong ca s ny. Ngoi ra, khi ma ln th suy hao do ma trong ca s tn s cn phi c tnh ton, xem xt thm kt qu tnh ton c chnh xc cao hn. 1.1.4.2. Phn nh tn s. Phn nh tn s cho cc dch v v tinh l mt qu trnh rt phc tp i hi s cng tc quc t v c quy hoch. Phn nh tn c thc hin di s bo tr ca Lin on vin thng quc t (ITU). tin cho vic quy hoch tn s, ton th gii c chia thnh ba vng: Vng 1: Chu u, Chu Phi, Lin x c v Mng C. SVTH : Nguyn Ngc Huy 12 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH Vng 2: Bc M, Nam M v o Xanh. Vng 3: Chu (tr vng 1), c v Ty nam Thi Bnh Dng, trong c c Vit Nam. Trong cc vng ny bng tn c phn nh cho cc dch v v tinh khc nhau, mc d mt dch v c th c cp pht cc bng tn khc nhau cc vng khc nhau. 1.1.4.3. Tn s s dng trong thng tin v tinh Cc tn s s dng trong thng tin v tinh nm trong bng tn siu cao SHF (Super High Frequency) t 3 n 30 GHz, trong ph tn s s dng cho v tinh ngi ta cn chia cc bng tn nh vi phm vi ca dy ph nh sau: Bng 1.2. Tn s s dng trong thng tin v tinh. Bng C X Ku Tn s (GHz) 3,400 7,075 7,025 8,425 10,90 18,10 Bc sng (cm) 8,82 4,41 4,41 3,56 2,75 1,66

Ka 17,70 36,00 1,95 0,83 Hin nay, bng C v bng Ku c s dng ph bin nht, bng C (4/6 GHz) nm khong gia ca s tn s, suy hao t do ma, trc y c dng cho cc h thng viba mt t. S dng chung cho h thng Intelsat v cc h thng khc bao gm cc h thng v tinh khu vc v nhiu h thng v tinh ni a. Bng Ku (12/14 v 11/14 GHz), c s dng rng ri tip sau bng C cho vin thng cng cng, dng nhiu cho thng tin ni a v thng tin gia cc cng ty. Do tn s cao nn cho php s dng nhng anten c kch thc nh, nhng cng v tn s cao nn tn hiu bng Ku b hp th ln do ma. Bng Ka (20/30 GHz) ln u tin s dng cho thng tin thng mi qua v tinh Sakura ca Nht, cho php s dng cc trm mt t nh v hon ton khng gy nhiu cho cc h thng viba. Tuy nhin bng Ka suy hao ng k do ma nn khng ph hp cho thng tin cht lng cao. 1.1.5. CU HNH H THNG THNG TIN V TINH Mt h thng thng tin v tinh bao gm hai phn c bn: Phn trn khng l v tinh v cc thit b lin quan. Phn mt t bao gm cc trm mt t .


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Lp 46K - TVT


ng ln 6GHz(14GHz)

ng xung 4GHz(11GHz)

iu ch

Nng tn

Khuch i cng sut

Khuch i tp m thp

H tn

Gii iu ch

Hnh 1.2. Lin lc gia hai trm mt t qua v tinh. Trong v tinh ng vai tr lp li tn hiu truyn gia cc trm mt t, thc cht k thut thng tin v tinh l k thut truyn dn m trong mi trng truyn dn l khng gian v tr vi khong cch ng truyn kh di. Ti y ta cng gp li mt s vn i vi mt bi ton truyn dn, l cc vn iu ch tp m v nhiu ng truyn, ng b gia hai u thu pht. ng hng t trm mt t pht n v tinh c gi l ng ln (Up link) v ng t v tinh n trm mt t thu gi l ng xung (Down link). Hu ht, cc tn s trong khong 6GHz hoc 14GHz c dng cho ng ln v tn s khong 4GHz hoc 11GHz cho ng xung. Ti u pht, thng tin nhn t mng ngun (c th l knh thoi, truyn hnh qung b, truyn s liu ...) s c dng iu ch mt sng mang trung tn IF. Sau tn hiu ny c a qua b chuyn i nng tn (Up Converter) cho ra tn s cao hn RF (Radio Frequency). Tn hiu RF ny c khuch i b khuch i cng sut cao HPA (High Power Amplifier) ri c bc x ra khng gian ln v tinh qua anten pht. Ti v tinh, tn hiu nhn c qua anten s c khuch i v chuyn i tn s xung (Down Converter), sau c khuch i cng sut ri c pht tr li trm mt t. trm mt t thu, tn hiu thu c qua anten c khuch i bi b khuch i tp m thp LNA (Low Noise Amplifier). Sau c chuyn i tn s xung trung tn qua b chuyn i h tn (Down Converter) v cui cng c gii iu ch khi phc li tn hiu bng gc. 1.1.5.1. Phn h thng tin a) B pht p B pht p bao gm tp hp cc khi ni vi nhau to nn mt knh thng tin duy nht gia anten thu v anten pht trn v tinh thng tin.


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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH500 4 36

Phn cc ng 6105 6145 6185

6085

6125

6165

Phn cc ngang

Hnh 1.3. Quy hoch tn s v phn cc Trc khi trnh by chi tit cc khi khc nhau ca b pht p, ta s xt ngn gn t chc tn s cho thng tin v tinh bng C. Bng thng n nh cho dch v bng C l 500 MHz v bng thng ny c chia thnh cc bng con, mi bng con dnh cho mt b pht p. rng bng tn thng thng ca b pht p l 36 MHz vi on bng bo v gia cc b pht p l 4MHz. V th bng tn 500 MHz c th m bo cho 12 b pht p. Bng cch ly phn cc, ta c th tng s b pht p ln hai ln. Cch ly phn cc cho php s dng cng mt tn s nhng vi phn cc ngc chiu nhau cho hai b pht p. thu c knh ca mnh, cc anten thu phi c phn cc trng vi phn cc pht ca knh tng ng. i vi phn cc tuyn tnh, ta c th cch ly phn cc bng phn cc ng v phn cc ngang. i vi phn cc trn, cch l phn cc nhn c bng cch s dng phn cc trn tay phi v phn cc trn tay tri. V cc sng mang vi phn cc i nhau c th chng ln ln nhau, nn k thut ny c gi l ti s dng tn s. Cng c th ti s dng tn s bng cc anten bp hp, v phng thc ny c th kt hp vi ti s dng theo phn cc cung cp rng bng tn hiu dng 2000 MHz trn c s rng thc t 500 MHz. i vi mt trong s cc nhm phn cc, hnh 1.3 cho thy chi tit hn s phn knh cho 12 b pht p. Di tn thu hay di tn ng ln l 5,925 n 6,425 GHz. SVTH : Nguyn Ngc Huy Lp 46K - TVT

15

TNH TON, THIT K TRUYN HNH S V TINH Cc sng mang c th c thu trn mt hay nhiu anten ng phn cc. B lc vo cho qua ton b bng tn 500MHz n my thu chung v loi b tp m cng vi nhiu ngoi bng (nhiu ny c th gy ra do cc tn hiu nh). Trong di thng 500MHz ny c th c rt nhiu sng mang c iu ch v tt c cc sng mang ny u c khuych i, bin i tn s trong my thu chung. Bin i tn s chuyn cc sng mang ny vo bng tn s ng xung 3,7 n 4,2 MHz vi rng 500 MHz. Sau cc tn hiu c phn knh vo cc rng bng tn ca tng b pht p. Thng thng rng bng tn cp cho mi b pht p l 36 MHz vi on bng bo v 4 MHz, v th 500MHz c th m bo knh cho 12 b pht p. B pht p c th x l mt sng mang c iu ch nh tn hiu TV chng hn hay c th x l nhiu sng mang ng thi vi mi sng mang c iu ch bi tn hiu in thoi hay knh bng gc no .B ghp knh u vo 1 2 3 4 5 6 7 8 9 10 11 12 3,72 3,76 Cc khi khuch i 3,80 cng sut 3,84 3,88 3,92 3,96 4.00 4.04 4.08 4,12 Hnh 1.4. Cc knh ca b pht p v tinh 4,16 n anten pht 3,7-4,2 GHz B ghp knh u ra

B lc bng thng My thu T anten thu bng rng 5,9256,425 GHz 3,7 4,2GHz

b) My thu bng rng S khi ca my thu bng rng c cho hnh 1.5. My thu c d phng kp phng trng hp s c. Bnh thng ch c my thu cng tc c s dng, khi c s c my thu th hai c t ng chuyn vo thay th. Tng u ca my thu l b khuch i tp m nh (LNA:low noise amplifier). B khuch i ny ch gy thm mt t tp m cho sng mang c khuch i, nhng vn m bo khuch i sng mang n c th vt qua c mc tp m cao hn trong tng trn tip sau. Khi tnh ton tp m do b khuch i gy ra, tin li ta thng quy i tt c cc mc tp m vo u vo LNA, y tng tp m thu c th c biu din vo nhit tp m tng ng. Trong mt my thu c thit k tt, nhit tp m c quy i vo u vo LNA


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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH thng c gi tr gn bng tp m ca ring LNA. Tng nhit tp m phi bao gm: tp m t anten. Nhit tp m tng ng ca anten c th ln n vi trm K. LNA tip tn hiu cho mt tng trn. Tng ny cn c tn hiu dao ng ni bin i tn s. Cng sut tn hiu cp t b dao ng ni cho u vo b trn khong 10dBm. Tn s ca b dao ng ni phi rt n nh v c t tp m. B khuch i th hai sau tng trn c nhim v m bo h s khuch i vo khong 60 dB. Cc mc tn hiu so vi u vo trn hnh v c cho dB. S phn chia khuch i ti 6GHz v 4GHz trnh dao ng xy ra nu khuch i qu ln trn cng mt tn s. My thu bng rng ch s dng cc thit b tch cc bn dn. Trong mt s thit k, cc b khuch i diode tunnel c s dng cho tin khuch i ti 6GHz trong cc b pht p 6/4-GHz v cho cc b khuch i thng s ti 14 GHz trong cc b pht p 14/12-GHz. Vi s tin b ca cng ngh Transitor trng (FET), cac b khuch i FET m bo hiu nng ngang bng hoc tt hn hin c s dng trong c hai bng tn. Cc tng trn diode c s dng. B khuch i sau b trn c th s dng cc transistor tip gip lng cc (BJT) ti 4GHz v FET ti 12 GHz hay FET cho c hai bng.My thu d phng

B tin khuch i

B trn

B khuch i

n b phn knh

- 1.5 dB

28.5 dB

23 dB

60 dB

B to dao ng 2.225 GHz

Hnh 1.5. My thu bng rng v tinh c) B phn knh u vo. My thu l thit b bng rng, trong khi khuch i tn hiu ti cng sut ln cho ng xung phi s dng cc b khuch i cng sut cao. Do vy, gim hi xuyn iu ch bng thng ca b pht p phi chia bng tn thnh cc bng con.

Nhim v: B phn knh vo phn chia u vo bng rng (3,7-4,2 GHz) thnh

cc knh tn s ca b pht p. Chng hn, trn hnh 1.4 cc knh ny c nh s t 1 n 12. Cc knh ny thng c t chc thnh cc nhm s chn v s l. Vic t


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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH chc ny cho php tng thm phn cch knh v gim nhiu gia cc knh ln cn trong mt nhm. Yu cu: Ti tn s trung tm c tuyn cng hng ca b lc khng c thay i trong sut qu trnh v tinh hot ng. Do vy, cc hc cng hng phi lm bng vt liu chu nhit, c h s dn n nh, dn tt.

Cu hnh: Bao gm cc b Circulator (xoay vng) v cc b lc bng di,

chnh cc b lc bng di ny quy nh bng thng ca b pht p. Trong hnh di, cc knh c t chc thnh hai nhm chn v l. S dng mt b lai ghp (Hybrid) chia thnh hai phn dng chung cng sut u ra my thu. Suy hao tn hiu trong b phn knh u ra ph thuc vo s ln tn hiu i qua b Circulator v s ln phn x ti b lc thng di. Do vy, suy hao tn hiu i vi cc knh khc nhau l khc nhau v t gi tr cc i ti knh xa u vo ca b IMUX nht. Bng cch phn chia b IMUX thnh nhiu phn, trong mi phn h tr mt s knh c th lm gim chnh lch suy hao gia cc knh.


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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINHKt cui phi 1 2 3 4 5 My thu 6

My

Gp 3dB

7 8 9 10 11 12

Kt cui phi

Hnh 1.6. B phn knh vo d) Cc khi khuch i cng sut y l thut b quan trng nht ca h thng pht p, bao gm cc b khuch i cng sut cao (HPA) m bo u ra cho tng b pht p. Nhim v: Thc hin cung cp cng sut theo yu cu cho ng xung v do Yu cu: Khuch i phi bo m tnh cht tuyn tnh hn ch tp m xc nh EIRP ca mi knh. xuyn iu ch phn b ti u vo cc trm mt t. Ngoi ra cn c cu hnh d phng v n d b hng.

Gii php:

- S dng cc b tin khuch i trc b HPA, do tn hiu u vo my thu sau khi khuch i b suy hao trong b IMUX khng ln iu khin b HPA.


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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH - Mc cc b suy hoa ni tip vi b khuch i vi h s suy hao c iu chnh t 0 n vi dB v bc nhy vi phn mi dB bng lnh t TT&C mt t. - S dng cc b cn bng (hot ng ch bo ho) b c tuyn bin tn s v pha tn s ca b HPA. (Nguyn l lm vic ca b cn bng thc cht l lm mo trc tn hiu sao cho c tuyn bin tn s, pha tn s ngc vi c tuyn trn ca b HPA. C hai loi HPA thng c s dng trn v tinh l b khuch i n sng chy (TWTA) v b khuch bn dn (SSPA). B khuch i n sng chy (TWTA) B khuch i n sng chy (TWTA) c s dng rng ri trong cc b pht p m bo cng sut ra cn thit cho anten pht. S n sng chy (TWT: travelling wave tube) c cho trn hnh 1.7. Trong n sng chy, sng tia in t gm: si nung, catt v cc in cc hi t to ra chm tia in t. Trng t gii hn tia in t truyn trong dy xon. i vi TWT cng sut cao hn c s dng cc trm mt t, trng t c th c to ra bi cun cm v c cp dng mt chiu. V kch thc kh ln v tiu th cng sut cao nn cun cm khng thch hp cho s dng trn v tinh, y cc TWT cng sut thp hn c s dng vi hi t bng nam chm t.

Hnh 1.7. B khuch i n sng chy (TWTA) Tn hiu v tuyn cn khuch i c cp cho dy xon ti u gn catt nht v to ra n hiu sng chy dc dy xon. Trng in ca sng s c thnh phn dc dy xon. Trong mt s vng trng ny s gim tc cc in t trong chm tia v trong mt SVTH : Nguyn Ngc Huy 20 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH s vng khc n s tng c cc in t trong chm tia. V th in t s co cm dc theo tia. Tc trung bnh ca chm ia dc xc nh bi in p mt chiu trn colector v c gi tr hi ln hn tc pha ca sng dc dy xon. Trong iu kin ny, s xy ra s chuyn i nng lng: ng nng trong chm tia c bin thnh th nng ca sng. Thc t, sng s truyn dc theo dy xon gn vi tc nh sng, nhng thnh phn dc trc ca n s tng tc vi chm tia in t. Thnh phn ny thp hn tc nh sng mt lng bng t s gia bc xon v chu vi. V s gim tc pha ny, nn dy xon c gi l cu trc sng chm. u im ca b khuch i ny so vi cc b khuch i n in t khc l n c th m bo khuch i trn mt rng bng tn kh rng. Tuy nhin cn iu chnh cn thn mc vo TWT gim thiu mo. Ti cc mc cng sut thp, quan h gia u vo v u ra l tuyn tnh, ngha l mt thay i dB cho trc u vo s gy ra cng mt s thay i dB u ra. Ti cc mc cng sut vo cao, cng sut ra s b bo ho. im cng sut ra cc i ny c gi l im bo ho. im bo ho l mt im tham chun tin li v cc i lng vo cng vi cc i lng ra thng c tham chun theo im ny. Vng tuyn tnh ca TWT c nh ngha l vng gii hn bi gii hn tp m nhit u thp v bi im nn 1dB. y l im m ti ng cong truyn t thc t thp hn ng thng suy din. e) Ghp knh u ra Nhim v: Kt hp tt c cc knh sau khi c khuch i a ra anten. Yu cu: Khng c lm tn hao cng sut ca cc b khuch i cng sut khng lm gim cng sut bc x ca anten. Gii php: Cc b lc u ra b khuch i cng sut s c ghp trc tip ln ng dn sng chung trong o ng dn sng c ngn mch ti u cui v u kia c ni vi cng tip sng ca anten.


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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINHT cc H PA

B lc knh 8

B lc knh 6

B lc knh 4

B lc knh 2

h iu ng thch ng v i b lc bng tn

1

T i cng tip sng ca A nten

dn sng ngn m ng ch B lc knh 7 B lc knh 5 B lc knh 3 B lc knh 1 N m gn ach

h iu ng thch ng v i b lc bng tn

8

T cc H A P

Hnh 1.8. B ghp knh u ra 1.1.5.2. Phn h Anten Anten trn v tinh thc hin chc nng kp: Thu ng ln v pht ng xung. Chng c nhiu loi: t cc anten dipole c c tnh v hng n cc anten tnh hng cao phc v cho vin thng, chuyn tip truyn hnh v pht qung b. Cc chc nng chnh ca anten trn v tinh : - La chn sng v tuyn c pht i trong bng tn cho vi phn cc cho t cc trm mt t nm trong vng ph sng ca v tinh. - Pht sng v tuyn bng tn v phn cc cho ln khu vc quy nh trn mt t. Yu cu : - Thu can nhiu cng nh cng tt. - Pht cng sut nh nht ra ngoi vng quy nh. Cc loi anten c c cc vng ph sng khc nhau anten trn v tinh thng s dng hai loi chnh anten loa v anten mt phn x vi cc b chiu x khc nhau c tip sng theo cc phng php khc nhau. a) Anten loa: C u dim tin cy v n gin nhng tnh hng km nn c s dng ph sng vi bp sng ton cu. H s tnh hng c tnh theo cng thc: D = 4pS/2 Trong : S l din tch ca ming loa l hiu sut s dng b mt bng 0,6 0,8.


22

Lp 46K - TVT


Hnh 1.9. Anten loa hnh ch nht

b) Anten phn x (reflector) : L loi anten thng c s dng nht to ra bp sng dng vt v dng hnh th ring r (shaped). Anten ny bao gm mt mt phn x parabol v mt hoc nhiu ngun pht x t ti tiu im ca mt phn x. iu chnh c hng chm sng ca anten trn qu o bng cc lnh iu khin t xa. Vic thay i bp sng c thc hin bng cch thay i pha ca cc phn t bc x. Vic lp t b pht x c t theo kiu ng trc hay lch trc (offset). to ra bp sng dng trn hay elp c thc hin bng cch thay i hnh dng mt phn x cho ph hp vi vng ph sng. Cn to ra bp sng dng hnh th ring r hay phc tp th c th c thc hin bng cch t mt dy cc phn t bc x ti tiu im ca mt phn x c tip in ca cng mt tn hiu nhng bin v pha lch nhau nh cc mch to bp sng.

Hnh 1.10. Anten phn x c) Anten dy (array) : S dng mt b rt nhiu cc phn t bc x to nn mt gc m bc x. Biu bc x ca anten ny c to ra bng cch kt hp bin v pha ca sng c bc x bi dy cc phn t bc x. Cc phn t bc x c t cch nhau 0,6, biu bc x c iu chnh bng cch thay i pha v bin ca tn hiu tip in bng mt dch pha, chia cng sut c th iu khin c. 1.1.6. SUY HAO TRONG THNG TIN V TINH Mt tuyn thng tin v tinh bao gm ng truyn sng t anten ca trm pht n v tinh (tuyn ln - uplink) v t v tinh n anten ca trm mt t thu (tuyn xung - downlink). Do suy hao trong thng tin v tinh gm cc loi suy hao sau: 1.1.6.1. Suy hao trong khng gian t do SVTH : Nguyn Ngc Huy 23 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH i vi v tinh i tnh cao 35.768km, c ly thng tin cho mt tuyn ln hay mt tuyn xung gn nht l 35.768km. Do c ly truyn sng trong thng tin v tinh ln nh vy nn suy hao trong khng gian t do l suy hao ln nht. Gi suy hao ny l Ltd , 4d ta c : Ltd = 2

Trong d[km] : l chiu di ca mt tuyn ln hay xung. [m] : bc sng cng tc.Ltd [dB]

Bc sng c i ra tn s cng tc vi quan h f = c/ . c : vn tc nh sng c = 3.108 m/s. f : tn s cng tc [GHz]. Trong n v dB 4d 4df 4df Ltd dB =10 lg =10 lg = 20 lg c c 2 2

Suy hao khng gian t do ca tuyn ln hay xung khi cng tc bng C (4/6GHz) vo khong 200dB. b vo suy hao ny, m bo cho my thu nhn c mt tn hiu ln c -90dBm n -60dBm, ngi ta s dng anten c ng knh ln hng chc mt c h s tng ch ln khong 60dB v my pht c cng sut ln hng trm dn hng ngn W. Xt trng hp mt my pht c cng sut bc x l 100W cho mi sng mang, cng tc bng C (6/4GHz). Nu ch tnh n suy hao khng gian t do l 200dB th cng sut thu c sng mang s l: PRx = 100/1020 = 10-18 (W) = 10-15 (mW) PRx = 10lg100 (dBW) - 200 (dB). Tnh theo dBw:= 20 (dBW) - 200 (dB) = -180 (dBW) = -150 (dBmW).

Vi cng sut nh nh vy th my thu khng th thu c tn hiu, c c cng sut u vo my thu khong -70dBm th ta phi s dng anten pht v thu c h s tng ch ln. Nu h s tng ch ca anten trm mt t l GR=50dB th anten thu trn v tinh c h s tng ch GT=30dB. Ngoi suy hao chnh trong khng gian t do cn c cc suy hao khc tuy khng ln nhng khi tnh ton tuyn thng tin v tinh m ta khng xt ht cc kh nng xu nht do nh hng ca mi trng truyn sng th khi xy ra cc hin tng cht lng thng tin s xu i v c th lm gin on thng tin. Cc suy hao c trnh by sau y. 1.1.6.2. Suy hao do tng i lu SVTH : Nguyn Ngc Huy Lp 46K - TVT

24

TNH TON, THIT K TRUYN HNH S V TINH Tng i lu l lp kh quyn nm st mt t ln n cao (10km-15km) (theo quy nh ca tng i lu tiu chun), bao gm cc cht kh chnh hp th sng gy ra suy hao nh hi nc, Oxy, Ozon, Cacbonic. Suy hao ny ph thuc nhiu vo tn s v gc ngng ca anten v ch ng k khi tn s cng tc t 10GHz tr ln, ngha l khi cng tc bng Ku (14/12GHz) hay bng Ka (30/20GHz). Anten c gc ngng cng ln th suy hao tng i lu cng nh, do ng truyn ca sng trong tng i lu cng ngn. Ti cc tn s 21GHz v 60GHz c cc suy hao cc i, l do s cng hng hp th i vi cc phn t hi nc v Oxy. 1.1.6.3. Suy hao do tng in ly Tng in ly l lp kh quyn nm cao khong 60km n 400km, do b ion ho mnh nn lp kh quyn cao ny bao gm ch yu l cc in t t do, cc ion m v dng nn c gi l tng in ly. S hp th sng trong tng in ly gim khi tn s tng, tn s trn 600MHz th s hp th khng ng k. 1.1.6.4. Suy hao do thi tit Suy hao do cc iu kin thi tit nh my, ma, sng m, suy hao ny ph thuc vo nhiu yu t nh cng ma hay sng m, vo tn s, vo chiu di qung ng i ca sng trong ma, chiu di ny ph thuc vo gc ngng anten. Khi gc ngng tng, suy hao gim, vi gc ngng anten khong 400 tr ln th suy hao khng ng k, lc suy hao do ma khong 0,6 dB, suy hao do sng m khong 0,2dB, cn suy hao trong cc cht kh rt nh c th b qua. Ni chung khi tn s v cng ma tng th suy hao tng nhanh, c bit trong khong tn s t 10GHz n 100GHz. Suy hao thc t tu thuc vo gc ngng anten, cao t anten so vi mc nc bin, chiu cao cn ma v sng m m on ng i thc t ca sng qua vng l khc nhau. Suy hao trn ton b on ng c chiu di Le sng i qua l:Ltt = .Le (dB)

Trong : l h s suy hao trn on ng 1km (dB/km), ph thuc tn s, mi trng gy suy hao nh cng ma hay dy ca sng m. Le: l chiu di thc t sng i qua vng gy suy hao (km), ph thuc gc ngng anten, cao t anten, c xc nh theo cng thc:Le = hm hs ( km ) sin E


25

Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH Vi hm l cao ca cn ma (km), theo khuyn ngh 564 ca CCIR v t 0 0 n 560 ly hm = 3 + 0,028 (km). hs l cao anten trm mt t so vi mc nc bin (km). E l gc ngng anten (). 1.1.6.5. Suy hao do t anten cha ng Khi anten pht v thu lch nhau th s to ra suy hao v bp chnh ca anten thu hng khng ng chm tia pht x ca anten pht. Thng th suy hao do t anten cha ng t 0,8 n 1 dB.Anten pht Anten thu

Gc sai lch

Hnh 1.11. Sai lch do t anten cha ng

T1.1.6.6. Suy hao trong thit b pht v thuTn hao Fi TX PTX LFTX PT GT GR PRX Tn hao Fi L LFRX PR

RRX

Hnh 1.12. Suy hao trong thit b pht v thuSuy hao trong thit b pht v thu cn gi l suy hao do h thng fi, c hai loi nh sau: Suy hao LFTX gia my pht v anten, anten pht c cng sut PT cn phi cung cp mt cng sut PTX u ra ca b khuch i pht, do vy: SVTH : Nguyn Ngc Huy Lp 46K - TVT

26

TNH TON, THIT K TRUYN HNH S V TINH PT = PTX - LFTX PR = PRX - LFRX = LFTX = 2dB. Suy ra LFTX = LFRX = 10-0,2 (ln). 1.1.6.7. Suy hao do phn cc khng i xng Suy hao do phn cc khng i xng xy ra khi anten thu khng ng hng vi s phn cc ca sng nhn. V ng truyn phn cc trn, sng pht ch c phn cc trn trn trc anten pht v n s tr thnh elip khi ra khi trc anten . Khi truyn qua bu kh quyn cng c th lm thay i phn cc trn thnh phn cc elip. Cn trong ng truyn phn cc thng th sng c th b quay mt phng phn cc ca n khi ng truyn i qua kh quyn, do anten thu khng cn mt phng phn cc ca sng ng v sng ti. Suy hao do lch phn cc thng ch 0,1dB. 1.1.7. TP M TRONG THNG TIN V TINH 1.1.7.1. Nhit tp m h thng Trong mt tuyn thng tin v tinh, tp m gy ra cho trm mt t do nhiu nguyn nhn v c tnh bng nhit tp m tng ng TSYS v c gi l nhit tp m h thng. Nhit tp m h thng c xem l tng ca bn thnh phn c biu din theo biu thc:TSYS = TS + T A + TF + TR LF

[dB] [dB]

Suy hao LFRX gia anten v my thu, cng sut PRX u vo my thu bng: Trong cc h thng v tinh hin nay, n gin thng ly h s tn hao fi LFRX

[0K]


27

Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINHDi ngn h Tng in ly Mt trng Mt tri

Tri ma

bp chnh

bp ngc

bp ph

Hnh 1.13. Cc ngun tp m nh hng n thng tin v tinh.

Trong : TSYS: nhit tp m h thng TS: nhit tp m bn ngoi TA: nhit tp m anten TF: nhit tp m ca h thng fi TR: nhit tp m hiu dng u vo my thu LF: Suy hao ca h thng fi, c a vo tnh ton vi nhit tp m bn ngoi, nhit tp m anten v nhit tp m fi v 3 loi tp m ny c lin quan n suy hao ca fi. Ta ln lt xt n cc loi tp m ny. 1.1.7.1.1. Nhit tp m bn ngoi TS v nhit tp m anten TA Nhit tp m bn ngoi v anten bao gm:

Nhit tp m khng gian: Gm cc thnh phn sau: - Nhit tp m v tr: Tc ng tn s v tuyn l do bc x t v tr cn d li

(khong 2,760K). - Nhit tp m ca di ngn h: Nu hng anten vo vng c s sao cc i ca di ngn h th nhit tp m c th ln n gn 1000K trong vng tn s t 0,3GHz n 1,2GHz.. - Nhit tp m ca mt tri: Mt tri bc x ra sng in t tt c cc tn s, c bit l di viba (microwave). Nhit tp m do mt tri gy ra cho trm mt t ph thuc vo hng anten, nu mt tri nm ngoi vng ph sng ca bp chnh anten th SVTH : Nguyn Ngc Huy Lp 46K - TVT

28

TNH TON, THIT K TRUYN HNH S V TINH nhit tp m di 500K. Cn khi mt tri chiu thng vo anten th nhit tp m ln n 10.0000K hoc c th hn tu thuc tn s cng tc, kch thc mt phn x v s vt en ca mt tri (s vt en th hin s hot ng mnh hay yu ca mt tri). Trng hp trm mt t - v tinh - mt tri nm trn cng mt ng thng ch xy ra mt vi ngy trong nm vo ma xun lm cho thng tin b gin on vi ba pht.

Nhit tp m do kh quyn (nhit tp m do tng i lu): n ph thuc vo

chiu di qung ng i ca sng trong tng i lu ( cao 15 km t mt t). Ni cch khc chnh l ph thuc vo gc ngng ca anten, tn s cng tc. + Nhit tp m do ma, c xc nh bng cng thc :TM = Tm ( 1 1 ) LM

Trong : TM : nhit tp m do ma (0K).LM : suy hao do ma, LM = 16,57. Tm : nhit trung bnh ca cn ma. Tm = 1,12 Txq - 50 (0K). Txq : nhit xung quanh trm mt t (0K).

Nhit tp m t trm mt t xung quanh trm: V anten ca trm mt t

hng ln bu tri nn nhit tp m ca mt t gy ra ch yu do bp ph v bp ngc, mt phn cho bp chnh khi anten c tnh nh hng km v gc ngng nh. Nhit tp m cho mi bp ph gy ra c tnh theo cng thc:Ti = Gi TD

Trong Gi : h s tng ch ca bp ph.TD : nhit chiu sng mt t do mt tri to ra.TD

= 1500K khi gc ngng t 00 n 100.

TD = 100K khi gc ngng t 100 n 900.

Ngoi ra cn nhit tp m do cc chng ngi gn nh to nh, cc mi che (v d nh vm cy), do cc b phn cn tr trong anten nh cc thanh , b tip sng (ngun bc x s cp) v b suy hao bp ph gy ra. 1.1.7.1.2. Nhit tp m h thng fi TF Nhit tp m h thng fi c tnh bi biu thc sau: SVTH : Nguyn Ngc Huy 29 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINHTF = T0 ( LF - 1)

(0K)

Trong T0 : Nhit mi trng (0K).LF : Suy hao ca h thng fi. TF tng khi LF tng dn n cng sut tp m bn trong gy ra bi h thng fi

tng ln. Do cn thit k cc phng tin sao cho c h thng fi l nh nht. Mt cch gn ng c th coi nhit tp m fi l 2900K. 1.1.7.1.3. Nhit tp m my thu TR Nhit tp m my thu bng tng nhit tp m gy ra trong mi phn ca my thu. N c tnh bi cng thc sau:TR = T1 + T3 T2 TK + + ... + (0K) G1 G1G 2 G1G 2 ...G K 1

Trong TR : Nhit tp m my thu (0K).G1 ,G2 ,...,G K : L h s khuch i tng phn. T1 ,T2 ,...,TK

: Nhit tp m u vo (0K).

Nu h s khuch i tng u ln th tp m ti cc tng tip theo c th b qua. Do yu cu tng u tin phi c h s khuch i ln v tp m thp, v vy trong thng tin v tinh dng cc b khuch i tp m thp (LNA-Low Noise Amplifier). Mt cch gn ng ta coi nhit tp m tng u ny cng l nhit tp m ca my thu. 1.1.7.2. Cng sut tp m nhiu 1.1.7.2.1. Can nhiu khc tuyn Cc tuyn thng tin v tinh c th b nhiu trong cc trng hp nh sau: + Tuyn viba mt t n v tinh thng tin. + Tuyn viba mt t n trm mt t. + V tinh thng tin khc n trm mt t. Can nhiu gia viba v trm mt t. C hai trng hp:

Trng hp th nht, ng thng tin viba mt t c cng tn s lm vic vi ng ln ca h thng thng tin v tinh, bi vy tn hiu viba mt t c trn vi tn hiu u vo my thu v tinh.


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TNH TON, THIT K TRUYN HNH S V TINH Trng hp th hai, ng thng tin vi ba mt t c tn s bng tn s ng xung ca h thng thng tin v tinh, bi vy u vo my thu trm mt t cng b trn vi tn hiu ca ng thng tin viba mt t.Qu o a tnh

Trmviba . Trm mt t.

Hnh 1.14. Can nhiu gia viba v trm mt t v v tinh

Trong thit k tuyn thc t, phi t trm mt t sao cho nhiu xy ra t nht, nhiu nh nht bng cch s dng anten c cc c tnh bp ph tt. Mc d mc tiu c bn thit k tuyn i vi v tinh thng tin l loi b nhiu, nu iu ny khng th thc hin c th trong thit k tuyn phi bao hm c lng nhiu cho php. Nhiu thng khng th nh gi c bng tnh ton, v d nh trong thnh ph c nhiu vt cn phn x quanh ni t trm mt t. V th phi dng phng php o thc t o nhiu. S can nhiu t v tinh thng tin khc n trm mt t: V tinh 1 V tinh 2 Qu o v tinh ng lin nt biu th ng i ca tn hiu mong mun.


Trm mt t 1

Trm mt t 2

ng t nt biu th ng i ca tn hiu can nhiu.

Hnh 1.15. Can nhiu gia cc h thng thng tin v tinh.

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TNH TON, THIT K TRUYN HNH S V TINH Hnh 1.15 cho ta thy can nhiu xy ra do cc v tinh t gn nhau. Ta xem xt tn hiu can nhiu t v tinh 1 tc ng ln trm mt t 2 v tn hiu can nhiu t v tinh 2 tc ng ln trm mt t 1. Khi gc cng nh ( tng ng vi 2 v tinh t cng gn nhau) th nh hng ca chng ln trm mt t cng ln. Trong thc t, t s cng sut ca sng mang trn sng can nhiu C/N gia cc v tinh c th ln hn hoc bng 30dB (1000 ln) khi hai v tinh t cch nhau khong 30 ngay trn qu o, ngay c nu cc anten ca chng cng chiu vo cng mt v tr. 1.1.7.2.2. Nhiu cng tuyn Nhiu c th to ra ngay trong tuyn, gi l nhiu cng tuyn, bao gm: + Tp m nhiu kh phn cc. Loi nhiu ny thng xy ra trong h thng thng tin phn cc kp, n b chi phi bi cc c tnh ca anten. trit tp m nhiu kh phn cc, chn loi anten c XPD (kh nng phn bit phn cc cho) ln, thng th XPD ca anten khong 30dB, nhng tn s 10GHz th XPD ca anten li gim do ma. + Tp m nhiu knh ln cn. Nhiu ny gy ra bi knh ln cn c cng phn cc vi tuyn v tinh ang xt. C th trit nhiu knh ln cn bng mt b lc c c tnh ct nhn. 1.1.7.2.3. Tp m mo xuyn iu chm c ra

mc vo

Hnh 1.16. c tnh vo ra ca TWT. Tp m mo xuyn iu ch l mt trong nhiu loi tp m ng truyn thng tin v tinh. Tp m xuyn iu ch trong v tinh sinh ra khi b pht p ca n khuch i ng thi nhiu sng mang. Cc c tuyn phi tuyn vo ra ca b pht p l nguyn


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TNH TON, THIT K TRUYN HNH S V TINH nhn sinh ra tp m xuyn iu ch. B khuch i n sng chy TWT c s dng l thnh phn chnh ca b pht p. Hnh 1.16 m t mi quan h gia u vo v u ra ca mt TWT, nu quan h vo ra tuyn tnh nh ng t nt th khng gy ra mo xuyn iu ch. Tuy nhin trong thc t c tuyn ca TWT khng tuyn tnh nn gy ra xuyn iu ch. Khi mc vo vt qu mt gi tr no , th mc ra ca TWT khng tng c na mc d mc vo vn tng ng k, hin tng ny gi l bo ho. mo do xuyn iu ch nh hn gi tr cho php, TWT phi lm vic mc thp hn im bo ho. Mc cng sut chnh lch gia im lm vic v im bo ho ti u vo v u ra tng ng gi l li u vo (IBO) v li u ra (OBO). Tp m xuyn iu ch sinh ra do cc sn phm xuyn iu ch hoc mo lt vo bng tn truyn dn khi nhiu sng mang c khuch i ng thi bng b khuch i TWT phi tuyn. Mc xuyn iu ch ph thuc vo s sng mang v s chnh lch tn s gia chng. 1.1.8. LI CA ANTEN V RNG CA CHM TIA Trong thng tin v tuyn, anten l mt phn rt quan trng ca lin kt truyn dn. N gip tp trung nng lng bc x v pha anten thu, nhng thit b thu nhn ch nhn c mt phn cng sut. Hu ht cng sut tri u ln trn mt vng rng. tng ch ca anten c biu th nh sau: 4 G =

Trong : =

C v vn tc ca nh sng l 3.108m/s v f l tn s sng in t, f

din tch anten l A= 4D2 vi D l ng knh. rng chm tia ca th bc x l: (dB ) = 70 C = 70 D fD


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TNH TON, THIT K TRUYN HNH S V TINH CHNG II: H THNG THU TRUYN HNH S QUA V TINH 1.2.1. S KHI H THNG THU TRUYN HNH S QUA V TINH. Truyn hnh s l truyn hnh c cht lng cao tha mn c nhu cu ca ngi xem cng nh gip cho cc trung tm truyn hnh d dng qun l cc thu bao. pht trin ln cng ngh truyn hnh s th cc trung tm truyn hnh cn phi ci thin h thng ca mnh, nng cp cc phng tin k thut cng nh cn phi lp t cc u thu truyn hnh s v iu ch. Cu to ca mt h thng truyn hnh s cng tng t nh h thng truyn hnh tng t. Tng qut ca mt h thng truyn hnh s bao gm cc khi chc nng nh: thu tn hiu s, nn v m ha, iu ch v sau s c ghp knh v truyn i n thu bao. Ti thu bao s c lp t mt Set-top-box s thu tn hiu v gii m tn hiu.

Tnh hiu thu

Nn v m ha

iu ch

Nt quang

Hub Headend (ni qun l mng)`

Khuch i

H thng truy cp c iu kin CA

Hnh 1.17: S khi h thng thu truyn hnh s Nguyn l hat ng ca h thng Theo s tng qut ca h thng truyn hnh s th tn hiu c pht i ti trung tm v i n thu bao s l tn hiu s. Ti trung tm ca h thng tn hiu s c thu nhn t nhiu ngun khc nhau. Cc tn hiu c my thu thu nhn s c a qua khi nn v m ha ti y tn hiu s c chuyn i hon tan thnh tn hiu s. Tn


34

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TNH TON, THIT K TRUYN HNH S V TINH hiu ny sau s c a qua b iu ch s iu ch tn hiu s thnh mt tn hiu hon chnh, sau tn hiu ny s c ghp knh v pht i trn si cp quang n node quang, t node quang tn hiu c khuch i v a n thu bao, ti thu bao ca truyn hnh s s c mt h thng truy cp c iu kin. Tin b ca truyn hnh s l c th kt ni gia my tnh vi my thu hnh v hp gii m Set-top-box s v c kh nng truyn trong Internet. Mng hat ng ca h thng trn u da trn c s ca mng HFC v c gi l HFC s, HFC l cng ngh cp quang lai ghp, s dng cu hnh mng dng cp quang v cp ng trc, c s dng phn phi li cc dch v bng rng. Cc dch v bng rng ny bao gm: in thai, a phng tin tng tc, truy cp Iternet tc cao, VOD (Video-on demand -video theo yu cu) v hc t xa, cc lai dch v cung cp cho thu bao thay i gia cc cng ty cp. Nhiu cng ty truyn hnh cp chnh Chu u, M v Chu M La Tinh, ng Nam s dng HFC s, Cc mng s dng cng ngh HFC c c trng: thc hin mt cch l tng cc dch v thng tin cho th h mi, HFC tha mn cc yu cu v tng kh nng m rng v thc hin cc dch v ph m khng cn thay i c s h tng. 1.2.2. H THNG HEADEND S 1.2.2.1. S h thng Headend s: Headend l trung tm thu v pht tn hiu. T y tn hiu s c thu nhn v qua qu trnh x l sau s c pht i. Khc vi Headend Analog l tn hiu ti trung tm pht i l tn hiu s, do s dng cng ngh mng HFC nn h thng Headend s vn da trn c s h tng c sn ch cn u t thm trang thit b x l tn hiu.AQUISITIONENCODE SMS P R O C E S S I N G EPG S C R A M B L I N G CAS

TRANSMISSIOND ASI E S R C SI O R U A T M SI I B N L G I N SI G IP&VOD

SATELLITE OFF - AIR NETWORK LOCAL

MOD PRIVATE LOCAL NETWORK

R E M U X I N G

Hnh 1.18. S h thng Headend s SVTH : Nguyn Ngc Huy 35 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH 1.2.2.2. Chc nng cc khi trong h thng Headend s: 1.2.2.2.1. Tn hiu thu Tn hiu thu y rt a dng, cc tn hiu thu gm: Tn hiu v tinh, truyn hnh s mt t, mng, cc i a phngty theo tng loi tn hiu m ta c cc b gii iu ch khc nhau bng cch s dng cc card ri gn trn cc zack cm. c im:-

Cc tn hiu sau khi thu c s c x l v truyn tn hiu s trn mt

bng tn c s (ASI). - Yu cu c trng ca tn hiu l: linh ng, n gin, nng lng thp v vn hnh mt cch c lp. - Cht lng hin th ca tn hiu: cn x l tn hiu mt cch y v chnh xc, trnh lan truyn tn hiu b trc trc. - Yu t d phng: c tm quan trng i vi cc tn hiu thu c v n s t ng backup d liu khi b phn trc xy ra s c.

Thu tn hiu t v tinh (card TITAN)

A SI QPSK DEM ODULATOR P R O C E S S IN G

Hnh 1.19. Thu tn hiu t v tinh Tn hiu thu t v tinh s c a qua b gii iu ch QBSK vi ng ra l tn hiu ASI. c im:-

Tc d liu t 1 n 45 Mbaud. C th la chn ch t ng hay l th cng cc thng s cn iu ch Hai ng ra ASI vi tc ln n 90 Mbps. Chc nng hin th c ci tin nh: SNR, mc ng vo, BER, khng sa c

li.


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Lp 46K - TVT


Thu tn hiu truyn hnh s mt t (card ATLAT II)

OFDM

D e m o d u la to ArS I

P R O C E S S IN G

Hnh 1.20. Thu tn hiu truyn hnh s mt t. Tn hiu thu c c a qua b gii iu ch C-OFDM. c im: - H tr FFT kch thc 2K v 8K. - C th chn c bng thng 7 MHz hoc 8MHz. - c h tr bng tn UHF v VHF. - Hai ng ra ASI vi tc ln n 31.7 Mbps. - Ch hin th c ci tin (MER). - Giao din s dng n gin.

Thu tn hiu cc i a phng (card SPECTRA)

QAM

D e m o d u la to r A S I (S P E C T R) A

P R O C E S S IN G

Hnh 1.21. Thu tn hiu cc i a phng Tn hiu s c anten Yagi thu nhn v c a qua b gii iu ch QAM (iu ch pha) c im: - Tn s RF ng vo t 45 n 860 MHz. - C th chn la ch t ng hay bng th cng cc thng s cn iu ch. SVTH : Nguyn Ngc Huy 37 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH - M rng chc nng hin th tn hiu. - Hai ng ra ASI. - Hon ton c ch to bi ROSA. 1.2.2.2.2. X l tn hiu Tn hiu sau khi thu c s c a qua khi PROCESSING x l. Gm cc khi: Decrambling, Routing, Remuxing, Processing, Scrambling.

Phn loi tn hiu

Cc tn hiu sau khi vo khi ny s c gii m x l. c im: - Phc hi tn hiu ch xy ra vi tn hiu l s. - S phc hi da trn chun m: + Cable- POD v DVB-CI. + Gn lin vi chun ASI. + C th thay i h thng CA v n d v r. - Vn hnh mt cch c lp v bn c th xo tn hiu sau khi hon tt cng vic truyn. nh tuyn L h thng chuyn mch thng minh c s dng rng ri trong nhiu lnh vc ni m im ni im hay a im ni a im th con ng d phng l rt cn thit. Mc ch chnh ca phn ny l chn ng i lm sao khi tn hiu truyn i trn ng ny b mt cn c ng khc thay th th c gi l Redundance swiching. Phng php d phng c xem l kh quan trng v: - Tt c cc thit b lm vic lp transport nn nhiu chng trnh c th b mt. - S gin on l nguyn nhn gay mt tn hiu. - Mng s thng cung cp nhiu dch v hn nh pay-per-view.

Phn knh

- S chn la v trn tn hiu li vi nhau trong 1 lung truyn. - Da trn giao din chun. + S thch hp gia cc thit b vi ASI. + Giao din s dng hin th v iu khin l Web v SNMP. - Qu trnh x l n gin. + Phn mm PSI/SI c th thc hin 1 cch t ng pha sau. SVTH : Nguyn Ngc Huy 38 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH + Ch cn 1 sai phm nh s hot ng sai.

X l

X l PSI/SI xem nh l 1 cch iu chnh ca tn hiu. Tht s PSI/SI rt ph hp vi ng TS (Transport Stream). a s c thc hin bi khi Re-multiplexer. Mt khc PSI/SI gip thng tin khng b gin on v mi 1 lun TS cha thng tin v chng trnh.

Xo trn

Chng trnh Scrambling xy ra trong s thiu lin lc vi iu kin truy cp h thng. B xo trn da trn tiu chun m. N cho php nhiu thnh phn CA trong h thng trn li vi nhau ging nh lun TS. Cc thit b vn hnh khng tt cng ging nh 1 b trn c ci t sn, khng c a cng v ch thng gi, chng trnh x l c gn vo chy vi thi gian thc OS. 1.2.2.2.3. Truyn ti tn hiu Vi phng thc a tt c cc tn hiu vo 1 ch truyn i vi mng. Vi cc k thut iu ch: + 64 & 256 QAM. + 8VBS &C-OFDM. + QPSK, 8PSK,16PSK. Tng thch vi mng: + PDH/SDH/SONET DS3/E3. + ATM. + a ch IP. K thut nn MPEG + Audio/Video. + SDI . 1.2.2.3. Nn v m ha tn hiu truyn hnh: Trung tm ca mt mng pht sng video s bao gm h thng nn, n cung cp chng trnh video v audio cht lng cao cho ngi xem bng cch ch s dng mt phn nh rng bng tn mng. Mc ch chnh ca nn l ti thiu ha kh nng lu


39

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TNH TON, THIT K TRUYN HNH S V TINH tr v truyn dn pht sng thng tin (ghp nhiu tn hiu chng trnh truyn hnh vo mt dng truyn). H thng nn tn hiu gm cc b m ha s (digital encodes) v cc b ghp knh (multiplexers), Cc b encodes c nhim v chuyn i tn hiu tng t (video/audio) sang dng s (digitizing), nn (compressing) v xo trn (scrambling) thnh mt dng audio, video v d liu (data) khc di dng s c nn. M ha s cho php truyn dn, pht sng nhiu chng trnh video/audio cht lng cao qua cng rng bng tn nh mt knh sng video/audio tng t (v d 8Mhz Vit Nam). Tn hiu c m ha v nn thnh mt dng tn hiu MPEG-2 (Moving Pictures Experts Group: Chun nn tn hiu video/audio ca Chu u, s dng cho Vit Nam) s c a n b ghp knh, nhm chuyn gia MPEG nh ngha cc tiu chun nn v nh dng file, bao gm c h thng ha video MPEG-2. Tiu chun nn tn hiu video s MPEG-2 c chp nhn 190 nc. C nhiu tiu chun nn video/audio: MPEG-1, MPEG-2, MPEG-3, MPEG-4, MPEG-5, MPEG-6, MPEG-7, MPEG-J(JAVA), Tuy nhin, truyn hnh s qung b ch s dng tiu chun MPEG-2, Chun nn MPEG-1 v MPEG-3 c hp nht vo tiu chun MPEG-2 (my VCD s dng chun nn MPEG-1 cn DVD s dng chun nn MPEG-2, do DVD c c c a VCD (MPEG-1) va a DVD (MPEG-2). 1.2.2.3.1. Khi qut v k thut nn nh s Cng vi s pht trin mnh m ca my tnh v s ra i ca Internet th vic tm mt phng php nn nh gim bt khng gian lu tr thng tin v truyn thng tin trn mng nhanh chng ang l mt yu cu cn thit. Trong nhng nm gn y, c rt nhiu cc phng php v ang c nghin cu rng ri thc hin nn nh, Tt c u vi mt mc ch chung l lm th no biu din mt nh vi t bit nht c th ti thiu ha dung lng knh truyn v khng gian lu tr trong khi vn gi c tnh trung thc ca nh, iu ny tng ng vi vic biu din nh c tin cy cao nht vi tc bit nh nht. Tc bit c o bng s bit trn mt im nh (pixel). Tc bit i vi nh en trng khi cha c nn l 8bit/pixel v i vi nh mu l 24bit/pixel, Cc k thut nn hin nay cho php dung lng nh c nn gim 30 n 50 ln so vi nh gc m nh vn gi c trung thc cao, trung thc ca nh c nh gi da trn tiu ch nh li trung bnh bnh


40

Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH phng (MSE: Mean Square Error) hoc t s tn hiu trn tp m (SNR: Signal-to-Noise rotio) gia nh gc v nh nn. Cc nguyn tc ca nn nh.

Mt tnh cht chung nht ca tt c cc nh s l tng quan gia cc pixel cnh nhau ln, iu ny dn n d tha thng tin biu din nh, D tha thng tin s lm cho vic m ha khng ti u, Do cng vic cn lm nn nh l phi tm c cc biu din nh vi tng quan nh nht gim thiu d tha thng tin ca nh, Thc t, c hai kiu d tha thng tin c phn lai nh sau: D tha trong min khng gian: Tng quan gia cc gi tr pixel ca nh, iu ny c ngha rng cc pixel ln cn ca nh c gi tr gn ging nhau (tr nhng pixel gip ng bin nh). D tha trong min tn s: Tng quan gia cc mt phng mu hoc di ph khc nhau. Trng tm ca vic nghin cu v nn nh l tm cch gim s bit cn biu din nh bng vic lai b d tha trong min khng gian v min tn s cng nhiu cng tt. Cc k thut nn nh c s dng.

Nn nh khng mt thng tin: Vi phng php ny sau khi gii nn ta khi phc c chnh xc nh gc, cc phng php nn ny bao gm m ha Huffman, m ha thut ton Nn nh c mt thng tin: nh gii nn c mt s sai khc nh so vi nh gc. Cc phng php ny bao gm: - Lng t ha v hng: PCM v DPCM. - Lng t ha vector. - M ha bin i: Bin i cosin ri rc (DTC), bin i Fourier nhanh. - M ha bng con. 1.2.2.3.2. Chun nn MPEG MPEG (Motion Picture Expert Group, tc nhm Lo - lng v hnh ng). Nhm ny do vin nh chun Quc T (ISO = International Standard Organization) thnh lp cui nm 1988 nhm nh ra cc dng thc video s ti u, dng c cho nhiu lnh vc. Khi u t dng thc DVI (Digital Video Interactive, 1988) ghi hnh trn CDROM ca vin i hc Sarhoff Princeton, n nm 1990 nhm son xong dng thc SVTH : Nguyn Ngc Huy 41 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH MPEG-1, ch p dng cho video s SIF (352x288x25 hoc 352x240x30 khng xen k), vi bit rate trung bnh c 1.5MHz. C th tm xem MPEG-1 l php nn ni hnh JPEG (Intra Frame) + php nn lin hnh (Inter Frame), bng cch suy ra mt hnh gia, t dch chuyn ca c hnh sau ln trc n (B = Bi-Directional Frame). ng dng ni bt ca MPEG-1 l vic ghi hnh cng lc vi hai ng m thanh ni trn a CD, t cht lng tng ng bng t VHS, gi l VCD, vn cn tn ti n nay. T l nn ca MPEG1 t khong 20:1 l mt thnh tu ln, m ra trin vng da vo MPEG1 pht trin ng dng cho truyn hnh i chng (Broadcast TV, tc PAL, NTSC, SECAM). Nm 1991, nhm son xong MPEG-2 vn trn nn c php ca MPEG1 nhng thm vo b cng c xen k (Interlace Tool). B cng c chn tm (Scalable Mode), chng hn chn t l khun hnh 4x3 hay 16x9, chn nhm hnh (GOP) nhiu hay t, chn tm d bo chuyn ng (macro block) rng hay hp. B cng c phm cp (Profiles-Levels), ty chn nhiu phm cp cht lng khc nhau, thnh cng ca MPEG2 m nn n , nh ban u chn MPEG-3 dnh ring cho HDTV khng cn lm na v chnh MPEG-2 p ng c tt c. Cui nm 1992, MPEG-4 c son xong, y l mt php nn hiu qu cc cao nhng ch dnh cho cc hnh c cnh nn c nh bit trc. Chng hn ton cnh sn banh trong mt trn bng , ton cnh vn phng ca mt in thai c hnh Nm 1995 MPEG-1 v MPEG-2 v nm 1999 MPEG4 c cng nhn l chun quc t. Cc c im cn lu :

MPEG khng phi tn hiu m ch l c php nn v gii nn lung bit. MPEG-2 khng nh ngha c nh cc thut tan, m m rng kh nng cho

tng pht kin ring, tng ng dng ring. Chng hn thng s nh gi chuyn dch ca hnh (Motion Estimation), thng s GOP (Group Of Picture: nhm hnh) , t l nn (Rate Control), u c th ty chn.

MPEG-2 m rng kh nng pht trin trong tng lai, tng thch c vi tt MPEG-2 khng i xng, nn MPEG-2 rt phc tp, trong qu trnh nn c

c cc b gii nn.

km sn thng s gii nn cho u thu, i hi tnh tan rt nhiu, gii nn MPEG-2 n gin, cn t tnh tan hn tc r tin hn.


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TNH TON, THIT K TRUYN HNH S V TINH Nhm hnh _ GOP (Group Of Picture):

MPEG t hiu qu nn cao, ch yu nh lai b d tha thi gian tc nn lin hnh (inter frame), bng cch khng truyn lp li phn ging nhau ca hnh sau so vi hnh trc, ni cch khc l ch truyn nhng ch a khc i (cc ch dch chuyn) ca hnh sau so vi hnh trc. C 3 lai hnh c MPEG truyn i: - Hnh I (Intra frame): C sau mt thi gian nht nh, li truyn i mt hnh y gi l hnh I. Ni cch khc, I l hnh c nn ni hnh v truyn i y . - Hnh P (Prediction frame): c suy ra t hnh I trc , tc suy ra t chiu thi gian pha trc. Ni r hn, trong thi gian ca hnh P, ch truyn thng tin khc nhau ca n so vi hnh I trc n m thi. - Hnh B (B- Directional frame): c suy ra t c hnh I (hoc P) trc v sau n, tc l suy ra t c hai chiu thi gian. Ni r hn, trong thi gian ca hnh B, truyn i cc ch khc nhau gia n vi hnh I (hoc P) trc n, hoc hnh P (hay I) sau n, hoc c hai, ty theo kt qu d tm dch chuyn. S hnh c trong khong t mt hnh I n hnh I k tip gi l mt nhm hnh, hay mt GOP. Tnh cht ca GOP k hiu bi NM, N l s hnh tng cng ca nhm, M l s hnh c t mt hnh P n hnh P k tip, MPEG cho php ty chn nhm hnh, GOP nh hay N nh, s hnh I c nhiu, hay s khng nn c nhiu. Nh vy, vi ng truyn c bit rate cho trc khng i, vi s hnh/giy c nh theo chun, GOP nh c ngha l hnh truyn i s km cht lng. Ngc li, GOP ln hay N ln, nn c nhiu hn gip hnh truyn i cht lng hn, nhng li gy kh khn cho cc b dng (phi gii nn ra tng hnh y ri mi ni, chn vo cc ch cn ni), M ln cn i hi mch MPEG tnh tan nhiu hn v lm chm (delay) video nhiu hn. 1.2.2.4. iu ch tn hiu s: Dng tn hiu s video/audio sau b ghp knh ch c 2 trng thi gi tr l 0 v 1.Tn hiu s s i qua khi iu ch trc khi pht sng. Khi tn hiu i qua qu trnh iu ch, th mt s trng thi s c cng li, lm tng tc truyn d liu. C 3 lai iu ch s chnh dng cho truyn hnh l: QAM (Quadrature Amplitude Modulation), QPSK (Quadrature Phase Shift Keying), COFDM (Coded Orthogonal Frequency Division Multiplexing). Trong truyn hnh cp s ta s dng hai dng iu ch l QAM v QPSK. 1.2.2.4.1. iu ch QAM: SVTH : Nguyn Ngc Huy 43 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH K thut iu ch QAM cho php truyn tn hiu s vi tc cao trong mt bng tn hp, n c th t tc truyn n 40Mbit/s v c tnh min nhiu tt i vi cc knh khc nhau cng truyn chung trn mt ng truyn. Thng thng ta c cc Mode iu ch: 16, 32, 64, 128 hoc 256 - QAM, c biu din trong s chm sao (s chm sao tng trng cho tn hiu truyn trong h thng cp). Mode iu ch thp c tnh min nhiu rt cao (thng thng dng mode 16-QAM). Ngc li, mode iu ch cao (mode 256-QAM) rt d b nhiu nn vic gii m tn hiu bn thu khng n nh. S chm sao ca mode iu ch 16-QAM c biu din trong hnh sau. Cc im trn chm sao thuc gc phn t th nht s c chuyn sang gc phn t th hai, th ba, th t bng cch thay i 2 bit c trng s cao nht MSB (c ngha l Ik v Qk) v quay bit c trng s thp nht LSB theo quy tc cho bi bng sau: Gc phn t 1 2 3 4 Q Qk Ik = 10 1011 1001 1010 1000 1101 1100 1111 1110 QkIk= 11 0010 0011 0000 0001 I 0100 0110 0101 0111 QkIk= 01 Hnh 1.22. S chm sao ca 16-QAM Vic xc nh mode QAM no c s dng c cn c vo tnh hnh c th v ty mi quc gia. V d ni cn truyn nhiu chng trnh th dng QAM mode cao (64QAM, 256-QAM), nhng lc i hi thit b phi c cht lng cao v di ng truyn thch hp. Ni khng cn truyn nhiu chng trnh th dng QAM mode thp c tin cy cao v tit kim gi thnh. Kt lun: SVTH : Nguyn Ngc Huy 44 Lp 46K - TVT QkIk= 00 MSB 00 10 11 01 /2 3/2 Xoay LSB

TNH TON, THIT K TRUYN HNH S V TINH Vic iu ch QAM cho truyn hnh s qua cp c nhiu u im: Vi mode iu ch QAM thp, ng truyn s qua cp c tnh khng nhiu cao. Nhiu ng truyn hu nh khng nh hng n cht lng tn hiu khi gii m (mode iu ch c th dng l 16-QAM). Tn hiu gii m pha thu ch c hai mc: Gii m c (cht lng hnh nh tt) v khng gii m c (hnh nh lc gii m lc c lc khng, dng hnh). Hay ni cch khc: Hoc thu c rt r hoc khng thu c. Vic khuch i tn hiu s iu ch QAM cn c quan tm, v y l tc nhn chnh gy nn li ng truyn, Tn hiu s iu ch QAM sau khi khuch i phi c x l sao cho trit c nhiu xy ra, Phi lai b nhiu sinh ra trong qu trnh khuch i, nu khng my thu s khng gii m c tn hiu. Cc knh s c th ghp k nhau (m khng tc ng ln nhau). i vi knh tng t khi ghp vi knh s iu ch QAM cn ch n chng ph trong sng mang hnh v sng mang ting. Tn hiu iu ch s QAM truyn i trn h thng phn phi b suy hao. Tuy nhin, nu mc khuch i va th tn hiu iu ch QAM vn c gii m tt. C th s dng h thng phn phi cp cho truyn hnh tng t phn phi knh truyn s iu ch QAM. i vi mt h thng truyn hnh cp thc t, mng phn phi thng dng l mng HFC. Cc knh tng t v knh s truyn chung trn mt h thng phn phi, khng xy ra s chng ph ca cc knh k nhau (d l knh tng t hay knh s), cc tn hiu sau khi c khuch i my pht cn ch trng n vic x l nhiu. Sau , c th ghp cc knh tng t v knh s ri truyn i trn cp quang. Khi ghp nn thc hin nh sau: + Nn ghp theo tng nhm cc knh tng t v nhm cc knh s. + Cc knh s ghp bng tng cao, cc knh tng t ghp bng tng thp hn do knh s c tnh khng nhiu tt. 1.2.2.4.2. iu ch Q-PSK: Q-PSK l k thut iu ch s c kh nng chng nhiu in t trng tt hn QAM, thng c s dng trong mi trng v tinh hoc knh phn hi (knh ngc) ca mng truyn hnh cp, Q-PSK lm vic da trn nguyn l dch tn hiu s khng SVTH : Nguyn Ngc Huy 45 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH c pha trong tn hiu ra, Q-PSK lm tng mnh ca mng. Tuy nhin s iu ch Q-PSK c th truyn d liu ti 10Mbit/s. Q-PSK s dng 4 trng thi pha t hiu qu s dng tn s cao hn so vi h thng B-PSK. Tn hiu truyn i c bin i t ni tip sang song song, hai song mang khc pha nhau /2 c iu ch hai tn hiu nh phn trn v chng c kt hp li vi nhau thnh tn hiu ra. Tc tn hiu trong ng truyn dn bng tc chui u vo. Qu trnh gii iu ch Q-PSK c coi l hai qu trnh gii iu ch B-PSK c lp. Hai tn hiu bng gc I(t) v Q(t) l kt qu so pha tn hiu thu c vi hai sng mang chun lch pha vi nhau mt gc /2. Hnh 1.23. a) Cu hnh b iu ch Q-PSK. LPF b) Cu hnh b gii iu ch Q-PSK./2 TS I(t)

c) Biu saoKhi phc tn hiu. BPFng h Khi phc

1.2.2.5. H thng qun l mng:/2 Trung tm pht sng bao gm nhiu thnh phn phc tp, sng mang cung cp nhiu dch v, cc Q(t) vn mng cn c pht trin nhanh chng v gii quyt cc vn iu khin v kim tra 01 11 LPF TS cc dch v cung cp cho thu bao. H thng kim tra v iu khin mng c t ti cc Q(t) I(t ) b) trung tm pht sng, mc ch chnh ca mt h thng nh vy l ti thiu ha s gin an a)

cc dch v i vi thu bao truyn10 s. H thng qun l mng s qun l cc knh truyn hnh 00 hnh cung cp cho thu bao,c) cc trung tm truyn hnh cp y l h thng c vai tr c ti bit quang trng, n cho php hay khng cho php thu bao xem cc kng truyn hnh. c trng ca mt h thng iu khin Headend s tiu biu bao gm: Kim tra kh nng cc thit b. Tp hp thng k. Thng bo v bo ng v cc vn thng k. D bo t xa. Hn nay cc h thng ny chy trn cc h iu hnh WindowsNT hoc UNIX. 1.2.2.6. H thng truy cp c iu kin CA: Ngy nay, cc nh iu hnh, sn xut chng trnh v pht sng truyn hnh c th trc tip tng tc vi ngi xem nhiu mc khc nhau, cung cp mt s lng ln chng trnh la chn. Mc tiu chnh ca h thng truy cp c SVTH : Nguyn Ngc Huy 46 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH iu kin CA l iu khin s truy cp ca thu bao i vi truyn hnh s c tr tin v bo mt cc dng video/audio s i vi nh qun l v iu hnh. Nh vy, ch cc thu bao trn h thng CA mi lin lc c hiu qu vi nh iu hnh mng truy cp vo dch v c th. S dng cc h thng cc nh iu hnh mng c th trc tip lp trnh (target programming), qung co, khuyn khch cho thu bao trong khu vc (th trng) hoc cho c nhn. Do h thng CA l hng quang trng ca truyn hnh s. Truy cp gii hn vo mt dch v c th c thc hin bng cch dng k thut mt m (cryptography). K thut mt m bo v dch v s bng cch bin i tn hiu thnh mt dng khng c c. Qu trnh bin i c gi l kha m (encryption) trong mi trng s v xo trn (Scrambling) trong mi trng tng t. Khi tn hiu c kha m (encrypted) th ch c th gii kha m (decrypted) bng b gii kha m gn vo Set-top-box s. Gii kha m l qu trnh bin i ngc tn hiu (message) v dng gc thng qua kha m (decryption). Kha (key) c hiu nh l mt gi tr mt m (secret), bao gm mt vng bit ngu nhin (s dng my tnh cng cc thut tan kha m v gii kha m thng tin). Set-top-box kt hp phn cng v phn mm cn thit thu nhn v gii m tn hiu. Cc thnh phn ny c thit k gm 1 chip gii kha m v kha m, 1 b x l bo mt (secure processor) v mt vi b iu khin phn cng (hardware driver) thch hp. Chip gii kha m v kha m m bo chn gii thut CA, b x l bo mt c th gn vo mch in trong Set-top-box hoc km theo card gii kha m (smart card-card gii kha m, cn gi l card thng minh). Card thng tin l card plastic, ging nh th tn dng ngn hng (credit card), b x l ny gm cc kha (key) cn cho gii m cc dch v khc nhau. Ngoi ra m ha cc dch v s, h thng truy cp c iu kin CA phi giao din vi cc h thng con SMS, SAS. 1.2.2.6.1. H thng SMS: SMS (Subscriber Management System) l h thng qun l thu bao, cn lm giao din cc h thng k thut ca cc nh iu hnh truyn hnh, SMS cung cp cc h tr cn thit qun l chng xc m hnh truyn hnh s thng mi. N thc hin c s SVTH : Nguyn Ngc Huy 47 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH d liu ca thu bao v gi cc yu cu ca thu bao cho h thng SAS (Subscruber Authorization System - l h thng qun l k thut ca h thng CA). Chc nng c trng ca SMS c cung cp bng mt h thng ng dng phn mm SMS, bao gm: Ghi dch (register), ci bin v xa cc cuc ghi ca thu bao. Phn u cho cc chin dch th trng. Qun l, kim k cc Set-top-box v card thng minh. Theo di vic s dng ca thu bao. Bn cho (cross-selling) cc dch v. Giao din vi ngn hng v cc cng ty th tnh dng.

Qun l s c. C kh nng dng nhiu lai ngn ng v nhiu lai tin t.

Sa v to dng ha n. Trnh by ha n in t.

Cc kh nng tnh tan. Nhiu phn mm trn th trng c kh nng h tr lm tng dch v tng tc cho thu bao. Mc ch chnh ca SMS l bo m thu bao xem c ng chng trnh m h tr tin.


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TNH TON, THIT K TRUYN HNH S V TINH 1.2.2.6.2. H thng SAS: SAS (Subscriber AUthorization System) l h thng quyn tc gi i vi thu bao, c nhim v truyn cc yu cu n t SMS vo EMM (Entitlement Management Message - bc in qun l quyn truy cp). Tip n, cc bc in quyn tc gi EMM ny c ghp knh s (digital multiplex) n card thng minh SC (smart card) t Set-top-box. Chng c gi n thu bao theo cc khong thi gian u n (v d mi thng) cp nht quyn truy cp cho thu bao trn SC. Trong trng hp dng PPV(pay-per-view), h thng SAS gi mt s du hiu in t nht nh (tokens) cho SC cho php thu bao tr tin cho cc s kin PPV khc nhau, SAS cha c s d liu, c kh nng lu tr cc thng tin sau: Thng tin v sn phm pay-TV. D liu h tr hng dn truyn hnh bng in t (EPG). S nhn dng SC. D liu danh mc (schedule).

Ngoi ra, c th nng cao vic bo mt SAS bng cch thay i theo chu k cc kha quyn tc gi truyn n c s thu bao. 1.2.3. K THUT A TRUY NHP Thy rng truyn dn v tinh s dng m hnh a truy nhp trong mi trng dng chung. C 3 dng chnh ca m hnh a truy nhp c trnh by Hnh 2.8 a truy nhp phn chia theo tn s (FDMA). a truy nhp phn chia theo thi gian (TDMA). a truy nhp phn chia theo m (CDMA).Tn s/bng thng n . . . . 4 3 2 1 FDM A Thi gian Tn s/bng thng 1 2 3 . . . . n M Tn s/bng thng

TDMA

Thi gian

CDMA

Thi gian

Hnh 1.24. K thut a truy nhp FDMA, TDMA v CDMA.


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TNH TON, THIT K TRUYN HNH S V TINH Ghp knh khc vi a truy nhp, n l chc nng tp trung vi vic chia s ti nguyn bng thng t cng mt v tr trong khi a truy nhp chia s cng mt ti nguyn t cc v tr khc nhau xem Hnh 2.9

Hnh 1.25. So snh gia khi nim ghp knh v a truy nhp 1.2.3.1. a truy nhp phn chia theo tn s (FDMA) Trong phng thc a truy nhp phn chia theo tn s (FDMA), bng thng ca

Thi gian knh trm lp c chia thnh cc bng con v c n nh cho tng sng mang pht it trm mt t. i vi kiu truy nhp ny cc trm mt t pht lin tc mt s sng mang cc tn s khc nhau v cc sng mang ny to nn cc knh ring. trnh nhiu gia cc fA fB cnfC ra fD phng thc iu ch, s khng hon thin ca cc knh ln gy do Tn s b dao ng v B pht p m bo khong bo v ga cc bng tn ca cc knh cc b lc, cn Hnh 1.26. FDMA. cnh nhau. Cc s truyn dn Cc s truyn dn khc nhau tng ng vi cc t hp ghp knh v iu ch khc nhau


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Hnh 1.27. Cc cu hnh truyn dn FDMA. a) FDM/FM/FDMA; b) TDM/PSK/FDMA; c) SCPC/FDMA 1.2.3.1.1. FDM/FM/FDMA cu hnh ghp knh theo tn s, iu tn (FM) v a truy nhp phn chia theo tn s (FDM/FM/FDMA trn hnh 1.27a) cc tn hiu bng tn gc ca ngi s dng l tn hiu tng t. Chng c kt hp to thnh mt tn hiu ghp knh phn chia theo tn s (FDM). Tn s tn hiu tng t c ghp knh ni trn s iu ch tn s (FM) cho mt sng mang, sng mang ny s truy nhp n v tinh mt tn s nht nh ng thi cng vi cc tn s khc t cc trm khc. gim thiu iu ch giao thoa, s lng ca cc sng mang nh tuyn lu lng c thc hin theo nguyn l mt sng mang trn mt trm pht. Nh vy tn hiu ghp knh FDM bao gm tt c cc tn s dnh cho cc trm khc. 1.2.3.1.2. TDM/PSK/FDMA cu hnh ghp knh theo thi gian, iu ch kho chuyn pha (PSK) v a truy nhp phn chia theo tn s (TDM/PSK/FDMA hnh 1.27b) tn hiu bng gc ca ngi s dng l tn hiu s. Chng c kt hp to ra mt tn hiu ghp knh phn chia theo thi gian (TDM). Lung bit th hin tn hiu c ghp ny iu ch mt sng mang theo phng php iu ch pha PSK, tn hiu ny truy nhp n v tinh mt tn s nht SVTH : Nguyn Ngc Huy 51 Lp 46K - TVT


A D C B A Thi gian 1 khung Thi gian

Hnh 1.28. a truy nhp phn chia theo thi giannh ng thi cng vi cc sng mang t cc trm khc cc tn s khc. gim ti thiu cc sn phm ca iu ch giao thoa s lng cc tn s mang nh tuyn lu lng c thc hin theo phng php mt sng mang trn mt trm pht. Nh vy tn hiu ghp knh TDM bao gm tt c cc tn hiu ph thuc thi gian cho cc trm khc. 1.2.3.1.3. SCPC/FDMA cu hnh mt knh trn mt sng mang (SCPC: Single Channel per Carrier) v a truy nhp phn chia theo tn s (SCPC/FDMA hnh 1.27c) tng tn tn hiu bng gc ca ngi s s iu ch trc tip mt sng mang dng s (PSK) hoc tng t (FM) tu theo tn hiu c s dng. Mi sng mang truy nhp n v tinh tn s ring ca mnh ng thi vi cc sng mang t cng trm ny hay t cc trm khc cc tn s SVTH : Nguyn Ngc Huy Lp 46K - TVT

52

TNH TON, THIT K TRUYN HNH S V TINH khc. Nh vy nh tuyn c thc hin trn nguyn l mt sng mang trn mt ng truyn. 1.2.3.2. a truy nhp phn chia theo thi gian (TDMA) TDMA l phng php a truy nhp trong cc trm mt t dng chung mt b pht p trn c s phn chia theo thi gian nh hnh 1.28. Trong trc honh ch tn s, trc tung ch thi gian. Trc thi gian c phn chia thnh cc khong thi gian gi l cc khung TDMA, mi khung TDMA c phn chia thnh cc khe thi gian, cc khe thi gian ny c n nh cho mi trm mt t. Tt c cc trm mt t u dng chung mt sng mang c tn s trung tm l f 0 v ch pht v thu tn hiu trong cc khe thi gian c n nh. V th, trong mt khong thi gian nht nh, ch c tn hiu t mt trm mt t chim ton b bng tn ca b pht p v tinh v khng bao gi xy ra trng hp tn hiu t hai trm mt t tr ln chim b pht p ca v tinh trong cng mt thi gian. di ca khe thi gian n nh cho mi trm mt t tu thuc vo lu lng ca trm. TDMA s dng cc sng mang iu ch s v cc sng mang c pht i t trm mt t cn phi c iu khin chnh xc sao cho chng nm trong khe thi gian c phn phi. lm c iu ny, cn phi c mt tn hiu chun pht i t mt trm chun v cc trm khc ln lt truyn tn hiu ngay sau tn hiu chun. Trong phng php a truy nhp ny, cc trm mt t phi truyn tn hiu mt cch gin on v cn phi d phng khong thi gian bo v gia cc sng mang cc tn hiu t cc trm mt t khng chng ln ln nhau khi n b pht p. u im ca phng php ny l c th s dng tt cng sut ti a ca v tinh v c th thay i d dng dung lng truyn ti bng cch thay i khong thi gian pht v thu, do n linh hot trong vic thay i, thit lp tuyn, c bit l hiu sut s dng tuyn rt cao khi s knh lin lc tng. Mt khc, TDMA khi kt hp vi k thut ni suy ting ni th c th tng dung lng truyn dn ln ba n bn ln. Tuy nhin, TDMA c mt s nhc im nh sau: Yu cu phi c ng b cm.

Mng TDMA cha cc trm lu lng v t nht mt trm chun. Cc cm c pht i t cc trm lu lng c gi l cc cm lu lng. S liu lu lng c pht bng cc cm lu lng. Trm chun pht mt cm c bit theo chu k gi l cm SVTH : Nguyn Ngc Huy 53 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH chun. Cm chun cung cp chun nh thi v chu k ca n ng bng mt khung TDMA. Mi trm lu lng pht cc cm lu lng trong cc khe thi gian c n nh v tinh bng cch iu khin nh thi pht cm theo cm chun, cm chun c s dng lm chun nh thi, cm chun v cc cm lu lng c t theo th t ng trnh chng ln trong mi khung TDMA. Nu khng c ng b cm th cc cm c pht c th trt khi cc khe thi gian c n nh v tinh. Nu xy ra chng ln cc cm v tinh th thng tin s b mt. c bit, hin tng thu triu l nguyn nhn lm cho cao ca v tinh bin thin khong 85Km dn n kt qu l quay 1 vng tr thay i khong 500s v s thay i tn s ca tn hiu c bit nh hiu ng Doppler. Tn hiu tng t phi c chuyn sang dng s khi s dng k thut TDMA.

Giao din vi cc h thng mt t tng t rt phc tp dn n gi thnh ca h thng cao. 1.2.3.3. a truy nhp phn chia theo m (CDMA) CDMA (Code Division Multiplex Access) l phng php truy nhp ng dng k thut tri ph, trong mi i tng c th : c php hot ng ng thi. Hot ng ti tn s nh nhau. S dng ton b bng tn ca h thng cng mt lc m khng gy nhiu sang

thng tin ca i tng khc. a truy nhp phn chia theo m CDMA l phng php a truy nhp m cc trm mt t c th pht tn hiu mt cch lin tc v ng thi, v s dng cng mt bng tn ca knh. Trong CDMA, mi sng mang pht c iu ch bng mt m c bit qui nh cho mi trm mt t v trm mt t thu c th tch c tn hiu cn thu khi cc tn hiu khc nh m c bit . Tp hp cc m cn dng phi c cc thuc tnh tng quan sau y: Mi m phi c th c phn bit mt cch d dng vi bn sao ca chnh n b

dch chuyn theo thi gian. Mi m phi c th c phn bit mt cch d dng bt chp cc m khc c

s dng trn mng.


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TNH TON, THIT K TRUYN HNH S V TINH Vic truyn dn tn hiu hu ch kt hp vi m i hi mt bng thng ln hn nhiu so vi bng thng yu cu truyn dn ch ring thng tin hu ch. l l do v sao ngi ta gi l truyn dn tri ph. c im ca CDMA Hot ng n gin, do n khng i hi bt k s ng b truyn dn no gia

cc trm. ng b duy nht l ng b ca my thu vi chui sng mang thu c. Nh vic tri ph pha pht v thu hp ph pha thu nn n c kh nng

chng li can nhiu gia cc h thng v nhiu do hin tng a ng truyn rt tt, ng thi c tnh bo mt ca tn hiu cao. Bn cnh cc u im nh trn, CDMA vn tn ti nhc im nh hiu qu s dng bng tn km, rng bng tn truyn dn yu cu ln. Tuy vy CDMA rt ph hp i vi cc mng c cc trm nh vi rng chm tia anten ln v i vi truyn thng v tinh vi cc my di ng. 1.2.3.4. Phng php a truy nhp phn phi trc v a truy nhp phn phi theo yu cu. 1.2.3.4.1. a truy nhp phn phi trc. a truy nhp phn phi trc l mt phng php a truy nhp m trong cc knh v tinh c phn b c nh cho tt c cc trm mt t khc nhau, bt chp c hay khng c cc cuc gi pht i. 1.2.3.4.2. a truy nhp phn phi theo yu cu. a truy nhp phn phi theo yu cu l phng php a truy nhp m trong cc knh v tinh c sp xp li mi khi c yu cu thit lp knh t cc trm mt t c lin quan. a truy nhp phn phi theo yu cu cho php s dng c hiu qu dung lng knh ca v tinh c bit khi mt s trm mt t c dung lng nh s dng chung mt b pht p nh trong trng hp h thng in thoi v tinh trn bin. 1.2.4. H THNG TV GIA NH, TVRO 1.2.4.1. S khi tng qut ca TVRO Theo quy nh truyn hnh qung b trc tip n my thu TV gia nh c thc hin trong bng tn Ku (12 GHz). Dch v ny c gi l dch v v tinh qung b trc


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TNH TON, THIT K TRUYN HNH S V TINH tip (DBS: direct broadcast satellite). Tu thuc vo vng a l n nh bng tn c th hi thay i. M, bng tn ng xung l 12,2 n 12,7GHz. Tuy nhin, hin ny nhiu gia nh s dng cc cho kh to (ng knh khong 3m) thu cc tn hiu TV ng xung trong bng C (GHz). Cc tn hiu ng xung ny khng ch nh thu gia nh m dnh cho vic chuyn i mng n cc mng phn phi truyn hnh (cc i pht VHF, UHF v cp truyn hnh). Mc d c v nh thc t thu cc tn hiu TV hin nay c thit lp rt tt, nhng nhiu nhn t k thut, thng mi v php lt ngn cn vic thu ny. Cc khc bit chnh gia cc h thng TVRO (TV recieve only: ch thu TV) bng Ku v bng C l tn s cng tc ca khi ngoi tri v cc v tinh dnh cho DBS bng Ku c EIRP (cng sut pht x ng hng tng ng) cao hn nhiu so vi bng C.12 .2 12 .7 G z H

950 1480 M z H

B khuch i tp mnh /b bin i

Loa thu v b lc phn cc

B phn x Parabol

Khi ngoi triB lc b v m b l h tn c K i tru g h n tn 7 M z 0 H G ch ii iu F M

M y thu v tinh

D n ao g iu ch ch h n c

C k hv h n n h th in

Ti ch iu A M

Hnh 1.29. S khi u cui thu thu DBS TV/FM gia nh Hnh 1.29 cho thy cc khi chnh trong mt h thng thu DBS ca u cui gia nh. Tt nhin cu trc ny s thay i trong cc h thng khc nhau, nhng s ny s cung cp cc khi nim c s v my thu TV tng t (FM). Hin nay TV s trc tip n gia nh ang dn thay th cc h thng tng t, nhng cc khi ngoi tri vn ging nhau cho c hai h thng. 1.2.4.2. Khi ngoi tri SVTH : Nguyn Ngc Huy 56 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH Khi ny bao gm mt anten thu tip sng trc tip cho t hp khuch i tp m nh/ bin i h tn. Thng thng b phn x parabol c s dng vi loa thu t tiu im. Bnh thng thit k c tiu im t ngay trc b phn x, nhng trong mt s trng hp loi b nhiu tt hn, b tip sng (Feed) c th c t lch nh thy trn hnh v. Kinh nghim cho thy rng c th thu cht lng m bo bng cc b phn x c ng knh t 0,6 n 1,6m (1,97-5,25 ft) v kch thc ch dn thng thng l 0,9m (2,95ft) v 1,2m (3,94 ft). Tri li ng knh b phn x bng C (4GHz) thng vo khong 3m (9,84 ft). Lu rng h s khuch i anten t l thun vi (D/) 2. So snh khuch i ca cho 3m ti 4GHz vi cho 1m ti 12 GHz, ta thy trong c hai trng hp t s D/=40, v th khuch i ca chng bng nhau. Tuy nhin mc d suy hao truyn sng ti 12 GHz cao hn nhiu so vi 4GHz, nhng ta khng cn anten thu c khuch i cao hn v cc v tinh qung b trc tip lm vic cng sut pht x ng hng tng ng cao hn nhiu. Bng tn ng xung di 12,2 n 12,7 GHz c rng 500 MHz cho php 32 knh TV vi mi knh c rng l 24 MHz. Tt nhin cc knh cnh nhau s phn no chng ln ln nhau, nhng cc knh ny c phn cc LHC v RHC an xen gim nhiu n cc mc cho php. S phn b tn s nh vy c gi l an xen phn cc. Loa thu c th c b lc phn cc c chuyn mch n phn cc mong mun di s iu khin ca khi trong nh. Loa thu tip sng cho khi bin i tp m nh (LNC: low noise converter) hay khi kt hp khuch i tp m nh (LNA: low noise amplifier) v bin i (gi chung l LNA/C). Khi kt hp ny c gi l LNB (Low Noise Block: khi tp m nh). LNB m bo khuch i tn hiu bng 12 GHz v bin i n vo di tn s thp hn c th s dng cp ng trc gi r ni n khi trong nh. Di tn tn hiu sau h tn l 950-1450 MHz (xem hnh 1.29). Cp ng trc hoc cp i dy c s dng truyn cng sut mt chiu cho khi ngoi tri. Ngoi ra cng c cc dy iu khin chuyn mch phn cc. Khuch i tp m nh cn c thc hin trc u vo khi trong nh m bo t s tn hiu trn tp m yu cu. t khi b khuch i tp m nh c t ti pha u vo khi trong nh v n c th khuch i c tp m ca cp ng trc. Tt nhin khi SVTH : Nguyn Ngc Huy Lp 46K - TVT

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TNH TON, THIT K TRUYN HNH S V TINH s dng LNA ngoi tri cn m bo n hot ng c trong iu kin thi tit thay i v c th b ph hoi hoc nh cp. 1.2.4.3. Khi trong nh cho TV tng t (FM) Tn hiu cp cho khi trong nh thng c bng tn rng t 950 n 1450 MHz. Trc ht n c khuch i ri chuyn n b lc bm chn knh cn thit (xem hnh 1.29). Nh ni, an xen phn cc c s dng v th khi thit lp mt b lc phn cc ta ch c th thu c mt na s knh 32 MHz. iu ny gim nh hot ng ca b lc bm v by gi cc knh an xen c t cch xa nhau hn. Sau knh c chn c bin i h tn: Thng t di 950 MHz vo 70 MHz, tuy nhin cng c th chn cc tn s khc trong di VHF. B khuch i 70 MHz khuch i tn hiu n mc cn thit cho gii iu ch. S khc bit chnh gia DBS v TV thng thng ch DBS s dng iu tn cn TV thng thng s dng iu bin (AM) dng n bin c nn (VSSB: Vestigal Single Sideband). V th cn gii iu ch sng mang 70 MHz v sau ti iu ch AM to ra tn hiu VSSB trc khi tip sng cho cc knh VHF/UHF ca my TV tiu chun. My thu DBS cn cung cp nhiu chc nng khng c th hin trn hnh 1.29. Chng hn cc tn hiu Video v Audio sau gii iu ch u ra V/A c th cung cp trc tip cho cc u V/A ca my thu hnh. Ngoi ra gim nhiu ngi ta cn b sung vo sng mang v tinh mt dng sng phn tn nng lng v my thu DBS c nhim v loi b tn hiu ny. Cc u cui cng c th c trang b cc b lc IF gim nhiu t cc mng TV mt t v c th phi s dng b gii ngu nhin ho (gii m) thu mt s chng trnh. 1.2.4.4. H thng TV anten ch H thng TV anten ch (MATV: Master- Antena TV) m bo thu cc knh DBS/TV cho mt nhm ngi s dng, chng hn cho cc ngi thu cn h trong to nh. H thng ny gm mt khi ngoi tri (anten v LNA/C) tip sng cho nhiu khi trong nh (xem hnh 1.30). H thng ny cn bn ging nh h thng gia nh trnh by trn nhng cho php tng ngi s dng truy nhp c lp n tt c cc knh. u im ca h thng ny l ch cn mt khi ngoi tri, nhng phi c cc LNA/C v cp tip sng ring cho tng phn cc. So vi h thng mt ngi s dng, cn c anten ln


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TNH TON, THIT K TRUYN HNH S V TINH hn (ng knh 2 n 3 m) m bo t s tn hiu trn tp m cho tt c cc khi trong nh.Nhm knh phn cc LHC Diplexer phn cc Nhm knh phn cc RHC B chia cng sut LNA /C LNA /C Khi ngoi tri Khi thu trong nh B chn nhm knh My thu 1 My thu 2 My thu 3 B iu ch 1 B iu ch 2 B iu ch 3 B phn x Parabol

Hnh 1.30. Cu trc h thng anten TV ch (MATV) 1.2.4.5. H thng TV anten tp th

950 1450 MHz t khi ngoi tri

My thu bng rng

My thu bng rng

1

3

5

Cc b lc knh Cc b gii iu ch B kt hp Cp phn phi

2

4

6

Hnh 1.31. Cu trc khi trong nh cho h thng TV anten tp th (CATV) SVTH : Nguyn Ngc Huy 59 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH H thng TV anten tp th (CATV: Community Atenna TV) s dng mt khi ngoi tri vi cc tip sng ring cho tng phng phn cc ging nh h thng MTAV c th cung cp tt c cc knh ng thi ti my thu trong nh. Thay v s dng mt my thu ring cho tng ngi s dng, tt c cc sng mang u c gii iu ch ti mt h thng lc - thu chung nh hnh 1.31. Sau tt c cc knh c kt hp vo mt tn hiu ghp chung truyn dn theo cp n cc thu bao. i vi cc vng xa, thay v dng cp phn phi, ngi ta c th pht li qung b tn hiu bng mt i pht TV xa vi s dung anten ng knh 8m (26,2 ft) thu tn hiu v tinh trong bng C. Cng c th phn phi chng trnh thu t v tinh bng h thng CATV. 1.2.5. CU HNH CA TRM MT T.B i u ch B b i n i n n g l n g

C huyn m ch

HPA

C huyn mch

A n te n

C huyn m ch P annen kt ni bng gc

C hui d phng B g i i B b i n i u c h i h t n

B c h ia

C huyn mch LNA

C huyn m ch

Hnh 1.32. Cc phn t cn bn ca mt trm mt t c d phng Trong cc phn trc ta xt cc trm TV ch thu. Tt nhin, mt ni no ta cn c mt tram pht hon thin ng truyn. Trong mt s trng hp ch cn trm ch pht, chng hn khi chuyn tip tn hiu truyn hnh n cc trm ch thu TV xa. Cc trm pht thu m bo c hai chc nng v thng c s dng cho vin thng vi


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Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH lu lng bao hm c mng TV. Cc phn t c bn ca mt trm mt t c d phng c cho trn hnh 1.32. Nhc li rng d phng c ngha mt s khi c nhn i. Mt khi c d phng kp ny khi b s c s t ng chuyn mch n khi d phng. Cc khi d phng c v trn hnh 1.31 dng ng ngt qung.. S khi chi tit ca trm pht thu mt t c cho hnh 1.32. Trong d nhn ta khng trnh by cc khi d phng.Tip sng antenDiplexer

Khuch i cng sut B kt hp Cc b bin i nng tn Cc b iu ch Cc sng mang viba s IF (trung tn) Cc bng gc c lp khun li

Khuch i LNA B chia Cc b bin i h tn Cc b gii iu ch

Cc b ghp/phn knh bao gm c x l tnh hiu s

Thit b dng kt ni ti trm mt t

Cc u vo/ra bng gc ca trm mt t

Hnh 1.33. S chi tit ca mt trm pht thu Nhn t pha di s , trc ht ta thy thit b kt ni trm v tinh mt t vi mng vin thng mt t. gii thch ta s xt lu lng in thoi. Lu lng ny c th gm nhiu knh in thoi c ghp vi nhau theo tn s, hoc thi gian. Ghp knh ny c th khc vi ghp knh cn thit truyn dn v tinh, v th khi tip theo l thit b ghp knh thc hin lp khun dng li cho lu lng. Sau lung ghp c iu ch trung tn (IF), thng l 70 MHz. Nhiu tng trung tn song song c s dng cho tng sng mang c pht. Sau khuch i IF 70 MHz, tn hiu sau iu ch c bin i nng tn n tn s sng mang cn thit. Nhiu sng mang c th c pht cng mt lc v mc d y l cc tn s khc nhau, cc sng mang c c t theo tn s: Cc sng mang 6GHz hay cc sng mang SVTH : Nguyn Ngc Huy 61 Lp 46K - TVT

TNH TON, THIT K TRUYN HNH S V TINH 14GHz. Cn lu rng mi sng mang c th c s dng cho nhiu im nhn. Ngha l chng mang lu lng n cc trm khc nhau. Chng hn mt sng mang vi ba c th mang lu lng n Boston v New York. Cng mt sng mang c thu ti hai im, c lc ra bi cc b lc ti trm mt t thu. Sau khi i qua b bin i nng tn, cc sng mang c kt hp v tn hiu tng bng rng c khuch i. Tn hiu bng rng sau khuch i c tip sng n anten qua b ghp song cng: Diplexer. Diplexer cho php anten x l ng thi nhiu tn hiu pht v thu. Anten trm lm vic c hai ch pht thu ng thi nhng ti cc tn s khc nhau. Trong bng C, ng ln c tn s pht l 6GHz v ng xung hay tn s thu l 4GHz. Trong bng Ku, tn s ng ln l 14 GHz v ng xung l 12 GHz. Do cc anten khuch i cao c s dng cho c hai ng, nn chng c cc bp sng rt hp. Bp sng hp ny cn thit ngn chn nhiu gia cc ng v tinh ln cn. Trong trng hp bng C, cng cn trnh nhiu n t cc tuyn vi ba mt t. Cc tuyn vi ba mt t khng hot ng ti cc tn s bng Ku. Trong nhnh thu (pha phi ca hnh 1.33), tn hiu thu c khuch i trong b khuch i tp m nh sau c chuyn n b chia tch thnh cc sng mang khc nhau. Cc sng mang ny c bin i h tn n bng IF ri c chuyn n khi ghp knh c chnh li khun dng cn thit cho mng mt t. Cn lu rng dng lu lng pha thu khc vi dng ny pha pht. S lng sng mang, khi lng lu lng c mang s khc nhau v lung ghp u ra khng nht thit phi mang cc knh in thoi c mang pha pht. Tn ti nhiu loi trm mt t khc nhau ph thuc vo cc yu cu dch v. Theo ngha rng c th phn loi lu lng thnh: Tuyn lu lng cao, tuyn lu lng trung bnh v tuyn lu lng thp. Trong knh tuyn lu lng thp, mt knh pht p (36 MHz) c th mang nhiu sng mang v mi sng mang lin kt vi mt knh thoi ring. Ch hot ng ny c gi l mt sng mang trn mt knh (SCPC: Single Carrier per Channel). Ngoi ra cn c ch a truy nhp. C th v cc ch ny s c xt chng cc h thng thng tin v tinh FDMA v TDMA. Kch thc anten thay i t 3,6 m (11,8ft) i vi cc trm di ng trn xe n 30 m (98,4ft) i vi u cui chnh.


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TNH TON, THIT K TRUYN HNH S V TINH Knh tuyn lu lng trung bnh cng m bo a truy nhp hoc theo FDMA hoc theo TDMA. Cc ch a truy nhp ny cng c xt trong chng tng ng. Kch thc anten t 30 m (89,4ft) cho trm chnh n 10 m (32,8 ft) cho cc trm xa. Trong h thng tuyn lu lng cao, mi knh v tinh ( rng bng tn 36 MHz) c th mang 960 knh thoi cho mt ng hoc mt knh TV kt hp vi knh ting. Nh vy knh pht p cho knh tuyn lu lng ln mang mt tn hiu bng rng: C th l TV hay lung ghp cc knh thoi. ng knh anten ca h thng ny t nht l 30 m (98,4ft) c thit k cho trm mt t tiu chun A ca INTELSAT. Cc anten ln ny c trng lng n 250 tn v th phi c nn rt chc chn v n nh. Cc anten ng knh ln ny m bo cc bp sng rt hp v v th phi trnh x dch khng lm lch hng anten. i vi vng c bng v tuyt ri cn c l si bn trong. Mc d cc anten ny c s dng cho cc v tinh a tnh, nhng vn xy ra tri v tinh. nh hng ny cng vi bp sng anten rt hp v th cn m bo mt gii hn nht nh v bm. iu chnh tng nc theo phng v v gc ngng c thc hin di s iu khin ca my tnh t c tn hiu thu cc i. Vic m bo lin tc ngun nui cng l mt vn quan trng khi thit k cc trm mt t pht thu. Tr cc trm nh nht, cn th s dng ngun d phng t in mng hoc acquy v cc my pht in. Nu in li b s c, cc acquy lp tc thay th. KT LUN Trm mt t v tinh bao gm phn pht v phn thu. My thu truyn hnh v tinh TVRO l trm mt t n gin nht. N ch c phn thu. Theo quy nh cc my thu gia nh ch lm vic ti bng Ku. Tuy nhin hin ny nhiu gia nh c th s dng cc cho kh to (ng knh khong 3m) thu cc tn hiu TV ng xung trong bng C (GHz) dng cho chuyn i mng n cc mng phn phi truyn hnh (cc i pht VHF, UHF v cp truyn hnh). Cc ta nh ln c th s dng h thng TV anten ch (MATV: Master- Antena TV) hoc h thng TV nten tp th (CATV: Community Atenna TV) cung cp chng trnh v tinh ng thi cho nhiu ngi s dng. Trm mt t thu pht thng l cc trm u cui s dng cho cc mng thng tin. Trm mt t ca h thng thng tin v tinh t hoc c th l cc trm di ng trn tu b, thng mi, qun s v hng khng. y l cc trm rt phc tp i hi cng sut pht ln v anten ln c th pht n v tinh. Phn pht ca cc trm ny bao gm SVTH : Nguyn Ngc Huy Lp 46K - TVT

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TNH TON, THIT K TRUYN HNH S V TINH phn giao tip vi cc h tng thng tin mt t, phn chuyn i khun dng tn hiu cho ph hp knh v tinh v phn ghp knh, phn iu ch v bin i nng tn, phn kt hp knh v tuyn v anten pht. Phn thu bao gm cc phn t ngc li vi phn pht: Anten thu, chia knh v tuyn, bin i h tn, gii iu ch, phn knh v giao tip vi h tng vin thng mt t. 1.2.6. TRUYN TN HIU TRUYN HNH QUA V TINH Tn hiu truyn hnh c th truyn qua v tinh bng 2 phng php: Tng t, nh iu tn (FM). S, nh iu ch PSK (phase shift keng - dch pha theo kha).

Nu xt trn quan im cng sut pht, th h thng s u vit hn h thng tng t khi tc bit 50MB/s. Theo quy nh quc t WARC (Worl Administrative Radio Conference), th rng knh v tinh truyn tn hiu truyn hnh bng Ku (12GHz) l 27MHz. rng bng tn 27MHz cho php truyn tn hiu s c tc khong 36MB/s. truyn tn hiu truyn hnh s cn s dng phng php m tit kim. C th dng hai knh ng thi truyn cc thnh phn tn hiu hnh, do tc bit ton b tia tn hiu c th nng gp 2 ln. Cc h thng truyn qua v tinh thng cng tc di tn centimetre (cm), tn s c GHz, v d bng Ku: Pht t mt t ln v tinh: 1414,5 GHz, pht t v tinh xung mt t: 11,712,5GHz. Bin i tn hiu t bng tn c bn ln bng tn knh truyn (cao tn) thng c thc hin qua mt vi ln iu ch. u tin t

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