Chng 1

Chng 1TNG QUAN K THUT THNG TIN V TINH1.Gii thiu chungThng tin v tuyn (radio communication) bng v tinh ra i nhm mc ch ci thin cc nhc im ca mng v tuyn mt t, n c dung lng cao hn, bng tn rng hn, em li cho khch hng nhiu dch v mi v thun tin vi chi ph thp hn.Trong chin tranh th gii th 2 to ra cc loi v kh, kh ti ngy cng hin i, cc nc tham chin buc phi thc y vic nghin cu hai k thut mi l tn la v truyn dn viba. Hai k thut ny lc ny ch l nhng k thut ring r. V sau ngi ta mi tm cch kt hp hai k thut ny li vi nhau v t c nhiu thnh cng ng k, t to iu kin cho thng tin v tinh ra i ca thng tin v tinh. Hin nay nhng dch v m h thng thng tin v tinh em li tr nn rt a dng v cc u im ca n so vi cc mng v tuyn mt t hai cc mng cp l khng th ph nhn.Nm 1957, nc Nga phng thnh cng v tinh nhn to u tin ca loi ngi vo qu o (v tinh SPUTNIK). Cc nm sau l nhng cuc th nghim din ra lin tc nh qung b li chc ging sinh ca tng thng Eissenhower qua v tinh SCORE nm 1985, phng thnh cng v tinh ECHO nm 1960, truyn tin qua v tinh COURIER nm 1960, cc v tinh di rng THELSTAR v RELAY nm 1962 v v tinh a tnh u tin SYNCOM nm 1963.n nm 1965 v tinh a tnh thng mi u tin (INTELSAT 1) hay cn gi l Early Bird c a ln qu o, nh du thi im chnh thc thng tin v tinh c th phc v ngi dng. Cng nm v tinh vin thng u tin ca nc Nga nm trong h MOLNYA cng c phng.Nhng h thng v tinh u tin ch c kh nng cung cp mt dung lng thp vi gi thu bao tng i cao, v d v tinh INTELSAT 1 ch c 480 knh thoi vi gi thu bao 32500 $ mt knh mt nm. Gi thnh qu cao ny l do thi im by gi kh nng tn la y cn thp nn ngi ta khng th a c mt v tinh qu nng c dung lng ln ln qu o. Dn dn vi s tin b ca khoa hc k thut, cc tn la y ln rt mnh. Thm na, ngi ta cng t c nhng tin b trong vic ch to cc anten a bp sng (Multi beam) ph hp vi hnh dng ca vng phc v cng vi cc k thut s dng li tn s v cng ngh bn dn cho php cc v tinh c mc pht tn hiu mnh hn v tit kim di tn hn. S pht trin ny lm cho chng ta c th phng c nhng v tinh nng vi dung lng ln v gi thu bao gim (v d v tinh ITELSAT 6) mang 80.000 knh thoi vi gi thu bao ch c 380 $ mt knh mt nm. Hin nay ngi ta khai thc n cc th h v tinh ITELSAT 7 v 8.*c im ca thng tin v tinh Ni ti mt h thng thng tin v tinh chng ta phi k n ba u im ni bt ca n m cc mng mt t khng c hoc khng hiu qu bng n: Kh nng qung b rng ln. C gii thng rng. Nhanh chng v d dng t li cu hnh khi cn thit (v d khi b sung trm mi hoc thay i thng s trm c)Ta bit i vi mng thng tin v tuyn mt t hai trm mun thng tin cho nhau th cc anten phi nhn thy nhau. gi l thng tin v tuyn c tm nhn thng (Line Of Sight LOS). Tuy nhin do tri t c hnh dng cu cho nn khong cch gia hai trm s b hn ch m bo cho cc anten trng thy nhau. i vi kh nng qung b cng nh vy, cc khu vc trn mt t khng cn nhn thy anten ca i pht s khng th thu c tn hiu. Trong trng hp bt buc phi truyn tin i xa ngi ta, c th dng phng php nng cao ct anten, truyn sng phn x tng in ly hoc xy dng cc trm chuyn tip. Trn thc t ngi ta thy rng c ba phng php u c nhiu nhc im. Vic nng cao ca ct anten gp rt nhiu kh khn v kinh ph v k thut m hiu qu khng c l bao nhiu (v d nu ct anten cao1km th cng khng th qung b qu 200km trn mt t). Nu truyn sng phn x tng in ly th cn c cng sut pht rt ln v b nh hng rt mnh ca mi trng truyn dn nn cht lng tuyn khng cao. Cn cc trm chuyn tip gia hai trm u cui s c ci thin c cht lng tuyn, nng cao tin cy, nhng chi ph lp t cc trm trung chuyn v rt khng thch hp khi c nhu cu m thm tuyn mi. Tm li, c th truyn tin i xa ngi ta mong mun xy dng c cc trm anten rt cao nhng li phi pht n nh v vng chc, s ra i ca v tinh chnh l tho mn nhu cu . Vi v tinh ngi ta ngi ta c th truyn sng i rt xa v d dng thng tin trn ton cu hn bt c mt h thng mng no khc. Thng qua v tinh INTEL SAT, ln u tin trn hai trm i din trn hai b i dng thng tin c cho nhau. Do c kh nng rng ln cho nn v tinh rt ph hp cho cc phng thc truyn tin a im n a im (cho dch v qung b) hay a im n mt im HUB (cho dch v thu thp s liu).Bn cnh kh nng ph sng ln, bng tn rng ca cc h thng v tinh rt thch hp vi cc dch v qung b nh truyn hnh s c phn di cao (High Difinition TV), pht thanh s hay cc dch v ISDN thng qua mng mt t, hoc trc tip n thu bao (Direct to Home DTH) thng qua mng VSAT. Cui cng do s dng phng tin truyn dn qua giao din v tuyn cho nn cc h thng thng tin v tinh l rt l tng cho kh nng cu hnh li nu cn. Cc cng vic trin khai trm mi, loi b trm c hay thay i tuyn u c th thc hin d dng, nhanh chng vi chi ph thc hin ti thiu.Tuy nhin h thng thng tin v tinh cng c nhc im l: Khng c nh . Khong cch truyn dn xa nn xuy hao ln, nh hng ca tp m ln . Gi thnh lp t h thng rt cao, chi ph cho trm mt t cng tng i tn km Tui th thp hn cc h thng mt t, kh bo dng, sa cha v nng cp.Ngi ta mun v tinh c vai tr nh mt ct anten c nh nhng trong thc t v tinh lun chuyn ng tng i vi mt t, d l v tinh a tinh i na th vn c mt s dao ng nh. iu ny bt buc trong h thng phi c cc trm iu khin nm gi v tinh mt v tr ti u cho thng tin. Tuy nhin k c khi c cc trm iu khin th v tinh vn khng hon ton c nh nn cc trm mt t ni chung cn cc h thng bm phc tp v iu ny lm gi thnh ca trm tng vt. Thm na, do cc v tinh bay trn cc qu o cch rt xa mt t cho nn vic truyn sng gia cc trm gp phi suy hao rt ln, b nh hng ca thi tit, i qua nhiu dng mi trng khc nhau. vn m bo c cht lng ca tuyn ngi ta phi s dng nhiu k thut b v chng li phc tp.Nh ta ni trc, chi ph phng v tinh l rt cao nn ni chung cc v tinh ch c kh nng hn ch. B li cc trm mt t phi c kh nng lm vic tng i mnh nn cc thit b phn ln l t tin, nht l chi ph cho mt anten ln (v d mt trm mt t trang b anten khong 30m gi thnh khong 10 triu $).Cc v tinh bay trong khng gian cch xa mt t, nng lng ch yu dng cho cc ng c phn lc iu khin l cc loi nhin liu lng hoc rn c v tinh mng theo trong boong. Lng nhin liu d tr ny khng th qu ln v kh nng ca cc tn la y c gii hn, ng thi n s lm cho kch thc v tinh tng ln ng k do phi tng th tch ca thng cha. Nu nh v tinh dng ht loi nhin liu ny th chng ta khng iu khin v tinh c na tc l khng cn duy tr c s n nh ca tuyn, khi coi nh v tinh hng v nh th ni chung tui th ca v tinh thng thp hn cc thit b thng tin mt t khc. lm cho v tinh hot ng tr li ngi ta cn thu hi v tinh sa cha tip thm nhin liu, sau li phng ln qu o. Vic khi phc cc v tinh ht tui th ny ht sc tn km v phc tp nn trong thc t ngi ta thng dng phng php thay th bng mt v tinh hon ton mi v vt b v tinh c i.Mt h thnh v tinh c th cung cp rt nhiu loi hnh dch v khc nhau v ngy cng c pht trin a dng hn. Tuy nhin nhn chung thng tin v tinh em li ba lp dch v nh sau:a. Trung chuyn cc knh thoi v cc chng trnh truyn hnh. y l s p ng cho cc dch v c bn nht i vi ngi s dng. N thu thp cc lung s liu v phn phi n cc mng mt t vi mt t l hp l. V d cho lp dch v ny l cc h thng INTELAT v EUTELSAT. Cc trm mt t ca chng thng c trang b anten ng knh t 15-30m.b. Cung cp kh nng a dch v, thoi, s liu cho nhng nhm ngi s dng phn tch nhau v mt a l. Cc nhm s chia s mt trm mt t v truy nhp n n thng qua mng. V d cho lp dch v ny l cc h thng v tinh TELECM 1, SBS, EUTELSAT 1, TELE X, v INTELSAT (cho mng IBS). Cc trm mt t y c trng b anten ng knh t 3 10m.b. Kt ni cc thit b u cui c gc m rt nh (VSAT) nhm truyn dn cc lung s liu dung lng thp v qung b cc chng trnh truyn hnh, truyn thanh s. Thng thng ngi ta dng s kt ni trc tip vi trm mt t c trang b anten ng knh t 0,6 1,2m. Cc thu bao di ng cng nm trong lp dch v ny. Tiu biu cho cc dch v ny l cc h thng EQUATORIAN, ITELNET hoc INTELSAT Cc dc v ca VSAT hin rt phong ph m ta c th k n nh cp v t ng qun l th tn dng, thu thp v phn tch s liu, cung cp dc v thoi, mt tha, truyn hnh hi ngh*S pht trin ca k thut thng tin v tinh

Th h v tinh thng mi u tin l INTELSAT 1 ra i vo nm 1965, n u nhng nm 70 cc h thng v tinh c th cung cp cc dch v thoi v truyn hnh gia hai lc a. Mi u v tinh ch p ng c cho cc tuyn c dung lng thp sau nhu cu gia tng tc cng nh s lng thng tin qua v tinh thc y nhanh chng vic hnh thnh cc h thng v tinh a bp sng v k thut dng li tn s cho sng mang. K thut u tin c a dng cho h thng thng tin v tinh l analog s dng cng ngh FDM/FM/FDMA. Sau p ng c nhu cu gia tng thng tin, ngi ta tin n cc phng thc truyn dn tin tin hn l PSK/TDMA v PSK/CDMA. Cc phng thc v sau da trn vic truyn dn s qua v tinh khai thc trit mi u im do k thut s em li. Trong tng lai khi dung lng ca tuyn v tinh cng nh s lng v tinh trn ton cu tng ln cc ln th vic s dng qu nhiu sng mang s lm cho mc can nhiu gia cc h thng th tin vi nhau vt qu mc cho php. gii quyt vn ny, nhng nh ch to bt buc phi ngh n vic p dng cc cng ngh sau. X l ti ch (onboard processing): Gii iu ch tn hiu ngay trn v tinh x l, sau iu ch li ri truyn cc tn hiu x l ny xung cc trm mt t thu. y l trng hp ca cc v tinh ti sinh (regenerative satellite).Chuyn mch trn v tinh hay cn gi l a truy nhp phn chia theo thi gian bng chuyn mch v tinh .- S dng cc tuyn thng tin ton cu (intersatellate Network). - S dng cc bc sng qot hoc nhy bc (Scanning hopping beam)cho ccS dng cc bc sng qot hoc nhy bc (Scanning hopping beam)cho cccell trn mt t . - S dng bng tn cao hn (30/20Ghz v 50/40Ghz ) mc d cc di tn ny khng nm trong di sng radio (300MHZ n 10GHZ) nn sng mang phi chu tc ng ln ca mi trng truyn sng v ma - Qung b trc tip t v tinh ti ngi s dng (direct to home).Khi thit b u cui ca ngi s dng s c kt ni thng trm mt t m khng phi thng qua mng .Hin nay cc nc chu u,M v Nht ang c nhiu chng trnh pht trin thng tin v tinh nhm tng cng kh nng ca v tinh nh dung lng, cng sut ,tui th v phng thc truyn dn. iu ny cho php kch c v gi thnh ca trm mt t ngy cng gim i v tr nn gn gi hn vi ngi s dng. Trong mt s trng hp chng ch n gin l cc trm nhn m n gin nht l trm thu truyn hnh trc tip t v tinh (TVRO).y l mt s tin b rt c ngha cho c hi pht trin ca v tinh trong tng lai.

Hnh 1 th cu trc tng qut ca mt h thng thng tin v tinh trong thc t. N c th chia thnh hai phn chnh l phn khng gian (space segment) v phn mt t(Ground segment)

2.phn khng gian2.1 Cu trc Phn khng gian l khi nim ch mt phn ca h thng bao gm v tinh v tt c cc thit b tr gip cho hot ng ca n nh cc trm iu khin v trung tm gim st v tinh .Ta cc trung tm ny cc hot ng nh bm st,o lng t xa v iu khin (Tracking telemetry and command- TT& C) s c thc hin nhm mc ch gi cho v tinh c nh ,ng thi kim tra c cc thng s hot ng ca v tinh nh nhit anten ngun in acquy, nhin liu

Tuyn m sng radio c pht t cc trm mt t n anten thu ca v tinh c gi l tuyn ln (uplink). Ngc li tuyn m v tinh pht cho cc trm mt t s c gi l tuyn xung (downling ). nh gi cht lng ca tuyn ngi ta hay dng i lng C/N l t s gia cng sut sng mang v cng sut tp m nh hng n sng mang. T s ny c quyt nh trn ton tuyn bi cht lng ca c tuyn ln v tuyn xung , tng ng vi iu kin truyn dn ring mi tuyn (nh mi trng trung gian, kiu iu ch , kiu m ho tnh cht ca thit b thu ) .

V tinh bao gm mt phn ti trng (payload)v mt phn nn (platform).Phn ti trng gm anten v cc thit b in t phc v cho truyn dn . Phn nn bao gm cc thit b bo m cho hot ng ca phn ti trng nh gi , cung cp ngun in, iu khin nhit , iu khin hng v qu o, cc thit b y phn lc, thng cha nhin liu v cc thit b TT& C. Trong qu trnh hot ng v tinh s nh dn do phi tiu tn nhin liu cho vic iu khin . cho v tinh khng b mt trng tm th qu trnh gim trng lng phi lun phn b u trn ton b th tch ca n. Do bao gi cng thit k sao cho cc thng cha nhin liu i sng vi nhau qua trng tm ca v tinh. Thc t nhng thng cha nhin liu nm trong phn nn(platform) chim phn ln khi lng v th tch ca v tinh.

Trong cc h thng hin nay , cc v tinh ch gi vai tr nh mt nt trung chuyn (repeater) hoc mt nt tip xc (relay) .V vy v tinh phi c chc nng khuych i sng mang t tuyn ln sau truyn li tuyn xung. Thng thng i vi v tinh cng xut ti u vo my thu nm trong khong t 10pw n 1nw, tng ng vi cng xut sng mang ti u raca b khuych i pht nm trong khong 10 n 100W. Do h s khuych i cng xut ca v tinh s vo khong 100 n 130dB. Ngoi ra v tinh cn c chc nng iu chnh tn s sng mang tuyn ln cao hn tuyn xung thng qua cc b i tn. Trn v tinh, yu cu ngn cch tn hiu u ra my pht v u vo my thu phi t c150dB. Hin nay v trong tng lai gn cc v sinh v d nh h thng ATC v ITALSAT s c cc chc nng nh gii iu ch, x l tn hiu bng gc v ti iu ch. Nh vy ta thy rng vic bin i tn s ca tuyn xung so vi tuyn ln vn c cc v tinh th h c thc hin bng cc b i tn (frequency concerter ) c th c thay bng cch iu ch mt sng mang mi tuyn xung i vi h thng v tinh ti sinh. tin cy ca phn khng gian l mt yu t quan trng nh gi kh nng hot ng ca c h thng. tin cy ca v tinh ph thuc vo tt c cc thit b ca n. Khi mt v tinh b hng th khng ch c ngha l cc thit b ca n b hng m c th l do v tinh ht tui th. Mt h thng c tin cy cao khi n c cc h thng d phng tt. Trong cc h thng cao cp, c mt v tinh hot ng th c mt v tinh d phng sn xng trn qu o v mt v tinh d phng di mt t (trong kho ).2.2, Cc dng qu o ca v tinh.Qu o l ng bay ca v tinh khi n trng thi cn bng gia hai lc trc i. l lc hp dnca tri t i vi v tinh c hng thng ti tm ca tri t v lc ly tm trc i vi lc hp dn c hnh thnh do cong ca hnh trnh v tinh. Hai lc ny c cng mt gi tr ti v tinh chng kt hp vi nhau gi cho v tinh bay trong mt mt phng theo mt dng ng cong no c th l ng trn hoc elip. C 3 dng qu o thng gp l qu o elip, qu o trn nghing v qu o trn ngang ( qu o xch o ).

a, Qu o elip: Qu o elip nghing mt gc 640 so vi mt phng xch o ca tri t. Loi qu o ny c n nh cao, ng bay ca v tinh nm trong mt mt phng c dng hnh elip vi bn trc ln bng khong cch t tri t n v tinh khi n im cc vin ( apogee ) v bn trc nh khi n im cc cn ( periee ). Vn tc ca v tinh s gim i khi khong cch gia n v tri t tng ln v ngc li.Qu o ny l trng hp cn bng c bit ca v tinh do lc ht khng u ca tri t. Nh qu o nghing ca mnh m v tinh c th ph sng c cc vng v o cao ca tri t khi n n gn im cc vin. Dng qu o ny c dng cho mt s h thng v tinh nh MOLNYA chu k 12h ( khong cch ti vin im 39957 Km v cn im l 548 Km ) v mt s v tinh c chu k 24h. Dng qu o ny c bit thch hp cho cc h thng Mobile dng thng tin v tinh cc ni m hiu ng che lp do chng ngi vt v fading ch nn ng k khi gc ngng ca anten nh hn 300 . Cc qu o elip nghing cho php to ra nhng tuyn v tinh v trung bnh v v cao vi gc ngng anten ca trm mt t gn bng 900 nn suy hao mi trng i vi tuyn ny rt thp. y l u im ni bt m kiu qu o v tinh a tnh khng c ( h thng ELLIPSAT bao gm 24 v tinh trn hai mt phng qu o khc nhau cng nghing 640 dng ph sng lin tc ton nc M cng thuc kiu qu o ny ). b, Qu o trn nghing:Trong trng hp ny v tinh c qu o trn v c cao khng i so vi mc nc bin. Vi kiu qu o ny v tinh bay trong mt mt phng c gc nghing gn 900 theo mt ng trn quanh tri t. cao khong vi trm Km tc l c chu k khong 1 gi ri. Cc v tinh c th bay qua mi vng ca tri t nn kiu qu o ny thng c s dng cho cc v tinh do thm v quan st ( Observation satellite ) v d v tinh SPOT c cao 830 Km qu o nghing l 98,70 chu k 101 pht. Hin nay c rt nhiu h thng v tinh ton cu s dng nhng chm v tinh bay theo kiu qu o ny cao thp khong 1000 Km IRIDIUM, GLOBAL STAR, ODYSSEY, ARIES, LEOSATc,Qu o trn ngang ( qu o xch o ):y l kiu qu o ph bin cho cc v tinh a tnh. Trong trng hp ny v tinh bay trn mt phng xch o c nghing bng 00 theo mt ng trn v 0 xung quanh tri t, tc ca v tinh ng bng chu k ca tri t quay quanh trc ca n. Do trong trng hp ny v tinh xut hin nh mt im c nh trn bu tri khi quan st t mt im trn mt t.Cc v tinh a tnh lun m bo c s chuyn tip thng tin liien tc trong thi gian thc gia trm mt t nm trong vng ph sng ca n. Nh vy trong trng hp ny ch cn 3 v tinh c kinh khc nhau 1200 l ta c th thit lp c mt h thng thng tin ton cu. Nhc im ca dng qu o ny l cc v tinh khng th ph sng c cc khu vc c v cao l hai vng cc ca tri t.*V tinh mc a, b cn c tn gi chung l v tinh khng a tnh, vic la chn qu o ny cn ph thuc vo cc ng dng c th, can nhiu m h thng c th chp nhn c v kh nng tm phng xa ca v tinh.* v tinh c th gi nguyn v tr ca mnh trn qu o, ngi ta thng dng mt trong hai k thut n nh l n nh quay hoc n nh 3 trc. i vi n nh quay th mt tr ngoi ca v tinh c quay to ra tc ng con quay hi chuyn n nh v tinh, cn i vi n nh 3 trc ngi ta x dng cc con quay hi chuyn bn trong v mt s ng c phn lc h tr thch hp.2.3, Vai tr ca trm iu khinTrn l thuyt, cc v tinh chuyn ng vi cc qu o c hnh dng l ng trn hoc ng elip, nhng trong thc t qu o ny khng c ho ton nh l thuyt do v tinh cn phi chu tc ng ca rt nhiu yu t khch quan nh s thay i ngu nhin lc ht ca tri t, lc hp dn ca cc hnh tinh ln cn V vy ngay i vi v tinh a tnh th vn lun c s giao ng xung quanh v tr cn bng ca n. Thm na qu o ca chng cn b nghing. iu ny dn n trong h thng phi c cc trm iu khin v trong cc trm mt t phi c cc h thng bm.

Ngi ta a ra khi nim ca s gi trm (Station Keeping WWindow) ni nn kh nng trm iu khin ch c th gi v tinh a tnh c nh trong mt phm vi nht nh (trong phm vi ca s ). Kch thc ca ca s thng thng l 0,10 theo kinh tuyn v 0,10 theo v tuyn tc l trm iu khin c kh nng gi v tinh giao ng trong phm vi 0,050 theo phng Bc-Nam v ng-Ty. S giao ng theo kinh v v ca cc v tinh tng ng vi mt dung sai v tr l 37,5 Km. ng thi cao ca v tinh so vi tri t cng lun b thay i theo thi gian vi lch tm cc i l 0,001. Do khong caothay i ca v tinh trong mt chu k 24h s l 2 x 0,001 x (35786 + 6378 ) = 85 Km. Trong gi tr 6378 l bn knh ca tri t tnh ra Km, nh vy v tinh a tnh s giao ng trong mt th tch c kch thc 75 x 75 x 85.S giao ng ca v tinh a tnh quanh v tr tng i r rng s l cho thi gian truyn dn gia trm v v tinh lun b thay i. ng thi n cn gy ra hiu ng Doppler i vi sng mang. Tt c nhng nh hng ny u gy ra nhng kh khn cho qu trnh truyn dn v ng b ca h thng, nht l trong cc h thng truyn dn s, ngoi ra trm iu khin cn c chc nng gi anten thu pht ca v tinh lun hng v vng ph sng trn mt t. Hot ng ca trm iu khin da trn c s cc thng tin o c nhn t rt nhiu b cm bin (sensor) t trn v tinh.2.4, Phn h thng tin ca v tinhTrn v tinh thng c hai phn h, l phn h thnh tin bao gm tt c cc thit b phc v cho vic truyn dn tin tc v phn h iu khin c nhim v o lng cc thng s lm vic v iu chnh li cc thng s ny khi c lnh t mt t cu trc ca phn h thng tin c th c biu din tng quan bng s khi sau:

*B khuych i tp m thpTrong s trn LNA l b khuych i tp m thp c t ngay sau anten thu ARX c nhim v khuych i bin in p tn hiu thu vi mc tp m k sinh rt nh (s trnh by chng 4 ). B LNA ca v tinh thng l kiu c lm lnh bng Nitrogen lng hoc hiu ng nhit in. B LNA ca v tinh cng ging nh b LNA ca trm mt t cho nn chng s c tho lun phn thit b ca trm mt t chng 3.*B i tn FC (Frequency converter):Sau khi c khuych i v bin tn hiu thu tuyn leensex c trn vi mt tn s chun FL0 c to ra bi mt b giao ng OSC- Ocsillator t ngay trn v tinh. Tn s sinh ra ng sau b trn MIX l t hp gia tn s tn hiu tuyn ln v tn s ngoi sai FL0. Do tn s sng mang ca tuyn ln bao gi cng cao hn tn s tuyn xung cho nn b i tn ca v tinh thng l b i tn xung (Down Converter). Nguyn tc ca vic trn tn l da vo c tnh truyn t khng tuyn tnh ca cc thit b bn dn. V d nh mt diode , sinh ra cc t hp tn s mi t hai tn s u vo, nguyn tc ny c th gii thch n gin nh sau:Gi s u vo c dng: I(t) = Acos[(2Fut)+u]V tn hiu ngoi sai c dng: LO(t) = B cos [(2FL0t) +L0]Sau khi qua b trn tn hiu u ra s l: O(t) = I(t) LO(t). Do O(t) = AB cos O(t) = (AB/2) Nh vy ti b ra ca b trn c tn hiu O(t) vi bin AB/2 bao gm hai thnh phn tn s l FU + FLO v bng cc b lc u ra ca b i tn ta c th chn ly mt thnh phn tn s m ta mong mun. Trong thng tin v tinh thng ngi ta chn thnh phn tn s FLO ln hn FU . Do ta c FD=FLO-FU. V d i vi bng tn C th FU=6Ghz nn FLO phi bng 10 Ghz FD= FL0 - FU* B khuych i tin cng sut PPAv chia HIBRID: B khuych i tin cng sut PPA (Prior Bower Amplifier) c chc nng khuych i s b cng sut tn hiu i ra t b i tn ti mc ln c th phn chia cho cc Transponder. Vic phn chia c thc hin nh b HIBRID gm c n u ra tng ng vi s Transpoder ca v tinh. Cng sut ti mi u ra ca n phi nh hn n ln so vi cng sut u vo.*Cc b pht p Transponder:

Bng tn rng ca tn hiu v tinh c chia lm cc bng nh hn ( v d rng khong 40MHz ). Mi bng ny c phn phi cho mt b pht p ca n cn gi l knh v tinh. Mi knh v tinh li c th mang rt nhiu knh s liu v knh thoi t ngi s dng. Trn thc t do phi c khong bo v gia cc b pht p cho nn di tn thc t m cc b pht p s dng thng nh hn 36MHz. Cc b pht p c vai tr nh l cc knh chuyn tip thng tin. Chng lm vic trong cc di tn ring nh cc b lc thng di BPF t ti u vo. Sau khi s l b nh b tr, b tn s Tn hiu trong mi Transponder s c a qua b khuych i cng sut cao HPA u ra khuych i ln trc khi pht li hng xung. B HPA ca mi knh v tinh thng l loi n sng chy TWTAvi d phng 5:1 v hin nay bt u s dng loi HPAbn dn SSPA. Vi cc phng php a truy nhp m ta s tho lun cc mc sau. Mt Transponder c th cng mt lc phc v nhiu tuyn khc nhau. D nhn thy rng trong tng trng hp h thng dng k thut s dng li tn s (Reuse) th s hnh 4 mi ch l mt na phn h thng tin ca v tinh dnh cho mt phn cc na cn li ca phn cc kia c s tng t.*B ghp cng sut (MUX):

Trc khi anten pht ATX tn hiu ca cc Transponder cc bng tn con khc nhau phi c ghp li vi nhau. Yu cu ghp phi m bo sao cho s can nhiu gia cc knh v tinh l thp v mc cng sut ca chng ng u nhau trong tn hiu tng hp. C nhiu k thut c ng dng cho b MUX m trong c th k n mt thit b thng dng l b CIRRCULATOR.3. Phn mt t (ground segment)

Phn mt t bao gm tt c cc trm mt t ca h thng. Thng thng chng c ni vi thit b u cui ca ngi s dng thng qua mt mng mt t c dy hoc khng dy. Trong mt s trng hp chng kt ni trc tip vi ngi s dng qua mng VSAT . Cc trm mt t i vi ngi s dng qua mng thng l cc trm ln c dung lng cao phc v nhiu khch hng mt lc. Ngc li cc trm VSAT l cc trm nh c dng lng thp v ch phc v mt s lng hn ch ngi s dng. Hin nay, cc dch v VSAT ng rt ph bin nn cc trm mt t VSAt c quan tm nghin cu rt nhiu.

Cc trm mt t c th phn bit theo kch c ca chn. Kch c ny ph thuc vo dung lng truyn ti v kiu tin tc ca mi trm (thoi, truyn hnh hay s liu). Cc trm ln nht c trang b anten c ng knh 30m nh cc trm mt t tiu chun A ca h thng INTELLSAT th h c. Cc trm nh nht anten ch c 0,6 m v d nh cc trm truyn hnh trc tip t v tinh. Nhn chung do k thut ngy cng pht trin nn kch c ca cc trm mt t ngy cng nh li. V d hin nay trm mt t tiu chun A ca INTELLSAT ch cn c anten ng knh t 15 18m .

Cc trm mt t thng c c my pht v my thu trao i tin tc vi v tinh. Mt s trm khc ch c my thu nh trong trng hp trm khai thc cc dch v qung b t v tinh hoc l trm phn phi cc dch vuh truyn hinh v s liu ti khch hng. Hinh 5 cho ta thy cu trc tng quan ca mt trm mt t thng dng.

Hinh 5: Cu trc c bn ca trm mt t

K thut v trm mt t c bit quan trng cho nhng ngi khai thc h thng thng tin v tinh bi v n gn lin vi h. Cc thng s ca trm mt t, cc tnh cht tn hiu v qu trnh x l tn hiu ti trm nh ghp knh, gy mo trc, gii mo trc s c ni ti phn sau.4. Phn cc sng mang trn thng tin v tinh.

Sng in t bao gi cng c mt thnh phn in trng v mt thnh phn t trng c hng vung gc nhau v vung gc vi phng truyn sng. Theo quy c, phn cc ca sng c nh ngha bi hng ca vector cng in trng. Ni chung hng ca in trng khng c nh v bin ca n cng khng phi l hng s. Khi truyn sng in t u mt ca vector cng in trng thng vch ra mt ng elip do gi l phn cc elip.

Phn cc ca sng in t c ba thng s c bn sau: Hng quay vector cng in trng: Theo tay phi hoc theo tay tri tc l thun hay ngc chiu kim ng h. T s trc AR (Axial Ratio): AR = EMAX/EMIN l t s gia trc ln v trc nh ca hnh elip phn cc. Khi AR=1 hay 0 dB th hnh elip tr thnh ng trn v phn cc c gi l phn cc trn. Khi AR= th hnh elip tr thnh mt ng thng v phn cc c gi l phn cc thng. nghing ca Elip phn cc: Khi cng ngh truyn dn s dng li tn s th ngi ta phi dng n 2 sng mng c phn cc vung gc nhau hay nghing ca hai elip lch nhau 900. Nhiu khi nhng tuyn gy xuyn cc ln ngi ta phi s dng thm s phn bit v chiu quay ca vector cng in trng. Mt sng mang quay theo tay phi cn sng mang vung gc vi n quay theo tay trai.

c bit khi dng phn cc trn th ch c th phn bit v chiu quay ca vector phn cc. Khi sng mang c vector E quay theo tay phi gi l RHCP (Right Hand Circular Polarisation) v sng mang c vector E quay theo tay tri gi l LHCP (Left Hand Circular Polarisation). Cc phn cc trn hin ang c dng rt ph bin trong thng tin v tinh, c bit trong cc h thng dng loi tn s.

Hai phn cc thng gi l vung gc nhau khi c phn cc hng theo chiu thng ng, phn cc kia hng theo chiu nm ngang trong mt h qui chiu no .5. Cc di tn s s dng trong thng tin v tinh.

phn phi tn s ngi ta chia th gii ra lm ba khu vc: Khu vc 1: Bao gm Chu u, Chu Phi, vng Trung ng v Nga. Khu vc 2: Bao gm cc nc Chu M. Khu vc 3: Bao gm cc nc Chu tr vng Trung ng, Nga v Chu i Dng.Tn s phn phi cho mt dch v no c th ph thuc vo khu vc. Trong mt khu vc. Mt dch v c th c dng ton b bng tn ca khu vc ny hoc phi chia s vi cc dch v khc. Cc dch v c nh s dng cc bng tn sau:a. Khong 6 GHz cho tuyn ln v 4 GHz cho tuyn xung c gi l bng C (hay bng 6/4 GHz), bng tn ny c cc h thng c s dng v d h thng INTELLSAT, cc h thng ni a ca M v hin nay c xu hng bo ho. b. Khong 8 GHz cho tuyn ln v 7 GHz chon tuyn xung c gi l bng X (hay bng 8/7 GHz). Bng tn ny c ginh ring cho chnh ph s dng.c. Khong 14 GHz cho tuyn ln v 11 hoc12 GHz cho tuyn xung c gi l bng tn Ku (hay bng 14/12 14/11). Bng tn ny c cc h thng mi hin nay s dng v d nh h thng EUTELSAT, TELECOM I v IId. Khong 30 GHz cho tuyn ln v 20 GHz cho tuyn xung cn c gi l bng Ka (hay bng 30/20). Bng tn ny hin mi s dng cho cc h thng cao cp, cc cuc th nghim v ginh cho tng lai.e. Cc bng tn cao hn 30GHz hin ang c nghin cu v chc chn s c dng rt ph bin trong tng lai.

Cc dch v di ng dng v tinh s dng bng tn khong 1,6GHz cho tuyn ln v 1,5 GHz cho tuyens xung, bng tn ny c gi l bng L.

Cc dc v qung b v tinh ch c tuyn xung v s dng bng tn vo khong 12GHz.

Mi trm mt t c v tinh phn phi cho mt bng tn nht nh. Trong thng tin v tinh ngi ta thng phn bit cc khi nim bng tn nh sau: Bng tn chim dng (Occupied bandwidth) BOCC. Bng tn doanh nh (Allocated Bandwidth) BALL. Bng tn tp m (Noise Bandwidth) BN. Bng tn phn tch ( Resolution Bandwidth) BRES. V bng tn cng sut tng (Epuibalent Power Bandwidth) BEqp.

Bng tn doanh inh BALL l bng tn thc s m v tinh cung cp cho trm mt t. nh gi sng mang trong BALL ngi ta phi o cng sut (dB) ca n ti mt v tr no c xc nh bi BRES. Nu l tng th BRES = 1Hz nhng trn thc t rng ca BRES bng khong 1% BALL.

Cc gii BALL ca trm mt t c t st cnh nhau cho nn gia chng phi c khong bo v no trnh s xuyn nhiu. V vy thc s cc sng mng ch lm vic vi gii tn chim dng BOCC nh hn v nm trong BALL, BOCC c xc nh bng tn gia hai BRES sao cho gi tr cng sut nh ln hn cng sut mi BRES ti thiu 40dB thng thng th c gi tr trpng khong 1/1,1 1/1,2 BALL.

Hnh 6: nh ngha cc bng tn .

Vi mt gi tr BOCC no cc trm mt t cn phi chu s qui nh v mc cng sut pht khng c ln hn mt gi tr cc i nht nh. Bi v mt trm pht mt cng sut qua ln th cc hi do n sinh ra s nh hng nghim trng n cc knh ln cn c mc cng sut nh hn. Do mt trm mun pht cng sut ln th tng ng vi mt gii tn chim dng ca n s m rng ra. Gi tr ca BEqp c trng cho s tng ng . Chingh v th m cc k thut truyn dn hin nay ang c gng lm cho BEqp =BOCC da trn vic gim mc cng sut pht l tng cng hiu qu ca cc k thut sa li trc (FEC). Khi d cng sut pht c nh (kh nng li) nhng do m ho chng li cho nn ta vn c th m bo c cht lng thng tin truyn dn t yu cu nh khi pht vi cng sut ln.

rng di tn BALL c v tinh cung cp cho cc trm mt t theo yu cu ca ring mi trm nhng theo qui inh bao gi n cng phi bng mt s l ln bc 22,5 KHz tc l BALL=n. 22,5KHz (vi n l). Quy nh ny nhm lm cho qu trnh phn b tn s trong h thng c thun li v hiu qu nht. Trn thc t ti thi im hin nay, quy nh ny tr nn c v s tin b ca k thut, tc lung s liu ngy cng nh hn, chim bng tn hp hn, cho php ta s dng nhiu liu bc chia cho BALL.6. Cc k thut iu ch v gii iu ch tn hiu6.1 Khi nim

iu ch tn hiu l bin i tin tc cn truyn sang mt dng nng lng mi c quy lut bin i theo tin tc v thch hp vi mi trng truyn dn. Qu trng iu ch l qu trnh dng tn hiu tin tc thay i mt hay nhiu thng s ca phng tin mang tin. Phng tin mang tin trong thng tin v tinh thng l sng in t cao tn (RF). Vic iu ch phi m bo sao cho tn hiu t b can nhiu nht l khi sng mang i qua mi trng trung gian.

Ngi ta phn bit hai loi iu ch l iu ch tng t cho tn hiu analog v iu ch s cho tn hiu s. i vi tn hiu tng t th kiu iu ch thng dng trong thng tin v tinh l iu tn FM ( dng cho thoi, s liu, truyn hnh). Cc phng php iu bin AM v iu bin pha QAM (iu ch cu phng) rt t dng bi khong cch truyn dn rt ln ca tuyn v tinh cng vi cc tp m ng truyn s lm cho bin sng mang thay i rt mnh gy nhiu kh khn cho qu trnh gii iu ch.

Cc k thut iu ch s da trn c s dng cc bin php ti cc ging bit ln sng mang. Tn hiu bng gc bao gi cng l tn hiu analog nn chng phi c chuyn thnh tn hiu s nh phng thc PCM (Pulse Coder Modullation) trc khi c iu ch. K thut iu ch s p dng trong thng tin v tinh thng l iu ch dch mc pha PSK (Phasing Shift Keying) v iu ch mc dch pha vi sai DE PSK (Diffferental PSK). u im ca k thut iu ch s l n khai thc c cc mt mnh ca tn hiu s so vi tn hiu tng t, t b can nhiu ca mi trng v d kt hp vi cc qu trnh s l nh m ho, bo mt, chng li v sa liNi chung nguyn tc ca vic iu ch s v tn hiu tng t l tng i ging nhau.6.2. K thut iu ch tn s (FM)A,Nguyn l:

iu ch tn s l dng tn hiu cn truyn bin iu tn s ca sng mang.

Gi v(t) l in th biu din tn hiu dng iu ch v FC l tn s ca sng mang. Khi dng v(t) iu ch sng mang, tn s ca sng mang s bin i theo v(t) v sinh ra mt lch F c cho bi cng thc: F(t) = F(t) F(c) = kFM v(t) (Hz)

Trong KFM (Hz/V) l h s ca b iu ch v F(t) l tn s sng mang b bin iu theo thi gian sau khi iu ch.

Nh vy s bin i bin ca in th v(t) c trng cho tin tc cn truyn i c ti ln sng mang F(t). Khi truyn sng sang trm thu b gii iu ch s cn c vo i lng khi phc li tin tc ban u.

B, Ch s iu tn:

Nu tn hiu iu ch c dng hinh sin vi tn s FM v bin A th n s gy ra lch tn s cc i ca sng mang l FMAX = kFM A khi ch s iu tn c nh ngha: mFM= FMAX / FM.

Ch s iu ch hin th ch mc iu ch ca tn hiu i vi sng mang. Gi tr mFM ln khi bin ca tn hiu ln lm tn s sng mang thay i nhiu v s nh khi A nh ko theo FMAX cng nh.

C, rng ph ca sng mang iu tn:

Ph ca sng mang b iu ch bi mt tn hiu chuaant hnh sin c tn s FM chim mt rng c tnh bng cng thc carsonB = 2(mFM + 1) FM .

Nu tn hiu dng iu ch khng c dng sin m chim mt gii tn t 0 n FMAX th ta phi thay i gi tr FM bng FMAX .

Vi t s iu tn mFM = FMAX / FMAX6.3. K thut gii iu ch sng mang FMa.Nguyn l:

Ngyn l ca vic gii iu ch sng mang FM ngc li vi qu trnh iu ch tc l gii iu ch s xc nh lch tn s tc thi ca sng mang v t khi phc in th tin tc u(t) theo phng thc sau:U(t)= FM F(t)

Trong FM l h s ca b gii iu ch tnh ra (V/ Hz)

b. T s tn hiu trn tp m ti u ra ucar b gii iu ch

nh gi mc tp m ngi ta thng s dng khi nim mt ph cng sut tp m N0 c suy t cng thc N = N0 B0. Trong N l cng sut tp m tng cng trn ton gii BN. i vi phng thc gii iu tn N0 s ph thuc vo tn s v c tnh bng :N0 (F) = N0 (FM/ A)2 (2F)2 [w/Hz]

y A l bin ca sng mang thu c v N0 l mt ph cng sut tp m ti u vo ca b gii iu ch. D thy rng mt ph tp m ti u ra ca b gii iu ch N0(F) c dng Parabol.

Nu tn hiu tin tc c rng ph trong khong (0 n FMAX) th t s S/N ti u ra ca b gii iu ch s c tnh bng cng thc:S/ N = [3/ (2 FMAX)] (FMAX/ FMAX)2 (C/ N0)T.

Trong (C/ N0)T l t s gia cng sut sng magn v mt ph cng sut tp m trn ton tuyn c o ti u vo ca b gii iu ch. N0 c nh gi trong gii BN = B = 2(mFM + 1) FMAX.c. Tng ch gii iu tn:

V N = N0 BN cho nn ta c C/ N = (C/ N0)T BN. Do vt m cng thc C/ N trn c th vit thnh:

S/ N = (3/2) (BN/ FMAX ) (FMAX / FMAX)2 (C/ N) = 3(1+mFM)mFM2 C/ N.

Khi mFM ln th ta d thy S/ N tng rt mnh theo mFM hay theo FMAX ca sng mang cho nn i vi nhng tuyn ch c th p ng c mt gi tr C/ N nh, ngi ta vn c th y c mt t s C/ N ln ti my thu nu m rng bng tn ca sng mang hay tng su iu ch. y chnh l nguyn l hon i bng tn C/ N (cng cn gi l nguyn l hon i bng tn cng sut: Bandwidth Power Exchange) ca phng php iu ch tn s 6.4. iu ch s

a, Nguyn l

Hnh 7 cho ta thy nguyn l c bn ca mt b iu ch s: Mt b to mc (Symbol Generater). Mt b m ho (Ecodinh ) Mt b to tn hiu sng mang ( Channel Signal Generater ).

B to mc s chuyn chui nh phn u ra M mc nht nh. C mt nhm m bit cnh nhau u vo s c th hin bi mt mc c trng ring cho cu trc logic ca nhm. Nh vy gia M v m c quan h M = 2m. B m ho s to ra mt s tng quan gia M mc ny vi M trng thi c th ca sng mang. Trn thc t trong b iu ch s c hai kiu m ho sau: M ho trc tip ( Direct Encodinh ): Mt mc s tng ng vi mt trng thi nht nh ca sng mang M ho vi sai ( Differental Encodinh ): Mt mc s tng ng vi mt s thay i nht nh gia hai trng thi lin nhau ca sng mang

Nu tc lung s ti u vo ca b iu ch l RC (bit/s) th tc iu ch l RS ti u ra hay s thay i trng thi ca sng mang trong mi giy l:

RS = RC/m = RC / log2 M (baud)

iu ch mc dch pha (PSK) c bit thch hp cho vic truyn dn s bi n t b nhiu lm nh hng v khi so snh vi iu ch dch mc tn s (FSK) th n s dng ph hiu qu hn hay c s lng bit/s trn mi gii tn n v ln hn. Hai kiu PSK ph bin nht trong thng tin v tinh l iu ch dch pha hai mc (BPSK) v iu ch dch pha bn mc (QPSK). Trong mi kiu li c hai kiu con tu theo cch m ho l pha trc tip hay vi sai phab. iu ch pha hai trng thi BPSK v DE- BPSK

Hnh 8 cho thy cu trc c, bn ca mt b iu ch pha 2 trng thi. y ta khng cn b to mc v trong trng hp ny mt mc tng ng vi mt bit chng ta k hiu l bK l gi tr logic ca bit ti u vo b iu ch tn ti trong khong thi gian [ kTC (k+1) TC ]. B m ho s chuyn gi tr bit bK thnh gi tr bit mK ca chui in th v(t), thng l NRZ, iu ch ln sng mang .

Hnh 8: B iu ch pha hai trng thi

Nu m ho trc tip (BPSK) th mK = bK cn nu m ho vi sai (DE-BPSK) th mK=bK +mK-1. B to sng mang c iu khin bi bit mK tn ti trong khong thi gian [kTC. (KHz+1)TC thng qua in th v(kTC) = V. Sng mang c iu ch c tn s FC= C /2 c 2 dng nh sau:

C(t) = Acos (Ct + K) = v(kTC )Acos (Ct)

Trong cng thc trn K = mK tc l K = 0 nu mK = 1 v K = . Nu mK = 0.

C hai kiu iu ch BPSK v DE BPSK ddeeuc dng phng trnh sng mang iu ch C(t) nh trn nhng v mK c xc nh khc nhau nn gi tr K ca mi kiu i vi cng mt bit bK c th khc nhau (xem bng 1.1). iu ch PSK da trn nguyn l dng cc mc mK iu pha ca sng mang bng cch nhn n vi mt sng mang chun nh v trn hnh 1.8 nhng pha ca n lun gi nguyn trng thi trong thi gian tn ti ca mi baud.BPSKDE BPSK

bKPhamk-1 Pha c mK Pha mi

01

0

010100011000

a. iu ch pha bn trng thi OPSKv DE OPSK

Hnh 9 cho ta hnh nh cu trc ca mt b iu ch pha bn trng thi. B to mc c vai tr nh mt b bin i ni tip song song to ra hai chui nh phn AK v BK t chui bK nh ban u. Nu bK c tc RC th AK v BK c tc RC/ 2. Trong b iu ch QPSK th mt mc c trng cho mt cp bit (dibit) ly t hai lung AK v BK, tn ti trong khong thi gian [ k.TS, (k+1)TS ] bng thi gian hai bit ca chui gc bK tc l TS = 2 TC. B m ho chuyn cp bit AK, BK thnh cp bit IK QK theo mt qui lut nht nh ri sau s dng mt trng thi pha no ca sng mang i din cho cp bit IK QK ny.

Hnh 9: B iu ch pha bn trng thi (QPSK)

Gi tr in th ca IK v QK em iu ch tng ng l VI(t) v VQ(t). Nu m ho trc tip th ta c kiu iu ch trc tip (QPSK) v IK.QK c xc nh t cng thc IK = AK ; QK = BK nu m ho vi sai th ta c kiu iu ch vi sai (DE QPSK Differental QPSK) v ngoi AK BK, gi tr IK QK cn c xc nh nh vo trng thi ca chng trc (IK-1 QK-1) theo cc phng trnh sau

IK = IK-1 AK BK + IK-1AKBK + QK-1 AK BK + QK-1 AK BK.

QK = IK-1 AK BK + IK-1AKBK + QK-1 AK BK + QK-1 AK BK.

B to sng mang s sinh ra hai sng mang c pha vung gc nhau thng qua mt b quay pha 900. Sau thnh phn sng mang cosin s c iu ch bin bi bt IK trong khong thi gian [ k TS, (k+1) TS ] v thnh phn sng mang sin s c iu ch bin bi bt QK cng trong khong thi gian ny. in th ca bit iu ch IK l :VI(kTS) = V v ca bit QK l VQ (kTS)=V chnh l cc gi tr ca IK v QK c xc nh bng cng thc trn ti thi im kTS . Cui cng hai thnh phn iu bin ny c cng vi nhau to ra sng mang tng hp C(t):

C(t) = VI(kTS)Acos(Ct) + VQ(kTS)AsinCt) Trong cng thc trn ta thy hai sng mang thnh phn c bin thay i hoc l + V hoc V nhng sng mang tng hp li c bin khng i trong khi gi tr pha ca n li c th ly 1 trong 4 trng thi 450, 1350, 2150 v 3150. Mi trng thi pha cch nhau 900 ny s i din cho mt mc IKQK no tu thuc vo kiu iu ch l QPSK hay DE-QPSK v c thng k trong bng 1.2.QPSKDE-QPSK

AKBKPhaGc i pha

000110115/43/47/4/40/23/2

6.5 K thut iu ch sng mang PSK

a, Nguyn l

Nhim v ca b gii iu ch l xc nh gi tr pha hoc di pha ca sng mang thu c t suy ra chui nh phn ging vi bn pht. Trong thnh tin v tinh, c hai bin php gii iu chsoos thng dng. l:- Kiu Coherent: B gii iu ch s x dng mt tn hiu hnh sin tham chiu c chnh n sinh ra, c tn s v pha ging ht nh ca tn hiu sng mang cha iu ch my pht. B gii iu ch s xc nh pha ca sng mang thu bng cch so snh n vi tn hiu tham chiu. Kiu Coherent c th khi phc c chnh xc chui nh phn trong c hai trng hp kiu iu ch PSK v DE-PSK. Kiu vi sai: B iu ch tin hnh so snh pha ca sng mang thu trong khong thi gian thu tn ti baud hin thi vi pha ca n trong thi gian tn ti baud trc . Do vy, b iu ch s xc nh c mc thay i pha gia cc baud v khi phc c tin tc ban u. V ch nh gi c s thay i pha gia cc mc m khng bit c pha chnh xc ca chng nn kiu gii iu ch ny ch c th lm vic dd][cjv[is sng mang DE-BPSK v DE-QPSK.

b, Cu trc ca b gii iu ch

Hnh 10 cho ta thy cu trc ca b gii iu ch Coherent v b gii iu cheesvi sai i vi sng mang iu pha hai trng thi BPSK v DE-BPSK:

Nguyn tc hot ng ca b gii iu ch coherent (hnh 10) c th tm tt nh sau: sng mang thu c dng iu khin b khi phc sng mang chun to ra tn hiu tham chiu c tn s v pha ging vi sng mang my pht. Tn hiu tham chiu c dng cos Ct li c thch hp vi chnh sng mang thu thng qua b nhn. Kt qu ti u ra b nhn ta c.

V(t) = C(t) cos Ct + K)cos Ct

V(t) = [A cos(K) + A cos (2Ct + K)] / 2

B lc thng thp ti u ra b nhn cho php ta loi b c thnh phn tn s 2FC . Sau khi ra khi b lc in th c dng tinV cosK V do K ch ly 2 gi tr hoc 0 hoc . in th ny s c so snh vi ngng 0 ca b xc nh ngng (Threshold Detector) vo im gia khong thi gian tn taijcuar mi bit nh b ng b bit c iu khin bi V(t). B xc nh ngng ny s cho ra gi tr bit 1 nu thy V(t) > 0 (Khi K = 0 hay V(t) = + V) v s cho ra gi tr bit 0 nu V(t) < 0 (khi K = hay V(t) = - V). B gii iu ch s gy li bit khi tp m lm i du gi tr V(t) so vi ngng 0 ca b nhn dng.

i vi b gii iu ch vi sai, nguyn l hot ng cng tng t vi b gii iu ch Coherent ch khc l b vi sai th khng c b to sng mang chun m thay th vo l b lm chm sng c thi gian tr bng thi gian tn ti ca mt baud. Sau khi lm chm mt baud, sng mang thu s c nhn vi tn hiu khng b lm tr. Ti u ra ca b nhn tn hiu tng hp c dng V2cos(Ct + K) cos (Ct + K 1). B lc s loi b thnh phn pha K +K 1 c tn s gc 2C v b xc nh ngng s suy ra mK ban u tu vu du ca thnh phn V2 / 2 [cos (K -K 1) ] co vi ngng 0 ca n.

Nguyn l hot ng ca b gii iu ch 4 trng thi Coherent v vi sai l s pht trin ca b gii iu ch 2 mc thnh hai knh ring bit hot ng song song vi hai sng mang vung pha nhau. Mi knh l mt b gii iu ch hai mc coherent hoc hai mc vi sai v trong hai knh c b quay pha 900 t u ra b to sng mang chun hoc u vo b tr. Ti u ra b gii iu ch, hai knh ny c ghp qua b bin i song song-ni tip khi phc li chui bit ban u.6.6 Cc thng s ca phng thc iu ch PSK

a, Hiu sut ph

Trong thc t, ngi khai thc tuyn s phi tr tin cho ton b di ph chim dng trn v tinh v thu li li nhun t nhng ngi s dng. Li ch ca ngi khai thc tuyn s tng ln khi chim ph gim i v lng khch hng tng ln. Tc l phi phn phi cho mi khch hng mt di ph ngy cng hp nhng tc s liu vn khng thay i m bo cht lng dch v. Khi nim hiu sut ph c nh ngha l t s gia tc ca s liu RC(bit/s) v di tn chim dng ca sng mang B(Hz). Do :

= RC / B [bit/sHz]

Vi iu ch BPSK, trn l thuyt c hiu sut ph l 1 bit/sHz cn i vi iu ch QPSK hiu sut ph l 2bit/sHz. Trn thc t do c s khng hon ho ca cc thit b cho nn hiu sut ph ca BPSK ch t 0,7- 0,8 bit/sHz v vo khong 1,4 1,6 bit/sHz i vi QPSK.

b, Kh nng li:

Tc ng ca tp m l nguyn nhn chnh gy nn li trong qu trnh gii iu ch. Mc li c nh gi bi t s li bit BER v c th d on bng kh nng li l thuyt EP (Error Probability). Thng trn thc t BER cao hn EP nhng vic a ra EP cho ta cc phng hng nh gi tuyn.

Khi iu ch PSK, ngi ta a ra hai khi nim l kh nng li mc SEP (Symbol EP) v kh nng li bit BEP (Bit EP). i vi BPSK v mt mc l mt bit cho nn BEP = SEP; i vi QPSK, mt mc bng hai bit cho nn BEP = SEP/2. Mt cch tng quan thifBEP v SEP quan h vi nhau theo cng thc:

BEP = SEP /log2 M

Trong M 2 l s trng thi ca phng php iu ch.

Cc gi tr ca BEP ph thuc vo t s gia cng sut mi bit v mt ph cng sut tp m EC / N0 theo cc cng thc phc tp c a ra trong cc bng Report ca CCIR. V EC = C/RC co nn ta c th vit:

EC/N0 = (C/RC)/N0 = (C/RC)/RC [dB]

Nh vy EC/N0 c xc nh thng qua gi tr (C/N0). Nu tuyn v tinh l kiu chuyn tip (Relay) th (C/N0) c nh gi ring trn hai tuyn (C/N0)U v (C/N0)D.

thun tin hn CCIR cng a ra cc bng i chiu gi tr gia BEP v EC/N0 tu theo kiu iu ch ngi ta thit k tuyn r dng quyt nh cc thng s cn thit nhng trn thc as ngi ta thy rng cn phi thm vo gi tr EC/N0 suy ra t cc bng ny 1,5dB na b cho s khng l tng ca cc thit b. V d c BEP =10-6 th EC/N0 cn thit ca BPSK l 10,5dB ca QPSK l 10,8dB. Khi dng n trn tuyn thc th cc gi tr EC/N0 cn thit tng ng s l 12 v 12,3dB.

Ngoi ra do s thay i pha t ngt nht l thay i 1800 ca sng mang QPSK lm cho b lc b gii iu ch c th gy ra li. V vy hin nay ngi ta ang nghin cu cc phng php iu ch QPSK lch (Offset QPSK) v MSK cn gi l iu ch dch tn nhanh (FFSK) c bc thay i pha nh hn.7.cc k thut a truy nhp 7.1. Khi nim

Ta bit rng v tinh thng c nhiu Transponder hot ng song song phc v cho nhiu tuyn khc nhau. Tuy nhin khi nhu cu thng tin ngy cng tng ln th ngi ta phi ngh n vic chia s kh nng hot ng ca mi b Transponder cho nhiu trm mt t trong mng. Khi ta c th thit lp nhiu tuyn ni trm vi trm ng thi trn mt knh v tinh. thc hin c iu ny ngi ta phi s dng n cc k thut a truy nhp.

Cc k thut a truy nhp l cc qu trnh s l trong thi gian thc v kh nng truy nhp ca mt h thng tu thuc vo s trm mt t c trong h thng .i vi mt v tinh n bp sng , c 3 k thut a truy nhp c bn sau: 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 Mi phng php trn c nhng c im ring v phc tp khc nhau. Trong trng hp v tinh l loi a bp sng (Multi Beam) th ta c thm phng php a truy nhp phn chia theo bp sng thng qua chuyn mch v tinh l mt k thut mi v ht sc phc tp m y cha th xem xt ti c .

Ngi ta c th phi hp ba k thut a truy nhp c bn trn trong mt h thng tuy theo iu kin c th to ra cc k thut a truy nhp tng hp rt hiu qu. C th k n mt s kiu thng dng nh l FD/CDMA, FD/TD/CDMA, FD/TDMA, v TD/CDMA.7.2 Cc vn v lu lng

Cng lu lng A ca mt tuyn c nh ngha l :A = RCALLTCALLTrong : RCALL l s cuc gi trung bnh trn mi n v thi gian TCALL l thi gian tn ti trung bnh mi cuc gi . Thng thng s cuc gi thc t ln hn s knh thng tin C m h thng c th p ng c nn gy ra tnh trng n tc. Vi mt gi tr A no v gi s cc cuc gi b n tc khng c lu tr trong b nh th cng thc Erlang th hin kh nng phc v h thng cho n knh (n C) l :

Dung lng ton phn gia mi cp trm ca mng l hm ca yu cu vi kh nng n tc t 0.51%. Dung lng trn tuyn t X ti Y c k hiu l CXY tnh ra s knh hoc s bt/s. Trong thng tin v tinh c 2 phng thc truyn dn c bn:Thit lp mt sng mang trn mi tuyn. Nh vy c mt tuyn (hoc i hoc v) gia hai trm bt k trong mng ta c mt sng mang ring. Nu s mng trong trm l N th s sng mang phi c l N(N 1). Dung lng ca mi sng mang ch bng dung lng tuyn s dng sng mang (CXY).Thit lp mt sng mang cho mi trm pht. nh vy c mt trm trong mng ch c mt sng mang duy nht d cho n ng thi pht tin ti nhiu trm trong mng. Trong trng hp ny d thy s sng mang bng s trm (N) . Dung lng ca mi sng mang bng tng dung lng tt c cc tuyn t trm pht n cc trm cn li trong mng hay

Trong phng n thit lp mt sng mang cho mi tuyn ta thy CXY TB cho nn L >RB tc l i vi TDMA cc trm mt t phi c ng tc nng cao tc ca dng s liu ban u ln.

Cu trc ca mt trm Burst gm mt phn mo u (Header hoc Preambe) v mt tng tin (Traffic Field) i sau. Trng tin bao gm Burst con (Sub Burst) cha tin ca tng User m v tr xc nh ca chng c lu trong phn mo u ca Burst. Phn mo u ca Burst c rt nhiu chc nng m ta s ni ti phn cu trc khung.c, Cu trc khung TDMA:

Di y ta c minh ho v mt khung 2ms TDMA chun INTELSAT.

Hnh 13: Cu trc khung TDMA(chun INTELSAT/ EUTELSAT )Khung c hnh thnh trn v tinh bao gm tt c cc Burst ca cc trm mt t c t cnh nhau. Gia chng c mt khong thi gian bo v (Guard Time GT) phng chng s can nhiu c th c. Chiu di ca khung l 2 ms. GT chim 64 mc sng mang hay 128 bit tng ng vi 1ns. Trong mt khung tn ti hai loi Burst.

Burst ca cc trm chuyn tin (Traffic Station) c phn mo u gm 280 mc sng mang hoc 560bit v mt trng tin di n x 64 mc (n nguyn dng). Burstca cc trm tham chiu (Referenee Station) gm 288 mc hay 576bit v khng c trng tin. Trm tham chiu khng c nhim v truyn tin m ch gi vai tr to ng b cho h thng bng vic chuyn vo khung Burst tham chiu ca n. Tt c cc trm chuyn tin phi ng b hot ng ca mnh i vi trm tham chiu bng cch t Burst ca chng sau mt thi gian tr (Delay) nht nh i vi Burst tham chiu. Do vai tr rt quan trng ca Burst tham chiu i vi s hot ng chnh xc ca ton h thng TDMA cho nn trong mt khung ngi ta thng t hai Burst tham chiu ly t hai trm tham chiu trong mng c ng b tng h vi nhau.

Khung TDMA c tc l 120,832 Mb/s v kiu iu ch l QPSK. Trong hnh 13, RB1 v RB2 l hai burst tham chiu ca mt khung, TBX l burst mang tin t trm X.

Phn mo u ca Burst c nhiu on tin nh c vai tr nh sau:

Carrier and Bit Timing gm 176 mc (352 bit) dng cho vic khi phc sng mang chun trong phng php gii iu ch Coherent QPSK pha thu v dng ng b ng h ca trm mt t (b TDMA Timing).

Unique Word: T kho, l mt nhm bit c bit nh du ch bt u ca cc phn s liu (data) trong Burst n di 24 mc hay 48 bit.SC: Knh phc v (Service channel) di 8 mc (16 biit) mang cc thng tin v bo hiu v qun l mng.CDC: Knh iu khin tr (Control And Delay Channel) di 8 mc (16 bit) mang cc thng tin tr (Dn) dng cho vic ng b cc Burst tin.TTY, VOW: Cc knh in bo v in thoi di tng cng 72 mc hay 144 bit ginh ring cho thng tin gia cc ES trong mng (knh nghip v).d. Qu trnh thu cc Burst v ng b:

Ti pha thu, qu trnh thu nhn cc Burst thng qua mt chu trnh ngc vi qu trnh sinh Burst. Ton b khung c gi n mi trm mt t. Sau khi h tn v gii iu ch QPSK c mt dng s liu tc R, b nh thi TDMA pha thu s tm ra cc t kho (Unique Word) nh du ch bt u cc Sup Burst tin tc gi n cho n nm ri rc trong khung. Nh thng tin trong tng kho, b nh thi cho php b nh m (Buffer) thu nht ra cc phn tin tc thuc v n nm trong chui s liu R v lu li. Sau b nh m s c ra mt cch lin tc cc lung s liu nhiu ngi nhn kt ni vi ES cng nh s iu khin ca b nh thi. Mt vn quan trong l my thu phi khng c nhn nhm cng khng c nhn st t kho nn ta phi tm mt di t kho v ngng tng quan ca b Detector thch hp.

Hnh 14: Nguyn l qu trnh ng b ca khung TDMA.

Ta bit rng mun pht mt Burst ES phi xc nh cc Burst tham chiu (RB) v sau t Burst sau mt tr dn i vi RB trong khung. RB thng c t u khung th hin bng cc Burst B0 (hnh 14). Do c tr ng truyn Rn /c cho nn ch bt u khung thu ti trm mt t (SORFn Start Of Transmit Frame n) s xy ra sm hn tng ng vi khung trn v tinh. Nh vy mi ES phi xc nh c SOTFn ca mnh v pht i Burst tin Bn sau thi gian tr nh l dn. Gi s ch bt u khung thu cch ch bt u khung pht m khung trn v tinh. Nh vy ta c:

SOTFn SORFn = Dn = mTF 2Rn/ c

Khi trm ES thu Burst tham chiu n s xc nh c SORFn thng qua t kho ca Burst tham chiu v xc nh c tr Dn knh CDC. T n s xc nh c ch bt u khung pht ti ES (SOTFn). Tm li ES s pht i Burst tin Bn ca n sau khi nhn c SORFn mt khong thi gian Dn + dn v y chnh l nguyn l ng b khung TDMA trong qua trinh thng tin, di ng truyn Rn lun bin ng. V tinh s o v a cc thng tin bin i ny vo Dn m bo cho cc Burst ca cc ES c sp xp chnh xc trong cc khung.e. Cc tnh cht ca TDMA:

*) Hiu sut ghp:

Gi s nu ton b knh v tinh dnh cho mt sng mang QPSK th dung lng tin tc ca n c th t c l R (R = B) [bit/s]. khi ghp khung TDMA cho nhiu sng mang th dung lng ca n s gim i v c nhng phn khng mang tin (phn mo u, khong bo v.). dung lng ca knh khi ghp s l R* = R(1 - ti/TF). Trong ti l tng thi gian khng mang tin trong mt khung. Vy ta c hiu sut tnh bng :

= R*/ R = 1- ti/TF = 1- (P+2)(p+g)/ R TF.

Trong P l s trm ES trong mng, P + 2 l thm hai trm tham chiu na, p v g tng ng l s bit trong phn mo u v bo v ca mi burst. V d vi cu trc khung hnh 13, p = 560, g = 128, P = 50 ta s c = 85%.

*) u nhc im ca TDMA:

Lun lun ch c mt sng mang chim dng knh v tinh mt thi im nn khng xy ra hin tng xuyn iu ch (Intermodulation).

Hiu sut TDMA vn gi c mc cao khi s ES truy nhp ln (85%) trong khi FDMA, n suy gim rt nhanh.

Khng cn c s iu khin cng sut pht gia cc trm nh FDMA.

Tt c cc trm lm vic cng tn s d l trm thu hay pht. Do khng cn phi d sng (Tunning) v tn s ca b dao ng ni c th c c nh.

C tnh linh hot cao. Khi b sung trm mi ngi ta ch vic m rng khung v thay i Dn trong CDC phn mo u v thng tin cho b nh thi TDMA.

Tuy nhin TDMA cn c s ng b phc tp, nu sai s gy ra li. Thit b t tin.

D cn c nhng bt li nhng TDMA ang c s dng rt rng ri v n cho php s trm truy nhp ln, hn na qu trnh truyn s liu cng c cht lng cao hn.7.5. K thut a truy nhp phn tn chia theo m CDMA.a. Khi nim.

a truy nhp phn chia theo m CDMA (Code Divisin Multiple Access) l k thut a truy nhp mi nht. Trong cc trm c th pht ng thi v chim cng mt bng tn s. Do chc chn phi c mt s can nhiu no gia cc trm vi nhau. phn bit gia cc sng mang cng tn s, ngi ta s dng mt loi m c cu trc c bit thng l m gii ngu nhin (Pseudo Random Code). M c chn phi: D phn bit vi chnh n khi b dich thi gian so vi m gc. D phn bit vi cc m khc s dng trong mng.

Vic truyn dn tn hiu kt hp vi phng thc CDMA s lm ph ca sng mang tri ra rt rng so vi di ph khi truyn ring tn hiu. V vy ngi ta cn gi CDMA l truyn dn di ph (Spread Spectrum Transmission). Trong CDMA c hai k thut thng c dng l: Truyn dn chui trc tip (DS CDMA Direct Sequence CDMA). Truyn dn nhy tn s (FH CDMA Frequency Hopping CDMA).b. Truyn dn chui trc tip (DS -CDMA)

Hnh 15: Nguyn l DS CDMA v ph ca sng mang

Hnh15 cho thy nguyn l ca phng php ny. Bn tin nh phn cn truyn dn m(t) c dng m NRZ c tc bt RB = 1/ TB c nhn vi mt chui m nh phn p(t) cng c dng NRZ vi tc RC = 1/ TC ln hn RB t 102 106 ln. Cc thnh phn nh phn ca p(t) c gi l chp phn bit vi bit ca m(t). Sau khi nhn tn hiu t hp c iu ch PSK ln sng mang vi mt tn s nh nhau i vi tt c cc trm khc trong mng. V d sau khi iu ch BPSK ta c tn hiu sng mang s(t):

s(t) = m(t) p(t)cos Ct [V]

Ti pha thu s(t) s c gii iu ch Coherent bng cch nhn vi sng mang chun cha iu ch 2 cos Ct. Ta c tn hiu ti u ra b DEMOD l:

r(t) = m(t) p(t) cos Ct 2cos Ct = m(t) p(t) +m(t) p(t)cos Ct

Sau khi dng b lc loi b thnh phn c tn s gc 2C, c m(t) p(t), ta a tn hiu qua b nhn nhn vi p(t). Nu p(t) c ng b vi m nh phn th sau b nhn ta s thu c m(t) v p(t)2 = 1 (tnh cht m NRZ). y cn gi l s nn ph.

x(t) = m(t) p(t) p(t) = m(t) p(t)2 = m(t).

Tn hiu m(t) sau c th c a qua b tch phn loi nhiu v khi phc ng dng bn tin pha pht.

V khi iu ch PSK tn hiu c dng m(t)p(t) vi tc chp RC ln hn RBcho nn di ph sng mang PSK s m rng so vi di tn ca n khi ch iu ch vi m(t) theo mt t s RC/ RB. V chui m p(t) l chui gi ngu nhin cho nn mt cng sut ca sng mang c tri u trong di ph v gi tr cng sut nh thp xung rt nhiu (hnh 15).

Khi N sng mang cng truy nhp v tinh cng tn s gc C, tn hiu r(t) sau b DEMOD c dng:

r(t) = [s(t) + si(t)] 2cos Ct (i = 1..N 1)

x(t) = m(t)p(t)2 + mi(t)pi(t) p(t) = m(t) + mi(t)pi(t)p(t)

Ti u ra b nhn my thu ta c:

r(t) = [m(t)p(t) cos Ct + mi(t)pi(t) cos Ct]2cos Ct

Lc ny bn tin m(t) b mt lng can nhiu ca cc sng mang ln cn. Tuy nhin nu ta tm c cc m p(t) v pi(t) c mt s khng tng quan ln th p(t) s lm cho cng sut ca mi(t) vn b tri rng ra bi pi(t) cng b tri rng hn na. Do mt cng sut ca mi(t) ti my thu s rt nh v ta c th ly c m(t) d dng. y chnh l nguyn l ca DS CDMA. Thc t ta c th iu ch PSK cho m(t) trc ri mi nhn vi p(t) nhng cch ny phc tp hn nn t c dng.c. Truyn dn nhy tn s (FH CDMA):

Lc ny bn tin m(t) khng c iu ch ln sng mang c tn s c nh na m iu ch ln sng mang c tn s thay i theo tng bc c to ra t b tng hp tn s (Synthesizer). B tng hp ny c iu khin bi mt chui m p(t) c tc RC c to ra t b sinh m (Code Generator). Gi s tn s ca sng mang c th c N gi tr nh th cn phi c cc chui m di log2N chp iu khin b tng hp sinh ra chng. Tc thay i tn s sng mang l RH =RC/log2N, chu k thay i tn s l TH ta thy RH c th bng, ln hn hoc nh hn RB nhng trn thc t ngi ta thng dng RH