Ky Thuat Viba So- Hoang Quang Trung

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  • TRNG I HC CNG NGH THNG TIN V TRUYN THNG

    KHOA CNG NGH IN T V TRUYN THNG

    ThS. HONG QUANG TRUNG

    K THUT VIBA S TP BI GING

    (Lu hnh ni b)

    THI NGUYN - 2011

  • 2

    CHNG 1. TNG QUAN V H THNG VIBA S

    1.1. Gii thiu

    HTTT siu cao tn (microwave) vi mi trng truyn dn v tuyn trn gii sng cc ngn, bao gm cc loi h thng thng tin v tinh, thng tin v tuyn

    tip sc (radio-relay) v thng tin di ng; Cc h thng thng tin v tinh c dung lng trung bnh song b li c c ly lin lc t ln n rt ln. Cc h thng ny c s dng lm trc xuyn lc a hoc phc v cho cc tuyn kh trin khai cc loi hnh lin lc khc (nh tuyn lin lc t lin-hi o, t lin-cc gin khoan du, t lin-cc tu vin dng...). Ngoi ra, cc h thng v tinh a tnh cn c s dng cho cc h thng pht qung b truyn hnh. Trong tng lai gn, khi h thng cc v tinh qu o thp v trung bnh c trin khai, cc h thng v tinh c th c s dng cho c thng tin di ng ph sng ton cu. Cc h thng thng tin di ng phc v cc u cui di ng, ni chung c dung lng thp. Kh nng di ng l u th ln nht ca cc h thng ny.Cc h thng v tuyn tip sc mt t (terrestrial radio-relay) c dung lng t thp ti cao, c kh nng thay th tt cc tuyn cp ng trc trong cc mng ni ht ln ng trc. Vi thi gian trin khai tng i thp, tnh c ng ca cc h thng v tuyn tip sc mt t hn hn mt s loi h thng khc. Mt u im na ca cc h thng ny l rt d trin khai, ngay c trong cc iu kin a hnh gy nhiu tr ngi cho vic trin khai cc loi h thng dung lng cao khc nh trong cc th, hoc qua cc vng c a hnh rng ni vi c ly chng lin lc ln n 70 km, trung bnh l t 40 dn 45 km.

  • 3

    Truyn dn viba tn nhn thng (LOS-Line Of Sight) cung cp kt ni bng

    rng tng i qua mt tuyn n hay mt chui cc tuyn ni ui. Mt

    tuyn kt ni mt thit b u cui v tuyn ti mt tuyn khc hoc trm lp.

    Tuyn Viba tm nhn thng c minh ha nh hnh 1.1. Khong cch ca

    tuyn ph thuc vo cao ca antenna.

    Hnh 1.1. M hnh truyn viba chuyn tip

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    1.2. H thng thng tin viba

    Thng tin sng cc ngn gia hai im bt u xut hin vo nhng nm 30 ca th k trc tuy nhin lc by gi do kh khn v mt k thut nn ch lm vic dI sng mt do vy u im ca thng tin siu cao tn cha c pht huy. Nm 1935 ng thng tin VTTS u tin c thnh lp Newyooc v Philadenphi chuyn tip qua 6 a im v chuyn c 5 knh thoi. V TTVTTS bng n sau chin tranh th gii ln th hai. H thng vi ba s bt u hnh thnh t u nhng nm 50 v pht trin mnh m cng vi s pht trin ca k thut vin thng.

    Hnh 1.2. M hnh h thng viba

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    M hnh chc nng ca h thng viba c minh ha nh hnh 1.2. Cc trm

    u cui l cc tng i ni b thc hin chc nng chuyn mch to kt ni

    gia cc thu bao trn khong cch xa. C th mt s lng ln cc thu bao

    (khong 2000 thu bao) c th c hp nht truyn trn mt tuyn viba.

    Ti , tn hiu c chuyn sang tn s sng viba (vo khong vi GHz ti

    vi chc GHz) v c truyn qua khong cch t 30 n 60 Km t trm A

    n anten nhn ca trm lp. Trm lp n gin l cc b khuch i tn hiu

    v chuyn tip s dng tn sng viba. Trm u cui B nhn tn hiu sng

    viba, x l ri phn b vo cc knh ring r.

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    Hnh 1.3. S khi chc nng ca h thng viba tm nhn thng

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    Di tn s hot ng ca cc h thng viba:

    Thng tin siu cao tn lm vic di sng cc ngn dng truyn tn hiu c di tn rng. V l thuyt, gii sng dng cho cc h thng vi ba l t 60MHz cho ti 60/80GHz. Trong thc t, i vi cc h thng vi ba dng thng phm thng lm vic trn gii sng t 60MHz n 20 GHz, cc h

    thng cng tc vi gii tn s cao hn (6080 GHz) hin vn ang cn trong giai on th nghim.

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    1.3. Phn loi h thng

    Theo dung lng (tc bt tng cng B u vo) cc h thng vi ba s c phn thnh: - Cc h thng dung lng thp: B < 10 Mb/s; - Cc h thng dung lng trung bnh: B = (10 - 100 Mb/s); - Cc h thng dung lng cao: B > 100 Mb/s. Theo c ly lin lc (haul) - Tuyn di (c ly lin lc ln hon 400km): thng l nhng ng trc c dung lng ln so snh c vi cp quang. Di tn c s dng rng ri t 4 n 6 GHz. - Tuyn ngn (c ly lin lc di 400km): dung lng thp, thng thng 1DS1, 4DS1, 1E1, 4E1 dng ni cc trung tm chuyn mch di ng. Di tn thng s dng khong 15 GHz v di tn ny cho php thu gn kch thc ca an ten v thit b. Do chng ngn nn khng cn phn tp chng li hin tng phaing. Nguyn nhn gy gin on lin lc ch yu gy do ma nn cn c h s khuych i ln v chng ngn. Vi chng ln hn thng s dng di tn L6GHz, U6GHz hoc 11GHz dung lng thp. Di tn ny khng chu nh hng pha ing do ma nn c th bo m c ly lin lc xa hn.

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    1.4. u, nhc im ca viba s

    u im: + Do lm vic di sng siu cao tn nn m bo c vic truyn nhng tn hiu di rng. + rng di tn siu cao khong 30GHz do nhiu i c th lm vic ng thi. + Hu nh khng b can nhiu kh quyn v cng nghip + Trong di sng SCT d dng to ra cc h thng an ten c tnh nh hng cao, bp sng hp nh vy my pht c th gim cng sut v trn cng mt phm vi ta c th trin khai nhiu h thng cng lm vic m khng gy nhiu ln nhau. + Trin khai nhanh v gi thnh r hn so vi cc h thng thng tin dng cp (cp quang hoc cp ng trc) v vic trin khai h thng cp l rt tn km v trong khu vc ng dn c c nhiu cng trnh xy dng th vic trin khai mt h thng cp l rt kh khn. + D dng qun l v h thng vi ba ch gii hn qun l trong phm vi ca trm v tuyn dc theo trc (trong khi h thng cp phi qun l ton b tuyn cp v cbit phi i u vi nguy c t cp).

  • 10

    Nhc im:

    Di sng SCT c nhc im l ch truyn c chc chn trong tm nhn thng c ly khng qu 50 km. V vy khi mun thng tin i xa cn thc hin chuyn tip nhiu ln.

    C tc nh hn nhiu so vi h thng cp quang v hin nay ng trc ch cn s dng nhng khu vc cha ko c cp quang do a hnh phc tp.

    Chu tc ng ca ng truyn: hp th do hi nc v xi, suy hao do ma v hin tng pha inh c bit i vi cc h thng bng rng phi chu tc ng ca pha inh a ng chn lc theo tn s.

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    1.5. Cc ch tiu c trng i vi cc h thng vi ba s

    1.5.1. T s li thp

    CCITT ngh mt ch tiu thit k t s li 10-10 trn Km vi h thng truyn dn ni s chun gi nh 25000Km. i vi HRDP 2500Km cho mt t s li 2,5x10-7 tr tc ng ca thit b ghp knh. Trong cc h thng viba s, t s li ny s ph hp vi tiu chun thit k hin nay, t c i vi t nht l 99% thi gian. Do gi tr thp ca t l li khong 10

    -7 l thch hp cho truyn dn s chun gi nh 2500Km. CCITT

    khuyn ngh l t l li bit i vi mt on bc cao 25000Km trong ng chun gi nh di nht khng ln hn 10-6 trong hn 4% ca tp m pht trong thng bt k.

    1.5.2. T s li cao

    Khong thi gian m t s li cao c th vt qu, lm nh hng rt ln n thit k mt h thng. Mt yu cu l t s li cao khng c vt qu 0,01% ca thng bt k i vi mt mch 2500Km, s c tc ng n tnh kinh t ca mt h thng thc. Tuy vy, mt yu cu l t s li ny khng c vt qu 0,1% ca thng bt k s to nn mt h thng kinh t hn, nhng ngi ta khng so snh vi cc h thng viba FDM-FM hin c. Cc s liu m n c chp nhn, cng l ph hp, mt ni no trong hai cc tr ny v l 0,054% ca thng bt k khi s dng thi gian hp thnh mt giy.

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    1.5.3. Ch tiu giy khng li trong mt thi gian di

    T l li bit (BER) khng cho mt nh gi chnh xc c tnh ca mch s liu tr khi bit phn b ca li. Tiu chun t s li ca khi nh gi tt hn, nhng li i hi v c ca khi s liu v tc s liu, m chng li thay i t ng s dng ny sang ngi s dng khc.

    Cc ch tiu giy khng li (EFS) l mt phng php trong c th gii quyt c cc gii hn trn. CCITT cng b rng i vi truyn a s liu th s t ti mc tiu EFS 95% i vi on chuyn mch ni ht ni chun gi nh 25000Km, mt gi tr 99,5% EFS i vi HRDP 2500Km l ph hp. Thc t y khng phi l mi tng quan n gin gi c tnh BER v c tnh EFS trong thi gian di i vi mt h thng v tuyn trong thi gian fading nhiu tia, v EFC l mt hm ca tc fading, trong lc BER khng ph thuc vo tc fading.

    1.5.4. Ch tiu t bin li

    Khi s dng vi cc thit b tch knh s th s xut hin ca cc t bin li trong cc h thng viba s dung lng ln s gy ra cc nh hng khng mong mun i vi mng li. Do , CCIR cng b mt gii hn v kch thc cho php v khong thi gian ca cc t bin li ny, ch yu iu khin giai on ban u ca nhiu cuc gi c ly ra trong mng in thoi.

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    1.6. Cc ch tiu c tnh li

    1.6.1. ng truyn s chun gi nh (HRDP)

    xc nh cc chun c tnh li ca mt h thng viba s, mt im cn thit l phi hiu nh ngha ng truyn s chun gi nh. iu ny s cho mt s hng dn thit k thit b v cc h thng s dng trong cc mng thng tin quc t. di ca ng truyn i vi cc h thng c tc bit trn ghp s bc th cp (2048 kbit/s hoc 1554 kbit/s) l 2500 Km. Di y l v d v ng truyn s chun gi nh HRDP gm 9 b ghp knh s ti mc phn bc theo khuyn ngh ca CCITT v bao gm 9 on v tuyn s ging nhau lin tip cng di.

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    2500 Km

    1 2 3 4 5 6 7 8 9

    64 Kbit/s 64 Kbit/s 64 Kbit/s 64 Kbit/s

    - Thit b ghp knh s bc nht

    - Thit b ghp knh s bc khc ti cc n-c phn bc do CCITT khuyn ngh

    on v tuyn s

    -ng truyn dn s chun gi nh i vi cc h thng viba c

    dung l-ng trn mc th cp (Theo Khuyn ngh 566-1, CCIR)

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    1.6.2. Cc on v tuyn s

    Theo khuyn ngh 556 ca CCIR, mt on v tuyn s bao gm hai thit b u cui v tuyn lin tip v mi trng truyn dn lin kt gia chng, cung cp ton b cc phng thc pht v thu gia hai khung phn b s lin tip. Do vic b sung cc bit mo u k c trong thit b v tuyn, nn tc bit tng on v tuyn s c th khc so vi mc phn cp do CCITT khuyn ngh hoc khc so vi vic ghp tng hp ca n.

    1.6.3. Cc t s li bit cho php

    Do nh hng lan truyn v cc nh hng khc, nn c tnh t l li bit c biu th bng k hiu thng k l phn trm thi gian. Trong khi thit lp cc ch tiu BER, mt s yu t khc c a vo xem xt, l o li bit yu cu trong mt khong thi gian nht nh ph thuc vo ln t s li do hin tng t bin li v cn phi a trt vo tnh ton. iu c th ni rng, trong khi hot ng bnh thng, s xut hin cc khong chu k t s li bit cao, chng s gy ra cc gin on ngn, ch yu do cc iu kin truyn lan bt li lm gim mc tn hiu mong mun hoc tng mc tn hiu tp m. HRDP tng ng vi mch ch tiu cht lng bc cao c nh r trong Khuyn ngh G.821 ca CCITT, n cng to nn mt phn ca ISDN v i vi n CCITT xc lp mt quy lut chia cho php suy gim tng cng ca mch s chun gi nh. S chia ny l:

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    1). t hn 0.00016%/km tng ng vi 25000 km ca nhng khong thi gian 1 pht c t l li bit xu hn 10-6 . (ch tiu cc pht suy gim).

    2). t hn 0.000128%/km, tng ng

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