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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 VIBAS TP BI GING

(Lu hnh ni b)

THI NGUYN - 2011


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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 giisng cc ngn, bao gm cc loi h thng thng tin v tinh, thng tin v tuyntip 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 lylin lc t ln n rt ln. Cc h thng ny c s dng lm trc xuyn lca 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 tuvin dng...). Ngoi ra, cc h thng v tinh a tnh cn c s dng chocc h thng pht qung b truyn hnh. Trong tng lai gn, khi h thng ccv 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 thngtin di ng phc v cc u cui di ng, ni chung c dung lng thp. Khnng di ng l u th ln nht ca cc h thng ny.Cc h thng vtuyntip sc mt t (terrestrial radio-relay ) c dung lng t thp ti cao, ckh nng thay th tt cc tuyn cp ng trc trong cc mng ni ht lnng trc. Vi thi gian trin khai tng i thp, tnh c ng ca cc hthng v tuyn tip s c mt t hn hn mt s loi h thng khc. Mt uim na ca cch thng ny l rt d trin khai, ngay c trong cc iu kina hnh gy nhiu tr ngi cho vic trin khai cc loi h thng dung lngcao khc nh trong cc th,hoc qua cc vng c a hnh rng ni vi c ly chng lin lc ln n 70 km, trungbnh l t 40 dn 45 km.


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Truyn dn viba tn nhn thng (LOS-Line Of Sight) cung cp kt ni bngrng tng i qua mt tuyn n hay mt chui cc tuyn ni ui. Mttuyn 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 30ca th k trc tuy nhin lc by gi do kh khn v mt k thut nn chlm vic dI sng mt do vy u im ca thng tin siu cao tn cha cpht 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 knhthoi. V TTVTTS bng n sau chin tranh th gii ln th hai. H thng viba s bt u hnh thnh t u nhng nm 50 v pht trin mnh m cng vis 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 trmu cui l cc tng i ni b thc hin chc nng chuyn mch to kt nigia 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 tivi chc GHz) v c truyn qua khong cch t 30 n 60 Km t trmAn anten nhn ca trm lp. Trm lp n gin l cc b khuch i tn hiuv chuyn tip s dng tn sng viba. Trm u cui B nhn tn hiu sngviba, 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 hiuc 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 dngthng 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 tronggiai 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 cdung 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 thng1DS1, 4DS1, 1E1, 4E1 dng ni cc trung tm chuyn mch di ng. Ditn thng s dng khong 15 GHz v di tn ny cho php thu gn kchthc 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 doma nn cn c h s khuych i ln v chng ngn. Vi chng ln hnthng s dng di tn L6GHz, U6GHz hoc 11GHz dung lng thp. Di tnny khng chu nh hng pha ing do ma nn c th bo m c ly lin lcxa hn.


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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 hinchuyn 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 hnhphc 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 chutc 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 hthng 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 hthng viba s, t s li ny s ph hp vi tiu chun thit k hin nay, tc i vi t nht l 99% thi gian. Do gi tr thp ca t l li khong10-7 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 ngchun gi nh di nht khng ln hn 10-6 trong hn 4% ca tp m phttrong 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 rtln n thit k mt h thng. Mt yu cu l t s li cao khng c vtqu 0,01% ca thng bt k i vi mt mch 2500Km, s c tc ng ntnh kinh t ca mt h thng thc. Tuy vy, mt yu cu l t s li nykhng c vt qu 0,1% ca thng bt k s to nn mt h thng kinh thn, 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 tronghai cc tr ny v l 0,054% ca thng bt k khi s dng thi gian hp thnhmt 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 tthn, nhng li i hi v c ca khi s liu v tc s liu, m chng lithay i t ng s dng ny sang ngi s dng khc.

Cc ch tiu giy khng li (EFS) l mt phng php trong c thgii quyt c cc gii hn trn. CCITT cng b rng i vi truyn as liu th s t ti mc tiu EFS 95% i vi on chuyn mch ni ht nichun 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 tnhBER v c tnh EFS trong thi gian di i vi mt h thng v tuyn trongthi 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 hngkhng mong mun i vi mng li. Do , CCIR cng b mt gii hnv 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 mngin 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 imcn 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 ccmng 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 ghpknh s ti mc phn bc theo khuyn ngh ca CCITT v bao gm 9 on vtuyn 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 giachng, cung cp ton b cc phng thc pht v thu gia hai khung phn bs lin tip. Do vic b sung cc bit mo u k c trong thit b v tuyn, nntc bit tng on v tuyn s c th khc so vi mc phn cp do CCITTkhuyn 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 thitlp cc ch tiu BER, mt s yu t khc c a vo xem xt, l oli bit yu cu trong mt khong thi gian nht nh ph thuc vo ln ts 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 khongchu k t s li bit cao, chng s gy ra cc gin on ngn, ch yu do cciu kin truyn lan bt li lm gim mc tn hiu mong mun hoc tng mctn hiu tp m. HRDP tng ng vi mch ch tiu cht lng bc cao cnh r trong Khuyn ngh G.821 ca CCITT, n cng to nn mt phn caISDN v i vi n CCITT xc lp mt quy lut chia cho php suygim 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 thigian 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 vi 25000 km ca nhng khongthi gian 1 giy c mt s li bt k. (ch tiu cc giy li trm trng).

3). t hn 0.05% ca khong thi gian 1 giy c mt t s li xu hn10-3 i vi HRDP 2500 km. (ch tiu cc giy li). V d i vi HRDP 2500 km s s dng nh sau: 1. t hn 0.4% cho cc khong thi gian 1 pht l c t l li bit xu hn

10-6.2. t hn 0.32% cho cc khong thi gian 1 giy c mt s li no

(tng ng vi 99.68% ca cc giy khng li). 3. t hn 0.05% ca khong thi gian 1 giy c t l li bit xu hn l

10-3.


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1.7.2 Vi ba s a i m

Mng vi ba sny tr thnh phbin trong mt svng ngoi v nngthn. Mng bao gm mt tr m trung tm pht thng tin trn mt an ten ngh ng phc v cho mt s tr m ngoi vi bao quanh. Nu cc tr m ngoi viny nm trong phm vi (bn knh) truyn dn cho php th khng cn dngcc tr m l p, nu khong cch xa hn th ss dng cc tr m lp a tnhiu n cc tr m ngoi vi. T y thng tin s c truyn n cc thu bao.Thit b vi ba tr m ngoi vi c th t ngoi tr i, trn ct.v.v... mi tr m ngoivi c th c lp t thit b cho nhiu trung k . Khi mt cao c th b sung thm thit b; c thit k hot ng trong cc bng tn 1,5GHz -1,8GHz v 2,4GHz s dng mt sng mang cho h thng hon chnh.

Hin nay cc h thng im ni n a im 19GHz c ch to vlp t Chu u cung c p cc dch v s liu (Kbit/s) Internet trongmng ni ht khong cch 10Km. Tr m trung tm pht tc bit khong8,2Mb/s v a ch mi tr m li s dng k thut TDMA.


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Hnh 1.5: Cu hnh viba im-im, im-a im


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1.8. Cc k thut iu chs dng trong viba s

1.8 .1. iu ch v gi i iu ch s

1.8 .2. Cc phng thc iu ch s trong viba

Trong viba sch yu s dng cc phng thc iu chM-PSK v M-QAMa mc.

1.9. Cc m truyn dn s dng trong viba s Trong viba sch yu s dng cc m HDB3 v CMI.


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Bu kh quyn chia ra lm 3 tng:

+ Tng i luu: l l p kh quyn t mt t ln n cao khong (10 -15)km. Cng ln cao mt phn t kh cng gim, lm thay i phngtruyn ca cc tia sng. Tng ny thch h p cho vic truyn sng ngn.

+ Tng bnh lu: l l p kh quyn nm trong min t tng i lu lnn cao khong 60km, tng ny c mt phn t kh th p, chit sutkh c tc dng lm khc x tia sng, i phng truyn, lm cho cc tiasng pht t mt t ln tng bnh lu sb i phng truyn quay v mt t. Do vy r t thch h p cho vic truyn sng cc ngn.

+ Tng in ly: l tng kh quyn cao nm t cao (60 - 2000)km,

min ny h p th nhiu tia t ngoi c nng l ng l n, cc tia ny c tcdng phn ly cc phn t

kh tr thnh cc ion t do, tng ny mt phn t kh gim th p. Khitia sng c pht ln gn tng in ly th cng bphn x b cong vquay tr li mt t do vy r t thch h p cho vic truyn sng ngn.


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- Sng tr c ti p: L sng truyn tr c ti p t anten pht n anten thu khng b phn x trn ng truyn. Trong iu kin truyn lan bnh th ng, n c bin l n nht so vi cc sng khc n my thu.

- Sng phn x t: Sng ny n an ten thu sau lc phn x mt vi ln t mt t hoc t cc vt th xung quanh. S phn x khng nhng ch xuthin trn mt phng ng m cn c th xut hin trn mt phng ngang.Sng phn x ti anten thu c bin v pha khc vi bin v pha casng tr c ti p, lm tn hiu thu khngn nh.

Nu hiu khong cch ng truyn ca tia phn x v tia tr c ti p bngs l ln na b c sng th anten thu sng phn x lch pha v i sng tr cti p mt gc

1800 v k t qu lm suy gim tn hiu sng tr c tip, n mt mc no ph thuc vo bin ca sng phn x.

- Sng phn x tng i lu: Do thay i ch s khc x ca khng kh theocao so v i mt t, nn sng c thb phn x, tu theo gc sng t i c th xy ra phn x ton phn t tng i lu. Trong tr ng h p ny xut hin mt bin gi i c tc dng ging nh mt bmt phn x, gi sng tr li mt t.

Mt s tia ny s n an ten thu, c th lm suy gim sng tr c ti p do s thay i pha v bin gy ra. Sng truyn theo tng i lu c thlan r ngn 10 dm (khong 15km).


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2.2. Cc yu t nh hng n s truyn lan ca sng v tuyn

2.2.1. Suy hao khi truy n lan trong khng gian t do

Khong khng m trong cc sng truyn lan b suy hao c gi lkhng gian t do. Mc suy hao ca sng v tuyn c pht i tanten phtn anten thu trong khng gian t do t l thun v i khong cch gia haianten v t lnghch vi di b c sng. Suy hao ny gi l suy hao truynlan trong khng gian t do, c tnh nh sau:

l n l t l khong cch truyn dn v b c sng casng v tuyn.


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2.2.2. nh h ng c a pha inh v ma

Pha inh c nh ngha l s thay i cng tn hiu sng mangcao tn thu c do s thay i kh quyn v phn x t, nc trong ngtruyn sng. Thc t cho thy nh hng do ma v pha inh nhiu tia lnhng nh h ng lan truyn ch yu i v i cc tuyn v tuyn tm nhnthng trn mt t lm vic trong di tn GHz. V chng quyt nh cc tnhao truyn dn v do quyt nh khong cch l p cng v i ton b githnh ca mt hv tuyn chuyn tip. Pha inh nhiu tia tng khi di c atuyn tng tuy nhin n khng phthuc nhi u vo t n s . Cn tiu hao doma tng ln khi tn s tng. Chng hn, i v i cc tuyn s dng tn s trn35GHz thng suy hao do ma ln do m bo cht l ng tn hiutruyn dn th cc khong cch lp th ng chn d i 20km, ngoi ra vicgim di ng truyn s lm gim ccnh h ng ca pha dinh nhiu tia.

Vy i vi cc ng truyn di v c tn s hot ng th p th phainh nhiu tia lnh hng chnh. Cn i v i cc tuyn ngn v c tn s hot ng cao hn th tiu hao do ma l nh h ng ch yu.


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Bng 2.1 K t qu thc nghim v suy hao do hi n c - kh hu theo tn s sng vtuyn ca Alcatel.

Cng mc d tr phadinh 40dB, mt ng truyn vi ba di tn 38GHz s b mt ihon ton do bo l n, trong khi tuyn vi ba lm vic tn s6GHz

vn ti p tc hot ng bnh th ng.2.2.3. S can nhi u ca sng v tuy n

Thng th ng nhiu xy ra khi c thnh phn can nhiu bn ngoi tr nln vo sng thng tin. Sng can nhiu c th trng hoc khng trng tn s v i sng thng tin. Chng hn h thng Vi ba s ang sdng b nh h ng b i s can nhiu t cc h thng vi ba s ln cn nm trong cng khu vc, ctn s sng v tuyn trng hoc gn bng tn sca h thng ny, ngoi ra ncn b nh h ng b i cc tr m mt t ca cc h thng thng tin v tinh lncn.


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2.3. Pha inh v cch phng chng

Pha dinh l s bin i cng tn hiu sng mang cao tn ti antenthu do c s thay i khng ng u v ch s khc x ca kh quyn, cc phn x ca t

v nc trn ng truyn sng v tuyn i qua. Sbin i ny l yu t xu i v i thng thng tin vi ba.

- Pha inh phng: lm thay i u tn hiu sng mang trong mt di tn s (thay i ging nhau i v i cc tn s trong di).

- Pha inh la chn tn s: lm thay i tn hiu sng mang v i mc thay i ph thuc vo tn s, pha inh ny nh h ng ln n tuyn vi ba s dung

l ng cao.Hai loi pha inh ny c thxut hin c l p hoc ng th i v vy dn

n lm ginon thng tin. S thay i tn hiu ti anten thu do phn x nhiu tia gi l pha inh nhiu tia.


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2.3.1. Pha inh phn x t

Nu ng truyn v tuyn i qua mt t hoc mt nc c phn x cao th pha inh do phn x mt t l pha inh chyu so vi pha inh do phn x t tng i lu. c bit vi cc ng truyn ngn th phn x mtt lm cho cc tn hiu thu thng ging ngu nhin do cc iu kin kht ng gy ra lm bin i cc tham s truyn dn.

Nu ng truyn v tuyn i qua cc vng nh bin, h, cc vng bng phng vm t, m ly,... th cc mc tn hiu phn x nh hn 10dB sov i mc tn hiu ca ng truyn tr c ti p. Nu trong tr ng h p tuyn vtuyn i qua a hnh c sng m bao phc thc s phn x ton phn.


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2.3.2. Cc k thu t gi m nh h ng c a pha dinh nhi u tia

Cc k thut c s dng gim ccnh h ng ca pha dinh phngv pha

inh la chn tn snhiu tia l dng phn t p khng gian v phn t p tn s nng cao cht l ng ca tn hiu thu.

Phn t p theo khng gian cng vi cc anten t cch nhau theo chiudc k t h p cc bkh giao thoa phn cc giao nhau. Hiu qu ca k thutny m bo khng lm gin on thng tin, thng c biu th bng mth snng cao. Nh p dng k thut phn t p khng gian v phn t p tn s thi gian gin on thng tin gim nh so v i th i gian yu cu h thng

t c ch tiu cht lng ra.


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Bng s m phng nhiu ln tm c v tr tt nht cho hai anten, khi khngth tnh c v tr, th khong cch hai anten phi ln hn 150. Thngth ng cng thc trn tnh gn ng cho mt tuyn c chiu di (20 - 70)Kmv tn s (2-11)GHz.

Hnh 2.1: Phn t p theo khng gian


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2.3.2.2. Phn t p theo t n s

nh ngha: phn t p theo tn s l k thut thu hoc pht mt tn hiutrn hai knh (hoc nhiu hn hai knh) tn ssng v tuyn.

Hsci thin phn t p tn sc th tnh:

Trong :

f : l tn s trung tm ca bng tn [GHz]

d: di ca ng truyn [km]

l khong cch tn s tng i biu th bng %

Fm: l d tr pha inh [dB]

Phng trnh trn ng v i cc gi tr tham ssau:

Mc d cc h thng thng tin v tuyn s phn t p theo tn s c th cho cc hsci thin tt hn nhng vic s dng ph tn khng t hiu qu cao.


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Ngoi ra tng hiu qu chng pha inh ng i ta s dng k t h p phn t pkhng gian v tn s.

Hnh 2.2: Phn t p khng gian v t n s s d ng 3 anten

2.3.2.3. Chuy n m ch b o v

Mc ch ca chuyn mch bo v l nng cao kh dng ca h thng bng cch chuyn sang knh d phng khi c hin t ng s c thit b chnh. Ngoi

ra cng c th t c l i ch khc khi thit b bo vchng li s gin onthng tin do pha dinh la chn tn sgy ra bng cch chuyn sang h thngd phng. (Ngha l knh d phng c s dng khi knh chnh b s c hoc b gin on thng tin do pha inh).

Cht l ng v kh nng sn sng ca h thng vi ba s c th nng caonh s dng mt hay 2 knh d phng thay thcc knh b s cnh thit b chuyn mch t ng. Thng th ng khi sknh truyn dn nh hn hoc

bng 7(n 7) th dng mt knh d phng, tng ng v i cu hnh (n+1).Trong thc t dng cu hnh (1+1) gm mt knh truyn dn v mtknh d phng nng HSB (Hot

Standby), c thhot ng cao tn RF hoc trung tn IF.


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Hnh d i m t mt tuyn vi ba s c chuyn mch bo v bng knh d phng

Hnh 2.3: M hnh tuyn c d phng


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Chuyn mch c thc hin khi my pht b s choc l khi c s la chnmy thu cho tn hiu tt nht trong 2 my ang hot ng.

Hnh 2.4: Phn pht v thu ca h thng d phng nng 1 + 1


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2.4. Nguyn l c bn truyn sng viba

Sng viba l sng c chiu di b c sng nm trong khong 1m - 1cm.K thut truyn sng viba va lin quan n tnh cht vt l li va mang tnhthng k. Suy hao khng gian t do khng ch ph thuc vo khong cchtuyn m cn phc thuc c vo tn s sng viba. V vy, v i mt tn s chotr c th tn hao khng gian t do c coi l hng s i v i v tr c nhca anten pht v thu. Hnh di y cho thy mi quan hgia li v mt mt cng sut tn hiu trn mt tuyn viba:


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Truyn sng viba b chi phi b i cc nguyn l: phn x (reflection),khc x (refraction) vnhi u x (diffraction). iu ny dn t i hai vn cn xemxt:

(1) S khc x (refraction) v ph n x (reflection) c a sng v tuy n .

Hnh 2.6: Tia khc xv tia phn x


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i qua mt khong cch di (t anten pht n anten thu), cc tia sngviba c th di theo cc ng khc khyu (do khc x lin ti p qua cc l pkh quyn theo mt phng thng ng). S ln khc x v ng i khckhyu ca ca cc tia sng l ph thuc vo cc l p (gradient) p sut kh

quyn v tn sca sng viba. K t qu l tia sng c th khng i theo ngthng. Cc gradient khc x l thay i chm theo th i gian, phc thuc votng ngy, ma, a l v kh hu. Chnh v th m cc ng khc khyu cacc tia sng t my pht n my thu cng c thay i mt cch tngng.

Ngoi ra, s khc xca cc tia sng viba cn dn n fading a ng.


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S phn x mt t ca cc tia sng ti nhng im gia ca tuyn gyra fading a ng ti my thu v dn t i mo tn hiu. Cn s phn x khu ch tn , s tn x do a hnh g gh s lm cho cc tia sng i theo cchng no mt cch ngu nhin nn cc tia ny khngnh h ng nhiu t i

fading a ng.cong ca cc tia sng c bit thng qua h s bn knh hiu dng

K ca tri t. C ngha l skhc xdo p sut kh quyn c mi lin hv igi tr trung bnh ca bn knhtri t. Nu gi ea l bn knh hiu dng catri t th ta c:

r aa e

111

Trong : a- bn knh thc ca tri t (6370 Km).

r - bn knh cong ca tia sng.


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gi s a K a e . , khi aa

K e c gi l h s bn knh hiu dng ca

tri t. Nu gi N l khc x, h l cao ca tia sng so v i mt t thk t qu tnh ton cho ta biu thc:

dhdN

K 157

157

Nh vy h sK l bin i v l phng php thng c s dng miu t kh nng un cong ca cc tia sng viba. Hnh di y cho thy s nh h ng ca hs K khc nhau n tia sng viba.

Hnh 2.7: Minh ha cong ca tia sng theo hsK


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(2) S nhi u x (diffraction)

Xt trn quan im vt l th chm sng viba c b r ng l mt ct ngang ly tia truyn tr c ti p lm tr c gia. Ga tr o br ng ca chm l iFresnel th nht - Mt ellipsoid cha hu ht cng sut tn hiu t my phtn my thu.

i v i mt tuyn c nh th i Fresnel s tr nn hp hn khi tn s viba tng ln v anten ln hn. Xt theo mt thng ng th i Fresnel xc

nh pha trong bin ca hnh ellipsoid, cc tia phn x khng gy nhiut i cc tia truyn tr c ti p ti anten thu. Cn t b mt bn ngoi ca iFresnel th nht tr ra th cc vt chn hay a hnh bt u gy ra tn hao donhiu x. Ngoi i Fresnel th nht, cn c cc h ellipsoid bao xung quanh ng bin ca n. l cc i Fresnel 2, 3, ... Chng cnh h ng t t ivic gy tiu hao nhiu x do cng sut tn hiu trong cc vng ny l khngng k.


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Bn knh i Fresnel th nht cho b i:

md f d d F

21

211 .

..32,17

d d d ,, 21 tnh theo Km. f l tn s tnh theo GHz.

i vi cc a hnh khc nhau c th lm chn min Fresnel th nhtgy ra cc tn hao nhiu xkhc nhau.

Hnh 2.8: Min Fresnel


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2.5.2. Cng su t pht

Cng sut pht cng ging nh vi ba tng t, ph thuc vo c ly vnhy my thu m bo t s li bit cho php.

n vcng sut pht tnh bngdBm. P0 = 1mw.

2.5.3. nh y my thu hay ng ng thu

L mc tn hiu cao tn ti thiu n u vo my thu n hot ng bnh thng, ngha l thomn t s li bit (BER) cho trc tng ng v itc bt nht nh.

2.5.4. T s bit l i BER

thng tin t c tin cy cao, m bo cho thit b hot ngkhng nhm li th t s ny cng nh cng tt, bnh thng cng phi t

310 , v i cht l ng tt hn phi t 610 . V i yu cu BER cho tr c mythu phi c mt ngng thu tng ng.


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2.5.5. Phng thc iu ch v gi i iu ch

Thng th ng trong vi ba s, ty theo tc bit (dung l ng knh)ngi ta thng dng cc phngthc iu ch nh QPSK (hoc 4PSK hay QAM) hoc QAM nhiu mc, chng hn (16QAM, 64QAM)...

Phng thc gii iu ch c chn tng ng vi phng thc iuch thc hin ti my pht. Thng th ng, trong vic gii iu ch c 2 phng php l tch sng k t h p (Coherent), hoc tch sng khng k t h p.Tch sng k t hp i hi my thu s khi phc li sng mang ng pha v ii pht nn cu hnh phc tp nhng cht l ng tn hiu cao hn so v i tchsng khng k t h p.

2.5.6. Tr khng vo my thu v tr khng ra my pht Vn phi h p tr khng i v i mch cao tn r t quan tr ng, cc b

phn k t ni vo my pht v my thu phi phi hp c tr khng. Nuvic phi h p tr khng tt s lmnh hng n cht l ng tn hiu, cngsut pht hoc thu khng t cc i, ngoi ra cn gy ra sng phn x, gymt cn bng lm gim nhy my thu. Thng th ng tr khng ra ca my pht v tr khng vo my thu c chun ho l 50 do tr khng vo ra

ca cc b lc,ng dn sng, phi phi l 50 .


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2.5.7. T c bng tn g c

Tc bng gc l tc dy s liu vo/ra my thu pht v tuyn

V d: Thit b vi ba sRMD 1502/4 HDB3 2*2048kb/s

9470LX HDB3 4*2048kb/sMini-link HDB3 2*2048kb/s

v i tr khng 75 khng cn bng.

2.5.8 Knh nghi p v

C cc ch tiu v iu ch, mc vo ra, t sS/N, tn s bo gi (knhnghi p v thng c iu chFM hoc FSK).

2.5.9 Knh gim st v iu khi n t xa

Cng c cc ch tiu nh knh nghi p v (c th c iu ch theo phng thc ASK ,FSK). Ng i ta s dng knh ny khai thc qun l vgim st thit b.


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2.6. Thit b ng-ten

Yu cu chnh ca thit b anten cho mt h thng v tuyn l c suy haotruyn dn nhv kinh t, hiu sut bc xanten cao, hskhuch i l n.

2.6.1 Anten

Anten l mt giao din chnh gia thit b in v mi tr ng truynsng, tu thuc vo tn s, cng nghv cng dng.

Anten YAGI c s dng cho tn s400MHz - 900MHz.

Anten Parabol c s dng cho tn s t 1GHz n 60GHz, b phn phn x c ch to bng kim loi hoc nha c phmt l p kim loi mng mt lm ca anten. Khi tn s nh hn 4GHz bphn phn x c th cch to bng vic ph kim loi trn cc thanh mng lm gim tr ng l nganten v lm cho gi l t xuyn.


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Phn chnh ca mt anten Parabol c th c minh ho di y:

Hnh 2.9: ng-ten Parabol

Trong :

D: ng knh anten [m]

d: Bsu lng cho, c tnh t tm n mt ming cho [m]

F: Tiu c ca cho, c tnh t tm cho n tiu im F ca n.


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2.5.2 Bi u b c x

Phn chnh ca nng lng c t p trung bp sng chnh nhng mt phn nng l ng s b bc x theo cc bp sng ph, iu ny dn n hint ng giao thoa ti cc im nt.

Gc m 3dB ph thuc vo ng knh anten v bc sng ctnh theo biu thc sau:

Trong : Dl ng knh an ten. l b c sng.


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Hnh 2.10: Biu bc x anten Parabol


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Bng sau cho thy gc pht x theo ng knh anten (s liu ca hngAlcatel):


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CHNG 3. THIT K TUYN VIBA

3.1. Cu trc h thng viba s

3.1.1. Cc thnh ph n c bn

a). S khi ca tuyn pht

Hnh 3.1: S khi ca bpht viba

Nhim v: - Bin i tn hiu bng gc thnh tn hiu dng sng - Chuyn i tn hiu ln bng tn cng tc- Khuch i tn hiu, hn ch ph tn hiu v bc x qua an ten.


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b). S khi tuyn thu

Hnh 3.2: S khi ca b thu viba

- Thu nhnv chuyn i tn hiu thu c v trung tn - Chuyn i thnh tn hiu bng gc - Khi phc xung clock


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d). S khi trm trung gian

My thu v my pht thc hin chuyn i tn s, tn hiu cao tn cchuyn i v tns trung tn my thu. Tn hiu c gii iu ch chuyn tn hiu thu c thnh tn hiu s bng gc sau c a n khiti to xung nhm gt b tp m tch lu. Tn hiu u ra ca khi ti to xungc a c a vo khi iu ch chuyn tn hiu bng gc thnh tnhiu dng sng. Ti my pht s thc hin vic chuyn i tn hiu trung tnthnh tn hiu cao tn v bc x ra anten. y l s trm trung gian phbin s dng cho nhng h thng c dung lng trung bnh v cao.

Hnh 3.4: My thu pht i tn vi b ti to tn hiu trung tnMy thu thc hin chuyn i tn s cao tn thnh tn hiu trung tn v giiiu ch nhmthu c tn hiu tn hiu s bng gc. Timy pht thc hiniu ch trc tip ti cao tnv pht ra an ten. S trm trung gian c thhin trn hnh v. thng c p dng cho nhng h thng dung lng nhv lm vic tn s cao hn.


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Hnh 3.5: My thu i tn vi b ti to tn hiu Ti trm trung gian khng thc hin vic ti to xung, gii iu ch v iuch. My thuthc hin vic chuyn i tn hiu v trung tn v chuyn sangmy pht. Ti my pht li chuyn i tn s tn hiu trung tn ln tn s tns cao tn tng ng v i phn hoch tn s. vi vic s dng trm trung giantheo s hnh v . h thng thng tin cho php s dng chung knh d phngvi h thng v tuyn tng t v ct gim nhng thit b cnthit.

Hnh 3.6: Trm trung gian dch chuyn tn s, khng c b ti to tn hiu


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Trm trung gian khng thc hin ti to tn hiu v dch chuyn tn s. Vbn cht trmtrung gian nh hnh v. l mt b khuch i tn hiu cao tn cbng tn gii hn chomi hng truyn dn.

Hnh 3.7:Trm trung gian khng c b ti to tn


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3.1.2. B tr t n s cho tr m l p

+ K hoch 2 tn s: Ti mt trm lp (A) s dng 2 tn s sng mang cholin lc hai hng. My thu trn c hai hng cng lm vic trn tn s f1,trong khi my pht trn c hai hng cng tc trn cng tn s f2

+ K hoch 4 tn s: Ti mt trm lp (A), theo mt hng thu trn tn s f1pht trn tn s f2, theo hng ngc li thu trn tn s f3, pht trn tn s f4.

i vi phng n b tr 4 tn s th thit b trm phc tp hn do phi lmvic trn 4 tn s song b li xuyn nhiu gia cc hng thu-pht rt nh.


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3.2. L thuyt v thit k tuyn

3.1.1. Xc nh cc tham s yu c u

- Xc nh yu cu vgi tr BER: Da trn cc tiu chun p dng c th iv i mi h thng khc nhau.

- Yu cu v dung l ng h thng: c kh nng pht trin trong vng 10 n15 nm t i. Yu tny phi da vo cc c im sau:

+ S pht trin dn s.

+ c im vng.

+ T lpht trin ca cc hot ng kinh t.

+ Tc ci thin iu kin sng trong tng lai. Tuy nhin cn phi dung ho chi ph v i h thng khi lp t. C th s

dng cc h thng c dung l ng va phi. Khi cn pht trin ln h thng cdung l ng cao th c th thay bng h thng m i hon ton cn h thng cs dng cho v tr cdung l ng nh hn.


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3.1.2. L a ch n bng tn v tuy n s d ng

i v i ccng dng ca k thut viba, bng tn hot ng ca n nmtrong khong 1GHz n 15GHz. Trong cc tn s v tuyn c c p phtcho cc dch v xc nh c quy nh b i CCIR (Consultative Committeeon International Radio).

3.1.3. K hoch t n s v tuy n

V mc khc bit v tn hiu v tuyn gia ng vo v ng ra ca mttr m lp thay i t 60 n 80 dB nn vic s dng cng mt tn sv tuyngia ng ra v ng vo s gy ra hin tng giao thoa ng do phn hi.Trong viba chuyn tip, ta th ng s dng k hoch 2 tn shoc k hoch 4

tn s.- K hoch 2 tn s: Ti mt tr m l p s dng 2 tn s sng mang cho

lin lc hai h ng. My thu trn c 2 h ng cng lm vic trn f 1 trong khi my pht trn 2 h ng cng lm vic v i tn s f 2.

- K hoch 4 tn s: Ti mt tr m theo mt h ng, thu trn tn s f1 phttrn tn s f2. Theo hng ng c li, pht trn tn s f3 thu trn tn s f4. iv i k hoch bn tn s th thit b phc tp hn khoch 2 tn s do philm vic trn 4 tn s. Tuy nhin xuyn nhiu gia hai h ng thu, pht l r tnhso v i k hoch 2 tn s.


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3.1.4. Ch n v tr v tnh ton ng truy n

(1) Kho st v tr t tr m

Kho st mt tuyn n c 2 tr m truyn dn. Tr c ht cn tin hnhcc b c sau:

- Xc nh tuyn trn bn : cn tm bn a hnh ca khu vc xy dngtr m.

- To nn cc bn vmt ct nghing ca tuyn.

T cc yu cu thc tca mt tuyn vi ba gm: v tr tr m, khong cchtr m, dung l ng truyn dn, a hnh tuyn s i qua... ta tin hnh nh duhai u cui ca tr m trn bn ca S o c xc nh chnh xc kinh, v ca mi tr m. Cc thng s to ny c s dng iu chnhcc anten mi tr m trong giai on lp t thit b. K hiu trn bn :tr m A l tr m th nht v tr m B l tr m th 2. Sau vmt mt ctnghing ca ng truyn:


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Hnh 3.8: Mt ct ngang ca mt tuyn viba

Mc d t c cong nhng n gin trong tnh ton,ng i tath ng v mt ct v i bn knh hiu dng ca tri t l K = 4/3. Phngtrnh sau cho ta xc nh ch li ca mt t:

m K

d d K r d d

h 211

21

51

41000

2

Trong : r 1 l bn knh qu t, K l h s bn knh hiu dng ca trit, d1, d2 l khong cch t tr m A v tr m B n im ang xt li camt t.


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Nh vy trn mt ct nghing ny thhin c bmt ca a hnh. Ngoi ran cng c thbiu din c c cao ca cy ci cc vt chn trn ngtruyn ni hai tr m A, B chng hn nh cc g, i, cc nh cao tng... iv i khong truyn dn di, cong ca mt t l n th cn phi tnh ton n

nng ca v tr tr m. nng c v dc cc ng thng ng nnkhng i dc theo ng bn knh xut pht t tm qu t.

(2) Tnh khong cch tia truyn pha trn vt chn

Sau khi chn c tn s lm vic cho tuyn, ta tnh min Fresnel th nht. Nu tn ti mt vt cn ra ca min Fresnel th nht th sng phn x s lm suy gim sng tr c ti p, mc suy gim tu thuc bin ca sng phn x. Do vic tnh ton i v i min Fresnel th nht i hi c tnhchnh xc vic thng tin gia hai tr m khng b nh hng ng kb isng phn x ny. Bn knh ca min Fresnel th nht (F1) c xc nhtheo cng thc sau:


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Trong : d1,d2 ln l t l khong cch t tr m A v tr m B n im bn knh min Fresnel c tnh ton, d l khong cch gia 2 tr m, f ltn sGHz.

Trong thc t, th ng gp ng truyn i qua nhng a hnh khcnhau c th chn min Fresnel th nht gy nn tn hao trn ng truyn. cc loi a hnh ny c th c vt chn hnh nm trn ng truyn v ccloi ch ng ngi khc. Hnh di y chra m hnh ca vt chn trn ng truyn dn, trong F1 l bn knh min Fresnel th nht, F lkhong h thc - l khong cch gia tia tr c ti p v mt vt chn hnh nmti im tnh ton min Fresnel th nht.


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Hnh 3.9: Thhin cc tham s trn mt ct ngang ca tuyn

Theo cc ch tiu thit k vkhong h ng truyn c khuyn ngh th cao ti thiu ca anten m bo sao cho tn hiu khng b nhiu x b ivt chn nm trong min Fresnel th nht l F = 0,577F1. Ngha l ngtr c ti p gia my thu v my pht cn mt khong h trn mt t hoc trnmt vt chn bt k t nht l vo khong 60% bn knh min Fresnel th nht t c cc iu kin truyn lan trong khng gian t do.


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(3) Tnh chn chiu cao ca thp antenna

tnh cao ca thp anten th tr c tin phi xc nh c caoca tia v tuyn truyn gia hai tr m. Trn c s ca cao tia c tnhcao ti thiu ca thp anten thu c tn hiu. Vic tnh ton cao catia v tuyn cng phi dng n s mt ct nghing ng truyn ni haitr m trong c xt n cao ca vt chn (O), cao ca cy ci (T) giatuyn v bn knh ca min Fresnel th nht (F1). Biu thc xc nh caoca tia v tuyn nh sau:

Trong : C l hsh .

Thng thng th cao ca tia B c tnh ton ti im c mt vtchn cao nht nm gia tuyn.


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Hnh 3.10:Xc nh cao tia B lm h mt vt chn

Cc cao ca cy ci v vt chn gia tuyn c xc nh t b ckho st ng truyn. Hnh di y biu din mt ct ng truyn catuyn cng v i cc vt chn gia tuyn v c xtn min Fresnel th nht.

Sau khi c c cao tuyn, ta tnh cao ca anten lm h mt vtchn nm gia tuyn (tc khng gy nhiu n ng truyn v tuyn). b c kho st nh v tr m, ta xc nh c cao ca hai v tr t tr mso v i mt n c bin tng ng l h1 v h2. Hai thng sny k t h p vi cao B ca tia nh tnh ton trn s tnh c cao ca ct an ten cn likhi bit trc cao ca mt ct an ten.


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Trong : ha1, ha2 [m] l cao ca mt trong hai anten cn c tnh.

d1, d2 [km] l khong cch t mi tr m n v tr tnh ton cao ca tiaB. Nh vy khi bit c cao ca mt an ten th c th tnh c caoca anten kia sao cho khng lm gin on tia truyn ca hai tr m. Hnh d iy minhho cch tnh ton cao ca an ten ni trn:


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Hnh 3.11: cao ca thp ng-ten


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Tuy nhin nh c p phn trc, m bo cho h thng hot ngkhng chu nh h ng ca cc yu t trong tng lai th cao an ten phi s dng mt khong d phng, ph thuc vo ng i thit k . Khi cc caoca cc anten thc tphi l har 1, har 2 do c cng v i mt lng cao

d phng l Ph1 hoc Ph2 nh sau:

(4) Tnh ton cc nhn t nh hng n ng truyn

Cng sut tn hiu truyn gia tr m pht n tr m thu b suy hao trn ng truyn. Khi pht mt cng sut Pt pha pht th bn thu s c

mt cng sut l Pt v do suy hao nn Pt > Pt. S mt mt cng sut ny docc yu tgy nhiu ng truyn; C th ta xem xt cc nhn tdi y:


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- d tr phading phng:

Do tc ng ca phadinh phng, mc tn hiu thu c c th b st iso v i mc tn hiu thu khng b pha inh, tr c khi h thng cn lm vicng. Nh vy tc ng ca pha inh l lm thay i mc ng ng thu camy thu. Do khi b nh h ng ca pha dinh phng my thu c th nhn c tn hiu r t yu t ng truyn v c th lm gin on thng tin nutr ng hp phainh mnh. Vic tnh ton mt l ng phadinh d tr l cnthit cho ng truyn v tuyn.

d tr phadinh phng Fm (dB) lin quan n mc tn hiu thu ckhng phadinh Wo (dB) v mc tn hiu thu c thc t thp W(dBm) tr clc h thng khng cn hot ng tnh theo biu thc:


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- Phadinh la chn:

Phadinh la chn ch yu nh hng n cc h thng vi ba s c dunglng trung bnh (34Mb/s) v dung l ng cao (140Mb/s).

- Tiu hao do ma:

Tiu hao do ma v phadinh l cc nh h ng truyn lan ch yu cc cctuyn v tuyn tm nhn thng trn mt t lm vic cc tn s trong di tnGHz, v chng quyt nh cc bin i tn hao truyn dn do quyt nhkhong cch l p cng v i ton b gi thnh ca mt h thng v tuynchuyn tip. Tiu hao do ma tng nhanh theo s tng ca tn s s dng, c bit v i cc tn s trn 35GHz th ng suy hao nhiu v do m bo th

khong cch l p phi nh hn 20km, ngoi ra vic gim di ca ngtruyn sgim ccnh h ng ca phadinh nhiu tia.


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3.1.5. Tnh ton cc tham s t n hao c a tuy n

Cc tham s c s dng trong tnh ton ng truyn nh: Mc suyhao trong khng gian t do, cng sut pht, ng ng thu, cc suy hao trongthit b...c vai tr quan tr ng xem xt tuyn c th hot ng c haykhng v hot ng mc tn hiu no.

- Tn hao trong khng gian t do:

Tn hao trong khng gian t do (A0) l tn hao l n nht cn phi cxem xt trc tin. y l stn hao do sng v tuyn lan truyn t tr m nyn tr m kia trong mi trng khng gian c tnh theo biu thc sau:

Trong : f l tn s sng mang tnh bng [GHz], d l di tuyn[Km].


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- Tn hao phi (feeder):

y l tn hao thit b (ng dn sng) truyn dn sng gia anten vmy pht/ my thu. Khi tnh ton suy hao ny th phi cn cvo mc suyhao chun c cho tr c b i nh cung c p thit b. Chng hn vi phi s dng loi WC 109 c mc tiu hao chun l 4,5dB/ 100m v cng v i 0,3dBsuy hao ca vng trn chuyn ti p ng dn sng th tn hao phi my pht (LTxat) v my thu (LRxat) c tnh nh sau:

Trong har 1 v har 2 l cao ca cc anten c tnh ton l ng d

phng.- Tn hao r nhnh:

Tn hao r nhnh xy ra ti b phn nhnh thu pht, tn hao ny cng c cho b i nh cung c p thit b. Mc tn hao ny th ng khong (2 -8)dB.


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- Tn hao h p th kh quyn:

Cc thnh phn trong kh quyn gy ra cc tn hao m mc ca nthay i theo iu kin th i tit, thay i theo ma, theo tn s s dng... Khitnh ton mc suy hao ny ta da theo cc ch tiu c khuyn ngh ccn c chu u. chng hn i v i h thng thit b v tuyn 18, 23 v 38GHzth mc suy hao chun Lsp0 c cho trong khuyn ngh vo khong 0,04dB/km -0,19 dB/km v 0,9 dB/m khi tn hao cho c tuyn truyn dnc xc nh l:

dBd L Loo sp sp

V i d l khong cch ca tuyn tnh bng Km.


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Phng trnh cn bng cng sut trong tnh ton ng truyn:

Trong : Pt l cng sut pht.

At: Tn hao tng = (tn hao trong khng gian t do + tn hao phi d + tn hao r nhnh + tn hao h p thkh quyn).

G: Tng cc l i = l i ca an ten A + l i ca an ten B.

Pr: Cng sut ti u vo my thu.

Pr l tham s quan tr ng khi thit k ng truyn vi ba, tham sny lmt ch tiu quyt nh xem tuyn c hot ng c hay khng khi em so

snh n v i mc ng ng thu ca my thu.


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3.1.6. Tnh ton cc tham s cht l ng c a tuy n

V cht lng ng truyn c nh gi da trn t sBER; cc t s BER khc nhau s cho mt mc ngng tng ng v cng c d tr phainh khc nhau. Cc ts BER thng c s dng trong viba s l:

63 10;10 BER tng ng v i hai mc ng ng RXa v RX b.

1. d tr fadinh ng v i RXa v RXb l FMa v FM b c tnh theo biu thc:

6

3

10

10

BERwith RX P FM

BERwith RX P FM

br b

ar a

2. Xc xu t fadinh ph ng nhi u tia (P0) l mt h s th hin kh nngxut hin fadinh nhiu tia c nh gia theo cng thc sau:

Trong : 8104,1 KQ ;

B = 1; C = 3,5 l cc tham s lin quan n iu kin truynlan vkh hu v a hnh ca sng v tuyn v cc gi tr c s dng theokhuyn ngh ca CCIR.


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3. Xc su t t n ng ng thu RXa; RX b.

Gi Pa; P b l xc sut t t i cc gi tr ngng thu tng ng RXa vRX b c tnh nh sau:

1010a FM

a P ; 1010b FM

a P

V i FMa v FM b l d tr fadinhng v i cc t s 310 BER v610 BER c tnh ton trn.

4. Khong th i gian fading

Ta v T b l cc gi tr c trng cho cc khong th i gian tn ti fadinhv cng ng v i FMa, FM b c tnh theo cng thc:

V i 5,0;5,0;3,10 222 d C ly theo khuyn ngh.


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5. Xc su t fading ph ng di hn 10 giy

P(10) v P(60) l Xc sut xut hin fadinh phng di hn 10s v 60stng ng v i cc t s BER khc nhau v c xc nh theo cng thc:

V i:

2

0

10 100,548ln ; 0,548ln ; 1 .

2.

a ba b

t t

Z Z erfc t erf t T T

erf t e dt

6. Xc su t BER v t 10 -3

Xc sut BER v t 10-3 thhin s gin on thng tin nhng trong th igian khng qu 10 giy.

103 1010a FM

oao P P P BER P


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7. Xc su t mch tr nn khng th s d ng c do fading ph ng

Pu l xc sut mch s c BER ln hn 10-3 trong khong th i gian l nhn 10 giy tc l mch tr nn khng s dng c v c tnh theo:

10 P P P P ou

8. Kh nng s d ng tuy n

Kh nng sdng tuyn c biu th bng phn trm v c xc nhtheo Pu nh sau:

9. Xc su t mch c 610 BER

Xc sut ny c tnh da theo Po v P b :

106 1010b FM

obo P P P BER P

10. Xc su t mch c 610 BER trong th i gian hn 60 giy do fading ph ng

606010 6 P P P st BER P bo


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Nh vy ton b cc tham s tnh cho ng truyn cng nh cc tham s nh gi cht l ng tuyn c s dng ng i thit k a ra cc quytnh vkh nng lm vic ca tuyn, tnh xem tuyn c cong sut cungcp cho my thu hay khng. Ngoi ra cng da vo cc tham s ny c th

hiu chnh li cng sut my pht, quyt nh dng cc bin php phn t p...3.1.7. Cc ch tiu k thu t nh gi cht l ng tuy n

Ba ch tiu ch yu nh gi cht l ng tuyn:

(1) khng s dng ng cho php (i vi ng tr c)

Km Lwith L P ucf 600%60006,0

V d: L = 30 Km%003,0%60030.06,0%60006,0 L P ucf

(2) khng s dng c ca mng ni ht

Gi tr cho php bng 0,0325% ti mi u cui.


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(3) khng s dng c (gi tr cho php) ca hnh trnhng c:0,0225%

Mc ch ca cc tnh ton ch tiu cht l ng l nhm xc nh xc sutv t cc ch tiu BER, bng cch s dng cc gi tri ca cc xc sut tm ratrong cc tnh ton ng truyn.

Cc mc tiu t l li bit BER c s dng sao cho BER khng cln hn cc gi trsau:

trong 610.1 hn %25004,0 d ca thng bt k i v i th i gian h pthnh mt pht, v i 280 Km < d < 2500 Km.

1.10-6 trong hn 0,045 % ca thng bt k i v i th i gian h p thnh 1 pht, v i d < 280km.

1.10-3 trong hn0,054.d / 2500 % ca thng bt k i v i th i gian h pthnh 1 giy, v i 280km < d < 2500km.

1.10-3 trong hn 0,006 % ca thng bt k i v i th i gian h p thnh 1giy, v i d < 280km.


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3.1.8. nh gi cht l ng tuy n, l p t thi t b a vo hot ng

y l mt bc c tin hnh sau khi tnh ton c kh nng lmvic ca tuyn v tnh xong cc tham s cn thit thit l p tuyn c nghal trn tnh ton thit k th tuyn hot ng. Tuy nhin vn cn nhiu vntn ti s tc ng ln tuyn v c th lm cho kh nng lm vic ca tuynkhng nh mong mun ca ng i thit k .

Ni chung vic nh gi cht l ng ca tuyn l da vo cc gi tr tnh c cc b c thit k trn.

Cng vic cui cng l lp t thit b a vo vn hnh. Tin hnh cnchnh anten thu c tn hiu t my pht. V y cng l lc i chiu

gia vic tnh ton gia thc tv l thuyt ph h p v i nhau hay khng bngvic o thcc tn hiu hai bn thu v pht.


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3.2. Thit k tuyn viba th c t

3.2.1. Yu c u bi ton

Tuyn viba s dng thit b c cc c tnh k thut:


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Gi s thng s tuyn cn thit k cho trc nh sau:

di tuyn: 28 Km.

Nhit trung bnh hng nm: 25oC.

Lng ma trung bnh hng nm: 150 mm/h. K = 4/3, C = 1 (chn theo khuyn ngh).

cao mt t so v i mc n c bin: Tr m A l 5m, tr m B l 12m.

cao anten: Gi thit tr m A l 30m, tr m B cn tnh ton.

cao: O + T = 12 ti v tr cch tr m A 14 Km.

Gi s a hnh bng phng.


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Yu cu tnh ton:

(1) Tnh cao thp anten ca tr m B.

(2) Vmt ct nghing ca tuyn.

(3) Tnh ton cc nhn t nh hng n tuyn.(4) Tnh cc tham s tn hao ca tuyn.

(5) Tnh cc tham scht l ng tuyn.

(6) nh gi cht l ng tuyn.


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Hnh 4.3: M hnh cc thit b MINI LINK c t ti cc tr m BTS


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4.2.2. Phn lo i thi t b u cu i

Cc sn phm MINI-LINK c hai loi l All-outdoor v Split c m t nh hnh di y:

Hnh 4.4: Thit b ngoi tr i


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Trong , MINI-LINK E Micro l mt thit b r t nhgn v tin l i cho viclp t ngoi tr i. N bao gm mt khi v tuyn ngoi tr i (RTU), mtantenna v mt h p k t ni v tuyn (RCB). Thit u cui ny c cc giaodin h tr lu l ng 2 Mbit/s hoc 2x2 Mbit/s, c kh nng ng dng trong

mng in thoi di ng, truy nhp dnh cho thng mi, PBX (Tng inhnh), v cc mng d liu khc v i cc thit b truyn thng c lp t bn trong nh hay ngoi tr i. Ngoi ra n cng c s dng nh l mt thit b k t cui, b l p hay mt tr m nhiu u cui.


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Hnh 4.5: K t ni terminal t i hp u ni v tuyn RCB


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Cc thit b u cui phn chia ca MINI-LINK E c th c tch h p trongmt module truy nh p chung. Kh nng ny to ra s tng thch cao cho cctr m cng nh to ra s chia s ti nguyn hiu qu gia cc thit b u cuiviba, nh cc bghp knh, cc giao din knh dch v v cc h thng h

tr . Loi thit b ny cung c p kh nng dung l ng ln t i 17x2 (34+2)Mbit/s.

Hnh 4.6: u ni v i module truy nh p


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Mt module truy nh p bao gm mt khi v mt t p cc khe c p khc nhau.Cc khi module truy nh p cho ccng dng khc nhau c thit k theotiu cc tiu chun c th l:

- Khi module truy nh p (AMM) 1U cho cc thit b u cui viba

- Khi module truy nh p (AMM) 2U-3 cho cc v tr u cui kp hocn, cha ng ln ti 4 n vkhe cm (plug-in).

- Khi module truy nh p 4U cho cc v tr nhiu u cui, phc t p, chang ln ti 7 n vkhe cm.

Ngoi ra phn thit b pha trong nh c th c nng cp hay c cuhnh vi cc n vtr khe cm c cung c p.

K t ni gia phn thit b bn ngoi (outdoor) v phn thit b phatrong nh l mt cable ng tr c mang lu l ng c hai h ng.


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Hnh 4.9: Mt tr c ca AMM 4U


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4.2.4. Cc c u hnh c a thi t b u cu i viba lo i Split

Cc thnh phn ca h thng bao gm:

- Mt khi v tuyn (RAU)

- Mt antenna- Mt khi module truy nh p (AMM 1U)

- Mt khi modem (MMU)

- Mt cp ng tr c cho k t ni lin k t (interconnection)

Kh nng lu l ng ca thit b ny l 8x2, 17x2 v 4x8+2.

Khi truy nh p dch v (SAU) cng c thm vo ti khi module truynhp (AMM) cung c p kh nng bo ng v cc giao din iu khin,cc knh dch v v ccng dng ring ca khch hng.


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Mt tr c ca MMU 34+2

Hnh 4.10: Cc thit b trong nh


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S cu hnh 1+1 (d phng nng):


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Hnh 4.11: Cu hnh tr m tch h p nhiu thit b u cui


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Cu hnh tr m l p (repeater):

Hnh 4.12: Thnh phn tr m l p


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Hnh 4.13: M hnh qun l mng viba.


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4.2.6. chc nng ca thi t b viba

MINI-LINK E Micro l mt thit b viba c khi v tuyn all-outdoor hotng bng tn 23 v 38 GHz, v i kh nng lu l ng l 2 hoc 2x2 Mbit/s.Trong loi 2x2 Mbit/s dnh cho bng tn 38 GHz.

Hnh 4.14:S khi ca khi truy nh p v tuyn (RAU)


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Ti liu tham kho:

[1] Viba s T1,2. Nxb: KHKT

[2] V tuyn chuyn tip. Nxb:KHKT 1995

[3] L thuyt vin thng. Nxb: KHKT 1997

[4] Telecmmunication Transmission Systems. Robert G. Winch

[5] General on Microwave Links, Alcatel, 1998

[6] Fundamentals of Telecommunications. John Wiley & Sons. ISBNs: 0-471-29699-6

viba so trung

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