- 1. TRNG I HC CNG NGH THNG TIN V TRUYN THNGKHOA CNG NGH IN T V
TRUYN THNGThS. HONG QUANG TRUNGK 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.2 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 tip3 4. 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 viba4 5. 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.5 6. Hnh 1.3. S khi chc nng ca h thng
viba tm nhn thng6 7. 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.7 8. 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.8 9. 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).9 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.10 11. 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.11 12. 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.12 13. 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.13 14.
2500 Km1264 Kbit/s34 64 Kbit/s567864 Kbit/s9 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)14 15. 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:15 16. 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 vi 25000 km ca nhng khong thi 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 hn 10 i vi HRDP 2500 km. (ch tiu cc
giy li). -3V d i vi HRDP 2500 km s s dng nh sau: -61. t hn 0.4% cho
cc khong thi gian 1 pht l c t l li bit xu hn10 . 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 10-3.16 17. 1.7. Cu hnh mng viba Thng cc mng vi ba s c ni cng vi
cc trm chuyn mch nh l mt b phn ca mng trung k quc gia hoc trung k
ring, hoc l ni cc tuyn nhnh xut pht t trung tm thu thp thng tin khc
nhau n trm chnh. (ng dng trong cc trung tm chuyn mch hoc t chc cc
mng Internet). 1.7.1 Viba s im ni im Mng vi ba s im ni im hin nay c
s dng ph bin. Trong cc mng ng di thng dng cp si quang cn cc mng quy
m nh hn nh t tnh n cc huyn hoc cc ngnh kinh t khc ngi ta thng s dng
cu hnh vi ba s im-im dung lng trung bnh hoc cao nhm tho mn nhu cu
ca cc thng tin v c bit l dch v truyn s liu. Ngoi ra, trong mt s
trng hp, viba dung lng thp l gii php hp dn cung cp trung k cho cc
mng ni ht, mng thng tin di ng.17 18. 1.7.2 Vi ba s a im Mng vi ba s
ny tr thnh ph bin trong mt s vng ngoi v nng thn. Mng bao gm mt trm
trung tm pht thng tin trn mt an ten ng hng phc v cho mt s trm ngoi
vi bao quanh. Nu cc trm ngoi vi ny nm trong phm vi (bn knh) truyn
dn cho php th khng cn dng cc trm lp, nu khong cch xa hn th s s dng
cc trm lp a tn hiu n cc trm ngoi vi. T y thng tin s c truyn n cc
thu bao. Thit b vi ba trm ngoi vi c th t ngoi tri, trn ct.v.v... mi
trm ngoi vi 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 v lp t Chu u cung cp cc dch v s liu
(Kbit/s) Internet trong mng ni ht khong cch 10Km. Trm trung tm pht
tc bit khong 8,2Mb/s v a ch mi trm li s dng k thut TDMA.18 19. Hnh
1.5: Cu hnh viba im-im, im-a im19 20. 1.8. Cc k thut iu ch s dng
trong viba s 1.8.1. iu ch v gii iu ch s1.8.2. Cc phng thc iu ch s
trong viba Trong viba s ch yu s dng cc phng thc iu ch M-PSK v M-QAM
a mc. 1.9. Cc m truyn dn s dng trong viba s Trong viba s ch yu s
dng cc m HDB3 v CMI.20 21. CHNG 2. TRUYN SNG VIBA S 2.1. C bn v sng
v tuyn Sng v tuyn l sng in t c tn s t 30KHz n 300GHz v c chia ra cc
bng tn LF, HF, VHF, UHF v bng tn cao dng cho thng tin v tinh. C hai
loi sng v tuyn l sng dc v sng ngang. Sng dc l sng lan truyn theo
phng chuyn ng ca n (tiu biu nh sng m thanh lan truyn trong khng kh)
cn sng ngang l sng in t c vect cng in trng v t trng vung gc vi nhau
v vung gc vi phng truyn sng. Cc sng v tuyn c th c truyn t anten pht
n anten thu bng hai ng chnh: bng sng b mt v sng khng gian.21 22.
2.1.1. Sng b mt Khi sng v tuyn lan truyn dc theo b mt tri t, th nng
lng truyn dn b tiu hao. Mc tiu hao ny ph thuc vo hng s in dn v in
mi hiu dng ca t. tng t nh khi sng i dc theo ng dy. Khi tn s sng trn
30MHz t c tc dng nh mt dy dn km gy tiu hao ln. Do , trong thc t khi
truyn sng trn mt t ngi ta thng chn sng c tn s thp. 2.1.2. Sng khng
gian Sng khng gian l mt loi sng quan trng trong thng tin VHF,UHF v
SHF. Nng lng truyn ca sng khng gian t anten pht n anten thu theo ba
ng truyn tng ng vi sng trc tip, sng phn x t mt t v sng phn x t tng
i lu.22 23. Bu kh quyn chia ra lm 3 tng: + Tng i luu: l lp kh quyn
t mt t ln n cao khong (10 15)km. Cng ln cao mt phn t kh cng gim, lm
thay i phng truyn ca cc tia sng. Tng ny thch hp cho vic truyn sng
ngn. + Tng bnh lu: l lp kh quyn nm trong min t tng i lu ln n cao
khong 60km, tng ny c mt phn t kh thp, chit sut kh c tc dng lm khc x
tia sng, i phng truyn, lm cho cc tia sng pht t mt t ln tng bnh lu s
b i phng truyn quay v mt t. Do vy rt thch hp cho vic truyn sng cc
ngn. + Tng in ly: l tng kh quyn cao nm t cao (60 - 2000)km, min ny
hp th nhiu tia t ngoi c nng lng ln, cc tia ny c tc dng phn ly cc
phn t kh tr thnh cc ion t do, tng ny mt phn t kh gim thp. Khi tia
sng c pht ln gn tng in ly th cng b phn x b cong v quay tr li mt t
do vy rt thch hp cho vic truyn sng ngn.23 24. - Sng trc tip: L sng
truyn trc tip t anten pht n anten thu khng b phn x trn ng truyn.
Trong iu kin truyn lan bnh thng, n c bin ln 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 xut hin 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 ca sng trc tip, lm tn hiu thu khng n
nh. Nu hiu khong cch ng truyn ca tia phn x v tia trc tip bng s l ln
na bc sng th anten thu sng phn x lch pha vi sng trc tip mt gc 1800
v kt qu lm suy gim tn hiu sng trc 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
theo cao so vi mt t, nn sng c th b phn x, tu theo gc sng ti c th xy
ra phn x ton phn t tng i lu. Trong trng hp ny xut hin mt bin gii c
tc dng ging nh mt b mt phn x, gi sng tr li mt t. Mt s tia ny s n an
ten thu, c th lm suy gim sng trc tip do s thay i pha v bin gy ra.
Sng truyn theo tng i lu c th lan rng n 10 dm (khong 15km).24 25.
2.2. Cc yu t nh hng n s truyn lan ca sng v tuyn 2.2.1. Suy hao khi
truyn lan trong khng gian t do Khong khng m trong cc sng truyn lan
b suy hao c gi l khng gian t do. Mc suy hao ca sng v tuyn c pht i t
anten pht n anten thu trong khng gian t do t l thun vi khong cch
gia hai anten v t l nghch vi di bc sng. Suy hao ny gi l suy hao
truyn lan trong khng gian t do, c tnh nh sau:ln lt l khong cch
truyn dn v bc sng ca sng v tuyn.25 26. 2.2.2. nh hng ca pha inh v
ma Pha inh c nh ngha l s thay i cng tn hiu sng mang cao tn thu c do
s thay i kh quyn v phn x t, nc trong ng truyn sng. Thc t cho thy nh
hng do ma v pha inh nhiu tia l nhng nh hng lan truyn ch yu i vi cc
tuyn v tuyn tm nhn thng trn mt t lm vic trong di tn GHz. V chng
quyt nh cc tn hao truyn dn v do quyt nh khong cch lp cng vi ton b
gi thnh ca mt h v tuyn chuyn tip. Pha inh nhiu tia tng khi di ca
tuyn tng tuy nhin n khng ph thuc nhiu vo tn s. Cn tiu hao do ma tng
ln khi tn s tng. Chng hn, i vi cc tuyn s dng tn s trn 35GHz thng
suy hao do ma ln do m bo cht lng tn hiu truyn dn th cc khong cch lp
thng chn di 20km, ngoi ra vic gim di ng truyn s lm gim cc nh hng ca
pha dinh nhiu tia. Vy i vi cc ng truyn di v c tn s hot ng thp th
pha inh nhiu tia l nh hng chnh. Cn i vi cc tuyn ngn v c tn s hot ng
cao hn th tiu hao do ma l nh hng ch yu.26 27. Bng 2.1 Kt qu thc
nghim v suy hao do hi nc - kh hu theo tn s sng v tuyn ca
Alcatel.Cng mc d tr phadinh 40dB, mt ng truyn vi ba di tn 38GHz s b
mt i hon ton do bo ln, trong khi tuyn vi ba lm vic tn s 6GHz vn tip
tc hot ng bnh thng. 2.2.3. S can nhiu ca sng v tuyn Thng thng nhiu
xy ra khi c thnh phn can nhiu bn ngoi trn ln vo sng thng tin. Sng
can nhiu c th trng hoc khng trng tn s vi sng thng tin. Chng hn h
thng Vi ba s ang s dng b nh hng bi s can nhiu t cc h thng vi ba s
ln cn nm trong cng khu vc, c tn s sng v tuyn trng hoc gn bng tn s
ca h thng ny, ngoi ra n cn b nh hng bi cc trm mt t ca cc h thng
thng tin v tinh ln cn.27 28. 2.3. Pha inh v cch phng chng Pha dinh
l s bin i cng tn hiu sng mang cao tn ti anten thu 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. S bin i ny l yu t xu i vi 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 vi cc tn s trong di). - Pha inh la chn tn s: lm thay i
tn hiu sng mang vi mc thay i ph thuc vo tn s, pha inh ny nh hng ln
n tuyn vi ba s dung lng cao. Hai loi pha inh ny c th xut hin c lp
hoc ng thi v vy dn n lm gin on thng tin. S thay i tn hiu ti anten
thu do phn x nhiu tia gi l pha inh nhiu tia.28 29. 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 ch yu so vi pha inh do phn x t tng i
lu. c bit vi cc ng truyn ngn th phn x mt t lm cho cc tn hiu thu
thng ging ngu nhin do cc iu kin kh tng 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 v m t, m ly,... th cc mc tn hiu phn x nh hn 10dB so vi mc tn
hiu ca ng truyn trc tip. Nu trong trng hp tuyn v tuyn i qua a hnh c
sng m bao ph c th c s phn x ton phn.29 30. 2.3.2. Cc k thut gim nh
hng ca pha dinh nhiu tia Cc k thut c s dng gim cc nh hng ca pha
dinh phng v pha inh la chn tn s nhiu tia l dng phn tp khng gian v
phn tp tn s nng cao cht lng ca tn hiu thu. Phn tp theo khng gian
cng vi cc anten t cch nhau theo chiu dc kt hp cc b kh giao thoa phn
cc giao nhau. Hiu qu ca k thut ny m bo khng lm gin on thng tin,
thng c biu th bng mt h s nng cao. Nh p dng k thut phn tp khng gian
v phn tp tn s thi gian gin on thng tin gim nh so vi thi gian yu cu
h thng t c ch tiu cht lng ra.30 31. 2.3.2.1 Phn tp theo khng gian
nh ngha: Phn tp theo khng gian l k thut thu hoc pht mt tn hiu trn 2
anten (hoc nhiu hn 2 anten) vi cng mt tn s v tuyn f. Khong cch cc
anten ca my pht v my thu c chn sao cho cc tn hiu ring bit c thu
khng tng quan nhau tng ng vi h s tng quan bng "0". Trong thc t khng
bao gi t c gi tr bng "0" ny. Trong h thng thng tin tm nhn thng ngi
ta a ra mt cng thc bn kinh nghim biu th h s tng quan khng gian theo
khong cch trc ng:Vi s: khong cch gia 2 tm ca anten [m] f: Tn s sng
v tuyn [GHz] d: khong cch truyn dn [km] Trong biu thc ny, ta b qua
sng phn x t.31 32. Theo khuyn ngh 376-4 ca CCIR, ngi ta chn khong
cch gia cc anten sao cho h s tng quan khng gian khng vt qu 0,6. Do
c th s dng h s ny lm ngng cho vic s dng phn tp. Kh nng ci thin tn
hiu thu do s dng mt cp anten c xc nh bng li phn tp IosTrong s:
khong cch gia 2 tm ca 2 anten [m] f: Tn s sng mang v" tuyn [GHz]
ar: H s khuch i in p tng i ca anten phn tp so vi anten chnh:Ad: l h
s khuch i cng sut anten phn tp [dB] Am: l h s khuch i cng sut anten
chnh [dB] d: di ca tuyn truyn dn [Km] Fm: d tr pha dinh phng32 33.
Bng s m phng nhiu ln tm c v tr tt nht cho hai anten, khi khng th
tnh c v tr, th khong cch hai anten phi ln hn 150. Thng thng cng thc
trn tnh gn ng cho mt tuyn c chiu di (20 - 70)Km v tn s
(2-11)GHz.Hnh 2.1: Phn tp theo khng gian33 34. 2.3.2.2. Phn tp theo
tn s nh ngha: phn tp theo tn s l k thut thu hoc pht mt tn hiu trn
hai knh (hoc nhiu hn hai knh) tn s sng v tuyn. H s ci thin phn tp
tn s c 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 vi cc gi tr tham s sau:Mc d cc h thng thng
tin v tuyn s phn tp theo tn s c th cho cc h s ci thin tt hn nhng
vic s dng ph tn khng t hiu qu cao.34 35. Ngoi ra tng hiu qu chng
pha inh ngi ta s dng kt hp phn tp khng gian v tn s.Hnh 2.2: Phn tp
khng gian v tn s s dng 3 anten 2.3.2.3. Chuyn mch bo 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 tng s c thit b chnh. Ngoi ra cng c th t c li ch khc
khi thit b bo v chng li s gin on thng tin do pha dinh la chn tn s
gy ra bng cch chuyn sang h thng d 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 lng v kh
nng sn sng ca h thng vi ba s c th nng cao nh s dng mt hay 2 knh d
phng thay th cc knh b s c nh thit b chuyn mch t ng. Thng thng khi s
knh truyn dn nh hn hoc bng 7(n 7) th dng mt knh d phng, tng ng vi
cu hnh (n+1). Trong thc t dng cu hnh (1+1) gm mt knh truyn dn v mt
knh d phng nng HSB (Hot Standby), c th hot ng cao tn RF hoc trung
tn IF.35 36. Hnh di m t mt tuyn vi ba s c chuyn mch bo v bng knh d
phngHnh 2.3: M hnh tuyn c d phng36 37. Chuyn mch c thc hin khi my
pht b s c hoc l khi c s la chn my 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 + 137 38.
2.4. Nguyn l c bn truyn sng viba Sng viba l sng c chiu di bc sng nm
trong khong 1m - 1cm. K thut truyn sng viba va lin quan n tnh cht
vt l li va mang tnh thng k. Suy hao khng gian t do khng ch ph thuc
vo khong cch tuyn m cn phc thuc c vo tn s sng viba. V vy, vi mt tn
s cho trc th tn hao khng gian t do c coi l hng s i vi v tr c nh ca
anten pht v thu. Hnh di y cho thy mi quan h gia li v mt mt cng sut
tn hiu trn mt tuyn viba:38 39. Hnh 2.5: Lc quan h gia tng ch v tn
hao39 40. Ti mi thi im nht nh, cc iu kin p sut dn ti fading l khng
th xc nh mt cch chnh xc do chng ta khng th bit chc chn mc tn hiu
thu s thay i nh th no theo thi gian. Nh cc phng php thng k ton hc,
chng ta c th quan st d on c cc kh nng xy ra s c v mc nh hng ca
fading.40 41. Truyn sng viba b chi phi bi cc nguyn l: phn x
(reflection), khc x (refraction) v nhiu x (diffraction). iu ny dn
ti hai vn cn xem xt: (1) S khc x (refraction) v phn x (reflection)
ca sng v tuyn.Hnh 2.6: Tia khc x v tia phn x41 42. i qua mt khong
cch di (t anten pht n anten thu), cc tia sng viba c th di theo cc
ng khc khyu (do khc x lin tip qua cc lp kh quyn theo mt phng thng
ng). S ln khc x v ng i khc khyu ca ca cc tia sng l ph thuc vo cc lp
(gradient) p sut kh quyn v tn s ca sng viba. Kt qu l tia sng c th
khng i theo ng thng. Cc gradient khc x l thay i chm theo thi gian,
phc thuc vo tng ngy, ma, a l v kh hu. Chnh v th m cc ng khc khyu ca
cc tia sng t my pht n my thu cng c thay i mt cch tng ng.Ngoi ra, s
khc x ca cc tia sng viba cn dn n fading a ng.42 43. S phn x mt t ca
cc tia sng ti nhng im gia ca tuyn gy ra fading a ng ti my thu v dn
ti mo tn hiu. Cn s phn x khuch tn, s tn x do a hnh g gh s lm cho cc
tia sng i theo cc hng no mt cch ngu nhin nn cc tia ny khng nh hng
nhiu ti 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 s khc x do p sut kh quyn c mi lin h vi
gi tr trung bnh ca bn knh tri t. Nu gi a e l bn knh hiu dng ca tri
t th ta c: 1 1 1 ae a rTrong : a - bn knh thc ca tri t (6370 Km). r
- bn knh cong ca tia sng.43 44. gi s ae K .a , khi K ae c gi l h s
bn knh hiu dng ca atri t. Nu gi N l khc x, h l cao ca tia sng so vi
mt t th kt qu tnh ton cho ta biu thc: K157 dN 157 dhNh vy h s K 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 hng ca h s K khc nhau n tia sng
viba.Hnh 2.7: Minh ha cong ca tia sng theo h s K44 45. (2) S nhiu x
(diffraction) Xt trn quan im vt l th chm sng viba c b rng l mt ct
ngang ly tia truyn trc tip lm trc gia. Ga tr o b rng ca chm l i
Fresnel th nht - Mt ellipsoid cha hu ht cng sut tn hiu t my pht n
my thu.i vi 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 nhiu ti cc tia
truyn trc tip ti anten thu. Cn t b mt bn ngoi ca i Fresnel th nht
tr ra th cc vt chn hay a hnh bt u gy ra tn hao do nhiu 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 c nh hng t ti vic gy tiu hao nhiu x do
cng sut tn hiu trong cc vng ny l khng ng k.45 46. Bn knh i Fresnel
th nht cho bi: F1 17,32. d1 .d 2 f .d 12md1 , d 2 , d tnh theo Km.
f l tn s tnh theo GHz.i vi cc a hnh khc nhau c th lm chn min
Fresnel th nht gy ra cc tn hao nhiu x khc nhau.Hnh 2.8: Min
Fresnel46 47. 2.5. Cc ch tiu k thut ca vi ba s 2.5.1. Phn b tn s
lung cao tn Tn s lung cao tn y l tn s thu pht ca thit b v tuyn, vic
la chn phng n phn b tn s ph thuc vo: - Phng thc iu ch s. - Cch sp
xp cc lung cao tn. - c tnh ca mi trng truyn sng. Theo khuyn ngh ca
ca CCITT v vi ba s th di tn lm vic nn chn t 2GHz n 23GHz. Nu sng
mang gia cc lung cao tn khng c phn chia ng th c s can nhiu gia chng
v tp m s tng ln. Cc lung ln cn nn cch nhau 29 n 40 MHz v phn cc trc
giao.47 48. 2.5.2. Cng sut pht Cng sut pht cng ging nh vi ba tng t,
ph thuc vo c ly v nhy my thu m bo t s li bit cho php. n v cng sut
pht tnh bng dBm. P0 = 1mw.2.5.3. nhy my thu hay ngng thu L mc tn
hiu cao tn ti thiu n u vo my thu n hot ng bnh thng, ngha l tho mn t
s li bit (BER) cho trc tng ng vi tc bt nht nh. 2.5.4. T s bit li
BER thng tin t c tin cy cao, m bo cho thit b hot ng khng nhm li th
t s ny cng nh cng tt, bnh thng cng phi t 10 3 , vi cht lng tt hn
phi t 10 6 . Vi yu cu BER cho trc my thu phi c mt ngng thu tng
ng.48 49. 2.5.5. Phng thc iu ch v gii iu ch Thng thng trong vi ba
s, ty theo tc bit (dung lng knh) ngi ta thng dng cc phng thc 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 iu ch thc hin
ti my pht. Thng thng, trong vic gii iu ch c 2 phng php l tch sng kt
hp (Coherent), hoc tch sng khng kt hp. Tch sng kt hp i hi my thu s
khi phc li sng mang ng pha vi i pht nn cu hnh phc tp nhng cht lng
tn hiu cao hn so vi tch sng khng kt hp. 2.5.6. Tr khng vo my thu v
tr khng ra my pht Vn phi hp tr khng i vi mch cao tn rt quan trng,
cc b phn kt ni vo my pht v my thu phi phi hp c tr khng. Nu vic phi
hp tr khng tt s lm nh hng n cht lng tn hiu, cng sut pht hoc thu
khng t cc i, ngoi ra cn gy ra sng phn x, gy mt cn bng lm gim nhy my
thu. Thng thng 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 50.49 50.
2.5.7. Tc bng tn gc Tc bng gc l tc dy s liu vo/ra my thu pht v tuyn
V d: Thit b vi ba s RMD 1502/4HDB3 2*2048kb/s9470LXHDB3
4*2048kb/sMini-link HDB3 2*2048kb/s vi tr khng 75 khng cn bng.
2.5.8 Knh nghip v C cc ch tiu v iu ch, mc vo ra, t s S/N, tn s bo
gi (knh nghip v thng c iu ch FM hoc FSK). 2.5.9 Knh gim st v iu
khin t xa Cng c cc ch tiu nh knh nghip v (c th c iu ch theo phng
thc ASK ,FSK). Ngi ta s dng knh ny khai thc qun l v gim st thit
b.50 51. 2.6. Thit b ng-ten Yu cu chnh ca thit b anten cho mt h
thng v tuyn l c suy hao truyn dn nh v kinh t, hiu sut bc x anten
cao, h s khuch i ln. 2.6.1 Anten Anten l mt giao din chnh gia thit
b in v mi trng truyn sng, tu thuc vo tn s, cng ngh v cng dng. Anten
YAGI c s dng cho tn s 400MHz - 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 ph mt
lp kim loi mng mt lm ca anten. Khi tn s nh hn 4GHz b phn phn x c th
c ch to bng vic ph kim loi trn cc thanh mng lm gim trng lng anten v
lm cho gi lt xuyn.51 52. 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: B su
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.52 53. Mi lin h gia tiu c, b su lng cho v ng
knh cho c biu in theo theo biu thc:Khi pha ca ngun s cp t ngay tm F
ca Parabol th cc sng bc x u ng pha. li ca anten parabol c tnh theo
biu thc:Trong : S: Din tch (tit din) b mt anten [m2] : Hiu sut ca
anten t (0,5 - 0,7).53 54. li ca anten theo hiu sut v tn s (s liu
ca hng Alcatel):S bin i ca hnh dng anten parabol hoc sai lch tiu c
u c th dn n suy gim tr s li ca n. Cc anten c th c dng pht hoc thu
nhn sng theo mt hoc 2 phn cc (phn cc ng hoc phn cc ngang).54 55.
2.5.2 Biu bc x Phn chnh ca nng lng c tp trung bp sng chnh nhng mt
phn nng lng s b bc x theo cc bp sng ph, iu ny dn n hin tng giao
thoa ti cc im nt. Gc m 3dB ph thuc vo ng knh anten v bc sng c tnh
theo biu thc sau:Trong : D l ng knh an ten. l bc sng.55 56. Hnh
2.10: Biu bc x anten Parabol56 57. Bng sau cho thy gc pht x theo ng
knh anten (s liu ca hng Alcatel):57 58. CHNG 3. THIT K TUYN VIBA
3.1. Cu trc h thng viba s 3.1.1. Cc thnh phn c bn a). S khi ca tuyn
phtHnh 3.1: S khi ca b pht 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.58 59. b). S khi tuyn thuHnh 3.2:
S khi ca b thu viba - Thu nhn v chuyn i tn hiu thu c v trung tn -
Chuyn i thnh tn hiu bng gc - Khi phc xung clock59 60. c). S khi ca
mt trm u cui thc tHnh 3.3: S khi ca mt trm u cui60 61. d). S khi
trm trung gian My thu v my pht thc hin chuyn i tn s, tn hiu cao tn
c chuyn i v tn s 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 khi ti to xung nhm gt b tp m
tch lu. Tn hiu u ra ca khi ti to xung c a c a vo khi iu ch chuyn tn
hiu bng gc thnh tn hiu dng sng. Ti my pht s thc hin vic chuyn i tn
hiu trung tn thnh tn hiu cao tn v bc x ra anten. y l s trm trung
gian ph bin 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 tn My thu thc hin
chuyn i tn s cao tn thnh tn hiu trung tn v gii iu ch nhm thu c tn
hiu tn hiu s bng gc. Ti my pht thc hin iu ch trc tip ti cao tn v
pht ra an ten. S trm trung gian c th hin trn hnh v. thng c p dng
cho nhng h thng dung lng nh v lm vic tn s cao hn.61 62. 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 iu ch. My thu thc hin vic chuyn i tn hiu v trung
tn v chuyn sang my pht. Ti my pht li chuyn i tn s tn hiu trung tn
ln tn s tn s cao tn tng ng vi phn hoch tn s. vi vic s dng trm trung
gian theo s hnh v. h thng thng tin cho php s dng chung knh d phng
vi h thng v tuyn tng t v ct gim nhng thit b cn thit.Hnh 3.6: Trm
trung gian dch chuyn tn s, khng c b ti to tn hiu62 63. Trm trung
gian khng thc hin ti to tn hiu v dch chuyn tn s. V bn cht trm trung
gian nh hnh v. l mt b khuch i tn hiu cao tn c bng tn gii hn cho mi
hng truyn dn.Hnh 3.7: Trm trung gian khng c b ti to tn63 64. 3.1.2.
B tr tn s cho trm lp + K hoch 2 tn s: Ti mt trm lp (A) s dng 2 tn s
sng mang cho lin 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 f1 pht 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 lm vic trn 4 tn s song b
li xuyn nhiu gia cc hng thu-pht rt nh.64 65. 3.2. L thuyt v thit k
tuyn 3.1.1. Xc nh cc tham s yu cu - Xc nh yu cu v gi tr BER: Da trn
cc tiu chun p dng c th i vi mi h thng khc nhau. - Yu cu v dung lng
h thng: c kh nng pht trin trong vng 10 n 15 nm ti. Yu t ny phi da
vo cc c im sau: + S pht trin dn s. + c im vng. + T l pht trin ca cc
hot ng kinh t. + Tc ci thin iu kin sng trong tng lai. Tuy nhin cn
phi dung ho chi ph vi h thng khi lp t. C th s dng cc h thng c dung
lng va phi. Khi cn pht trin ln h thng c dung lng cao th c th thay
bng h thng mi hon ton cn h thng c s dng cho v tr c dung lng nh
hn.65 66. 3.1.2. La chn bng tn v tuyn s dng i vi cc ng dng ca k
thut viba, bng tn hot ng ca n nm trong khong 1GHz n 15GHz. Trong cc
tn s v tuyn c cp pht cho cc dch v xc nh c quy nh bi CCIR
(Consultative Committee on International Radio). 3.1.3. K hoch tn s
v tuyn V mc khc bit v tn hiu v tuyn gia ng vo v ng ra ca mt trm lp
thay i t 60 n 80 dB nn vic s dng cng mt tn s v tuyn gia ng ra v ng
vo s gy ra hin tng giao thoa ng do phn hi. Trong viba chuyn tip, ta
thng s dng k hoch 2 tn s hoc k hoch 4 tn s. - K hoch 2 tn s: Ti mt
trm lp s dng 2 tn s sng mang cho lin lc hai hng. My thu trn c 2 hng
cng lm vic trn f1 trong khi my pht trn 2 hng cng lm vic vi tn s f2.
- K hoch 4 tn s: Ti mt trm theo mt hng, thu trn tn s f1 pht trn tn
s f2. Theo hng ngc li, pht trn tn s f3 thu trn tn s f4. i vi k hoch
bn tn s th thit b phc tp hn k hoch 2 tn s do phi lm vic trn 4 tn s.
Tuy nhin xuyn nhiu gia hai hng thu, pht l rt nh so vi k hoch 2 tn
s.66 67. 3.1.4. Chn v tr v tnh ton ng truyn (1) Kho st v tr t trm
Kho st mt tuyn n c 2 trm truyn dn. Trc ht cn tin hnh cc bc sau: -
Xc nh tuyn trn bn : cn tm bn a hnh ca khu vc xy dng trm. - To nn cc
bn v mt ct nghing ca tuyn. T cc yu cu thc t ca mt tuyn vi ba gm: v
tr trm, khong cch trm, dung lng truyn dn, a hnh tuyn s i qua... ta
tin hnh nh du hai u cui ca trm trn bn ca S o c xc nh chnh xc kinh ,
v ca mi trm. Cc thng s to ny c s dng iu chnh cc anten mi trm trong
giai on lp t thit b. K hiu trn bn : trm A l trm th nht v trm B l
trm th 2. Sau v mt mt ct nghing ca ng truyn:67 68. Hnh 3.8: Mt ct
ngang ca mt tuyn viba Mc d t c cong nhng n gin trong tnh ton, ngi
ta thng v mt ct vi bn knh hiu dng ca tri t l K = 4/3. Phng trnh sau
cho ta xc nh ch li ca mt t: hd1 d 2 4 dd 1000 1 2 2r1 K 51 KmTrong
: r1 l bn knh qu t, K l h s bn knh hiu dng ca tri t, d1, d2 l khong
cch t trm A v trm B n im ang xt li ca mt t.68 69. Nh vy trn mt ct
nghing ny th hin c b mt ca a hnh. Ngoi ra n cng c th biu din c c
cao ca cy ci cc vt chn trn ng truyn ni hai trm A, B chng hn nh cc
g, i, cc nh cao tng... i vi khong truyn dn di, cong ca mt t ln th
cn phi tnh ton n nng ca v tr trm. nng c v dc cc ng thng ng nn khng
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 trc tip, mc suy gim tu thuc bin ca sng phn
x. Do vic tnh ton i vi min Fresnel th nht i hi c tnh chnh xc vic
thng tin gia hai trm khng b nh hng ng k bi sng phn x ny. Bn knh ca
min Fresnel th nht (F1) c xc nh theo cng thc sau:69 70. Trong :
d1,d2 ln lt l khong cch t trm A v trm B n im bn knh min Fresnel c
tnh ton, d l khong cch gia 2 trm, f l tn s GHz. Trong thc t, thng
gp ng truyn i qua nhng a hnh khc nhau 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 cc loi chng ngi khc. Hnh di y ch ra m hnh ca vt chn trn
ng truyn dn, trong F1 l bn knh min Fresnel th nht, F l khong h thc
- l khong cch gia tia trc tip v mt vt chn hnh nm ti im tnh ton min
Fresnel th nht.70 71. Hnh 3.9: Th hin cc tham s trn mt ct ngang ca
tuyn Theo cc ch tiu thit k v khong h ng truyn c khuyn ngh th cao ti
thiu ca anten m bo sao cho tn hiu khng b nhiu x bi vt chn nm trong
min Fresnel th nht l F = 0,577F1. Ngha l ng trc tip gia my thu v my
pht cn mt khong h trn mt t hoc trn mt 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.71 72. (3) Tnh chn chiu cao ca thp antenna tnh cao ca thp
anten th trc tin phi xc nh c cao ca tia v tuyn truyn gia hai trm.
Trn c s ca cao tia c tnh cao ti thiu ca thp anten thu c tn hiu. Vic
tnh ton cao ca tia v tuyn cng phi dng n s mt ct nghing ng truyn ni
hai trm trong c xt n cao ca vt chn (O), cao ca cy ci (T) gia tuyn v
bn knh ca min Fresnel th nht (F1). Biu thc xc nh cao ca tia v tuyn
nh sau:Trong : C l h s h. Thng thng th cao ca tia B c tnh ton ti im
c mt vt chn cao nht nm gia tuyn.72 73. 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 bc kho st
ng truyn. Hnh di y biu din mt ct ng truyn ca tuyn cng vi cc vt chn
gia tuyn v c xt n min Fresnel th nht. Sau khi c c cao tuyn, ta tnh
cao ca anten lm h mt vt chn nm gia tuyn (tc khng gy nhiu n ng truyn
v tuyn). bc kho st nh v trm, ta xc nh c cao ca hai v tr t trm so vi
mt nc bin tng ng l h1 v h2. Hai thng s ny kt hp vi cao B ca tia nh
tnh ton trn s tnh c cao ca ct an ten cn li khi bit trc cao ca mt ct
an ten.73 74. Trong : ha1, ha2 [m] l cao ca mt trong hai anten cn c
tnh. d1, d2 [km] l khong cch t mi trm n v tr tnh ton cao ca tia B.
Nh vy khi bit c cao ca mt an ten th c th tnh c cao ca anten kia sao
cho khng lm gin on tia truyn ca hai trm. Hnh di y minh ho cch tnh
ton cao ca an ten ni trn:74 75. Hnh 3.11: cao ca thp ng-ten75 76.
Tuy nhin nh cp phn trc, m bo cho h thng hot ng khng chu nh hng ca
cc yu t trong tng lai th cao an ten phi s dng mt khong d phng, ph
thuc vo ngi thit k. Khi cc cao ca cc anten thc t phi l har1, har2
do c cng vi 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 trm pht n trm 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 do
cc yu t gy nhiu ng truyn; C th ta xem xt cc nhn t di y:76 77. - d
tr phading phng: Do tc ng ca phadinh phng, mc tn hiu thu c c th b
st i so vi mc tn hiu thu khng b pha inh, trc khi h thng cn lm vic
ng. Nh vy tc ng ca pha inh l lm thay i mc ngng thu ca my thu. Do
khi b nh hng ca pha dinh phng my thu c th nhn c tn hiu rt yu t ng
truyn v c th lm gin on thng tin nu trng hp phainh mnh. Vic tnh ton
mt lng phadinh d tr l cn thit cho ng truyn v tuyn. d tr phadinh
phng Fm (dB) lin quan n mc tn hiu thu c khng phadinh Wo (dB) v mc
tn hiu thu c thc t thp W(dBm) trc lc h thng khng cn hot ng tnh theo
biu thc:77 78. - Phadinh la chn: Phadinh la chn ch yu nh hng n cc h
thng vi ba s c dung lng trung bnh (34Mb/s) v dung lng cao
(140Mb/s). - Tiu hao do ma: Tiu hao do ma v phadinh l cc nh hng
truyn lan ch yu cc cc tuyn v tuyn tm nhn thng trn mt t lm vic cc tn
s trong di tn GHz, v chng quyt nh cc bin i tn hao truyn dn do quyt
nh khong cch lp cng vi ton b gi thnh ca mt h thng v tuyn chuyn tip.
Tiu hao do ma tng nhanh theo s tng ca tn s s dng, c bit vi cc tn s
trn 35GHz thng suy hao nhiu v do m bo th khong cch lp phi nh hn
20km, ngoi ra vic gim di ca ng truyn s gim cc nh hng ca phadinh
nhiu tia.78 79. 3.1.5. Tnh ton cc tham s tn hao ca tuyn Cc tham s c
s dng trong tnh ton ng truyn nh: Mc suy hao trong khng gian t do,
cng sut pht, ngng thu, cc suy hao trong thit b...c vai tr quan trng
xem xt tuyn c th hot ng c hay khng 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 ln
nht cn phi c xem xt trc tin. y l s tn hao do sng v tuyn lan truyn t
trm ny n trm 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].79
80. - Tn hao phi (feeder): y l tn hao thit b (ng dn sng) truyn dn
sng gia anten v my pht/ my thu. Khi tnh ton suy hao ny th phi cn c
vo mc suy hao chun c cho trc bi nh cung cp thit b. Chng hn vi phi s
dng loi WC 109 c mc tiu hao chun l 4,5dB/ 100m v cng vi 0,3dB suy
hao ca vng trn chuyn tip ng dn sng th tn hao phi my pht (LTxat) v
my thu (LRxat) c tnh nh sau:Trong har1 v har2 l cao ca cc anten c
tnh ton lng d phng. - Tn hao r nhnh: Tn hao r nhnh xy ra ti b phn
nhnh thu pht, tn hao ny cng c cho bi nh cung cp thit b. Mc tn hao
ny thng khong (2 8)dB.80 81. - Tn hao hp th kh quyn: Cc thnh phn
trong kh quyn gy ra cc tn hao m mc ca n thay i theo iu kin thi tit,
thay i theo ma, theo tn s s dng... Khi tnh ton mc suy hao ny ta da
theo cc ch tiu c khuyn ngh cc nc chu u. chng hn i vi h thng thit b
v tuyn 18, 23 v 38GHz th mc suy hao chun Lsp0 c cho trong khuyn ngh
vo khong 0,04 dB/km - 0,19 dB/km v 0,9 dB/m khi tn hao cho c tuyn
truyn dn c xc nh l:Lspo Lspo ddBVi d l khong cch ca tuyn tnh bng
Km.81 82. 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 hp th kh quyn). G: Tng cc li
= li ca an ten A + li ca an ten B. Pr: Cng sut ti u vo my thu. Pr l
tham s quan trng khi thit k ng truyn vi ba, tham s ny l mt ch tiu
quyt nh xem tuyn c hot ng c hay khng khi em so snh n vi mc ngng thu
ca my thu.82 83. 3.1.6. Tnh ton cc tham s cht lng ca tuyn V cht lng
ng truyn c nh gi da trn t s BER; cc t s BER khc nhau s cho mt mc
ngng tng ng v cng c d tr pha inh khc nhau. Cc t s BER thng c s dng
trong viba s l: BER 10 3 ; 10 6 tng ng vi hai mc ngng RXa v RXb.1.
d tr fadinh ng vi RXa v RXb l FMa v FMb c tnh theo biu thc:FM a Pr
RX awithBER 10 3FM b Pr RX bwithBER 10 62. Xc xut fadinh phng nhiu
tia (P0) l mt h s th hin kh nng xut hin fadinh nhiu tia c nh gia
theo cng thc sau:Trong : KQ 1,4 10 8 ; B = 1; C = 3,5 l cc tham s
lin quan n iu kin truyn lan v kh hu v a hnh ca sng v tuyn v cc gi
tr c s dng theo khuyn ngh ca CCIR.83 84. 3. Xc sut t n ngng thu
RXa; RXb. Gi Pa; Pb l xc sut t ti cc gi tr ngng thu tng ng RXa v
RXb c tnh nh sau:Pa 10FM a 10;Pa 10FM b 10Vi FMa v FMb l d tr
fadinh ng vi cc t s BER 10 3 v BER 10 6 c tnh ton trn. 4. Khong thi
gian fading Ta v Tb l cc gi tr c trng cho cc khong thi gian tn ti
fadinh v cng ng vi FMa, FMb c tnh theo cng thc:Vi C2 10,3d ; 2 0,5;
2 0,5 ly theo khuyn ngh.84 85. 5. Xc sut fading phng di hn 10 giy
P(10) v P(60) l Xc sut xut hin fadinh phng di hn 10s v 60s tng ng
vi cc t s BER khc nhau v c xc nh theo cng thc:Vi:Z a 0,548ln 10 ; Z
b 0,548ln 10 ; erfc t 1 erf t . T T a b erf t 2te t 2dt.06. Xc sut
BER vt 10-3 Xc sut BER vt 10-3 th hin s gin on thng tin nhng trong
thi gian khng qu 10 giy.P BER 103 P P oa Po 10 FM a 1085 86. 7. Xc
sut mch tr nn khng th s dng c do fading phng Pu l xc sut mch s c
BER ln hn 10-3 trong khong thi gian ln hn 10 giy tc l mch tr nn
khng s dng c v c tnh theo: Pu Po P P108. Kh nng s dng tuyn Kh nng s
dng tuyn c biu th bng phn trm v c xc nh theo Pu nh sau:9. Xc sut
mch c BER 106 Xc sut ny c tnh da theo Po v Pb :P BER 106 P P ob Po
10 FM b 106 10. Xc sut mch c BER 10 trong thi gian hn 60 giy do
fading phngP BER 10 6 t 60s Po Pb P6086 87. Nh vy ton b cc tham s
tnh cho ng truyn cng nh cc tham s nh gi cht lng tuyn c s dng ngi
thit k a ra cc quyt nh v kh nng lm vic ca tuyn, tnh xem tuyn c cong
sut cung cp 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 tp...
3.1.7. Cc ch tiu k thut nh gi cht lng tuyn Ba ch tiu ch yu nh gi
cht lng tuyn: (1) khng s dng ng cho php (i vi ng trc)Pucf 0,06L
600% withL 600KmV d: L = 30 KmPucf 0,06L 600% 0,06.30 600% 0,003%
(2) khng s dng c ca mng ni ht Gi tr cho php bng 0,0325% ti mi u
cui.87 88. (3) khng s dng c (gi tr cho php) ca hnh trnh ngc:0,0225%
Mc ch ca cc tnh ton ch tiu cht lng l nhm xc nh xc sut vt cc ch tiu
BER, bng cch s dng cc gi tri ca cc xc sut tm ra trong cc tnh ton ng
truyn. Cc mc tiu t l li bit BER c s dng sao cho BER khng c ln hn cc
gi tr sau: 1.10 6trong hn 0,4d 2500% ca thng bt k i vi thi gian
hpthnh mt pht, vi 280 Km < d < 2500 Km. 1.10-6 trong hn 0,045
% ca thng bt k i vi thi gian hp thnh 1 pht, vi d < 280km. 1.10-3
trong hn 0,054.d / 2500 % ca thng bt k i vi thi gian hp thnh 1 giy,
vi 280km < d < 2500km. 1.10-3 trong hn 0,006 % ca thng bt k i
vi thi gian hp thnh 1 giy, vi d < 280km.88 89. Cc mc tiu c trng:
+ Cc pht suy gim cht lng: l khong thi gian 1 pht trong BER trung
bnh xu hn 10-6, ngha l trong 1 pht ny xut hin t nht 5 li (gi s
64Kbps). + Cc giy li nghim trng: l khong thi gian 1 giy, trong BER
trung bnh xu hn 10-3, ngha l c hn 64 li trong giy ny. + Cc giy li:
l khong thi gian 1 giy trong xut hin t nht mt li bit.89 90. 3.1.8.
nh gi cht lng tuyn, lp t thit b a vo hot ng y l mt bc c tin hnh sau
khi tnh ton c kh nng lm vic ca tuyn v tnh xong cc tham s cn thit
thit lp tuyn c ngha l trn tnh ton thit k th tuyn hot ng. Tuy nhin
vn cn nhiu vn tn ti s tc ng ln tuyn v c th lm cho kh nng lm vic ca
tuyn khng nh mong mun ca ngi thit k. Ni chung vic nh gi cht lng ca
tuyn l da vo cc gi tr tnh c cc bc thit k trn. Cng vic cui cng l lp
t thit b a vo vn hnh. Tin hnh cn chnh anten thu c tn hiu t my pht.
V y cng l lc i chiu gia vic tnh ton gia thc t v l thuyt ph hp vi
nhau hay khng bng vic o th cc tn hiu hai bn thu v pht.90 91. 3.2.
Thit k tuyn viba thc t 3.2.1. Yu cu bi ton Tuyn viba s dng thit b c
cc c tnh k thut:91 92. 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 vi
mc nc bin: Trm A l 5m, trm B l 12m. cao anten: Gi thit trm A l 30m,
trm B cn tnh ton. cao: O + T = 12 ti v tr cch trm A 14 Km. Gi s a
hnh bng phng.92 93. Yu cu tnh ton: (1) Tnh cao thp anten ca trm B.
(2) V mt 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 s cht lng tuyn. (6)
nh gi cht lng tuyn.93 94. CHNG 4. THIT B VIBA S V H THNG 4.1. THIT
B VIBA PASO INK 4.1.1. M hnh mng viba DHHnh 4.1: M hnh mng viba SDH
4.1.2. Tham s h thngHnh 4.2: Thit b ng-ten94 95. 95 96. 4.2. THIT
BINI-LINK4.2.1. Gii thiu MINI-LINK E v E Micro l cc thit b viba
im-im, cho php truyn dn t 2 n 34 Mbit/s, v hot ng trong gii tn t 7
GHz n 38 GHz. H thng v tuyn viba MINI-LINK c th tng thch vi nhiu
mng vin thng hin ti, ngoi ra cn c nhiu tin ch khc. Thit b ny cho
php truyn dn tin cy. Dng thit b ny to ra cc kt ni mang lu lng Nx2
Mbit/s v c thit k ch yu cho h thng truyn dn t bo v cc mng c nh.96
97. Hnh 4.3: M hnh cc thit b MINI LINK c t ti cc trm BTS97 98.
4.2.2. Phn loi thit b u cui 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 tri98
99. Trong , MINI-LINK E Micro l mt thit b rt nh gn v tin li cho vic
lp t ngoi tri. N bao gm mt khi v tuyn ngoi tri (RTU), mt antenna v
mt hp kt ni v tuyn (RCB). Thit u cui ny c cc giao din h tr lu lng 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 i nhnh), v cc mng d liu khc vi
cc thit b truyn thng c lp t bn trong nh hay ngoi tri. Ngoi ra n cng
c s dng nh l mt thit b kt cui, b lp hay mt trm nhiu u cui.99 100.
Hnh 4.5: Kt ni terminal ti hp u ni v tuyn RCB100 101. Cc thit b u
cui phn chia ca MINI-LINK E c th c tch hp trong mt module truy nhp
chung. Kh nng ny to ra s tng thch cao cho cc trm cng nh to ra s
chia s ti nguyn hiu qu gia cc thit b u cui viba, nh cc b ghp knh,
cc giao din knh dch v v cc h thng h tr. Loi thit b ny cung cp kh
nng dung lng ln ti 17x2 (34+2) Mbit/s.Hnh 4.6: u ni vi module truy
nhp101 102. 4.2.3. Lp t h thng a) Pha ngoi triHnh 4.7: Ghp ni
ng-ten102 103. Khi v tuyn v anten c th c lp t theo cc cu trc khc
nhau. Trong c th lp trc tip khi v tuyn vi anten hoc t ring r sau kt
ni bng ng dn sng ph hp. Trong c hai trng hp th anten phi c lp t sao
cho c th d dng hiu chnh hng. b) Pha trong nh (Indoor) Cc phn thit b
pha trong nh c t vo trong cc rack 19.Hnh 4.8: Gi cm dy trong nh103
104. Mt module truy nhp bao gm mt khi v mt tp cc khe cp khc nhau.
Cc khi module truy nhp cho cc ng dng khc nhau c thit k theo tiu cc
tiu chun c th l: - Khi module truy nhp (AMM) 1U cho cc thit b u cui
viba - Khi module truy nhp (AMM) 2U-3 cho cc v tr u cui kp hoc n,
cha ng ln ti 4 n v khe cm (plug-in). - Khi module truy nhp 4U cho
cc v tr nhiu u cui, phc tp, cha ng ln ti 7 n v khe cm. Ngoi ra phn
thit b pha trong nh c th c nng cp hay c cu hnh vi cc n v tr khe cm
c cung cp. Kt ni gia phn thit b bn ngoi (outdoor) v phn thit b pha
trong nh l mt cable ng trc mang lu lng c hai hng.104 105. Hnh 4.9:
Mt trc ca AMM 4U105 106. 4.2.4. Cc cu hnh ca thit b u cui viba loi
Split Cc thnh phn ca h thng bao gm: - Mt khi v tuyn (RAU) - Mt
antenna - Mt khi module truy nhp (AMM 1U) - Mt khi modem (MMU) - Mt
cp ng trc cho kt ni lin kt (interconnection) Kh nng lu lng ca thit
b ny l 8x2, 17x2 v 4x8+2. Khi truy nhp dch v (SAU) cng c thm vo ti
khi module truy nhp (AMM) cung cp kh nng bo ng v cc giao din iu
khin, cc knh dch v v cc ng dng ring ca khch hng.106 107. Mt trc ca
MMU 34+2Hnh 4.10: Cc thit b trong nh107 108. S cu hnh 1+1 (d phng
nng):108 109. Cu hnh trm nhiu u cui (Multi-Terminal): Mt vi thit b
u cui MINI-LINK E c th c tch hp trong cng khi module truy nhp
(AMM). Cc cu hnh khc, kh nng lu lng v cc tn s v tuyn c th c kt hp.
Mt trm c th c nng cp d dng bng cch b sung thm cc khi ghp ni. Trong
: - Mt AMM 2U-3 c th h tr hai u cui (khng d phng) - Mt AMM 4U c th
h tr ln n 4 thit b u cui (khng d phng) hoc hai u cui (1+1).109 110.
Hnh 4.11: Cu hnh trm tch hp nhiu thit b u cui110 111. Cu hnh trm lp
(repeater):Hnh 4.12: Thnh phn trm lp111 112. 4.2.5. Qun l mng viba
Cc chc nng bo dng to kh nng tng thch v d dng ci t v tin li trong
vic tm v sa cha li. Cc chc nng qun l mng bao gm: - Knh truyn cnh bo
- Gim st cht lng - Kim tra vng phn hi (loop-back) u cui gn v xa -
Cc knh dch v thoi v d liu - Kh nng thit lp mm nh tuyn lu lng - Kh
nng la chn mm cng sut v tn s u ra - Nng cp phn mm112 113. Hnh 4.13:
M hnh qun l mng viba.113 114. 4.2.6. chc nng ca thit b viba
MINI-LINK E Micro l mt thit b viba c khi v tuyn all-outdoor hot ng
bng tn 23 v 38 GHz, vi kh nng lu lng 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 nhp v
tuyn (RAU)114 115. 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:
0-47129699-6115
