thông tin vsat-cau 6

7/31/2019 Thng tin VSAT-cau 6

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PHN I: TNG QUAN V MNG VSAT

I. Tng quan v mng VSAT

1. Mng VSATVSAT: Very-Small-Aperture Terminal

Lch s pht trin ca ngnh thng tin v tinh tri mt thi gian di

pht trin, t s khi u n gin cho n nhng h thng phc tp, ngy cng

c hon thin v khng ngng pht trin trong tng lai.

Cng vi vic h gi thnh v kch thc, s lng trm v tinh mt t

tng ln khng ngng. Cc trm v tinh c nh, vi kch thc an-ten t 1,2mn 1,8m tr nn quen thuc vi tn gi VSAT (trm v tinh c gc m cc

tiu) c pht trin t nhng nm 1980 bi Cng ty Telcom General (M).

VSAT nh l thit b u cui ca mng vin thng (thoi, fax, Internet),

ca mng qung b (xem truyn hnh) hoc nh l thit b chuyn i lu lng

trong ni b mng VSAT.

Nu phn tch theo kha cnh v kch thc trm v tinh th trm VSATc xem l bc trung gian ca qu trnh pht trin t trm v tinh mt t c

ln n my u cui thng tin di ng v tinh.

C th xem mng VSAT l mng c nh v tinh v dch v VSAT l

dch v c nh v tinh cho php ngi s dng vi an-ten v tinh c nh c th

s dng cc loi hnh dch v vin thng, truyn thng trc tip t mng VSAT

thng qua ng truyn dn v tinh.

2. ng dng ca mng VSAT.

Mng VSAT c s dng cung cp rt nhiu loi ng dng v c

phn loi ch yu da trn hai hnh thc cung cp dch v:

+ Qung b (hay ng dng mt chiu).

+ ng dng tng tc (hay ng dng hai chiu).

ng dng qung b: Qung b l ng dng c ph bin sm nht c cung

cp bi mng VSAT (Hnh 1). thc hin truyn tn hiu hnh n ngi sdng dch v truyn hnh v tinh, cc i truyn hnh c th s dng tiu chun

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truyn hnh truyn thng l NTSC, PAL,SECAM hoc s dng tiu chuntruyn hnh s mt t DVB-S (Digital Video Broadcasting Satellite) truyntn hiu.

Hnh 1: M hnh cung cp ng dng truyn hnh qung b bng mng VSAT

ng dng tng tc: (thoi, Internet, truyn d liu, ...) l ng dng thng tin

hai chiu cung cp cho ngi s dng c thc hin thng qua mng VSAT.

Ngoi ra, mng VSAT cn c ng dng lm truyn dn trong mng vinthng v cc ng dng khc. Minh ha ng dng kt hp mng VSAT v mng

WLL cung cp dch v cho vng nng thn.

Hnh 2: M hnh cung cp ng dng tng tc

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3. Cu hnh mng VSAT.

Mng VSAT s dng v tinh a tnh c cao 35 786 km so vi b mt

tri t v tr ng truyn cho mt bc nhy khong 0.25 s (theo ngtrm mt t - v tinh - trm mt t). C ba cu hnh tiu biu ca mng VSAT:mng sao (STAR), mng li (MESH) v cu hnh kt hp c mng sao vmng li (Hnh 3).

Hnh 3: Cu hnh mng VSAT- Trong cu hnh hnh sao, mi mt trm VSAT truyn v nhn tn hiu

thng qua trm ch HUB. iu ny khng cho php cc trm VSAT c kt

ni trc tip vi nhau. Hu ht cc mng VSAT u s dng cu hnh ny v vi

tng ch an-ten ca trm ch HUB s cho php ti u phn khng gian v

gim nh kch thc an-ten ca ngi s dng. Nhc im ln nht ca cu

hnh hnh sao l tr ng truyn thng thng tin gia trm VSAT v VSAT.- Cu hnh li cho php cc trm VSAT c lin h trc tip vi nhau.

Mt trm ch HUB c thit lp iu khin cc qu trnh thit lp lin lc

nhng khng cn thit cho iu khin lu lng. Trong mt s trng hp c

th, mt trm VSAT c kt hp cng cc chc nng qun l v iu khin.

Cu hnh li i hi mi mt trm VSAT phi c cng sut ln, an-ten ln

lin lc gia cc trm VSAT. Thch hp ng dng m yu cu tr nh nh

in thoi.

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- Cu hnh hn hp cho php nhm cc trm VSAT lin lc vi cu hnh

li vi cu hnh cn li s dng cu hnh sao. Cu hnh ny ph hp vi mng

m mt s trm s dng c nhu cu v lu lng ln hn hn cc trm VSAT

khc trong mng. Cc trm c nhu cu lu lng cao c cung cp bi cu

hnh li (MESH) v gim chi ph do khng cn phi thit lp thm phn thit

b trm ch HUB v phn mng cn li vn s dng cu hnh mng hnh sao.

4. Tn s s dng

Mng VSAT thng s dng bng tn s nghip v c nh v tinh (FSS)

c quy nh bi ITU (tr trng hp mng VSAT c s dng cho cung cp

dch v pht thanh hoc truyn hnh qung b th s dng bng tn nghip vqung b v tinh BSS) l bng tn C v bng tn Ku. i vi bng tn s dng

ca mng VSAT l bng tn Ku v bng C th cc u, nhc im c tm tt:

Bngtn

u im Nhc im

CThit b sn c, t b nh hng

ca thi tit

An-ten ln (1 n 3 m) nh hngbi nhiu v tinh v nhiu mt t.

Dng chung bng tn vi nhiu hthng vi-ba

KuKch thc an-ten nh (0.6m -1.8m) Nhiu mt t thp

Thit b khng sn c nh hngnhiu bi thi tit

5. u nhc im ca mng VSAT.

- u im:

+ Loi tr c yu t a hnh, khong cch.

+ Cung cp c nhiu loi hnh dch v khc nhau, t cung cp dch v

qung b n dch v vin thng, internet; t vic s dng lm truyn dn cho

mng vin thng n s dng lm truyn dn cho thit lp mng dng ring.

+ C tnh kh chuyn cao trong thay i loi hnh dch v cung cp v cu

hnh mng.

+ C cht lng truyn dn tt (t l li bt


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- Nhc im:

+ Nhy cm vi nhiu: Do mng VSAT s dng an-ten c nh nn bp

sng ca an-ten rt ln v kh nng gy nhiu cng nh can nhiu ln.

+ Kh nng bo mt km: do vng ph sng ca v tinh rt ln; c bit

trong truyn hnh v tinh. iu ny bt buc cc nh cung cp dch v truyn

hnh v tinh (DTH) phi c phng php m ho, bo mt ph hp bo v

vic thu thng tin bt hp php.

+ Kh nng khi phc h thng khi c s c v tinh; iu ny xy ra do

mng VSAT ch s dng mt v tinh thit lp knh truyn dn n ngi s

dng dch v. Trong trng hp xy ra s c nh trn, bt buc phi chuynsang v tinh d phng v dn n ngi s dng cng phi iu chnh an-ten

n v tr ph hp.

- Kt lun:

Vi nhng u, nhc im trn, c bit l u im khng ph thuc a

hnh v vic gim gi thnh ca thit b u cui v tinh, chng ta khng ngc

nhin l trong thc t mng VSAT c pht trin rt nhanh cung cp ccdch v vin thng, qung b v l mt trong nhng gii php cho cc t chc,

c nhn s dng thit lp mng dng ring.

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PHN II: CC YU T NH HNG NCHT LNG TN HIU

I. Suy hao trong thng tin v tinh

1. Suy hao trong khng gian t do

i vi v tinh i tnh cao 35.768km, c ly thng tin cho mt tuyn

ln hay mt tuyn xung gn nht l 35.768km. Do c ly truyn sng trong

thng tin v tinh ln nh vy nn suy hao trong khng gian t do l suy hao ln

nht. Gi suy hao ny l Ltd, ta c

2

4

td

dL

=

Trong d[km] : l chiu di ca mt tuyn ln hay xung.

[m] : bc sng cng tc.

Bc sng c i ra tn s cng tc vi quan h f = c/ .

c : vn tc nh sng c = 3.108 m/s.

f : tn s cng tc [GHz].Trong n v dB ta c cng thc :

24 4 4

10 lg 10lg 20 lgtd

d f df L dB

c c

= = =

Suy hao khng gian t do ca tuyn ln hay xung khi cng tc bng C

(4/6GHz) vo khong 200dB. b vo suy hao ny, m bo cho my thu

nhn c mt tn hiu ln c -90dBm n -60dBm, ngi ta s dng anten

c ng knh ln hng chc mt c h s tng ch ln khong 60dB v

my pht c cng sut ln hng trm dn hng ngn W.

Ngoi suy hao chnh trong khng gian t do cn c cc suy hao khc tuy

khng ln nhng khi tnh ton tuyn thng tin v tinh m ta khng xt ht cc

kh nng xu nht do nh hng ca mi trng truyn sng th khi xy ra cc

hin tng cht lng thng tin s xu i v c th lm gin on thng tin.

Cc suy hao c trnh by sau y.

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2. Suy hao do tng i lu

Tng i lu l lp kh quyn nm st mt t ln n cao (10km-15km)

(theo quy nh ca tng i lu tiu chun), bao gm cc cht kh chnh hp th

sng gy ra suy hao nh hi nc, Oxy, Ozon, Cacbonic. Suy hao ny ph thuc

nhiu vo tn s v gc ngng ca anten v ch ng k khi tn s cng tc t

10GHz tr ln, ngha l khi cng tc bng Ku (14/12GHz) hay bng Ka

(30/20GHz). Anten c gc ngng cng ln th suy hao tng i lu cng nh, do

ng truyn ca sng trong tng i lu cng ngn. Ti cc tn s 21GHz v

60GHz c cc suy hao cc i, l do s cng hng hp th i vi cc

phnt hi nc v Oxy.3. Suy hao do tng in ly

Tng in ly l lp kh quyn nm cao khong 60km n 400km, do b

ion ho mnh nn lp kh quyn cao ny bao gm ch yu l cc in t t

do, cc ion m v dng nn c gi l tng in ly. S hp th sng trong

tng in ly gim khi tn s tng, tn s trn 600MHz th s hp th khng

ng k. 4. Suy hao do ma

Suy hao do cc iu kin thi tit nh my, ma, sng m, suy hao ny ph

thuc vo nhiu yu t nh cng ma hay sng m, vo tn s, vo chiu

di qung ng i ca sng trong ma, chiu di ny ph thuc vo gc ngng

anten. Khi gc ngng tng, suy hao gim, vi gc ngng anten khong 400 tr

ln th suy hao khng ng k, lc suy hao do ma khong 0,6 dB, suy hao

do sng m khong 0,2dB, cn suy hao trong cc cht kh rt nh c th b

qua. Ni chung khi tn s v cng ma tng th suy hao tng nhanh, c

bit trong khong tn s t 10GHz n 100GHz.

Suy hao thc t tu thuc vo gc ngng anten, cao t anten so vi mc

nc bin, chiu cao cn ma v sng m m on ng i thc t ca sng

qua vng l khc nhau. Suy hao trn ton b on ng c chiu di L e sng

i qua l: Ltt = .Le (dB)

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Trong : l h s suy hao trn on ng 1km (dB/km), ph thuc tn

s, mi trng gy suy hao nh cng ma hay dy ca sng m.

Le : l chiu di thc t sng i qua vng gy suy hao (km), ph

thuc gc ngng anten, cao t anten, c xc nh theo cng thc:

( )sin

m se

h hL km

E

=

Vi hm: l cao ca cn ma (km)

Hs: l cao anten trm mt t so vi mc nc bin (km).

E : l gc ngng anten ().

5. Suy hao do mt ng chnh antenKhi anten pht v thu lch nhau th s to ra suy hao v bp chnh ca anten

thu hng khng ng chm tia pht x ca anten pht.

Thng th suy sao do mt ng chnh khong 0.8 n 1 dB.

Hnh 4: Suy hao do mt ng chnh anten

6. Suy hao do lch phn ccSuy hao do phn cc khng i xng xy ra khi anten thu khng ng hng

vi s phn cc ca sng nhn. V ng truyn phn cc trn, sng pht ch

c phn cc trn trn trc anten pht v n s tr thnh elip khi ra khi trc

anten . Khi truyn qua bu kh quyn cng c th lm thay i phn cc trn

thnh phn cc elip. Cn trong ng truyn phn cc thng th sng c th b

quay mt phng phn cc ca n khi ng truyn i qua kh quyn, do

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anten thu khng cn mt phng phn cc ca sng ng v sng ti. Suy hao do

lch phn cc thng ch 0,1dB.

7. Suy hao trong my thu v my pht

Hnh 5: Suy sao trong my thu v my pht

Suy hao trong thit b pht v thu cn gi l suy hao do h thng fi, c hai

loi nh sau:- Suy hao LFTX gia my pht v anten, anten pht c cng sut PT cn

phi cung cp mt cng sut PTX u ra ca b khuch i pht, do vy:

PT = PTX - LFTX [dB]

- Suy hao LFRX gia anten v my thu, cng sut PRX u vo my thu bng:

PR = PRX LFRX [dB]

Trong cc h thng v tinh hin nay, n gin thng ly h s tn hao

fi khong 2dB

II. Tp m trong thng tin v tinh

1. Nhit tp m h thng

Trong mt tuyn thng tin v tinh, tp m gy ra cho trm mt t do nhiu

nguyn nhn v c tnh bng nhit tp m tng ng T sys v c gi l

nhit tp m h thng.

Nhit tp m h thng c xem l tng ca bn thnh phn c biu din

theo biu thc:

Trong :

Tsys: nhit tp m h thng Ts : nhit tp m bn ngoiTA : nhit tp m anten TF : nhit tp m ca h thng fiTR : nhit tp m hiu dng u vo my thu

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0

ys[ ]S A F

s R

F

T T TT K T

L

+ += +


10/17

LF: Suy hao ca h thng fia. Nhit tp m bn ngoi TS v nhit tp m anten TA

* Nhit tp m khng gian: gm cc thnh phn sau:- Nhit tp m v tr: tc ng tn s v tuyn l do bc x t v tr cn

d li (khong 2,760

K)- Nhit tp m ca di ngn h: nu hng anten vo vng c s sao cc ica di ngn h th nhit tp m c th ln n gn 1000K trong vng tn s t0,3GHz n1,2GHz.

- Nhit tp m ca mt tri: mt tri bc x ra sng in t tt c cc tns, c bit l di viba (microwave). Nhit tp m do mt tri gy ra cho trmmt t ph thuc vo hng anten, nu mt tri nm ngoi vng ph sng ca

bp chnh anten th nhit tp m di 500K. Cn khi mt tri chiu thng voanten th nhit tp m ln n 10.0000K hoc c th hn tu thuc tn s cngtc, kch thc mt phn x v s vt en ca mt tri (s vt en th hin shot ng mnh hay yu ca mt tri).

Trng hp trm mt t - v tinh - mt tri nm trn cng mt ng thngch xy ra mt vi ngy trong nm vo ma xun lm cho thng tin b gin onvi ba pht.

* Nhit tp m do kh quyn (nhit tp m do tng i lu): n ph thucvo chiu di qung ng i ca sng trong tng i lu ( cao 15 km t mtt). Ni cch khc chnh l ph thuc vo gc ngng ca anten, tn s cng tc.

* Nhit tp m t trm mt t xung quanh trm: V anten ca trm mtt hng ln bu tri nn nhit tp m ca mt t gy ra ch yu do bp ph

v bp ngc, mt phn cho bp chnh khi anten c tnh nh hng km v gcngng nh.

Ngoi ra cn nhit tp m do cc chng ngi gn nh to nh, cc miche (v d nh vm cy), do cc b phn cn tr trong anten nh cc thanh ,

b tip sng (ngun bc x s cp) v b suy hao bp ph gy ra.b. Nhit tp m h thng fi TF

Nhit tp m h thng fi c tnh bi biu thc sau:TF [

0K] = To(LF-1)Trong To: nhit mi trng.

LF: suy hao do phi .TF tng khi LF tng dn n cng sut tp m bn trong gy ra bi h thng

fi tng ln. Do cn thit k cc phng tin sao cho c h thng fi lnh nht. Mt cch gn ng c th coi nhit tp m fi l 2900K.c. Nhit tp m my thu TR

Nhit tp m my thu bng tng nhit tp m gy ra trong mi phn ca mythu. N c tnh bi cng thc sau:

Trong TR: nhit tp m my thu.

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0 321

1 1 2 1 2 1

[ ] ...

...

KR

k

TT TT K T

G G G G G G = + + + +


11/17

T1...Tk: nhit tp m u vo.G1 Gk: h s khuch i tng phn.

Nu h s khuch i tng u ln th tp m ti cc tng tip theo c thb qua. Do yu cu tng u tin phi c h s khuch i ln v tp m

thp, v vy trong thng tin v tinh dng cc b khuch i tp m thp (LNA-Low Noise Amplifier). Mt cch gn ng ta coi nhit tp m tng u nycng l nhit tp m ca my thu. 2. Cng sut tp m h thng

Cng sut tp m h thng N c biu th bi biu thc:N = k.Tsys.B [W]

Trong N: cng sut tp m trong bng thngk: hng s Boltzman

k=1.38.10-23W/Hz.0k=-228,6 (dBW/Hz.K)

B: bng thng cp cho mi sng mangTrng hp tp m tnh trong rng bng tn 1Hz th cng sut tp m sinhra gi l mt ph tp m N0 v bng : N0 = N/B (W/Hz), nhit tp m tngng s bng :

Tsys = N0/k

III. nh hng ca nhiu n ng truyn 1. Nhiu do cc trm vibaCan nhiu gia trm vi ba v trm mt t c hai trng hp:

- Trng hp th nht: ng thng tin viba mt t c cng tn s lm vic ving ln ca h thng thng tin v tinh, bi vy tn hiu viba mt t c trnvi tn hiu u vo my thu v tinh.- Trng hp th hai: ng thng tin vi ba mt t c tn s bng tn s ngxung ca h thng thng tin v tinh, bi vy u vo my thu trm mt tcng b trn vi tn hiu ca ng thng tin viba mt t .

Hnh 6:Nhiu t cc trm vi ba

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Trong thit k tuyn thc t, phi t trm mt t sao cho nhiu xy ra t nht,nhiu nh nht bng cch s dng anten c cc c tnh bp ph tt. Mc dmc tiu c bn thit k tuyn i vi v tinh thng tin l loi b nhiu, nuiu ny khng th thc hin c th trong thit k tuyn phi bao hm c

lng nhiu cho php. Nhiu thng khng th nh gi c bng tnh ton, vd nh trong thnh ph c nhiu vt cn phn x quanh ni t trm mt t. Vth phi dng phng php o thc t o nhiu. 2. Nhiu do cc v tinh bn cnh

Hnh di y cho ta thy can nhiu xy ra do cc v tinh t gn nhau. Taxem xt tn hiu can nhiu t v tinh 1 tc ng ln trm mt t 2 v tn hiucan nhiu t v tinh 2 tc ng ln trm mt t 1. Khi gc cng nh ( tngng vi 2 v tinh t cng gn nhau) th nh hng ca chng ln trm mt tcng ln.

Hnh 7: Nhiu t cc trm v tinh khc

Trong thc t, t s cng sut ca sng mang trn sng can nhiu C/N gia ccv tinh c th ln hn hoc bng 30dB (1000 ln) khi hai v tinh t cch nhaukhong 30 ngay trn qu o, ngay c nu cc anten ca chng cng chiu vocng mt v tr. 3. Nhiu xuyn iu ch

Tp m mo xuyn iu ch l mt trong nhiu loi tp m ng truyn

thng tin v tinh. Tp m xuyn iu ch trong v tinh sinh ra khi b pht pca n khuch i ng thi nhiu sng mang. Cc c tuyn phi tuyn vo raca b pht p l nguyn nhn sinh ra tp m xuyn iu ch. B khuch in sng chy TWT c s dng l thnh phn chnh ca b pht p.

Tp m xuyn iu ch sinh ra do cc sn phm xuyn iu ch hoc molt vo bng tn truyn dn khi nhiu sng mang c khuch i ng thi

bng b khuch i TWT phi tuyn. Mc xuyn iu ch ph thuc vo ssng mang v s chnh lch tn s gia chng.

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PHN III: A TRUY NHPI. Nguyn nhn, mc ch

Nguyn nhn: Do qu o v tinh a tnh ch c mt, s lng v tr b gii hn. Bng tn cho thng tin v tinh b gii hn. Chi ph phng mt v tinh ln qu o l rt ln, thi gian hot ng cng

ngn; do lm sao s dng hiu qu nht ti nguyn l mt vn .Mc ch

Dng chia s ti nguyn chung ca h thng thng tin v tinh: bngtn, b pht p.II. Cc cng ngh a truy nhp

1. a truy nhp phn chia theo tn s (FDMA)

y l phng php c s dng rng ri nht. Trong h thng ny mitrm mt t c dng ring mt tn s pht khng trng vi cc trm khc saocho khong cch tn s gia cc trm khng b chng ln ln nhau. FDMA cth s dng cho tt c cc h thng iu ch (iu ch s cng tng t).

Cc trm thu mt t mun thu c tin tc phi dng cc b lc ditng ng vi tn s cn thu.

Phng php ny cho php cc trm truyn dn lin tc m khng cniu khin nh thi ng b, thit b s dng kh n gin. Hiu qu cng sutca v tinh khng qu ti.

Nhn xt:Phng php ny thiu linh hot trong vic thay i cch phn phiknh do: cc knh truyn dn c phn chia theo tn s quy nh, khi muntng s knh bt buc phi gim nh bng thng ngha l thay i cc b lc dii vi trm thu. ng thi phng php ny tn km knh truyn.

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M hnh v nh sau:

f1 f2 f3 f4 f5 f6A -> B A-> C A->D B ->A C -> A D -> A.

2. a truy nhp phn chia theo thi gian (TDMA)

L mt h thng cc trm thu mt t dng chung mt b pht p trnc s phn chia thi gian. Trc ht phi s dng mt sng mang iu ch s.H thng ny thng nh ra mt khung thi gian gi l khung TDMA. Khungthi gian ny s chia ra lm nhiu khong tng ng vi mi trm mt t.

Mi trm s pht sng theo theo khe thi gian ca khung quy nh.ngthi gia cc khe thi gian cn mt khong thi gian trng tn hiu cc trmkhng chng nhau v thi gian ti trm pht p.

Tng t ti cc trm thu mt t, ly c tin tc cn c xc nh

ng khe thi gian ly sng mang ca chnh n.y l phng php c th sdng tt nht cng sut ca v tinh. N c th thay i s khe cng nh rng

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Pht : f1 ,f2 ,f3Thu : f4 ,f5

,f6.

Pht f4thu f1

V tinhthng tin

Pht f5thu f2

Pht f6thu f3


15/17

ca khe thi gian trong khung m khng nh hng g ti cc thit b phncng.Hnh nh khung TDMA nh sau:

3. a truy nhp phn chia theo m (CDMA)

Tt c cc trm VSAT truyn ng thi trong cng tn s, bng thng vTt c cc trm VSAT truyn ng thi trong cng tn s, bng thng vcng sut c ch nh.cng sut c ch nh.

Mt chui gi ngu nhin s m ha tn hiu gc bng vic tri tn hiuMt chui gi ngu nhin s m ha tn hiu gc bng vic tri tn hiutrn mt bng tn rng.trn mt bng tn rng.

khi phc tn hiu gc, b thu s kt hp vi cc t hp u vo vi khi phc tn hiu gc, b thu s kt hp vi cc t hp u vo vi chui m ha gc bit.chui m ha gc bit.

m bo trit nhiu th cc chui chip (m) tc cao cho cc sng m bo trit nhiu th cc chui chip (m) tc cao cho cc sng mang cn phi trc giao vi nhau.mang cn phi trc giao vi nhau.

V dV d-Khi cn gi i d liu dng nh phn (hnh a), thc hin iu ch

PSK cho tn hiu ny trc ht ngi ta m ha cc bit 0, 1 thnh m tc cao hn, sau a vo iu ch PSK nh hnh v, nh th s tri ph ca tnhiu ra c bng tn.

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Mt khe

D CA B D A

T trm chunThi gian bo v

Mt khung


16/17


17/17

III. Nhn xtBn tnh nng ca cc h thng a truy nhp cho ta s la chn thch hp nhsau:

Hthng u im Nhc im Nhn xt

FDMA - Th tc truy nhpn gin.- Cu hnh trm mtt n gin.

- Thiu linh hottrong thay i thitlp tuyn.- Hiu qu thp khis knh tng.

- D ng dng trongphn phi theo yu cuv kch hot bng tingni trong SCPC dunglng nh.

TDMA - Hiu qu s dngtuyn cao.- Linh hot trong vic

thay i thit lp tuyn.

- Yu cu ng bcm.- Cng sut trm

pht mt t cao.

- C th ng dng:SS-TDMA

CDMA - Chu c nhiu vmo.- Chu c s thay icc thng s khc nhauca ng truyn dn.- Bo mt ting nicao.

- Hiu qu s dngbng tn km.- rng bng tntruyn dn yu culn.

- Ph hp vi cc hthng c trm thu dunglng nh.

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thông tin vsat-cau 6

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