kỹ thuật viễn thông 2

8/3/2019 K thut vin thng 2

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HC VIN CNG NGH BU CHNH VIN THNG

K THUT VIN THNG

(Dng cho sinh vin ho to i hc txa)

Lu hnh ni b

H NI - 2007


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HC VIN CNG NGH BU CHNH VIN THNG

K THUT VIN THNG

Bin son : TS. NGUYN TIN BAN


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Chng 1. Csk thut truyn dn

3

CHNG 1:CSK THUT TRUYN DN1.1. K thut iu ch v ghp knh1.1.1.Cc phng php m ha v iu ch

M ha

Trong cc h thng truyn dn s thng tin c chuyn i thnh mt chui cc t hpxung, sau truyn trn ng truyn. Khi , thng tin tng t (nh ting ni ca con ngi)

phi c chuyn i vo dng s nhcc b bin i A/D. chnh xc ca chuyn i A/Dquyt nh cht lng lnh hi ca thu bao. T hp s phi chi tit sao cho ting ni (hocvideo) tng t c thc ti to m khng c mo v nhiu lon thit b thu. Hin nay, mongmun ca chng ta l gim khi lng thng tin s s dng tt hn dung lng mng.

Cc b m ho c phn lm 2 loi chnh: m ho dng sng v m ho thoi (vocoder).Ngoi ra, cn c cc b m ho lai t hp c tnh ca 2 loi trn. Hnh 1.1 minh ho s khcnhau v cht lng thoi v cc yu cu tc bit i vi cc loi m ha khc nhau.

Hnh 1.1: Cc phng php m ho v mi quan h cht lng thoi/tc bit

M ho dng sng c ngha l cc thay i bin ca tn hiu tng t (ng thoi)c m t bng mt s ca gi trc o. Sau cc gi tr ny c m ho xung v gi ti

u thu. Dng iu tng t nh tn hiu c ti to trong thit b thu nhcc gi tr nhn c.Phng php ny cho php nhn c mc cht lng thoi rt cao, v ng tn hiu nhn cl bn sao nh tht ca ng tn hiu bn pht.

1 6482 4 16 32

Cht lng thoi

Tuyt vi

Tt

Kh tt

Km

Cc b m ho thoi

Cc b m ho li

Cc b m ho dng sng

Bit Rate(Kbit/s)


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M ho thoi l b m ho tham s. Thay cho vic truyn tn hiu m t trc tip dng cang tn hiu thoi l truyn mt s tham s m tng cong tn hiu c pht ra nh thno. Cch n gin gii thch s khc nhau gia hai phng php ny l s dng php ndng: nhc ang c chi v cc bn nhc th c cc nhc cng s dng. Trong m ho dng

sng chnh nhng m thanh nhc ang chi c truyn i, cn trong m ho tham s th cc bnnhc c gi ti bn nhn. M ho tham s yu cu c mt m hnh xc nh r ng tn hiuthoi c to nh th no. Cht lng smc trung bnh (m thanh ca thoi nhn c thucloi tng hp) nhng mt khc cc tn hiu c thc truyn vi tc bit rt thp.

B m ho lai gi mt s cc tham s cng nh mt lng nht nh thng tin dng sng.Kiu m ho thoi ny a ra mt s tho hip hp l gia cht lng thoi v hiu qu m ho,v n c s dng trong cc h thng in thoi di ng ngy nay.

iu ch

iu ch l mt k thut cho php thng tin c truyn nh s thay i ca tn hiu

mang thng tin. iu chc s dng cho c thng tin s v tng t. Trong trng hp thngtin tng t l tc ng lin tc (s bin i mm). Trong trng hp thng tin s, iu ch tcng tng bc (thay i trng thi). Khi kt hp iu ch v gii iu chc gi l modem.Trong truyn dn tng t c th s dng hai phng php iu ch theo bin v theo tn s

Hnh 1.2: iu ch theo bin v theo tn s

iu bin c s dng truyn ting ni tng t (300-3400 Hz). iu tn thng cs dng cho truyn thng qung b (bng FM), knh m thanh cho TV v h thng vin thngkhng dy.

1.1.2.iu chxung m PCMHin nay c nhiu phng php chuyn tn hiu analog thnh tn hiu digital (A/D) nh

iu xung m (PCM), iu xung m vi sai (DPCM), iu ch Delta (DM), ... Trong thit b ghp

knh s thng s dng phng php ghp knh theo thi gian kt hp iu xung m (TDM -PCM).

Sng mang

Tn hiu ang iu ch

Tn hiu c iuch bin

Tn hiu c iuch theo tn s


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chuyn i tn hiu analog thnh tn hiu digital dng phng php PCM, cn thchin 3 bc nh hnh 1.3.

Hnh 1.3: Qu trnh chuyn i A/D dng phng php PCM

Trc ht phi ly mu tn hiu thoi, tc l ch truyn cc xung tn hiu ti cc thi imnht nh.

Bc th hai l lng t ho bin , ngha l chia bin ca xung mu thnh cc mcv ly trn bin xung n mc gn nht.

Bc th ba m ho xung lng t thnh t m nh phn c m bit.Ly mu tn hiu analog

Bin ca tn hiu analog l lin tc theo thi gian. Ly mu l ly bin ca tn hiuanalog tng khong thi gian nht nh. Qu trnh ny ging nhiu ch bin , trong ccdy xung c chu kc iu ch bin bi tn hiu analog. Do vy cc mu ly c s ginon theo thi gian. Dy mu ny gi l tn hiu PAM (iu ch bin xung).

thc hin qu trnh ly mu tn hiu bt k phi da vo nh l Nyquist, ni dung canh l c pht biu nh sau:

Nu tn hiu gc l hm lin tc theo thi gian c tn ph gii hn t 0 n fmax khi ly

mu th tn s ly mu phi ln hn hoc bng hai ln tn s ln nht trong tn hiu gc, ngha l:fm 2fmax.

Mt yu t quan trng trong ly mu l pha pht ly mu cho tn hiu analog theo tn sno cho pha thu ti to li c tn hiu ban u. Theo nh l Nyquist, bng cch ly mu tnhiu analog theo tn s cao hn t nht hai ln tn s cao nht ca tn hiu th c th to li tnhiu analog ban u t cc mu .

i vi tn hiu thoi hot ng bng tn 0,3 3,4 kHz, tn s ly mu l 8kHz png yu cu v cht lng truyn dn: pha thu khi phc tn hiu analog c mo trong phmvi cho php. Qu trnh ly mu tn hiu thoi nh hnh 1.4.

Ly mu

Lng t ho

M ho

t

t

t

1

0


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Hnh 1.4: Qu trnh ly mu tn hiu thoi

(a) Th hin ng cong tn hiu thoi.

(b) Dy xung iu khin hot ng b ly mu c chu k Tm = 125s.(c) Tn hiu u ra b ly mu (tn hiu iu bin xung- PAM)

Lng tho

Lng t ho ngha l chia bin ca tn hiu thnh cc khong u hoc khng u,mi khong l mt bc lng t, bin tn hiu ng vi u hoc cui mi bc lng t gil mt mc lng t. Sau khi c cc mc lng t th bin ca cc xung mu c lm trnn mc gn nht.

C hai loi lng t ho bin : lng t ho u v lng t ho khng u.

Lng tho uBin tn hiu c chia thnh nhng khong u nhau, sau ly trn cc xung mu

n mc lng t gn nht. Qu trnh lng t ho u th hin nh hnh 1.5.

Bin

0 t(a)

Bin

+1+2

+3

-1-2-3

0 t(c)

Bin

t(b)

T


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Hnh 1.5: Qu trnh lng tho u

Bc lng tu bng . Nh vy, bin ca tn hiu gm c 7 bc lng t v 8

mc (nh s t -3 +3). Mi quan h gia s mc lng t v s bc lng t nh sau:

Tng s mc lng t = Tng s bc lng t + 1.

Do phi ly trn n mc lng t gn nht, chnh lch gia bin xung lng t vgi tr tc thi ca xung ly mu s gy ra nhiu lng t Qd(xem hnh 1.6).

Bin xung nhiu lng t lun tho mn iu kin sau:

2Qd

2 K

+

Cng sut trung bnh nhiu lng tu c xc nh nh sau:

PQd =

12

2

T biu thc ny cho thy cng sut nhiu lng t ch ph thuc vo bc lng tm khng ph thuc vo bin tn hiu.

i vi tn hiu mnh, t s:

=Nhiu

Tnhiu

N

Ss ln hn t s ny ca tn hiu yu.

Mun san bng t s ny gia tn hiu mnh v tn hiu yu phi s dng lng t ho khng u.

Hnh 1.6: Nhiu lng t

Bin

+1

+2

+3

-1

-2

-3

0 t

Bin

+1

+2

+3

-1

-2

-3

0 t

Qd Qd

Qd

Qd

Qd

Qd

Qd

Qd

Qd = nhiu lng t (Quantising distortion)


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Lng tho khngu

Lng t ho khng u da trn nguyn tc: khi bin tn hiu cng ln th bc lngt cng ln (hnh 1.7).

Hnh 1.7: Qu trnh lng tho khng u

Trong th d trn hnh 1.7 bin ca tn hiu analog c chia thnh 4 bc lng t, k

hiu l 1, 2, 3, 4. Nh vy: 1 < 2 < 3 < 4 < ... Cc ng thng song song vi trc honh(t) gi l cc mc lng t, c nh s t 0 ti gc to.

Cc xung ly mu ti cc chu k nTm (trong n=0,1,2,...) c ly trn n mc lng

t gn nht.Mun lng t ho khng u c th s dng mt trong hai phng php: nn - dn

analog hoc nn - dn s.

Nn - dn analog

Qu trnh nn - dn analog c thc hin bng cch t b nn analog trc b m hou pha nhnh pht ca thit b ghp knh, trong min tn hiu thoi analog v t mt b dnanalog trc b gii m u nhnh thu ca thit b ghp knh, cng trong min tn hiu thoianalog.

Trong thit b ghp knh s ch to theo tiu chun Chu u s dng b nn - dn theo

lut A. Cn theo tiu chun Bc M v Nht s dng b nn theo lut .c tuyn ca b nn lut A (s ph thuc in p u vo v u ra b nn) biu th

bng biu thc

Y=

Trong x =0

V

Uu

vi uV l bin in p u vo b nn, cnU0 l in p vo bo ho.

Bin

+1

+2

+3

-1

-2

-3

0 t1

+4

-4

2

3

4

A

1x0

Aln1

Ax

+

1xA

1

Aln1

Axln1

++


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Theo khuyn ngh ca ITU-T ly A = 87,6.

c tuyn ca b nn lut biu th bng biu thc

Y=

Theo khuyn ngh ca ITU-T ly = 255.

T cc biu thc trn c th xy dng c cc ng cong th hin c tuyn b nn A

v . c tuyn b nn phi i xng vi c tuyn b dn khng gy mo khi khi phc tnhiu. Dng ng cong c tuyn ca b nn v b dn nh hnh 1.8.

Hnh 1.8: c tuyn b nn v b dn analog

Nhiu th nghim v lng t ho tn hiu thoi a ra kt lun:

Mun t c t s:

=Nhiu

Tnhiu

N

Skhong 25 dB th s mc lng tu phi bng

2048. Nh vy mi t m cn c 11 bit (khng k bit du). V 211 = 2048 l s mc lng t ca

bin dng hoc m ca tn hiu thoi. Sau khi nn, tn hiu thoi ch cn 128 mc. Nu k cbit du ch cn t m 8 bit. l l do ti sao phi thc hin nn tn hiu.

Nn - dn s:

B nn sc t trong min tn hiu s ca nhnh pht v b dn sc t trongmin tn hiu s ca nhnh thu ca thit b ghp knh. c tuyn b nn v b dn s da trn csca b nn v b dn analog. Bng cch gn ng ho ng cong c tuyn b nn - dn

analog theo lut A v thnh cc on thng gp khc.

c tuyn ca b nn s lut A c tt c 13 on thng c dc khc nhau v ly tn lb nn s A = 87,6/13 c th hin trong hnh 1.9.

Bin ra

+1

Bin vo+1

-1

-1

c tuyn b dn

c tuyn b nn

( )( )

1x01ln

x1ln

++

( )( )

0x11ln

x1ln

+


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Cc on thng c dc khc nhau, do vy trong cng mt on tn hiu khng b nn.Khi chuyn ton ny sang on khc th tn hiu b nn v khi bin cng ln s b nn cngnhiu.

Hnh 1.9: c tnh bin b nn s A=87,6/13

xy dng c tnh bin ca b nn s cn tin hnh cc bc sau y:Trc x c trng cho bin chun ho ca tn hiu u vo b nn (-1 x 1 tng

ng vi 4096 bc lng tu) v trc y c trng cho tn hiu u ra.

Trn trc x chia theo khc logarit cs hai, na dng gm cc im 0,128

1,

64

1,

32

1,

16

1,

8

1,

4

1,

2

1v 1; cn na m c chia ngc li.

Trn trc y chia thnh cc khong u nhau v na dng gm cc im 0,

8

1,

8

2,

8

3,

8

4,

8

5,

8

6,

8

7v 1 ; cn na m c chia ngc li.

128

1

64

1

32

1

16

1

8

1

4

1

2

1

8

1

82

8

3

8

4

8

5

8

6

8

7

1

on 13

12

11

10

9

8

on 1

2

3

4

5

6

-1

7

(Tn hiu

u ra

(Tn hiu u v o1-1

H

G

F

E

D

C

B

A

x

y

Tn hiuu vo

Tn hiu

u ra


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Tip nh du cc im c bit A, B, C, D, E, F, G v H, trong na dng ca ngc tnh, trong im H l im ct nhau ca on thng vung gc vi trc x ti im c x= 1v on thng vung gc vi trc y ti im c y= 1. im G l im ct nhau ca on thngvung gc vi trc x ti im c x=1/2 v on thng vung gc vi trc y ti im c y=7/8, ....

im A l im ct nhau ca on thng vung gc vi trc x ti im c x=1/128 v on thngvung gc vi trc y ti im c y=1/8. Ni hai im k nhau bng mt on thng. Nh vy na dng ca ng c tnh bin c tt c 8 on thng, mi on c c trng bng tm 3 bit. Trong mi on c chia thnh 16 mc, mi mc phn phi t m 4 bit. Na m cang c tnh bin c ly i xng vi na dng qua gc to O. Do 4 on gn gcto 0 c dc nh nhau (trong na dng c hai on OA v OB). Nh vy ton bng c tnh bin c 13 on thng c dc khc nhau.

Na m v na dng ca ng c tnh bin c phn phi t m 1 bt. Bt 0 tngng vi na m ca ng c tnh bin v bt 1 tng ng vi na dng ca ng c tnh

bin .Tm li, khi cha nn th tn hiu thoi c chia thnh 4096 mc, sau khi dng b nn

A=87,6/13 th ch cn li 256 mc (tc l s bt trong mt t m gim t 12 xung 8).

Chuyn i tn hiu digital thnh tn hiu analog

Ti pha thu, tn hiu s PCM c chuyn i thnh tn hiu analog qua hai bc l: giim v lc. Tng hp hai qu trnh x l ny gi l qu trnh chuyn i D/A v c biu dinnh hnh 1.10.

Hnh 1.10: Qu trnh chuyn i D/A

Gii m l qu trnh ngc li vi m ho. Trong gii m, bt u bng vic tch cc m nhphn 8 bit t tn hiu PCM (trong hnh 1.10 tng trng t m 3 bit). Tip theo, chuyn mi t mnh phn thnh mt xung lng t c bin tng ng vi s mc lng t ca t m . Hnh1.11 minh ho gii m cc t m 3 bt. Tn hiu xung c lng t ho u pht c toli u thu bng cch gii m nh vy. Tn hiu xung sau khi gii m c bin chnh lch vi

bin xung mu ti pha pht. Hin tng ny gi l mo lng t v pht sinh do lm trn bin khi lng t ho.

Lc

Gii m

t

t

t

1

0


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Hnh 1.11: Qu trnh gii m

Sau , tn hiu xung lng tc a qua b lc thng thp. u ra b lc ny nhnc tn hiu analog l tn hiu lin tc theo thi gian nhni suy gia cc mu k tip nhau nhhnh 1.12.

Hnh 1.12: Qu trnh lc tn hiu tcc xung PAM

Cc phng php m ho mi

PCM tn ti trong 1/4 thp k v cc cng ngh mi bt u thu ht s ch . Trongthp k cui, m ho thoi tinh vi trln hin thc nhs pht trin ca VLSI (mch tch hprt ln). PCM ti 64 Kb/s khng cn l cng ngh duy nht na. Vic m ho 32 v 16 kbit/s

c pht trin, v cc phng php vocoder cng c pht trin m ch yu cu 4.8 Kb/s v thn. Chng ta c th bng mi cch t ti 800bit/s m vn nghe hiu c, nhng ti tc

bit ny khng c kh nng nhn dng c li ni ca ngi ni.

Cc phng php m ho mi gi ra rt nhiu li ch, v chng cho php cc nh khaithc tng gp 2 hay 4 ln dung lng truyn dn thoi trong mng ca h m khng cn philp t thit b truyn dn mi. Mt trong nhng phng php c th dng l iu ch xung m visai thch ng, ADPCM. ADPCM cho php truyn thoi vi cht lng gim ti thiu ti 32Kbit/s.Khuyn ngh ca ITU v ADPCM c gi l G.726.

PCM vi sai (DPCM)

Tn hiu c ly mu cho thy mc tng quan cao gia cc mu k cn. Hay nicch khc, hai mu gn nhau l kh tng t nh nhau. Ngha l s c nhiu li ch nu m ho s

t

t10

Tn hiu PCMnhn c

Tn hiu xunglng t

1 1 1 0 1 10 10 1 0 0 1 0 01

3

6

53

12

tTn hiu xung

Lng t

in p tngu ra b lc

Lc tn hiu analog

t


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khc nhau gia cc mu k cn thay cho m ho gi tr tuyt i ca mi mu. Trn hnh 1.13 chothy 4 bit c thc s dng thay cho 8 bit. y l tng n trong PCM vi sai (DPCM), y chnh xc vn c gi li mc d khng cn bng tn rng. DPCM u tin da trn bnquyn t 1952.

Mu s 1 2 3 4 5 6 7

1

2

3

4

5

6

t

Gitr

mutuyti

Mu s 1 2 3 4 56 t

lchgia cc

gi tr mu

Hnh 1.13: PCM vi sai (DPCM)

PCM vi sai c nhc im l nu tn hiu u vo tng t m thay i qu ln gia ccmu, th n khng thc biu din bng 4 bit m s b ct.

DPCM thch ng (ADPCM)

PCM vi sai thch ng (ADPCM) t h p phng php DPCM v PCM thch ng.ADPCM c ngha l cc mc lng t ho c thch ng vi dng ca tn hiu u vo. Kch cca cc bc lng t tng ln khi c lin tip dc ng trong tn hiu ko di. Trong hnh1.14, s mu l 6 c thc m t bng 5 bc lng t ln thay cho 10 mu nh. Phng phpny c tn t kh nng thch ng y, tc l n to ra kh nng gim cc bc lng t.


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Mu s 1 2 3 5 64Thi gian

Hnh 1.14: PCM thch ng

Trong m ho ADPCM, sau khi tn hiu vo tng t i qua m ho PCM thng thng,th lung cc mu 8 bit c gi tip ti b m ho ADPCM. Trong b m ho ny, mt thutton ch vi 15 mc lng tc s dng gim di t 8 bit xung 4 bit. 4 bit ny khng

biu din bin ca mu na, nhng nhc m ho vi sai m 4 bit vn cha thng tin chophp tn hiu gc sc ti to b thu.

Mc ca mt mu c don da trn mc ca mu ng trc. S khc nhau gia mudon v thc t l rt nh v v vy c th m ho bng 4 bit. Nu c vi mu tip theo thay

i ln, th cc bc lng tc thch ng nh m ttrn.

1.1.3.Kthut ghp knhGhp knh csPCM-30

Phn trn trnh by nguyn l cbn ca phng php iu ch xung m PCM. By gi,chng ta s xem xt nhng nguyn l c s dng nh th no thit l p cc h thngtruyn dn PCM thc t. Trc ht l xt n nguyn l ghp knh phn chia theo thi gian v nlm cho cc h thng truyn dn tn hiu thoi bng PCM c u im v mt kinh t.

S nguyn lcc h thng PCM, qu trnh ghp knh phn chia theo thi gian thng c thc hin

trc khi m ha dy xung, tc l cc mu ca tn hiu tng t ring c kt hp li trn mtng truyn PAM chung. Theo phng php ny, thit b m ha c thc dng trong qutrnh ghp knh phn chia theo thi gian. y khng thc hin ghp tng xung mt m ghptng t m PCM mt, cch ny thng c gi l ghp khe thi gian. Cc h thng PCM hu htl cc h thng TDM.

S b ghp knh PCM-30 nh hnh 1.15.


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Hnh 1.15: B ghp knh PCM-30

S ny ghp 30 knh thoi, knh ng b v knh bo hiu thnh lung bit c tc bng 2048 kbit/s. i dy m tn c ni vo my u cui thu bao nh my in thoi, thit btruyn s liu v.v. Sau y phn tch hot ng ca b ghp tn hiu thoi.

B sai ng S tch tn hiu thoi thu v pht ring bit. Ti nhnh pht c b lc thngthp hn ch bng tn ting ni t 300 n 3400 Hz, u ra b lc thng thp ni n mch ly

mu. Mch ly mu l mt chuyn mch in tng m theo chu k 125s, u ra nhn c

cc xung mu c chu k bng 125s. B m ho bin i mi xung ly mu thnh 8 bit v khighp knh tn hiu thoi, tn hiu ng b v tn hiu bo hiu thnh mt khung c thi hn

125s. u ra cc mch ly mu u song song vi nhau, v vy xung ly mu ca cc knh cghp theo thi gian v ln lt a vo b m ho. Trong b ghp knh PCM-30 dng b m honn s A= 87,6 v c tnh bin c 13 on. Dy xung ly t b to xung pht qua b chia to ra xung iu khin cc mch ly mu 8 kbit/s, iu khin cc b m ho v iu khin b ghpknh. Bo hiu t cc thu bao c a ti khi x l bo hiu. Ti y bo hiu c chuyn

i thnh cc bit ghp vo khung tn hiu. Dy bit hai mc u ra khi ghp knh qua khi lpm ng chuyn thnh dy bit ba mc ri i ra ngoi.

Ti nhnh thu ca b ghp knh PCM-30 tip nhn dy bit ba mc n v chuyn vo khigii m ng chuyn thnh dy bit hai mc. Mt phn tn hiu u ra khi gii m ng,a vo khi tch xung ng b tch ra xung ng b v a ti khi to xung thu kchthch b chia xung v to ra cc khe thi gian ng b vi pha pht. Phn tn hiu cn li ca vo khi tch knh tch lung bit u vo thnh 30 knh thoi, knh bo hiu. Khi bohiu chuyn cc bit bo hiu thnh tn hiu bo hiu ban u, chng hn bo hiu a tn, cc digitb s thu bao, xung iu khin rle v.v. Cc t m 8 bit ca 30 knh thoi a ti b gii m

chuyn thnh cc xung lng t, qua b chn xung knh v b lc thng thp tch ra tn hiuthoi analog ca tng knh. Tn hiu analog qua b sai ng i vo my in thoi. B chn xung

Ghpknh

B to

xung pht

B toxung thu

M ho

S

S

XB v Bo hiu

Lymu Lp m-ng

Chn

xung knh

Lymu

TchknhGii m

Chnxungknh Gii m-ng

Tch

XB

u ra

u

1

30

1

30

1

30


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knh l mt chuyn mch in tng m theo tc v pha ca b ly mu pha pht. uvo b chn xung knh u song song vi nhau v mi b ch cho xung knh mnh i qua, tc ltch knh theo thi gian c thc hin ti y.

Cu trc khung v a khung

Cu trc khung v a khung ca b ghp PCM-30 nh hnh 1.16.

Hnh 1.16: Cu trc khung v a khung ca b ghp knh PCM-30

Khung c thi gian 125s c chia thnh 32 khe thi gian bng nhau v nh s th t t

TS0 n TS31. Mi khe thi gian TS di 3,9s gm mt t m 8 bt. Mi khung gm c 256 bitv chu k lp li ca khung bng 8000 Hz.

Cc khe TS0 ng u cc khung chn gm bit Si c s dng cho quc t (nu khngdng th ci t bng 1) v by bt cn li l t m ng b khung 0011011. Cc khe TS0 ngu cc khung l gm bit th nht Si dng cho mng quc t, nu khng s dng t Si= 1, bit thhai lun c logic 1 trnh phng to t m ng b khung, bit th ba dng cho cnh bo xa khimt ng b khung, nm bit S cn li dnh cho quc gia. Khi trm u xa khng thu c t m

ng b khung st A=1 v truyn v trm gc.Mi a khung ko di trong 2 ms v cha 16 khung. Cc khung c nh s th t t F0

n F15, trong 8 khung mang ch s chn v 8 khung cn li mang ch s l.

Khe thi gian TS16 ca khung F0 truyn t m ng ba khung vo v tr cc bit th nhtn bit th t, bit th 6 truyn cnh bo xa khi mt ng ba khung (A=1), cc bit S dnh choquc gia, nu khng s dng t S=1.

Cc khe thi gian TS16 ca khung F1 n khung F15 dng truyn bo hiu. Bo hiuca mi knh thoi c m ho thnh 4 bit a, b, c, d v ghp vo na khe thi gian TS16. Na

bn tri truyn bo hiu ca cc knh thoi th nht n 15 v na bn phi truyn bo hiu ccknh thoi th 16 n 30 nh bng 1.1.

F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10F11F12F13F14 F15

TMF=125s 16= 2ms

a khung16 khung

313029282726252423222120191817161514131211109876543210

302928272625242322212019181716151413121110987654321

TF=125s

Si 0 0 1 1 0 1 1Cc khung chn

Si 1 AS S S S SCc khung l

b1b2b3b4b5b6b7b8

Khung F0

0 0 0 0 S A S SCh thch:TS - khe thi gianCH- knh thoi

TS

CHKhung

a khung

16 khung


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Bng 1.1. Ghp tn hiu bo hiu ca 30 knh thoi

b1b2b3b4 b5b6b7b8

a b c d a b c d

Khung 1

Khung 2

Khung 3

Khung 4

Khung 5

Khung 6

Khung 7

Khung 8

Khung 9

Khung 10

Khung 11

Khung 12

Khung 13

Khung 14

Khung 15

Knh 1

Knh 2

Knh 3

Knh 4

Knh 5

Knh 6

Knh 7

Knh 8

Knh 9

Knh 10

Knh 11

Knh 12

Knh 13

Knh 14

Knh 15

Knh 16

Knh 17

Knh 18

Knh 19

Knh 20

Knh 21

Knh 22

Knh 23

Knh 24

Knh 25

Knh 26

Knh 27

Knh 28

Knh 29

Knh 30

Nh vy phi c 16 khe thi gian TS16 trong mt a khung mi truyn bo hiu vng ba khung. cng l l do ti sao mi a khung cha 16 khung. Nu cc bt a b c dkhng dng cho bo hiu th t b=1, c=0 v d=1. Ngoi ra cng cn lu cm s dng t hp0000 truyn bo hiu v n trng vi t m ng ba khung. Phng thc bo hiu trnh

by trn y gi l bo hiu knh kt hp CAS. Ngoi phng thc bo hiu knh kt hp CAS,

trong tng i in t s cn c phng thc bo hiu knh chung CCS, trong bo hiu cacc knh thoi c truyn trn mt ng ring. in hnh ca CCS l h thng bo hiu s 7(CCSS-7).

Trong trng hp PCM-30 c s dng truyn s liu th bit Si trong khe thi gian TS0l bit kim tra d vng CRC (xem bng 1.2).

Khe thi gian

TS16


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Bng 1.2. Chc nng cc bit trong TS0 ca mt a khung

Bit 1 n bit 8 ca TS0Th t khung

Si 2 3 4 5 6 7 8

0 C1 0 0 1 1 0 1 11 0 1 A S S S S S

2 C2 0 0 1 1 0 1 1

3 0 1 A S S S S S

4 C3 0 0 1 1 0 1 1

5 1 1 A S S S S S

6 C4 0 0 1 1 0 1 1

7 0 1 A S S S S S

8 C1 0 0 1 1 0 1 19 1 1 A S S S S S

10 C2 0 0 1 1 0 1 1

11 1 1 A S S S S S

12 C3 0 0 1 1 0 1 1

13 E 1 A S S S S S

14 C4 0 0 1 1 0 1 1

15 E 1 A S S S S S

Bng 1.2. tm tt chc nng cc bit ca khe thi gian TS0 trong mi a khung 16 khung.Cng c th xem a khung gm 2 a khung con; a khung con th nht gm khung 0 n khung 7v a khung con th hai gm khung 8 n khung 15. Bit Si trong cc khung chn ca mi akhung con l cc bit kim tra d chu trnh C1 C2 C3 C4 (CRC-4). Bit Si trong cc khung l caa khung to thnh t m ng ba khung CRC-4, bt E trong khung 13 ch th li bit ca CRC-4 ca a con th nht v bit E trong khung 15 ch th li bit ca CRC-4 ca a khung con th hai.

Phn cp s cn ng b PDH

Sau khi gii thiu v phng php ghp knh csPCM-30, phn ny s trnh by v cch thng ghp knh bc cao. Ghp knh bc cao l ghp nhiu lung s c tc thp tothnh mt lung s c tc cao hn. Thit b thc hin nhim v ni trn c gi l my ghpknh bc cao.

PDH l mt trong nhng h thng ghp knh s bc cao thng dng. Trong mng thng tinPDH khng s dng ng b tp trung, ngha l tt c cc phn t trong mng khng b khng chbi mt ng h ch. Mi thit b ghp knh hoc tng i trong mng ny c mt ng h ring.Chnh v vy m cc lung s do chng to ra c s chnh lch v tc bit.

Chng hn tng i th nht a ra lung s (2048 kbit/s + 510-5); trong khi mt tng

i khc li a ra lung s (2048 kbit/s - 510-5). Mun ghp cc lung s c tc bit khcnhau ny thnh mt lung s c tc cao hn th phi hiu chnh cho tc bit ca chng bng


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tc bit ca ng h b ghp nhchn bit. Sau khi chn bit th cc lung su vo b ghpxem nhng b v tc bit, nhng pha ca chng khng ng b vi nhau. Kiu ghp nhvy c gi l ghp cn ng b (hnh 1.17).

Hnh 1.17: Nguyn tc ghp cn ng b

V tiu chun tc bit PDH, hin nay trn th gii c 3 tiu chun: Chu u, Bc M vNht Bn. Sau y l c im chnh ca cc tiu chun ny.

Tiu chun Chu uChu u da trn tc bit cs2048 kbit/s ghp xen bit thnh cc tc bit cao hn

v gm c 4 mc. S hnh thnh cc mc theo tiu chun Chu u nh hnh 1.18a.

Mc 1 (DS1): Ghp 30 knh thoi thnh lung 2048 kbit/s. Cc lung s c sny ccung cp t thit b ghp knh PCM-30 hoc t tng i in t s.

Mc 2 (DS2): Ghp 4 lung s csthnh lung s mc 2 l 8448 kbit/s, gm 120 knh thoi.

Mc 3 (DS3): Ghp 4 lung s mc 2 thnh lung s mc 3 l 34368 kbit/s, gm 480 knhthoi.

Mc 4 (DS4): Ghp 4 lung s mc 3 thnh lung mc s 4 l 139268 kbit/s, gm 1920knh thoi.


Tiu chun Bc M

Bc M s dng lung s cs1544 kbit/s t thit b PCM-24 hoc t tng i in t s ghp xen bit thnh cc lung s c tc bit cao hn v gm c 4 mc. S hnh thnh ccmc theo tiu chun Bc M nh hnh 1.18b.

Mc 1 (DS1): Ghp 24 knh thoi thnh lung 1544 kbit/s.

Mc 2 (DS2): Ghp 4 lung s mc 1 thnh lung s mc 2 l 6312 kbit/s, gm 96 knh thoi.

ChnBit

ChnBit

123456

12345J

J- Cc bit chn

B to xung ng h

B chuynmch

Tn hiu ra

B ghp

123456

12345

Cc bit dliu u vo

Lung s 2Mbit/s ctc bit nh mc

Lung s 2Mbit/s c tc bit thp hn nh mc


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Tiu chun ca Nht Bn

Hai mc u tien theo tiu chun Nht Bn hon ton ging tiu chun Bc M v gm ctt c l 5 mc nh hnh 1.18b.

Mc 1 (DS1): Ghp 24 knh thoi thnh lung 1544 kbit/s.

Mc 2 (DS2): Ghp 4 lung s mc 1 thnh lung s mc 2 l 6312 kbit/s, gm 96 knh thoi.




ITU-T cng nhn 4 mc u tin theo tiu chun Chu u v 3 mc u tin theo tiuchun Bc M l cc mc truyn dn PDH quc t.

Hnh 1.18: Qui nh cc mc truyn dn PDH

2048

kbit/s

8448

kbit/s4

34368

kbit/s4

139264

kbit/s4

564992

kbit/s4

ITU-T

Tiu chunChu u

(a)

1544

kbit/s 4

6312

kbit/s

44736kbit/s

274176kbit/s6

7

32064kbit/s

97728kbit/s

400352kbit/s5

43

Tiu chunNht

Tiu chunBc M

(b)ITU-T


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Phng php ghp

C ba phng php ghp cc lung s l:

- Ghp xen bit

- Ghp xen byte- Ghp xen chu trnh

Nhng trong PDH ch s dng k thut ghp xen bit. Sau y s trnh by phng phpghp ny.

Hnh 1.19 m t qu trnh ghp xen bit bn lung s DS1 thnh lung s DS2.

Th t ghp nh sau:

Trc tin ghp xung ng b (XB), tip theo ghp bit th nht ca lung s DS1 thnht, bit th nht ca lung s DS1 th hai, bit th nht ca lung s DS1 th ba, bit th nht calung s DS1 th t. Sau ghp bit th hai cng theo trnh t trn. C ghp nh vy cho ht mt

chu trnh 125s. n chu trnh sau trc ht phi ghp xung ng b v sau phi ghp tng bittheo th t trn.

Trong 125s phi ghp ht s bit trong chu trnh c 4 lung vo. Nh vy th tc lung su ra DS2 mi tng t nht gp 4 ln tc mt lung su vo DS1.

Hnh 1.19: Ghp xen bit bn lung s DS1 thnh lung s DS2

Khi ghp cc lung s PDH c tc bit thp thnh lung s c tc bit cao hn th cc thitb ghp thng hot ng theo kiu cn ng b. V cc lung su vo b ghp c tc bit

T

tDS-1#1

t

DS-1#2

t

DS-1#3

t

DS-1#4

t

XB

Tt

DS-2


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tc thi c th khc nhau vi tc bit danh nh cht t, nn ghp cc lung su vo nythnh lung su ra c lin quan n qu trnh chn. Qu trnh ny hot ng nh sau:

Khi thc hin ghp cc bit ca cc lung nhnh, trc ht cc bt ny c ghi ln lt vo nh trong cc b nh tng ng ca cc lung nhnh (di siu khin ca ng h tch t

dy xung vo, cn gi l ng h ghi). Sau cc bt ny c ly ra (di siu khin cang hc ly t b to xung ca b ghp knh MUX) v a vo b MUX thc hin ghpxen bt. C dy bit c v y bit ghi u c a vo b so snh pha. Khi hai dy bit lch phavi nhau t gi tr ngng t trc th xy ra qu trnh chn. Nhn c thng bo chn th khiiu khin chn s pht tn hiu iu khin chn, khi khi MUX s tin hnh chn bit vo v tr qui nh trong khung.

Trong trng hp mt lung su vo b nhc tc bit tc thi chm hn tc btng hc ca MUX s xut hin nh k mt sim b trng trong tn hiu u ra b nhm v gy ra li bit ti pha thu. Mun trnh li bit bt buc phi chn thm cc bit mang thng

tin gi vo cc im b trng v truyn thng bo ti pha thu xo cc bit cc bt chn ny, nhvy gi l chn dng.

Ngc li, nu tc tc thi ca lung su vo b nhnhanh hn tc bit ng hc ca b MUX s xut hin nh k cc thi im m ti hai bit d liu c c bi mt bitca ng hoc, gy ra li bit ti u ra b nh. Do , phi tch bit d liu c c sau ghp vo v tr qui nh trong khung v c thng bo gi ti pha thu pha thu khng xo bitd liu ny. y chnh l chn m.

Chn c xem nh qu trnh lm thay i tc xung ca tn hiu smc iu khincho ph hp vi tc xung khc vi tc xung vn c ca n m khng lm mt thng tin.

Phn cp sng b SDH

Cc c im chnh

Nh trnh by trong phn trn, hin nay trn th gii tn ti 3 phn cp s cn ng bPDH (Chu u, Bc M v Nht Bn). Song cc phn cp s cn ng b ny khng c giao dintiu chun ho quc t nn khng p ng c nhu cu pht trin cc dch v vin thng tronggiai on hin ti v tng lai. Ngoi ra qu trnh tch/ghp cc lung s trong h thng truyndn cn ng b rt phc tp, yu cu thit b cng knh lm gim cht lng truyn dn v khnng gim st, qun l mng cn km.

H thng truyn dn ng b SDH c xem l giai on pht trin tip theo ca phn cptruyn dn cn ng b. SDH to ra mt cuc cch mng trong vic truyn cc dch v vinthng, th hin mt k thut tin tin c thp ng rng ri cc yu cu ca thu bao, nh khaithc cng nh cc nh sn xut, tho mn cc yu cu t ra cho ngnh Vin thng trong thi imi.

Trong tng lai, h thng truyn dn ng b s ngy cng c pht trin nhcc u imvt tri so vi h thng truyn dn cn ng b, c bit SDH c kh nng kt hp vi PDHtrong mng li hin hnh, cho php thc hin vic hin i ho mng li theo tng giai on

pht trin.Cc tiu chun ca SDH bt u hnh thnh t nm 1985 ti M. Khi u l n lc to

ra mt mng giao tip quang c th hot ng vi tt c cc h thng truyn dn khc nhau ca


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cc sn phm khc nhau (theo tiu chun Chu u hoc Bc M). Dn dn sau cc tiu chunny c s dng rng ri c th x l cho mng hin ti v cho c cc loi tn hiu trongtng lai, cng nh cho c phng din khai thc v bo dng.

Trong hon cnh , thng 2 nm 1985 cng ty BELLCORE l cng ty con ca cng ty

BELL ti M ngh mt phn cp truyn dn mi nhm mc ch khc phc cc nhc imca h thng cn ng b. Phn cp mi ny c tn l mng quang ng b (SONET). SONETda trn nguyn l ghp knh ng b, trong cp quang c s dng lm mi trng truyndn. V sau cc tiu chun v giao din thit b cng c nghin cu, kt ni cc loi thit bkhc nhau c tiu chun khc nhau m khng gy trngi khi p dng phn cp ng b SDHvo mng li hin ti. p ng yu cu cn phi lu n qu trnh t chc cc tn hiu

bo dng, gim st, chuyn mch bo v tng v c vn qun l mng li ca cc loithit b khc nhau .

ngh ca hng BELLCORE c Vin cc tiu chun quc gia Hoa K ANSI nghin

cu v n nm 1988 ph chun SONET l tiu chun ca Hoa K. Cc tiu chun caSONET c hnh thnh theo hai giai on. Giai on mt qui nh cc tiu chun v cc tc

bit truyn dn (bng 1.3), khun dng tn hiu, cc thng s giao din quang v th t sp xp titrng trong khung tn hiu. Giai on mt hon thnh vo nm 1988. Giai on hai caSONET qui nh cc giao thc s dng cc knh nghip v vo vic iu hnh, qun l, bodng, gim st v c hon thnh nm 1991. ng thi SONET cng gy c s ch vcng c nghin cu, pht trin ti Chu u.

Bng 1.3. Tc bit ca SONET

Cc mc tn hiuquang (OC)

Cc mc tn hiung b (STS)

Tc bit

(Mbit/s)

OC-1 STS-1 51,84

OC-3 STS-3 155,52

OC-9 STS-9 466,56

OC-12 STS-12 622,08

OC-18 STS-18 933,12

OC-24 STS-24 1244,16

OC-36 STS-36 1866,24

OC-48 STS-48 2488,32

Thng 11 nm 1988, trn cs tiu chun ca SONET v xt n cc tiu chun khc Chu u, Bc M v Nht Bn, ITU-T a ra tiu chun quc t v cng ngh truyn dn theo

phn cp sng b SDH dng cho truyn dn cp quang v vi ba. Cc tiu chun ca SDH c ITU-T ban hnh trong cc khuyn ngh sau y:

G.702 - S lng mc trong phn cp sng b

G.707 - Cc tc bit ca SDHG.708 - Giao din nt mng SDH


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G.709 - Cu trc ghp ng b

G.773 - Giao thc ph hp vi giao din Q (Qun l h thng truyn dn)

G.774 - M hnh thng tin qun l SDH

G.782 - Cc kiu v cc c tnh ch yu ca thit b ghp SDHG.784 - Qun l SDH

G.803 - Cu trc mng truyn dn SDH

G.825 - iu khin rung pha v tri pha trong mng thng tin SDH

G.957 - Cc giao din quang ca cc thit b v h thng lin quan n SDH

G.958 - H thng truyn dn SDH s dng cho cp si quang

M.30 - Cc nguyn tc qun l mng vin thng

M.3010- Nguyn l hot ng ca TMN

Hin nay cc khuyn ngh G.707, G.708 v G.709 kt hp li thnh khuyn ngh G.70x.

V tc bit ca SDH bao gm nh sau:

STM-1 = 155,52 Mbit/s

STM-4 = 4 STM-1 = 622,08 Mbit/s

STM-8 = 8 STM-1 = 1244,16 Mbit/s

STM-12 = 12 STM-1= 1866,24 Mbit/s

STM-16 = 16 STM-1= 2488,32 Mbit/s

STM-64 = 64 STM-1= 9953,28 Mbit/s

Cc tc bit STM-1, STM-4 v STM-16 trng vi cc tc bit STS-3, STS-12 v STS-48 ca SONET.

So vi PDH th SDH c cc u im cbn sau y:

Giao din ng b thng nht. Nhgiao din ng b thng nht nn vic ghp v tchcc lung nhnh t tn hiu STM-N n gin v d dng. ng thi trn mng SDH c

th s dng cc chng loi thit b ca nhiu nh cung cp khc nhau.

Ghp c cc loi tn hiu khc nhau mt cch linh hot. Khng nhng tn hiu thoim c tn hiu khc nh ATM, B-ISDN v.v. u c th ghp vo trong khung SDH

Dung lng cc byte dnh cho qun l, gim st v bo dng ln. Lm cho mng hotng linh hot, tin cy cao v gim c chi ph rt ln cho vic qun l.

Mng c kh nng p ng c tng lai, c ngha l cung cp cho nh khai thc mtgii php p ng c tng lai, cng vi kh nng cp nht phn mm v mrngc dung lng ca cc thit b hin c. C th thay th h thng SDH tng phn vo

trong mng theo nhu cu ca dch v mi.B ghp SDH


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B ghp SDH theo khuyn ngh G.709 ca ITU-T c cu trc nh trn hnh 1.20.

Hnh 1.20: Cu trc b ghp SDH ca ITU-T

Chc nng cc khi

C-n (n=1,...,4) : Container mc n

Container l mt khi thng tin cha cc byte ti trng do lung nhnh PDH cung cp trong

thi hn 125s cng vi cc byte n (khng mang thng tin).

VC-n : Containero mc n

Containero mc n l mt khi thng tin gm phn ti trng do cc nhm khi nhnh(TUG) hoc Container mc n (C-n) tng ng cung c p v phn mo u tuyn (POH). POHc s dng xc nh v tr bt u ca VC-n, nh tuyn, qun l v gim st lung nhnh.Trong trng hp sp xp khng ng b cc lung nhnh vo VC-n th phi tin hnh chn bit.C hai loi VC-n l VC-n mc thp (n= 1; 2) v VC-n mc cao (n = 3; 4).

TU-n : Nhm khi nhnh mc n

Nhm khi nhnh mc n l mt khi thng tin bao gm mt Containero cng mc v mtcon tr khi nhnh (TU-PTR) ch th khong cch t con tr khi nhnh n v tr bt u caVC-3 hoc VC-n mc thp.

TUG-n (n = 2; 3) : nhm cc khi nhnh mc n

Nhm cc khi nhnh mc n c hnh thnh t cc khi nhnh (TU-n) hoc t nhm cckhi nhnh (TUG) mc thp hn. TUG-n to ra s tng hp gia cc Containero (VC) mcthp v Containero (VC) mc cao hn.

AU-n : khi qun l mc n

STM-N AUGN 1

AU-4 VC-4

TUG-3 TU-3 VC-3

C-3

C-4

VC-3

TUG-2 TU-2 VC-2 C-2

AU-3

TU-12 VC-12 C-12

TU-11 VC-11 C-11

3

77

3

3

4

139264 kbit/s

44736 kbit/s34368 kbit/s

6312 kbit/s

2048 kbit/s

1544 kbit/s

X l con tr

Ghp knhng chnh

Sp xp

Ghi ch:


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Khi qun l mc n (AU-n) l mt khi thng tin bao gm mt Containero mc n (VC-n)cng mc v mt con tr khi qun l (AU-PTR) ch th khong cch t con tr khi qun ln v tr bt u ca Containero (VC) cng mc.

AUG : nhm cc khi qun l

Nhm cc khi qun l (AUG) gm mt AU-4 hoc 3 AU-3.

STM-N (N=1, 4, 16, 64) : module truyn ti ng b mc N

Module truyn ti ng b mc N (STM-N) cung cp cc kt ni lp on trong SDH, bao

gm phn ti trng l N AUG v phn mo u on (SOH) ng b khung, qun l v gimst cc trm lp v cc trm ghp knh.

S khc nhau ch yu gia SONET v SDH c th thy mt cch r rng ti lung bc cao.Trong SONET lung bc cao l VC-3, trong khi bc cao ca SDH li da vo VC-4. S khcnhau ny to iu kin thun li cho truyn dn cc lung PDH ca Chu u v ca Bc M quamng SDH. Tuy nhin, qu trnh phi hp hot ng ti bin gii lc a s s dng lp lung bccao VC-4. Trong SONET, mt VC-3 c th ghp cc tn hiu STS-1. Nhng ITU-T khng quinh tc bit STS-1 l tc bit thp nht ca SDH. ITU-T qui nh ba lp phng tintruyn dn chung cho c SONET v SDH bt ngun t module truyn dn ng b mc 1 (STM-1) v hai mc tip theo l STM-4 v STM-16.

Trong SDH, VC-3 c s dng truyn cc tc bit mc 3 l 34368 kbit/s; VC-4 cs dng ghp 3 VC-3 hoc 63 VC-12. VC-4 c xc nh l lp truyn dn trgip cho cclung ATM trong SONET v SDH. Ngoi ra, SDH trgip chuyn ti VC-11 trn cc kt ni lpVC-12 bng gii php tng thch c bit.C hai phng php hnh thnh tn hiu STM-N.Phng php th nht qua AU-4 v phng php th hai qua AU-3. Phng php th nht cs dng ti Chu u v cc nc khc trong c Vit Nam. Phng php th hai c s dngti Bc M, nht Bn v mt s nc khc. Tn hiu AU-4 c hnh thnh t mt lung nhnh139264 kbit/s, hoc 3 lung nhnh 34368 kbit/s, hoc 63 lung 2048 kbit/s thc phn cp s PDHca Chu u. AU-3 c hnh thnh t mt lung nhnh 44736 kbit/s, hoc 7 lung nhnh 6312kbit/s, hoc 28 lung 1544 kbit/s. Cng c th s dng 63 lung 1544 kbit/s thay th cho 63 lung2048 kbit/s ghp thnh tn hiu STM-1 qua TU-12, ..., AU-4.

C th coi qu trnh hnh thnh STM-N bao gm hai bc c l p. Bc th nht hnhthnh module truyn dn ng b mc 1 (STM-1) t cc lung nhnh PDH. Bc th hai hnhthnh cc module truyn dn ng b bc cao mc N (STM-N), thc hin bng cch ghp xen

byte cc module truyn dn ng b mc 1 (STM-1) hoc cc module truyn dn ng b mcthp hn STM-M (M


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Ghp cc khung VC vo STM-1

Giai on 2: Hnh thnh mc STM-N t mc STM-1 hoc mc STM thp hn.

Tuy nhin ch xt qu trnh ghp knh SDH c ly t cc lung nhnh PDH theo tiu

chun Chu u. hiu r qu trnh ghp knh SDH, sau y s trnh by v cu trc khung SDH.

Cu trc khung SDH

Theo khuyn ngh G709, cc khung tn hiu trong SDH c t chc thnh khi thng tin

c 9 dng n ct v c thi hn l 125s.

Cu trc khung VC-3 v VC-4

Cu trc khung VC-3 v VC-4 nh hnh 1.21.

Hnh 1.21: Cu trc khung VC-3 (a) v VC-4 (b)

Khung VC-3 c trc 9 dng 85 ct. Ni mt cch khc l khung c 9 dng mi dng ghp85 byte, mi byte ghp 8 bit.

Khung VC-4 c trc 9 dng 261 ct, ngha l l khung c 9 dng mi dng ghp 261byte, mi byte ghp 8 bit.

Cu trc khung VC-3 v VC-4 gm 2 phn chnh:

Phn th nht ghp cc byte POH t dng 1 n dng 9 thuc ct 1 dng cho qun l,gim st tuyn mc cao. (chc nng v v tr ca cc byte ny sc trnh by trongmc 2.4.2.5)

Phn th hai l phn ti trng ghp cc lung nhnh PDH. Vi khung VC-3 cghp t dng 1 n dng 9 thuc ct 2 n ct 85, i vi khung VC-4 c ghp tdng 1 n dng 9 thuc ct 2 n ct 261.

Trnh t truyn cc byte trong khung l t tri sang phi v t trn xung di. Trnh ttruyn cc bit trong mt byte l bit c trng s ln nht truyn i trc v bit c trng s b nhttruyn sau cng. Nguyn tc truyn ny p dng cho mi loi khung tn hiu trong SDH.

Cu trc khung v a khung VC-n, TU-n mc thp

Cu trc khung v a khung VC-n, TU-n mc thp nh hnh 1.22.

Vng ti trng

P

O

H125s

85 ct

9

dn

VC-3 POH(a)

Vng ti trng

P

O

H125s

261 ct

9

dn

VC-4 POH(b)


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Hnh 1.22: Cu trc khung v a khung VC-n, TU-n mc thp

c im ca cc khung VC-n v TU-n mc thp l s byte rt t so vi VC-n v TU-n mccao. V vy phi sp xp thnh a khung c 4 khung s dng mt s byte mo u tuyn vmt con tr.

Cu trc a khung VC-n nh hnh 2.24a, gm 4 khung VC-n (mi khung VC-11 c 26 dngv 1 ct, mi khung VC-12 c 35 dng v 1 ct, mi khung VC-2 c 107 dng v 1 ct). Trongmi khung VC-n ca a khung VC-n gm c 2 phn:

Phn th nht l cc byte POH c ghp vo dng 1, ct 1 dng cho qun l v gimst tuyn mc thp. Nh vy trong mi a khung VC-n mc thp c 4 byte VC-n POH,c k hiu l V5, J2, N2 v K4.

Phn th hai l phn cn li trong mi khung VC-n dng sp xp cc lung nhnhPDH.

Cu trc trc a khung TU-n nh hnh 1.22b, gm 4 khung TU-n. c hnh thnh bngcch thm con tr TU-n PTR vo trong a khung VC-n (dng1, ct 1 trong mi khung VC-n).

Nh vy trong mi a khung TU-n c con tr TU-n PTR gm 4 byte; k hiu l V1, V2, V3, V4.

Cu trc khung STM-1

Cu trc khung STM-1 nh hnh 1.23.

Khung STM-1 c 9 dng 270 ct, ngha l, khung c 9 dng mi dng ghp 270 byte,mi byte ghp 8 bit.

Cu trc gm 3 phn:

Phn th nht dng ghp cc byte RSOH v MSOH. Cc byte RSOH ghp t dng1 n dng 3 thuc ct 1 n ct 9 dng cho qun l, gim st cc trm lp. Cc byte

V1

V2

V3

V4

TU-n

125s

250s

375s

500s

V5

VC-n

J2

N2

K4

VC-11 VC-12 VC-2

26 35 107

26 35 107

26 35 107

26 35 107

104 140 428

(b) (a)


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MSOH ghp t dng 5 n dng 9 thuc ct 1 n ct 9 dng cho qun l, gim st cctrm ghp knh.

Phn th hai dng ghp con tr khi nhnh AU-3 PTR hoc AU-4 PTRt ti dng4 thuc ct 1 n ct 9 (c 9 byte).

Phn th ba l phn ti trng c 9 dng 261 ct c s dng ghp 1 VC-4 hoc 3VC-3 hoc 63 VC-12 v.v.

Hnh 1.23: Cu trc khung STM-1

T cu trc ca khung STM-1 tnh c tc bit ca lung STM-1.

VSTM-1 = 9 dng/khung 270 byte/dng 8 bit/byte 8000 khung/s

= 15552104 bit/s = 155,52 Mbit/s

Cu trc khung STM-N

Khi ghp xen byte N tn hiu STM-1 to ra tn hiu STM-N th cu trc khung ca STM-N nh hnh 1.24.

Hnh 1.24: Cu trc khung STM-N

Vng ti trng

AU-n PTR

125s

270 ct

9

dn

RSOH

MSOH

9 261

Vng ti trng

AU-n PTR

125s

270 N ct

9dng

RSOH

MSOH

9 N 261N


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So vi kch thc ca khung STM-1 kch thc ca khung STM-N s tng N ln. Ngha l,

c 9 dng (270 N) ct.

Cu trc khung STM-N cng tng t nh cu trc khung STM-1, gm c 3 phn chnh:

Phn th nht dng ghp cc byte qun l, gim st cc trm lp RSOH v cc trmghp knh MSOH.

Phn th hai dng ghp con tr khi nhnh AU-3 PTR hoc AU-4 PTR

Phn th ba l phn ti trng.

1.2. Thng tin quangTrong nhng nm gn y cng ngh thng tin quang pht trin rt nhanh. Thng tin

quang c trin khai trong c mng ng di (lin tnh v quc t) v mng ni ht.

Trong mng thng tin quang th mi trng truyn dn si quang v cp si quang ng vaitr ht sc quan trng. c tnh ca si quang v cp si quang nh hng rt ln n cht lngca h thng. V vy cn nghin cu k lng v cu to, tnh cht v cc thng s ca si quang la chn, thit k, xy dng v bo dng tuyn thng tin cp si quang theo cc tiu chun vyu cu t ra.

Mun hnh thnh mt tuyn thng tin quang, ngoi cp si quang phi c thit b thng tinquang. Thit b thng tin quang c cc b phn ch yu nh chuyn i m nhnh pht, bchuyn i m nhnh thu, chuyn tn hiu in thnh tn hiu quang, chuyn tn hiu quang thnhtn hiu in, cc knh nghip v, v.v.

Chuyn tn hiu in thnh tn hiu quang ch yu s dng cc ngun quang bng bn dn.Hin ti c 2 loi ngun quang ch yu, l diode pht x nh sng (LED) v laser diode (LD).Mi loi ngun quang ny c u im v nhc im ring v c ng dng vo tng tuynthng tin quang c th.

Chuyn tn hiu quang thnh tn hiu in nhdiode tch quang (PD). Trong thc t thngdng PIN diode hoc APD. Mi loi diode tch quang cng c c tnh ring v s dng thchhp cho mi tuyn c th.

Ni cch khc, phi la chn ngun quang v diode tch quang khi thit k mt tuyn thng

tin cp si quang no .

1.2.1.M hnh hthng thng tin quangM hnh h thng thng tin quang nh hnh 1.25.


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Hnh 1.25: M hnh h thng thng tin quang

Chc nng cc phn t

B ghp Mux: C chc nng chuyn tn hiu thoi analog thnh tn hiu s, chuyn tnhiu truyn hnh TV v tn hiu Fax thnh tn hiu s, chuyn tn hiu s liu data t dng tn hiusu ra thit b truyn s liu thnh tn hiu s tng ng. Cc loi tn hiu ny c ghp thnhmt lng chung c tc bit cao a vo khi chuyn i tn hiu in thnh tn hiu quang(E/O).

Khi E/O: Khi ny chuyn tn hiu in thnh tn hiu quang. Mun vy phi dng

ngun quang nh LED hoc laser diode c bc sng thch hp. Nu tn hiu s bm trc tipvo ngun quang th gi l iu ch cng quang. Nu tn hiu s v tn hiu quang t laserdiode a vo mt biu ch th gi l iu ch ngoi. iu ch ngoi c th l iu ch cng, iu ch bin , iu ch tn s hoc iu ch pha. Tn hiu quang u ra khi E/O a vosi quang truyn i xa.

Trm lp: Trong hnh v l trm lp in. Ti y phi chuyn i tn hiu quang thnh tnhiu in (ti hng thu), ti to xung, khuch i xung v chuyn i tn hiu in thnh tn hiuquang (ti pha pht). Nu dng b lp quang th khng cn chuyn i quang - in - quang.Trm lp s dng trong trng hp hai trm u cui hoc xen/r vt qu c ly cho php. Thng

thng khi s dng laser diode truyn qua si quang n mode th c ly l 60 - 80 km (ph thucbc sng).

Khi O/E: Khi ny c chc nng chuyn tn hiu quang thnh tn hiu in. Mun vydng diode tch quang PIN hoc APD. Dng tch quang chnh l dy tn hiu sc a vo bkhuch i nng cng sut tn hiu thu.

Khi DEMUX: Khi ny tch lung tn hiu su vo thnh cc knh tiu chun, sau gii m chuyn thnh tn hiu thoi, tn hiu truyn hnh TV v a n thit b thu bao.

Cp si quang: Trong thng tin quang ch dng mt cp si quang. S si quang trong cp

ph thuc dung lng ca tuyn v phng thc d phng. Mi h thng thng tin quang cn 2si quang, mt si pht v mt si thu. Trong phng thc d phng 1 + 1 th mi h thng hotng (2 si quang) c mt h thng d phng (2 si quang).

MUX E/O O/E E/O O/E

DE-

MUX

Trm u cui Trm u cuiTrm lp

Telephone

Fax

Data

TV

Telephone

Fax

Data

TV


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1.2.2.Cc loi cp si quangSi quang l loi si in mi c ch s chit xut thp. Si c cu trc hnh tr ca vt liu

in mi trong sut, gm li truyn nh sng v bao quanh li l v c ch s chit sut nhhn ch s chit sut ca li. iu ny nhm to ra iu kin nh sng truyn c trong li.V cn c tc dng bo v li. Vt liu cbn ch to li v v l Silica (SiO2). Thng dngGermani dioxide (GeO2) b sung vo Silica lm tng ch s chit xut ca li. Mun lm gimch s chit xut ca v phi dng cht b sung l Fluorine. trnh try xc v v tng bnchc, si quang thng c bao bc thm mt lp cht do tng hp. Lp v bo v ny sngn chn cc tc ng chc vo si, gia cng thm cho si, bo v si khng b nt do kodn hoc xc do c xt b mt; mt khc to iu kin bc si thnh cp sau ny. Lp v bcny c gi l lp v bc scp. Cu trc y ca mt si quang cho vin thng nh hnh1.26.

Tu thuc tng loi si m c s phn b chit xut khc nhau trong li si. Nu chit xutphn bu th gi l si chit xut bc, nu phn b theo qui lut tng dn dn gi l si chit

xut gradient. Kch thc ca si ph thuc loi si, loi th nht li c ng knh 2a = 50m

gi l si a mode, loi th hai li c ng knh 2a 10m gi l si n mode. ng knh v

d ca cc loi si u bng 125m.

Hnh 1.26: Cu trc tng th ca si quang sdng trong vin thng

Tng hp c phn b chit xut v kch thc ca li chia thnh ba loi si, l:

Si a mode chit xut bc.

Si a mode chit xut gradient.

Si n mode (chit xut bc).

Ngoi ra, khi phn loi theo cu trc vt liu si quang c chia thnh cc loi sau:

Si thu tinh (loi si thng thng)

Si li thu tinh v cht do.

Si thu tinh nhiu thnh phn Si cht do

Li V Lp v bc scp


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Sau y s nghin cu chc nng cc thnh phn ca si thng thng.

Li si ng vai tr ht sc quan trng, l mi trng truyn dn nh sng. ngknh li ln hay b v tr s cng nh phn b chit xut ca li nh hng trc tip n cht

lng tn hiu thu. ng knh li ln (50 m) truyn nhiu mode (nhiu tia) nn gi l si a

mode. Ngc li, nu ng knh li b ( 10 m) th truyn ch mt mode (mt tia) nn gi lsi n mode.

Tr s ca chit xut li (n1) phi ln hn tr s chit xut v (n2) to ra phn x tonphn ti tip gip li - v. iu ny c th hin trong hnh 1.27.

Gi thit nh sng truyn t mi trng c chit xut ln sang mi trng c chit xut

b. Trong hnh 1.27a tia ti hp vi php tuyn mt gc b hn gc ti hn, ngha l 0 < < c

nn c tia khc x v gc khc x 0 < , < / 2. Theo qui tc Snell vit c:

n1 sin = n2 sin , (1.1)

Khi gc ti tng cho n khi tia khc x trng vi tip gip gia hai mi trng th gc ti = c gi l gc ti hn v gc khc x

,= / 2 (hnh 1.27b). V vy:

sin c= n2/ n1 (1.2)

Khi r> c s xy ra phn x ton phn (hnh 1.27c).

Hnh 1.27: Khc x v phn x ton phn ti tip gip li v v

ca si a mode chit xut bcNh vy, iu kin xy ra phn x ton phn l:

Cc tia sng phi truyn t mi trng c chit sut ln sang mi trng c chit sutnh hn.

Gc ti ca tia sng phi ln hn gc ti hn.

Cc nh lut phn x v khc x nh sng l nguyn l cbn p dng cho vic truyn tnhiu nh sng trong si quang. si quang, cc tn hiu nh sng c truyn da vo hin tng

phn x ton phn.

1.2.2.1.Truyn nh sng trong si quanga modeSi quang a mode chit xut bc

c r r

A

A

B

B

C C

n1

n2

n1>n2

(a) (b) (c)


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Mt ct chschit xut:

Loi si ny gi l si quang a mode chit xut bc. Si c c trng bi vng li ngnht c chit xut l hng s n1 v xung quanh n l v c chit xut l n2 < n1. Mt ct dc casi v mt ct chit xut ca n c th hin tng ng trong cc hnh 1.28a v 1.28b. V mt

ct chit xut c hnh bc thang nn gi l chit xut bc. Li si c ng knh 2a=50m, v sic ng knh d=125m.

Hnh 1.28: Si a mode chit xut bc

Khu s:

hiu r khu s (NA - Numerical Aperture) ca si, xem xt mt mt ct dc qua trcsi quang chit xut bc nh hnh 1.29.

Hnh 1.29: Mt ct dc si chit xut bc

T ngun quang c 3 tia sng truyn n si quang. Ti u si cc tia sng i vo li sit mi trng c chit xut na=1. Mun tia sng truyn trong li si th gc ti ti tip gip li v

v phi ln hn gc ti hn c. Mun vy th tia khc x ti u si phi nghing vi trc si mt

gc m = / 2 - m b hn c = / 2 - c v gc ti ca tia t ngun quang i vo li si m phi b

hn c. xc nh c v c cn p dng qui tc Snell:

i vi cc tia khc x ti tip gip li - khng kh vit c:

sin m = n1sin m = n1cos m

Cc tia khc x ny truyn c trong li si nu tho mn iu kin phn x ton phnti tip gip li - v, ngha l:

sinm= sin c = n1sin c = n1cos c

sin c = n2/ n1

r

n

a2

d

n1n22

d

a

(a) (b)

m

n2

n1

na =1n1>n2

na

A

B C

C

A

B

m

m

m

l

l/ cos m


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cos c = (n12- n22)1/ 2/ n1

V vy:sin c = (n12- n22)1/ 2

Cc tia t ngun quang i vo li si c gc ti ln hn c khng truyn c trong li

si. Gi trc cng ln th cng sut nh sng ca cc tia truyn c trong li si cng ln. c l

gc n nh sng, sin cc gi l khu s ca si quang, k hiu l NA.

NA = sin c = (n12- n22)1/ 2 = n1 (2)1/ 2 (1.3)

trong : = (n12- n2

2)/ 2n12 (n1- n2)/ n1 l lch tng i ch s chit xut li v v.

Mt tham s khc c lin quan n NA, l tn s chun ho V c xc nh theo(1.4).

( )NAa2

V

= = 222

1 nna2

(1.4)

Trong : a l bn knh li si quang, l bc sng ca nh sng.

V khu s c lin quan ti gc vo ln nht, cho nn n th hin s tip nhn nh sngv kh nng tp trung cc tia sng ca si. Ngoi ra, qua khu s cho php tnh c hiu quca qu trnh ghp ngun pht vo si quang.

Quo truyn lan ca cc tia sng

Hnh 1.28a th hin quo truyn lan cc tia sng trong li si. Do cu trc ca si nhtrn nn s truyn lan nh sng c m t nhphn x ton phn bn trong lm cho cc tia sngkhi truyn trong li c dng l nhng on thng gp khc. Mi tia l mt mode sng, nh vytrong si a mode chit xsut bc truyn c s lng ln cc tia. S lng mode truyn trong

li si c xc nh theo biu thc

N 2

V2(1.5)

Trong : V l tn s chun ho c xc nh bi (1.4).

Si quang a mode chit xut Gradient

Mt ct chit sut

Khc vi si a mode chit xut bc, si a mode chit xut Gradient c mt ct chit xut

nh hnh 1.30b.Trong si quang chit xut Gradient, chit sut ca li khng ng u nh si a mode

chit xut bc, m n gim dn t tm li ra ranh gii phn cch li - v theo chiu tng ca cabn knh r. Cn chit xut v l mt hng s. Phn b chit xut ca si quang a mode chit xutGradient nh biu thc (1.6).

2/12

1 a

r21n

Khi 0 r a

n2 Khi a < r d/2

Trong r l bn knh ca si quang; a l bn knh li si; n1 l chit sut ti trc li si;

n2 l chit xut ca v; c gii thch ti biu thc (1.3).

n(r) = (1.6)


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Hnh 1.30: Si a mode chit sut Gradient

Khu s:

So vi si chit sut bc th vic xt khu s ca si chit xut gradient c phc tphn. si Gradient, khu s NA ph thuc vo v tr mt ct ngang u li si. Ni mt cch

khc, v chit sut ca si a mode Gradient ph thuc vo bn knh r ca si, do vy gc n nhsng max v khu s NA cng l hm ca bn knh. Xt v quang hnh hc th nh sng ti lisi ti v tr r s lan truyn c trong li ch khi n trong khu s cc b NA(r) ti im .Khu s cc b NA(r) c xc nh nh biu thc (1.7).

( )[ ] 2/1222 nrn ( )2

a

r10NA

Khi 0 r a

0 Khi a < r d/2

Vi NA(0) l khu s ti trc si v c xc nh nh biu thc (1.8).

NA(0) = ( )[ ] 2/1222 n0n (1.8)T biu thc (1.7) cho thy NA ca si Gradient gim t gi tr NA(0) (v ti li c r = 0)

ti khng (v ti tip gip li - v r = a). Nh vy, cc tia sng i n li si quang gn tip gipli - v si quang phi song song vi tip gip ny th mi truyn c vo li.

Quo truyn lan ca cc tia sng

Quo cc tia sng truyn trong li si a mode Gradient nh hnh 1.30b. V ch s chitsut ca li si l ng cong Parabole nn cc tia sng i hng lin tc v to thnh ngcong hnh sin v ct nhau ti cc im cch u trn trc si.

u im ni bt ca si a mode Gradient l rng bng tn ln hn si a mode ch sbc v tc truyn ca cc mode khc nhau trong li si hu nh c cn bng nhcu tomt ct ch s chit sut thch hp.

S lng mode truyn trong li si c xc nh theo biu thc (1.9).

N 4

V2(1.9)

Trong : V l tn s chun ho c xc nh nh biu thc (1.4).

NA(r) = (1.7)

r

n

a2

d

n1n22d

a

(a) (b)


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1.2.2.2.Truyn nh sng trong si quangn modeTrong si a mode chit xut bc tn sc mode c nh hng ln nht v lm hn ch kh

nng truyn tn hiu. loi tr hon ton tn sc ny cn ch to si sao cho trong li ch truynmt mode cbn nh hnh 1.31.

Hnh 1.31: Si n mode chit sut bc

iu kin truyn mt mode cbn trong si n mode l tn s chun ho V phi thomn biu thc (1.10).

405,2nna2

V2

22

1

= (1.10)

u im ca si n mode l bng tn ln hn so vi si a mode do khng c tn scmode. Nhn t ch yu lm hn ch bng tn ca si n mode l tn sc sc th. Ngoi ra sin mode cn c cc u im khc nh: Suy hao thp, dung lng ln nn p ng c nhu cutruyn tn hiu bng rng trong tng lai.

T biu thc (1.10) nu a, n1 v n2c chn th s lng mode N ph thuc vo bc

sng . Nu bc sng t c t gi tr no tr ln th trong li si ch truyn mt mode,ngc li nu bc sng nh sng b hn gi tr gii hn th trong li si truyn nhiu mode.

Bc sng ti thiu m bo cho si quang hot ng n mode gi l bc sng ct (c). Khinim v bc sng ct l rt quan trng i vi cc si n mode bi v n xc nh vng hotng n mode ca si. Bc sng ct c xc nh theo biu thc (1.11).

22

21c nn405,2

a2

= (1.11)

Nu si c ng knh li 2a = 9m, NA= (n12- n22)1/2 = 0,11 th bc sng ct s l:c = 3,14 9m 0,11/ 2,045 = 1293 nm

Mun t c tn s ct nh biu thc (1.12) th phi tng di bc sng cng tc ca

ngun quang hoc gim ng knh li si hoc gim hiu s chit xut gia li v v (). Tuy

nhin nu gim thp hn gii hn cho php th si rt nhy cm vi suy hao do un cong.

T biu thc (1.10) cng xc nh c ng knh ca li si:

( ) 2/12221 nn405,2

a2

r

n

a2

d

n1n22

d

(b) (a)


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Mc d ng knh li gim b hn 10 m nhng ng knh v vn phi m bo

125m bo v li si t cc tc ng chc v gim nhy cm i vi suy hao do un cong.Cc ch tiu k thut ca si n mode tiu chun nh bng 1.4.

Bng 1.4. Cc ch tiu k thut ca si n mode tiu chun

Cc tham s n v Gi tr danh nh Sai s

ng knh trng mode m 910 10%

ng knh v m 125 3

lch tm gia li v v m 1

mo ca v %


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Trong cc suy hao trn y, suy hao do hp th c lin quan ti vt liu si bao gm hpth do tp cht, hp th vt liu v hp th vng hng ngoi v vng cc tm. Suy hao do tn xc lin quan ti c vt liu si v tnh khng hon ho v cu trc ca si.

Nguyn nhn bn ngoi gy ra suy hao c th l do ghp ni, lp t v mi trng gy ra.

Trn mt tuyn thng tin quang, cc suy hao ghp ni gia ngun pht quang vi si quang, gia siquang vi si quang v gia si quang vi u thu quang cng c coi l suy hao trn tuyn truyndn. Bn cnh , suy hao cn do vi un cong (vi bn knh un cong rt nh) v un cong qu giihn cho php. Un cong l khng th trnh khi trong iu kin hot ng hin ti cho c bn trongcp v ti cc hp cha mi hn. Vi un cong ch yu hoc do lc p vo b mt g gh ca v sihoc do on si bn trong cp. Ngoi ra s thay i nhit cng gy ra vi un cong. gim suyhao vi un cong ti mc nh nht l bc mt lp v c kh nng chu nn cho si. Khi c lc bnngoi tc ng vo th v ny s b bin dng trc nhng si vn nh hng tng i thng.

Khi si b cong qu mc th nh sng khng phn x ti tip gip li - v, m truyn vo

v v gy ra suy hao. V l thuyt suy hao cng sut quang ti on si a mode b cong t l viexp(R/ Rc), trong R l bn knh cong, Rc a/ (NA)2 = a/ 2n12 l bn knh cong ti hn, a l

bn knh li si. Ti on cong c bn knh Rc suy hao l ng k, nhng suy hao dng hm m sgim rt nhanh khi cong gim. Trong si n mode suy hao un cong ph thuc vo phm vim in t trng thm nhp vo v, v v vy ph thuc vo mt ct h s chit xut v bcsng. Suy hao do un cong chnh l trng mrng vo v v suy bin theo hm m theo khongcch bc x. Mt phng pha vung gc vi trc si. Tc pha ca mode dn bt k l thp hntc pha ca cc sng phng trong v (c/n2). Nhng bn ngoi on cong tc pha tng theokhong cch bc x v t c c/n2.

Trong si n mode si b un cong snh hng n c tnh chc nhiu hn nhhng n suy hao. Nu si b un cong thi qu th mt si pha ngoi b dn thm 0,2% v sib nt, cn mt si pha trong s b nn v gy. Mun ngn nga phi t si trong cp.

2) Tn sc ca si quang

Hin tng tn sc lm ni rng xung nh sng theo thi gian v mo xung nh sngtruyn dc theo si. Tn sc lm hn ch kh nng truyn tn hiu ca si hoc ni ng hn lhn ch bng tn cng tc v c ly truyn dn ca si.

C cc loi tn sc ch yu sau y: Tn sc Mode, tn sc vt liu v tn sc ng dnsng. Tu loi si m tn sc no trong s cc tn sc ny vt tri.

Tn sc mode ch ph thuc vo kch thc ca li si, n tn ti trong cc si a mode.Cc si n mode gn nh khng c tn sc mode m ch tn ti tn sc vt liu v tn sc ngdn sng (tng hai loi tn sc ny gi l tn sc sc th).

Tn sc mode cn gi l tn sc gia cc mode. Nguyn nhn l trong si a mode c skhc nhau v tc nhm gia cc mode v do cc mode xut pht tu si ti cng mtthi im nhng n cui si khng ng thi. lch thi gian gia mode nhanh nht v modechm nht c trng cho tn sc mode.

Tn sc sc th (hay tn sc tng) gm c hai thnh phn, l: Tn sc vt liu v tnsc ng dn sng nh ni trn y. Tn sc vt liu l do chit xut ca vt liu ph thuc vo

bc sng. Tn sc ng dn sng l do s ph thuc khng tuyn tnh ca hng s truyn lan vo tn s (bc sng) trong ng dn quang.


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3) rng bng tn cng tc ca si

Hin nay trn mt s tuyn thng tin quang cn s dng si a mode chit xut gradient. rng bng tn ca loi si ny b hn ch ch yu do tn sc mode hoc tn sc vt liu tuthuc vo bc sng ca ngun quang. Khi ngun quang l LED c c tnh ph rng v hot

ng ti = 850 nm th tn sc vt liu ng vai tr ch yu. Ngc li, nu s dng laser diodec c tnh ph hp hn v hot ng ti = 1300 nm th tn sc mode li ng vai tr ch yu.

rng bng tn cng tc ca si n mode ph thuc ch yu vo tn sc tng.

4) Khu s

Khi nim khu s NA c trnh by trn. NA c xc nh theo cc biu thc(1.3) v (1.7).

5) Bc sng ct

Khi nim bc sng ct c trnh by trn. Bc sng ct c xc nh theo biu

thc (1.11).

1.2.3.My pht tn hiu quang1.2.3.1.Cc loi ngun quang

Ngun quang trong thit b thng tin quang l linh kin c kh nng chuyn i tn hiuin thnh tn hiu quang di bc sng truyn trong si quang. C hai loi ngun quang, ldiode pht x (LED) v laser diode (LD). Sau y trnh by mt s khi nim lin quan n chcnng ca ngun quang.

Nu trong nguyn t (hoc phn t) chuyn ng ca cc in tc gii hn trong mt

phm vi hp cbng kch thc nguyn t, th trong cc cht rn ni chung v cht bn dn niring, cc in t ho tr c th chuyn ng t nguyn tnt mng tinh th ny n nguyn tntmng tinh th khc v l shu chung ca c mng tinh th. V cc mc nng lng ca in t phthuc vo v tr tng i ca n so vi mng tinh th, m sin t li rt nhiu, do s cc mcnng lng ca cc in t ho tr trong ton mng tinh th l mt s v cng ln. Ngoi ra trong chtrn, cc nguyn tc phn b st nhau, cc lp vin t ca chng c bit l nhng lp phangoi che ph ln nhau v tng tc vi nhau rt mnh. S tng tc ny gy nn nhng dch chuynv tr v lm tch cc mc nng lng in t ra thnh nhiu phn mc khc nhau.

i vi cc cht bn dn, nhng vng nng lng cho php c ngn cch vi nhau bi

tp hp cc gi tr nng lng vng cm. Thng thng rng vng cm ca cc cht bn dnin hnh khong 0,11,0 eV. Trong s cc vng nng lng cho php, vng trn cng dn ycc in t ho trc gi l vng dn. Vng tip theo cn hon ton trng nhit 0K gil vng cm v vng di cng gi l vng ho tr. V qu trnh vt l xy ra trong cc cht bndn ch lin quan n cc in tvng ho tr hoc y vng dn do khi v gin nnglng ca bn dn ch n hai vng ny nh hnh 1.32.


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Hnh 1.32: S vng nng lng ca bn dn

nhit thp th bn dn tr thnh cht in mi. Khi nhit tng th bn dn trthnh cht dn in. Bi v khi cc in t ca vng ho tr nhn c nng lng ln vt qua vng cm ln vng dn v trthnh cc in t dn. Khi vng ho tr, ti ni int va i khi s xut hin cc l trng nh hnh 1.33.

Hnh 1.33: S phn b mt cc in tv l trng

Qu trnh ny c gi l qu trnh to cp in t v l trng bng nhit. Qu trnh xy rakhng ch do nung nng bn dn, m c th hnh thnh di tc dng cc dng kch thch khc. Vd nh bng nh sng, dng in, bn ph bi cc in t v ion bn ngoi.

Song song vi qu trnh trn, trong tinh th bn dn cn xy ra qu trnh ngc li gi l

qu trnh ti hp in t-l trng, cc in t ca vng dn c th thc hin chuyn di t phtxung vng ho tr v chim ly cc mc nng lng t do .

Cc thc nghim v quang phu cho thy khi cc nguyn t hp th v bc xu hnhthnh ph vch. Hin tng ny c gii thch da vo mc nng lng ri rc tng ng vicc trng thi ca nguyn t. K hiu E1 v E2 l hai mc nng lng ca mt nguyn t. y,E1 l nng lng trng thi nn v E2 l nng lng trng thi kch thch. Ti trng thi cn bngnhit th cc in tmc nng lng thp E1 (hnh 1.34a). Theo nh lut Planck th s dchchuyn gia hai trng thi ny c lin quan ti qu trnh h p th v pht x ca cc photon c

nng lng h12= E2-E1. Bnh thng, h thng trng thi nn. Khi c mt nng lng h12 tcng vo h thng th mt in t trng thi E

1s hp th nng lng ny v c kch thch

ln trng thi E2 (hnh 1.34b). V y l trng thi khng bn vng nn in t s nhanh chngquay li trng thi ban u v s gii phng mt nng lng bng E2-E1. Hin tng ny gi l

Vng dn

Vng ho tr

Vng cmEg

Vng dn

Vng ho tr

Vng cmChuyn

dchin t

in t

L trng

Mt ccin t

Mt cc ltrng

Nng lngcc in t

EC

EV

E0


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pht x t pht (hnh 1.34c) v khi nng lng c gii phng di dng nh sng th gi l nhsng pht x t pht. Pht x ny ng hng, c pha ngu nhin. Mt s cht d dng pht sng,v mt s cht khc khng pht sng.

Hnh 1.34: Biu mc nng lng v qu trnh pht x

Theo chc lng t th bc sng nh sng khi pht x hoc hp thc xc nh theo

biu thc sau y:E2-E1 = h12,12 = (E2- E1)/ h

Vy: = c/12 = hc/(E2-E1) (1.13)

Trong : h = 6,626 10-34 J.s l hng s Planck

c = 3108 m/s l tc nh sng trong khng gian t do.

Khi nh sng c nng lng bng E2 - E1 tc ng vo h thng trong khi in tang trng thi kch thch th in t hp th nng lng nh sng ti buc n trv mc nng lngE1v gii phng ra nng lng. Nng lng nh sng c gii phng ti thi im ny s lnhn nng lng nh sng pht x t pht v pha ca n l pha ca nh sng ti. Hin tng nygi l pht x cng bc (hnh 1.34d). Bc sng pht x cng bc cng c xc nh theobiu thc (1.13).

1.2.3.2.Diode pht quang (LED)LED pht x mt (SLED)

Diode pht x mt c cu trc d th kp c k hiu l DH SLED. Mt ct ngang caDH SLED kiu chn nh hnh 1.35.

Hnh 1.35: Mt ct ngang ca DH SLED kiu chn

E1

E2

in t

a)

L trng

b) c) d)

nh sng

Bc x t pht Bc x cn bc

nh sng

ti x c

Lp tip xc

L nn n-GaAs

150 m

50 m

60 m

-GaAs


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Lp hot tnh c b rng gn bng ng knh li si a mode v pha ni vi si quangkhot mt h su chn u si quang gn lp hot tnh. Nh vy s hng c nhiu tia sngi vo li si, ng thi gim suy hao cng sut nh sng. Lp cch in SiO2 ph ln lp tipxc dng ch tr mt vng i din vi lp hot tnh tp trung mt dng qua lp hot tnh

v s nng cao c hiu sut pht x. Lp tip xc dng t gn lp hot tnh s to nhit vm bo cho nhit ca ngun quang khng vt gii hn cho php. Nu nhit lp hot tnhvt qu phm vi cho php s gy ra ba hu qu l bc sng bc x thay i theo nhit , h slng t bn trong gim do tng tc ti hp khng bc x khi nhit tng, gim tui th caLED. Qua tnh ton v thc nghim thy rng cng sut pht ca LED gim 50% nu nhit

trong phng tng ti 900C 1000C. LED ch to t GaAlAs v InGaAsP th nhit nh ca tip

gip phi duy tr thp hn 600C 700C.

Hnh 1.36 minh ho nguyn l hot ng ca LED.

Hnh 1.36: Nguyn l hot ng ca LED

LED gm o mt cc ht ti in, bc x t pht v pht nh sng vo si. LED sdng ngun phn cc thun, tc l cc dng ca ngun ni vi lp tip xc dng. Khi c dngbm qua LED th cc in t t di ho tr nhy ln di dn. Di tc ng ca in trng phn

cc thun, cc in t t lp N chuyn dch vo lp hot tnh, cn cc l trng t lp P chuyndch vo lp hot tnh. Cc cp in t l trng ti hp vi nhau v bc x photon.

NP

Lp hot tnh

Vng dn

Vng ho tr

Vng cm

in tL trn

Hn ro d th

Hng ro d th

Lp hot tnh

V

NP

Ti hpPht x t pht

p

hoc


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LED pht x cnh (ELED)

Diode pht x cnh c cu trc d th kp c k hiu l DH ELED v c cu to nh hnh1.37.

Cng sut quang c truyn dc theo lp hot tnh nhphn x bn trong ti cc tip

gip d th v i ti mt bn ca diode. Lp hot tnh c qui nh bi lp tip xc v rnh sucui lp hot tnh. Nhvy m lp hot tnh c thu ngn nhng kch thc ca chip li khngqu b. T hp th ca lp hot tnh gim do lp ny rt mng. Cng sut quang b hp th lnnht xy ra ti di bc sng ngn. Nhvy m thu hp b rng ph so vi LED pht x mt.

rng ph gim t 35nm xung 25nm ti bc sng 0,9m v t 100nm xung 70nm ti 1,3m.

nh sng u ra ca DH ELED c dng hnh chp elip, gc mtheo chiu ng l 300 vtheo chiu ngang l 1200. Gc pht x nh vy s ghp ni ELED vi si a mode c hiu quhn so vi SLED v cng c th phng vo si n mode mt cng sut quang ng k. So viSLED th ELED kh to nhit hn. Nhng so vi laser diode th ELED d ch to hn, hot ng

n gin hn, tin cy cao hn v r hn. V vy n c s dng rng ri trong cc h thngm tin cy v gi thnh c u tin hn cht lng.

Hnh 1.37: S cu trc ca diode pht x cnh

Cc lp bao gm: Lp cch in; p+-GaAs; P-Ga0,6Al0,04As; Lp hot tnh n-Ga0,9Al0,1As; N- Ga0,6Al0,4As; n-GaAs

Cc tham s ca LED

Cc tham s cbn ca LED nh bng 1.5.

350m

L nn n-GaAs

Lp tip xc v to nhit Au

150m1200

30050m

3m2m0,05m2m3m


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Bng 1.5. Cc tham s ca LED

Cc tham sGii bc sng

800 nm 850nm

Bc sng 1300nm

Vt liu lp hot tnh Ga Al As Ga In As P

rng ph, nm 30 60 50 150

Cng sut pht, mW 0,5 4,0 0,4 0,6

Cng sut phng vo si, mW:

- a mode 2a = 50m

SLED

ELED

n mode ELED

0,01 0,05

0,05 0,13

0,015 0,035

0,03 0,06

0,003 0,03Dng iu khin, mA 50 150 100 150

Thi gian tng sn xung, ns

SLED

ELED

4 14

2 10 2,5 10

Tn siu ch, GHz 0,08 0,15 0,1 0,3

Tui th, 106 h 1 10 50 100

1.2.3.3.Laser diode c khoang cng hng Fabry- perotLaser diode c cu trc d th kp nh LED, nhng c kh nng khuch i. t c

mc ch ny thng dng khoang cng hng Fabry - Perot, bng cch mi nhn hai u d thkp thnh hai gng phn x nh hnh 1.38a. Cu trc ny ca laser diode c vit tt l FP-LD.

Khong cch hai gng trong Laser diode Fabry-Perot l L. Cc gng ny c c khnng to ra hi tip tch cc, tc l s quay li ca cc photon kch thch trong vng hot tnh skch thch nhiu photon hn. nh sng i ra ngoi qua hai gng phn x.

Xt iu kin khuch i trong laser diode Fabry-Perot: mt sng truyn t gng bn tri

ti gng bn phi, nh hnh 1.38b. Ti gng bn phi, sng ny s phn x v tip tc truynnh th. Dng sng ny gi l sng ng. trong bung cng hng ch c sng vi bc sngn nh th n phi l sng ng. Yu cu vt l ny c thc vit nh biu thc

2L/ =N (1.14)

Trong : L- khong cch hai gng

N- s nguyn.

tho mn iu kin cng hng, hai gng phn x phi cch nhau mt qung l Lbng s nguyn ln na bc sng.

Qu trnh pht x ca FP-LD c thc hin khi mt vi bc sng cng hng nmtrong ng cong khuch i c h s khch i ln hn suy hao nh hnh 1.38c.


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Hnh 1.38: Laser diode Fabry-Perot: (a) Cu to ca khoang cng hng;

(b)Hnh thnh sng ng trong khoang cng hng;(c) ng cong tn hao-khuch i; (d) Ph pht x ca FP-LD

1.2.3.4.My pht tn hiu quangChc nng chuyn i in-quang ca my pht quang c th hin trong hnh 1.39.

Hnh 1.39: S khi my pht tn hiu quang

B lp m c chc nng chuyn m ng thnh m thch hp vi hot ng ca ngunquang v ng truyn. Biu khin chuyn in p tn hiu n cc thnh dng bm Ip chongun quang. Nu dng Ip t gi tr cc i th cng sut pht ca ngun quang cng t gi trcc i. Ngc li, khi Ip cc tiu th cng sut pht ca ngun quang gn bng zero. y l

phng thc iu ch cng bc x ca ngun quang.

1.2.4.My thu tn hiu quang1.2.4.1.Cc loi photodiode

Nguyn tc tch quang

Ngunquang

B

iu khin

Tn hiu vo Tn hiuquang ra

IPB

lp m

(b)

nm

Cng

sutura(mW)

Tn hao

(nm)

Khu

chi/Tnhao

NN+1 N-1N+2 N-2

(d)

Vng hot tnhGngGng

L

(a)

Vng hn ch

Vng hn ch

(c)


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Trong thit b thng tin quang s dng hai loi diode tch quang (PD), l PIN diode vdiode quang thc (APD). Trc khi phn tch chi tit cc loi, cn hiu r nguyn tc chung vtch quang.

C hai loi PD u da vo tip gip p-n phn cc ngc. Khi nh sng chiu vo PD c

bc sng trong khng gian t do b hn bc sng ct c xc nh theo biu thc

( )]eV[E

]m.eV[24,1m

gc

= , (1.15)

trong Eg l rng di cm, th bn dn s hp th cc photon.

Hp th mt photon s kch thch mt in t trong di ho tr nhy ln di dn v litrong di ho tr mt l trng. Nh vy mi photon c hp th s to ra mt cp in t - ltrng. Tuy nhin biu thc (1.15) ch l iu kin cn cho tch quang. iu kin tch quangl cc cp in t-l trng c to ra do hp th photon s khng ti hp trc khi hnh thnh

dng in qua mch ngoi (hnh 1.40). Khng phi tt c photon c hp th trong diode tchquang u tham gia vo s hnh thnh p ng ca diode. Cn ch l, khc vi laser diode hotng trong mt di bc sng rt hp, cn PD li hot ng trong di bc sng rt rng. Dicm l mt tham s ch cht ca PD.

Nh nhn xt trn l silic khng s dng cho laser diode, v c di cm gin tip.Nhng silic li c th s dng ch to PD hot ng trong di bc sng ti ca s th nhtca si quang. Mt im khc nhau cbn na gia LD v PD l PD c nh thin ngc. Docng sut quang thu rt b nn khng lm nng PD, do vic ch to cc PD di dng mchtch hp quang in rt d dng.

1.2.4.2.Diode tch quang p-nDiode tch quang p-n nh hnh 1.40.

Hnh 1.40: Diode tch quang p-n

Tip xc P-N khng c in p phn cc

Trong vng bn dn n nng in t cao hn nng l trng; trong vng bn dn pnng l trng cao hn nng in t. V vy ti vng ln cn tip gip p- n xut hin hintng khuch tn cc ht ti in: l trng khuch tn t vng p sang vng n v cc n t

khuch tn t vng n sang vng p. Dng khuch tn hng t p sang n. Cc in t khuch tnqua tip gip s ti hp vi cc l trng trong vng p v hnh thnh cc ion m trong vng p lncn vi tip gip. Cc l trng khuch tn qua tip gip s ti hp vi in t trong vng n v hnh

V

p n

_

_

_

+

+

+

+

nh sng ti

ETXEngoi

(a)


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thnh cc ion dng trong vng n ln cn vi tip gip. v hai pha ca tip gip xut hin haikhi in tch cnh tri du v gi l vng in tch khng gian hay vng ngho (nng cc

ht ti in khng ng k). dy vng ngho ny khong 1m. Hai khi in tch tri du tora in trng tip xc (ETX) c vc tcng in trng hng t cc in tch dng c

nh sang cc in tch m cnh. Tip xc P-N phn cc ngc

Do vng ngho hu nh trng rng cc ht ti in nn c in trln hn cc vng nmbn ngoi vng ngho. Do ngun phn cc ngc (V) to ra trn vng ngho mt in trngngoi (Engoi) c tr s gn bng V v chiu ca vc tcng in trng ngoi trng vi chiuvi vc tcng in trng tip xc. in trng tng ny to ra trn tip gip p-n mt hngro th ngn trs khuch tn ca cc ht ti in a s qua tip gip. Ngc li, cc ht ti inthiu s tri qua tip gip d dng to ra dng in ngc hay cn gi l dng ti. Do nng cc ht ti in thiu s thp nn dng in ngc nhanh chng t gi tr bo ho.

Khi c nh sng tinh sng i vo PD qua lp p rt mng, qua vng ngho c hnh thnh ti tip gip

p-n v tip tc truyn vo lp n. Cc photon b hp th sut chiu dc ca PD. Cng nh snggim theo hm m khi truyn qua cc lp bn dn.

Trong diode tch quang iu kin quan trng kch thch cc in t nhy ln di dn lhp th photon t nh sng u vo hoc tng nhit .

Hu ht nh sng i vo bn dn sc hp th trong diode tch quang, nu diode tchquang c di. Tuy nhin ch c hp th photon xy ra trong lp ngho mi c vai tr quantrng. Hiu qu ca hp th photon trong vic to ra dng tch quang c nh gi theo h s

lng t v p ng R.

1.2.4.3.Diode tch quang p-i-nCu to v hot ng

Diode tch quang p-i-n da trn cu trc ca ca PD p-n bng cch xen vo gia lp p vlp n mt lp bn dn thun i nh hnh 1.41. Vng ngho trong diode tch quang p-i-n bao gmton b lp i. Trong vng ngho hnh thnh mt hng ro th ngn cn cc ht ti in a sivo vng ngho. in trng ny c to ra nhcc khi in tch cnh nm v hai pha catip gip gia lp i vi cc lp p v n. Cc l trng v cc in t trong lp i c hnh thnh khi

vng ngho hp th photon.

u im quan trng ca p-i-n diode l ci thin c p ng tn s do in dung cadiode CD rt b . t c iu ny nhxen thm lp i. gii thch vn ny, s dng biu

thc xc nh in dung ca mt tin phng song song: C = 0r(A/ d), trong 0 l hng s

in mi ca khng gian t do, r l hng sin mi tng i ca mi trng gia hai m tin, A l din tch ca m tin, d l khong cch hai m tin. Mt khc p ng tn scng ph thuc vo thi gian chuyn dch ca cc ht ti in qua vng ngho. Vng ngho mrng ca p-i-n diode lm chm thi gian chuyn dch. Vng ngho cng rng th thi gian chuyndch cng di. in trng trong vng ngho ca p-i-n diode yu hn in trng trong cu trc

diode p-n. Tuy nhin cu trc p-i-n diode c p ng nhanh nht so vi cc diode khc.


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Hnh 1.41: Cu to ca diode tch quang p-i-n

Cn c mt su im khc ca p-i-n diode so vi diode p-n. Th nht, hu nh ton bphoton i vo vng ngho u c hp th ti v rt t photon i ti lp n. Nh vy gimc hp th photon trong cc vng bn ngoi vng ngho v hn ch ko di sn sau ca png. Lung dng trong cc vng n v p ch yu l do khuch tn, trong khi dng chnh trongvng ngho l dng tri c iu khin bi in trng. Hu ht lung dng u di dng dngtri nhanh, nn p ng thi gian ca p-i-n diode c ci thin hn so vi diode tch quang p-n.Mun nhn c p ng tn s trn 50 GHz, lp i v mt ct ngang phi b. Nhng mt ctngang hp s g p kh khn khi a nh sng vo PD. Diode tch quang p-i-n gim c nhhng ko di sn sau ca xung v p ng tn s tt hn so vi diode tch quang p-n nn c

s dng trong thit b thng tin quang.

i vi p-i-n diode cng c th s dng cu trc d th. Nhng khc vi cu trc d thca laser diode, p-i-n diode cu trc d th hot ng trong mt di rng ca bc sng. V vyvic ch to p-i-n diode d th hot ng n mode l khng cn thit. Di cm ca lp i phi thit

k sao cho c phi ln hn bc sng ca tn hiu quang 1,55m, nh vy th diode tch quang sc s dng cho mi trng hp.

Tp m trong p-i-n diode

Tp m n (shot noise)

nhy ti a ca PD c xc nh bi in p ngu nhin v thng dng dng xy rati u ra PD khi c hoc khng c tn hiu quang. Tr trung bnh ca dng tch quang l Iph,thng dng ngu nhin quanh tr s trung bnh ny gi l tp m n. Cng c th gii thch chi tithn nguyn nhn gy ra tp m n nh sau.

Tp m n trong cc mch in do s lng cc ht ti in i qua mt im ring bittrong mch l hm ngu nhin ca thi gian gy ra. Trong tch quang, s cp in t - l trngc to ra do hp th photon cng l hm ngu nhin ca thi gian, do s lng ht ti inqua mch bn ngoi cng ngu nhin. Loi tp m n ny c xem nh qu trnh Poisson ngnht hoc khng ng nht ph thuc vo dng iu chc s dng trong h thng thng tin

quang.Trung bnh bnh phng ca tp m n Ish

2 t l vi tr trung bnh dng tch quang I v

rng bng tn ca diode tch quang f . V vy:

pGaAs

nGaAs

_

_

__

+

+

++

+V

E

x

iInGaAs

w2w1

nh sng ti


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2 = 2e I f (1.16)

e = in tch ca in t

Tp m dng ti

Dng u ra diode tch quang xut hin ngay c khi khng c tn hiu quang hay cn gil dng ti Id. Trung bnh bnh phng ca tp m dng ti c xc nh theo biu thc

2 = 2e Idf (1.17)

Tp m nhit

Tp m nhit cn gi l tp Johnson hoc tp m Nyquist l do chuyn ng ngu nhinca cc in t t do qua mch in. S lng cc in t l v cng ln v v vy tp m nhit lmt qu trnh ngu nhin Gauss. Qua thc nghim xc minh c mt ph ca tp m nhit

phn bu trn trc tn s trong min xa cc tm. Nu in trl R[], ti nhit Kelvin T vB l rng bng tn my thu th tr trung bnh bnh phng ca tp m nhit l:

RfkT4

I2

th= (1.18)

Trong k = 1,38 10-23 J/ K l hng s Boltzmann.

1.2.4.4.Diode quang thc (APD)Cu to v qu trnh thc

APD c cu to nh hnh 1.42.

Hnh 1.42: APD v phn bin trng bn trong

nh sng i vo APD qua lp p+ rt mng. Hu nh ton b hp th photon u xy ra

trong vng ngho l bn dn p pha tp nh. Cng nh trong p-i-n diode, in trng trong vngngho ca APD iu khin cc l trng v in t chuyn ng ngc hng vi nhau. Di tc

+V

E

x

p+ pi n+

Min hp th

Min nhn

nh sng ti


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ng ca in trng phn cc ngc, cc l trng trong lp ny hng ti lp p+, cn cc in thng ti lp n+.

in p phn cc ngc t ln PD gn vi mc nh thng zener to ra in trngln (hay min tng tc) ti tip gip p - n+. Khi cc in t v l trng qua min in trng ln

ny sc tng tc, va p mnh vo cc nguyn t ca bn dn v to ra cc cp in t - ltrng th cp thng qua qu trnh ion ho do va chm. Cc ht ti in th cp qua min intrng ln li c tng tc v chng c ng nng to ra cc cp in t - l trng miv.v. chnh l hiu ng thc, hay cn gi l hiu ng nhn. Qu trnh ny lm tng dng in

bn ngoi v cng chnh l tng nhy ca APD .

Hnh 1.43 minh ho qu trnh nhn trong min tng tc.

T hnh v cho bit t mt cp in t - l trng ban u, hiu ng nhn to ra su cp

khc. C thnh ngha h s ion ho ca cc in te v ca l trng h l xc sut ca mt vachm gia mt ht ti in c tng tc v mt nguyn t bn dn sinh ra mt cp in t -

l trng. H s ion ho tng rt nhanh khi cng in trng tng.S ph thuc ca M vo nhit c th hin trong hnh 1.44. H s nhn M c o

nh l hm ca in p nh thin V:

nBph ]V/)RIV(1[

1M

= (1.19)

Trong Iph l dng tch quang trung bnh, R= RS + Rth l tng in trni tip RS vin trgia tng do nhit Rth, VB l in p nh thng.

Hnh 1.43: Qu trnh nhn

Mt sng cong in hnh ca M(V) nh hnh 1.44.

hf


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H s nhn M t cc i khi s dng cc vt liu c ln. Tuy nhin thit b nh vy cp ng tn s chm. Trong thc t APD c thit k tho mn tiu chun p ng tn s.iu ny c th thc hin nu hoc cc in t hoc cc l trng chim u th trong min tng

tc. Mong mun k 0.

T hnh 1.44 thy rng i vi APD yu cu rt nghim ngt c vn nh ngun nhthin v nhit ca mi trng.

Hnh 1.44: S thay i ca M khi V v nhit thay i

Tp m trong APDHiu ng nhn ch xy ra i vi dng tch quang v dng ti nhng khng khuch i

tp m nhit. Thnh phn b mt ca dng ti chy dc mt ngoi ca diode cng khng ckhuch i. V vy tp m ch yu trong APD l tp m nc khuch i. Tp m ny cxc nh nh sau:

2 = M2[2e F (RP) f] (1.20)

Trong : F- H s tp m, M- H s nhn, R- p ng ca APD v P- Cng sut nhsng u vo APD.

1.2.4.5.Cc tham sca diode tch quang1) H s lng t

H s lng t ca diode tch quang c nh ngha nh sau:

= S lng in t trn mch ngoi/ S lng photon ti

V Iph/e l s lng in ti qua mch ngoi ca diode tch quang trong mt giy do hp

th photon ti, cn P0/ hf l s lng photon ti trong mt giy nn c vit li nh sau:

=

==

e

hcR

e

hc

P

I

hf/P

e/I

0

ph

0

ph(1.21)

Mun nhn h s lng t cao phi c cc iu kin sau y:

- Phn x nh sng ti b mt diode tch quang l cc tiu.

V

M

1000

100

10

10 100 200 300 400

T=2730K

T=3200KT=3000K

T=3400K


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- Hp th trong lp ngho l cc i

- Trnh ti hp cc ht ti in trc khi chng c tp trung li.

H s lng t 0,8 hoc ln hn c tht c v c th ti u ti mt bc sng bngcch thay i b dy lp n- .

2) p ng

Dng tch quang Iph tng tuyn tnh vi cng sut quang ti P. T s Iph trn P gi l png R ca diode tch quang :

hc

e

hf

e

P

IR

ph =

== (1.22)

Trong e l in tch ca in t, l h s lng t, l bc sng ca nh sng ti, h

l hng s Plank v c l tc nh sng trong khng gian t do. p ng c o bng A/W, A/

W hoc nA/ nW v.vp ng thng c th hin bng th nh hnh 1.45.

Nhng ng cong ny l gi tr gii hn, tuy nhin p ng cng ph thuc vo kchthc ca thit b, v chng phn x v cc yu t khc. Mt khc p ng cn ph thuc vo ccc tnh ca bn dn cu thnh diode. Ti u vo my thu ca h thng thng tin quang c mccng sut quang rt thp nhng dng tch quang vn t l tuyn tnh vi cng sut quang. V vy

cp ng R v h s lng tu l hm ca m khng ph thuc vo mc cng sutquang. i vi cc laser mnh s c hin tng bo ho v dn ti ph thuc khng tuyn tnhca p ng vo cng sut quang.

Hnh 1.45: p ng ph thuc vo bc sngTrong trng hp l tng p ng tng tuyn tnh vi bc sng v nng lng ca mt

photon gim khi bc sng tng v sau gim t ngt xung zero ti = c. p ng thc tkhc vi p ng l tng, v h s hp th t n l hm ca bc sng. Bc sng ct pha trnc xc nh bi di cm, trong khi bc sng ct pha di ph thuc vo h s hp th.

3) rng bng tn

p ng ca APD c cu trc n+ - p -i - p+ nh hnh 1.42 c chia lm ba phn:

(a) Thi gian dch chuyn ca in t qua min tri (ttr)e = w2/ vse

(b) Thi gian yu cu ca qu trnh nhn tA

(c) Thi gian dch chuyn ca l trng cui cng c to ra trong min tng tc ti raca khng gian tri (ttr) = w2 / vsh.

L tng

Thc t

R


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Phn (b) v (c) c trng cho tr b sung vo thi gian p ng ca diode khng c hiung thc. rng bng tn b hn ch cn trvic s dng APD trong cc h thng tc bitrt cao.

Thi gian tr tA l hm ca ca h s ion ho k. Khi k = 0 th hiu ng nhn tin trin ph

thuc thi gian dch chuyn ca in t qua min tng tc (ttr=wA/vse). Gi thit wA 0 th hiu ng nhn c s tham gia ca cin t v l trng, thi gian tAc xc nh theo

biu thc sau y:

se

AA v

Mkwt (1.23)

p ng thi gian ton b l:

sh

A2

se

A2

v

)ww(

v

)Mkww( ++

+ (1.24)

rng bng tn c xc nh theo biu thc

f(-3dB) 0,44 / (1.25)

Biu thc (1.23) mt ln na cho bit ti sao phi tm kim vt liu c k 0 ch toAPD trong cc in t khi u hiu ng thc. Tm li khi tc bit cao hn 2-5 Gbit/s sdng APD khng c li.

1.2.4.6.My thu tn hiu quangTrong my thu tn hiu quang ca h thng IM-DD, nh sng t si quang chiu vo b

tch sng quang. Do u ra b tch quang nhn c tn hiu in. Sau tn hiu in qua

cc bc x l tip theo khi phc li tn hiu ban u nhu vo my pht. S khi camy thu quang in hnh nh hnh 1.46.

B tch sng quang l PIN hoc APD thc hin chuyn i cng sut quang u vothnh tn hiu in. B khuch i iu chnh thc hin bin i dng tch quang thnh tn hiuin p vi mc ph hp. Sau khi khuch i tn hiu qua b cn bng hiu chnh hm truynt ca b khuch i. B lc y gii hn bng tn ca my thu trong phm vi yu cu i vi

ph tn hiu v nh ra p ng tn s ca my thu nhm ti u ho cht lng my thu (lm gimti thiu tp m pht ra t b tch sng v khuch i). Xung ng h (clock) c ly t b tchng h, thc hin bng cch trch ly ra t lung d liu s chung v c dng ti to li tn

hiu s trong mch quyt nh. Tn hiu sn cc u ra b quyt nh c a vo b giim chuyn thnh m ng lng cc tng ng.

Hnh 1.46: S khi b thu quang in hnh trong h thng truyn dn s

Tch sngquang

Khuch i

iu chnhB

quyt nhB

gii m

B tch

ng h

Bcn bng

Blc

Tn hiuvo

D liura

clock


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1.3. Thng tin v tuyn1.3.1.Cc phng php a truy nhp v tuyn

Cc phng thc a truy nhp v tuyn c s dng rng ri trong cc mng thng tin di

ng. Phn ny gii thiu tng quan cc phng php a truy nhp s dng trong thng tin vtuyn. M hnh ca mt h thng a truy nhp c cho hnh 1.47.

Hnh 1.47. Cc h thng a truy nhp

a) cc u cui mt t v b pht p, b) cc trm di ng v cc trm gc

Thng thng mt h thng thng tin a truy nhp v tuyn c nhiu trm u cui vmt s cc trm c nhim v kt ni cc trm u cui ny vi mng hoc chuyn tip cc tnhiu t cc trm u cui n mt trm khc. Cc trm u cui trong cc h thng thng tin di

ng mt t l cc my di ng cn cc trm u cui trong cc h thng thng tin v tinh l cctrm thng tin v tinh mt t. Cc trm kt ni cc trm u cui vi mng hoc chuyn tip cctn hiu t cc trm u cui n cc trm khc l cc trm gc trong thng tin di ng mt thoc cc b pht p trn v tinh trong cc h thng thng tin v tinh. Do vai tr ca trm gctrong thng tin di ng mt t v b pht p v tinh cng nh my di ng v trm mt tging nhau cc h thng a truy nhp v tuyn nn trong phn ny ta s xt chng i ln chonhau. Trong cc h thng thng tin a truy nhp v tuyn bao gicng c hai ng truyn: mtng t cc trm u cui n cc trm gc hoc cc trm pht p, cn ng khi theo chiungc li. Theo quy c chung ng th nht c l ng ln cn ng th hai c gi l

ng xung. Cc phng php a truy nhp c chia thnh bn loi chnh: a truy nhp phn chia theo tn s (FDMA: Frequency Division Multiple Access). a truy nhp phn chia theo thi gian (TDMA: Time Division Multiple Access).


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a truy nhp phn chia theo m (CDMA: Code Division Multiple Access). a truy nhp phn chia theo khng gian (SDMA: Space Division Access).

Cc phng php a truy nhp cbn ni trn c th kt hp vi nhau to thnh mtphng php a truy nhp mi.

Cc phng php a truy nhp c xy dng trn csphn chia ti nguyn v tuyn chocc ngun s dng (cc knh truyn dn) khc nhau.

Nguyn l ca ba phng php a truy nhp cbn u tin c cho hnh 1.48. Miknh ngi s dng v tuyn trong h thng v tuyn t ong mt t hay mt tram u cui trongh thng thng tin v tinh a trm s dng mt sng mang c ph nm trong bng tn ca knhvo thi im hot ng ca knh. Ti nguyn dnh cho knh c thc trnh by dng mthnh ch nht trong mt phng thi gian v tn s. Hnh ch nht ny th hin rng ca knhv thi gian hot ng ca n (hnh 1.48). Khi khng c mt quy nh trc cc sng mang ngthi chim hnh ch nht ny v gy nhiu cho nhau. trnh c can nhiu ny cc my thu

ca trm gc (hay cc phy thu cu cc trm pht p trn v tinh) v cc my thu ca cc trmu cui phi c kh nng phn bit cc sng mang thu c. t c s phn bit ny ccti nguyn phi c phn chia:

Nh l hm s ca v tr nng lng sng mang vng tn

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