1

H.Q.Trung.TVT

CHNG 1. TNG QUAN V TRUYN DN S

1.1. S PHT TRIN CA CC H THNG TRUYN DN

Mng in thoi c xy dng da trn c ch truyn ting ni gia cc my in thoi. n nhng nm 1970, mng ny hon thin bng vic thc hin truyn tn hiu tng t trong cp ng xon i v ghp knh phn chia tn s (FDM-Frequency Division Multiplexing) dng trong cc tuyn ng di kt hp truyn nhiu knh thoi trong mt cp ng trc. Thit b truyn dn loi ny rt t so vi gi ca mt tng i in thoi, v vy, chuyn mch c xem nh mt thit b nhm tit kim s dng ti nguyn khan him lc by gi l bng thng truyn dn.

Vo u nhng nm 1970, cc h thng truyn dn s bt u xut hin, s dng phng php iu ch xung m (PCM-Pulse Code Modulation) do Alec Reeves nu ra ln u tin vo nm 1937. PCM cho php truyn tn hiu tng t (nh ting ni ca con ngi) dng nh phn. S dng phng thc ny, tn hiu thoi tng t chun 4 kHz c th truyn di dng lung tn hiu s 64 kbit/s.

Cc nh k thut nhn thy kh nng h gi thnh sn xut cc h thng truyn dn bng cch kt hp mt s knh PCM v truyn chng trong mt i cp ng xon m trc y ch dng truyn mt tn hiu tng t duy nht. Hin tng ny c gi l li dy. Do gi thnh thit b in t s bt u gim nn s dng cc cng ngh ny tit kim c rt nhiu chi ph.

Phng thc ghp knh 64 kbit/s thnh mt lung bit tc cao duy nht cn c gi l Ghp knh phn chia theo thi gian TDM (Time Division Multiplexing). Mt cch n gin, mi byte ca mi knh u vo theo th t c a vo knh tc cao u ra. Qu trnh x l ny cn c gi l "chn byte tun t".

chu u v sau l rt nhiu ni trn th gii, s TDM chun c p dng ghp knh 64 kbit/s, cng vi hai knh thng tin iu khin kt hp to thnh mt knh c tc 2,048 Mbit/s. Do nhu cu s dng in thoi tng ln, lu lng trn mng tng, knh chun tc 2 Mbit/s khng p ng cho lu lng ti trn mng trung k. trnh khng phi s dng qu nhiu kt ni 2 Mbit/s th cn to ra mt mc ghp knh cao hn. Chu u a ra chun ghp 4 knh 2 Mbit/s thnh mt knh 8 Mbit/s. Mc ghp knh ny khng khc bao nhiu so vi mc ghp knh m cc tn hiu u vo c kt hp tng bit ch khng phi tng byte, ni cch khc l mi p dng chn bit ch cha thc hin chn byte. Tip , do

2

H.Q.Trung.TVT

nhu cu ngy cng tng, cc mc ghp knh cao hn na c xy dng thnh chun, to ra mt phn cp y cc tc bit l 34 Mbit/s, 140 Mbit/s v 565 Mbit/s.

1.2. H THNG TRUYN DN S

1.2.1. Cc thnh phn c bn

Truyn dn l chc nng truyn mt tn hiu t mt ni ny n mt ni khc. H thng truyn dn gm cc thit b pht v nhn, v phng tin truyn cng b lp li gia chng nh hnh 1.1.

Hnh 1.1: Cc thnh phn c bn ca mt h thng truyn dn.

Nhng phng tin pht s truyn v pht i nhng tn hiu u vo (tn hiu gc) truyn chng mt cch hiu qu qua phng tin, thit b nhn tch ra nhng tn hiu gc trong nhng tn hiu thu c. ng thi b lp li x l vic b li trong qu trnh truyn. Cc phng tin truyn bao gm dy dn kim loi, cp ng trc, radio, ng dn sng v cp si quang.

Truyn dn bao gm phn truyn dn thu bao ni lin my thu bao vi tng i v phn truyn dn tng i ni tng i vi tng i. Truyn dn gm truyn bng cp, truyn radio, lin lc v tinh, truyn TV, lin lc si quang, ng dn sng, lin lc di t cng b chuyn tip phc hi s dng cc phng tin truyn dn, kt cu kt hp v mng ng b ha ca cc thit b ny, vic bo dng v phn qun l ca mng truyn dn v.v..

* Truyn dn s dng si quang (fiber)

Mi trng quang si c rng bng gn nh khng gii hn. c im ca n l suy hao khng ng k, ch vo c 0,25 Db/Km. y chnh l u im vt tri ca si quang so vi cp ng trc. Ngoi ra truyn dn trn si quang cn c cc u im khc na l: Khng b nh hng bi nhiu in t trng, an ton, kch thc nh v nh,

Gii tn s c s dng trong truyn dn si quang c m t nh hnh di:

3

H.Q.Trung.TVT

Cu trc ca si quang:

1.2.2. Cc ngun nh hng ti tn hiu truyn dn

1.2.2.1. Mo tn hiu qua knh (distortion) Knh truyn thc t l khng l tng, do tn hiu i qua knh t hay nhiu cng b nh hng n dng tn hiu, c ngha l b mo so vi tn hiu gc.

Ngoi ra, s khng th trnh khi mo phi tuyn i vi nhng tn hiu lm vic ti cc tn s cao. iu ny xut pht t mt thc t rng vi cc tn s cao s b nh hng do s xo ng ca cc iu kin kh quyn, bi vy gy ra s thay i v tn s. Chng hn vi cc h thng radar doppler s dng trong vic gim st thi tit l mt trng hp c th.

Mo tuyn tnh c th gy ra cc nh hng trong cc h thng truyn dn xung. Loi mo ny c c trng bi s phn tn thi gian (lm ko di xung), dn ti hiu ng a ng.

4

H.Q.Trung.TVT

1.2.2.2. Tp m (noise) Thut ng tp m (noise) m t cc tn hiu in khng mong mun xut hin

trong h thng. S xut hin ca tp m lm gim kh nng tch chnh xc cc tn hiu pht, v, v vy, lm gim tc truyn dn thng tin. Tp m c to ra t cc ngun khc nhau nhng c th c phn ra thnh hai loi chnh l ngun tp m nhn to v tp m t nhin. Tp m nhn to xut hin t cc ngun nh la, chuyn mch hay pht x in t. Tp m t nhin xut hin trong cc mch hay linh kin in t.

1.2.2.3. Nhiu Nhiu c hiu l cc thnh phn tn hiu khng mong mun c thm vo tn hiu bn tin khi n c truyn t my pht n my thu. Trong thc t, vic truyn tin c th b nh hng bi nhiu ngun nhiu khc nhau: nhiu iu ch, nhiu xuyn knh (Crosstalk), nhiu xung (ISI), ...

1.2.3. Cc knh truyn dn

Knh truyn dn l mi trng kt li gia b pht v b thu, c th l cc si dy dn kim loi, cp ng trc, cp si quang, ng dn sng, bu khng kh

5

H.Q.Trung.TVT

hay s kt hp gia cc mi trng trn. Tt c cc knh u c mt bng tn gii hn cho php tn hiu c th i qua. Do cc c tnh vt l m mi knh c th c tn s ct gii hn trn (tn s cao) hay gii hn di (tn s thp). Trong trng hp knh b chn di (tn s ct gii hn di ca bng knh) th knh c m t nh l mt b lc thng di. Cn nu bng thng ca knh khng b chn di th knh c m t nh l mt b lc thng thp.

Knh truyn dn c phn loi theo rng bng. C 3 loi knh ph bin l: Knh bng hp (narrow band), bng thoi (voiceband) v bng rng (wideband).

Cc knh bng hp: i vi nhng knh c rng bng ln ti 300 Hz th c gi l bng hp, v thuc vo loi truyn in tn. Nhng knh nh th c s dng cho truyn dn d liu tc chm mc l 600 bit trn giy (bps). Nhng knh bng hp khng tin cy s dng cho truyn dn tn hiu thoi.

Cc knh thoi c rng bng gii hn trong khong t 300 Hz n 4 kHz. Thit k ban u ca knh thoi l phc v cho mc ch truyn dn tng t (analog) tn hiu thoi (voice), mc d vy cc knh ny thng c s dng truyn d liu tc 10 kil bits trn giy (kbps). M s dng tn hiu video nn cng c th c truyn trn cc knh thoi. Cc mch vng khp kn thu bao trong h thng in thoi cng cng truyn thng s dng bng thoi.

Cc knh bng rng c rng bng ln hn 4 kHz. Cc knh ny c th c dnh cho mt n v truyn thng (chng hn mt cng ty in thoi) v c th s dng cho mc ch truyn d liu tc cao, video, hay cc knh thoi hp nht.

Bng tn hot ng ca tn hiu c phn b theo cc di tn s nh sau:

6

H.Q.Trung.TVT

1.2.3. Tham s cht lng ca h thng truyn dn s

Cc tham s cht lng c bn ca h thng truyn dn s c nh gi thng qua t l li bit (BER) v dung lng truyn dn.

i vi cc h thng truyn dn s hin ti, cc tn hiu s nhn gi tr trong mt tp hu hn cc gi tr c th c v c thi gian tn ti hu hn. Khi tp cc gi tr c th c ca tn hiu gm hai phn t 0 v 1 th h thng c gi l nh phn v tn hiu khi c gi l bit. Khi s gi tr c th c ca tn hiu khc 2, tng qut l M th h thng c gi l h thng M mc v tn hiu c gi l k hiu (symbol). Gi gi tr ca symbol th k l kD v thi gian tn ti ca n l kT (i

vi cc h thng thng thng hin nay, kT T= v l hng s vi mi k). u thu

tn hiu khi phc li l kD v c rng l kT , nu k kD D th tn hiu th k

c gi l b li, nu k kT T th tn hiu th k c gi l c jitter. Cc tham s k

thut chung nht i vi cc loi h thng truyn dn s khc nhau, th hin ch tiu cht lng c bn ca h thng, l t l li bit BER v jitter (rung pha). i vi h thng nh phn, xc sut li BER c nh ngha l:

{ } k kBER P D D= =

Khi kT T T= + d th d c gi l jitter, tnh theo phn trm.

7

H.Q.Trung.TVT

Trong trng hp h thng nhiu mc th { } k kP D D= c gi l t l li symbol (SER) v c quan h cht ch vi BER.

1.3. CC KHI NIM C BN

1.3.1. Tm hiu truyn dn

a) Tn hiu tng t (analog signal)

Tn hiu tng t c th c xem nh l mt dng sng c tnh cht lin tc v thi gian trong phm vi tn hiu tn ti.

Hnh 1.5: Minh ha dng sng v ph tng ng ca tn hiu tng t.

b) Cc tn hiu mu

Tn hiu mu nhn c t tn hiu tng t bng cch ly mu ti cc thi im nht nh. Hm biu din tn hiu mu c bin thi gian ri rc.

Hnh 1.6: Minh ha dng sng ri rc nhn c t vic ly mu tn hiu tng t.

c) Tn hiu s (Digital signal)

Tn hiu s l mt dng ca tn hiu mu hay tn hiu ri rc trong mi mt con s trong chui tn hiu tng ng vi mt gi tr xc nh. Tn hiu s c th c c t li ra ca nhiu thit b. V d, khi ta quay s my in thoi th s to ra

8

H.Q.Trung.TVT

cc tn hiu s ph thuc vo nt c nhn, tn hiu s c c t u ra ca bn phm my tnh hoc t cc b chuyn i tng t sang s (ADC).

1.3.2. Cc phng php truyn thng tin

a) Truyn tin nh phn

- Truyn tin nh phn dng cp n

Tc truyn dn ph thuc vo tc thay i ca in p (hay cc kiu k hiu khc) trn knh truyn trc khi thnh phn tn s l qu ln c th lc suy hao knh truyn v dn n mo tn hiu. Ni theo cch khc, tc truyn dn b gii hn bi bng thng ca tuyn truyn.

- Truyn tin nh phn dng nhiu cp song song

Bng cch s dng nhiu cp, tn hiu truyn qua knh c th s tng t l vi s cp (knh) s dng. Tn hiu truyn qua c th duy tr nh tuyn truyn nh phn n, cho php thay th bi cc tuyn c bng thng nh hn (dn ti chi ph thp hn).

b) Truyn tin a mc

- Truyn tin a mc s dng cp n

Truyn dn d liu khng bt buc phi gii hn c s hai (nh phn), theo l thuyt c th s dng mt s mc in p hay mt s kiu k hiu.

9

H.Q.Trung.TVT

V d: s dng 4 mc in p, chng ta c th m ha mi t hp hai bit nh phn bi mt trong 4 mc in p (00 ~ mc A, 01 ~ mc B, 10 ~ mc C v 11 ~ mc D). Khi ta c th gi thng tin nhanh gp 2 ln xt trn cng mt rng bng thng.

- Truyn tin a mc s dng nhiu cp

Vic s dng cc knh truyn dn song song truyn d liu cho php tng kh nng (dung lng) truyn tin trn bng thng b gii hn.

c) K hiu a mc

V nguyn tc chng ta c th s dng mt s k hiu (trng thi k hiu) cho bn tin s. V d, ti sao s dng 1024 trng thi in p khc nhau, mi trng thi (k hiu) m ha s bit l 101024log2 = bits. Chng ta thm ch c th s dng 1048576 trng thi k hiu, khi vi mi k hiu m ha 20 bits thng tin.

R rng c mt gii hn thc t trn s trng thi c s dng, ph thuc vo kh nng phn bit chnh xc cc trng thi (cc mc in p, tn s, ) ca thit b thu.

V d: mt s modem in thoi hot ng tc 56 kbps s dng 1024 trng thi k hiu khc nhau (t hp bin v pha ca sng mang) bo hiu trn knh thoi, trong khi cc h thng in thoi t bo s ch s dng 4 trng thi do thit b phi hot ng trong cc mi trng chu nhiu n hn.

10

H.Q.Trung.TVT

1.3.3. Tc truyn d liu

Tc truyn thng tin ca mt knh truyn dn thng c xc nh theo lng thng tin nh phn (bit). C ngha l tc truyn dn c o theo n v bit/giy (bps). V d: nu nh c 6 bit thng tin c truyn i sau mi khong thi gian 6 giy, th tc truyn tin s l

6 its 1000 its s6

bR bms

= =

Ngoi ra tc truyn dn cn c xc nh thng qua tc k hiu. Trong th tc k hiu l tc thay i trng thi cc k hiu mang thng tin nh phn qua knh truyn. Chng ta c th m ha mt s bit trong mi k hiu. Tc k hiu khng nht thit phi bng tc truyn thng tin. n v o tc k hiu l k hiu/giy hay (baud). V d: mt h thng s dng 4 tn s m ha cc t hp 2 bit nh phn qua mt knh, v tn s (k hiu-symbol) c thay i sau mi 0.5 ms, khi tc k hiu s l:

1 2000 / 2000 ( ).0.5symbolR symbol s baud= = =

Tc truyn thng tin bi vy s l: 2 x 2000 = 4000 bps.

1.4. CC TIU CHUN TRUYN DN

1.4.1. nh ngha

Lnh vc truyn thng lin tc pht trin thay i mt cch nhanh chng, cc h thng truyn thng c pht trin bi nhiu nh sn xut khc nhau trn th gii, chnh v vy cn c s tng thch v cc tiu chun v cc khuyn ngh

11

H.Q.Trung.TVT

phm vi quc gia, khu vc v quc t. Theo ISO, cc nh ngha v tiu chun v khuyn ngh dnh cho truyn thng nh sau:

Tiu chun: Chi tiu k thut hay vn bn qui nh c kh nng ph bin rng ri c xy dng bi s hp tc v thng nht hay s chp thun ni chung ca tt c nhng vn lin quan ti n da trn cc kt qu nghin cu khoa hc, cng ngh v thc nghim.

Khuyn ngh: Ti liu vn bn lin quan quy nh cht ch cc th tc thc hin c thng qua v ph bin rng ri bi mt c quan (t chc) chu trch nhim c quyn hn nht nh.

1.4.2. Cc t chc tiu chun v khuyn ngh

ISO: International Standardization Organization (OrganizationT chc tiu chun ha quc t).

ITU: International Telecommunications Union (Hip hi Vin thng quc t).

IEC: International Electrotechnical Commission (y ban in t quc t).

INTELSAT/INMARSAT: International Telecommunications Satellite Organization /International Maritime Satellite Organization.

12

H.Q.Trung.TVT

ANSI Vin Tiu chun quc gia Hoa K

CEPT The European Conference for Posts and Telecommunications.

CCIR Consultative Committee for International Radiocommunication (y ban T vn quc t v v tuyn in).

CCITT Consultative Committee for International Telephone and Tele-

Graph (y ban T vn quc t v in thoi v in bo).

13

H.Q.Trung.TVT

ETSI European Telecommunications Standards Institute (Vin Tiu chun Vin thng Chu u).

14

H.Q.Trung.TVT

CHNG 2. TRUYN DN S CC TN HIU TNG T

2.1. NH L LY MU V H THNG TIN XUNG

2.1.1. nh l ly mu

nh l ly mu c mt ngha su sc trong l thuyt thng tin. nh l ny c pht biu nh sau:

Mt tn hiu c di tn gii ni l B Hz ( )( )0 2G khi Bw w p= > c th c khi phc mt cch chnh xc (m khng b li) t cc mu ca n c ly u n vi tc R > 2B mu trn giy. Hay ni theo cch khc, tn s ly mu ti thiu phi l 2B Hz.

Xt tn hiu ( )g t (hnh 2.1) c ph gii hn l B Hz. Ly mu ( )g t vi tc sf Hz ( sf mu trn giy) tng ng vi vic nhn ( )g t vi on xung

( )sT

td gm cc xung n v lp li vi chu k 1s sT f= . Tn hiu mu nhn c

( )g t s l:

( ) ( ) ( ) ( ) ( )sT s s

ng t g t t g nT t nTd d= = - (2.1)

15

H.Q.Trung.TVT

Hnh 2.1: Tn hiu mu v ph ca n

Tc ly mu ti thiu 2sf B= c gi l tc Nyquist cho tn hiu

tng t ( )g t v khong thi gian ly mu 1 2sT B= c gi l khong Nyquist cho ( )g t .

2.1.2. Khi phc tn hiu

Qu trnh khi phc mt tn hiu tng t ( )g t t cc mu ca n c xem nh l php ni suy.

Hnh 2.2: Qu trnh ni suy tn hiu

Mi mt mu ( )g t l mt xung, hnh thnh mt xung ca c cao bng vi ln ca mu. Xung th k l xung c ln ( )sg kT xc nh ti v tr st kT= , v c th biu din nh l ( ) ( )s sg kT t kTd - . Cho xung ny qua b lc, u ra nhn c s l ( ) ( )1s sg kT rect T . y l xung ca c cao ( )sg kT , tm xc nh ti

st kT= . Mi xung ( )g t s to ra mt xung ca tng ng, v kt qu s l:

(2.2)

2.1.3. ng dng ca l thuyt ly mu

16

H.Q.Trung.TVT

L thuyt ly mu c tm quan trng trong phn tch, x l, v truyn dn tn hiu. V ta c th chuyn tn hiu lin tc theo thi gian thnh chui ri rc cc s. X l tn hiu thi gian lin tc bi vy c chuyn v x l chui ri rc cc s. V cng v th m c th s dng cc b lc s. Trong lnh vc truyn thng, truyn dn bn tin tng t c gim bt thnh truyn dn mt chui cc s. iu ny lm xut hiu nhiu k thut mi cho truyn thng cc tn hiu lin tc. Bng cch ly mu tn hiu tng t v lm thay i cc thng s v bin , rng v v tr xung ca cc mu nhn c ta c cc k thut iu ch xung tng t tng ng l: iu ch bin xung (PAM), iu ch rng xung (PWM), iu ch v tr xung (PPM). Trong c vai tr quan trng nht trong phng thc iu ch xung ngy nay l iu ch m xung (PCM).

Hnh 2.3: Cc tn hiu iu ch xung

Thay v truyn tn hiu tng t ( )g t , chng ta truyn tn hiu iu ch xung. Ti b thu, chng ta c thng tin ca tn hiu iu ch xung v khi phc li tn hiu tng t ban u.

Mt trong nhng u im ca vic s dng iu ch xung l cho c th cho php truyn mt s tn hiu da trn vic chia s ti nguyn v thi gian bng cch

17

H.Q.Trung.TVT

s dng k thut ghp knh phn chia theo thi gian (TDM-Time Division Multiplexing).

Hnh 2.4: Ghp knh phn chia theo thi gian vi hai tn hiu PAM

2.2. IU CH M XUNG (PCM)

2.2.1. Nguyn tc

iu ch xung m PCM c thc hin theo mt quy trnh gm bn bc c tnh nguyn tc l:

- Lc nhm hn ch ph tn ca tn hiu lin tc cn truyn: Bin i Fourier ca cc tn hiu lin tc thc t l v hn theo bin tn s, do thi gian tn ti hu hn ca chng. Chnh v vy, cc tn hiu lin tc cn truyn nht thit phi c lc nhm hn ch ph ti tn s cc i W no nhm tha mn tnh gii hn v bng tn ca nh l ly mu.

- Ly mu: Tn hiu lin tc sau lc c ri rc ha bng cch ly mu thng qua chui xung nhp c tn s sf tun theo nh l ly mu c c cc tn hiu

iu bin xung (PAM-Pulse Amplitude Modulation).

- Lng t ha: S ha gi tr c th c ca tn hiu PAM sau ly mu l v hn, do vy s bit cn thit m cc gi tr ca cc xung PAM l v hn v iu ny khng th thc hin c. hn ch s bit m cn s dng, gi tr ca tng xung PAM cn c lm trn thnh mt trong cc gi tr mu xc nh gi l cc mc lng t (c s lng hu hn) v qu trnh ny c gi l lng t ha.

- M ha: Cc gi tr mc lng t ng vi cc xung PAM c m ha bng cc t hp m nh phn truyn i trn h thng truyn dn s.

S m t cng on iu ch xung m c th hin nh hnh di y:

18

H.Q.Trung.TVT

Hnh 2.5: H thng truyn dn PCM

Qu trnh khi phc phn thu c thc hin nh sau: gii m c chui xung PAM lng t ha ri cho qua lc thng thp c tn s ct bng mt na tn s ly mu.

Sai s trong truyn dn PCM: Nh ta ni trn, trong thc t, cc tn hiu li vo iu ch m xung l

cc tn hiu c ph tri rng v hn. Sau lc hn ch ph tn tn hiu, tn hiu c ph hn ch v do vy thi gian tn ti tri rng ti v hn, ngha l v l thuyt, vic ly mu phi c thc hin vi v hn mu (tuy nhin s khng c nh th). T chng ta c th thy rng tn hiu lin tc khi phc li c phn thu, ngay c trong trng hp khng tnh n mo v tp nhiu trn ng truyn, cng ch l mt phin bn gn ng ca tn hiu lin tc cn truyn i phn pht m thi. Sai s gia cc tn hiu phin bn v nguyn bn gy bi cc nguyn nhn sau:

(a) Vic ly mu khng th tin hnh trong thi gian di v hn c.

(b) Sai s do lm trn (lng t ha).

(c) Cc c tnh lc khng hon ton l tng.

(d) Phin bn l mt tn hiu c ph hn ch, khng nh tn hiu nguyn bn.

Ngoi ra, cc sai lch qu mc v ng b cng c th dn n sp xp sai cc t hp m thu c v iu ny dn n cc sai lch v cng trm trng. Cc

19

H.Q.Trung.TVT

chi tit v cc cng on trong quy trnh PCM v cc bin php khc phc sai s s c trnh by trong phn tip theo.

2.2.2. Lc gii hn bng

Ta xt tn hiu thoi, c ph tp trung trong di t 0,3 n 3,4 kHz. Vic ct b cc thnh phn tn s ngoi di ni trn khng gy ra nhng mo th cm qu ln, tc l khng gy nn nhng tr ngi c bit i vi qu trnh thng thoi. hn ch ph tn hiu c th tin hnh loi b cc thnh phn tn s ln hn 3,4 kHz trong tn hiu in thoi bng lc thng thp, tc l c th chn tn s cc i W ca tn hiu thoi l 3,4 kHz. Trong trng hp ny, sai s do lc hn bng gy ra ch yu l bi khng th ch to c mch lc thng thp l tng m ch c th ch to c cc mch lc vi c tnh lc khng dc ng ti tn s ct. khng gy nn nhng mo th cm r rt c, tn s ct ca mch lc hn bng phi chn cao hn 3,4 kHz. Cc mch lc tiu chun trong thc t (c c tnh tha mn cc khuyn ngh ca CCITT cho cc mch thoi) c tn s ct rt st vi 4 kHz.

2.2.3. Ly mu

Qu trnh ly mu c thc hin bng cch nhn tn hiu thoi lin tc vi mt chui xung nhp c tn s 2sf W . Vic chn tn s nhp ln hn hai ln W s

lm m rng bng tn chim ca tn hiu s, do vy tn s nhp phi chn nh nht m khng gy mo tn hiu. Sai s ly mu gy bi vic khng th ly mu trong mt thi gian di v hn thng khng ng k v c th b p bng vic chn

2sf W> mt cht. Kt hp vi thc t pht sinh do lc hn bng nh ni trn

y, tn s ly mu tiu chun cho tn hiu thoi c chn l 8 kHz. Qu trnh ly mu tn hiu in thoi c m t nh hnh di y:

20

H.Q.Trung.TVT

Hnh 2.6: Qu trnh ly mu tn hiu lin tc

2.2.4. Lng t ha

Lng t ha c thc hin n gin nht bng cch chia di ng tn hiu

[ ],a a- + thnh Q mc cch u nhau, c gi l lng t ha u. Khong cch gia cc mc lng t l 2a QD = . Cc gi tr ca mu tn hiu (cc xung PAM)

c lm trn thnh gi tr mc lng t gn nht. Sai s lng t ca cc gi tr

mu l mt bin ngu nhin qe , nhn cc gi tr trong khong [ ],a Q a Q- + , c th xem nh mt lng tp m gi l tp m lng t v c th nh gi c thng qua cng sut tp m lng t:

( )2 2a Q

eq q q q qa Q

P e e pdf e de-

= = (2.3)

Trong ( )pdf l hm mt xc sut.

21

H.Q.Trung.TVT

Hnh 2.7: Qu trnh lng t ha u

Do khng bit c phn b thc s ca bin tn hiu in thoi trong khong gia hai mc lng t st nhau, ngi ta buc phi gi thit rng tn hiu in thoi nhn cc gi tr bin trong khong gia hai mc lng t st nhau vi xc sut nh nhau. Do tp m lng t c xem l bin ngu nhin phn b

u, tc l ( ) 2qpdf e Q a= . Thay vo phng trnh (2.3), ta c: 2 2 23 12eqP a Q= = D (2.4)

T (2.4) chng ta c th thy khi tng s mc lng t Q th cng sut tp m lng t gim. Chng hn, khi tng s mc lng t ln hai ln, cng sut tp m lng t gim 4 ln, tc l khong 6 Db. Tuy nhin, vic tng qu mc s mc lng t dn n hai h qu: a) S mc lng t ln dn n s bit dng m cc mc lng t tng (c tng s mc lng t ln hai ln th phi thm 1 bit trong t hp m) lm tng tc bit v do vy tng ph chim dng ca tn hiu s; b) Vi cng mt di ng tn hiu, vic tng qu mc s mc lng t s c th dn n mc lng t khi phc li phn thu b nhn nhm di tc ng ca tp m nhit trong cc mch in t. Thm vo , nu lng t ha u th vic chia cc mc vi s mc ti thiu (nhm gim s bit m cn dng) xc nh theo chnh xc cho i vi cc mc cao ca tn hiu li dn n sai s phm phi li ln i vi cc mc thp. iu ny dn n sai s tng cng ln do trong thc t cc mc tn hiu thp ca tn hiu thoi thng xy ra nhiu hn so vi cc mc cao. Cc mu thun ni trn trong thc t c khc phc nh p dng lng t ha khng u, trong khong cch gia cc mc lng t c chn ln i vi cc tn hiu ln cn vi cc mc tn hiu nh th khong cch gia cc mc lng t chn nh. Gii

22

H.Q.Trung.TVT

php ny l kh t nhin do i vi mc tn hiu ln th t s tn hiu trn sai s (tn hiu trn tp m lng t) vn kh nh d sai s lng t tuyt i c ln. Vic chia cc mc lng t khng u nh th tuy vy li kh kh thc hin trong thc t v mt gii php tng ng thng c p dng l thc hin lng t ha u cc tn hiu c nn. Lut nn c p dng trong iu ch m xung tn hiu in thoi l lut logarit, trong tn hiu li ra y ca mch nn bin thin theo lut logarit ca tn hiu li vo x. phn thu, tn hiu c gin tr li. Vic duy tr nn-gin chnh xc l mt yu cu rt ngt ngho nhm trnh cc mo tn hiu mang vo do qu trnh nn-gin.

Cc lut nn logarit c p dng trong h Chu u v h M kh khc nhau, iu ny l do lch s qu trnh pht trin vin thng trc y trn cc khu vc khc nhau li. Lut nn c p dng l lut m i vi h M, trong khi h

Chu u s dng lut nn A. Biu thc gii tch xc nh cc lut nn m v A l:

Lut nn m (H M):

( ) ( )( )ln 1

, 1 1ln 1

xy sign x x

mm

+= -

+ (2.5)

Lut nn A (H Chu u):

( )

( )

, 0 11 ln

1 ln, 1 1

1 ln

A xsign x x A

AyA x

sign x A xA

+=

+ +

(2.6)

Hnh 2.8: c trng nn-gin theo lut m v lut A

Trong cc biu thc (2.5) v (2.6), x v y ln lt l cc gi tr ca cc tn hiu li vo v li ra ca b nn c chun ha theo gi tr cc i ca chng. Theo

23

H.Q.Trung.TVT

khuyn ngh G.711 ca CCITT, cc gi tr ca cc tham s c chn l: A=87,6 v 255m = . i vi h M, 100m = cng c s dng trong mt s h thng, tuy

nhin khng phi l gi tr m CCITT chn lm gi tr tiu chun.

Trong thc t hay s dng k thut lng t ha phi tuyn t c hiu qu cao v t s tn trn tp. c bit trong h thng truyn dn s cn p dng k thut nn-gin s da trn cc c trng nn-gin tng t. Thut ton nn-gin c p dng cho hai h thng M v Chu u da trn vic xp x cc ng cong c trng nn-gin tng t tng ng vi hai chun m v A. C th vi h M, ng cong c trng nn-gin tng t c xp x bng 15 on thng (bao gm 7 on dng, 7 on m v mt on qua gc). Vi h Chu u, c xp x thnh 13 on thng (bao gm 6 on dng, 6 on m v mt on qua gc c chia thnh 4 phn on).

2.2.5. M ha

Vic m ha cc mc lng t u (sau nn) to thnh tn hiu PCM c thc hin bng cc t hp 8 bits i vi c h M ln Chu u v cng c dng PXYZABCD. Trong , bit P ch th cc tnh gi tr lng t ca mu tn hiu: P=1 vi tn hiu dng v P = 0 vi tn hiu m. Ba bits XYZ dng m cc on thng (hoc phn on) lm gn ng tuyn tnh cc lut nn ( m hay A), bn bit ABCD dng m 16 mc lng t u trong tng on.

Hnh 2.9: c tuyn xp x tuyn tnh ha lut A 13 on

Do c tuyn tnh ha theo s on khc nhau, cch nhn c cc m 8 bits PCM i vi h M v h Chu u cng khc nhau. i vi h Chu u, trc tin vic m 12 bit c thc hin, chia di bin tn hiu vo mt cch tuyn tnh

24

H.Q.Trung.TVT

thnh 4096 bc. Cc mch logic sau c s dng to ra cc t m 8 bit theo mt quy tc m c m t trong bng thut ton di y:

on

th

Gii hn di ca on (tnh theo bc

bin vo)

Gii hn trn ca on (tnh theo

bc bin vo)

T m

PXYZABCD

ln ca mi bc lng t u (tnh theo s

bc bin li vo)

1 2048 4096 1111ABCD 128

2 1024 2048 1110ABCD 64

3 512 1024 1101ABCD 32

4 256 512 1100ABCD 16

5 128 256 1011ABCD 8

6 64 128 1010ABCD 4

7

(on gc)

32 64 1001ABCD 2

0 32 1000ABCD 2

-32 0 0000ABCD 2

-64 -32 0001ABCD 2

8 -128 -64 0010ABCD 4

13 -4096 -2048 0111ABCD 128

Bng 2.1: Thut ton m ha ha theo lut A

T bng thut ton trn, chng ta c th thy rng mi mt on c chia thnh 16 mc lng t u. Gi tr ln ca mi mt mc lng t u (tnh theo s bc ca mc bin li vo) c ghi ct cui cng ca cc bng l thay i theo gi tr mc li vo.

Li ch ca nn-gin c th thy c thng qua xt lm v d i vi h Chu u. Chng ta thy rng b lng t u s dng nn-gin (cng cn gi l b lng t phi tuyn) c phn gii tng ng vi phn gii ca b lng t u (lng t tuyn tnh) vi 4096 mc s dng cc t m 12 bit. Nh vy so vi vic s dng b lng t tuyn tnh 8 bit (256 mc) cng sut tp m lng t gim c 4x6 = 24 dB.

2.3. IU CH M XUNG VI SAI

2.3.1. Nguyn tc

25

H.Q.Trung.TVT

Nh ta bit, PCM c thc hin bng cch m ha cc gi tr mu c lng t ha ca tn hiu lin tc li vo. S bit m cn thit, chng hn cho tn hiu in thoi, nh phn trc trnh by, c CCITT xc nh phi l 8. Tc tn hiu thoi PCM nh vy l 64 kb/s, chim ph tn kh ln.

Nhng rung ng chun chu k ca thanh huyn v cc chuyn ng hn ch ca cc b phn thuc c quan pht m nh mi v li thc t to ra cc mu m thanh rt tng quan trong tn hiu m hu thanh. Ngc li, cc m v thanh th c xu hng khng tng quan. Trong m thoi, mt ngi m thoi tiu biu thng ni trong 40% thi gian v i vi cc giai on ting ni tch cc (giai on c ni trong cuc m thoi) th cc m hu thanh xy ra gp 4 ln so vi cc m v thanh. S tri hn hn ca cc m hu thanh c ngha l tnh tng quan trong cc tn hiu m thoi c th khai thc c mt cch c hiu qu. Do tnh tng quan cao ca cc mu ting ni, sai lch gia hai mu k nhau thng kh nh so vi gi tr ca tng mu.

1n n ns s s--

26

H.Q.Trung.TVT

(Hnh 2.11: Cu trc m v gii m DPCM vi b d on)

Theo cu trc m v gii m nh trn, mt b d on c s dng to ra gi tr c lng ca mu tip theo v sai lch ca s c lng ny vi gi tr thc ca mu tn hiu k tip c m ha PCM. Tn hiu li vo ca b d on c hnh thnh t mt b gii m PCM. pha thu, cc thut ton ngc li c thc hin. Tn hiu li vo b m ha PCM pha pht l:

( )n n nR s t X= - (2.8)

Trong , ch s n pha trn biu th nhp ly mu th n.

Gi tr c lng ca mu tip theo (tn hiu li ra ca b d on) c xc nh (ngoi suy) t L mu trc theo biu thc:

( )1

Ln n

ii

X a s t=

= (2.9)

Cc h s ia l cc trng s ca thut ton d on, c la chn mt cch

thch hp. Phn t c s dng rng ri trong thc hin b d on l b lc gin bao gm cc on dy gi chm T (l khong cch gia cc mu) v mng kt hp trng s.

27

H.Q.Trung.TVT

(Hnh 2.12: B lc gin dng lm mch d on)

Bc ca b d on thng c gii hn ti 3 hoc 4 v tng ch ca cc b

d on tiu biu l 15 dB. Cn ch y rng ci gi phi tr cho s gim ph

chim dng ca tn hiu, c c nh gim d tha trong tn hiu, l mc

nhy cm i vi li truyn dn t l vi mc gim d tha. Do tn hiu thc s

c m ch l sai s gia tn hiu thc v tn hiu d on, nu c sai li trong

truyn dn th sai li ny s c cng vo tn hiu khi phc trong sut qu trnh

khi phc bi v my thu ch thc hin gii m chnh lch, tch phn li v cng

kt qu vi mu tn hiu c khi phc trc , v c vy sai li s lan ra

ton tn hiu c khi phc.

2.3.3. Hin tng qu ti sn

DPCM da trn tin l cc mu tn hiu lin tip ni chung c chnh

lch nh, v vy ch cn t bit m. iu ny ng vi cc loi tn hiu bin thin

tng i chm. Vi cc tn hiu bin thin kh nhanh th vic s dng tng i it

bit m chnh lch s gy mo tn hiu. Hin tng ny c gi l qu ti

sn.

thy r hin tng ny, ta xt trng hp n gin nht ca DPCM l iu

ch delta. (DM: Delta Modulation), trong chnh lch gia hai mu lin tip

c m ch bng mt bit: khi chnh lch dng th bit m l 1, ngc li khi

chnh lch m, bit m l 0. Ti pha thu, mt lng c nh D c cng vo hay

tr i vi mc tn hiu trc (tnh tch ly) ty theo cc tnh ca bit nhn c l

28

H.Q.Trung.TVT

1 hay 0, hnh thnh mt tn hiu dng bc thang. Sau tn hiu bc thang c cho

qua lc.

Hnh 2.13: Qu trnh iu ch Delta

Hnh 2.14: B iu ch Delta

Hnh 2.15: B gii iu ch Delta

2.4. IU CH M XUNG VI SAI THCH NG (ADPCM)

Nh ta trong phn trn, tc k hiu hoc tc bit ca tn hiu PCM gm cc t m 8 bit v tn hiu ting ni c ly mu vi tn s 8 kHz bng 64 kbit/s. Nu s lng bit ca t m gim cn 4 nh trong iu ch m xung vi sai (DPCM) th tc bit gim v ch bng 32 kbit/s. C xu hng tiu chun ha quc

29

H.Q.Trung.TVT

t tc 32 kbit/s i vi tn hiu m ting ni nh s dng ADPCM. Vn ny c phn nh trong khuyn ngh G.721 ca CCITT, c lin quan n iu ch m xung vi sai t thch nghi 32 kbit/s v cc ti liu khc.

Vi mt qu trnh ngu nhin dng c xc nh nh mt qu trnh m nhng c tnh thng k ca cc php o qu trnh khng thay i theo thi gian. Nhiu ngun tn hiu thc t l khng dng nhng ta dng. Qa trnh ny c th hin qua phng sai v hm t tng quan thay i chm theo thi gian. Cc b m ha trong cc h thng PCM v DPCM c tnh ton trn c s tn hiu vo dng v c m hnh ha i vi cc ngun tn hiu ta dng. Nu b lng t u PCM c s dng th tr trung bnh ca tp m lng t bng 0, phng sai

hoc cng sut tp m lng t bng 2 12D . Nu phng sai thay i do sai s

lng t thay i gy ra bi tn hiu vo ta dng, th phng php trung ha l thay i bc lng t D . y l mt trong nhng phng php hot ng ca b lng t ha t thch nghi. B lng t ha t thch nghi thay i bc lng t ca n ph hp vi phng sai ca cc xung ly mu tn hiu i qua. Cc thut ton c pht trin cho iu ch m xung vi sai khi m ha tn hiu ting ni bng cch s dng b lng t ha v b d on t thch nghi, trong cc h s thay i c chu k phn nh thng k ca tn hiu vo. Hn na truyn cc h s d on n my thu, v nh vy lm tng s bit truyn v tc bit, b d on thu tnh cc h s ring ca n.

Hnh 2.15: Lng t ha t thch nghi vi:

(a) nh gi thun v (b) nh gi ngc mc tn hiu vo.

30

H.Q.Trung.TVT

C hai loi h thng t thch nghi. Th nht l h thng DPCM c lng t t thch nghi (thng gi tt l DPCM-AQB). Loi th hai kt hp c b lng t ha t thch nghi v b d on t thch nghi. Loi ny gi tt l DPCM-APB-AQB.

AQF: Thng tin mc tn hiu c truyn n b m ha xa khi s dng 5-6 bit cho mt xung ly mu trn c bc. Cho php bo v thng tin c bc pha pht bng cch thm bit d. tr nh gi c to ra trong hot ng m ha (bng 16 ms cho ting ni). Yu cu chn cc mu vo khng lng t ha. T thch nghi khi hoc t thch nghi nh k; ngha l c bc D ca n i mi mi ln mi khi v gi khng i sut trong thi gian mt khi ca N mu. nh gi da trn c s cc mu khng lng t.

AQB: Thng tin v c bc D c tch ra t trng thi trc ca b lng t ha. Khng c tr ca nh gi. Tp m lng t lm gim t tnh bm st mc v gim c tnh hn na khi tng c ca bc. y l h thng phi tuyn c hi tip v c th khng trnh khi vn v s n nh. Cc h thng AQF i hi cc khi m t tin c cu trc phc tp v cng gy ra tr, cc h thng DPCM t phc tp thng dng cc mch AQB nn c li ch khng cn cc bit ngoi cung cp thng tin v c ca bc. Hot ng ca cc b m ha DPCM-AQB ti 32 kbit/s c chp nhn truyn ting ni. Kt qu l khng tt nh h thng PCM 7 bit s dng phng php lng t ha logarit m ch so snh vi h thng PCM 6 bit lng t ha logarit. Nu yu cu c mt h thng DPCM 32 kbit/s cht lng cao th b d on t thch nghi (APB) phi a vo thit k.

31

H.Q.Trung.TVT

Hnh 2.16: S DPCM vi (a) d on t thch nghi thun

V (b) d on t thch nghi ngc.

32

H.Q.Trung.TVT

CHNG 3. X L TN HIU TRUYN DN S

3.1. RUNG PHA V TRI PHA TRONG MNG TRUYN DN S

3.1.1. Vn rung pha v tri pha

Rung pha v tri pha c nh ngha tng ng l s bin thin ngn v di cc thi im c ngha ca tn hiu s so vi cc thi im l tng ca chng. Cc thi im c ngha c th ly im gia hoc im bt k c nh no c nhn bit mt cch r rng trn mi xung. S bin thin cc thi im c ngha nh hng n tn hiu s ging nh trong trng hp khi tn hiu nh thi gc s dng to ra dng bit b iu pha vi mt tn hiu iu ch, l dng sng rung pha. Dng sng rung pha c th c biu din bng mt hm lin tc theo thi gian v c lp vi tn hiu s b nh hng. iu ny c th minh ha nh hnh 3.1. Cc tn hiu rung pha c ngha nht l cc tn hiu khong tn s t vi chc Hz n vi kHz. Cc n v biu th bin i ca rung pha ph thuc vo mi trng ang o n, v c th l cc n v thi gian, pha hoc cc khong thi gian n v (UI).

Hnh 3.1: Minh ha rung pha v nh hng ca n n tn hiu s

Chng ta s s dng UI, CCITT nh ngha khong n v l chnh lch danh nh v thi gian gia cc thi im c ngha k tip nhau ca mt tn hiu chim thi gian bng nhau. iu ny c ngha l rng ca mt chu k tn hiu s bng khong thi gian n v. V d, bin tc thi ca rung pha c o l 1 sm di dng sng vung 100 kHz; v chu k ca dng sng 100 kHz l 10 sm , i vi tn hiu nh thi phi phn bit c du v khong cch, thi gian c ngha i vi tn hiu ny cch nhau 5 sm . Nh vy khong n v s l 5 sm v

bin rung pha s l 1 sm /5 sm = 0,2 UI.

3.1.2. Cc ngun rung pha

33

H.Q.Trung.TVT

Trong nhiu trng hp rung pha khng th xc nh c v ch c tnh bng cch s dng cng c thng k ton hc nh hm mt xc sut ca bin (pdf). Gi tr ca hm ny chnh l xc sut tm thy bin rung pha, ln hn hoc nh hn mt gi tr xc nh no . Sau khi c hm mt xc sut ca bin rung pha, chng ta tnh c gi tr trung bnh v phng sai ca n. Nu dng sng rung pha c th xc nh c th mt tham s quan trng l tc bin i ca rung pha trong tn hiu s. Rung pha c th xut hin do cc nguyn nhn khc nhau trong mt h thng truyn dn s. Tuy vy, ch yu cc nguyn nhn ny c th phn thnh cc loi sau:

(1) Rung pha hoc tri tn s rt thp do s thay i thi gian tr truyn dn ca cc cp... M s thay i li do s bin i chm ca nhit ni t cp gy ra.

(2) Rung pha tn s thp do s bt n nh khng phi hp ca cc ngun ng h gy ra.

(3) Rung pha do tp m gy ra, n xut hin do tp m pha trong cc mch ca b dao ng iu khin bng tinh th c s dng lm ng h trong ton h thng, cng nh tp m trong cc mch logic gy ra rung pha cho cc chuyn tip s v ng h trong khi c hin tng choi sn xung.

(4) Rung pha sinh ra do ghp tn hiu, xut hin do vic a vo hoc ly ra cc bit hiu chnh v cc khung s.

(5) Nhiu giao thoa gia cc k hiu gy nn mo dng xung ca mi bit. iu ny gy ra s thay i v mc pht hin ca mi xung, gy nn hin tng rung pha cho lung bit c ti sinh.

(6) Rung pha ca b ti sinh xut hin do s khi phc thi gian khng ph hp trong cc trm lp ti sinh s.

Tr rung pha sinh ra do cc b ti sinh, cc ngun rung pha k trn c mc thp v khng tng quan. Trong cc trng hp ny, rung pha cng vi nhau trn c s cng sut v tch ly dc h thng truyn dn s. Tuy vy rung pha ca cc b lp l do tng mu sinh ra v do tng quan vi nhau. V cng mu hoc cc bin i trng mu a n mi b lp, nn trong trng hp ny rung pha s c tch ly trn c s bin .

3.1.2.1. Rung pha do cc phn t nh thi sinh ra

34

H.Q.Trung.TVT

Rung pha ny c bin v tn s tng i thp v n xut hin do tp m pha khng phi hp trong mch dao ng ca ng h v do thay i rng sn xung trong cc mch logic. Trong nhiu trng hp rung pha sinh ra c th b qua. Cc yu t m chng ta c th to ra cc dng rung pha hoc tri, l cc s bin i chm trong ngun cung cp v cc thay i v nhit ln lt nh hng n cc mc ngng kch thch trong cc mch logic.

3.1.2.2. Tri do s thay i nhit gy ra Thng rung pha bin thin chm c xem nh tri xut hin do s bin i v nhit ca thit b v knh truyn dn gy ra s thay i v thi gian i vi tn hiu i qua chng. V tri c tn s thp nn cc vng kha pha c xu th lm tng s tri ny v nh vy thc t khng th dng cc vng kha pha loi b hoc gim s tri c, tr khi thit k chng c hng s thi gian rt di. C th s dng cc b nh m c dung lng ln, v chng c kh nng hp th c cc s bin i ca cc rung pha tng i di hn ny.

Bng 3.1: Bng gi tr nh gi i vi tri

Bng 3.1 cho mt gi tr nh gi ca tri i vi cc h thng khc nhau Anh. Cc bin tri ging nh rung pha, c th c biu th bng cc khong n v, nhng ph bin hn thng dng cc n v thi gian.

Thng th ngi ta s dng cc b nh m iu tit s tn ti u vo ca cac thit b s nhm ti thiu ha cc hin tng tri iu khin c hoc khng iu khin c.

3.1.2.3. Rung pha do hiu chnh to ra Nh trnh by trong phn trn, hiu chnh l mt qu trnh trong cc bit c chn u n vo lung bit n nng tc bit ln n tc m thit b ghp knh yu cu. u cui xa nh c cc tn hiu iu khin hiu chnh, cc bit

35

H.Q.Trung.TVT

c chn thm vo b gt ra khi phc li tn hiu thng tin tr li gi tr danh nh ca n. Vic loi b mt bit hiu chnh u cui thu ngha l tn hiu s xy ra t bin, c lc ra li cc ch trng ca mt khong khe thi gian. Tn hiu bt thng ny ngoi vic i vo b nh n hi, cn c a n u vo mch vng kha pha, mch ny tch tn hiu nh thi t tc trung bnh ca tn hiu bt thng ny. Tn hiu nh thi tng hp s dng c tn hiu ra t b nh n hi (n hp th s bt thng ny).

Nu s nh thi khng tng thch, s cn li thnh phn rung pha d tn

hiu ra. Ga tr RMS ca rung pha ny c th biu th bng ( )max0,2887 f , y

maxf l tc bit u ra ca b ghp knh. Ngoi rung pha hiu chnh, cn c mt

thnh phn rung pha b sung gi l rung pha thi gian ch i. N xut hin do s tr gia thi gian hiu chnh c. Dng rung pha ny l mt loi tch ly trong thit b ghp knh v c th c cc thnh phn cc tn s nm trong bng thng ca vng kha pha ca b tch knh, l rung pha tn s thp. Trong thi gian ch i bin rung pha tng tuyn tnh, to nn nhng mu rng ca c trng. S

tch ly ca rung pha thi gian ch i c tc nm gia ( )1/ 4N v ( )1/ 2N , y N l s cp ghp knh. Mt m hnh c cng b ph hp vi cc php o tin hnh trn mt on s gm mt thit b ghp knh c lin quan ch ra rng rung pha ca b lp cng thm vo rung pha thi gian ch i do b tch knh v do trong x on s i vi mt h thng hu tuyn hot ng 140 Mbit/s v i vi mt h thng viba hot 34 Mbit/s, th tng rung pha XJ bng cng sut do

mt on n nhn vi cn bc 3 ca s on sut ng ni. V ngi ta cng chng minh l bin rung pha ca thi gian ch i ln hn gi tr ca rung pha mu ph thuc.

3.1.2.4. Rung pha do hot ng ca b ti sinh Hu ht cc b ti sinh s s dng hin nay u t nh thi, tc l tn hiu ra c nh thi li di s iu khin ca mt tn hiu nh thi c tch ra t tn hiu vo. Dng rung pha ng k nht xut hin do s khng hon thin trong mch in, ph thuc vo dy xung trong tn hiu s ang c truyn i. Rung pha ny c gi l rung pha mu ph thuc.

Trong cc lung bit s mang s liu, khng th tch trc tip thng tin nh thi c. l v lung bit khng cha cc xung trong mi khe thi gian. tch thng tin nh thi, cn phi tip tc x l v trong qu trnh x l cn to ra mt dng xung ng h u n, c rung pha nh thi xen vo. Mt phng php

36

H.Q.Trung.TVT

c s dng tch thng tin nh thi l s dng lung bit vo thit lp dao ng trong mch iu hng c Q cao. Nu Q cao, vo lc thi gian dao ng bt u tt s c mt xung khc n v bt u mch li dao ng mt ln na.

V thc t Q ca mch l hu hn cho nn tn hiu ra s bao gm vch ph c tch cng vi mt thnh phn rung pha do s chuyn i bin pha ca tp m ngu nhin trong tn hiu vo. Rung pha do tp m ny to ra s hon ton ngu nhin v khng tng quan vi tn hiu vo. V vy, cc b lp khc dc ng dy to nn rung pha mt cch c lp v c xu hng tch ly, d cho mi b lp s nn iu pha trong tn hiu vo mt phm vi no .

Tuy vy, bt k rung pha hoc tp m no nm trong bng tn ca mch khi phc ng h s xut hin trong tn hiu nh thi li s thm vo rung pha do chnh mch khi phc ng h gy ra. Nh vy lng iu pha b gim ph thuc vo gi tr Q ca mch iu hng v lng mch b lch tn s danh nh. ng trn quan im ca rung pha, s hot ng ca mt b lp ti sinh tng ng nh mt b lc thng thp i vi rung pha c tn hiu vo, nhng ng thi cng to nn rung pha, nh ni trn. C th biu th iu ny bng mt ngun rung pha b sung u vo. Nu rung pha ny thc s l ngu nhin, v n c sinh ra t nhng mu ph thuc hoc rung pha tng quan, th tng rung pha RMS NJ tn ti

trong tn hiu s sau N b ti sinh s c biu th bng biu thc gn ng:

1/ 4NJ J N=

y J l rung pha RMS to nn t mt b ti sinh n do cc ngun rung pha khng tng quan. Ngun rung pha to nn do mt mu ngu nhin l ngu nhin v bn cht v hm phn ph xc sut ca bin (pdf) c th coi gn nh phn b Gauss. Do , vi mt lch chun hoc bin rung pha RMS cho, ta c th tnh c xc sut rung pha vt bt k bin nh nh no chn. Vi nhng mc ch xc nh, gi thit t s nh nh trn RMS l 12 n 15, lc xc sut vt rt thp.

3.1.2.5. Rung pha ng chnh Rung pha ng chnh l hiu tc thi gia rung pha tn hiu nh thi v rung

pha tn hiu tn tc. l rung pha rng i, ngc vi rung pha tuyt i. Cng c th xem l hiu gia rung pha vo v rung pha ra ca mt b ti sinh bt k trong mt chui di ca cc b lp ti sinh, s c gi tr khng i i vi tt c cc b lp, v gi tr cc i xut hin trong chui.

37

H.Q.Trung.TVT

3.1.3. Cc nh hng ca rung pha

Ti mt giao din lin kt, dung sai vo i vi rung pha ca thit b tip theo s c thit k iu tit rung pha do thit b pha trc sinh ra; ni cch khc, nu khng iu khin mt cch thch ng th s tch ly rung pha s gy ra nhng vn sau:

(1) Tng xc sut cc li s a vo trong cc tn hiu s ti cc im ti sinh tn hiu do cc tn hiu nh thi b lch v thi gian so vi cc v tr ti u ca chng.

(2) a cc rung pha iu khin c vo cc tn hiu s do kh nng nh s c thit k phc v cho cc mc ch khc s dng ht, v th gy ra s trn b nh, vi hiu ng ngc li l lm cn b nh. Hin tng trn v cn xut hin trong cc loi no ca cc b m v cc b so pha ca thit b u cui, v d cc b gim rung pha v thit b ghp knh s no .

(3) Suy gim tin tc tng t, c m ha s do s iu pha ca cc mu c khi phc trong thit b bin i s/tng t u cui ca ng ni. Trong trng hp ny, rung pha thi gian nh hng n tnh ng u v khong cch gia cc mu ca cc tn hiu PAM c khi phc, ngi ta thng gi l rung pha tuyt i. Tn hiu ting ni c m ha s th nhy hn nhiu.

3.1.4. Cc gii hn ca rung pha v tri

Trong mt mng thng tin s, cn phi iu khin c s tch ly rung pha v tri. Cc l do iu khin ny l phi m bo cc mc tiu v li s v tri khng c vt cc ch tiu nu ra trong khuyn ngh G821 v G822 ca CCITT v cht lng thng tin tng t m ha ly ra t cc mu c khi phc trong qu trnh bin i s thnh tng t khng b gim st ng k.

iu khin rung pha hn ch s li trong mt phm vi cho php bng cch gii hn li ng chnh trong mi mt qu trnh khi phc thi gian. iu ny c th t c bng cch thit k mch khi phc ng h c rng bng tn ca rung pha c th so snh c vi bng tn ca rung pha vo. Bin rung pha tuyt i thng khng ng k khi xut hin cc li do rung pha to ra.

Vic iu khin tri nm trong phm vi cho php ch c th t c nu xt n vic to ra rung pha v tch ly rung pha trong tt c cc thit b trn ton mng.

38

H.Q.Trung.TVT

iu ny ni ln yu cu i vi iu khin ton b rung pha v cc c im k thut ca rung pha.

V iu khin rung pha ngha l iu khin rung pha trong mt gii hn xc nh cn phi nh r cc gii hn ny bng cch sao cho c th thc hin c s iu khin ton b. c im k thut ca cc gii hn ny c xc nh nh sau:

Cc thnh phn lin kt khc nhau nh cc tuyn viba s hoc cc phn ring bit ca thit b nh thit b ghp knh, cc tng i s, ... Mng tng th vi nhng cu hnh mng khc nhau. Cc mch ni quc t.

Quan im iu khin rung pha phi m bo l cc mc tiu li v tri ... s p ng i vi rung pha xy ra trn phn t bt k ca mng u vo ca n, khng k n v tr ca n trong mng hoc tn hiu xut pht t u trong mng li. Hn na cc mc tiu ny phi tip tc p ng d rng mng c m rng v cu hnh ca n thay i.

Khuyn ngh G823 ca CCITT tp trung vo vic iu khin rung pha v tri trong cc mng s da trn cp 2048 kbit/s v c gii quyt trc lc xt cp 1544 kbit/s.

Bng 3.1: Rung pha cho php cc i ti giao din phn cp da trn cp 2048 kbit/s. (Rung pha u vo c th chu c).

Bng 3.2: Rung pha cc i khi khng c rung pha vo i vi mt on s c di mt HRDS (rung pha ni ti).

39

H.Q.Trung.TVT

Bng 3.3: Cc c tnh truyn t rung pha ca b tch/ghp s.

3.1.5. Cc phng php o rung pha

Nhm nh mc c tnh rung pha ca thit b v cc h thng v so snh vi cc gii hn cho trn, ngi ta phi tm ra mt cch o rung pha. Trc lc m t cc phng php o, cc bi o ta miu t mt cch vn tt mt s thit b o c dng trong thc t.

3.1.5.1. Thit b o rung pha - B hin sng: Phng php s dng b hin sng l phng php n gin nht trong tt c cc phng php nhng ch c s dng xc nh gi tr nh-nh ca rung pha i vi cc ngun lp nh tn hiu thi gian. Khng th s dng cho cc lung s vo khng lp v khng th kch thch my hin sng. Phng php ny bao gm vic s dng mt tn hiu ng chnh, l bi ca tn s vo hoc trng vi tn s vo. Chun ny c s dng cho u vo kch thch ca my hin sng. Tn hiu thi gian lp a n u vo ca my hin sng v iu chnh my to sng hin th n nh.

40

H.Q.Trung.TVT

Nu tn hiu trn mn hnh biu th rung pha nh-nh khng r nt, c th xc nh n bng cch so snh vi gc thi gian nh ca b hin sng. C th s dng phng php ny xc nh tri bng cch ch ti s chuyn ng ca mt tn hiu thi gian ch o hoc m nt trong mt thi gian di, hoc ch n bin thi gian nh nh i vi cc bin i chm. Mt khuyt im ca phng php ny l khng th xc nh c cc thnh phn tn s ca rung pha bt k c o. Tn s thp b nh hng ca tri c th coi l tri hn hn v c th a vo tnh ton trong giai on thit k thit b l f1.

- B tch pha nh c: Mt trong cc thit b thng dng nht thuc loi ny l vn k vector Hewlett Packard. Cng nh my hin sng, n ch c th s dng cho nhng tn hiu ng h hoc nhng lung bit lp, trong tn hiu chun l tn hiu hoc c cng tc vi tn hiu vo hoc bi ca tc tn hiu vo.

V c bn, loi tch sng ny so pha ca ng h chun vi pha ca ng h tn hiu. Mi u vo c x l trc khi so pha nhm loi tr cc s bin i v bin v m bo dng sng thch hp.

Theo thit k, b so pha c th l mt mch mu-gi c s dng trong vn k vector. u ra ca b so pha c x l tip tc, sau c a n ng h o, b phn tch ph hoc b ghi th, mi b u c chun cng vi thit b cho php c trc tip rung pha nh-nh.

- B hin sng x l s: K thut ny cung cp mt b nh gi lng t ha thi gian v bin i vi hm mt xc sut cc ln chuyn tip ca tn hiu s, khc vi nhiu k thut o rung pha bnh thng, cho php o tt c cc mc phn cp v cc tc s liu trung gian. Ngoi ra k thut ny cng m t chi tit hin tng rung pha, cho php o mt chuyn tip trong dy s liu tun hon, hoc o ton b ca tt c cc chuyn tip. Phng php ny cng cho cc phng php nhn dng v nghin cu s phn b ca rung pha khng tng quan v rung pha mu ph thuc.

tn hiu b rung pha c th hin trn mn hnh my hin sng x l s (DPO), ta a mt tn hiu chun khng c rung pha n u vo kch thch.

Bng cch t tc dc cc i ca tn hiu trung tm ca mn hnh, tn hiu c to ra cho qua mt ca s in p trong mt thi gian nh nht.

41

H.Q.Trung.TVT

S dng nhng phch cm tc cao, c th m rng trc thi gian cho n khi tn hiu chim y ton b mn hnh, mt khi t c giai on ny, nhy in p b gim xung, nh vy bng cch to ra mt ca s in p th n ch to nn mt phn thay v cho ton b biu th trc.

Mc ch ca ca s in p ny l to ra lng t ha bin v nh vy nh c khong thi gian trong cc im trn dng sng ri vo trong ca s ny.

Nu cho mt trng s, bt k mt chuyn tip no xut hin trong ca s u bng 1 v ngoi ca s u bng 0, c th t c mt nh gi ca hm mt xc sut rung pha. Xc nh hm mt xc sut ny bng cch ghi vch gi tr ca n c sau m chuyn tip. Cc gi tr pdf gin on t c bng cch truy nhp sau mi m chuyn tip, mt tp nh n t tch ly s chuyn tip ti thi im chuyn tip ca tn hiu xut hin trong ca s l N(t), v chia gi tr ny theo thi gian k t khi bt u th nghim, l tng s cc chuyn tip nhn vi thi gian gia cc chuyn tip. Ngoi hm mt xc sut ny, cng c th xc nh c lch chun rung pha RMS.

i vi h s tin cy cao hn c th s dng cc phng php thng k xc nh hm mt xc sut i vi cc phn khc nhau ca tn hiu vo, cng nh cc gi tr rung pha RMS. V cc kt qu thu c ph thuc vo vic thit lp ch lm vic kch thch cng nh ti u ha p tuyn, nn cn tin hnh lm thm nhng th nghim ph, yu t ch quan ny lm kh khn trong vic thu nhn cc kt qu lp i lp li.

- Mt s thit b o c trn th trng: + B phn tch truyn dn s HEWLETT-PACKARD 3764A.

Thit b ny cho php o li n cp 139,268 Mbit/s theo khuyn ngh O151 ca CCITT, v to rung pha, o theo Khuyn ngh O171 ca CCITT 139 Mbit/. C th s dng thit b o rung pha nh sau:

Rung pha u ra nh-nh cc i.

cho php i vi rung pha vo.

Hm truyn t rung pha.

m tc ng ca rung pha tng cng.

m trong nhiu giy tc ng ca rung pha tng cng.

42

H.Q.Trung.TVT

m trong nhiu giy khng tc ng ca rung pha tng cng.

Tt c cc o lng rung pha tin hnh ng thi, v la chn cc kt qu mong mun biu th.

+ Thit b o li WANDEL&GOLTERMANNPF - 4 BIT V B IU CH RUNG PHA PFJ-4.

Khi s dng ng thi hai thit b ny cho php nghin cu rung pha trn cc h thng thng tin 140 Mbit/s.

Ngoi ra hng sn xut thit b o lng ni trn, cn c mt s hng c ting l Siemens, Marconi,...

3.1.5.2. Thit lp o rung pha Trong iu kin hot ng bnh thng c th coi tn hiu thng tin l ngu nhin v ph thuc vo ti lu lng thng tin. pht hin s bt thng trong h thng vi cc iu kin ny cn phi tin hnh o trong mt thi gian di, t bit nu h thng l mi. Cn gii hn ca mng cho trong bng trn ch ra rng xc sut vt cc mc rung pha nh vy l rt nh.

Tuy nhin, thi gian quan st thc t thu c cc mc c tin cy cao hi mt thi gian o di khng th chp nhn c. i vi phng th nghim, nh my v cc mc ch y thc, th ngi ta tin hnh cc o lng trn h thng bng mt lot php o th c bit. Trong o lng cn s dng cc dy th gi ngu nhin sao cho kt qu thu c phi gn ging vi hon cnh thc t trong ni dung thng tin hu nh gn ngu nhin hn I V nh-nh vi tr khc 75.

B tr thit b o th theo khuyn ngh ca CCITT

43

H.Q.Trung.TVT

Hnh 3.2: Thit lp o rung pha ca CCITT

44

H.Q.Trung.TVT

pht tn hiu ny th cn mt b to ng h v mt ngun iu ch nh hnh 3.2(A). Ngun iu ch c th ly trong b to ng h hoc b to mu hoc cng c th ly ring r nh nu. B to ng h c th b iu pha bng ngun iu ch v ch ra lch nh-nh ca tn hiu c iu ch. Cc u ra ca b to ng h gm c tn hiu ng h c iu ch v mt tn hiu chun thi gian, cc tn hiu yu cu khng c nh hn

3.1.5.3. o rung pha Thng thng cc thnh phn ca h thng c thi k chu c cc mc rung pha xc nh trong khong tn s thit k: neus rung pha ca mt tn hiu s vt cc gi tr ny, s xut hin li truyn dn.

Ghi bin rung pha v nh du thi im m rung pha vt ngng ny v sau chiu cc thi im ny vi cc c tnh ca h thng s thy c mt cch su sc nh hng ca rung pha. Trong mt s ng dng, mt iu rt b ch l xem xt php o rung pha ph thuc vo tn s, v thng thng cc h thng chu ng c bin rung pha tn s thp rt tt hn l i vi cc thnh phn tn s cao.

Cc c tnh v cc hm truyn t rung pha ca cc thnh phn h thng cng l cac tham s quan trng, chng c th h tr cho vic nng cao cht lng h thng. Trong cc h thng s tng i di ni qua cc trm lp, mc cho php ca mt u vo b lp i vi nhng lnh bin rung pha l hm ca tn s rung pha. C th s dng n v trng s tn s trong php o rung pha, n biu th ln rung pha t l vi tc hi ca rung pha vo h thng, khng k n lng tn s ca rung pha .

Trong o rung pha, cn thc hin ba php o ch yu, l:

- Xc nh rung pha ni ti u ra khi khng c rung pha vo.

- Xc nh hm truyn t rung pha.

- o rung pha u vo cc i cho php.

Nh ta ni trn, thit b o c bn gm mt b to mu c thit b iu ch rung pha, mt ng h o rung pha v mt b pht hin t l li bit. Thit b c o c th l mt thnh phn ca h thng v d nh mt phn ca thit b ghp knh, mt b ti sinh hoc mt h thng truyn dn y . Rung pha ni ti c o bng tt iu ch rung pha trn thit b o. xc nh hm truyn t, th b to sng b iu pha hnh sin vi cc tn s khc nhau v vi mt bin rung pha cho

45

H.Q.Trung.TVT

php. Nh s h tr ca b pht hin t l li bit c th xc nh c rung pha u vo cc i cho php. Rung pha u vo c tng cho n lc xut hin li. iu ny c lp li i vi nhng tn s rung pha khc nhau. Bnh thng xc nh s dng dy nh phn gi ngu nhin cho tt c ba php o ny.

3.1.6. Cc phng php ti thiu ha rung pha

Thng s dng hai phng php c bn. Phng php th nht l tin hnh cc bc ngn chn vic sinh ra v tch ly mt cch c h thng ca rung pha bng cch s dng cc b trn ngu nhin. Cc thit b ny lm cho tn hiu tr thnh ngu nhin v nh vy gim c nh hng ca bt k c ch gy rung pha ca mu ph thuc. Phng php th hai l gim ln ca rung pha c bng cch s dng mch khi phc li thi gian c rng bng tn nh hn rng bng tn ca tn hiu. Mch ny c gi l b gim rung pha. Nh ni trn, cc tn s rung pha thp hn tn s ct ca b gim rung pha khng b gim v c th gy ra trong mt s mch mt lch khng iu khin c, do bin rung pha ti cc tn s ny c tch ly v ln lm nh hng ti cc thit b s khng trong sut i vi rung pha. Tuy nhin trong hon cnh thc t vic chn cc b nh m c dung lng ln u vo ca cc thit b s gim nh bt vn ny.

3.1.7. Cc b trn ngu nhin (Scrambling)

Mt s u im ca cc b trn ngu nhin khi s dng nh l mt b phn hp thnh ca h thng truyn dn s.

- m bo s tch ly rung pha khng tng quan vi tn hiu.

- Gim c nh hng tch ly ca rung pha tn thp.

- Gim c cc nh hng ca nhiu trn cc i cp i xng sinh ra do cc h thng ng b bng cch nn cc thnh phn ph ri rc ca cc mu bit tun hon.

Ni chung, b xo trn nhm lm cho d liu ngu nhin hn bng cch loi b cc chui bit 1 hay bit 0. Vic xo trn (scrambling) lm cho d liu tr nn thng sut hn c ngha l d liu s khng tn ti mt chui di cc bit 1 hay cc bit 0.

46

H.Q.Trung.TVT

Hnh 3.3. B trn v gii trn gi ngu nhin c bn

B xo trn bao gm thanh ghi dch, v b kh xo trn tng ng cha thanh ghi dch phn hi thun. Mi ln thc hin dch ng vi vic lm tr i mt bit. phn tch b xo trn v kh xo trn, ta xt chui d liu T li ra ca m xo trn. Nu nh gi S l chui d liu li vo b xo trn th:

Trong D l ton t tr. DnT l chu T b lm tr i n n v. T phng trnh trn ta c:

thit k b gii xo trn (descrambler) ti my thu, ta bt u vi chui d liu T (t li ra b xo trn), chui ny c a ti li vo ca b gii xo trn (descrambler).

T phng trnh trn, ta nhn c chui d liu ban u S t chui d liu thu c T, c thc hin bi b gii xo trn descrambler).

Cn ch rng nu xy ra mt li n trong chui d liu thu c T s gy nh hng ti 3 bit li ra R. Bi th, xo trn c nhc im l c th gy ra mt lot cc li t mt li bit n.

47

H.Q.Trung.TVT

V d: Cho li vo ca b xo trn l mt chui bit S = 101010100000111. Hy tm li ra T, gi s rng cc thanh gi c khi to bng 0.

Gii: T s b xo trn (scrambler), ta quan st thy khi to T = s, v chui S vo thanh ghi v phn hi li c dng:

Chui mi FS tip tc vo thanh ghi v tr li c dng F2S, c th tip tc. Tng qut ta c:

Do

Nn

=>

Tng t, ta c:

V vy: nD S l chui S b tr n bit. Do ta c th tm chui T nh sau:

48

H.Q.Trung.TVT

3.2. CC M NG TRUYN

Mt s l do m ha s l:

- a vo d bng cch m ha cc t s liu nh phn thnh nhng t di hn. Cc t nh phn di hn ny s c nhiu t hp hn do tng s bit. Chng ta c th chn nhng t hp xc nh c cu trc theo mt qui lut t m hp thnh, cho php tch thng tin nh thi mt cch d dng hn v gim chnh lch gia nhng con s 1 v nhng s 0 xut hin trong mt t m ( l gim s chnh lch). Vic gim chnh lch ny dn ti gim thnh phn mt chiu. Nu chnh lch ny gim n khng i vi tt c tp hp t m th thnh phn mt chiu ca chng cng gim n khng. iu ny l cn thit v khng th truyn thnh phn mt chiu ca tn hiu s i c. C th s dng cc t m ph trong m d truyn s liu ph nh truyn bit chn l trong m pht hin li v truyn cc knh ph tr. Tuy vy vic tng di ca t m nh phn s lm tng tc bit v do tng rng bng tn. Tc bit tng t l vi t s di cu t m ra trn di ca t m vo. m 5B6B tc bit u ra tng 6/5 ln so vi tc bit vo.

- M ha tn hiu nh phn thnh tn hiu nhiu mc gim rng bng tn. Loi m ny quan trng khi truyn s liu c tc cao trn i dy kim loi c di tn hn ch. Vic gim rng bng tn cn thit ca knh hoc tng tc bit vi mt rng bng tn cho s lm tng t s tn hiu trn tp m t c xc sut li cho trc.

- Bo mt tin tc nhng ni yu cu tnh an ton cao c bit trn cc tuyn truyn s liu ca cc h thng my tnh x l s liu.

- To ph tn hiu nhm ng dng cho nhng mc ch c bit nh ng b gim thnh phn bin tn s khng n 0, hoc gim cc thnh phn cao v thp ca tn s trc lc lc . C th a nhng s 0 c bit vo cc lung s c m lng cc bc cao b chn v cc lung s b chn.

Trong qu trnh m ha PCM nh trnh by trong chng trc, tt c cc bit thng tin c ngm gi thit l nh phn n cc. Gi thit ny l hp l min l cc bit thng tin c xc nh trong mt cng on nht nh no ca thit b x l v dy ni khng c di qu vi mt. Vi nhng ng dy ni tng i di, ng cp i xon bc kim, hoc cp ng trc th khng nn s dng lung bit nh phn.

49

H.Q.Trung.TVT

Mt s l do lm cho di ca cc dy ni gia cc b phn ca thit b di hn hai mt l:

+ Thit b ghp knh PCM s cp c b tr nhng v tr khc so vi thit b ghp knh bc 2 hoc cao hn.

+ Thit b PCM s cp hoc thit b ghp knh bc 2 hoc cao hn c th c ni n thit b truyn dn x l tip tc tn hiu trc lc truyn qua knh thng tin.

+ Thng tin PCM s cp hoc thng tin ghp knh bc 2 tr ln c th c truyn trc tip qua cp i bc kim, hoc cp ng trc n u cui xa.

D l do g i na, th tn hiu s ri khi mt cng on duy nht ca thit b x l cng phi giao tip chnh xc phi hp vi knh truyn dn, thm ch knh truyn dn ch l mt on ni tip gia cc thit b c s dng trong x l tn hiu trc lc truyn ln knh thng tin. Cc l do ti sao mt tn hiu nh phn n cc, (tc l mt tn hiu ch c hai trng thi trong mt trng thi mc 0) phi bin thnh dng khc trc lc truyn qua cc ng nh ni trn l:

(1) N c cha thnh phn mt chiu.

(2) N c cc thnh phn tn s thp mc cao.

(3) Khi truyn mt dy 0 lin tip th khng c im chuyn tip tn hiu.

Ngi ta s dng cc bin p ngn chn dng mt chiu, i vi cc thit b lp trung gian n c s dng ni cc u vo v u ra ca thit b x l. Tn hiu nh phn n cc c thnh phn mt chiu, v vy khng thch hp truyn qua bin p. Ngoi ra v trong tn hiu nh phn n cc c cha nng lng cao trong ph tn s thp nn khi truyn qua bin p chun, thc t s b mo dng xung v nh vy n s to thm li truyn dn. V my thu cn phi tch thng tin nh thi ra khi dng bit thu c, vic xut hin mt dy 0 lin tc trong tn hiu nh phn n cc s nhiu lon qu trnh tch thng tin nh thi v dn n mt ng b.

Trong thit b x l c hai dng tn hiu nh phn n cc. l RZ tr v 0 v NRZ khng v 0. Nu s dng trc tip chng truyn dn th s gp phi mt s kh khn nh nhau. Hnh 3.4 th hin dng sng ca hai tn hiu nh phn n cc ny. Ch rng mc ca tn hiu RZ biu th bit gi tr 1, n ch mc cao trong mt na u tin ca khong thi gian bit, trong mt na khong thi gian bit cn li tn hiu quay tr v 0.

50

H.Q.Trung.TVT

Hnh 3.4: Biu din m nh phn NRZ, RZ v bin i nh phn AMI

u im ca tn hiu RZ l mt chuyn tip tn hiu ca n ln hn so vi NRZ. khc phc nhng vn cn tn ti trong tn hiu nh phn n cc, ngi ta xut ra m ng truyn. Cc m ng truyn khng c thnh phn mt chiu ng thi cng khng c hin tng nng lng ph ca n tp trung vng tn s thp. i khi c mt s m ng truyn c thm u im l trong cu trc ca n khng c mt dy di nhng s 0 hoc nhng s 1, iu lm cho vic tch thng tin nh thi s d dng hn.

Nhng yu cu ca m ng cn phi c bin i tn hiu nh phn thnh mt dng thch hp hn truyn trn knh thng tin l:

- Phi phi hp c tnh ph ca tn hiu vi cc c tnh ca knh (nh tp m, rng bng tn, ...)

- m bo cc dy bit phi c lp thng k vi nhau gim lng rung pha phu thuc mu do cc mu lp gy ra.

- m bo d dng tch ng h v ti sinh tn hiu.

- m bo d phng cn thit gim st li ca ng truyn dn v nh v s c ca thit b.

- Phi duy tr d phng mc thp c th c sao cho tc bit l nh nht v do rng bng tn truyn l b nht v gim phc tp ca mch. Gim thnh phn mt chiu ca tn hiu n 0.

- Gim cc thnh phn tn s thp nhm gim xuyn m v kch thc cc linh kin mch. y l mt tiu chun do CCITT khuyn ngh v c cc nh sn xut khc p dng.

Ch l hin nay c hai trng hp tn ti trong h thng thng tin s v vy c cc loi m khc nhau. Trng hp th nht l m giao tip, trong u ra

51

H.Q.Trung.TVT

ca thit b ghp knh s tr thnh mt nhnh vo ca thit b ghp knh s mc cao hn tip theo. di ng dy trong trng hp ny khng qu 100m. Trng hp th hai l u ra ca thit b ghp knh hoc thit b s khc c gi trc tip ln ng dy. Trong trng hp ny m ng ph thuc vo thit b c s dng truyn tn hiu thng tin trn knh truyn dn.

Bng 3.5: Cc v d m ng ca h thng

3.2.1. M tam phn la chn cp

Bin i tn hiu nh phn n cc RZ thnh tn hiu tam phn la chn cp c tin hnh bng cch ghp hai bit ln cn v s dng quy lut bin. i vi c hai u ra mode dng v mode m cc t hp tam phn c to nn t cc cp nh phn 00 v 11 khng c mc DC, mc du cc cp nh phn 01 v 10 to nn mc DC tng ng vi u ra 2A+ v 2A- ph thuc vo mode ca u

ra. B m hot ng to ra u ra mode dng cho n khi u vo xut hin trng thi 01 hoc 10. Sau to nn mt nh thin 2A+ u ra thay i sang

mode m. S xut hin tip theo ca 10 hoc 01 to nn nh thin 2A- , xa nh

thin 2A+ v i mode tr li dng. V mt dy di lin tip cc s 0 hoc 1

khng xut hin, nn n cn bng c dng mt chiu u ra, d tch nh thi cc thit b lp v thit b thu u cui.

Bng 3.6: Quy lut bit i ca m tam phn chn cp

52

H.Q.Trung.TVT

3.2.2. M 4B3T

M ny lm cho vic s dng cc mc tam phn c hiu qu hn bng cch nn 4 bit s liu vo thnh 3 bit ca s liu ra. Nh vy tc ban u bng 0,75 ln tc ca tn hiu vo. Bng 3.7 ch ra thut ton bin i ca n. Nng lng ph i vi m ny tng i rng v u, tuy vy n gim nhanh i vi nhng tn s ph b hn 0,1 ln tc bit. Ngui ta c gng gt nhng thnh phn tn ph rt thp cu m ny bng cch to ra mt m 4B3T ci tin nh MS43. K hiu 4B trong m 4B3T tng ng vi cc khi 4 s nh phn ca tn hiu vo. K hiu 3T trong khi 3 s tam phn biu th m ra. M ny thuc nhm m khi

nNmT, y 3 2m n> , m l s tam phn, n l s s nh phn trong khi. Bng 3.7 cho chng ta nhn thy rng lch cc i hoc tng s sai khc s l 3 i vi m vo 0111. Nu tn hiu ra trung bnh l dng th mode s chuyn thnh m v v.v... ch ci ca mode c sp xp nh thm dy ra tin n tch ly tng s sai lch s dng hoc m v chuyn cc mode nhm duy tr tng s tin n 0, nh vy s lm ti thiu ha nng lng ph tn thp ca tn hiu ra. Ngi ta s dng m ny trong cc h thng Ethernet, ISDN v cc h thng truyn s liu.

Bng 3.7: Quy lut bin i m 4B3T

53

H.Q.Trung.TVT

3.2.3. M MS43

Mt m 4B3T khc l m MS43, c s dng truyn tn hiu trong cc h thng truyn dn s trn cp bc kim. M ny s dng 3 ch ci 1 2 3, ,R R R , c

chn tng ng vi lch ca tng bin i s ca m. mt thi im no tng s c th thay i trn 6 gi tr, t 0 n 5, vi nhng gi tr cc i xut hin trong thi gian hoc ca phn t th nht hoc phn t th hai ca mt t tam phn. iu ny hn ch cc trng thi cui t 1 n 4. Nu trng thi cui l 1 th ch ci

1R c chn cho t m tip theo. Nu n l 2 hoc 3, chn ch ci 2R v nu n l

4 th chn ch ci 3R . Bng di y ch ra quy lut bin i m ny. Cc ch ci

c sp xp sao cho mi mt nhm 3 k hiu tam phn c lin h duy nht vi 1 nhm 4 bit nh phn v do n khng cn tit b gii m xc nh mode hoc ch ci c my pht s dng. M ny cho php xut hin 4 s 0 lin tip trong 1dayx tam phn hoc ln n 5 k hiu dng hoc m lin tip. T tam phn bao gm 3 s 0 khng s dng v mt dy bt k 3 hoc 4 s 0 cn phi ni bin gii gia hai t k cn nhau. S dng c im ny t c ng chnh khung. Ngoi ra v mi t cha mt hoc nhiu s 1 nn vic tch ng h n gin.

54

H.Q.Trung.TVT

Bng 3.8: Tut ton m MS43

3.2.4. M 6B4T

M 6B4T bin i 6 s nh phn thnh 4 s tam phn. M ny c 62 hoc 64

t nh phn 6 bit vo 43 80= t tam phn khng k 0000. Nhm cc m c din t nh nNmT, l mt lp m ch ci tam phn phi tuyn. Nh ni trn, trng hp n gin nht ca m ny khi n=m=2, nh m tam phn la chn cp (PST).

3.2.5. Cc m BNZS

Cc m c trin khai, l m lng cc khi thay i mt dy N s 0 lin tip trong m nh phn bng nhng vi phm lut lng cc. S thay th ny gim bt chnh lch. Cc m ny thuc mt lp m khng tam phn phi tuyn v c th coi l m AMI ci tin hoc m lng cc. Ba loi m quan trng nht trong lp ny l B3ZS, B6ZS, B8ZS. Tt c cc m ny c CCITT khuyn ngh trong G703 s dng trong mt s giao tip no s trnh by di y.

3.2.5.1. M B3ZS

Cng nh ni trong khuyn ngh G703 ca CCITT, m B3ZS l mt dng m lng cc ci tin hoc dng xung AMI c thay 3 s 0 lin tip bng nhm vi phm lut lng cc. Cc bit logic 1 c thi gian tn ti 50% chu trnh v ni chung l thay i lun phin dng v m i vi mc logic 0.

55

H.Q.Trung.TVT

Trng hp ngoi l l khi c 3 logic 0 xut hin (ng thi) trong lung bit. Trong dng B3ZS mi khi 3 logic 0 lin tip c thay th bng t hp B0V hoc 00V, y B biu th 1 xung hp vi lut lng cc, hoc l qui lut cc tnh du lun phin ca AMI v V biu th 1 xung phm qui lut ny.

Vic chn B0V hoc 00V c tin hnh sao cho s cc xung B gia hai xung V k tip nhau l l. M ny thng c s dng trong thit b giao tip ca b ghp knh bc 3. Cc bit ng b khung cha trong qu trnh m ha.

Hnh 3.5: Cc tn hiu nh phn m HDB3, CMI v B3ZS

3.2.5.2. M B6ZS

Trong m ny mt t hp 6 s 0 lin tip trong tn hiu vo c thay th bng mt t hp 0 0+ - - + nu xung pha trc l +; cn nu xung pha trc l th lc 6 s 0 c thay bng 0 0- + + - . Nh vy xut hin mt dy 0 di lin tip trong m AMI thng thng c loi tr v thay th vo l cc xung hoc t hp xung phm li quy lut cc tnh ca xung an du. nh ph nng lng ca m ny ti 0,5 tc bit, v hu ht nng lng ph nm trong khong 0,2 0,8 ln tc bit. M ny c khuyn ngh s dng trong giao tip s 6312 kbit/s nu s dng 1 i dy i xng cho hng truyn dn.

3.2.5.3. M B8ZS

Trong m ny mt nhm 8 s 0 lin tip trong tn hiu vo c thay th bng 000 0+ - - + nu xung trc l dng (+). Nu xung trc l m (-) th 8 s 0 c thay bng 000 0- + + . p li quy lut cc tnh xung du lun phin ca AMI b vi phm CCITT khuyn ngh s dng m ny cho cc giao tip s tc 1544 kbit/s nu s dng cp i xng, hoc tc 6312 kbit/s nu s dng cp ng trc.

56

H.Q.Trung.TVT

3.2.6. M HDB3

M HDB3 l m tam phn mt cao c nhiu nht 3 s 0 lin tip. Thut ton m mt tn hiu nh phn thnh mt tn hiu HDB3 ph thuc vo cc quy tc sau y:

1- S 0 nh phn c m bng mt trng thi trng trong tn hiu HDB3. Tuy nhin i vi mt dy 4 s 0 lin tip th s dng cc quy lut c bit theo quy tc 3 di y.

2- S 1 nh phn c m bng k hiu dng hoc m v c du ngc vi xung trc (o du lun phin).

3- Cc dy 4 s 0 lin tip trong tn hiu nh phn c m nh sau:

(1) S 0 u tin ca dy c m bng trng thi trng nu du trc tn hiu HDB3 c cc ngc vi cc ca vi phm pha trc v bn thn n khng vi phm.

(2) S 0 u tin ca dy c m bng du A m khng vi phm (+ hoc -), nu du trc ca tn hiu HDB3 c cng cc vi du vi phm trc hoc chnh bn thn n vi phm.

Cc quy lut (1) v (2) m bo cc vi phm lin tip c cc tnh o nhau sao cho thnh phn mt chiu c th gp li bng khng.

(3) S 0 th 2 ca dy 4 s 0 nh phn lin tip lun c m bng trng thi trng.

(4) S 0 th t trong dy ca 4 s 0 nh phn c m bng mt du m cc tnh ca n vi phm an du. Nhng vi phm an du nh vy c k hiu bng V- hoc V+ tng ng vi cc tnh ca n.

Phn b nng lng ph ca tn hiu u vo ngu nhin c m ha thnh HDB3 ging phn b nng lng ph ca AMI, trong nng lng ph cc i nm khong 0,5 tc bit v hai nh nh khong 0,45 v 0,55 ln tc bit. M ny c s dng ch yu cho cc giao tip ghp knh 2048, 8448 v 34368 kbit/s theo nh CCITT khuyn ngh. Ngi ta s dng n trong cu hnh mng cc b Ethernet v truyn a s liu.

3.2.7. M CMI

CMI-Code Mark Inversion (o du m). N l mt m NRZ hai mc trong bit 0 nh phn vn c m bng hai mc 1A v 2A tng ng, mi mc chim mt

57

H.Q.Trung.TVT

na khong thi gian n v T/2. Bit 1 c m bng cc mc bin 1A v 2A

mi mc chim ton b thi gian n v T, cc mc ny lun phin thay i theo cc bit 1 k tip nhau. Ch rng i vi bit 0 lun lun c mt chuyn tip dng ti im gia ca khong thi gian bit, v i vi bit 1 c mt chuyn tip dng ti im khi u ca khong thi gian n v bit mc trc l 1A v mt

chuyn tip m ti thi im khi u ca khong thi gian n v bit nu bit 1 sau cng c m ha bng mc 2A . Lu rng bit khng c k hiu bng 01

v bit 1 k hiu bng 11 v 00 trong khong thi gian ca khe thi gian.

3.3. M IU KHIN LI

Nh ta bit, d l dung lng truyn dn khng c s dng ca mt tn hiu m. Tuy vy, s dng d ny cho php pht hin v cng c th sa c cc li do tp m sinh ra my thu, iu ny nng cao c tin cy ca h thng. Trong h thng thng tin s c th xut hin 3 loi li. l: li thay th, trong mt bit s gc c thay bng mt s trng thi khc; cc li b st, trong mt k hiu b xa i khi lung bit; v cc li trong mt k hiu gi mo c xen vo dng bit. Li hay xut hin nht l li thay th. Tt c cc loi li c th xut hin trong cc lung bit nh phn hoc trong mt k hiu nhiu mc. V xc sut li trong truyn dn s l mt hm trc tip ca t s tn hiu /tp m, mt vn xut hin l nu v mt s l do no cng sut tn hiu b gii hn bi mt s gi tr cc i no , iu dn ti xut hin li vi tn sut khng chp nhn c. C th gim cc li ny bng cch s dng cc mch pht hin li v sa li. Trong truyn dn s liu in bo v Fax ngi ta quan tm nht vic s dng m bo v li, cn i vi ting ni th khng c tiu chun nh vy. V hin nay ang hnh thnh mt dng s liu tng hp lm cho chng ta kh m xc nh c phm vi no, vng no cn bo v li, phm v no, vng no khng cn. Nhng iu khng th trnh c l phi a ra m iu khin li cho ton b cc h thng m bo tnh linh hot ca chng. D nhn thy c hai loi iu khin li khc nhau xut hin trong truyn dn. Loi u tin l li ngu nhin, gia cc bit s li khng tng quan. Loi th hai l cc t bin li, y phn ln cc bit s kt tip nhau b sai lch. Hai loi ny c tnh cht ph bin v th ngi ta s dng chng l yu t quyt nh ca loi m iu khin li. Cc t bin li thng xut hin t nhng thit b nh cc b trn hoc cc thit b m ng dy, mt li duy nht c m thnh mt t khc vi cc t gc ca n. iu ny dn ti xut hin thm cc li khc ging nh nhn hoc m rng li. Trong thc t tn thng gi loi li ny l li m rng. Hiu qu ca li m rng c c trng bng khi nim T l li bit

58

H.Q.Trung.TVT

tng ng (EBER) trong t l li bit c o vi iu kin bit nguyn nhn do li m rng c gim bng mt h s m rng li ti h s v t l li bit tng ng. Cc t bin li cng l mt c th ca truyn dn s qua mng chuyn mch in thoi trong m xung chim u th hn so vi tp m Gauss.

S dng rng ri cc m bo v li m bo v cc mch s liu v cc mch in bo. Ngi ta s dng cc h thng m ha trang b cho cc h thng qun s, my tnh cng vi thng tin in thoi bo mt bo v cc thng tin bng cch chn cc loi m thch hp. S dng cac h thng ny trong phm vi m cho thng tin c s d liu khc vi m bo v li hoc pht hin li. M iu khin li thng c s dng trong cc modem u ni ca in thoi hoc mt knh thng tin ca ng dy, t khi dng trong h thng v tuyn chuyn tip ng di, ngi ta cng nh gi v th nghim mt s m sa sai khc nhau trn cc h thng HF, i lu v v tinh.

3.4. CN BNG

Trong nhng h thng thc t v thit k b lc khng hon thin v do s thay i cc c tnh ca knh thng tin theo thi gian cho nn lun lun tn ti mt lng giao thoa gia cac k hiu (ISI). Nhm b hin tng ni trn v gim ISI n mc b nht cn phi s dng mt mch la chn tn s, c hm truyn t bin v pha ngc vi hm truyn t tng hp ca knh v b lc knh. Mt mch nh th gi l mt b cn bng v qu trnh b ca n gi li qu trnh cn bng p o. Nu s thay i c tnh ca knh do thay i nhin hoc i vi tuyn viba l thay i cc iu kin kh hu, mi trng, ... dn ti fading nhiu tia; hoc thay i cc tham s b lc ca thit b, ... khi s dng b cn bng c c tnh bin i ngc so vi cc s bin i ni trn th s hn ch c hin tng ISI n mc b nht. S thay i ca ISI t b nht n ln nht trong tt c cc iu kin c th ln n ni khng chp nhn c. c bit i vi nhng knh c c tnh thay i theo thi gian vi tc kh ln trong thi gian knh b chim. khc phc tnh trng ny, ngi ta s dng mt phng php t ng iu chnh mo do knh sinh ra cn gi l cn bng t thch nghi. Trong phn ny chng ta s trnh by mt cch vn tt cc loi b cn bng v ng dng ca chng.

3.4.1. B cn bng ngang Hnh 3.6 di y ch ra b cn bng ngang bao gm mt ng dy tr 2 1M + nhnh tr, mi nhnh tr c thi gian trn sT giy, v n c ni qua mt

thit b c tng ch bin i c, k hiu cc h s tng ch tng ng vi tng

59

H.Q.Trung.TVT

thit b l 0 1, , , , ,m mC C C C- L L cho ( )2 1M + nhnh tr; tt c cc u ra c ni n mt b khuch i tng. Nu bit dng xung i vo ca b cn bng l

( )rP t v u ra ca n l ( )eqP t , th quan h gia chng l:

Gi s ( )rP t c nh ti t = 0, v giao thoa gia cc k hiu c hai pha, tc ly mu u ra cho bi: ( )k st k M T= +

Ta nhn c:

Hnh 3.6: B cn bng ngang c 2M+1 t

loi b c ISI, v l tng chng ta s c:

V ch c 2M+1 nhnh tr, trong khi s nhnh tr cn thit phi l v cng,

vy gi tr ca ( )eq kP t ch c th nh r ti 2M+1 im:

60

H.Q.Trung.TVT

T cc phng trnh trn, mt tp hp gm ( )2 1M + phng trnh tuyn tnh ng thi, chng ta c th gii c mC . Tp ny cho di dng ma trn.

Ma trn trn c gi l ma trn cng bc bng khng ca cc b cn bng v chn cc h s ca nhnh tr mo nh l b nht bng cch p b cn bng tin ti 0 ti M im ly mu c hai pha ca xung yu cu.

V d: S dng b cn bng 3 nhnh tr iu chnh sao cho gim c giao thoa gia cc k hiu do xung ( )rP t gy ra. Hnh 3.7 biu din th dng xung vo v ra khi b cn bng. Xc nh cc gi tr tng ch ca nhnh tr v cc gi tr ca xung c cn bng ti cc im ly mu.

61

H.Q.Trung.TVT

Hnh 3.7: Dng sng vo v ra b cn bng ngang 3 nhnh tr

Cn c vo hnh 3.7, s dng ma trn cng bc bng 0:

T phng trnh ca ( )eq kP t ta xc nh c cc gi tr ca xung cn bng v lp thnh bng:

Xung cn bng ch ra trn hnh 3.7. Ch rng cc pha khc nhau ca 0t = (ngha l st T= ), l im 0 ca xung ra.

3.4.2. Cc b cn bng t ng

C th phn cc b cn bng t ng thnh 2 nhm: loi nh trc iu chnh ph thuc vo s truyn dn mt mu th, v loi t thch nghi, loi ny t iu chnh lng li lin tc trong khi truyn s liu ty theo lng li gia tn hiu thc t thu c v nh gi tn hiu pht i trc ti my thu. Trn c s lng li thu c lin quan hu c vi lung s liu, chng ta nh gi c cc h s tng ch. y l mt qu trnh lp, sao cho bin ca li l b nht.

3.4.2.1. Cc b cn bng nh trc

tin hnh iu chnh cn bng t ng phi truyn i mt mu kim tra v da vo nhng iu bit trc ca mu ti my thu tnh p ng xung tn hiu thu c. T p tuyn xung, ta tnh c cc h s ca nhnh tr, hoc s dng phng php lp tnh cc h s cho n lc thu c gi tr ti u. Loi b cn bng ny tc hi t nhanh, nhng nu c tnh ca knh thay i tc th phi pht li mu kim tra ti nhng thi im thch hp thit lp li cc h s. S dng phng php lp li i vi phng trnh:

I XC=

Trong C, I l vec t ct c 2M+1 phn t, X l ma trn vung cp 2M+1.

62

H.Q.Trung.TVT

Mc ch lp t c 0I XC- = , bc lp th k tn ti mt vc t sai

s: k ke XC I= - .

Thut ton ni trn c s dng ti u cc h s ca b cn bng mC

v ang l i tng c th xem xt nghin cu. C 2 tiu chun c s dng rng ri. Mt l tiu chun mo nh, hai l tiu chun trung bnh bnh phng (MSE).

2.4.2.2. Cc b cn bng t thch nghi

Trong cc h thng thng tin v tuyn s s dng b cn bng t thch nghi khc phc cc nh hng ca truyn lan nhiu tia; mt khc nh chng ta bit v s lan truyn ca knh gy ra hin tng tri rng cng sut tn hiu theo thi gian, khi s dng phn tp theo khng gian cho php b sung phi hp cc tn hiu v b cn bng trong knh phn tp c th t ng iu chnh s chnh lch v thi gian tr gia cc knh. B cn bng t n cng c th khc phc cc hiu ng lan truyn bng cch nng cao d tr fading hiu dng.

B cn bng t thch nghi khng s dng mt mu kim tra nh b cn bng nh trc m s dng nh gi sai s gia tn hiu thc thu c v nh gi tn hiu pht i trc , nh ni trn. Mt c tnh lm vic i vi b cn bng t thch nghi l: tn hiu vo phi l mt dy bit ngu nhin. Nu tn hiu khng cn bng b mo nghim trng, b cn bng t thch nghi to ra rt nhiu li v qu trnh lp khng hi t. D nhin iu ny c ngha l khng t c s cn bng. Trong hon cnh nh vy xut hin phng php c th t c l s dng b cn bng nh trc chuyn sang phng thc t thch nghi sau khi t c mt s o cn bng.

63

H.Q.Trung.TVT

Hnh 3.8: Cc phn t ca b cn bng thch nghi

3.5. TI SINH TN HIU S

Qu trnh cho php truyn dn s tt hn truyn dn tng t, l qu trnh ti sinh tn hiu. Ti sinh l mt qu trnh trong mt tn hiu s b mo v b tiu hao c ti to li thnh bin v dng sng ng ca n. Qa trnh ny c th a n mt nh ngha khc ca truyn dn s, v tt c truyn dn s u cn c ti sinh ti mt s im trn ng truyn dn. i vi mt chui tuyn truyn dn s, cc b phn ti sinh c mt mch ca my thu bng gc tin khuch i v sa dng tn hiu vo b tiu hao, tn x v tp m xm nhp vo trc lc tch sng. B tch sng thc cht l mt b tch sng ngng, n to nn u ra mt dng xung xc nh r rng. Mch tch sng cng c th hot ng da vo cc dng tn hiu m n gn ging hnh sinh. Mch ny bao hm mt b khuch i bng gc, b lc bng gc m rng bng tn ca chng khng cn phi ln nh lc yu cu truyn dn mt xung v nh vy tp m u ra ca bng gc gim xung. Gim rng bng tn ca b khuch i bng gc ca my thu lm ni rng ph xung, dn n gim liu lng cho php giao thoa gia cc k hiu v nh vy tng kh khn khi to nn ngng quyt nh. Kt qu tt yu s lm cho th hnh mt ng li. Thit k b ti sinh phi dung ha gia vn gim giao thoa khi ni rng bng tn v tng tp m nhit, iu cng s tng cc nh hng ca trt thi gian do nh hng ca tp m n ngng quyt nh.

64

H.Q.Trung.TVT

Hnh 3.9: (a) S ca mt b ti sinh s in hnh

Hnh 3.9: (b) B lp ng dy

B ti sinh ng dy cho chng ta thu hiu s hot ng ca cc b ti sinh thc t.

Trong v tuyn s, b ti sinh thng thng c b tr trong cc mch tch bit, n a vo sau b gii iu ch v cc tng cn bng ngang. Ph hp vi hnh 3.9 (b) cc tn hiu ng dy vo qua bin p ng dy v cc mch cn bng n mt b khuch i. T b khuch i ny c hai u ra ngc pha nhau a vo cc b tch sng ngng. ng thi 2 u ra ny cng c mc v a n mch nh thi.

65

H.Q.Trung.TVT

Hnh 3.9: (c) B ti sinh s v tuyn

3.5.1. Khi phc nh thi v tch sng ngng

Mch khi phc nh thi hay cn gi l mch khi phc ng h c ch ra nh hnh 3.9 (b), n nhn cc bit s liu vo tc bit ng dy v n lm hot ng mt mch cng hng ni tip hoc a n mt vng kha pha. Mch c thit k sao cho u ra khng c cc xung ng dy. Mch nh thi trong trng hp ny to ra 2 sng vung u ra ngc pha nhau iu khin cc mch ti to thi gian. Khi u vo ca mt b tch sng ngng c tn hiu 1 th n hot ng. Sau 1 c truyn n u ra ca n. V tn hiu u vo ca cc b tch sng ngc pha nhau, v vy ti thi im xc nh th ch c mt b tch sng hot ng. u ra ca b tch sng hot ng sau c a n mt mch flip-flop loi D bao gm mch ti to thi gian. Khi u vo D mc cao 1 v 2 trng thi n nh c cht, 1 ny c truyn n u ra Q. Trng thi ny

66

H.Q.Trung.TVT

c duy tr cho n khi u vo xut hin mt xung xa ng i vo. V cc xung nh thi v cc xung xa l ngc pha nhau, nn cc u ra Q t cc mch ti to thi gian to ra dng tn hiu thi gian c khi phc ca tn hiu vo. S dng bin p ra to nn tn hiu song cc ging nh xut hin trn u vo ca b ti sinh v phi hp tr khng vi ng dy. Hnh 3.9 (c) ch ra s khi ca mt b lp IF v tuyn s 140 Mbit/s, trong chng ta cng c th nhn thy cc mch tch bit cha b ti sinh.

Hnh 3.9: (d) u cui v tuyn

67

H.Q.Trung.TVT

CHNG 4. K THUT GHP KNH S

(Tham kho: K thut tuyn dn s, Viba s T1)

4.1. NGUYN L GHP KNH THEO THI GIAN

Khi c hai tn hiu tng t tr ln c truyn dn trn mt knh thng tin, thng s dng mt trong hai phng php c in lin kt hai tn hiu ring r ny. Phng php th nht, cc tn hiu c x l chim cc khong tn s khc nhau trong di tn nhng u c truyn trong cng mt thi gian. Ni cch khc cc tn hiu truyn i ng thi nhng tn s chuyn i. y chnh l ghp knh theo tn s. Phng php c in th hai, tt c cc tn hiu u c cng tn s nhng chim khong thi gian khc nhau trong di thi gian. Trong trng hp ny mi bng tn, chng hn ting ni u nh nhau nhng truyn trn knh thng tin vo cc thi im khc nhau. thc hin vic ny, mi tn hiu tng t c ly mu ti cc thi im khc nhau v xung ly mu mang thng tin v bin ca mi tn hiu ring c pht ln ng dy. Kt qu l mt dy xung PAM c ghp li, trong mi xung iu bin tun hon bt ngun t mt tn hiu khc nhau. iu ny c th thc hin c, v b rng xung ly mu ca tn hiu ngn hn nhiu so vi thi gian tri qua cho n trc khi tn hiu 1 c ly mu ln na. Xt hnh 4.1 (a), 4 tn hiu tng t t cc ng dy khc nhau i n kha chuyn mach kiu quay trn c tc c nh, kha ny c gi l b gp. ng dy th 5 c in p V ln hn gi tr cc i ca bt c tn hiu no trong 4 tn hiu. Dng sng to nn nh hnh 4.1 (b). Xung ln nht F gi l xung ng b khung v c s dng xc nh mt khung hoc mt tp cc tn hiu trong mt vng quay. Nhm 5 xung trong hnh 4.1 c gi l mt khung. Trong thc t thng mt khung c nhiu xung.

Nu mi tn hiu tng t c bng tn l B (Hz) c xc nh bi b lc thng thp th tn s ly mu thp nht phi bng 2B (Hz). Khi c n tn hiu PAM ghp theo thi gian th tc truyn xung bng 2Bn xung /s. iu ny c ngha l bng tn ca b lc l tng l nB. Trong thc t n rng hn, v khng c kh nng ch to b lc nh vy v rng hn ngn nga nhiu gia cc k hiu hoc nhiu gia cc xung ln cn.

pha thu, cc mu ca knh ring bit c tch ra v c phn phi nh mt b chuyn mch kiu quay trn gi l b phn phi.

Sau cc mu ca mi knh c lc tch v to tn hiu tng t ban u. B chuyn mch my pht v my thu c ng b mt cch cht ch.

68

H.Q.Trung.TVT

ng b l ch tiu u tin ca mi qu trnh ghp knh theo thi gian (TDM). Trong TDM c hai dng ng b l ng b khung v ng b theo mu (hoc bit). ng b xung cn thit xc didnhj mt cch chnh xc im bt u ca mt nhm xung mu v ng b bit cn thit phn bit mt cch chnh xc cc xung mu trong mi khung. Qa trnh ng b cng cho php ng b thit b u cui pht v thu khi cc tn hiu thi gian trong hai thit b u cui c cng tc trung bnh. Trong thit b u cui pht, ng h to ra cc khung thi gian theo yu cu iu khin cc chc nng khc nhau v iu khin tc bit tn hiu s cho mc ch truyn dn trong nhiu trng hp s khng hot ng cng tc nh ng h c to ra trong thit b u cui thu. khc phc s khc nhau ca tn s ng h, thit b u cui thu thng nhn s nh thi t lung bit s thu c.

Hnh 4.1. Nguyn l ghp knh theo thi gian (TDM)

iu ny m bo cho hai thit b u cui lin quan n lung bit ring, hot ng cng tc trung bnh v c th xem nh ng b. Nh vy thit b u cui thu ph thuc vo thit b u cui pht. ng h pht trong thit b u cui ph thuc cng ph thuc vo mch thu to ra s ng b gia pht v thu ca hai thit b u cui tr ln mang lung bit ny. Thit b u cui thu tch thng tin thi

69

H.Q.Trung.TVT

gian t lung bit s a n nh cc mch khi phc ng b, cc mch ny hot ng da vo cng mt nguyn l nh cc mch s dng trong cc b ti to s. Cui cng, trong cc h thng TDM c s, cc xung PAM ghp c th c lng t x l tip trc khi truyn dn bng PCM hoc c th c x l khng qua lng t v PCM truyn trc tip bng c s. Cc xung PAM c ghp nh trong hnh 4.1 (b) c m ha trc khi truyn. u ra b m l t m nh phn, mi t m thay cho mt xung ly mu ; S ny c gi l s ghp theo t hay ghp theo k hiu. Cc t m c gp li vi cc bit bo hiu v cc bit ng b khung. Cc bit ng b khung c sp xp sao cho chng to thnh mt cu trc lp hoc t m ng b khung cho php thit b u cui nhn dng chnh xc mi bit hoc khe thi gian v phn chia lung tn hiu s n thnh cc tn hiu thnh phn. Cc tn hiu thnh phn sau i n knh ra tng ng. Mt on ca tn hiu pht c hnh thnh bi t m ng b khung v cc bit tin gi l mt khung. Nu thit b u cui thu pht hin c t m ng b khung ti v tr bit trc tc l c ng b v n tip tc lm vic ch ng. Nu sau v tr qua nhiu khe thi gian khng pht hin c t m ng b th thit b u cui cho rng ng b khung b mt.

4.2. GHP KNH S CP

4.2.1. c im

Ghp knh c s PCM 30 v PCM 24 l cc h thng knh PCM thc t cho cc lung bit s cp 1 theo tiu chun chu u v Bc M - Nht Bn vi tc c s PCM 30 l 2048 Kil bit/s v PCM 24 l 1544 Kb/s.

70

H.Q.Trung.TVT

71

H.Q.Trung.TVT

4.2.2. Ghp knh s cp theo tiu chun M-Nht

a) a khung 12

b) a khung 24

4.2.3. Ghp knh s cp theo chun Chu u

72

H.Q.Trung.TVT

4.3. GHP KNH BC CAO

4.3.1. Phn cp tc s PDH

a) Tiu chun Chu u

73

H.Q.Trung.TVT

Da trn cc khuyn ngh ca ITU-T th cc cp ghp knh s theo tiu chun Chu u da trn vic ghp cc lung s cp 1 c tc bit 2048 Kb/s bng vic ghp cc knh (khe thi gian 8 bit). Cp ghp knh nh sau:

- Cp 1: L mt khung PCM gm 32 knh (30 knh thoi + 2 knh bo hiu) v ng b c tc 2048 Mb/s.

- Cp 2: Thc hin ghp knh 4 lung cp 1, c tc 8,448 Mb/s.

- Cp 3: Thc hin ghp 4 lung cp 2, c tc 34,368 Mb/s

- Cp 4: Thc hin ghp 4 lung cp 3, c tc 139,264 Mb/s.

- Cp 5: Thc hin ghp 4 lung cp 4, c tc 565, 128 Mb/s.

b) Tiu chun Nht Bn

c). Tiu chun Bc M

- Cp 1: Ghp knh 24 knh thoi thnh lung s S1 c tc 1,544 Mb/s

74

H.Q.Trung.TVT

- Cp 2: Ghp 4 lung cp 1 thnh lung cp 2 l 6,312 Mb/s

- Cp 3: Ghp 5 lung cp 2 thnh lung cp 3 l 32, 064 Mb/s

- Cp 4: Ghp 3 lung cp 3 thnh lung cp 4 l 97,728 Mb/s

- Cp 5: Ghp 4 lung cp 4 thnh lung cp 5 l 400,352 Mb/s

Cp ghp knh ca Bc M v Nht Bn c s thng nht cp ghp 1 v 2, cp ghp Bc M c hng AT & T s dng. Tuy nhin, cp ghp Bc M cn c hai cp na l cp ghp 4 v cp ghp 5, trong , cp 5 c th ghp ti 8064 knh thoi PCM, l cp ghp khng c trong tiu chun ca ITU-T.

Trong phn cp s ca Nht Bn c thm mt cp so vi phn cp ca ITU-T l 5760 knh thoi, tng dung lng l 400,352 Mb/s.

Bng tm tt cc cp ghp knh s PDH

Nhc im ca PDH

Ngy nay, nhu cu s dng cc dch v phi thoi bng ln mt cch ph bin v rng ri, ngha l ngi iu hnh mng mong mun c nhiu lung 2 Mb/s cho

75

H.Q.Trung.TVT

thuy v c th tch/xen cc lung mt cch n gin v d dng. Nhng PDH khng p ng c cc yu cu . Chng hn khi mt tuyn truyn dn c tc 140 Mb/s i qua mt im no c nhu cu s dng cc lung 2 Mb/s th ti phi lp t mt khi lng ln thit b tch/xen.

Tip n phi tch mt lung 34 Mb/s thnh 4 lung 8 Mb/s. Bc tip theo phi tch mt lung 8 Mb/s thnh 4 Mb/s lung 2 Mb/s th phi xen vo by nhiu lung 2 Mb/s. V phi qua nhiu cp tch/xen nh vy k thut tng i phc tp, h thng cng knh, gi thnh dch v tng ln theo s cp ghp/xen, mt khc phng thc trn cn gim tin cy ca h thng. y l mt hn ch th nht PDH.

4.3.2. H thng SDH

Mng in thoi t trc nhng nm 1970 ch yu truyn tn hiu tng t v k thut FDM (Frequency Division Multiplex), phng tin truyn dn ch yu l cp ng. u nhng nm 1970 h thng s tng i pht trin km theo n l k thut ghp knh theo thi gian vi cc cp ghp knh khc nhau, thc hin ghp nhiu lung s c tc thp thnh lung s c tc cao. K thut ny c gi l phn cp s cn ng b PDH (Plesiochronous Digital Hierarchy). Cng ngh cng pht trin yu cu truyn dn tc cao, tin cy ln th PDH cha phi l h thng ti u, mt phn cp s ra i l k thut SDH (Synchronous Digital Hierarchy).

SDH ra i vo nm 1980 khc phc c nhng nhc im ca PDH. thi k u SDH c trin khai trn h thng cp ng trc. Nm 1985, vit tiu chun Quc gia Hoa K (ANSI) a ra mng thng tin quan ng b (SONET Synchronous Optical Network) cho php ha mng thit b ca cc hng khc nhau ti mc quang.

76

H.Q.Trung.TVT

4.3.2.1. Nguyn l ca SDH

Nguyn l ca SDH l ghp xen k tng byte d liu PDH vo cc Container (Ci), sau c gn cc t mo u POH trong container o VC, con tr Pointer, v thng tin mo u SOH to thnh khung d liu STM-1.

4.3.2.2. Cu trc khung thi gian SDH

4.3.2.3. Truyn cc byte trong khung STM-1

77

H.Q.Trung.TVT

4.3.2.4. Cu trc khung STM-N

4.3.2.5. S cu trc h thng ghp knh ng b STM-1 theo ITU-T

78

H.Q.Trung.TVT

Chc nng ca cc khi c bn:

4.3.2.6. Cc vng ca mng truyn dn

79

H.Q.Trung.TVT

- Vng ghp knh trong mng SDH bao gm cc phng tin truyn dn v cc b lp lin kt vi nhau. Vng ny cung cp mt phng tin truyn dn thng tin gia hai nt mng lin tip nhau (v d: cc b ghp knh SDH, cc chuyn mch kt ni cho s DCC). Mt trong nhng nt mng l ni to ra t mo vng ghp knh MSOH v mt trong nhng nt mng cn li l ch cui cng ca t mo ny.

Trong mng SDH, vng ghp knh c ngha quan trng. N l mc m mng SDH c th hnh thnh cc chc nng khc nhau trong cc trng hp khc nhau nh hng thit b hay l gim cht lng ng truyn dn. Cc chc nng ny mang nhim v chung l bo v ng truyn c c cht lng tt nht v khng b gin on.

Cc byte mang chc nng bo v c trong t mo vng ghp knh MSOH c gn vo lung tn hiu v truyn n u cui khc. Nu pht hin li, mng SDH s chuyn mch lin kt cc VC n mch truyn dn d phng. Mch d phng l cc knh d phng bao gm cc phng tin truyn dn, cc b lp v cc thit b u cui vng ghp knh.

Cc kt ni cho:

80

H.Q.Trung.TVT

4.3.2.7. Cu hnh h thng mng truyn dn SDH

- Mng im im

- Mng chuyn tip trc tip

- Chuyn mch c rt v xen knh

81

H.Q.Trung.TVT

- Chuyn mch c rt v xen knh s dng thit b phn lung

- Cu hnh kn (mng vng ring)

- Cu hnh nt mng:

Thit b u cui STM-1.

82

H.Q.Trung.TVT

Thit b u cui STM-4.

Thit b u cui STM-16.

- Cu hnh xen/r

4.3.3. Ghp knh theo bc sng

4.3.3.1. Khi nim

83

H.Q.Trung.TVT

4.3.3.2. Nguyn l c bn ghp bc sng quang

4.3.3.3. Thit b ghp/tch knh quang

84

H.Q.Trung.TVT

CHNG 5. CC H THNG TRUYN DN VIN THNG

5.1. NG DY THU BAO S Xdsl

5.1.1. Khi nim

ng dy thu bao s bt i xng (ADSL Asymmetric digital subcriber) l mt cng ngh modem cho php chuyn i cc ng in thoi cp xon i truyn thng thnh cc ng truyn thng truy nhp tc cao.

85

H.Q.Trung.TVT

ADSL c th cho php truyn vi tc hn 6 Mbps t pha nh cung cp ti khch hng truy nhp cc dch v Video theo yu cu, v truy nhp mng LAN. Theo hng ngc li, t pha khch hng n nh cung cp c th truyn tc hn 640 kbps (hay c hai hng). Kh nng ny lm tng dung lng truy nhp hn rt nhiu so vi kh nng truyn thng tin cua mng cng cng truyn thng.

5.1.2. Modem

5.1.3. B ghp knh truy cp DSL (DSL Access Multiplexer) ghp nhiu khch hng s dng DSL vi mt mng backbone tc cao, c quan

bu in s dng b ghp knh truy cp DSL (DSLAM). Thng thng b DSLAM kt ni ti mng truyn khng ng b (ATM Network) m tng tc c th t ti Gigabit. pha nhn, c mt thit b phn knh tn hiu v s chuyn cho cc kt ni DSL ring r.

86

H.Q.Trung.TVT

5.1.4. Cc bin php iu ch Trong sn phm ADSL, cc m ng truyn CAP, QAM, DMT c s

dng ph bin nht, ngoi ra cn mt s loi m khc ang trong qu trnh th nghim.

CAP: Carrierless Amplitude and Phase modullation

M ho

B lc ng pha

B lc lch pha 900

D/A LPF

A/D

B lc thch ng I

B lc thch ng II

B x l Gii m

-

ng tr

uyn

Thu pht tn hiu theo ph-ng php iu ch CAP

QAM: Quadrature Amplitude Modullation

87

H.Q.Trung.TVT

f1(t)

f2(t)

2E0

0000 0001 0011 0010

1000 1001 1011 1010

1100 1101 1111

0100

1110

0101 0111 0110

Chm tn hiu 16-QAM

DTM: Discrete MultiTone Modullation

S liu tc

cao

S liu tc thp

S liu tc thp

Phn knh

iu ch

iu ch

iu ch

iu ch

iu ch

.

.

.

Kim sot

S

Lc

Lc

Lc

Lc

Lc

Gii iu ch

Gii iu ch

Gii iu ch

Gii iu ch

Gii iu ch