truyen hinh cap(font time new roman)

8/7/2019 truyen hinh cap(font time new roman)

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MC LC

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BNG CH VIT TT

ADSLAGCAPDATMBERCATVCNRCODFBDWDM

DBSDSLDSLAM

EQFDMFTTCFTTB

FTTHFITLGIGVDHDTHFCHFPCHFWHFRHPF

ISDNLPFMDFMMDS

NAONU

Asymetric Digital Subcriber LoopAutomatic Gain ControlAngled Physical ContactAsynchronous Transfer ModeBit Error RateCommunity Antenna TelevisionSystemCarrier-to-Noise RatioCentral OfficeDistributed Feedback laser

Dense Wavelength DivisionMultiplexingDirect Broatcast SatelliteDigital Subscriber LineDSL Access Multiplexing

EqualizerFrequency Division MultiplexingFiber To The Curb

Fiber To The BuildingFiber To The HomeFiber In The LoopGratded IndexGroup Velocity DispersionHost Digital TerminalHybrid Fiber/ Coaxial networkHybrid Fiber Passive/ Coaxial networkHybrid Fiber/ Wireless networkHybrid Fiber/ Radio network

Hight Pass FilterIntergrated Services Digital NetworkLow Pass FilterMain Distribution FrameMultipoint Multichanel DistributionService

Numerical Aperture

Vng Thu bao s khng i xngT iu chnh h s khuch iTip xc gcPhng thc truyn khng ng bT s li bitH thng truyn hnh cp cng ngT s sng mang trn nhiuTng i trung tmLaser hi tip phn tnGhp knh phn chia theo mt

bc sngV tinh qung b trc tipng thu bao sGhp knh truy nhp ng thu basKhi cn bngGhp knh phn chia theo tn sCp quang n khu vcCp quang n to nh

Cp quang n gia nhCp quang trong mng thu baoCh s chit sut GradientTn sc vn tc nhmThit b u cui s trung tmMng lai cp quang/ cp ng trcMng HFC th ngMng lai cp quang/ khng dyMng lai cp quang/ v tuynB lc thng cao

Mng lin kt s a dch vB lc thng thpGi phi dy chnhDch v phn phi a im a knh

Khu sn v mng quang

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OTUPOTSQPSKQAM

RFSDHSISTBSMFSTMVOD

Optical Network UnitOptical Terminal UnitPlain Old Telephone ServiceQuadrature Phase Shift Keying

Quadrature Ampliture ModulationRadio FrequencySynchronous Digital HierarchyStep IndexSet Top - BoxSingle Mode FiberSynchronous Transfer ModeVideo On Demand

n v u cui quangDch v thoi thng thngiu ch kho dch pha cu phngiu ch bin cu phng

Tn s cao tnPhn cp s ng bCh s chit sut phn bcu thu tn hiu sSi quang n modePhng thc truyn ng bTruyn hnh theo yu cu

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LI NI U

Truyn hnh cp (CATV) t lu khng cn xa l i vi ngi dn ccnc pht trin trn th gii. Tuy nhin vic pht trin v m rng cc mngtruyn hnh cp vn cha c quan tm nhiu bi v trc y mng truyn hnhcp ch n thun cung cp cc dch v v truyn hnh, khng th cung cp ccdch v khc nh thoi, s liuThut ng CATV xut hin ln u tin vo nm1948 ti M khi thc hin thnh cng h thng truyn hnh cp hu tuyn v thutng CATV c hiu l h thng truyn hnh cp hu tuyn (Cable TV).

Mt nm sau, cng ti M h thng truyn hnh cng ng s dng anten

(Community Antenna Television-CATV) cung cp dch v cho thu bao bngng truyn v tuyn c lp t thnh cng. T thut ng CATV cdng ch chung cho cc h thng truyn hnh cp v tuyn v hu tuyn.

Nhng nm gn y, do tng nhu cu thng thc cc chng trnh truynhnh cht lng cao, ni dung phong ph cng nh s tin b trong cng ngh,cc mng truyn hnh cp c nhng bc pht trin mnh m. Gi y khngch cung cp cc chng trnh truyn hnh tha mn nhu cu ngy cng cao cangi xem m chng cn tr thnh mt tim lc cnh tranh ng k i vi cc

mng vin thng khc trong cung cp cc dch v vin thng.Ti Vit Nam hin nay c cc dch v truyn hnh nh truyn hnh qung

b, dch v truyn hnh MMDS v dch v truyn hnh cp hu tuyn. Truynhnh qung b s dng mi trng hon ton khng kh truyn tn hiu v ccthu bao ch vic cm anten thu tn hiu t anten pht ca cc i truyn hnhl c th xem chng trnh nn cc thu bao khng cn phi ng cc dch vv cc nh sn xut chng trnh cng khng phi tn km v phng tin truyndn. Tuy nhin v l chng trnh truyn hnh tng t v s dng di tn s

ngoi khng gian nn ti nguyn b hn hp dn n s lng knh pht ra cadch v truyn hnh qung b rt hn ch v n chu nh hng mnh m bi ccngun nhiu ca mi trng truyn dn nh: nhiu cng nghip, nhiu t cc i

pht ln cn v n cng chu nh hng rt ln ca thi tit. Dch v truynhnh qung b khng th tng thm knh chng trnh khc do bng thng b hnch. V ti nguyn tn s khng gian l mt ti nguyn qu gi i vi mi quc

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gia v ngoi vic dnh cho dch v truyn hnh n cn dnh cho nhiu dch vkhc na nh: thng tin lin lc trong qun i, thng tin di ng

Cn dch v truyn hnh MMDS th s dng sng mang ph ca thng tinvi ba (900MHz) truyn ti cc knh truyn hnh v ko cp t trung tm truyn

hnh n trm vi ba, s dng anten pht ca trm vi ba pht sng n cc vngxung quanh trm trong mt phm vi bn knh nht nh, n c chia thnh cccell hnh di qut ph sng. i vi dch v ny th thu bao cng ch cndng ct anten l c th thu c chng trnh truyn hnh v gii m xem.Tuy nhin y l phng thc truyn trong tm nhn thng nn anten thu ca thu

bao bt buc phi nhn thy anten pht ca trm vi ba gn n th mi thu c tnhiu. y l mt nhc im ca dch v v n s hn ch i vi cc vng dnc trong khu vc c nhiu to nh cao tng che chn (nh l cc khch sn) hoccc khu vc dn c c nhiu cy ci che ph. Cc khu vc khng th bt ctn hiu do tn hiu khng th xuyn qua chng ngi vt hoc i cong xung.Cn na n cng tng t nh dch v truyn hnh qung b ch bng thng bhn ch nn knh truyn hnh pht ra cng b hn ch v n cng chu nh hngmnh m ca cc ngun nhiu cng nghip, nhiu ca cc i pht ln cn vchnh n cng gy nhiu cho cc i pht khc, cng chu nh hng mnh cathi tit.

Do cc hn ch ca cc dch v truyn hnh nh trn nn vic pht trintruyn hnh cp hu tuyn HFC l iu tt yu v: Mng HFC s dng cp quang mng truyn dn v phn phi tn hiu nn s dng c cc u im cacp quang so vi cc phng tin truyn dn khc nh: Bng thng ca cp quangrt ln (1014 ~ 1015 Hz), suy hao ng truyn rt nh, khng chu nh hng binhiu ca mi trng ngoi v nhiu in t, c th tch hp c nhiu dch vtrn cng mt ng truyn

Ti H Ni, nhu cu pht trin mng truyn hnh cp hu tuyn qui m,hin i cung cp nhiu chng trnh cho ngi dn Th c lp k hoch

pht trin v ang c trin khai trn din rng.Cng vi s pht trin ny, ti tt nghip Phng php thit k mng

truyn hnh cp hu tuyn trnh by nhng ni dung c bn nht cc cng nghs dng trong mng truyn hnh cp hu tuyn v kin trc mng HFC, hng

pht trin ca mng v so snh cc u nhc im ca mng HFC vi cc dch v

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truyn dn cnh tranh khc. Ni dung bn n gm nm chng c giithiu s lc sau y:

Chng I: Gii thiu tng quan v truyn hnh cp hu tuyn ni chung, vtr ca truyn hnh cp trn th trng thng tin v xu hng pht trin ca n

trong thi gian tip theo sau ny. Ngoi ra cn im qua mt s cng ngh truynhp cnh tranh vi mng truyn hnh cp.

Chng II: Gii thiu v cc mng truyn hnh cp truyn thng v mngtruyn hnh kt hp. Gii thiu v so snh gia cc cu trc mng khc nhau.

Chng III: Gii thiu v mt s thit b quan trng s dng trong victhit k v lp t mng quang (mng truyn dn v mng phn phi tn hiutruyn hnh). Nu nguyn tc lm vic ca mt trm trung tm truyn hnh cp c

bn, cu to ca thit b trung tm.Chng IV: Gii thiu v cc thit b chnh dng trong mng cp ng

trc (mng truy nhp tn hiu).Chng V: Nu nguyn tc thit k mng truyn hnh cp hu tuyn v

thit k mt mng truyn hnh cp c th trn a bn thnh ph H Ni. Ngoi rac thit k thm th d v kin trc mng HFPC so snh vi kin trc mngHFC v a ra kt lun v vic la chn kin trc mng no th ph hp cho tnhhnh nc ta hin nay.Trong chng ny c tnh ton chi tit tn hiu t trung tmn tn thit b nh thu bao. Tuy nhin ch chn la th im mt s vng nhtnh.

Trong qu trnh lm n do thi gian hn hp nn khng trnh khinhng s sut v mt s ni dung cha c chi tit, mong cc thy c gio gp

v thng cm.Em xin chn thnh cm n thy gio cng nh cc anh ch trong phngthit k ca cng ty truyn hnh cp H Ni tn tnh gip trong qutrnh lm n.

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CHNG I -TNG QUAN V TRUYN HNH CP HU TUYN

1.1 Tng quan v truyn hnh cp

Mng truyn hnh cp bao gm 3 thnh phn chnh: H thng thit b ti trungtm, h thng mng phn phi tn hiu v thit b thu bao. H thng thit b trung tm

H thng trung tm (Headend System) l ni cung cp, qun l chng trnhh thng mng truyn hnh cp. y cng chnh l ni thu thp cc thng tin quanst trng thi, kim tra hot ng mng v cung cp cc tn hiu iu khin.

Vi cc h thng mng hin i c kh nng cung cp cc dch v truyntng tc, truyn s liu, h thng thit b trung tm cn c thm cc nhim v

nh: m ho tn hiu qun l truy nhp, tnh cc truy nhp, giao tip vi ccmng vin thng nh mng Internet... Mng phn phi tn hiu truyn hnh cp:

Mng phn phi tn hiu truyn hnh cp l mi trng truyn dn tn hiu ttrung tm mng n cc thu bao. Tu theo c trng ca mi h thng truynhnh cp, mi trng truyn dn tn hiu s thay i: vi h thng truyn hnh cpnh MMDS mi trng truyn dn tn hiu s l sng v tuyn. Ngc li, ivi h thng truyn hnh cp hu tuyn (Cable TV) mi trng truyn dn s lcc h thng cp hu tuyn (cp quang, cp ng trc, cp ng xon ...). Mng

phn phi tn hiu truyn hnh cp hu tuyn c nhim v nhn tn hiu pht ra tcc thit b trung tm, iu ch, khuch i v truyn vo mng cp. Cc thit bkhc trong mng c nhim v khuch i, cp ngun v phn phi tn hiu hnhn tn thit b ca thu bao. H thng mng phn phi tn hiu truyn hnh cp l

b phn quyt nh n i tng dch v, khong cch phc v, s lng thubao v kh nng m rng cung cp mng.

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HEADEND

HUB SCP

HUB THCP

NODEQUANG

TAP

Mngtruyn dn

Mng phnphi

Mng truynhp

NODEQUANG

Hnh 1.2 Cu hnh mng truyn dn v phn phi tn hiu


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Thit b ti nh thu baoVi mt mng truyn hnh cp s dng cng ngh tng t, thit b ti thubao c th ch l mt my thu hnh, thu tn hiu t mng phn phi tn hiu. Vimng truyn hnh cp s dng cng ngh hin i hn, thit b thu bao gm cc

b chia tn hiu, cc u thu tn hiu truyn hnh (Set-top-box) v cc cp dn...Cc thit b ny c nhim v thu tn hiu v a n TV thu bao s dng ccdch v ca mng: Chng trnh TV, truy nhp Internet, truyn d liu...

1.2 - V tr cc mng truyn hnh cp v xu hng pht trin

Cc mng CATV tri qua cc giai on pht trin t mng tng tqung b mt chiu ng trc ti mng HFC tng tc 2 chiu truyn ti cc knhVideo tng t/ s v d liu tc cao. Mng ng trc bng rng kin trc cyv nhnh truyn thng c h tr bi cng ngh RF phc v tt cc dch vqung b v cc dch v im-a im. Dng nhiu b khuch i (30 40), cth lm gim cht lng v tnh nng ca knh Video AM-VSB, lm gim thhiu ca khch hng. Vic s dng cc kt ni vi ba mt t gim s lng cc

b khuch i, ci thin c hiu nng truyn dn cc knh qung b tng t.S tin b vt bc trong cng ngh si quang t cui nhng nm 80

khin cho cng nghip truyn hnh cp pht trin mnh m. S ra i ca laseriu ch trc tip DM-DFB 550 MHz v cc b thu quang hot ng di bcsng 1310 nm lm thay i kin trc truyn thng mng cp ng trc. MngHFC cho php truyn dn tin cy cc knh Video tng t qung b qua si nmode SMF ti cc node quang, do s lng cc b khuch i RF c

gim i rt nhiu. Hn na cc nh iu hnh cn thc hin trin khai thit bheadend s dng cc Ring si quang kt ni gia headend trung tm v ccheadend th cp hoc cc Hub ti nhng v tr quan trng. Do vy, cc nh iuhnh cp c th h gi thnh v ci thin hn na cht lng v tnh hu dng cacc dch v qung b truyn thng.

S pht trin ca nhiu thit b quan trng nh: Cc b iu ch QAM, ccb thu QAM gi thnh h, cc b m ha v gii m tn hiu Video s, cho php

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cc nh iu hnh cp cung cp thm khong 10 dch v Video s mi trong ccknh Video AM/VSB dng vi STB s. Vic trin khai nhanh chng mng HFC750 MHz v mt s dch v vin thng cung cp kh nng cnh tranh truy nhpv nhiu loi hnh kinh doanh cho khch hng ti cc th trng quan trng.

Vo gia thp k 1990, kin trc mng HFC bt u c hng pht trinmi. Cuc cch mng ny l do nhng p lc sau ca th trng:- Bng n nhu cu truy nhp d liu tc cao trong cc khu vc dn c.- Nhu cu chuyn pht cc dch v s tng tc.- Gia tng cnh tranh t nhiu nh cung cp dch v vin thng v cc nh cung

cp dch v DBS (Direct Broadcast Satellite).- S tin b trong cng ngh si quang, c bit l laser v b thu quang v

qun l mng cp.Nhng nhu cu v p lc ca th trng tc ng ti cc nh iu hnh cp

xem li kin trc mng HFC hin ti v tin ti mng truy nhp CATV DWDM

(Dense Wavelength Division Multiplexing).

1.3 - Cc cng ngh truy nhp cnh tranh

C nhiu cng ngh truy nhp c th phc v cc dch v bng rng tithu bao. Phn ny s cung cp tng quan mt s cng ngh cnh tranh cngnhng u nhc im tng loi.

1.3.1 - Cng ngh ADSL (Asymetric Digital Subscriber Line)

Cng ngh ADSL s dng ng dy thoi xon i hin c cung cpbng thng yu cu cho cc dch v bng rng nh truy nhp Internet, thoi hingh, a phng tin tng tc v VOD. Cng ngh ADSL c thit k giiquyt tnh trng tc nghn nghim trng hin nay trong cc mng thoi gia tngi trung tm (CO: Central Office) v thu bao. ADSL c th chuyn pht tc d liu trong khong t 64 kb/s n 8,192 Mb/s cho knh ng xung v tc trong khong 16 kb/s ti 768 kb/s cho cc knh ng ln trong khi vn ng thidng cc dch v thoi truyn thng POTS (Plain Old Telephone Service).

ADSL rt ph hp p ng nhu cu truy nhp Internet tc cao.

ng truyn dn ADSL cung cp tc d liu ti 8 Mbit/s xung khch hngv 640 Kbit/s lung ln m rng dung lng truy nhp m khng cn lp t thmcp mi. Ngoi ra, vic s dng ADSL s chuyn lu lng dch v Internet quacc mng chuyn mch gi hoc ATM gip hot ng hiu qu hn, gii quytc vn tc nghn trn mng thoi.

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Cu trc mng c bn :

Mch vng thu bao l mt i dy ng xon i ni cm thu bao vtng i trung tm. i vi ADSL full-rate (cung cp tc 6 8 Mbit/s lungxung), b Splitter c lp t ti c hai u cui mch vng. Pha khch hngmodem ADSL m dy ADSL kt ni ti gi l khi kt cui ADSL u xa (ATU-R). pha tng i, cc b Splitter c lp t ni cc mch vng thu bao ktcui trn gi phi dy chnh MDF (Main Distribution Frame), u ra c hai idy. i th nht kt ni ti mng chuyn mch thoi cung cp dch v thoitruyn thng. i dy th hai kt ni ti khi kt cui ADSL trung tm (ATU-C).

truyn dn hiu qu, cc khi ATU-C c kt hp vi chc nng ghpknh to nn b ghp knh truy nhp DSL (DSLAM: Digital Subscriber LineAccess Multiplexer) trong tng i trung tm v c kt ni ti mng cc nhcung cp dch v. S liu qua ADSL c ng gi trong cc t bo ATM.DSLAM cn c kh nng x l cc t bo ATM thc hin ghp knh lu

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Internet

Chuyn mch ATM

SDLAM

ATU-C

SPLITT

ERMDF

PSTN

Chuyn mch CO

POTS/ISDN

SPLITTER

ATU-

R

PC

MchvngTB

Cc ngkhng phi

xDSL

POTS/ISDN

Hnh 1.4 Cu hnh h thng s dng cng ngh ADSL


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lng thng k. Tng tc cc ng ADSL qua tt c cc khi ATU-C c thln hn tc ng STM-1. ng dng ca ADSL

c im truyn tc hai chiu khng i xng ca ADSL lm cho k thut

ny ph hp vi hu ht cc ng dng yu cu bng thng lung xung ln hnbng thng lung ln. VoD l hng pht trin ban u ca ADSL nhng sau truy nhp Internet tc cao nhanh chng tr thnh hng pht trin ch yu.

Ngoi ra cn mt s ng dng khc ang c pht trin v s dng cng nghny nh sau:- Telecommuting: Dch v thoi v truy nhp d liu t xa, cho php ngi s

dng lm vic ti nh v kt ni ti c s d liu ti ni lm vic.- Dch v truyn video hoc thng tin thi gian thc: ADSL cho php phn phi

nhng ng dng bng rng theo thi gian thc nh tin tc, token chng khon,thi tit ...

- Chng trnh o to t xa: ADSL full-rate vi cht lng dch v m bo cth cung cp lung video theo tiu chun MPEG-2 cho php cc trung tmging dy gi video minh ho bi dy v trao i trc tip vi hc vin tnhiu v tr.

- Cha bnh t xa: Cc bc s c th chn on v khm cha bnh t xa.- Hi ngh truyn hnh: Mc d dch v ny yu cu bng thng hai chiu i

xng nhng ADSL full-rate c th cung cp mt knh H0 (384 384Kbit/s)chuyn dng ngoi bng thng c sn ca ADSL cho ng dng ny trong khivn m bo phc v cc ng dng khc.

1.3.2- Fiber-In-The-Loop(FITL)

Cng ngh truy nhp FITL thng dng cp quang theo kin trc hnh sao(im-a im), gm mt h cc kin trc nh:- Cp quang ti tn node FTTN- Cp quang ti tn h dn c FTTC- Cp quang ti tn h thu bao FTTH

Cc h thng FITL c pht trin theo hng tng thch vi cc dch v,h thng truyn dn, h thng iu hnh ca cc nh khai thc ni vng (LEC).

Kin trc nguyn thy FITL c ch ra trong hnh 1.5.Mt mng FITL gm mt kt cui host s (HDT: Host Digital Terminal)

vi cc khi ONU trong kin trc hnh sao, c HDT qun l. HDT cung cpcc hot ng v giao din cn thit ca h thng FITL cho phn cn li ca mngLEC.

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V d, HDT c th tch ring lu lng chuyn mch ni ht v ra bnngoi qun l nh tuyn. Cc dch v bng rng nh Internet, tng tc a

phng tin v thoi c pht ti HDT, HDT c th c t ti CO hoc tiu xa, nh tn hiu bng gc. iu ny tri ngc vi cc mng HFC, trong

cc dch v bng rng c iu ch RF. Ti HDT, tn hiu s bng gc cchuyn mch v gi ti cc khi mng bng rng qua cp quang. ONU c tgn thu bao v phc v nhiu khch hng. ONU thc hin chuyn i quang inv cc chc nng quan trng khc. Tn hiu in sau c pht ti thu baoqua cp ng trc hoc cp ng xon i. Mt khi giao din mng t ti phathu bao s tch tn hiu Video, tn hiu d liu, v tn hiu thoi nh ch ra tronghnh 1.5.Tn hiu Video s c tch knh v gii m bi mt STB.

Mt kin trc FITL khc l FTTH. Nh tn gi ca n, cp quang s thayth cp ng trc hoc cp ng xon i t ONU n thu bao. S khc nhaugia kin trc FTTH v FTTC nm v tr lp t ca ONU.

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Videoserver

SPL

ITT

ER

Video

servertng tc

CngInternet

H thng truynhp chuyn

mch s

ONU

Cpn

g

trc

Mng

ATM

Mng

PSTN

Hnh 1.5 Cu hnh h thng Fiber in the loop


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Trong kin trc FTTH, ONU c t ti nh thu bao. V vy, nhu cu dngcng ngh cp xon i ngoi nh thu bao c loi b trong kin trc FTTH.Phn mng gia HDT v cc ONU tr thnh mng quang th ng, iu ny trnn rt quan trng khi nng cp trong tng lai. Bng thng rng sn c (hng

THz) ca si quang chuyn pht cc dch v bng rng cho thu bao l mttrong nhng u im ca kin trc mng ton quang ny. Vi vic dng cngngh WDM, cc h thng FTTH c kh nng truyn ti tc hng Gb/s v dOC-48/STM-16 (2.5GHz) hoc cc dch v bng rng tng thch viSONET/SDH ti thu bao. Hn na, v mi ONU c t ti nh thu bao, dovy khng cn thit cng sut ngoi hoc bo dng thm.

Nhc im chnh ca FTTH l gi thnh mi OTU tng i cao v chiph lp t cp quang ban u. Bt chp nhc im ny, kin trc FTTH cnhiu tim nng ha hn cung cp cc dch v bng rng cho thu bao.

1.3.3 - V tinh qung b trc tip DBS (Direct Broadcast Satellite)

Cng ngh DBS da trn cc v tinh ng b a tnh cung cp cc chngtrnh truyn hnh a knh cho cc thu bao c trang b cc b thu DBS. Tuy nhin Vit Nam hin nay cha c v tinh ring v cng cha c cc knh truyn hnhtr tin qua v tinh ring. Cc chng trnh truyn hnh qua v tinh c cung cp

bi cc nh cung cp dch v nc ngoi v thng qua cc v tinh nc ngoi.Ngi dn mun ng k dch v truyn hnh qua v tinh cn phi c s chophp ca B Vn Ho Thng Tin. V vy, truyn hnh tr tin qua v tinh Vit

Nam gp nhiu hn ch nh: khng c knh truyn hnh v ngn ng ting vitng k dch v phc tp, chi ph thu bao cao. Chnh v nhng l do ny m slng ngi xem truyn hnh qua v tinh Vit Nam rt t.

1.3.4 - Dch v phn phi a im a knh (MMDS)(MMDS: Multipoint Multichanel Distribution Service)

Cng ngh truy nhp MMDS l mt cng ngh khng dy (wireless) khcc da trn cc knh Video tng t v s qung b mt t. Kin trc c bnMMDS gm cc khi pht v tuyn MMDS t ti cc thp radio cng vi anten,

mt anten ca thu bao, mt b h tn v mt STB. Mi vng phc v c chiathnh cc cell c phn giao nhau, mi cell c bn knh 40 km. i vi truyn dnyu cu mc tin cy cao, tm nhn gia anten pht v thu c yu cu bnhthng. V tm nhn lun khng thun li nn nhn t nh hng ti cht lngch yu trong h thng MMDS l tn hiu fading nhiu ng. S dng cngngh MMDS c nhng thun li v kh khn sau:

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1 Thun liTrin khai mng n gin, chi ph thp: Do mi trng truyn dn tn hiu

MMDS l sng vi ba (sng v tuyn) cho nn khi trin khai mng n thu baokhng cn phi ko cp ti tn h thu bao, m ch cn dng ct anten thu ti thu

bao sao cho c th nhn thy ct anten pht (ti ct anten ca i THVN) l c ththu c tn hiu v gii m xem. c im ny s gip nh cung cp dch vMMDS khng mt thi gian, cng sc v chi ph o ng ri cp, m bo mquan th.

2 Kh khn- Hn ch vng ph sng: Do s dng sng viba ti di tn 900MHz truyn

tn hiu Video, MMDS i hi anten pht v anten thu phi nhn thy nhau thmi thu c tn hiu tt. V vy i vi cc h dn c nm pha sau cc khukhch sn cao tng, cc khu cao c, vic thu tn hiu MMDS rt kh thc hin.

iu ny cng xy ra i vi cc gia nh trong cc ngi nh thp, b cc vtcn nh cy ci che chn. t chnh l tr ngi ln cho vic cung cp dch vMMDS n vi mi ngi dn s dng dch v.

- Chu tc ng mnh bi nhiu cng nghip: Do s dng phng thc iu chtn hiu truyn hnh tng t (analog) khng c kh nng chng li, li truyn

bng sng v tuyn, tn hiu MMDS b nh hng rt mnh bi cc ngunnhiu cng nghip: nhiu t mng in li, nhiu t cc thit b in: mtin, qut in m khng c cch hn ch v khc phc. iu ny dn ngim cht lng hnh nh ca dch v MMDS.

- Chu nh hng ln bi thi tit: khi thi tit xu, v d nh ma to, st tnhiu MMDS v tuyn b suy hao rt ln trong khng gian, dn n gim mnhcht lng tn hiu hnh nh.

- Yu cu ph tn s v tuyn qu ln: Mun c kh nng cung cp nhiuchng trnh truyn hnh tng t, MMDS i hi phi c di tn ln . Vd: c th cung cp 13 knh truyn hnh tng t, MMDS i hi phi cdi tn ti thiu l 13knh x 8MHz/knh = 104MHz. y l mt di tn vtuyn rt ln, v khi cng tng s lng chng trnh th yu cu rng bngtn cng tng theo. Trong khi ph tn v tuyn l ngun ti nguyn qu gii vi mi mt quc gia. Khng ch c dch v truyn hnh MMDS, truyn

hnh qung b mt t s dng ngun ti nguyn qu gi ny, m cn rt nhiucc dch v vin thng khc cng nh cc trm pht v tuyn thuc nghip van ninh quc phng tham gia s dng ngun ti nguyn qu gi ny. V thvic chim dng ph tn v tuyn qu ln ca MMDS l khng hiu qu.

- Gy can nhiu cc i pht v tuyn khc: Mc d c phn mt di tnring, nhng my pht MMDS cng nh cc my pht v tuyn khc lun sinh

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15/88

ra cc tn s hi bc cao c th nh hng n cc trm pht v tuyn nghipv khc.

- Kh khn trong vic cung cp dch v truyn hnh s: Hin nay vic khng sdng MMDS cung cp dch v truyn hnh l xu hng thc t trn th gii

(do cc nhc im trn) . Chnh v th vic cc thit b pht tn hiu truynhnh s MMDS khng c pht trin, thm na, cc thit b gii m phathu bao cng khng c cc nh sn xut thit b nghin cu v sn xut. Do vic ng dng truyn hnh s nng cao cht lng hnh nh, dch v caMMDS s khng c tnh kh thi.

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16/88

Mt cng ngh gn vi MMDS l cng ngh lai ghp gia cp quang vkhng dy (HFW) hay cn gi l lai ghp gia quang v v tuyn (HFR). Kintrc ny tng t nh HFC mt headend trung tm pht cc dch v bngrng ti nhiu cell RF qua cp n mode SMF, ti thu bao c thc hin qua 2chiu MMDS.

C nhiu u im trong kin trc ny:- Tng tin cy truyn dn 2 chiu gia thu bao v headend so vi kin trc

MMDS truyn thng.- Gim lp t vng RF v chi ph bo dng.

- Kin trc ny thng ph hp trin khai trn din rng trong cc khu vcthnh th ti mng cp quang c xy dng.

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Khi pht vtuyn MMDS

Mitrngtruyn

dn

STB

Trungtm

Khi h tn

Hnh 1.6 Cu hnh mng dch v phn phi a knh a im MMDS


17/88

CHNG II KIN TRC MNG TRUYN HNH CP

2.1 - Kin trc mng CATV truyn thngHnh 2.1 l s n gin ca mt mng cp ton ng trc. Cc chng

trnh thu c t v tinh hoc viba ti headend, headend thc hin nhim v sau:

- Thu cc chng trnh (v d t NBC, CBS, v cc mng cp nh MTV&ESPN)

- Chuyn i tng knh ti knh tn s RF mong mun, ngu nhin ha ccknh khi c yu cu.

- Kt hp tt c cc tn s vo mt knh n tng t bng rng (ghp FDM).- Pht qung b knh tng t tng hp ny xung cho cc thu bao .

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Hnh 2.1Kin trc n gin mng CATV truyn thng

Headend

Cp fid

Cpthu bao

Cptrung k

Thubao

Thubao

Thubao

Ch thch

PadB khuch

iSpliter

Tap


18/88

H thng mng truyn dn bao gm:- Cp chnh trung k (Trunk cable).- Fid cp: Cp r ra t cc cp trung k- Cp thu bao (Drop cable): Phn cp kt ni t cp nhnh fid n thu bao h

gia nh.Lu lng Video tng ng xung pht t headend v c a ti cccp trung k. cung cp cho ton mt vng, cc b chia tn hiu (spliter) s chialu lng ti cc cp nhnh fid t cp trung k. Tn hiu a n thu bao ctrch ra t cc cp nhnh (fid cp) nh b trch tn hiu Tap.

Mc tn hiu suy hao t l vi bnh phng tn s trung tm khi truyn quacp trc (cp trung k, cp fid v cp thu bao). Do vy tn hiu tn s cngcao suy hao cng nhanh so vi tn s thp. l l do ti sao cc nh cung cpmong mun t knh. Mc tn hiu cng b suy gim khi i qua cc b Spliter vTap .

Trn ng i ca tn hiu, cc b khuch i tn hiu c t cckhong cch ph hp khi phc tn hiu b suy hao. Cc b khuch i ccp ngun nh cc b cp ngun t ri rc trn ng i ca cp, cc b ngunny c nui t mng in s ti. Cc b khuch i xa ngun c cp nguncng chnh bng cp ng trc: dng in mt chiu c cng chung vi tn hiunh b cng. n cc b khuch i, dng mt chiu s c tch ring cpngun cho b khuch i.

V cc knh tn s cao tn hiu suy hao nhanh hn nht l trn khong cchtruyn dn di, cc knh tn s cao cn c mc khuch i cao hn so vi cc

knh tn s thp. Do cn phi cn bng cng sut trong di tn pht ti nhngim cui gim mo. ph cho mt vng, mt b khuch i c th t mc cao, kt qu l c mc tn hiu v mo u ln. Do vy ti nh thu bao gnheadend cn mt thit b th ng lm suy gim bt mc tn hiu gi l Pad.

Cc h thng cp ng trc ci thin ng k cht lng tn hiu thu caTV. Mc d nhiu vng tn hiu truyn hnh v tuyn qung b thu c c chtlng kh tt nhng CATV vn c la chn ph bin v kh nng la chnnhiu chng trnh. Tuy nhin mng ton cp ng trc c mt s nhc imsau: Mc d t c mt s thnh cng v cung cp dch v truyn hnh, cc h

thng thun ty cp trc khng th tha mn cc dch v bng rng tc cao. Dung lng knh ca h thng khng p ng cho pht v tinh qung b

trc tip DBS. H thng cp ng trc c th cung cp hn 40 knh nhng ccthu bao DBS c th thu c gp 2 ln s knh trn, cho h la chnchng trnh. Cc mng cp yu cu cn thm dung lng knh tng cnhtranh.

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Truyn dn tn hiu bng cp ng trc c suy hao rt ln, nn cn phi tnhiu b khuch i tn hiu trn ng truyn. Do vy phi c cc chi phkhc km theo: ngun cp cho b khuch i, cng sut tiu th ca mng tngln dn n chi ph cho mng ln.

Cc h thng cp ng trc thiu tin cy. Nu mt b khuch i gnheadend khng hot ng (v d nh mt ngun nui), tt c cc thu bao dob khuch i cung cp s mt cc dch v. Mc tn hiu (cht lng tn hiu) s khng p ng cho s lng ln cc

thu bao. Do s dng cc b khuch i b suy hao cp, nhiu ng truyntc ng vo tn hiu v nhiu ni b ca b khuch i c loi b khng htv tch t trn ng truyn, nn cng xa trung tm, cht lng tn hiu cnggim, dn n hn ch bn knh phc v ca mng.

Cc h thng cp ng trc rt phc tp khi thit k v vn hnh hot ng.Vic gi cho cng sut cn bng cho tt c cc thu bao l vn rt kh.

gii quyt cc nhc im trn, cc nh cung cp cng i ti tng s dngcp quang thay cho cp trung k ng trc. Ton h thng s c c cp quang vcp ng trc gi l mng lai gia cp quang v ng trc (mng lai HFC). Yucu i vi h thng quang tng t l duy tr s tng thch vi cc thit b cpkim loi hin c.

2.2 - Kin trc mng ccu trc2.2.1 - Cc c im c bn mng HFC

Khi nim:Mng HFC (Hybrid Fiber/Coaxial network) l mng lai gia cp quang vcp ng trc, s dng ng thi cp quang v cp ng trc truyn v phn

phi tn hiu. Vic truyn tn hiu t trung tm n cc node quang l cp quang,cn t cc node quang n thu bao l cp ng trc.

Mng HFC bao gm 3 mng con (segment) gm:- Mng truyn dn (Transport segment)- Mng phn phi (Distribution segment)- Mng truy nhp (Acess segment)

Mng truyn dn bao gm h thng cp quang v cc Hub s cp, nhim v

ca n l truyn dn tn hiu t headend n cc khu vc xa. Cc Hub s cp cchc nng thu/pht quang t/n cc node quang v chuyn tip tn hiu quang ticc Hub khc.

Mng phn phi tn hiu bao gm h thng cp quang, cc Hub th cp vcc node quang. Tn hiu quang t cc Hub s c chuyn thnh tn hiu in ticc node quang truyn n thu bao. Ngc li trong trng hp mng 2 chiu,

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tn hiu in t mng truy nhp s c thu ti node quang v chuyn thnh tnhiu quang truyn n Hub v headend.

Mng truy nhp bao gm h thng cp ng trc, cc thit b thu pht caotn c nhim v truyn ti cc tn hiu cao tn RF gia node quang v cc thit b

thu bao. Thng thng bn knh phc v ca mng con truy nhp ti a khong300m.

Hot ng ca mng:Tn hiu Video tng t cng nh s t cc ngun khc nhau nh: Cc b

pht p v tinh, ngun qung b mt t, Video sever c a ti headendtrung tm. Ti y tn hiu c ghp knh v truyn i qua Ring si n mode(SMF). Tn hiu c truyn t headend trung tm ti thng thng l 4 hoc 5Hub s cp. Mi Hub s cp cung cp tn hiu cho khong hn 150.000 thu bao.C khong 4 hoc 5 hub th cp v headend ni ht, mi hub s cp ch cung cpcho khong 25000 thu bao. Hub th cp c s dng phn phi ph thmcc tn hiu video tng t hoc s ghp knh vi mc ch gim vic phtcng knh video ti cc headend s cp v th cp khc nhau. Cc knh s vtng t ca headend trung tm c th cng c chia x s dng trn mng

backbone. Mng backbone c xy dng theo kin trc Ring s dng cng nghSONET/SDH hoc mt s cng ngh c quyn.Cc c im ca SONET/SDH c nh ngha cp tc s liu chun t tc OC-1 (51,84 Mb/s)/STM-1 (155,52 Mb/s) ti cc tc gp nguyn ln tc ny.

Trong mng SONET/SDH, tn hiu Video tng t c s ho, iu ch,ghp knh TDM v c truyn cc tc khc nhau t OC-12/STM-4 (622

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Mng truyn dn(backbone)

Mng phn phi Mng truy nhp

Hnh 2.2 Kin trc mng HFC


21/88

Mb/s) ti OC-48/STM-16 (2448 Mb/s). y s dng k thut ghp knh thngk TDM tng rng bng tn s dng. Ghp knh thng k TDM thc hincp pht ng cc khe thi gian theo yu cu thc hin cc dch v c tc

bt thay i qua mng SONET/SDH. gim chi ph lp t, phn ln cc nh

iu hnh CATV la chn s dng thit b tng thch vi chun SONET/SDH,tu theo cc giao din mng. Dung lng node quang c xc nh bi s lngthu bao m n cung cp tn hiu. Node quang c th l node c nh vi khong100 thu bao hoc c ln vi khong 2000 thu bao.

2.2.2- u v nhc im ca mng HFC

- S dng cp quang truyn tn hiu, mng HFC s s dng cc u im vttri ca cp quang so vi cc phng tin truyn dn khc: Di thng cc ln,suy hao tn hiu rt thp, t b nhiu in t, chng lo ha v n mn ha hc

tt. Vi cc si quang c sn xut vi cng ngh hin i ngy nay, cc siquang cho php truyn cc tn hiu c tn s ln ti hng trm THz (10 14 1015 Hz). y l di thng tn hiu v cng ln, c th p ng mi yu cu dithng ng truyn m khng mt phng tin truyn dn no khc c th cc.

- Tn hiu quang truyn trn si quang hin nay ch yu nm trong 2 ca sbc sng quang l 1310 nm v 1550 nm. y l 2 ca s c suy hao tn hiurt nh: 0,3 dB/km vi bc sng 1310 nm v 0,2 nm vi bc sng 1550 nm.Trong khi vi mt si cp ng trc loi suy hao thp nht cng phi mt

43 dB/km ti tn s 1 GHz.- Tn hiu truyn trn si cp l tn hiu quang, v vy khng b nh hng bicc nhiu in t t mi trng dn n m bo c cht lng tn hiu trnng truyn. c ch to t cc cht trung tnh l Plastic v thy tinh, ccsi quang l cc vt liu khng b n mn ha hc dn n tui th ca si cao.

- C kh nng d phng trong trng hp si quang b t.Trc y cc mng con truy nhp thng s dng cc thit b tch cc l

cc b khuch i tn hiu nhm b suy hao cp truyn tn hiu i xa. Theokinh nghim ca cc nh iu hnh mng cp ca chu u v chu M, trc trcca mng truyn hnh cp phn ln xy ra do cc b khuch i v cc thit b

ghp ngun cho chng. Cc thit b ny nm ri rc trn mng, v th vic nh v,sa cha thng thng khng th thc hin nhanh c nn nh hng n chtlng phc v khch hng ca mng. Vi cc mng truy nhp ng trc, khi cungcp dch v 2 chiu, cc b khuch i cn tch hp phn t khuch i tn hiucho cc tn hiu ngc dng d n n nh ca mng gim. Hin nay xuhng trn th gii ang chuyn dn sang s dng mng truy nhp th ng, ti

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22/88

khng s dng bt c mt thit b tch cc no na, m ch cn cc b chia tnhiu, cc b ghp nh hng v cc b trch tn hiu th ng. Mt mng HFCch s dng cc thit b cao tn th ng c gi l mng HFC th ng HFPC(Hybrid Fiber/Passive Coaxial) nh th hin trong hnh 2.3. S dng mng truy

nhp th ng hon ton s to ra cc u im sau:- Cht lng tn hiu c nng cao do khng s dng cc b khuch i tnhiu m hon ton ch dng cc thit b th ng nn tn hiu ti thu bao skhng b nh hng ca nhiu tch t do cc b khuch i.

- S c ca mng s gim rt nhiu dn n tng n nh v cht lng phcv mng v trc trc ca mng truyn hnh cp phn ln xy ra do cc bkhuch i v thit b ghp ngun cho chng.

- Cc thit b th ng u c kh nng truyn tn hiu theo 2 chiu v th nnh ca mng vn cao khi cung cp dch v 2 chiu.

- S dng hon ton cc thit b th ng s gim chi ph rt ln cho vic cp

ngun bo dng, thay th v sa cha cc thit b tch cc dn n gim chiph iu hnh mng.

- Nu s dng mng ng trc th ng, s lng thu bao ti mt node quangs gim i, dn n dung lng ng truyn cho tn hiu hng ln s tngln, to ra kh nng cung cp tt cc dch v 2 chiu tc cao cho thu bao.Tuy nhin, mng truy nhp cp ng trc th ng HFPC cng c mt s

nhc im sau:- Do khng s dng cc b khuch i tn hiu cao tn, tn hiu suy hao trn cp

s khng c b dn n hn ch ln bn knh phc v ca mng.

- Do khng ko cp ng trc i xa, s lng thu bao c th phc v bi mtnode quang c th gim i. c th phc v s lng thu bao ln nh khi sdng cc b khuch i tn hiu, cn ko cp quang n gn thu bao hn vtng s node quang dn n tng chi ph rt ln cho mng.

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2.2.3 - Kt lun

Nh trnh by trn, u im ca mng ny l nhc im ca mngkia. Tu thuc vo m hnh kinh t, iu kin a l p dng loi mng no cho

ph hp. Nu xt trong cng mt phm vi phc v, mng HFPC yu cu s lngnode quang ln hn mng HFC. V vy:- Trong iu kin mng quang c sn, nn chn phng n xy dng mng

HPFC nhm mc ch gim chi ph u t cho mng ng trc, y nhanh tc trin khai mng, nng cao cht lng tn hiu v hiu qu khai thc.

- Trong iu kin mng quang cn hn hp, nn chn phng n xy dng mngHFC. Khi , y nhanh tc m rng mng phi vn di mng ngtrc bng cch s dng cc b khuch i cao tn.

i vi tnh hnh nc ta hin nay th cu trc mng HFC hp l hn v Vit Nam mng truyn hnh cp vn ang cn mi m, mng mi c a vos dng trong khong thi gian ngn nn c s h tng cn thiu thn. H thngmng hu nh phi ko mi nn gim chi ph lp t cho c nh khai thc lncc thu bao th mng HFC l hp l nht nn trong phm vi n ny em ch xtcu trc mng HFC v phng php thit k mng ny.

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Hnh 2.3 Cu trc mng HFPC

Mng truyn dn(Backbond)

Mng phn phi Mng truy nhp

B chia

B chia


24/88

CHNG III CC THIT B CHNH TRONG MNG QUANG

3.1 Cu to v nguyn l hot ng ca Headend3.1.1 - S khi c bn ca Headend

1/ Khi RF/IF l khi chuyn i t tn hiu cao tn (RF) ca truyn hnh qungb ln tn hiu trung tn (IF) ca h thng truyn hnh cp (hay cn gi l bupconverter).2/ Khi thu tn hiu v tinh l khi c chc nng chuyn i t tn hiu v tinh (lhai tn hiu audio v video tch bit) c tn s cao xung tn hiu trung tn (IF)ca h thng truyn hnh cp (gi l b downconverter).3/ Khi IF/IF l b lc trung tn c chc nng lc ng tn s ca knh truyn

hnh cn thu.4/ Khi IF/RF l khi chuyn i t tn hiu trung tn ln tn hiu cao tn trongdi tn ca h thng truyn hnh cp ghp knh v truyn ln mng n thu

bao.5/ Khi combiner l khi kt hp knh hay cn gi l khi ghp knh n c chcnng ghp cc knh truyn hnh thu c t truyn hnh qung b v t v tinhvo mt di tn ng xung (65MHz ~ 862MHz) ca h thng truyn hnh cptheo phng thc ghp knh theo tn s (FDM).6/ Khuch i RF l b khuch i tn hiu cao tn trc khi a vo b chia tn

hiu cao tn vo my pht.7/ My pht quang c chc nng chuyn i t tn hiu in thnh tn hiu quangv ghp n vo si quang truyn i.

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RF R

FRF

RF

R

F

R

F

R

F

R

F

R

F

R

F

R

F

R

F

R

F

R

F

IFIFIFIFIFIFIFIFIF IF

IF IF IFIFIFIFIFIFIFIF

IF IF IF IFA/V

A/V

A/V

A/V

A/V

A/V

IF IF IF IFThuv

tinh(IF)

Thuv

tinh(IF)

Thuv

tinh(IF)

Thuv

tinh(IF)

Thuv

tinh(IF)

Thuv

tinh(IF)

Combiner

RF

IS

Pht quang(Tx)

Si quangCp ng trc

Hnh 3.1 Trung tm Headen

IS

ISIS

IS


26/88

3.1.2 - Nguyn l hot ng ca Headend

Cc chng trnh qung b mt t (VTV1, VTV2, VTV3, ) c thu

qua cc anten VHF (very hight friquency), mi mt knh truyn hnh c thuqua mt anten ring, cc knh truyn hnh thu c sau a vo khi chuyni t tn hiu cao tn RF thnh tn hiu trung tn IF (upconverter). Lc ny tnhiu thu c t mi anten l mt di tn bao gm knh tn hiu cn thu v ccknh tn hiu khc lt vo (v d: anten VHF cn thu knh VTV3 nhng trong tnhiu thu c c c cc knh khc nh HTV, VTV2). Tn hiu trung tn chungny c a qua b lc trung tn lc ly knh tn hiu cn thu (VTV3). Mi

b lc trung tn c iu chnh ch thu mt knh tn hiu. Tn hiu trung tnra khi b lc ch c mt knh duy nht. Cc knh tn hiu ny s c i ln tns RF qua b chuyn i IF/RF c tn hiu RF nm trong di tn ng

xung ca mng CATV. Sau tn hiu RF ny c a vo b kt hp(combiner 16:1) ghp knh vi cc knh tn hiu khc theo phng thc ghpknh theo tn s (FDM: Friquency Division Multiplexing).

Cc tn hiu v tinh c thu qua anten parabol l cc tn hiu truyn hnhbao gm nhiu knh ghp li vi nhau, tch cc knh ny ra thnh cc knhc lp th chng c chia thnh nhiu ng bng cc b chia v tinh. Sau mi ng s c a vo b thu v tinh (downconverter) chuyn t tn scao thnh tn s thp, tn hiu ra khi b thu l tn hiu A/V. y cha phi l tnhiu m CATV cn nn sau chng c a vo b chuyn i A/V thnh

IF.Tn hiu ra l tn hiu IF trn c Audeo v Video. Tn hiu trung tn ny vn ls kt hp ca nhiu knh tn hiu , ly ra mt knh theo yu cu th chngc a qua b lc trung tn ging nh khi thu cc chng trnh truyn hnhqung b v tn hiu ra l knh tn hiu cn thu. Cc knh ny tip tc c avo b chuyn i IF/RF c tn hiu RF nm trong di tn CATV. Sau c a vo combiner 16:1 ghp knh vi cc knh truyn hnh khc thu tv tinh v cc knh truyn hnh qung b trong di tn ng xung (70MHz ~862MHz). Tn hiu ra l tn hiu RF ghp knh bao gm nhiu knh c ghpli vi nhau. Tn hiu ny c th a vo my thu hnh ca thu bao gii mv xem c, nhng truyn i xa v theo nhiu hng khc nhau th n c

a vo b khuch i khuch i ln sau chia ra bng b chia tn hiu caotn (b chia k hiu ISV hoc IS). Tn hiu sau b chia mi ng c a vomt my pht quang, ti y tn hiu RF c chuyn thnh tn hiu quang vghp vo si quang truyn n thu bao qua mng HFC.

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27/88

3.1.3 - Cu to v nguyn l hot ng ca my pht quang3.1.3.1 Cu to

My pht quang bao gm 3 khi chnh nh sau:

+ B lp m c chc nng chuyn cc m ng truyn khc nhau (RZ, NRZ,AMI) thnh m ng truyn thch hp trn ng truyn quang, thng l mManchester.+ B iu khin c chc nng chuyn tn hiu vo biu din theo p thnh tn hiu

biu din theo dng ph hp vi ngun laser. V ngun laser ch lm vic vi tnhiu dng.+ Ngun quang trong trng hp ny dng ngun laser loi phn b phn hi(DFB) nng cao cht lng tn hiu. C cc loi ngun quang thng dngsau:

1/ Laser iode c khoang cng hng Fabry perotLaser ioe 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 cchmi nhn hai u d th kp thnh hai gng phn x nh hnh 3.2a. Cu trc nyca laser diode c vit tt l FP-LD.

Khong cch hai gng trong laser diode Fabry-perot l L. cc gng nyc kh nng to ra hi tip tch cc, tc l s quay li ca cc photon kch thchtrong vng hot tnh s kch thch nhiu photon hn. nh sng i ra ngoi qua haigng phn x.

Xt iu kin khuch i trong laser diode Fabry-perot: mt sng truyn tgng bn tri ti gng bn phi, nh hnh 3.3b. Ti gng bn phi, sng nys phn x v tip tc truyn nh th. Dng sng ny gi l sng ng. trong

bung cng hng ch c sng vi bc sng n nh th n phi l sng ng.Yu cu vt l ny c th c vit nh sau:

N = (3.1)

n tt nghip 27

B lpm

B iukhin

Ngunquang

Hnh 3.2 S khi my pht tn hiu quang

Tn hiu

vo

Tn hiu

quang ra

Ip


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Trong :L: l khong cch hai gng

N: l s nguyn tho mn iu kin cng hng, hai gng phn x phi cch nhau mt

khong l L bng s nguyn ln na bc sng.Qu trnh pht x ca FP-LD c thc hin khi mt vi bc sng cng hngnm trong ng cong khuch i c h s khuch i ln hn suy hao, nh tronghnh 3.3c

n tt nghip 28

Vng hn ch

Vng hn ch

GngVng hot tnh

(nm)

Cngsutura(mW

)

L

(a)

(b)

(c)

Hnh 3.3 Laser Fabry-perot: (a) Cu to ca khoang cnghng; (b) Hnh thnh sng ng trong khoang cng hng;

(c) Ph pht x ca FP-LD


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c tnh P I ca laser diode nh hnh 3.4. c tnh ph ca FP LD nh hnh3.3d.

2/ Laser diode DFB (Distributed FeedBack)Nguyn l hot ng ca laser DFB l s dng hin tng phn x Bragg

vo mc ch nn cc mode bn trong v chn lc tn s. Trong thit b nybung cng hng Fabry-perot c thay th bng cch t nhiu x (Hnh 3.5).

Sng quang lan truyn song song vi cch t, do cch t c cu trc tunhon chu k to hin tng giao thoa gia hai sng ghp la truyn ngc nhau. hin tng giao thoa sng xy ra th sau mt chu k ca cch t pha ca sng

phi thay i 2 m, trong m l s nguyn, c gi l bc nhiu x Bragg tc

l:2 m = 2 (3.2)

Vi neff l chit sut hiu dng ca mode (neff 3,4 i vi bc sng1550nm InGaAsP laser). l chu k cch t, h s 2 xut hin trong biu thctrn v nh sng phi phn x hai ln trng pha vi pha ca sng ti.

n tt nghip 29

Dng (mA)

Cngsutra(mW)

Hnh 3.4 c tnh P I ca laser diode

0


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Nu iu kin (3.2) khng tho mn th nh sng tn x t cch t s giaothoa trit tiu ln nhau, kt qu l sng khng th lan truyn i c. Khi m = 1th bc sng B c gi l bc sng Bragg bc 1 v:

B = 2 neff (3.3)

Cng thc (3.3) l trng hp c bit ca nh lut Bragg v nu m = 1n v th sng c gi l tho mn iu kin Bragg th nht. Ngoi ra cng c

th nh sng phn x tho mn iu kin Bragg th hai. Thc t th khi m = 2,chu k cch t tng ln v d ch to hn. Lu rng cch t khng nm tronglp hot tnh nguyn nhn l do cch t nm trong vng hot tnh s gy ra schuyn v b mt v iu ny s lm tng t l ti hp khng bc x. Lp cch tc thc hin nh cng ngh quang khc hoc cng ngh n mn ho hc.

c tuyn P-I ca laser c th hin nh trong hnh 3.4 v chc nng iuch tn hiu tng t c th hin nh trong hnh3.6a v iu ch tn hiu sc th hin nh trong hnh 3.6b. y logic 0 tng ng vi trng thi ti vlogic 1 tng ng vi trng thi sng ca nh sng laser. Mt tn hiu thay i

thng t gi tr di ngng n gi tr trn ngng ca c tuyn laser (dngbm).

n tt nghip 30

Hnh 3.5 Cu trc diode laser hi tip phn tn DFB

P

Lp tip xc

Cch t

Lp tip xc

Vng hottnh

n


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n tt nghip 31

Tn hiu quang u ra

Tn hiu in u vo

Hnh 3.6.(a) iu ch tn hiu s

IF

(mA)Ith

Ib

P(mW)

Tn hiu quang u ra

Tn hiu in u vo

Hnh 3.6.(b) iu ch tng t

IF (mA)I

th

Ib

P(mW)


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3.1.3.2 Hot ng ca my pht

Tn hiu cao tn RF qua b lp m (nu l tn hiu s th n s c chuyni m ng truyn hin ti thnh m ng truyn thch hp cho ng truyn

quang thng l m Manchester) sau tn hiu c a vo b iu khin chuyn tn hiu in p thnh tn hiu dng bm thch hp cho ngun laser vngun laser c chc nng chuyn tn hiu in thnh tn hiu nh sng v ghpvo si quang qua b ni.

3.2 Cu to v hot ng ca node quang

1 - S khi ca node quang 4 cng ra:

2 - Cu to ca node quang bao gm cc khi c bn sau:

n tt nghip 32

01

03

02

04 05

06TP1

RF1

0708

RF2

RF3

RF4

TP2

TP3

TP4

Hnh 3.7 S khi ca node quang 4 cng ra


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(01) Khi thu quang c chc nng thu tn hiu t tuyn n v sau chuynthnh tn hiu cao tn (RF)(02) Khi khi phc tn hiu: khi ny bao gm cc b chia tn hiu, b suy hao(pad), b khuch i, chng C ch nng ln lt l chia u tn hiu cho cc

cng khc, iu chnh mc tn hiu ph hp vi yu cu u ra v khuch i tnhiu.(03) Khi khuch i cng sut trc khi a ra u ra.(04) Khi Diplexer ba cng: c chc nng r tn hiu ng xung v ng ln.Tn hiu c ng xung s i theo cng H (Hight) cn ng ln s theo cng L(Low).(05) L cc b r tn hiu (trch tn hiu ra ) kim tra.(06) L khi kt hp (Combiner) tn hiu t hai cng theo hng ln (Hng trv trung tm)TP (Test Point): l u kim tra,ti mi u ra s c mt u kim tra tn hiu

c trch ra bng khi chia tn hiu.

3 - Nguyn l hot ng ca node quangTn hiu quang ti u vo c chuyn thnh tn hiu cao tn (RF) qua

it quang in vo b khuch i, tn hiu cao tn (RF) c chia u thnh haihng vo hai khi tng t nhau. Ti y tn hiu c khi phc li nh b cnchnh v khuch i ln a vo b chia, tn hiu li tip tc c chia thnh haihng vo b khuch i cng sut trc khi a ra cng. Tn hiu hng xungi qua khi Diplexer s i qua cng H ra cng ra. Cn tn hiu cao tn hng ln

(i t pha thu bao) s i qua cng L vo khi Combiner v c kt hp vi tnhiu n t cc cng khc qua b lc, b lc s lc ly khong tn hiu trong bngtn hng ln (5MHz 65MHz) sau c khuch i v c a vo khi

pht quang. Ti y tn hiu cao tn (RF) s c chuyn thnh tn hiu quangqua it in quang truyn v trung tm trn cc si cp hng ln.

4 - Chc nng ca node quangChc nng chnh ca node quang l chuyn i tn hiu quang thnh tn

hiu cao tn (RF) v ngc li. ng thi n cng khuch i tn hiu v cnchnh li tn hiu tng t nh tn hiu ti my pht. V tn hiu khi truyn trn

si quang b suy hao v cc xung b gin ra do hin tng tn sc ca si quangm c bit l truyn trn si n mode nn s nh hng ny li cng ln.Chng lm suy gim cht lng tn hiu v vy cn cn chnh v khuch i. Tnhiu vo ca node quang nm trong khong 2.5dBm +2dBm v tn hiu rathng thng ca mt node quang trong khong 108dB V. Khong bc sng

n tt nghip 33


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hot ng l t 1270 1550nm, trong truyn hnh cp dng ca s quang 1310nm c suy hao trn si quang thp.

3.3 Si quang

3.3.1 - Cu to v dng si quang

truyn lan c nh sng, mt si quang c bn c cu to nh hnh3.8. Mc d trn thc t, si quang c th c nhiu lp nhng ch c hai lp c

bn l lp li (core) v lp v (cladding) ng vai tr quan trng trong qu trnhtruyn lan nh sng. C lp li v lp v c ch to t thu tinh Silic, tuy nhin

chng c chit xut khc nhau, li c chit xut ln hn v m bo iu kinphn x c th duy tr vic truyn lan nh sng bn trong li si quang. Chnhlch chit xut gia lp li v lp v thng khong mt vi phn trm. Hu htcc loi si quang u c ng knh lp v khong 125 m. Bn ngoi lp vny thng l mt vi lp bc bo v trnh s tc ng c hc vo si gy bini cc c tnh c l ca si.

3.3.2 - Si n mode v si a mode

Khi nh sng truyn lan bn trong li ca mt si quang ph thuc vo h

s khc x ca li (h s khc x l hng s hoc thay i), c th c cc phn btrng in t khc nhau qua mt ct ca si. Mi mt phn b thng tho mn

phng trnh Maxwell v cc iu kin bin ti mt phn cch li-v c gi lmt mode quang (Transverse mode). Cc mode khc nhau truyn lan dc trn siquang cc vn tc khc nhau. Si quang cho php lan truyn ch mt mode duynht c gi l si quang n mode (single mode fiber). Si quang cho php

n tt nghip 34

Lp v bo v

Hnh 3.8 Cu to si quang

V

Li


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truyn lan nhiu mode ng thi c gi l si quang a mode (Multimodefiber).

iu mu cht trong vic thit k, ch to si truyn n mode l ngknh li si phi nh, xut pht t mi quan h gia bc sng ct ca si ving knh li. Bc sng ct c l bc sng m trn si ch c mt mode ctruyn v c tnh nh sau:

c = (3.4)Trong :

V = 2,405 i vi si c chit xut bc (SI fiber)a[ m]: l bn knh ca li

n tt nghip 35

(a)

(c)

(b)n1

n2

2a

2a

2a

n1

n1

n2

n2

Hnh 3.9 Mt ct v cc tia sng truyn trong (a) si a mode chitxut phn bc, (b) si a mode chit xut Gradien v (c) si n

mode chit xut phn bc


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n1 l chit xut ca lin1 l chit xut ca v

Khi ng knh li ca si n mode khng ln hn nhiu so vi bcsng th s c mt s phn chia cng sut ng k lp v. V th cn phi nh

ngha mt tham s khc c gi l ng knh trng mode (Mode FieldDiameter). Mt cch trc gic, chnh l rng ca trng mode. c bit,ng knh trng mode chnh l trung bnh bnh phng rng ca trng nu

phn b trng theo Gauss. Khi phn b trng khng phi dng Gauss th cnhiu cch nh ngha ng knh trng mode.

3.3.3 - Cc c tnh ca si quang3.3.3.1 Suy hao

- Cng sut quang truyn ln si gim theo quy lut hm s m:

P(z) = P(0).10(-z) (3.5)Trong :

P(0)[mW]: l cng sut quang u siP(z)[mW]: l cng sut quang c ly z l h s suy hao

- suy hao ca si quang c tnh bi cng thc:A[dB] = -10lg (3.6)

Trong :P1[dBm]: l cng sut quang u vo

P2[dBm]: l cng sut quang u ra- H s suy hao trung bnh (suy hao trn mt n v chiu di): [dB/km] = (3.7)

Trong :A[dB]: l suy hao ca si quangL[km]: l chiu di si quang

3.3.3.2 Cc nguyn nhn gy nn suy hao

c 4 yu t gy suy hao c bn nh sau:

1 Suy hao hp th ca vt liunh sng truyn lan trong si quang b hp th do cc vt liu si v c

bin i thnh nhit gy nn suy hao m khng lt nh snh ra ngoi. C hai dngsuy hao hp th vt liu c bn l suy hao do bn thn si quang v suy hao do ctp cht trong thu tinh ch to si quang. Suy hao do bn thn vt liu si l do

n tt nghip 36


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cc cng hng nguyn t ca vt liu si. Trn hnh 3.9 cho chng ta thy s hpth xy ra c hai min hng ngoi v cc tm. S hp th khng phi do bnthn vt liu ch to si gy ra bi cc cng hng nguyn t ca cc thnh phnkhc cn gi l tp cht nh cc ion kim loi, ion Hydroxyn v cc lin kt OH

m tn s cng hng c bn ca n bc sng 2.8 m. Do lin kt OH c thhp th nh sng ti tn s cng hng v cc sng hi, do c nhiu nh hpth ti cc bc sng 2.8/(n + 1) m, v d nh 1.4 m, 0.93 m v 0.7 m(vi n = 1,2 v 3) nh th hin hnh 3.10. Cc nh hp th khc nh l nh 1.24 m l do tc ng gia lin kt OH v SiO2.

2 Suy hao do tn sc

n tt nghip 37

Suy hao tng

Tn x Rayleigh

Hp thcc tm

Hp th

hng ngoi

OH

850nm1.ca s

1300nm2.ca s

1550nm3.ca s

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.60

1

2

3

4

5

ion OH

Bc sng

( m)

Suyhao(d

B/km)

Hnh 3.10 Suy hao tng ca si quang


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C 4 loi suy hao tn sc trong si quang l Rayleigh, Mie, Brillouin vRaman m trong quan trng nht l suy hao do tn sc Rayleigh. Suy hao dotn sc Rayleigh t l thun vi 1/ 4 v c tnh nh sau:

R = CR[dB/km] (3.8)

y, CR c gi l h s tn sc Rayleigh. Gi tr thc t o c thhin hnh 3.10 v nm trong khong t 0.8 1.0 (dB/km)/( m)2 v l mt hmca chnh lch chit xut giav li vi v, ng knh li v kiu ca vt liu.

Ni mt cch tng qut, chnh lch chit xut gia li v v cng ln th suy haodo tn sc Rayleigh cng ln.

Suy hao tng bao gm suy hao vt liu v suy hao do tn sc Rayleigh cth hin nh hnh 3.10 cho thy c hai ca s thp 1.3 m v 1.55 m. Dovy hu ht cc ngun quang thng hot ng cc bc sng c suy haol nh nht.

Cc tn sc Rayleigh v Mie l cc tn sc tuyn tnh, trong cng sutmt mode ca tng loi thit b bin thnh mode bc x bi tnh khng ng nhtca ch s chit xut (Rayleigh) hoc tnh khng ng nht ca b mt dn sng(Mie). Cc tn cc Brillouin v Raman l cc tn sc khng tuyn tnh , trong cng sut mt mode ca tng loi b bin thnh mt mode c tn s khc. Tn scBrillouin c th coi nh mt s iu ch sng mang nh sng bi s dao ng

phn t nhit, tn s ca nh sng b iu ch s b dch ln hoc dch xung sovi tn s sng mang vn c. Tn sc Raman ging nh tn sc Brillouin. Thct, c hai tn sc Brillouin v Raman cn cng sut ln, thng thng 100mW viBrillouin v 1W vi Raman. V th chng khng ng k khi cng sut c

truyn ch c vi mW.

3 Suy hao un congTn hiu trong si quang cn chu suy hao bc x ti cc im un cong bi

cc mode vi phn c to ra. Trong thc t, suy hao un cong khng ng k trkhi bn knh un cong si qu nh, do vy thng b qua suy hao un cong ny.Tuy nhin khi tuyn truyn dn quang di v c nhiu im un cong th suy haodo un cong c th ng k. Khi cn s dng cc si c ng knh trngmode nh hn gim suy hao un cong.

4 Suy hao ghp ni v mi hnTn hiu quang cn b suy hao ti im kt ni gia hai si bng b ghp

ni hoc mi hn. Suy hao ny gy ra bi nhiu nguyn nhn sau y:- Suy hao bi cc yu t bn ngoi:

+ Khng ng tm gia hai li si+ Mt ct si b nghing

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+ C khe h gia hai u si c ni vi nhau+ B mt u si khng phng

- Suy hao bi cc yu t ni ti:+ Li si b elip

+ Khng tng thch v chit xut+ Khng ng nht v ng knh trng modeThng thng suy hao ni ghp khong 0.2dB v suy hao mi ni khong

0.05dB.

3.3.4 - nhy thu v qu cng sut

Do suy hao si quang, cng sut nh sng s b suy gim khi lan truyn vsuy hao si s hn ch c ly lin lc v tc bt. Gii hn suy hao c thc thy r thng qua khi nim nhy thu v qu cng sut.

1 - nhy thuTrong mi h thng vin thng, mt cng sut thu ti thiu cn thit phi

c t c cc c tnh nht nh, cng sut thu ti thiu c gi l nhy thu. Nu cng sut tn hiu thu c thp hn cng sut ti thiu cn thitth h thng s khng tho mn cc ch tiu k thut hoc thm ch c th khnglm vic c.

2 BER ca truyn dn s

Trong truyn dn s, phm cht c nh gi da trn thng s BER mn l phn trm cc bt li thu c. Mt nguyn nhn c bn gy ra cc bt lichnh l tp m. Cng sut tn hiu cng ln hn cng sut tp m th BER cngnh. T kt qu cc nghin cu, BER i vi cc tp m trng phn b Gauss(chnh l tp m nhit) c tnh nh sau:

Vi t s tn hiu trn tp m SNR >>1.Nu BER yu cu nh hn 10-9 th SNR s phi t nht l 36 ln tc l

16dB. V vy, vi mt cng sut tp m tng cng cho ca h thng, nhythu s phi gp 36 ln cng sut tp m.

3 Qu cng sut

n tt nghip 39

BER = )

SNR

e - dx = Q() ) e-

(3.9)


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Qu cng sut c nh ngha l hiu s gia mc cng sut pht v cngsut thu cn thit v c tnh theo cng thc:

Bp = (3.10)Hoc:

BD[dB] = PTx[dBm] PRxmin[dBm] (3.11)Vi: PTx[dBm]: l cng sut phtPRxmin[dBm]: l cng sut thu ti thiu cn thit ( nhy thu)

Nh vy, suy hao tng cng trn ng truyn phi thp hn qu cng sut.Trong si quang, suy hao c tnh theo dB/km. Nu mt si quang c diL[km] v c suy hao si[dB/km] th suy hao tng cng ca si l si.L[dB]. Vvy ta cn c:

si.L + ghp ni.N + Aloss Qu cng sut (3.12)Trong :

si[dB/km]: l suy hao si ghp ni[dB/mi hn]: l suy hao mi ghp ni

N: l tng s im ghp ni trn tuyn truyn dnAlos[dB]: l cc suy hao khc

Qu cng sut c th c ci thin bng mt s cch, v d nh: c thtng PTx bng cch tng cng sut ra ca laser hoc gim PRxmin bng cc b tchsng quang dng thc l (Avalanche Photodetector). Qu cng sut cn c thtng ln bng cch s dng cc b khuch i quang m chng c th rt quantrng trong cc h thng thng tin xuyn i dng bi cc h thng ny th suyhao l yu t v cng quan trng nh hng n h thng.

3.3.5 - Cc gii hn bi suy hao

T biu thc v qu cng sut (3.12) ta thy c ly truyn dn b hn chbi:

Lmax = {10lgPTx 10lgPRxmin - Aloss} (3.13)Trong :

si[dB/km]: l suy hao siPTx[dBm]: l cng sut pht

PRxmin[dBm]: l nhy thu m bo cht lng truyn dn nhtnh.Alos[dB]: l cc suy hao khc

Nu cng sut ra PTx ca b pht v suy hao si quang si l cho trc thkhong cch lin lc c th t c ph thuc c bn vo PRxmin.

V d: Nu PTx = 0dBm, PRxmin = -45dBm, si = 0.2dB/km, N = 2 v ccsuy hao khc trn h thng Aloss = 5dB th ta c:

n tt nghip 40


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Qu cng sut = 45dB.T ta tnh c khong cch ti a:

Lmax = (45 5 -2)/0.2 = 190[km]

3.3.6 - Truyn lan nh sng trong si quangNgoi vn suy hao, tn sc si (Dispersion) cng l mt yu t hn ch

khc n vic truyn dn sng nh sng. Tn sc l mt hin tng m cc photon(tc l cc mode) c tn s khc nhau truyn lan vi cc vn tc khc nhau. Dovy, mt xung nh sng s tr nn rng hn v chng ln ln nhau khi n truynlan trn si quang. Trong phn ny s i vo c s vt l ca vic truyn lan nhsng trong si quang, sau s cp n cc dng tn sc khc nhau trong siv cc hn ch do tn sc. Vic truyn lan tn hiu trong si quang c th c mt bng phng php quang hnh hoc bng cc hm Maxwell c th th hin mt

cch chnh xc, tuy nhin rt phc tp. n gin trong n ny ch yu xemxt bn cht vt l ca vic truyn sng vi mt mc ton hc n gin nht.

3.3.6.1 Truyn lan tn hiu trong si quang

truyn c trong si quangth tia sng phi tun theo nh lut phn xton phn ca Snell. Trong si quang c hai loi tia c th truyn i trong l tiakinh tuyn (Hnh 3.11) v tia nghing (Hnh 3.12). Tia kinh tuyn l tia truyndc theo si quang theo ng zic zc v cc tia ny ct ngang trc ca li si sau

mi ln phn x, cn tia nghing l tia truyn dc theo si quang theo ng xonc v ng i ca cc tia ny thng di hn cc ng kinh tuyn do suyhao mnh hn. V vy, vic i vo phn tch loi tia nghing ny l khng cnthit v n khng phn nh v cc tia lan truyn trong si. Nn ch xem xt cc tiakinh tuyn mi c ngha trong mc ch ny. Tuy nhin, cc tia nghing cnggp phn vo vic kt lun qu trnh tip nhn cc tia sng v suy hao tn hiu casi quang.

Cc tia kinh tuyn c th hin trong hnh 3.11 l xt cho loi si c chs chit xut phn bc. Vi gc vo ca tia sng hp vi li trc mt gc 0.Theo nh lut Snell th gc min to ra s phn x ton phn s c xc nhnh sau:

sin min = (3.14)Trong :

min l gc ti hnn1 l chit xut lin2 l chit xut v

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3.3.6.2 Cc mode truyn lan

Phng php quang hnh trn ch cho thy mt cch gn ng s truyn lannh sng trong thc t. nghin cu chnh xc hn chng ta phi dng cc hmMaxwell. Tuy nhin nu ch hiu cc c tnh truyn lan trong si v tn scca si chng ta c th s dng cc phng php ton hc khng phc tp lm.Vi iu kin bin ti giao din li-v (tng ng vi nh lut Snell cng thc(3.14) trong phn phn tch quang hnh), ch c mt tp xc nh cc hm sngtho mn phng trnh Maxwell mi c th truyn lan trong si quang, mi mthm sng c gi l mt mode truyn lan v c biu din di dng:

i(r, ,z) = Ai(r, )ej( t - ziZ) (3.18)

Trong :i l ch s ca mode truyn lan iAi(r, ): l phn b trng ngang zi: l hng s truyn lan theo trc z

Biu thc (3.18) cho thy hm sng l hm theo thi gian v cc tham skhng gian r, , z. H to tr c dng ay bi si quang chnh l mt ngdn sng trn. Trong biu thc (3.18), tham s ej( t - ziZ) biu th vic truyn lansng dc theo trc z ca si.

Hng s truyn lan hng z: T khi nim cc mode truyn lan zi tho mn biuthc tn sc nh sau:

12 = = zi2 + Ki2 (3.19)

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Hnh 3.12 Tia nghing trong si chit xut phnbc

n1

n2

Trc


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Vi 12 = l hng s truyn lan ca mt sng hi tn s v trong mitrng in mi ng nht c chit xut n1. Ki l hng s truyn lan theo phngngang ca mode truyn lan th i.T ta c, mi mode truyn lan c mt cp Ki, zi l s thc tho mn bt ng

thc sau: 12 - zi2 = Ki2 > 0 (3.20)Cc mode truyn lan bc cng cao (i cng ln) th Ki cng ln v zi cng

nh. Khi Ki vt qu 1 th zi tr thnh s o v mode c mt s suy gimhm m khi n truyn lan.

Biu thc (3.20) l iu kin truyn lan cho cc sng bn trong li si. Cmt s tng t cho lp v nhng vi iu kin khc:

zi2 - 22 > 0 (3.21)Vi 2 =

iu kin ny c ngha l khng c truyn lan trn lp v, ni cch khc lsng trong lp v b suy hao.Cc biu thc (3.20) v (3.21) u yu cu zi nm trong khong:

< < 1 (3.22)iu kin (3.22) chnh l tng ng iu kin phn x ton phn trong phn

phn tch quang hnh. Biu thc (3.17) cho thy khi n2 xp x n1 th NA cng nh,do ch c rt t mode lan truyn c.Vn tc truyn lan theo trc z: Tng t vi nhn xt c nu trong phn phntch quang hnh, vn tc ca tia sng ph thuc vo gc ti . Vn tc theo trc zca mode lan truyn th i l hm ca hng s truyn lan zi ca n v c tnhnh sau:

vgi = (3.23)Vi vgi thng c gi l vn tc nhm v cho thy rng cng sut ca tn

hiu quang lan truyn nhanh nh th no. Vn tc nhm khc vi vn tc pha vpi= / zi. Vn tc pha vpi cho bit pha ca tn hiu quang thay i nhanh nh thno.Mc d vic tnh ton chnh xc vn tc nhm vgi cn phi bit zi l mthm ca , biu thc (3.20) c th c tnh xp x khi s ph thuc tn s caKi l nh, khi :

Bi v 1 = n1 /C

= = (n1 + ) = = (3.24)Vi n1g l ch s khc x nhm:

n1g = n1 + (3.25)T kt qu trn, dng php rng buc ta c:

vgi = = ()-1 = ()-1 ()-1 = = vg (3.26)

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So snh (3.26) vi (3.16) ta thy t s zi/ 1 l tng ng vi sin 1hoc t l vi C/n1 trong biu thc (3.10). Bi mi mode truyn lan c zi ringnn mi mode c tr truyn lan khc nhau. iu ny khng nh iu nhn xtqu trnh truyn lan trong phn phn tch quang hnh rng cc tia ti vi gc ti

khc nhau c vn tc theo trc z l khc nhau.

3.3.7 - Tn sc si quang

Nh nu trn, vn tc nhm vgi trong biu thc (3.26) l mt hm catn s v ca cc mode truyn lan. Nu mt xung quang c cha cc thnh phntn s khc nhau v cc mode truyn lan khc nhau th cc tr truyn lan khcnhau ca cc thnh phn ny s lm xung b gin rng v chng ln ln nhau cui si quang.

Tng qut, c ba loi tn sc si nh sau:

- Tn sc vt liu (Material Dispersion).- Tn sc ng dn sng (Waveguide Dispersion).- Tn sc mode (Modal Dispersion).

Hai loi tn sc u (tn sc vt liu v tn sc ng dn sng) c th quycho s ph thuc tn s ca vn tc truyn lan v gp chng li gi chung l tnsc trong mode (intramode) hay cn gi l tn sc vn tc nhm (Group VelocityDispersion - GVD). Loi th ba (tn sc mode) c gi l tn sc gia cc mode(intermode) do s ph thuc ca cc vn tc truyn lan vo cc mode truyn lankhc nhau. T cch phn loi ny, si n mode ch c tn sc trong mode.

3.3.7.1 Tn sc trong mode (Intramode Dispersion)

Tn sc trong mode c th hiu nh sau: bi vn tc nhm ca mt mode lph thuc tn s nn tr truyn lan n v (tc l nghch o ca vn tc nhm)cng ph thuc tn s. Khai trin chui Taylor i vi tr n v ti bc sngxc nh ta c:

g = g ( 0) + ( - 0) + 0.5( - 0)2 + (3.27)Trong g ( 0) l tr truyn lan khong cch n v bc sng trung

tm 0. T khai trin trn, tn sc trong mode c tnh bi:Dintra = = () = () (3.28)Trong ch s i i vi zi c b i cho n gin. Biu thc (3.27) c

th tnh gn ng nh sau: g = g ( 0) + ( - 0)Dintra + 0.5( - 0)2 (3.29)

Nu ch gi li hai ton hng u, rng xung tng do tn sc bn thnmode Dintra s c tnh bi:

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g = Dintra (3.30)Vi l rng ph tn hiu.

tm Dintra ta dng nh ngha (3.28) v php rng buc ta c:Dintra = () (3.31)

T (3.24) ta c Dintra:Dintra = + () = Dmaterial + Dwaveguide (3.32) y tn sc vt liu:

Dmaterial = (- ) (- ) (3.33)V tn sc ng dn sng:

Dwaveguide = () (3.34)T cc nh ngha trn, lu rng Dmaterial l tham s khng ph thuc mode

lan truyn m ch ph thuc vo n1. Tn sc vt liu ca si quang Silic thngthng c coi l mt hm ca bc sng v c m t hnh 3.12

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1.1 1.2 1.3 1.4 1.5 1.6 1.7

0

10

20

-20

-10

Bc sng ( m)

Ps/(nm.km)

Hnh 3.13 Tn sc vt liu trong si quang


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Tn sc ng dn sng th ngc li, n ph thuc vo mode lan truyn th ic xc nh bi cu trc ng dn sng quang. Ni mt cch tng qut, vic tnhton tn sc ng dn sng l rt phc tp.

3.3.7.2 Tn sc modeTn sc mode l do s tr truyn lan khc nhau ca cc mode khc nhau.

T (3.20) zi l khc nhau i vi cc mode khc nhau. Do vy, vn tc nhmtng ng vgi trong (3.23) cng khc nhau. C th, ta c th tnh tn sc mode nhsau:

Dmode = - = g.max - g.min (3.35)Vi g.max v g.min l tr truyn lan nhm n v cc i v cc tiu.

Vi si chit xut bc (SI): c th tnh ton tn sc mode ca si SI bngphng php quang hnh. Theo cng thc (3.16) ta c:

g.max = (3.36)Vy tn sc mode:

Dmode = g.max - g.min ( - 1) = (3.37)Kt qu (3.37)cho thy rng tn sc mode trong si SI t l thun vi

chnh lch chit xut . Do NA t l thun vi nn c mt s tng ng giahiu sut ghp ni v tn sc.

Vi si chit xut Gradient (GI): Mt ct ch s chit xut ca si GI c thc biu th tng qut bi biu thc sau:

Trong l tham s c ti u ho sao cho c th t c tn scmode cc tiu.

Tn sc mode trong si GI rt kh xc nh c trong phng php quanghnh. Theo cc kt qu nghin cu th khi:

= 2(1 - )(3.39)

th tn sc mode l cc tiu v c xc nh bng biu thc:Dmode = (3.40)

Khi tn sc mode trong si GI l nh hn nhiu so vi si SI c tnhbi (3.37) do yu t 2 (v < 1 nn 2


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3.3.7.3 Tn sc tng cng ca si

T tn sc trong mode v tn sc mode c th tnh c tn sc tng cngca si quang. Thay v cng trc tip chng, ngi ta dng cng thc tng bnh

phng nh sau (do tn sc c tnh ngu nhin):D2total = D2intra 2 + D2mode(3.42)

Vi l rng ph ca nh sng c tnh bng nm.Do mt si quang c th hin cho mt knh thng tin, tp m tng ca

si quang thng c dng c trng cho bng thng truyn dn:Bsi = (3.43)

Biu thc (3.43)cho thy tn sc tng v di si quang cng ln th tc bt c th truyn cng thp.

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3.3.7.4 S hn ch do tn sc

Tng t nh vi tn sc si, tn sc si lm hn ch c ly truyn cc i

mt tc bt nht nh c gi l gii hn tn sc v c th c hiu nh: Khitn hiu c truyn i trn si quang n u thu chng tr nn rng hn vchng ln ln nhau, nu khong cch truyn dn qu ln th n mt lc no takhng th phn bit c cc xung v khng th tch tn hiu c ch ra c (thhin trong hnh 3.14). S can nhiu ny trong truyn dn s c gi l can nhiugia cc bt v kt qu l lm tng BER (th hin trong hnh 3.15l hnh xungs).

Ni chung BER khng tng ng k tr khi T = T T0 l qu ln. Mtnguyn tc c bn l nu tho mn:

T = T T0 = DtotalL = (3.44)th BER khng xung cp ng k.

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Cc xung tch nhau t1

Cc xung cn phn bit

c t2

Kh phn bit xung t3

Khng phn bit cxung t

4

Giao nhau

Dngxung

vbin

Chiu di si

Hnh 3.14 S gin xung v suy hao ca hai xung k nhau khichng c truyn dc theo si


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Trong h thng thng tin cp si quang, s gin xung khng ch gy ra bitn sc si m cn do mt yu t khc l rng sn trc ngun pht quangv ca b thu cng gy ra gin xung.

Tng t vi tng bnh phng ca tn sc si tng cng, gin xung

tng cng l tng bnh phng ca tt c cc yu t: T = t2 + r2 + (DtotalL)2(3.45)

Vi t v r l rng sn trc ca my pht v my thu.Kt hp (3.44) v (3.45) ta c:

t2 + r2 + (DtotalL)2 < ()2 (3.46)y chnh l biu thc tng qut cho gii hn tn sc.

3.4 - Cc mi hn v cc b kt ni (Connector) trong mng quang.

Mt nhn t quan trng trong vic lp t h thng thng tin quang l skt ni qua li gia cc thit b hoc gia cc thnh phn vi nhau. Cc lin kt xy ra ti ngun quang, thit b tch quang hay cc im trung chuyn trongng truyn khi c hai si quang hoc cc si cp ni vi nhau. Vic s dngcc mi hn hay dng cc b kt ni (connector) tu thuc vo lin kt l tmthi hay lu di. Lin kt lu di l cc im ni him khi thay i nh ni hai siquang, ni gia si quang v dy nhy. Cn lin kt tm thi l lin kt gia ccthit b c th thay i c nh: Gia si quang v node quang ,si quang vhub, gia si quang v cc b chia,..Ni chung, cc mi hn c suy hao thphn so vi cc connector nhng li yu cu thit b hn t tin v nhiu nhn lchn connector.

i vi si n mode, yu cu chnh xc rt cao ti im kt ni giacc si quang. Cc nguyn nhn gy nn suy hao trong c connector v mi hnc th chia lm hai loi c bn: Suy hao bi cc yu t bn ngoi v suy hao bi

n tt nghip 50

Tn hiu pht

Tn hiu thu

Hnh 3.15 Dng cc xung pht v thu c

T0

T


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cc yu t ni ti. Suy hao bi cc yu t bn ngoi nh: S khng ng tm giahai li si, cht lng mt ct si v c khe h gia hai u si.

C 3 yu t gy suy hao bn ngoi trong mi hn quang c bn sau (Hnh3.16)

+ C khe h gia hai si quang+ Trc ca hai si b lch+ Trc ca hai si to gc

Trong trng hp c khe h th trc ca hai si quang trng nhau nhng

mt ct ca hai si cch nhau mt khong s. Khong cch ny to ra mt vngkhng gian to ra s phn x v nu s phn x ny ln th ngi ta gi l phn xFresnel v gy ra suy hao. Cn trong trng hp lch trc l trc ca hai si songsong vi nhau nhng lch nhau mt khong d. Trng hp to gc l trc ca haisi khng song song vi nhau m to vi nhau thnh mt gc hay mt ct gia haiu cui si to thnh vi nhau mt gc nh trong hnh 3.16. khc phcsuy hao bi cc yu t bn ngoi th u cui hai si quang phi mn ,trc ca haisi phi trng nhau v mt ct hai si phi kht nhau.

Suy hao bi cc yu t ni ti do cc nguyn nhn gy ra nh: ng knhhai si khng bng nhau, li si hnh elip,

C hai loi suy hao bn trong v bn ngoi u nh hng n hiu sutghp ca si quang, hiu sut gia ngun v si quang. Trong si a mode, thsuy hao do b gc l ln nht ri n suy hao do lch trc v suy hao t nht l suyhao do c khe h. i vi si n mode th suy hao do lch trc nhiu hn so visuy hao do c khe h v do b gc nh c th hin trong hnh 3.16. V trong sin mode ch c mt mode c truyn lan v n truyn dc theo trc ca si nn

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s d

Hnh 3.16 Cc mi hn gy suy hao gia hai si quang (a) c khe h, (b) Trchai si b lch, (c u cui hai si to gc)

(a) (b)(c)


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nh sng ghp vo si khng ng tm s b suy hao mt cch nhanh chng. Do, i vi kt ni suy hao thp ( 0.5dB) th gim suy hao do lch trc phic iu chnh chnh xc n m (vi n l s nguyn) cn i vi si a modeth iu chnh chnh xc n m.

3.4.1 - Phng php hn cp

Hn bng cch lm nng chy si quang: Trong phng php ny vic u tin

cn phi lm l gia cng b mt li si ni cn lm mn b mt si v mt ct phivung gc vi trc ca si. Sau c hai u cui ca si phi c t vo ci gic rnh hnh ch V v chm u vi nhau nh trong hnh 3.18. Tip theo chngc c nh bng cc thit b c iu khin bng tay hoc bng b vi x l.Bc tip theo ti im tip xc c lm nng chy bng n h quang hoc

bng laser v vy u cui si quang b chy ra mt cch nhanh chng v lin ktli vi nhau. K thut ny c th c s dng cho cc si n mode v si a

n tt nghip 52

0

00

0

10 20 30 40

4321

1

2

3

0.5 1.0 1.5 2.0

lch d ( m)

Gc ()

Khong h s ( m)

Suyhaoghp(dB)

Lch trc

B gc

Kheh

Hnh 3.17 Suy hao ghp trong si n mode i vi ngunMDF v bc sng 10 m trong cc trng hp: Khe h,

lch trc v b gc


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mode vi suy hao nh hn 0.1dB. hn na, cc si quang nng chy c th baoph gn nh l kch thc si quang ban u. Hin nay hu nh cc my hnquang u s dng phng php hn bng cch lm nng chy si quang. Cc siquang c a vo my hn v n c iu chnh hon ton t ng bng b vi

x l v hn cng hon ton t ng. Cc nhn vin ko cp ch thc hin mt sbc nh sau:- u tin dng dao chuyn dng ct cp.- Sau tut v bo v ngoi cng, tip theo l tut lp v ch nh mu ca

si quang v lp v trong sut.- Ra sch si quang bng cn (thng u si cp tut v khong 1 1.5m)

v ct bng u cui si.- Sau khi xong cc khu chun b th si quang c a vo my hn quang c

rnh ch V c nh cp.- Sau khi nhn nt set th my s t ng iu chnh v tr tng i ca hai si

quang cn hn v hai tia h quang s lm nng chy si quang v chng clin kt li vi nhau mt cch nhanh chng.

n tt nghip 53

in cc Si quang

Cc thanh c nh cp

Hnh 3.18 Phng php hn si quang nng chy

Rnh ch V


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3.4.2 - Cc Connector

Trong mng truyn hnh cp hu tuyn thng dng hai loi connectorchnh l FC/APC (Angled Physical Contact) v SC/APC. FC/APC l loiconnector u trn c ren vn c nh u cp. Cn SC/APC l loi u vungkhng c ren m ch c khp c nh khi cm vo. Tuy SC/APC n nh hn

FC/APC nhng li c suy hao ln hn. Cu to ca hai loi ny c th hin nhtrong hnh 3.19. Nguyn tc hot ng ca hai loi connector l bc x chm tia si quang pha truyn v hi t li ti li ca si quang pha thu sau khi i qua haithu knh. Chc nng ca cc connector l kt ni gia cc thit b quang ni cth d dng thay i v chuyn tuyn sau ny.

n tt nghip 54

Hnh 3.19 Cu to ca conector

Thu knhThu knh

Si quang ra

Keo dnh

Si quang vo

V bc c nh


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CHNG IV - CC THIT B CHNH TRONG MNG NG TRC

4.1 - Cp ng trcCp ng trc c s dng rng ri cho vic phn phi tn hiu cc trng

trnh truyn hnh . Hnh 4.1 v s cu trc cp ng trc s dng trong CATV.

Phn li ca dy dn trong thng lm bng ng vi in tr nh thun licho vic truyn dng in cng cao. Lp v ngoi cp v phn li trongthng lm bng nhm. Vt liu gia hai lp nhm thng l nha. Gia li v

phn ngoi c cc ti khng kh gim khi lng v trnh thm nc. Ngoicng l mt lp v bc chng cc tc ng c hc.

ng knh tiu chun ca cp l 0.5; 0.75; 0.85 v 1 inch, tr khng ctnh ca cp thng l 75 . Tn hiu s b suy gim khi truyn theo chiu di cacp. Lng suy gim ph thuc vo ng knh cp, tn s, h s sng ng vnhit .

Cc thng s ca cp ng trc:

4.1.1 - Suy hao do phn x

Suy hao do phn x l i lng c o bng khc bit ca tr khngc tnh cp so vi gi tr danh nh. N bng t s gia cng sut ti trn cngsut phn x:

n tt nghip 56

Lp v nha

V bc nhm

Dy dn trong( ng bc nhm)

Lp bt nha

Hnh 4.1 Cu to cp ng trc


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Lr[dB] = 10log[dB] (4.1)Khi tr khng thc hin cng gn vi gi tr danh nh, cng sut phn x

cng nh v suy hao phn x cng nhiu. Khi phi hp l tng ta c P r= 0. Tuynhin, trong thc t gi tr Lrvo khong 28dB 32dB. Nu suy hao phn x qu

nh, phn hi s xut hin trn ng dy v s to nn tn hiu c ting .4.1.2 - Tr khng vng

Cng sut t cc b khuch i b li suy gim trn ng truyn thngc cung cp bi dng mt chiu/xoay chiu in p thp truyn trong cp theotn s RF. Do mc in p thp, thng thng khong 45V, tr khng vng(trkhng phi hp ca dy dn trong v ngoi ca cp) l mt c tnh quan trng.Dng in ny chy qua trong ton b thit din ca cp, v v vy tr khng cady dn trong i vi n s cao.

C ba loi cp ng trc khc nhau c s dng trong mng cp phnphi:

+ Cp trung k+ Cp fid+ Cp thu bao

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Khuch itrung k

Khuch icu/trung k

Cp fid

Khuch ing dy

Tap

Cp thu

bao

Hnh 4.2 Phn cp ng trc trong kin trc cy v nhnh trong mng HFC

Cc loi cp v cc loi b khuch i

Nodequang


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Cp trung k: ng knh t 0.5 1 inch dng truyn tn hiu bt u tnode quang. Tn hao truyn dn i vi loi cp 1 inch l 0.89dB/100m tn s50MHzv 3.97dB/100m 750MHz .

Cp fid c s dng ni gia cc b khuch i ng dy v cc b

chia tn hiu, cn cp thu bao c ng knh nh hn cp fid dng kt ni tcc b chia ti thit b thu bao. V tr lp t trong mng c ch trong hnh 4.2.

4.2 - Cc b khuch i RF (Radio Friquency)4.2.1 - c im cc b khuch iCc b khuch i ng truyn b li suy gim tn hiu, chng ng vai tr quantrng khi thit k h thng. Mi b khuch i c cha mt b n nh b lisuy gim cc tn s khc nhau. Trong h thng truyn hnh cp thng s dng

b khuch i cu. Vi tr khng ln, tn hiu t ng trung chuyn c th cly ra m khng nh hng n cht lng ton b knh truyn. Yu cu i vi

b khuch i l n nh phi cao do c s tch lu suy hao ca nhiu thnhphn mc ni tip :- Chng phi lm vic c trn mi phm vi di tn rng, h s khuch i phit c gi tr ph hp ti cc min tn s cao.- B n nh c kh nng b li suy gim theo tn s mt cch ph hp.- B khuch i c c tuyn tuyn tnh cao trnh xuyn m.- T ng iu chnh h s khuch i v c tuyn tn s b li s thay i donhit .- T s CNR ca ring mt b khuch i phi ln chng c mc nhiu

tng ca cc b khuch i.C ba loi b khuch i c s dng trong mng CATV HFC tu thucvo v tr ca chng:

+B khuch i trung k.+B khuch i fid.+B khuch i ng dy.

V tr ca tng loi trong mng c nu trong hnh 4.2.c im ca tng loi:

4.2.1.1 - B khuch i trung k

c t ti im suy haoln ti 20 22dB tnh t b khuch i trc ,mc u ra thng khong 30 36dBmV.u im:

Mc CNR cao(300MHz). V cp ng trc khi truyn dn tn hao ph thuc nhiu vo tn s

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nn bin tn hiu Video pht i cn phi c gi cn bng nhm duy tr sng u trong ton vng ph tn hiu RF pht. Cc b gi cn bng ngxung c thit k b cho cc on cp ng trc c di c nh. Bngcch tng suy hao tn s thp, b cn bng cho php cc b khuch i trung k

duy tr mc khuch i thch hp vi tng khong tn s trong ph tn hiu truyndn. Ngoi ra, mt s b khuch i trung k cn c trang b b cn bng don trc (Bode Equalizer) b tn hao cp gy ra do s thay i ca nhit :

Cc b khuch i trung k thng dng mch t iu chnh h s khuchi (AGC: Automatic Gain Control). Khong iu chnh chnh lch mc khuchi thng trong khong 6 10dB. Cc khi AGC trong b khuch i trung ktch tn hiu mu ca cc knh hoa tiu ti u ra b khuch i, tn hiu mu nythng c dng to ra mc in p ph hp iu khin mc khuch i(Gain) v dc (Slope) c tuyn ca b khuch i, cc tn s hoa tiu chunkhc nhau i vi tng nh sn xut. Tt c cc b khuch i trong truyn hnhcp u dng mt s mch khuch i y ko gim thiu hi mo bc hai.

4.2.1.2 - B khuch i fid

c s dng khng ch pht xung nhnh knh tn hiu Video ti ccb khuch i trung k m cn chia tn hiu ti cc fid cp khc nhau (thng l4 cp fid). Mc tn hiu ra thng khong 40 50dBmV(cao hn 12dB so vi

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Phn u(Flatness)

Khuch i(Gain)

Postamp

Tinkhuch i

Tn hiuvo

Tn hiura

24 V

Khi cnbng(EQ)

Hnh 4.3 S khi n gin b khuch i trung k

D on(Bode)

HPF

LPF


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b khuch i trung k). Tuy nhin, u ra c mo phi tuyn mc cao hn sovi b khuch i trung k.

4.2.1.3 - B khuch i ng dy

Khong cch gia cc b khuch i ny khong 120m 130m, t phagn thu bao. gim hiu ng mo phi tuyn tn hiuVideo pht i cng nhduy tr s ng u trong ton di tn tn hiu, ti a ch s dng 2 4 b khuchi ng dy, tu thuc vo s lng Tap (b trch tn hiu) gia cc b khuchi ng dy di rng. Trong cc h thng CATV hai chiu c s dng mtthit b c bit l b lc Diplexer (hnh 4.4) cho php tch ring tn hiu ngln v ng xung. Ti cc h thng truyn hnh cp ti Bc M cc knh tnhiu ng ln c t di tn s 5 65MHz. Di tn s tn hiu ngxung l 70 862MHz. Diplexer c cch ly gia cc di tn khong 60dB.

Diplexer l mt thit b c ba cng: cng H, Cng L, Cng chung C.ng t cng chung C ti cng thp L l mt b lc thng thp cho php tnhiu ng ln bng tn thp hn c pht i. ng i t cng chung C ticng cao H l mt b lc thng cao cho php pht cc knh tn hiu ng xung.Trong mt b khuch i ng dy (khuch i trung k v khuch i cu) cctn hiu ng xung chuyn qua cng H, tn hiu ng ln chuyn qua cng L.

4.2.2 - CNR ca mt b khuch i n v nhiu b khuch i ni tip.

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Cng C

Cng H

Cng L

Cc b

lc

Hnh 4.4 Minh ha n gin mt diplexer


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S n gin ca b r tn hiu Tap cng ra suy hao 20dB:

Tap c s dng a tn hiu ti cc thu bao. Mt Tap in hnh baogm mt khi ghp nh hng RF v cc khi chia cng sut .

Khi ghp nh hng r ra mt phn nng lng tn hiu u vo, cn cckhi chia cng sut (Spliter) chia tn hiu ti thng l 2,4,8 cng ra. Cng suttn hao gia cng vo so vi cng ra gi l suy hao xen (Insertion Loss), cn vicc cng ra khc (cng r) gi l suy hao cch ly (Isolation Loss).

Suy hao xen ca Tap thng c lp vi tn s v nhit .Suy hao cch ly ln hn rt quang trng i vi cc h thng CATV hai

chiu ngn tTn hiu ng ln ca mt thu bao ny lt vo tn hiu ng xung ca

thu bao khc. Thng thng suy hao cch ly vo khong 20dB gia di tnng ln v ng xung.

Tap c c trng bi gi tr r, c nh gi bng t l gia cng sut rav cng sut tn hiu vo. Gi tr r thng trong khong 4 35dB.

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Hnh 4.6 S khi n gin ca Tap 4 ng suy hao 20 dB

ng ngunAC

Khi ghpnh hng

-12 dBng RF

-4 dB-4 dB

Chia tnhiu

ngvo

ng ra


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CHNG V PHNG PHP THIT K MNG TRUYN HNH CP HUTUYN

5.1 La chn cc thng s k thut cho mng truyn hnh cp hu tuyn5.1.1 - Phn b di tn tn hiu

Hu ht cc h thng HFC ti M hin nay u phn b di tn nh sau: t5MHz 40MHz dnh cho di tn ngc (truyn s liu t thu bao n trungtm), t 52MHz 750MHz dnh cho di tn hng xung trong : di tn t52MHz 550MHz dng cho cc knh truyn hnh tng t, t 550MHz 750MHz dng cho cc knh truyn hnh s v cc knh truyn s liu hngxung (downstream data). Vy di thng dnh cho cc knh truyn s liu hng

ln ch l 35MHz, trong khi di thng ca knh truyn s liu hng xung cth t n 120MHz 150MHz. iu c th nhn thy di thng cho knh hngln nh hn nhiu so vi knh hng xung. Ngoi ra hu ht nhiu h thngcng u xut hin ti cc tn s thp, phn cui ca ph tn, iu ny li cnglm cho di thng hng ln nh hn.

Do yu cu di thng cng ngy cng tng, c bit l di thng tn hiuhng ln, hin nay cc thit b truyn hnh cp mi ra i u h tr cho phngthc phn b di tn mi, trong di tn hng xung ln n 860 hoc870MHz, ng thi di tn s hng ln cng c tng t 5 65MHz. Vi phn

b di tn ny, di thng hng ln tng gp i, trong di thng hng xungcng tng hn 100MHz. Ta s la chn phn b tn s theo phng n ny chomng truyn hnh cp hu tuyn.

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Tn s750MHz

Hnh 5.1 Phn b di tn cho cc mng truyn hnh cp ti M

550MHz

500MHz

40MHz

5MHz

50MHz

Data, interativeupstream

Analog VideoDowstream

Digital video,data

Dowstream


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5.1.2 - Tnh ton kch thc node quang cho yu cu hin ti

Kch thc node quang l s lng thu bao c cung cp dch v ti mtnode quang. S lng thu bao ti mt node quang l thng s quan trng hngu khi thit k mng, v nh hng n mt lot cc thng s quan trng camng, nh: tc bt tn hiu hng ln ca mi thu bao (i vi mng haichiu), cu hnh mng truy nhp, kh nng cung cp dch v cho thu bao hin tiv tng lai, kh nng pht trin v nng cp mng trong tng lai.

Mt h thng mng c xy dng thng thng phi p ng c yu

cu s dng t nht trong 15 n 20 nm, v phi c kh nng nng cp png trong nhng nm tip theo. V vy trin khai mng truy nhp ng trc cntnh n mc tp trung ca thu bao v cu trc mng cp quang ca khu vcnode quang bao ph, cn kt hp cc yu t nh cu trc a l thnh ph, hngng ph v nh hng pht trin ca thu bao trong thi gian tip theo, cn cvo tnh hnh thc t xc nh s lng thu bao ca mi node quang.

Nu thu bao tp trung th 1000 ~ 1500 thu bao/ node quang l ph hp.Khi s dng 1500 thu bao /1 node quang th nn chn node quang c 4 cng ra,mng phn phi cp ng trc th nn chn cu trc dng sao v c gng cn is lng thu bao cc cng ra ca mi node quang tin cho vic nng cp sau

ny.

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Hnh 5.2 Phn b di tn cho cc mng truyn hnh cp Chu u

Data,interativeupstream

5MHz

65MHz

500MHz

70MHz

550MHz

862MHz

Analog VideoDowstream

Digital video,data

DowstreamData, interative

upstream

Tn s


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5.2 Thit k

Thit k mt h thng cp quang l mt quy trnh phc tp. Ngi thit k

phi nh gi nhiu yu t nh: tc d liu, suy hao ng truyn, mi trng,loi cp, thit b c c, cc loi giao din in t, b ni, hn quang, cc giaothc

Mt quy trnh hon chnh s rt phc tp, tuy nhin n c th c ngin ho nh cc ch dn km theo cc thit b quang do cc nh sn xut cungcp. Nhng ch dn ny thng mang y cc thng tin c th chn cloi cp quang thch hp v d lp t. Nhng yu t khc nh loi cp, panel,

jumper, mi trng, th ngi thit k phi t nh gi.Phn ny s trnh by phng php thit k n gin da trn cc khuyn

ngh ca nh sn xut. Cc phng php tnh ton chn la loi cp quang s

dng trong mng.

5.2.1 - La chn si quang

iu u tin phi c quyt nh l lp t h thng quang n mode haya mode. C hai h thng u c nhng u nhc im ca mnh. Cc u im ca h thng si quang n mode1. Si quang n mode c bng tn truyn dn rt ln, l tng cho truyn dn

c ly xa.

2. Si quang n mode c suy hao nh hn si quang a mode.3. Cp si quang n mode r hn si quang a mode.4. Si quang n mode hin c hot ng bc sng 1310nm hoc 1550nm u im ca h thng si quang a mode1. Si quang a mode thch hp cho cc c ly di 2km.2. H thng si quang a mode c bng tn t ph thuc vo chiu di ca si .

C th truyn s liu tc 100Mbps, c ly nh hn 2km, s dng si quangFI tiu chun 62,5/125.

3. Thit b quang a mode thng r hn n mode. Cc LED r thng cdng lm ngun quang.

4. Cp si quang a mode thng t hn n mode, nhng i vi nhng ngdng c ly ngn th s tit kim chi ph t cc thit b quang c th b li chi

ph cp quang a mode.5. Si quang a mode hin c hot ng bc sng 850nm hoc 1310nm.

C th kt lun chung rng cc u im nu trn ca si quang n modethch hp hn cho mng truyn hnh cp hu tuyn. Cn si quang a mode ch

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c th c lp t cho h thng phc v cho cc ngnh cng nghip khc c lyngn.

5.2.2 - Tnh ton suy hao ca h thng

Tng suy hao tuyn l suy hao cng sut trong mt tuyn si quang do ttc cc yu t bao gm: cc b ni, mi hn, suy hao si quang, cong ca cpSuy hao cng sut quang do cc b ni gn vi thit b quang c th b qua v n c tnh gp vo thit b. Tng suy hao tuyn phi nm trong phm vi cho

php ca thit b quang th h thng mi c th hot ng tt. iu ny c xcnh nh vic lp k hoch chi tit cc suy hao cng sut cho ton b h thngquang. Tt c cc yu t c lin quan hoc c th s lin quan n suy hao tuynu cn phi c tnh n.

Cc nh sn xut thit b quang thng khuyn ngh mt hoc nhiu loi

si quang c th s dng ph hp vi thit b ca h. Nhng loi si quang ny c th nghim vi thit b ca h trong mt cu hnh im - im tiu chuncho cc ch s di v suy hao cc i ca si quang. Thit b s hot ng ttnu ta s dng loi si quang c khuyn ngh trong cc gii hn v chiu div suy hao ca si.

Mt phng php dng xc nh tng suy hao tuyn l nh gi phn bsuy hao tuyn quang. nh gi ny lit k tt c cc yu t lin quan hoc s linquan n suy hao quang ca h thng. Kt qu s a ra tng suy hao tuyn yucu cho h thng si quang. Sau s so snh vi suy hao cc i ca thit b

xc nh liu thit k c t ch tiu v suy hao hay khng.1. Cc yu t nh gi h thng quang- Cc khuyn ngh v bn knh si quang (125 m)- Khuyn ngh v suy hao cc i ca si quang (0,35dB/km)- Khuyn ngh v m s cc i ca si quang (NA)- Khuyn ngh v bng tn si quang cc i (MHz*km) ti bc sng hot ng

(1310nm)- Khuyn ngh v chiu di cc i ca si quang- Cc thng s suy hao cc i ca thit b- nhy thu ca thit b

- Cng sut ra trung bnh ca b pht ca thit b- Di ng ca b thu

Nu suy hao cc i c cung cp v b thu c di ng l ton b dicng sut lm vic ca b pht th khng cn cc thng tin v nhy b thu vcng sut ra trung bnh ca b pht.Suy hao cc i = cng sut ra trung bnh b pht - nhy b thu

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2. T k hoch lp t si quang, cn xc nh- Tng chiu di tuyn si quang- S lng mi ni cn thit v suy hao ca mi mi ni- D tr cho thit k

- Cc suy hao quang do cc thnh phn khc trong h thng3. Kt lun phn b suy hao tuyn quang- Suy hao si quang ti bc sng hot ng: dB/km- Suy hao mi hn: s mi hn*dB/mi hn- Suy hao mi ni: s mi ni*dB/mi ni- Cc suy hao thnh phn khc- D tr cho thit k- Tng suy hao tuyn- Cng sut u ra trung bnh ca b pht- Cng sut u vo b thu

- Di ng ca b thu- nhy ca b thu- D tr cn li4. D tr cn li nn ln hn 0. Nu khng t c iu ny th cn phi xem

xt li suy hao ton tuyn gim suy hao ton tuyn.

5.3 Nguyn tc thit k phn mng quang

Mng quang l mng truyn dn khng th thiu trong tng th mng

truyn hnh cp xy dng theo cu hnh HFC hay HFPC nhm mc ch truyndn tn hiu quang t Trung tm n cc im t node quang (thc cht l bchuyn i quang in O/E) v trong tng lai l truyn dn tn hiu ngc dngt cc u cui thu bao (Modem cp, thoi, h thng bo chy, Set-top-box...).Tu theo quy m pht trin mng m mng quang c th c chia thnh nhiucp khc nhau, y ta ch xem xt v tnh ton quy hoch cho mt mng truynhnh cp c quy m va phi v ch s dng mt cp truyn dn quang v ch tnhton cho hng tn hiu ng xung (t Trung tm n my thu thu bao).

Vic quy hoch v pht trin mng quang gn lin vi c im a hnh,dn c v tnh hnh pht trin cc dch v gi tr gia tng trong tng lai v iu

ny lin quan n vic quyt nh s s dng cu hnh mng quang, mng ngtrc v h thng thit b ph hp vi tng cu hnh mng. V d Vit Nam, vics dng cu hnh HFC hay HFPC ph thuc rt nhiu vo mt bng quy hoch th . trin khai dch v truyn hnh cp trn cc tuyn ph trn a bn thnh

ph H Ni hay mt s thnh ph c s lng dn c ng c, h thng giaothng phc tp ng thi c s h tng, ng mng, ct in lc, ct n chiusng... khng n nh th vic trin khai mng theo cu hnh HFPC l khng c

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tnh kh thi m thch hp hn l trin khai mng theo cu hnh HFC. Do tnhkhng n nh nu trn cho nn s nh hng rt nhiu n mng quang dy ctrn a bn thnh ph (nu xy dng theo cu hnh HFPC) m thc t thi cng,khc phc s c mng quang cc k phc tp. Tuy nhin, hin nay cc khu vc

ngoi thnh vic quy hoch cc khu chung c th mi ang c trin khai mtcch rng ln v t s l ni rt thch hp trin khai mng truyn hnh cp vcc dch v gia tng trn mng cp theo cu hnh mng HFPC. Chi tit vic quyhoch, phn tch v tnh ton mt mng quang cho cc khu quy hoch ny s cxem xt k hn phn sau. Ni nh vy khng c ngha l ch cc khu quyhoch mi mi c th trin khai c mng theo cu hnh HFPC, m khu vcni thnh cng c th trin khai thnh cng nhng cn c thi gian v phi lp khoch trin khai tng bc (tc l nng cp dn mng HFC ln thnh mng HFPCda trn c s h tng quy hoch th ca thnh ph).

Cc s liu cn thit thit k c tng hp t nhiu ngun khc nhau,

bao gm cc s liu v tnh hnh kinh t chnh tr, mt dn c, trnh dn trv cc yu t khc lin quan n vic trin khai c hiu qu dch v truyn hnhv a hnh cng l iu kin quan trng lin quan n vic la chn cu hnhmng.

Xc s lng v nh v tr t node quang: S cc node quang c nhgi theo ton vng dn c v vng dch v thu bao thun li nht. nh gi nyc s dng lm c s xem xt cu trc mng. Do vy cn thit xc nh cvng dch v thu bao ti u, xem xt nh gi cho ng thut bao v nodequang. V tr t node quang ni thu bao tp trung l hiu qu nht. Vng

trung tm c xc nh ph hp vi s thng nht v kinh t v vn ho trongkhu vc. i vi cc khu vc thnh ph Vit nam hin nay, hu ht cc tuyncp ng trc sau node quang u ang trin khai treo trn cc h thng ct i

truyen hinh cap(font time new roman)

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