1

Tiu Lun

OFDM v ng dng trong truyn hnh s

mt t DVB-T.

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MC LC

1- Gii thiu tng quan v h thng OFDM..........Trang 4

2- L thuyt v knh v tuyn ...........................Trang 7

3- ng dng OFDM trong truyn hnh s mt t DVB-T...Trang31

4- Kt qu v nhn xt...Trang 40

5- Kt lun.Trang 44

6- Ph lc ..Trang 46

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LI M U

K thut iu ch a sng mang trc giao l mt trong nhng lnh vc

c nghin cu hng u hin nay. K thut ny ang c nghin cu,

trin khai v ng dng vo nhiu lnh vc khc nhau nh: Wimax, Wlan,

ADSL, DVB-Tvv. K thut ny cng l ng c vin quan trng nht

cho th h di ng th 4 (4G). Vit Nam K thut Ofdm c ng

dng thnh cng trong lnh vc ADSL v cng ang c trin khai th

nghim trong nhiu lnh vc khc nh: DVB-T, Wimaxvv.

V vy chng em quyt nh chn OFDM l ch nghin cu

thc hin bi tp ln mn h thng v tuyn. Bn cnh OFDM chng em

cng tp trung nghin cu mt lnh vc tng i in hnh cho ic ng

dng k thut OFDM vo thc t l Truyn hnh s mt t DVB-T.

Chng em hi vng s nhn c nhiu kin ng gp qu bu t

pha thy c gio v cc bn sinh vin ti ngy cng c hon thin.

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PHN 1: GII THIU TNG QUAN V H THNG OFDM

(Overview about OFDM)

1.1 Lch s pht trin

Trong nhng nm gn y, Phng thc ghp knh phn chia theo tn

s trc giao OFDM (Orthogonal Frequency Division Multiplexing) khng

ngng c nghin cu v m rng phm vi ng dng bi nhng u im

ca n trong tit kim bng tn v kh nng chng li Fading chn lc theo

tn s cng nh xuyn nhiu bng hp.

K thut iu ch OFDM l mt trng hp c bit ca phung php

iu ch a sng mang trong cc sng mang ph trc giao vi nhau, nh

vy ph tn hiu cc sng mang ph cho php chng ln ln nhau m pha

thu vn c th khi phc li tn hiu ban u. S chng ln ph tn hiu lm

cho h thng OFDM c hiu sut s dng ph ln hn nhiu so vi cc k

thut iu ch thng thng. Nh OFDM l chia dng d liu tc cao

thnh cc dng d liu tc thp hn v pht ng thi trn mt s cc

sng mang, ta thy rng trong mt s iu kin c th, c th tng dung

lng ng k cho h thng OFDM bng cch lm thch nghi tc d liu

trn mi sng mang tu theo t s tn trn tp SNR ca sng mang .

K thut OFDM do R.W Chang pht minh nm 1966 M. Tri qua

40 nm hnh thnh v pht trin nhiu cng trnh khoa hc v k thut ny

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c thc hin khp ni trn th gii. c bit l cc cng trnh ca

Weistein v Ebert, ngi chng minh rng php iu ch OFDM c th

thc hin bng php bin I IDFT v php gii iu ch bng php bin i

DFT. Pht minh ny cng vI s pht trin ca k thut s lm cho k thut

iu ch OFDM c ng dng rng ri. Thay v s dng IDFT ngi ta c

th s dng php bin i nhanh IFFT cho b iu ch OFDM, s dng FFT

cho b gii iu ch OFDM.

1.2. S ng dng ca k thut OFDM

1.2.1. Cc ng dng quan trng ca OFDM trn th gii

K thut OFDM l nn tng ca cc k thut truyn dn v tuyn. Cc ng

dng c th ca OFDM trn th gii.

H thng truyn hnh s mt t DVB-T (digital video broadcasting for

terestrial transmission) (1995)

H thng pht thanh s ng di DRM ( Digital Radio Mondiale)

Truy cp internet bng thng rng ADSL ( Asymmetric Digital Subscriber

line)

Cc chun IEEE 802.11a (1999) IEEE 802.11g.

Mng my tnh khng dy vi tc truyn dn cao HiperLAN/2 (High

Pefomance Local Area NetWork type 2)(2000)

c bit OFDM l ng c vin trin vng nht cho h thng thng tin 4G (

h thng truy cp Internet khng dy bng rng theo tiu chun Wimax )

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Hnh1.1. Cc ng dng ca OFDM

1.2.2 ng dng hin ti ca k thut OFDM Vit Nam

Bn cnh mng cung cp dch v Internet ADSL, hin c ng

dng rt rng ri Vit Nam, c h thng thng tin v tuyn nh mng truyn

hnh mt t DVB-T cng ang c khai thc s dng. Cc h thng pht

thanh s nh DAB v DRM chc chn s c khai thc s dng trong tng

lai khng xa. Cc mng v thng tin my tnh khng dy nh hiperLAN/2,

IEEE 802.11a, g cng s c khai thc mt cch rng ri Vit Nam. Hin

ti trong thng tin di ng c mt s cng ty Vit Nam th nghim Wimax

ng dng cng ngh OFDM nh VDC, VNPT.

1.3 Cc hng pht trin trong tng lai

K thut OFDM hin c c lm phng php iu ch s dng

trong mng thng tin thnh th bng rng Wimax theo tiu chun

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IEEE.802.16a v cc h thng thng tin di ng th 4 (4G). Trong h thng

thng tin di ng th 4, K thut OFDM cn kt hp vi cc k thut khc

nh k thut anten pht v thu (MIMO technique) Nhm nng cao dung lng

knh v tuyn c kt hp vi cng ngh CDMA nhm phuc v a truy cp

ca mng. Mt vi hng nghin cu vi mc ch thay i php bin FFT

trong b iu ch OFDM bng php bin I Wavelet nhm ci thin s nhy

cm ca h thng i vi hiu ng dch tn do mt ng b gy ra v gim

di ti thiu ca chui bo v trong h thng OFDM. Tuy nhin kh nng ng

dng ca cng ngh ny cn c kim chng.

Phn 2: L thuyt v knh v tuyn

2.1 T iu ch n sng mang n iu ch trc giao OFDM

2.1.1 Phng php iu ch n sng mang

BbbbbBb

bbbbbbbdfgfgfgfgfggfgfgg

Hnh 2.1. Biu din tn hiu trong min thi gian

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Hnh 2.2 H thng n sng mang

Hnh 2.2 H thng n sng mang

Vi mu tn hiu c chu k l T th tn hiu s chim ton b bng thng l

1/T.

Trong phng php iu ch n sng mang, dng tn hiu c truyn i trn

ton b bng tn B, c ngha l tn s ly mu ca h thng bng rng bng

tn v mI tn hiu c di l

T sc= 1/B PT(2.1)

Trong thng tin v tuyn bng rng, knh v tuyn thng l knh ph

thuc tn s (frequency selective channel). Tc ly mu thn tin bng

rng s rt ln, do chu l ly mu Tsc s rt nh. Do phng php iu

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ch n sng mang c nhng nhc im sau:

nh hng ca nhiu lin tn hiu ISI gy ra bi hiu ng phn tp

a ng i vi tn hiu thu l rt ln.iu ny c gii

thch do di ca 1 mu tn hiu Tsc l rt nh so vi trng

hp iu ch a sng mang. Do vy nh hng ca tr truyn

dn c th gy nhiu lin tn hiu ISI nhiu mu tn hiu

thu. C 5 loi nhiu trong thng tin v tuyn

Gaussian Noise

Interchannel Interference

Co-channel Interference

Inter-symbol Interference

Multiple Access Interference

nh hng ca s ph thuc knh theo tn s l rt ln i vi h

thng. Do bng thng rng knh ph thuc vo tn s

Hai l do nu trn lm cho b cn bng knh v lc nhiu my thu

l phc tp.

Phng php iu ch n sng mang hin nay vn c s dng ch yu

trong thng tin bng hp nh h thng thng tin di ng ton cu GSM. Trong

thng tin bng rng, phng php iu ch a sng mang ra i ci thin

cc nhc im trn.

2.1.2 Phng php iu ch a sng mang FDM

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Hnh 2.3 Mt ph ca tn hiu a sng mang

Hnh 2.4 H thng a sng mang

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Phng php iu ch a sng mang c hiu l ton b bng tn

ca h thng c chia ra lm nhiu bng con vi cc sng mang ph cho

mi bng tn con l khc nhau. Chi tit ca phng php ny xem hnh

2.2.

Phng php iu ch a sng mang cn c bit nh phng

php phn knh theo tn s FDM, trong ph ca tn hiu ca h thng

chia lm Nc = 2L+1 knh song song. V vy di ca mu tn hiu trong

iu ch n sng mang :

Ts=1/Fs=Ts.Nc PT(2.2)

H qu l t s tng i gia tr truyn dn i vi di mu

tn hiu trong iu ch a sng mang cng gim i Nc ln. do vy nh

hng ca nhiu lin tn hiu gy ra bi tr truyn dn s gim ( gim nh

hng ca phn tp a ng). T chng ta c th nu ra mt s cc u

im c bn ca iu ch a sng mang so vI cc phng php iu ch

n sng mang l:

nh hng ca nhiu lin tn hiu ISI (Inter-symbol Interference)

gim

nh hng ca s ph thuc knh vo tn s gim do knh c

chia lm nhiu phn ( Bng thng gim-> B

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Tuy nhin phng php ny cn mt s nhc im c bn sau

H thng nhy cm vi hiu ng ph thuc thi gian ca knh

(time selectivity). iu ny c bit n l do di ca mt

mu tn hiu tng ln ( T tn hiu tng ln-> T>Tc -> knh

ph thuc thi gian). Dn n s bin i v thi gian ca

knh v tuyn c th xy ra trong mt mu tn hiu.

Phng php iu ch a sng mang khng lm tng hiu qu s

dng bng tn ca h thng so vi phng php iu ch n tn, ngc li

nu cc knh ph c khong cch nht nh th s lm gim hiu qu s

dng ph. va tn dng ht bng tn v c c cc u im ca iu

ch a sng mang -> ngi ta s dng phng php iu ch OFDM vi cc

sng mang ph trc giao nhau.

2.1.3 Phng php iu ch a sng mang trc giao OFDM

a)FDM b)OFDM

OFDM l mt trng hp c bit ca FDM ( Frequency division

Multiplex). C th hnh dung knh FDM ging nh dng nc chy trong

vi nc hnh a). Cn OFDM ging nh nc chy trong vi hoa sen.

Trong vi nc hnh a) nc s chy thnh nhng dng ln nhng khng

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th chia nh. Cn vi hoa sen (OFDM) li c th chia thnh nhiu dng nh.

Chng ta c th t ngn tay dng dng nc vi hnh a) nhng khng

th lm nh vy vi vi hnh b). V vy tuy c hai lm nhng cng vic khc

nhau nhng p ng vi nhiu ca chng l khc nhau.

Mt cch khc nhn trc quan. Gi s chng ta vn chuyn mt kin

hng bng xe ko. C hai cch. Cch th nht chng ta vn chuyn ht kin

hng trong mt chic xe. Cch th hai chng chia kin hng thnh nhng

phn nh rI mang i trn nhiu chuyn xe khc nhau. C hai u mang

chnh xc mt lng d liu. Nhng trong trng hp xy ra tai nn, ch

d liu trong kin hng OFDM b hng.

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Hnh 2.5 Biu din tn hiu OFDM trong min tn s

Hnh 2.6 Biu din tn hiu OFDM trong min thi gian

H thng OFDM l h thng s dng nguyn l ghp knh phn

chia theo tn s trc giao, hot ng trn nghuyn l pht d liu bng cch

phn chia lung d liu thnh nhiu lung d liu song song c tc bt

thp hn nhiu v s dng cc lung con ny iu ch sng mang vi

nhiu sng mang con c tn s khc nhau. Cng nh cc h thng a sng

mang thng thng, h thng OFDM phn chia di tn cng tc thnh cc

bng tn con khc nhau cho iu ch, c bit tn s trung tm ca cc bng

con ny trc giao vi nhau v mt ton hc, cho php ph ca cc bng con

chn ln nhau tng hiu qu s dng ph tn m khng gy nhiu.

Nguyn l c bn :

Knh vi bng thng ln s c chia lm nhiu knh ph gim

nhiu ISI v fading theo tn s.

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Mi sng mang phu s trc giao vi nhau trong min tn s. S lm

tng hiu qu s dng knh.

Hnh 2.7 : u im ca OFDM so vi iu ch n sng mang

l hn ch nhiu lin knh ISI

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FDM vi 9 sng mang ph s dng b lc

OFDM vi 9 sng mang ph

Hnh 2.8 : u im ca OFDM so vi FDM

l tng hiu qu s dng knh

a)Tn hiu gi trong knh khng ph thuc vo tn s

b) Tn hiu gi trong knh fading theo tn s

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c) Vi tn hiu OFDM gi trong knh fading theo tn s

Hnh 2.9 nh hng ca knh i vi tn hiu ofdm

u im chng li fading theo tn s

Khi knh c p ng tn s tt tn hiu c th truyn qua.Khi knh fading

vi mt vi tn s( knh la chn tn s) th tn hiu khng th i qua .Vi

OFDM chng ta c rt nhiu sng mang ph v vy ch mt lng nh sng

mang ph mt d liu do fading

2.2 Phng php iu ch OFDM

2.2.1 Khi nim v s trc giao ca 2 tn hiu

V mt ton hc xt tp hp cc tn hiu vI p l phn t th p

ca tp, iu kin cc tn hiu trong tp trc giao vI nhau i mt l

,*( ) ( )

0,

a k p qt t dt

p q p qb

PT(2.3)

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Trong *( )t

q l lin hp phc ca ( )t

p . Khong thI gian

t a n b l chu k ca tn hiu, cn k l hng s.

2.2.2 B iu ch OFDM

Da vo tnh trc giao, ph tn hiu ca cc sng mang ph cho php

chng ln ln nhau. S chng ln ny lm cho hiu sut s dng ph ca

ton b bng tn tng ln mt cch ng k

Hnh 2.4. Ph ca mt sng mang OFDM con v ca tn hiu OFDM

S trc giao ny thc hin nh sau: ph ca tn hiu sng mang ph th p c dch vo mt knh con th p thng qua php nhn vi hm m

ip tse

. Trong s = 2 fs l khong cch tn s gia 2 sng mang. Thng qua php nhn vI s phc ny m cc sng mang ph trc giao vI nhau. Tnh trc giao ca 2 sng mang ph c kim chng nh sau:

( )( 1) ( 1)*( )

j p qk Ts k Ts sip t iqs t

kTs kTs

e e dt e dt

1( ) ( 1)

( )

j p q t t k Tsst kTs

j p qe

0, ,p qTs p q PT(2.5)

phng trnh trn ta thy hai sng mang ph p va q trc giao vi nhau do tch phn ca mt sng mang vi lin hip phc ca sng mang cn li

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bng 0 nu chng l hai sng mang khc bit. Trong trng hp tch phn vi chnh n s cho kt qu l mt hng s. S trc giao ny l nguyn tc ca php gii iu ch OFDM.

Hnh 2.10 : B iu ch OFDM

Cc bc thc hin iu ch tn hiu OFDM

Chuyn i dng bt ni tip thnh dng bit song song

Chuyn i dng bit thnh tn hiu phc

Tin hnh iu ch sng mang ph

Nhn vi hm phc jn tse

To khong bo v

2.2.2.1 Xt khi th nht

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Hnh 2.11 Khi bin i serial to parallel ri iu ch s

Khi ny c nhim v bin i mt chui tn hiu ni tip thnh cc

chui tn hiu di dng song song. Khi ny thc hin chc nng ging vi

iu ch FDM. hiu r hn ta xt mt vi d.

Trong OFDM c N sng mang , N c th l bt c gi tr no trong

khong t 16 n 1024 ty thuc vo mi trng m h thng ang s dng.

Chng ta tin hnh th nghim truyn bit m chng ta mun truyn

bng vic s dng cng ngh OFDM vI 4 sng mang ph. Tn hiu c tn

s ly mu l 1sample/ 1 symbol.

Hnh 2.12 Dng bit m c iu bin s dng 4 sng mang

Nhng bit u tin l : 1,1,-1,-1,1,1,1,-1,1,-1,-1,-1,-1,1,-1,-1,-1,1..