OFDM Report Upload

8/3/2019 OFDM Report Upload

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I HC QUC GIA TP H CH MINHTRNG I HC BCH KHOA

KHOA IN IN T

Ti Mn Hc Thng Tin S

TM HIU V OFDM

TP H Ch Minh, 10/2011

i Hc Bch Khoa TP.HCM


2/19

2

uluTng qut ti............................................................................................................................. 3

Gii thiu ........................................................................................................................... .......... .. 4

Ni dung ................................................................................................................... ..................... 6

1 Khi qut v gii thut OFDM ................................................................................................ 6

1.1 Tng quan vgiai thut OFDM ........................................................................... 6

1.1.1 Ph ng phap iu ch n song mang ....................................................... 71.1.2 Ph ng phap iu ch a song mang (FDM) ............................................. 81.1.3 Ph ng phap iu chtr c giao (OFDM) .................................................. 9

1.2 Tnh tr c giao (Orthogonality) ........................................................................... 111.3 S hthng OFDM ....................................................................................... 11

1.3.1 i u ch s d ng IFFT (Modulation using IFFT) .................................... 131.3.2 Kho ng th i gian b o v ........................................................................... 141.3.3 B cn b ng (Equalization) ...................................................................... 14

1.4 Thu n l i ............................................................................................................ 141.4.1 u im .................................................................................................... 151.4.2 Nh c im .............................................................................................. 15

2abALAB .................................................................................. 17

l .............................................................................................................. .................. 18

References ..................................................................................................................... .............. 19



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3

Tng qt ti

ti ch yu i vo tm hiu cc vn c bn cng nh cc khiaapppepsraOthogonl Fquncy Diviion Multilxing (OFDM). Tong qu tnh tm,araapppda

Tong ti ny chng t t tung vo hi vn u:

1Ga+Na+a+N

+N ii thut IFFT

+Cda

2ALAB



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Gii thiOFDM l mt hng h tuyn kh hc t tn knh vt l

nguyn l c bn c hng h l ng k thut ng mng tuyn mt lng ln k t ti cng mt thi im. S ng k thutOFDM c t nhiu u im l hiu qu ng h t co kh nngchng gio tho ng tt (c bit tong h thng khng y) v t lc b nhiu (nu mt knh tn b nhiu cc tn ln cn b bqu khng ng). goi tc tuyn Ulink v Downlink c ththy i ng bng vic thy i lng ng mng ng. Mt uim qun tng c h thng ng ng mng l cc ng mnging c th hot ng tc bit nh n n chu k c k t tngng c ko i . V nu mun tuyn vi tc l hng tiu bittn giy bng mt knh n chu k c mt bit hi nh hn micogiy. iu ny gy kh khn cho vic ng b v loi b gio tho

ng. u cng lng thng tin tn c ti cho ng mngchu k c mi bit c tng ln ln lc vic x l vn nhthi ng n gin hn.

K thut iu ch OFDM o R.W hng ht minh nm 966 M.Tong nhng th k v qu nhiu cng tnh kho hc v k thut ny c thc hin kh ni tn th gii. c bit l cng tnh khohc c Witin v Ebt chng minh ng h iu ch OFDM cth thc hin c thng qu h bin i IDFT v h gii iu chOFDM c th thc hin c bng h bin i DFT. Pht minh nycng vi ht tin c k thut lm cho k thut iu ch OFDM

c ng ng t nn ng i. Thy v ng IDFT ngi t c th ng h bin i IFFT cho b iu ch OFDM ng FFT cho b giiiu ch OFDM.

th ni mng intnt bng ng DS (ymmtic DigitlSubcib in) t qun thuc Vit m nhng t ngi bit ng nng co tc ng tuyn tong h thng DS chnh l nh cngngh OFDM. h k thut iu ch ng mng v cho h chngh gi cc ng mng m tc tuyn n tong h thng DS tngln mt cch ng k o vi cc mng cung c ch v intnt thngthng.

n cnh mng cung c ch v DS hin ng c ng tng i Vit m hin ny cc h thng thng tin v tuyn nh mngtuyn hnh mt t DVT cng ng c khi thc ng. c hthng ht thnh nh D v DRM chc chn c khi thc ng tong mt tng li khng x. c mng v thng tin my tnh khngy nh Hi/ IEEE 80. g cng c khi thc mt cchng i Vit m.



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gy ny k thut OFDM cn kt h vi cc hng h mknh ng tong thng tin v tuyn. c h thng ny cn c gi vikhi nim l OFDM (o OFDM). Tong cc h thng ny tn hiutc khi c iu ch OFDM c m knh vi cc loi m khcnhu vi mc ch chng li cc li ng tuyn. Do cht lng knh (

fing v t l tn hiu tn t m) c mi ng mng h l khcnhungi t thc hin iu ch tn hiu tn mi ng mng vi cc mc iuch khc nhu. H thng ny m khi nim v h thng tuyn n ng k thut OFDM vi b iu ch tn hiu thch ng (tivmoultion tchniqu). K thut ny hin c ng tong h thngthng tin my tnh bng ng Hi/ hu u. Tn th gii hthng ny c chun h tho tiu chun IEEE.80..



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Ni dng

1 Khi qt v gii tht OFDM

1.1 aiu ch l qu tnh chuyn i mt tn hiu thng tin lm vic vi

n ng hn. Tong lnh vc vin thng tn hiu iu ch th cchn mng mt thng nht nh nh l: ch thc thi hc t(imlmnttion comlxity) tc liu cho h (uot tt) v nh hng c nhiu cng nh kh nng chng nhiu cknh tuyn (obutn gint chnnl n noi ffct).

iu ch ng mng l mt thy th h n o vi iu ch bngthng ng n ng mng tn cc knh c mo tn chn lc. tn tng chi tng bng thng n thnh nhiu bng thng conh hn m vic chuyn knh lin tc xut hin tn mi bng thngthng con. ng cch lm nh vy cn thit hi cn bng cc vngthi gin c loi b. Thy vo cc bng thng con hi hn cchtho khu vc tn khc nhu. hnh bi th vic ng ng bin i FFT(Ft Foui Tnfom) c ng.

K thut iu ch th tn m hnh OFDM. hn u ni chitit v thng tin iu ch nhiu ng mng con bng cch ng mtm hnh iu ch c bn cc b tuyn nhn hc t v cung c nhngkh nng cho vic gim ti knh tuyn.

OFDM l m hnh hn chi nhiu tn (fquncy-iviionmultilxing FDM) ng nh l hng h iu ch ngmng. Mt lng ln cc ng mng con tc gio vi khong cchgn nhu c ng mng liu i. D liu c chi thnh cclung liu ong ong mt lung liu tng ng cho mi ng mngcon. Mi ng mng con c iu ch vi m hnh iu ch thngthng(chng hn nh iu ch bin cu hng(Qutu-QM) hoc iu ch h (Ph Shift Kying-PSK) ) tc th v uy t tc ton liu tng t vi m hnh iu ch n ng mng thngthng tn cng mt bng thng. Eo: Rfnc ouc not foun chothy vic ng bng thng n c cho mt 7 tn hiu OFDM ng mng

con.



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Hnh 1.1 Bng thng s dng trong tn hi OFDM

1.1.1 ppma

rpsa,drrB,asard

plama a Eo: Rfnc

ouc not foun, r rsa0rr,psrsarsr,dSs



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rpppsau:

arappar

aspsr

adrppsrpasa

1.1.2 ppama()

rr,pppasaraapppsaasaa

3plaamappasappp

eppas,rrpaaN=2L+1ssapr

darasaNsdrsa

sarrdadrasaNssa



9/19

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rarrdsCppaCappp

asaspppsasa

a aasar

sdarap

ppa

pppasa

paa

sdda,saarar

ppasasdasppp,ppassdpaaapppasa,

pppasarara

1.1. ppa()

rpardasa,daapprsardsdsprsa,ddr,asa,prs

asasadsa asar dssr,saNsasaNepsNpprspasaradapepa



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sasaasasarasaNsrapapeppasdpapasa

4ky

Eo: Rfnc ouc not founsaaasapNrapep5%aprsaasa,aasraasaa

5aaa1;a4

Eo: Rfnc ouc not founara,rpdsCprsasaaa



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raapdpaa

1.2 Tnh trc gio Orthogonit

Mt tong cc nhn t chnh c OFDM l cc ng mng con cchn hi tc gio ln nhu. iu c ngh l ti v t tn tung tmc ng mng k l fk th cc bin c cc ng mng con khc th btit tiu. (Eo: Rfnc ouc not foun). Vic ny th tun t cho cctn tung tm c cc ng mng con tng ng tnh v bo vvic cc ng mng chng ln nhu khng cn thit. iu ny gi vhovic thit kt b tuyn v nhn t nn n gin h tong vn hnchi cc b lc cho mi bng thng con

Vn tc gio yu cu khong cch tn gi cc ng mngl f /T Hz (Eo: Rfnc ouc not foun) vi T () l khong thi

gin mu ng v l nguyn ng (thng l ). h vy t c K.f Hz l tng tn cc ng mng con

Mt tong nhng thun li nht c tnh tc gio gi cc ngmng l cho h ng hiu qu co v h tn khi m tt c ccbng tn c ng

Mt iu khng thun li l kt qu c vic ng tnh tc gioth cn chnh xc co cho vic ng b tn gi my thu v my

ht. S hn chi tn m h thng OFDM c th m nhn l t nh.i vy khi m cc ng mng khng cn tc gio ny inh hintng chng ln ng mng hy cn gi l o-tlk.

lch tn (fquncy offt) l nguyn nhn chnh c hintng ch tn Dol hy cn gi l khng tng ng o ng cb nhn v b thu. Tong khi vic ch tn Dol c th khc hc bib nhn th tnh hung t nn ti t hn khi c kt h tuyn ng. Vic i g t nhiu kh khn.

1.



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6

S h thng OFDM c cho nh Eo: Rfnc ouc not

foun. my ht chui liu ni ti qu b S/P c bin i thnh chui con ong ong mi chui ny qu mt b iu ch. ng ccb iu ch t thu c mt chui hc D 0 D D- tong Dk k jk. hui hc ny i vo b IFFT.

[ ] [ ] [ ]

=

=

==1

0

1

0

2

.1

2

.1 N

k

N

k

nt

kfj

ekDN

nN

kjekD

Nnd

(.3_)

(o nksks

k tfnTfnf

fn

N

k=== vi T l chu k k hiu fk l tn cc ng

mng)

g b IFFT l cc mu i c c k hiu OFDM tong minthi gin.

})fsin2fcos2).(({ReN

1{d[n]}Re)( kk

1

0

nnkk

N

k

tjtjBAny ++==

=

=

=1

0

kk )fsin2f2cos(1

N

k

nknk tBtAN

(.3_)



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c mu y(n) ny c chn thm khong bo v cho qu b bini D/ t thnh tn hiu lin tc y(t) c khuch i ln tn co i ht ln knh tuyn.

=

+=1

0

kk )fsin2fcos2(N

1)(

N

k

kk tBtAty (.3_3)

Tong qu tnh tuyn tn cc knh c cc ngun nhiu gynh hng nh nhiu uin tng cng W.

my thu t lm qu tnh ngc li: Tn hiu OFDM c i tnxung bin i /D loi b khong bo v i c vo b FFT. Su gii iu ch bin i t ong ong ng ni ti thu li chui liu bn u.

[ ] [ ]n

N

kjN

n

endkD21

0

.

=

= (.3_)

1.3.1 iu ch s dng IFFT (Modulation using IFFT)h bit OFDM l mt k thut iu ch ng mng tong

liu c tuyn ong ong nh v ng mng h mng cc bitthng tin. ng cch ny t c th tn ng bng thng tn hiu chng linhiu gi cc k t....Tuy nhin iu bt li l mt ng mng cn cmt my ht ng in mt b iu ch v gii iu ch c ing n iuny l khng th ch nhn c khi ng mng h t ln i vivic thi cng h thng. hm gii quyt vn ny thut ton IDFT/DFTc vi t ging nh hng lot cc b iu ch v gii iu ch.

i tn hiu x(n) c chiu i l (n 0 -). ng thc

c h bin i DFT l[0]

=

=

1

0

2

)()(N

n

jNkn

enxkX

k 0 - (.3._)

- ng thc c h bin i IDFT l

=

=1

0

12

)()(N

k

j

NNkn

ekXnx

k 0 - (.3._)

- huyn i Foui nhnh (FFT) l thut ton gi cho vic tnh tonDFT nhnh v gn hn.T cng thc (.3._) (.3._) t thy thi gintnh DFT bo gm: Thi gin thc hin h nhn hc. Thi gin thc hin h cng hc.

Thi gin c cc h Nje2 .

Thi gin tuyn liu.



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Tong ch yu l thi gin thc hin h nhn hc. V vy mungim thi gin tnh ton DFT th ngi t t tung ch yu vo vic gimthi gin thc hin h nhn hc. M thi gin thc hin h nhnhc t l vi h nhn. Do gim thi gin tnh DFT th ngi thi gim c lng h tnh nhnh bng cch ng thut ton

FFT. tnh tc ti cn 2N h nhn. Khi tnh bng FFT h nhn

ch cn NN

2log2

. V vy tc tnh bng FFT nhnh hn tnh tc ti l

N

N

2log

2.

goi FFT cn c u im gi tit kim b nh bng cch tnh tich.

1.3.2 Khong thi gian bo v

Mt tong nhng li th c cc m hnh OFDM l k t khi cc liu tong mi khi c chi thnh K lung k t tuyn ong ongvi tc k t th h thng c th t nn mnh m hn cho vn chngchng ln k t (Int-Symbol Intfnc - ISI) gy bi cc tuynthng ng (multith chnnl).

Khi khong thi gin c mi k hiu i hn tong mt tc lungn vic chn mt khong thi gin bo v gi cc khi OFDM (hoc khiu) l kh thi o lm gim ISI c th. goi khong thi gin bov cng loi b cn thit cho b lc ng ung v n hn ch cc vn ny inh v ng b thi gin.

1.3.3 B cn bng (Equalization)S cn thit hi cn bng v min thi gin hc t th c tnh

tong vic tin khi h thng OFDM. Thc t lng K knh bng hc ng cung c cho cc k t gi gim thiu nh hng ccc iu kin chn lc tn cho knh tuyn. hng hn nh fing gy bi tuyn ln. Tong thi hn bng thng mt ng mng con cxm nh l l lin tc nu lng ng mng con K l ln o lm cn bng knh n gin hn nhiu o vi tong cc h thng n ngmng.

u c ht hin khc bit hoc iu ch khc (nh DPSK hocDQPSK) c ng cho cc ng mng con b cn bng c th honton b qu khi cc m hnh khng nht qun th khng th ng vivic thy i chm mo ng v h v bin

1. Thn i



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Qu bn cht c OFDM t c th tm tt nhng u im vnhc im c OFDM nh u:

1.4.1 i

OFDM tng hiu ut ng bng cch cho h chng lnhng ng mng con.

ng cch chi knh thng tin thnh nhiu knh con finghng bng h cc h thng OFDM chu ng fing l chntn tt hn nhng h thng ng mng n.

OFDM loi t nhiu ISI v xuyn nhiu gi cc ng mng(II) bng cch chn thm vo mt khong thi gin bo vtc mi ymbol.

S ng vic chn knh v m knh thch h h thng OFDMc th khi hc li c cc ymbol b mt o hin tng lchn tn c cc knh.

K thut cn bng knh t nn n gin hn k thut cn bngknh thch ng c ng tong nhng h thng n ngmng.

S ng k thut DFT b ung vo cc chc nng iu chv gii iu ch lm gim chc nng hc t c OFDM.

c hng h iu ch vi i (iffntl moultion) gitnh yu cu vo b ung b gim t knh.

OFDM t b nh hng vi khong thi gin ly mu (mltiming offt) hn o vi h thng n ng mng.

OFDM chu ng tt nhiu xung vi v nhiu xuyn knh kt

h.goi nhng u im tn th OFDM cng c nhng hn ch.

1.4.2Nhc i

Symbol OFDM b nhiu bin vi mt khong ng ln. V ttc cc h thng thng tin thc t u b gii hn cng ut t PRR co l mt bt li nghim tng c OFDM nu ng bkhuch i cng ut hot ng min bo h u khuch itn hiu OFDM. u tn hiu OFDM t PRR ln hn th gy nn nhiu xuyn iu ch. iu ny cng tng hc

t c cc b bin i t nlog ng igitl v t igitl ngnlog. Vic t ngn (cliing) tn hiu cng lm xut hin cmo nhiu (itotion) tong bng ln bc x ngoi bng.

OFDM nhy vi tn offt v tt c ng mng hn cch thng n ng mng. Vn ng b tn tong h thngOFDM hc t hn h thng n ng mng. Tn offt cng mng gy nhiu cho cc ng mng con tc gio v gy



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nn nhiu lin knh lm gim hot ng c cc b gii iu chmt cch tm tng. V vy ng b tn l mt tong nhngnhim v thit yu cn hi t tong b thu OFDM .



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2bL



18/19

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l

-C Na

M hng thnh cng mt h thng OFDM vi nhiu gi ngunhin.-c im tn ti: Cap h hin thc c h thng OFDM tng ng nh m

hng. Cassda

-Hng ht tin ti:D vo nhng g thc hin c t c th chi lm hi hng

ht tin ti nh u:

arV

dddrrd,drWAX,drdCA

CAd



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References

[] Shinuk H & Rmj P tch- MulticiTchniqu fo Mobil ommuniction

[] NV-NBKKLdaK

[3] Phn Minh Dc K Thut Sng Mng Tc iowww.book.u.vn

[] Phm Quc Hng ng gh OFDM V ng Dng TongTuyn Hnh S Mt tun Vn Tt ghi_Kho in in T_iHc K Thut ng gh TP.HM -003

[5] T.S inh c nh V in i Foui hnh FFT hng 6 Sli bi ging mn hc x l tn hiu

i Hc Bch Khoa TP HCM
http://www.ebook.edu.vn/http://www.ebook.edu.vn/

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