nghien cuu su ket hop giua cong nghe cdma va ofdm

8/8/2019 Nghien Cuu Su Ket Hop Giua Cong Nghe CDMA Va OFDM

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Chng 2: iu ch a sng mang trc giao ghp theo tn s

n mn hc:Truyn thng s nng cao 5

CHNG 2: CNG NGH OFDM

2.1. GII THIU CNG NGH OFDM

OFDM l mt trng hp c bit ca truyn dn a sng mang. OFDM cn c tn gikhc l iu ch a sng mang trc giao (OMCM - Orthogonal Multicarrier Modulation).OFDM c th gi l k thut iu ch hay k thut ghp, da trn nguyn tc phn chia lung d

liu tc cao thnh nhiu lung d liu tc thp, truyn trn nhiu sng mang trc giaonhau.

Trong h thng a knh (Multi-Channel) c s dng FDM, tt c bng thng c sn cah thng s c chia nh thnh N knh con c tn s con khng trng lp nhau. Mi knh conny c m ho vi cc lung symbol ring bit v N knh con ny c ghp knh theo tn sli vi nhau. Mc d vic ngn chn vic chng ph ca cc sng mang con gim (hay loitr) nhiu xuyn knh ICI (Inter-Channel Interference) n dn n gim hiu qu s dng ph.Bng tn bo v hai bn ca sng mang con s lm lng ph ngun bng thng qu bo ca hthng. khc phc vn lng ph bng thng, chng ta s dng N sng mang con khng trnglp nhau (trc giao), tc baud ca mi sng mang con l 1/T v khong cch mi bn l 1/T.Bi v khong cch cc sng mang con c la chn, cc sng con s trc giao nhau v mtton hc. iu ny s cho php vic gii iu ch lung d liu chnh xc m khng cn i his tch bit ph. Mt cch khc th hin iu kin trc giao gia cc sng mang con l mi sngmang con cn c mt s nguyn chu k trong mt khong thi gian T. M ho cc sng mangtrc giao c th biu hin bng vic bin i IFT (Inverse Fourier Transform). S dng bin iDFT v cc b lc thng thp s to ra c tn hiu OFDM.

2.2. TNH A SNG MANG.

Tn hiu c truyn i trn nhiu sng mang, mi sng mang ti mt phn d liu c chv tri u trn c bng thng th khi c mt d liu mi knh s ch mt mt phn nh khng nh hng n d liu trn knh. Tuy nhin khng c can nhiu gia cc sng mang cn phic khong tn bo v, chng lm gim i hiu qu s dng ph. Nu s dng cc sng mang trc

giao th khong cch gia cc sng mang c th gim n mc cho php chng chng ph lnnhau, tit kim khong 50% di thng.

Hnh 2.2.1: Hiu qu s dng ph trong FDM(a) v OFDM(b)


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2.3. TNH TRC GIAO (Orthogonal)

Thut ng trc giao y ch ra rng c mt mi quan h ton hc chnh xc gia cctn s ca cc sng mang trong h thng OFDM. Trong h thng FDM, cc sng mang ccch nhau mt khong ph hp tn hiu thu c th nhn li bng cch s dng cc b lc. Khi, cc khong bo v cn c d liu trc gia cc sng mang v vic ny lm gim hiu qus dng ph ca h thng. Tuy nhin, trong OFDM c th sp xp cc sng mang sao cho cc di

bin ca chng che ph ln nhau m cc tn hiu vn c th thu c chnh xc m khng c scan nhiu gia chng. Mun c nh vy cc sng mang phi trc giao v mt ton hc. Mythu OFDM gm cc b gii iu ch, dch tn; tn hiu nhn c ly tch phn trn mt chu kca symbol phc hi d liu gc. Nu v l do g m tt c cc sng mang khc u c dchxung tn s tch phn ca sng mang ny (trong mt chu k symbol ), th kt qu tnh tch phncho cc sng mang khc s l zero. c gi l trc giao, cng c ngha l cc sng mang clp tuyn tnh vi nhau. iu ny t c nu khong cch gia cc sng l bi s ca 1/. Btk s phi tuyn no gy ra bi can nhiu bi cc sng mang ICI (Inter-Carrierinterference) cnglm mt i tnh trc giao.

V mt ton hc, trc giao c ngha l cc sng mang c ly ra t nhm trc chun(Orthonomal basis ) {{ (t) / i = 0,1,...} c tnh cht sau ;

_ a _ aTtmTtnki

kidttt

ti

ikti

T

T

/2cos,/2sin

,0

,1)()(

)(

2

1

TT

H

!*

{

!!!** (2.1)

Khong cch gia 2 sng mang trc giao cnh nhau s l f=1/ T.

Nu biu din di dng phc

{

!!!==

ki

kidttt ik

T

T

ki

,0

,1)()(

2

1

* H (2.2)

y du (*) ch s lin hip phc.

Vic x l tn hiu OFDM c thc hin trong min tn s.

Khi nim trc giao xut pht trong ton hc, khi hai vect vung gc vi nhau v tchca chng bng khng th c gi l hai vect trc giao. Ta xt vic nhn hai hm s ca sngmang vi nhau, hm sng sin tun hon c gi tr trung bnh trong mt chu k bng 0; trong tonhc th hin l hm tch phn.

Zk

dtt

2

0

0)sin(

(2.3)sint

[t

Hnh 2.3.1: Dng sng hnh sin

0


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Khi nhn hai sng sin khc tn s v cng tn s, ta c:

Nu hai sng sin c cng tn s nh nhau th dng sng hp thnh lun dng, gi trtrung bnh ca n lun khc khng (hnh trn). y l c cu rt quan trng cho qu trnh giiiu ch OFDM. Cc my thu OFDM bin i tn hiu thu uc t min tn s nh dng k

thut x l tn hiu s gi l bin i nhanh Fourier (FFT). Vic gii iu ch cht ch cthc hin k tip trong min tn s bng cch nhn mt sng mang thu c vi mt sng manguc to ra trong my thu c cng chnh xc tn s v pha. Sau php tch phn, tt c cc sngmang khc s bng 0 ngoi tr sng mang c cng tn s vi sng c to ra my thu, nc dch ln phn dng trc honh, cui cng l tch symbol ra khi sng mang. Ton b qutrnh ny c lp li kh nhanh chng cho mi sng mang, n khi tt c cc sng mang c gii iu ch.

2.4. BIU DIN TON HC TRONG OFDM

Phng php iu ch OFDM truyn mt s ln sng mang c di thng hp c tcch nhau chnh xc trong min tn s. trnh vic s dng mt s lng ln b iu ch v b

lc my pht cng nh mt s lung ln b lc v b gii iu ch my thu th phng phpny phi s dng cng ngh x l tn hiu s hin i th hin qua cc php bin i nhanhFourier.

Trong ton hc, mi sng mang c m t nh mt sng phc:

tj

CCCCetAtS

)]([)()( *! [ (2.4)

sint

sin2t

sint*sin2t

sint

sint

sint*sint

Hnh 2.3.2a: Tch phn hai hm sin khc tn s Hnh 2.3.2b: Tch phn hai hm sin cng tn s

0

0

0 0

0

0t

t

t t

t

t


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Phng php iu ch OFDM s dng rt nhiu sng mang, v vy tn hiu phc S S(t)

c th hin bi cng thc:

[[[

[

(!

!

!

*

n

etAN

tS

n

N

n

tj

CSCn

0

1

0

)]([)(1

)((2.5)

i vi mi sng, AC(t) v C(t) c nh nn ta c th xem AC(t) = An v C(t) = n

Nu tn hiu c ly mu vi tn s 1/T th tn hiu hp thnh c th hin bi cngthc:

!

*(!1

0

])[( 01)(N

n

kTnj

nSneA

NkTS

[[

(2.6)

Chn 0 = 0, ta c

!

(*!1

0

)(1

)(

N

n

kTnjj

nS eeANkTS

n [

(2.7)

Nu so snh biu thc (2.6) vi bin i Fourier ngc

!

!1

0

)/2()(1

)(N

n

Nnkje

NT

nG

NkTg

T (2.8)

Hai biu thc (2.7) v (2.8) l tng ng nu f = 1/NT = 1/. y cng l iu kinyu cu cho tnh trc giao. Do vic s dng bin i Fourier khng lm mt i tnh trc giao.C th xem tp hp cc sng mang pht i l mt mng trc giao c cho bi cng thc:

t

k

et

k

tj

kk

T[[

[

2

)(

0 !!

!=

(2.9)

Nhng tn hiu ch trc giao nu chng c lp vi nhau. S trc giao l mt thuc tnhcho php truyn tn hiu mt cch hon ho trn mt knh chung v pht hin chng m khngc can nhiu.

Ghp knh theo thi gian (TDM) cho php truyn nhiu tn hiu thng tin trn mt knhn nh vic gn khe thi gian ng nht cho mi tn hiu thng tin ring bit ngn nga s cannhiu gia nhiu ngun thng tin. Do vy tn hiu TDM cng trc giao.Trong min tn s, a scc h thng FDM trc giao v mi tn hiu truyn ring bit cch ly nhau theo tn s. Mc d

nhng phng php ny l trc giao nhng thut ng OFDM c dnh ring cho mt dng tbit ca FDM.Cc tn hiu OFDM c to thnh t tng cc tn hiu hn sin, mi hnh sin cmt di tn khc nhau, c chn sao cho chu k ca chng l s nguyn ln trong mt chu ksymbol.

Khi xt trong min tn s, mi sng mang th cp (subcarrier) OFDM c p tyn tn ssinc (sinx/x). th sinc gm mt bp sng chnh v nhiu bp ph c cng gim dn khi ira khi tn s trung. Tnh cht trc giao t c khi nh ca sng ny l gi tr 0 ca tt cnhng sng mang khc, chng uc pht hin nh s dng bin i Fourier ri rc (DFT). Ph


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ca chng khng lin tc (hnh 2.4.1) m l nhng mu ri rc (hnh 2.4.2), cc mu chng lpgia cc subcarrier khng lm nh hng n kt qu u thu.

2.5. TO V THU TN HIU OFDM

(Hnh 2.5) l s khi ca thit b u cui OFDM tiu biu. Phn my pht bin i dliu s cn truyn s dng bin i nhanh Fourier o (Inverse Fast Fourier Transform), ngha ltn hiu trong min thi gian c nh x sang min tn s. My thu thc hin thut ton

ngc li vi my pht. Trc tin n dch tn hiu xung bng c s, sau s dng bin iFourier nhanh (FFT) phn tch tn hiu trong min tn s. T bin v pha c chn rav uc bin i ngc li thnh d liu s.

Hnh 2.4.2: Subcarrier qua bin i IFFT

Hnh 2.4.1: Subcarrier trn min tn s


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Hnh 2.5: Khi pht v thu tn hiu OFDMD liu cn truyn thng dng ni tip. Trong OFDM, mi symbol ti nhiu bit nn

cn bin i dng d liu ni tip thnh song song ti vo symbol, n ph thuc vo kiu iuch s dng. Ti my thu qu trnh l ngc li, d liu t cc symbol s c bin i tr lithnh dng d liu ni tip gc.

Trn mi trng lan truyn a ng (lan truyn a ng), fading chn lc tn s c thlm cho nng lng ca subcarrier suy gim, gy ra li. Vic sa li s c hiu qu hn nu likhng nhm li thnh cm m c tri rng ra. V vy ci thin ch tiu k thut phn lncc h thng dng thut xo trn d liu (scrambing) nh mt phn ca giai on bin i nitip thnh song song, y l qu trnh ngu nhin ho v tr cc bit d liu trn cc symbol lin

tip. Ti my thu qu trnh xo trn cng c s dng gii m tn hiu, khi phc dy bit dliu gc nhng tri rng cc cm li bit lm cho chng c phn b gn u theo thi gian. Nhvy s ngu nhin ha v tr ca cc li bit ci thin kh nng sa li nhn chung ca c hthng.

2.6. IU CH A SNG MANG

Tn hiu OFDM bao gm tng ca cc sng mang con c iu ch bng PSK (PhaseShift Keying) hoc QAM (Quadrature Amplitude Modulation). Nu d i l cc symbol QAM, Ns ls sng mang con, T l chu k symbol v fc l tn s sng mang, th mt tn hiu OFDM bt u thi im t = ts c th c vit nh sau:

TtttttS

TtttttT

ifjdtS

ss

sssc

N

i

i

i

"!

ee

!

!

;0)(

,))(5.0

(2exp(Re)(

1

0 T (2.10)

Nu ta biu din di dng phc, cc phn thc v phn o tng ng vi phn ng phav vung pha ca tn hiu OFDM, ngha l chng phi c nhn vi hm cosine hay sine tns sng mang mong mun to tn hiu OFDM.


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TtttttS

TtttttT

ijdtS

ss

sss

N

i

i

i

"!

ee!

!

;0)(

)),(2exp()(1

0

T(2.11)

S khi ca b iu ch OFDM c minh ho nh sau:

Trong minh ho ny, tt c cc sng mang c cng pha v bin , nhng trong thc t,mi sng mang c iu ch vi pha v bin khc nhau. Mi sng mang trong khong thigian T phi c s nguyn chu k v s chu k ca cc sng mang k cn nhau hn km nhau ngmt chu k. Tnh cht ny gii thch cho s trc giao gia cc sng mang.

V d, nu sng mang th j l:

? A

? A

ee

ee

ee

!

FTFFTFFTFTFFT

FFTGI

GIGIFTFTGI

TTtTTTt

TtT

TtTTTTt

tW

,/)(cos12

1

,1

,/)(cos12

1

)(

T

T

(2.12)

c iu ch vi tn s j/T trong khong thi gian T, kt qu c:

TtttttS

TtttttT

ijdtS

ss

sss

N

i

i

s

"!

ee!

!

;0)(

)),(2exp()(1

0

T(2.13)

Kt qu trung gian:

TddtttT

ijd

dtttT

ijdtt

T

jj

js

N

i

Tt

t

i

s

N

i

is

Tt

t

i s

s

is

s

!!

!

!

))(2exp(

))(2exp())(2(exp

1

0

1

0

T

TT

(2.14)

L sng mang phc c tch phn trong thi gian T. i vi sng mang j, tch phn nycho ng ra l dj nhn vi T. i vi cc sng mang khc, tch phn ny l bng khng, do tn

Hnh 2.6.1: S khi ca b iu ch OFDM

SerialTo

Parallel

.

.

.

)/)(exp( TttNj sS T

)/))((exp( TttnNj sS T

QAM data OFDM Signal


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s khc nhau gia sng mang j v cc sng mang khc l (j - i)/T l mt s nguyn ln chu ktrong khong thi gian T. Do , tch phn ny lun lun l khng.

(Hnh 2.6.2) m t s chng chp ca ph hnh sinc ca cc sng mang con ring l. Tibin cc i ca mi ph sng mang, tt c cc sng mang khc c bin bng khng. Biv b thu OFDM ch tnh ton cc gi tr ph nhng im tng ng vi cc sng mang conring l. N c th gii iu ch mi sng mang con c sn mc d c nhiu t sng mang conkhc. Ch rng dng xung c trnh by trong min tn s ch khng phi trong min thigian. Do vy, thay v l nhiu xuyn k hiu ISI (InterSymbol Interference), th li l nhiu xuynsng mang ICI (InterCarrier Interference). ICI c loi b bi ti bin cc i ca sng mangny tng ng vi cc sng mang khc c bin cc tiu. Ph ca OFDM p ng nguyn lca Nyquist i vi ISI c m t trn (Hnh 2.6.3).

Tn hiu OFDM di nn phc c nh ngha nh trn, thc cht l bin i Fourierngc ca Ns k hiu QAM ng vo. Tng ng vi ri rc thi gian l bin i Fourier ri rcngc (IDFT, Inverse Discrete Fourier Transform) c cho bi:

Hnh 2.6.2. Tn hiu OFDM c 4 sng mang.

Hnh 2.6.3. Ph ca tn hiu OFDM.


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)2exp()(1

0 T

injdnS

iN

i

i T

!

! (2.15)

OFDM s dng k thut iu ch MPSK hoc MQAM, ty theo s mc iu ch M mmi symbol s c nh x bao nhiu bit, v d vi 16-QAM mi symbol mang 4 bit thng tin vc biu th bng mt vect duy nht (th hin bng mt chm) trong khng gian chm sao iuch. Misymbolli c truyn trn mt subcarrier, tn hiu sau khi iu ch c nng ln tns cao ri pht i. u thu, trc tin phi h tn s tn hiu n mc gc, sau gii iu chtch ra chui bit gc. iu ch v gii iu ch trn c th thc hin bng k thut tng t, tuynhin hin nay th h iu ch s I/Q cho kt qu chnh xc hn.

Nh kt qu trnh by phn trn, sng mang c th c biu din thnh s phc viphn thc v phn o. Chng s c chuyn t ni tip sang song song ri a n b iu ch.Mi sng mang c mt cp I/Q xc nh v c th hin bng mt vect duy nht (I+iQ) trn s

. V d 256-QAM c 256 im (I,Q) trong mt chm sao c cu trc thnh hnh vungvi 16 ct t cch u nhau trong trc thc v 16 hng trong trc o. S bit c th c truynti trn mt symbol l log2M vi M l s cc im trong s , do vy 256-QAM truyn 8 bittrong mt symbol. Mi symbol c nh x vo mt v tr IQ duy nht trong chm sao. Vectphc hp thnh I+jQ c bin (I2+Q2)1/2 v pha arctg(Q/I) vi j2= 1. S im trong s ncng nhiu cng lm tng hiu qu s dng bng thng, 256-QAM cho hiu qu ph l 8b/s/Hzso vi 1b/s/Hz ca BPSK. Tuy nhin s im trong gin chm sao cng ln bao nhiu th vicgii m my thu cng kh by nhiu v khi cc v tr (I,Q) c t cng gn nhau nn ch cnmt gi tr nh nhiu l c th gy ra li truyn. iu ny dn n s dung ho trc tip giadung sai nhiu v hiu qu ph trong s iu ch, c tng kt trong l thuyt thng tin

Shannon. L thyt ny pht biu rng thng tin cc i ca knh C c bng thng W vi cngsut tn hiu S b xo trn bi nhiu trng c cng sut trung bnh N th c xc nh theo cngthc:

)/(1log2 sbN

SWC

! (2.16)

Hiu qu ph ca knh l php o s bit c truyn trong mt giy trong mi Hz bngthng. Do vy hiu qu ph uc tnh theo cng thc:

Hnh 2.6.4: th phn b QAM - 32 Hnh 2.6.5: th phn b QAM - 64


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)//(1log2 HzsbN

S

W

C

! (2.17)

y c tn hiu v nhiu c tnh trn thang tuyn tnh, hiu qu ph c o bngb/s/Hz.

2.7. NHIU PHA TRONG H THNG OFDM

Gi s c N im trong php bin i DFT c s dng iu ch v gii iu ch tnhiu OFDM. Do h thng s c N subcarrier, mc d mt ssubcarrier hai bin ca ph sc t bng khng trong my pht cung cp bng tn bo v cho vic thc hin cc b lctng t d dng hn

Ta c th m t tn hiu pht i nh sau:

!

!1

0

)( 0)(N

k

tkj

kueStS

[[

(2.18)

Trong : Sk l bin phc ca sng mang th k

u l khong cch sng mang (c gi tr 2/Tu, Tu l chu k symbol) c n vl rad/s ;

0 l tn s gc ca sng mang th 0

i vi mt s gi tr k (tng ng vi cc bin ca ph) th Sk=0.

Trc ht ta kho st mt my thu l tng, tc khng c nhiu pha trong dao ng ni.Tn hiu thu l tng r(t) ch chu tc ng ca p xung knh h(t). V th ta c th vit:

!

!1

0

)( 0)(N

k

tkj

kkueSHtr

[[

(2.19)

Hk l p ng tn s phc ca subcarrier th k

t k= HkSk l bin subcarrierl tng khi khng c nhiu pha, lc ny

!

!1

0

)( 0)(N

k

tkj

kueRtr

[[

(2.20)

Trong mt my thu thc t, tn hiu thu x(t) chu tc ng ca knh truyn c p ngxung h(t) v ca nhiu pha (t) c to ra bi cc b dao ng ni.

!

!

1

0

)()( 0

)(

N

k

tkjtj

ku

eetx[[N

(2.21)

Ti my thu, chng ta tng quan tn hiu x(t) ny vi mt sng mang xc nh bin ca sng mang gii iu ch.V d sng mang th l ta c cng thc sau :


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Ti my thu, chng ta tng quan tn hiu x(t) ny vi mt sng mang xc nh bin ca sng mang gii iu ch.V d sng mang th l ta c cng thc sau :

dteeT

dtetxT

X

N

k

tlkj

k

T

tj

u

tlj

T

u

lu

u

u

u

!

!!1

0

)(

0

)()(

0

1)(

10 [N[[

(2.22)

Phng trnh ny m t qu trnh c s dng trong mt my thu gi nh vi cc bgii iu ch tng quan tch phn v loi tr (Correllator/Integrate and Dump Demodulators)cho mi sng mang. My thu thc t s dng bin i FFT ca mt bin i Fourier ri rc(DFT), bng cch thay th thut ton tch phn bi s tch ly ca nhiu mu ri rc v kt hptt c cc b gii iu ch trong mt thut ton hiu qu. Vic pht trin tip theo biu thc ()cho trng hp tng qut l rt kh. Tuy nhin mt cch n gin ha n, ch rng chng tamun my thu hot ng m khng c d phng ca tn hiu OFDM khi tnh ti nhiu phaca b dao ng ni. V th chng ta gi s rng gc (t) lun lun nh.

ej(t) 1 + j(t) (2.23)

(ex = 1 + x + x2/2! + x3/3! +. ..) (2.24)

T y ta c th vit biu thc cho ng ra ca b gii iu ch i vi sng mang th lnh sau:

ll

N

k

T

tlkj

k

u

N

k

T

tlkj

k

u

N

k

tlkj

k

T

u

l

Y

dtetjT

dteT

dtetjT

X

u

u

u

u

u

u

!

!

!

!

!

!

1

0 0

)(1

0 0

)(

1

0

)(

0

)(11

)(1(1

[[

[

N

N

(2.25)

S hng u tin l gi tr thu l tng l m ta hy vng khi phc, trong khi s hng thhai Yl th hin N ng gp l kt qu s hin din ca nhiu pha (t).

2.8. CC KHONG BO V

i vi mt gi tr bng thng c nh, tc symbolca tn hiu OFDM thp hn nhiutc symbolca s truyn sng mang n. V d i vi iu ch n sng mang BPSK tc symboltng ng vi tc bit. Tuy nhin vi OFDM bng thng h thng c chia cho Nng, tm gi l ti ph, thnh tc symbolnh hn N ln so vi truyn sng mang n. Tc symbol thp ny lm cho OFDM chu ng c tt vi can nhiu ISI (inter- Symbolinterference) gy ra bi truyn lan a ng. Mc d vy chng ta vn c th gim nh hng ISIhn na bng cc thm vo khong bo v trc ca mi symbol. Khong bo v ny l bncopy tun hon theo chu k, lm m rng chiu di ca dng sng symbol.


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2.8.1. Bo v chng li offset v thi gian (time offset)

gii m tn hiu OFDM my thu phi nhn uc FFT ca misymbolthu c, tch rabin v pha. i vi h thng OFDM dng cng mt tn s ly mu cho c my pht v mythu, phi dng cng mt kch thc FFT cho c my thu v tn hiu pht duy tr s trc giao.Mi symbolthu c c cc di TG+TFFT do b sung khong bo v. i vi knh l tngkhng c m rng tr my thu c th d tm c lch thi gian bt k (ln nht l bngkhong bo v TG) m vn cn t c s cc mu ng. Do bn cht tun hon ca s thay ikhong bo v lch thi gian (time offset) ch dn n s quay pha ca tt c cc ti ph trong tnhiu. Gi tr quay pha t l vi tn s ti ph. Vi ti ph tn s Nyquist th s thay i l 180 0cho mi time offset. chng minh rng time offset c duy tr khng i t symbolny tisymbolkhc, nn s quay pha cho time offsetc th c loi b nh mt phn ca cn bngknh trong mi trng ISI a ng, gim di ca khong bo v dn n li time offset.

2.8.2. Bo v chng li ISI

Trong tn hiu OFDM bin v pha ca ti ph phi c duy tr khng i trong chuksymbol bo m tnh trc giao cho mi sng mang. Nu chng b thay i c ngha l dngph ca cc ti ph s khng c dng sine ng v nh vy im khng (Null) s khng tn sng, dn n can nhiu gia cc sng mang ICI. bin ca symbolbin v pha thay i btthnh lnh ti gi tr mi cn thit bi symbol d liu tip theo. Trong mi trng ISI a nggy ra s tri rng nng lng gia ccsymbol, dn n s thay i nhanh bin v pha ca tiph im usymbol. di ca nhng nh hng thay i nhanh tng ng vi s m rng tr ca knh v tuyn. Tn hiu thay i nhanh l kt qu ca mi thnh phn trong knh ang cc thi im khc nhau mt t, thay i vect ti ph thu c, th hin hnh 2.8.Vic a vo cc khong bo v cho php c thi gian phn tn hiu thay i nhanh ny b suyhao. Tr li trng thi ban u, do vy FFT c ly t phn trng thi ng ca symbol. iuny loi b nh hng ca ISI. khc phc ISI th khong bo v phi di hn s m rng tr ca knh v tuyn. Cc nh hung cn li m knh a ng gy ra, nh thay i bin vquay pha c sa bi cn bng knh.

Khong bo v chng li cc nh hng thay i nhanh do knh a ng loi b cc nhhng ca ISI. Tuy nhin trong thc t cc thnh phn nhiu c khuynh hng suy gim chm

theo thi gian, dn n vn cn ISI ngay c khi khong bo v tng i di.Vic a vo khong bo v lm gim tc symbol, tuy nhin n khng nh hng ti

khong cch ti ph m my thu, c xc nh bi tn s ly mu FS.

f =FS/NFFT

Vi f l khong cch ti ph [Hz],

FS l tn s ly mu [Hz]

NFFT l kch thuc FFT.


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2.9. VN CNG SUT NH TRONG OFDMMt trong nhng hn ch chnh trong h thng OFDM chnh l t s cng sut nh trn

cng sut trung bnh do s tng hp mt lng ln sng mang con lin tip nhau. V s sngmang con N tng ln, cng sut nh cc i c th ln gp N ln cng sut trung bnh. Gi trPAP ln i hi khong tn khuch i phi ln trnh mo dng tn hiu. Tuy nhin, mt bkhuch i khng ch m bo vng khuch i tuyn tnh m c vng bo ho cng phi tuyntnh. V vy, m bo b khuch i trong h thng OFDM hot ng vng tuyn tnh thcng sut ra trung bnh ca tn hiu phi nh hn N ln cng sut cc i. Vic gim PAP lmt trong nhng yu cu quan trng nht khi s dng h thng OFDM. gim PAP ta c thdng phng thc tng vng khuch i tuyn tnh ca b khuch i. Tuy nhin, vn nythuc lnh vc in t ta ch xem xt phng thc gim PAP bng x l tn hiu.

2.9.1. nh ngha:T s cng sut nh trn cng sut trung bnh (PAP ) ca tn hiu trong h thng OFDM

nh ngha nh sau:

_ a ? A

!

T

Tt

dttST

tSPARR

0

2

,0

2

)(1

)(max(2.26)

Trong : S(t) c cho trong phng trnh (2.4)T l chu k tn hiu.

Nng lng trung bnh ca tn hiu s(t) c xc nh da theo cng thc nh sau:

_ a dttST

tSavgT 2

0

)(1

)( ! (2.27)

tNT

nt

NT

nmxnxt

NT

nnxt

NT

nnx

T

dttNT

nnxt

NT

nnx

T

dtenxT

T N

n bb

N

n

N

m bb

T N

n

N

n bb

T N

n

NTntj d

!

!

!

!

!

!

!

!

!

0

1

0

1

0

1

0

2222

0

21

0

1

0

2

0

1

0

/2

)2sin()2cos()()(2)2(sin)()2(cos)(1

)2sin()()2cos()(1

)(1

TTTT

TT

T

Xem x(n) l tn hiu BPSK, nh vy |x(n)| = 1 khi t [0,T] v T=NTb. Ta c:

{ {

{

{

!

!

!

mn

T

mn b

b

T

mn b

b

T

mn b

dttNT

mnmxnx

T

dtt

NT

mt

NT

nmxnx

T

dttNT

mt

NT

nmxnx

TI

0

0

0

0)2cos()()(2

)2sin()2sin()()(21

)2cos()2cos()()(21

T

TT

TT

(2.28)


14/21


n mn hc:Truyn thng s

Nng lng trung bnh ca tn hiu S(t) l:

_ a !!

-

!

!

TT N

n

NdtnxT

Ndtnx

TtSavg

0

2

0

1

0

2 )()(1

)( (2.29)

Do nng lng trung bnh ca tn hiu S(t) l mt hng s c nh N nn gim t sPAP th cn phi gim nng lng tn hiu tc thi , tc l gim s knh thnh phn. Nu phnng lng S(f) ca tn hiu S(t) trong phng trnh (2.4) l hng s nng lng cc i ca tn

hiu S(t) bng vi nng lng trung bnh ca tn hiu S(t), th t s PAP = 1, gi l t s ltng. PAP biu din di bin ca cc mu to ra bn my pht tn hiu OFDM. Ni cchkhc, PAP biu din khong cch n gc ca cc k t trong khng gian tn hiu. minh hor hn, xt 3 v d hnh di y cho cc h thng thng tin thng thng nh sau:

H thng nh phn {0,1}: PAP .- H thng iu ch pha M mc (M-PSK): do cc tn hiu trong khng gian ch khc nhau

v pha trong khi ln bng nhau (khong cch n gc nh nhau) nn PAP =1. y chnh lgi tr nh nht ca PAP trong mi h thng.

- Vi h thng dng 16-QAM: PAP =1.8.2.9.2. Cc phng php gim PAPR

Chng ta c th chia lm 3 loi:- Th nht l nhng k thut lm mo tn hiu. Cc k thut ny gim bin nh ch

n gin bng cch lm mo tn hiu OFDM ti xung quanh nh u ca tn hiu. Cc k thutny gm c: Clipping, Peak Windowing v Peak Cancellation.

- Loi th hai l cc k thut m ho s dng b m sa sai c bit nhm loi ra nhngsymbol OFDM c PAP cao.

- Loi th ba l nhng k thut da trn vic ngu nhin ho mi symbol OFDM vi cc

chui gi ngu nhin khc nhau v la chn chui c t l PAP nh nht.2.9.3. Clipping v Peaking WindowingMt trong nhng c tnh quan trng ca PAP trong OFDM l phn trm cc k hiu c

PAP ln th t (v phn trm ny gim theo s ca mt s sng mang ph). V vy trong trnghp ny, mt gii php n gin nht gii quyt vn cng sut nh l Clipping, n gii hnbin nh ti mt mc ngng ln nht. Tuy n gin nhng phng php ny c mt vikhuyt im sau:

- To ra mt loi t xuyn nhiu gy ra s suy gim cht lng BE .

Hnh 2.9.1: Khng gian hnh sao ca iu ch SPSK v 16-QAM


15/21



- Gy ra mo phi tuyn do Clipping lm tng lng bc x ngoi bng. S tng bc xngoi bng ch yu l do clipping chnh l vic nhn k hiu OFDM vi hm ch nht l 1 nubin thp hn mt mc ngng v nh hn nu bin trn mc ngng. Dng sng ch nhtny lm tng bc x ngoi bng dn n ph c rolloff t l nghch vi tn s.

Vn rolloff ph chm c th c khc phc n mt chng mc nht nh bng vicwindow dng sng ch nht. Mt s window thng dng nht l: Gaussian, Cosine, Hamming,Kaiser

2.9.4. Trit nh (Peak Cancellation)Mt phng php khc loi b nh khi thc hin OFDM l tch hm tham chiu t tn

hiu mi hm tham chiu c tch ra gim cng sut nh ca t nht mt mu tn hiu. Bngvic la chn hm tham chiu chnh xc vi bng thng xp x bng thng tn hiu pht, c thm bo vic gim cng sut nh khng gy ra nhiu ngoi bng. Hm trit nh c s dngthng dng nht l hm sine do c tnh mong mun trong min tn s ca n. Tuy nhin hmsine s c nhn vi hm window raised-cosine loi b nhiu ngoi bng khng mong mun.

Mt u im ca k thut trit nh l n c th c thc hin mt cch s ho theo sauIFFT pha pht.

Trit nh c thc hin trn nn tng tng k hiu mt. i vi mi k hiu OFDM, btch nh xc nh mu no vt qu bin c xc nh trc, sau , i vi mi nh tnhiu, mt xung c to ra c pha bng pha nh v bin bng bin nh tr i bin tia mong mun. Nhng xung ny sau qua b lc thng thp, b lc thng thp t c trongmin tn s nh thc hin FFT, t tt c cc ng ra m tn s vt qu tn s sng mang phcao nht v 0, sau bin i tn hiu tr li nh b IFFT.

M ha S/P IFFT CP& Windowing

P/S Trit D/A FTX

Hnh 2.9.4.1: Trit nh bng b bin i P/S

M ha S/P IFFTCP&

Windowing P/S D/A

Hnh 2.9.4.2: Trit nh dun FFT v IFFT

Tr

+

-

Tchnh

Toxung

FFT LPF IFFT


16/21



2.9.5. Cc m gim PAPRMt gii php khc cho PAP l s dng cc k thut m ho, PAP c th gim nh s

dng mt m m ch to ra cc k hiu OFDM c PAP thp hn mc mong mun. Tt nhinmc PAP mong mun cng nh th t l m ho cng nh. Ngi ta thy rng c th s dng bm c t l c c PAP ti a l 3dB. C th s dng chui Golay to ra nhng mny. Chui Golay l cc cp chui c hm t tng quan l 0 i vi tt c cc dch tr khc 0.Cc c tnh tng quan ca cc chui b bin i thnh PAP nh tng ng 3 dB khi cc mc s dng iu ch mt tn hiu OFDM. Nh c tnh ny, cc m Golay vi k thut giim gim PAP ng thi cho kh nng sa li tt.

2.9.6. Ngu nhin ho k hiuK thut ngu nhin ho k hiu gim PAP ca tn hiu OFDM pht c th xem nh

l trng hp c bit ca k thut m ho. Khc bit y l ngu nhin ho k hiu khng cgng kt hp sa li vi gim PAP nh m ho.

tng c bn ca ngu nhin ho l i vi mi k hiu OFDM, chui ng vo cngu nhin ho bi mt s chui ngu nhin xc nh. Tn hiu ng ra vi PAP nh nht sc truyn i. i vi cc chui ngu nhin khng tng ng, tn hiu OFDM nhn c vPAP tng ng s khng tng quan, v vy, nu PAP ca mt k hiu c xc sut P vt qumt mc cho trc khng ngu nhin, xc sut n vt qu qua mt mc c ngu nhin gimti Pk khi s dng k m ngu nhin.

V vy, ngu nhin ho s gim xc sut xy ra PAP cao hn l gim PAP ny.2.10. TNH TON THNG S OFDM

Gi s tc truyn bit l v thi gian tri tr ca knh a ng l th cc thng sca h thng OFDM c xc nh nh sau:

- Thi gian bo v t nht phi ln hn 2 ln thi gian tr : G 2 (2.30)- lm gim ti thiu t s SN mt do thi gian bo v th chu k tn hiu phi ln

hn rt nhiu thi gian bo v. Tuy nhin, cc symbol c chu k di d b mc phi s triDoppler, nhiu pha, v lch tn s. Nh ta bit th thi khong ca symbol OFDM Tsys t nhtphi gp 5 ln thi gian bo v tc l:

Tsys 5G (2.31)- Khong cch gia hai sng mang k nhau l:(f Tsys (2.32)- Vi tc bit l th s bit thng tin trn symbol OFDM Binfo s l:Binfo= .Tsys (2.33)- Vi Binfo bit v s bit trn mt symbol trn mi sng mang con l Rsub , ssng

mangcon l N th:

Rsub

oBN

inf! (2.34)

Trong : Rsub = 2bit/symbol/subcarrier vi QPSKRsub = 4bit/symbol/ subcarrier vi 16QAM

-

Bng thng ca tn hiu OFDM c nh ngha:BW = N(f (2.35)T nhng tnh ton trn ta rt ra 2 nhn xt sau:* Khi tng thi khong ca symbol s lm gim khong cch tn s gia cc sng mang

con. Nhvy, vi mt bng thng tn hiu cho trc, c th c rt nhiu sng mang con. Mtkhc, vi s sng mang con c sn vic tng thi khong symbol s lm gim bng thng tnhiu.

* Vic tng s sng mang con s lm tng s ln ly mu trn mt symbol OFDM. Tuynhin, n khng bao hm vic tng thi khong symbol. Nu thi khong ca symbol vn duy tr


17/21



khng i, kt qu l thi khong gia hai ln ly mu s gim. iu ny c ngha l tng bngthng tn hiu OFDM. Mt khc, nu bng thng ca tn hiu OFDM c nh, vic tng s sngmang con lm gim khong cch gia hai sng mang con, iu ny lm tng thi khong symbol.Thi khong gia hai ln ly mu s khng i trong trng hp ny.2.11. LA CHN CC THNG S CHO OFDM

Vic la chn cc thng s cho OFDM bao gm nhiu yu t c nh hng n kinhdoanh. Thng thng c ba thng s chnh l: di thng, tc bit v tr lan truyn. Trong thi gian tr truyn l thi gian bo v tng ng. Thng thng th thi gian bo v nn bng 2 4 ln tr lan truyn. Tuy nhin, n cn ph thuc vo loi m v loi iu ch QAM. M hovi s symbol cng ln th cng nhy cm vi ISI v ICI hn l BPSK.

Khi thi gian bo v c thit lp, chu k tn hiu n nh. trnh lm gim tithiu t s SNR gy ra do thi gian bo v th chu k ca symbol phi ln hn rt nhiu so vithi gian bo v. Tuy nhin, gi tr ny khng th ln tu c, bi v thi gian k t ln hnngha l nhiu sng mang hn vi khong cch gia cc sng mang nh hn, s phc tp s giatng, d gy ra nhiu v cc nh hng khc. Trong thc t, nn chn thi gian k t t nht bng5 ln thi gian bo v (suy hao 1 dB SNRdo thi gian bo v).

Sau khi thi gian bo v v thi gian k t c nh, s sng mang c quyt nh davo bng thng yu cu 3dB c chia ra theo khong cch ca cc sng mang con, n ngcvi chu k ca symbol v nh hn thi gian bo v. S sng mang c quyt nh da vo tc bit yu cu chia cho tc bit trn mi sng mang con. Tc bit trn mi sng mang conc nh ngha bng loi iu ch (nh 16 QAM), tc m v tc k hiu.

Gi s chng ta mun thit k mt h thng vi cc yu cu sau:+ Tc bit: 20 Mbps.+ Tr lan truyn c th chu c: 200 ns.+ Di thng < 15 MHz.Thi gian tri tr yu cu l 200ns v ta chn thi gian bo v an ton gp 4 ln l 800ns.Chn chu k symbol OFDM gp 6 ln thi gian bo v (4.8ns), suy hao do thi gian bo

v nh hn 1dB. Khong cch gia cc sng mang con by gi gim xung 4.8-0.8 = 4(s) chnhbng 250kHz.

xc nh s sng mang ph cn thit chng ta cn xt n tc bit yu cu v tc symbol OFDM. t c 20Mbps, mi symbol OFDM mang 96 bit thng tin (v96/4.8(s=20Mbps).

lm c nh th ta c mt s cch la chn:+ S dng 16 QAM vi tc m nhn c 2 bit trn k t (trn sng mang). Do

, 48 sng mang cn n c 96 bit trn k t.+ Cch la chn khc l C th dng QPSK vi tc m , nhn c 1.5 bit trn k t

(trn sng mang). Do , cn 64 sng mang t c 96 bit trn k t.Tuy nhin, 64 sngmang con th bng thng s l 64*250kHz=16MHz s ln hn bng thng yu cu. t cbng thng nh hn 15MHz th s sng mang con phi nh hn 60.

Do , s la chn u tin l 48 sng mang con v m ho 16 QAM s p ng y cc yu cu.

Mt yu cu na nh hng n vic chn cc thng s l phi c mt s nguyn cc mutrong chu k FFT/IFFT v chu k symbol. Chng hn, vi th d trn chng ta c th c chnhxc 64 mu trong thi khong FFT/IFFT bo m tnh trc giao gia cc sng mang con.Chng ta c th t c iu ny bng cch ly mu vi tc 64/4(s=16MHz. Tuy nhin, vitc ly mu ny th s khng c mt s nguyn mu trong thi gian 4.8(s) ca symbol. Giiphp duy nht gii quyt vn ny l cn thay i mt cht cc thng s t c s nguyn.Chng hn, s mu trn mt symbol c th thit lp bng78, khi tc ly mu


18/21



78/4.8(s=16.25Mhzv thi khong FFT l 64/16.25MHZ=3.9385(s, v th c thi gian bo v vkhong cch gia cc sng mang s ln hn thi khong 4s ca FFT so vi trng hp c.

2.12. SO SNH HIU NNG KHI S DNG SNG MANG N V OFDM

- Trong cc phng php iu ch mt sng mang, d liu c a trc tip ti b iuch hot ng vi tn s sng mang chnh gia di tn truyn dn f0, f1,, fN-1, ngha l ti[fN-1+ f0]/2. Tn hiu c iu ch, khi , chim ton b di tn W. Khi d liu c a vomt cch ni tip, hin tng fading su s gy ra li cm. (Hnh 2.12.1 ) m t vic truyn nitip N k hiu, S0, S1, ,SN-1. Trong phn t m ch ra v tr li cm nh hng ln (K

nghien cuu su ket hop giua cong nghe cdma va ofdm

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