ofdm ofdma va sung dung trong cong nghe truy cap bang rong khong day wimax 8 9024

7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024

1/9

Bng cc t vit tt

b)Knh fading chon lc tn s

Hnh 3.5: Pilot trong gi OFDM

3.3.2 ng b k t da vo CP

Xt hai tn hiu thu cch nhau N bc:

d(m) = r (m) r (m + N),

Vi N l sng mang ph. N bng s im ly mu tng ng vi phn c ch

ca symbol OFDM, chng phi l bn sao ca nhau nn d(m) thp. Nu r(m) v

r(m-N) tng ng vi cc mu pht nm trong thi khong ca cng mt symbol

OFDM, d(m) l hiu ca hai bin ngu nhin khng tng quan. Cng sut ca

d(m) trong trng hp ny bng hai ln cng sut trung bnh ca symbol OFDM.

Nu s dng mt ca s trt c rng thi gian bng khong thi gian

ca CP (im cui ca ca s trng vi im bt u ca symbol OFDM) th khi

ca s ny trng vi thnh phn CP ca symbol OFDM s c mt cc tiu v cng

sut trung bnh ca cc mu d(m) trong ca s ny. Do , c th c lng c

thi im bt u ca symbol OFDM, v ng b thi gian c thc hin.

3.3.3 ng b khung k t da trn m ng b khung (FSC)

ng b khung k t nhm nhn bit v tr bt u ca khung k t tm

thy v tr chnh xc ca ca s FFT. Cc thut ton ng b khung symbol truyn

thng (dng symbol pilot, dng CP,) da vo quan h gia khong bo v GI v

phn sau ca symbol. Nhng cc thut ton ny khng th pht hin chnh xc v tr

bt u ca k t do nhiu ISI trong knh fading a ng. Cu trc khung c th


2/9

Bng cc t vit tt

c chia thnh vng m ng b khung FSC cho ng b khung symbol v vng

d liu cho truyn dn symbol OFDM (Hnh 3.6).

Hnh 3.6: Mt kiu cu trc khung symbol OFDM

C th biu din tn hiu khung OFDM nh sau:

)()()( FSCdataFSCframe TtStStS (3.16)

Trong , TFSC : Khong thi gian symbol FSC

Ti pha pht, chui cc mudng sc pht gm c chuiCA(n) ca

FSC v cc mu d liu khng c GI qua FFT l:

1

0

2

:1...,,1,0)(1

)(

:...,,2,1)()(

N

k

N

nkj

mm

LA

tadaNkekXN

ns

FSCCnnCns

(3.17)

Trong , CL : di bit ca FSC

sm(n) : Chui cc mu ca symbol OFDM th m trong min

thi gian khi khng thm GI.

xm(k) : Symbol truyn dn phc th m trong min tn s.

N : S sng mang ph

Cc muCA(n)cng dng trc tips(n) l s btu khung


3/9

Bng cc t vit tt

Tn hiu FSC l mt chui tun t cc mu, )()( nCns A , vi n = 1,2, CL

c to thnh t vector FSC C(n) = {C(1), C(2), ..., C( LC )} gm cc CL gi tr nh

phn. i vi m C(n) c gi tr "1" , chng ta thc hino cc tnh lun phin

to ra tn hiu 3 mc )(nCA . V d: Cho C(n) = {1, 0, 0, 1, 1, 1, 0, 1} th )(nCA =

{1, 0, 0, -1, 1, -1, 0, 1}. Bng cch ny, ta c th duy tr s gi tr '1' v '-1' bng

nhau ti pha pht hn ch khong dch DC v duy tr mt mc cnh cho di

ng.

Cu trc ng b khung symbol OFDM gm: B nhn bit cng sut, b

nhn bit bit '0'/ '1' , thanh ghi dchCL, b cng Modulo -2 c gim bt, b tng,

b nhn bitnh.

Thut ton ng b khung symbol nhFSC gm c 3 bc: Nhn bit FSC,

xc nh cc mc ngng tiu Th1 v Th2 tng cng xc sut nhn bit v tr

u khung symbol.

Hnh 3.7: ng b khung k t dng FSC

3.3.3.1 Nhn bit FSC


4/9

Bng cc t vit tt

u tin, bng b khung symbol s nhn bit cng sut bng cch dng

mi mu thu. Gi s nu chui mu tn hiu ti u th i sau knh a ng v

AWGN l )(~

is , chng ta c th biu din mt tn hiu vi khong dch tn s v

pha thnh cc knhIv Q ring r nh sau:

jQI eisisiy )).()(()(~~

)sin)(cos)(()sin)(cos)(( isisjisis~

I

~

Q

~

Q

~

I (3.18)

Trong , )(~

is I : KnhIcas(i)

)(~

is Q : Knh Q cas(i)

: Biu din tng pha 02 Ni , gm khong dch tn

s ( fT ) v khong dch pha 0 .

Nu chng ta thc hin nhn bit cng sut cho chui mutrn ng b

khung symbol nh trong Hnh 3.7, chng ta c th thu c cng sut m khng

ph thuc vo khong dch tn s v pha nh sau;

)()()()(2~2~

22isisiyiy QIQI (3.19)

3.3.3.2 Xc nh mc ngng Th1

Theo php phn tch, chng ta s thu c mt mc ngng tiu Th1 trong

mi trng AWGN xc nh '0' v '1' t cng thc (3.19). thu c mt mc

ngng tiu trong mi trnga ng l rt kh khi n ph thuc vo kiu FSC.


5/9

Bng cc t vit tt

Hnh 3.8: Ngng ti u Th1 vi gi tr SNR

Cc ngngTh1 c thc vit:

2/1

0

4

1 )(2

2

P

eIP

Th (3.20)

(.)10

I : Hm ngc ca Bessel bc 0: (.)0I ,

2

: Phng sai ca cc bin ngu nhin Gaussian trong cc knhIv Q

P : Gi tr bin c nh ngha trong tn hiu

Hnh 3.8 so snh gia m phng v phn tch t cng thc (3.20) gi tr ca

ngng ti u vi cc SNR khc nhau.

Cc gi tr '0' v '1' c xc nh ri a n u vo thanh ghi dch ca b

nhn bit FSC ph hp vi tc ly mu Ts v b php ton cng modulo-2 thc

thi CL thi im vi kiu FSC bit. y, u ra b cng modulo-2 sa i l '1'

nu cc bit ging nhau ti v tr hin ti, nu khng s c gi tr '-1'. Cc gi tr


6/9

Bng cc t vit tt

tng quan ny s c cng tt c cc khi tng v kt qu c so snh vi

ngng Th2 ca b nhn bit nh d tm FSC.

3.3.3.3 Xc nh mc ngng Th2

Nu gi tr nh chnh xc ca u ra b nhn bit nh l nh hn ngng

Th2 m thit lp cho b nhn bit nh, FSC khng c pht hin. y gi l s

nhn bit trt PM. Nu thit lp Th2 thp, tng quan u ra ca cc vng d liu

khc c th trn Th2 v c xem nh l FSC, gi l xc sut d phng sai PF.

i vi ng b khung symbol, xc sut nhn bit trt PM kh nng pht

hin li chnh xc PC. PC l xc sut nhn bit FSC khi s lng li trong FSC

tr nn ging nhau hoc t tng s li cc i (vi 2/)( 2ThCL ) ca qu trnh

nhn bit nh. V vy, kh nng nhn bit FSC ng PC c th c tm bng cch

cng xc sut ca cc li bit FSC di ngng li . Xc sut nhn bit trt c th

c tm bng cch tr tt c cc xc sut nhn bit ng ra khi ton b cng sut.

Khi ngng li v chiu di CL ca FSC tng, xc sut nhn bit trt

gim. Gi s nu chiu di FSC lL

C bit, mi kh nng kt hp d liu ngu nhin

l LC2 . Nu = 0, kh nng pht hin li l 1/ LC

2 . y l kh nng pht hin ngu

nhin chnh xc vi kiu FSC. PF c th c gim bng cch tng s bit FSC, CL

hoc gim ngng nhn bit . Nh vy, PM v PF c th trao i vi nhau khi cho

CLc nh v bin i gi tr hoc Th2.

Trong trng hp tng qut PM l rt nh cn PF l rt ln. iu ny c th

khc phc bng k thut ca s. Trong k thut ny, qu trnh nhn bit FSC ch


7/9

Bng cc t vit tt

trong mt khong c bit, s tnh ton trc cao c xem nh l mt nh. Vic

thc hin tng i n gin v cho hiu qu tt.

Nh vy, thut ton ng b khung symbol c th chn chiu di v kiu

FSC. iu ny ph thuc vo mi trng knh v hiu sut h thng. Khi mi

trng knh xu, ta c th m rng chiu di v gim WFP v MP .

3.4 ng b tn s trong h thng OFDM

Trong k thut ng b tn s cn quan tm n li tn s v thc hin c

lng tn s. Li tn s y l s lch tn s nguyn nhn do s sai khc gia hai

b to dao ng bn pht v bn thu, dch tn Doppler v nhiu pha do knh

khng tuyn tnh. Hai nh hng li tn s lm gim bin tn hiu (do tn hiu c

dng hnh sine) c ly mu khng phi ti nh v to ra xuyn nhiu knh ICI

gia cc knh ph do mt tnh trc giao ca cc sng mang ph .

Vn ng b tn s trong h thng OFDM gm c ng b tn s ly mu

v ng b tn s sng mang.

3.4.1 ng b tn s ly mu

Ti bn thu, tn hiu thu lin tc c ly mu theo ng h my thu. S

chnh lch nhp ng h gia my pht v my thu gy ra xoay pha, suy hao thnh

phn tn hiu c ch, to ra xuyn nhiu knh ICI.

khc phc vn ny, gii php th nht l s dng thut ton iu khinb dao ng iu chnh bi in p VCO; gii php th hai l thc hin x l s

ng b tn s ly mu trong khi gi c nh tn s ly mu.

3.4.2 ng b tn s sng mang


8/9

Bng cc t vit tt

ng b tn s l vn quyt nh i vi h thng thng tin a sng

mang. Nu vic thc hin ng b khng bo m, hiu sut ca h thng cng nh

u im ca h thng ny so vi h thng thng tin n sng mang gim i ng

k. thc hin ng b tn s sng mang phi c lng khong dch tn sng

mang CFO.

Cng nh ng b thi gian (symbol), c th chia cc gii php c lng

tn s thnh cc loi : da vo tn hiu d liu, da vo tn hiu pilot, da vo CP,..

3.4.2.1 c lng khong dch tn s sng mang CFO da vo pilot

Trong thut ton ny, mt s sng mang c s dng truyn dn tn hiu

pilot. Tn hiu thng c chn l cc tn hiu PN. Bng cch s dng mt thut

ton thch hp, bn thu s xc nh c gi tr xoay pha ca tn hiu gy ra bi sai

lch tn s. Nu sai lch tn s nh hn mt na khong cch tn s gia hai

sng mang ph k nhau, nh x gia gi tr xoay pha v lch tn s l nh x 1-1

nn c th xc nh duy nht chnh lch tn s.

3.4.2.2 c lng tn s sng mang s dng CP

Xt sng mang ph c iu ch bng mt dng d liu:

1...,,1)(1

)(1

0

2

NLnekSN

nuN

k

N

nkj

Tn hiupha pht: n

snTggnutx )()()(

Tn hiupha thu: )()()()( tnnTghnutyn

s , vi h(t) l p ng knh;

n(t)l p ng nhiu.


9/9

Bng cc t vit tt

Tn hiu CP vi chiu di L (Hnh 3.9), tn hiu pha thu s l:

)()()( /2 iniueiy Nijm

i vi 0...,,1 LI , Ii hm

Nle

lliyiyE

j

s

ns

mm

22

220

)()(

Hnh 3.9: CP trong mt symbol OFDM

Hm c lng: y

2

1, vi

0

1

)()(Li

mmNiyiyy

Gi trc lng ch tha mn khi 5,0 , khi 5,0 phi thc hin

li mt gi nh ban u.

3.4.2.3 c lng CFO da trn d liu

Tn hiupha thu c biu din:

12,1,0;)(1)( /)(2 NneHkSN

nyNknj

km

Ta c th tch hai phn sau khi qua FFT:

1

0

21 )(

1)(

N

n

Nnkj

m enyN

kY

0n1Nn

1 Ln

ofdm ofdma va sung dung trong cong nghe truy cap bang rong khong day wimax 8 9024

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