ofdm ofdma va sung dung trong cong nghe truy cap bang rong khong day wimax 8 9024
Post on 04-Apr-2018
222 Views
Preview:
TRANSCRIPT
-
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
-
7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024
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
-
7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024
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
-
7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024
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.
-
7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024
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
-
7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024
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
-
7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024
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
-
7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024
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.
-
7/31/2019 Ofdm Ofdma Va Sung Dung Trong Cong Nghe Truy Cap Bang Rong Khong Day Wimax 8 9024
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
top related