chuong 2
Chng 2 K thut iu ch OFDM
CHNG 2 K THUT IU CH OFDM2.1 Gii thiu k thut iu ch OFDM
2.1.1 Khi nim
K thut iu ch OFDM, v c bn, l mt trng hp c bit ca phng php iu ch
FDM, chia lung d liu thnh nhiu ng truyn bng hp trong vng tn s s
dng, trong cc sng mang con (hay sng mang ph, sub-carrier) trc giao
vi nhau. Do vy, ph tn hiu ca cc sng mang ph ny c php chng ln ln
nhau m pha u thu vn khi phc li c tn hiu ban u. S chng ln ph tn hiu
ny lm cho h thng OFDM c hiu sut s dng ph ln hn nhiu so vi cc k thut
iu ch thng thng.
Hnh 2.1 So snh gia FDMA v OFDM
S lng cc sng mang con ph thuc vo nhiu yu t nh rng knh v mc nhiu.
Con s ny tng ng vi kch thc FFT. Chun giao tip v tuyn 802.16d (2004)
xc nh 256 sng mang con tng ng FFT 256 im, hnh thnh chun Fixed
WiMAX, vi rng knh c nh.Chun giao tip 802.16e (2005) cho php kch c
FFT t 512 n 2048 ph hp vi rng knh 5MHz n 20MHz, hnh thnh chun
Mobile WiMAX (Scalable OFDMA ), duy tr tng i khong thi gian khng i
ca cc k hiu v khong dn cch gia cc sng mang vi rng knh.
a) Tn hiu OFDM
b) Ph OFDM
Hnh 2.2 Tn hiu v ph OFDM
2.1.2 Lch s pht trin:
D thut ng OFDM mi ph bin rng ri gn y nhng k thut ny c xut hin
cch nay hn 40 nm:
Nm 1966, R.W. Chang pht minh ra k thut OFDM M.
Nm 1971, mt cng trnh khoa hc ca Weisteins v Ebert chng minh rng
phng php iu ch v gii iu ch OFDM c th c thc hin thng qua php bin i
IDFT (bin i Fourier ri rc ngc) v DFT ( bin i Fourier ri rc). Sau ,
cng vi s pht trin ca k thut s, ngi ta s dng php bin i IFFT v FFT
cho b iu ch OFDM.
Nm 1999, tp chun IEEE 802.11 pht hnh chun 802.11a v hot ng ca
OFDM bng tn 5GHz UNI.
Nm 2003,IEEE cng b chun 802.11g cho OFDM hot ng bng tn 2.4GHz v
pht trin OFDM cho h thng bng rng, chng t s hu dng ca OFDM vi cc h
thng c SNR( t s S/N) thp.
Ngy nay, k thut OFDM cn kt hp vi cc phng php m ha knh s dng
trong thng tin v tuyn, gi l Coded OFDM, ngha l tn hiu trc khi iu ch
s c m ha vi nhiu loi m khc nhau hn ch cc li xy ra trn knh truyn. Do
cht lng knh ( fading v t s S/N) ca mi sng mang con ph l khc nhau,
ngi ta thc hin iu ch tn hiu trn mi sng mang vi cc mc iu ch khc
nhau, gi l iu ch thch nghi (adaptive modulation) hin ang c s dng
trong h thng thng tin my tnh bng rng HiperLAN ca ETSI Chu u.2.1.3
Cc u v nhc im ca k thut OFDMNgoi u im tit kim bng thng knh truyn k
trn, OFDM cn c mt s u im sau y :
H thng OFDM c th loi b hin tng nhiu xuyn k hiu ISI (Inter-Symbol
Interference) nu di chui bo v (guard interval) ln hn tr truyn dn ln
nht ca knh truyn.
OFDM ph hp cho vic thit k h thng truyn dn bng rng. Cu trc my thu
n gin. Tuy nhin, bn cnh , OFDM cng c mt s nhc im sau :
Vic s dng chui bo v gip gim hin tng ISI do phn tp a ng nhng chui
bo v khng mang thng tin c ch, chim mt phn bng thng ca ng truyn lm
gim hiu sut ng truyn.
Do yu cu v tnh trc giao gia cc sng mang ph nn h thng OFDM kh nhy
cm vi hiu ng Dopler, dch tn (frequency offset) v dch thi (time
offset) do sai s ng b.
ng bao bin ca tn hiu pha pht khng bng phng, gy ra mo phi tuyn cc
b khuch i cng sut u pht v u thu.2.2 Nguyn l iu ch OFDM2.2.1 S trc
giao ca hai tn hiu
Nu k hiu cc sng mang con c dng trong h thng OFDM l si(t) v
sj(t). m bo tnh trc giao cho OFDM, cc hm sin ca sng mang con phi
tha mn iu kin sau :
f= l khong cch tn s gia hai sng mang con, T l thi gian k hiu, N
l s cc sng mang con, N.f l bng thng truyn dn v ts l dch thi gian.Du
* trong cng thc (2.1) ch s lin hp phc.V d: nu tn hiu l sin(mx) vi m
= 1,2. th n trc giao trong khong t - n . Trong ton hc, s hng trc
giao c c t vic nghin cu cc vector. Theo nh ngha, hai vect c gi l
trc giao vi nhau khi chng vung gc vi nhau (to nhau mt gc 900) v tch
ca 2 vect l bng 0.
Hnh 2.3 Tch ca hai vect vung gc
2.2.2 S iu ch Hnh 2.4 B iu ch OFDMGi s bng thng h thng l B chia
thnh Nc knh con, vi ch s knh con l n, n , nn NFFT=2L+1. Dng d liu u
vo chia thnh NFFT dng song song vi tc d liu gim i NFFT ln thng qua
b chia ni tip/song song. Dng bit trn mi lung song song li c iu ch
thnh mu ca tn hiu phc a mc, n l ch s song mang ph, i l ch s khe thi
gian tng ng vi Nc bit song song sau khi qua b S/P, k l ch s khe thi
gian ng vi Nc mu tn hiu phc.Cc mu tn hiu pht c nhn vi xung c s gii
hn ph ca mi sng mang, sau c dch tn ln n knh con tng ng bng vic nhn
vi hm phc ejLt , lm cc tn hiu trn cc sng mang trc giao nhau. Tn hiu
sau khi nhn vi xung c s v dch tn cng li qua b tng v cui cng c biu
din nh sau: mk(t)= (2.3)Tn hiu ny c gi l mu tn hiu OFDM th k, biu
din tng qut tn hiu OFDM s l: m(t)==
(2.4)Trc khi pht i th tn hiu OFDM c chn thm chui bo v chng nhiu
xuyn k hiu ISI.
Php iu ch OFDM c th thc hin c thng qua php bin i IDFT v php gii
iu ch OFDM c th thc hin c bng php bin i DFT. 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. iu ch OFDM bng phng php bin i ngc Fourrier nhanh
cho php mt s lng ln cc sng mang con vi phc tp thp.2.2.3 Thc hin b
iu ch bng thut ton IFFTTn hiu sau b gii iu ch OFDM khi chuyn i tng
t thnh s, lung tn hiu trn c ly mu vi tn s ly mu: ta=== (2.5)
ti thi im ly mu t=kT+lta,, S(t-kT) =S0, do vy (2.3) vit li :
mk(kTs+lta) = S0
=S0 (2.6) Do SkTS = 2, kt qu
Tng t nh vy, vi , (2.6) c vit li: mk(kTs+lta)=S0 (2.7)
Php biu din (2.7) trng vi php bin i IDFT. Do vy b iu ch OFDM c
th thc hin mt cch d dng bng php bin i IDFT.2.2.4 Chui bo v trong h
thng OFDMu im ca phng php iu ch OFDM khng ch th hin hiu qu s dng
bng thng m cn c kh nng lm gim hay loi tr nhiu xuyn k hiu ISI (Inter
Symbol Interference) nh s dng chui bo v (Guard Interval- GI ). Mt
mu tn hiu c di l TS, chui bo v tng ng l mt chui tn hiu c di TG pha
sau c sao chp ln phn pha trc ca mu tn hiu nh hnh v sau:
Hnh 2.5 Chui bo v GI
Do , GI cn c gi l Cyclic Prefix (CP). S sao chp ny c tc dng chng
li nhiu xuyn k hiu ISI do hiu ng phn tp a ng.Nguyn tc ny gii thch
nh sau: Gi s my pht i mt khong tn hiu c chiu di l Ts, sau khi chn
thm chui bo v c chiu di TG th tn hiu ny c chiu di l T = TS+TG. Do
hiu ng a ng multipath, tn hiu ny s ti my thu theo nhiu ng khc nhau.
Trong hnh v m t trang bn,hnh a,tn hiu theo ng th nht khng c tr, cc
ng th hai v th ba u b tr mt khong thi gian so vi ng th nht. Tn hiu
thu c my thu s l tng hp ca tt c cc tuyn, cho thy k hiu ng trc s
chng ln vo k hiu ngay sau , y chnh l hin tng ISI.Do trong OFDM c s
dng chui bo v c di TG s d dng loi b hin tng ny. Trong trng hp TG
MAX nh hnh v m t th phn b chng ln ISI nm trong khong ca chui bo v,
cn thnh phn tn hiu c ch vn an ton. pha my thu s gt b chui bo v trc
khi gi tn hiu n b gii iu ch OFDM.Do , iu kin cn thit cho h thng
OFDM khng b nh hng bi ISI l:
TG MAX (2.8)vi MAX l tr truyn dn ti a ca knh.
a ) Khng c GI
b) C GI
Hnh 2.6 Tc dng ca chui bo v Vic s dng chui bo v m bo tnh trc
giao ca cc sng mang con, do vy n gin ho cu trc b nh gi knh truyn, b
cn bng tn hiu my thu. Tuy nhin, do chui bo v khng mang thng tin c
ch nn tng ph ca tc truyn nn ph tn hiu s tng, tiu tn bng thng, lm
gim hiu sut s dng bng thng mt lng l:
(2.9)2.2.5 Php nhn vi xung c bn
Trong a s cc h thng v tuyn, tn hiu trc khi truyn i u c nhn vi
xung c bn. Mc ch chnh l gii hn ph tn hiu pht sao cho ph hp vi rng
knh truyn.Trong trng hp rng ph tn hiu ln hn rng knh truyn th s gy
nhiu xuyn knh cho h thng khc. Trong OFDM, tn hiu trc khi pht i c
nhn vi xung c bn c b rng ng bng b rng ca mt mu tn hiu OFDM, xung c
bn thng l xung vung hay xung ch nht. Sau khi chn thm chui bo v th
xung c bn k hiu l S(t) c rng l TS + TG.
Hnh 2.7 Xung c bnTrong thc t xung c bn thng c s dng l b lc cos
nng (Raise cosine filter).2.3 Nguyn l gii iu ch OFDM2.3.1 Truyn dn
phn tp a ng
Knh truyn dn phn tp a ng,v mt ton hc, c biu hin qua p ng xung
h(, t) v hm truyn t H(j , t).i vi p ng xung, bin l tr truyn dn ca
knh, l khong thi gian tn hiu i t my pht n my thu. Bin i Fourier ca
p ng xung cho ta hm truyn t ca knh
H(j,t) = (2.10)Gi s khng c AWGN, mi lin h gia tn hiu thu u(t),
tn hiu pht m(t) v p ng xung:
Hnh 2.8 M hnh knh truyn
Trong min thi gian l tch chp ca tn hiu pht v p ng xung ca
knh:
u(t) = m(t) * h()
= (2.11) 2.3.2 Nguyn tc gii iu ch
S
Hnh 2.9 B thu tn hiu OFDM
Cc bc thc hin y u ngc li so vi pha my pht. Tn hiu thu s c tch
chui bo v, gii iu ch khi phc bng tn gc, gii iu ch cc sng mang con,
chuyn i mu tn hiu phc thnh dng bt (tn hiu s) v chuyn i song song
sang ni tip
Hnh 2.10 Tch chui bo v
Sau khi tch chui bo v khi lung tn hiu u(t), lung tn hiu nhn c l:
u(kTS+t)=u(kT+t) (2.12) Thc hin gii iu ch bng thut ton FFT
Gi thit mt mu tin OFDM Ts c chia thnh NFFT mu tn hiu, tn hiu c
ly mu vi chu k ly mu l ta. Khi rng mt mu l: ta = (2.13)Sau khi ly
mu, tn hiu nhn c s tr thnh lung tn hiu s: u(t) => uk(kTs + nta)
, n=0,1,2,....,NFFT 1 (2.14)Mu tn hiu sau khi gii iu ch c biu din
di dng s: = (2.15)Tch s biu din thnh phn m thnh tch hai thnh phn
(2.15) c vit li di dng: = (2.16)Vi , th
Mt khc, nn (2.16) vit li: = ( 2.17)Biu thc trn chnh l php biu
din DFT vi chiu di NFFT.2.4 ng dng v hng pht trin ca k thut iu ch
OFDMNgy nay, k thut OFDM c tiu chun ho l phng php iu ch cho cc h
thng pht thanh s nh DAB (Digital Audio Broadcasting), DRM (Digital
Radio Mondiale - h thng pht thanh s ng di thay cho h thng AM), cc h
thng truyn hnh s mt t DVB-T (Digital Video Broadcasting for
Terrestrial Transmission Mode), DVB-H (Digital Video Broadcasting
for Handheld) v t ngi bit rng s nng cao tc ng truyn trong h thng
ADSL l nh k thut OFDM.Nh k thut iu ch a sng mang v cho php chng ln
ph gia cc sng mang m tc truyn dn trong ADSL tng ln ng k.2.4.1 H
thng DRM
Hnh 2.11 H thng DRM
DRM l h thng pht thanh s thay th cho h thng pht thanh truyn thng
AM. Tn s sng mang cho h thng DRM tng i thp, nh hn 30MHz, ph hp cho
vic truyn sng khong cch ln. Mi trng truyn sng ca h thng l knh phn
tp a ng c s tham gia phn x mt t v tng in li nn phm vi ph sng ca DRM
rt ln, c th ph sng a quc gia hay lin lc a. Cc tham s c bn ca DRM
theo ETSI, nh sau:
rng bng: B=9.328kHz
di FFT: NFFT= 256.
S sng mang c s dng truyn tin: NC=198.
Do tr truyn dn tng i ln nn h thng DRM c thit k ch dnh cho cc my
thu tnh hay xch tay. iu ny khc hn so vi h thng DAB hay DVB c thit k
cho my thu c tc di chuyn tng i ln nh t, tu ho.
2.4.2 Cc h thng DVB
DVB-T
Gii thiu :
Th h my pht s ra i khc phc nhc im ca my pht tng t nh kh nng mang
nhiu chng trnh trn mt knh RF, h tr kh nng thu tn hiu a ng v thu di
ng My pht s DVB-T v my pht hnh tng t ging nhau, ch khc nhau phn iu
ch.
Hnh 2.12 S khi b DVB-T
c im :
Tn hiu truyn i c t chc thnh tng khung, c 4 khung lin tip to thnh
1 siu khung.L do ca vic to khung l phc v t chc mang thng tin tham s
ca pha pht bng cc sng mang bo hiu thng s pha pht (Transmission
Parameters Signalling carriers- TPS). Vic hnh thnh siu khung l chn
s nguyn ln gi m sa sai reed-Solomon 204 byte trong dng truyn ti
MPEG-2 d ta chn bt k cu hnh no trnh vic chn thm cc gi m khng cn
thit. Mi khung cha 68 symbol OFDM trong min thi gian c nh s t 0 n
67.Mi symbol ny cha hng ngn sng mang (6817 vi ch 8K, 1705 vi ch 2K)
nm dy c trong di thng 8MHz ( nc ta chn di thng 8MHz, mt s nc khc
chn 7MHz). Nh vy, mt symbol ODFM s cha:
Sng mang d liu: c iu ch M-QAM, vi mode 8K l 6048 sng mang v mode
2K l 1512.
Sng mang dn ng (pilot symbol, mang thng tin pha pht khi phc tn
hiu: cc pilot ny thng c iu ch BPSK vi mc cng sut 2.5dB Pilot lin
tc: gm 177 pilot vi mode 8K, 15 vi mode 2K, c v tr c nh trong 8MHz
pha thu sa li tn s v pha, t ng iu chnh tn s.
Pilot ri rc: 524 vi mode 8K, 131 vi mode 2K, khng c v tr c nh
trong min tn s nhng c ri u trong di tn 8MHz, gip u thu t ng iu chnh
t p ng knh tt nht.
Sng mang thng s pht TPS: cha nhm thng s pht c iu ch BPSK, gm 68
sng mang trong mode 8K, 17 trong mode 2K lun c v tr c nh trong biu
chm sao BPSK v trong di thng 78MHz.
trnh nhiu gia cc k hiu ISI v nhiu tng h gia cc sng mang ICI,
ngui ta thc hin chn thm chui bo v GI vo mi symbol. Vic chn thm ny c
thc hin bn pha pht vi thi gian bo v TG khc nhau theo quy nh ca DVB:
1/4 TU, 1/8 TU, 1/16TU, 1/32 TU (TU: chiu di phn tn hiu c ch).
DVB-H : in thoi di ng truyn hnh
S :
Hnh 2.13 S thu ca DVB-H
Cu trc my thu ca in thoi di ng DVB-H c cho trn hnh gm 2 phn:
Mt b gii iu ch DVB-H (gm khi iu ch DVB-T, module Time slicing v
module MPE-FEC) v mt u cui DVB-H.
Tn hiu vo l tn hiu DVB-T. Khi iu ch DVB-T thu li cc gi dng truyn
ti MPEG-2, tn hiu ny cung cp cc mode truyn dn (2K, 8K v 4K) vi cc
tn hiu mang thng s truyn dn - TPS tng ng.Module Time Slicing gip
tit kim cng sut tiu th v h tr vic chuyn giao mng linh hot hn.
Module MPE-FEC cung cp m sa li tin cho php b thu c th ng u vi cc iu
kin thu c bit kh khn. Tn hiu ra khi gii iu ch DVB-H c dng cc gi ca
dng truyn ti TS hoc cc IP Datagrams (khi thu tn hiu DVB-H).u cui
DVB-H gii m cc IP Datagrams,hin th ni dung ca cc chng trnh
DVB-H.
Hin nay nhiu hng sn xut in thoi c cc th h TD DVB-H u tin: NOKIA
7700 v 7710, PHILIPS HoTMAN 2, SIEMENS
Kin trc ban u ca cc my TD DVB-H hin nay gm: in thoi tch hp 3 bng
tn s: GSM, GPRS v UMTS (3G).
B thu DVB-H. Camera 1.3M pixel. Mn hin th VGA (640 x 480). Mn
hnh cm bin - touch screen. m thanh ng ra Stereo. H tr chun khng dy
Bluetooth. B nh trong c dung lng 1Gbit
Hng NOKIA tuyn b t nay hng s tung ra th trng khong 60 thit b
sang trng c tch hp DVB-H .
DVB-H ang c nhng u th vt tri ca mnh: Tit kim nng lng Pin ti 90%,
thu tn hiu trong mi trng di ng tt, tn hiu c ng gi dng IP v truyn tn
hiu di dng qung b ti cc my in thoi di ng. Bi vy ng dng cng ngh qung
b DVB-H cho ng xung (downlink) ca cc my in thoi trong mng di ng dng
nh l mt gii php mang tnh t ph m cc th h mng vin thng 2G (GSM); 2,5G
(GPRS) v 3G (UMTS) hin nay cha th khc phc ngay c. l khng b hn ch v
bng thng khi ti cng mt thi im s thu bao s cc dng dch v truyn hnh
trc tuyn tng vt.
S hi t gia cng ngh qung b DVB-H v vin thng ang c cc hng truyn
thng ln trn th gii th nghim ti nhiu nc trn th gii (m hnh DVB-H
& GPRS hay DVB-H & UMTS) v chnh thc a ra sn phm ca mnh vo u
nm 2005 nh Nokia, Philips, Siemens...cc hng sn sut my pht s hng u
th gii (Harris, Intelco, Rohde&Schwarz...) cng xut xng cc thit
b tch hp cng ngh DVB-H. Cuc ua gia in thoi di ng 3G v in thoi di ng
truyn hnh DVB-H bt u! Vi nhng u th ca mnh, TD truyn hnh cng ngh
DVB-H ang m ra nhng trin vng mi cho ngi s dng.
STTc imDVB-TDVB-H
1Tc bit ca mt knh truyn hnh c nt tiu chun SDTV4-5 Mbit/s128-384
Kbit/s
2Mn hin thMn hnh TV c trung bnh v ln.Mn hnh in thoi nh.
3AntenAnten trn mi nh (anten Yagi), trong nh (anten roi) hoc
anten trn t.Anten bn trong in thoi.
4Ngun cung cpC nh v l ngun lin tc.Ngun nng lng Pin v c gii
hn.
5Ch thuThu c nh, thu xch tay trong nh v thu trn phng tin giao
thng.Cc my cm tay di ng.
Bng 2.1 So snh gia DVB-T v DVB-H
Hin nay vi cng ngh pht s mt t (DVB-T) chng ta c th pht c khong
6-7 chng trnh TV (SDTV) trn mt knh sng (vi tc tng l 27,14 Mbit/s).
Trong khi cng ngh IP Datacast (DVB-H) d dng tng thch vi cc mn hnh c
nh (vi inch) ca cc u cui cm tay. Vi mn hnh nh th ch vi tc 128-384
Kbit/s trn mt knh (hay mt chng trnh TV yu cu) c th phn phi mt knh
video cht lng cao. Chnh cng ngh ny lm tng hiu qu ca qu trnh pht
qung b v c th truyn c t 10 n 55 chng trnh TV trn mt knh sng. Ngoi
ra, trong h thng thng tin di ng th h th 4 (4G), k thut OFDM cn cn
th kt hp vi cc k thut khc nh phn tp anten (MIMO- Multi In Multi
Out- a anten pht thu) nhm nng cao dung lng knh v tuyn v kt hp vi
CDMA nhm phc v dch v a truy nhp vo mng.Mt vi hng nghin cu vi mc ch
thay i php bin i FFT trong b iu ch OFDM bng Wavelet nhm ci thin s
nhy cm ca h thng i vi hiu ng dch tn do mt ng b v gim di ti thiu ca
chui bo v trong OFDM; tuy nhin, kh nng ng dng nhng k thut ny cn c
kim chng trong tng lai.
Xung c s
Xung c s
Xung c s
X
X
X
ejL EMBED Equation.3 t
ejn EMBED Equation.3 t
Hnh 1.10 Tch ca hai vect vung gc bng 0
kTS
kT
(k-1)TS
(k-1)T
t
GI
Phn tn hiu c ch
Phn tn hiu c ch
e- jL EMBED Equation.3 t
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
EMBED Equation.3
m,(t)
m(t)
PAGE 27
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