Thut ng vit tt
CHNG 5: BNG SIU RNG UWB5.1 Gii thiu v bng siu rng UWB
5.1.1 nh ngha:
UWB m t cc h thng truyn dn tri ph ti 500 MHz hay t s bng tn ln
hn 20%. (1.1)
Trong B:=fH-fL ch bng tn 10 dB ca h thng, v tn s trung tm h thng
UWB vi fc=(fH+fL)/2 vi fH l tn s cao vi cng sut thp hn 10 dB so vi
tn s c cng sut cc i, v fL l tn s thp vi cng sut thp hn 10 dB so vi
tn s c cng sut cc i.
5.1.2 Cc u im ca UWB
Cc u im ca UWB c th tng kt l:1. Tc cao
2. Gi thnh thit b thp3. Chng a ng
4. o c (nh v) v truyn thng trong cng mt thi im Tc cao cho php a
ra cc ng dng v cc thit b mi m hin ti cha c. Tc ln hn 100 Mb/s t c v
c kh nng vt qua tc trn khong cch ngn.
Biu thc Shannon c biu din:
(1.2)
Trong C l dung lng ti a ca knh, vi n v [b/s]; B l bng tn knh
[Hz]; S l cng sut tn hiu [W] v N l cng sut tp m [W].
Biu thc ny ni cho thy c ba cch c th lm tng dung lng knh. C th
tng bng tn, tng cng sut tn hiu hay gim tp m. C th thy rng dung lng
knh tng tuyn tnh vi bng tn B nhng ch theo hm loga vi cng sut tn hiu
S. Knh UWB c bng tn rt ln v thc t c th hy sinh (tng) rng bng tn gim
cng sut pht v nhiu n cc ngun v tuyn khc. Qua biu thc Shannon c th
thy cc h thng UWB c kh nng cung cp tc rt cao cho cc h thng truyn
thng khng dy. .5.2 Cc phng php iu ch trong truyn thng UWB5.2.1 Gii
thiu chung: Trong h thng UWB, cc dy xung tn s cao ng vai tr sng
mng.
Mt xung UWB n bn thn n khng cha thng tin. Chng ta cn a thng tin
s vo cc xung, bng cc phng php iu ch. Trong cc h thng UWB c mt vi
phng php iu ch c bn. Ta phn loi phng php iu ch thnh hai loi c bn.
Chng c ch ra trong hnh 3.1 l cc k thut da trn thi gian v da trn dng
xung. Phng php iu ch ph bin nht trong l thuyt v UWB l iu ch v tr
xung (PPM) trong mi xung c tr c iu chnh lch so vi v tr truyn dn
danh nh mt khong thi gian chnh xc. Do , c th thit lp h thng truyn
thng nh phn bng cch dch cc xung chnh xc ln pha trc hoc v pha sau.
Mt khc, bng vic xc nh cc tr xc nh cho mi xung, c th to h thng iu ch
M trng thi.
Hnh 3.1: Phn loi cc phng php iu ch trong truyn thng UWB
Mt phng php iu ch ph bin khc l o xung: l, to mt xung vi pha ngc
li. y l phng php iu ch pha hai trng thi (BPSK). Mt k thut iu ch th
v l iu ch xung trc giao, n yu cu cc dng xung c bit c to ra trc giao
vi nhau. c cc phng php ni ting khc. V d, kho bt tt (OOK) l k thut
trong s c mt hay vng mt biu th thng tin s l 1 hay 0 tng ng. iu ch
bin xung (PAM) l mt k thut trong bin ca xung khc nhau mang thng
tin. Mt vi k thut iu ch truyn thng khng thch hp vi truyn thng UWB.
V d, phng php iu tn (FM) kh p dng cho UWB bi v mi xung mang rt nhiu
thnh phn tn s lm n rt kh iu ch. Ch rng khng nn nhm vi ghp knh phn
chia theo thi gian (FDM-frequency division multiplexing) l k thut
hon ton khc phn bit cc knh truyn thng da vo cc khong tn s ln.
Hnh 3.2 Minh ho PPM v BPSK trong truyn thng UWB
Chng ta hy kim tra cc k thut iu ch ln lt c th: Th nht, chng ta
kim tra hai k thut ph bin nht: PPM v BPSK. Mt so snh n gin ca hai
phng php c ch ra trn hnh 3.2. Trong hnh 3.2(a) a ra mt chui xung
khng iu ch so snh. Mt v d cho PPM, xung biu din thng tin 1 c pht cc
khong thi gian c xc nh bng chu k lp xung. Xung biu din thng tin 0 c
pht tr so vi v tr thng thng ny mt khong nh nh trong hnh 3.2(b). Vi
BPSK xung o biu din bit 0 trong khi xung khng o biu din bit 1. iu
ny c minh ho trong hnh 3.2(c)5.2.2 iu ch v tr xung (PPM)
Xt trng hp iu ch nh phn, trong khi bit 0 c biu din bi mt xung
thi im quy nh, bit 1 c tr mt khong thi gian tng i so vi thi im quy
nh (bit 0).
V mt ton hc c th biu din tn hiu l :
(3.1)
Trong wtr(t) l dng xung v: (3.2)
Gi tr ca c th chn thu thuc vo c im t tng quan ca xung. Hm t tng
quan ca xung c th nh ngha
(3.3)
Hnh 3.3: Cc dng xung PPM vi cc bit 1 v 0
Chng hn khi chng ta mun ci thin PPM chun vi cc tn hiu trc giao,
gi tr ti u ca gi tr(chng ta gi l ) l gi tr tho mn
(3.4)
Hnh 3.3 ch ra trng hp c bit trong bit d liu 1 c pht thi im tr so
vi thi im danh nh mt lng , trong quy nh bit 0 c pht thi im danh nh.
5.2.3 iu ch dng xung (PSM)Trong truyn thng sng dng sin bng hp, cc
hm sin v cos trc giao to nn tng cho truyn sng. Trong UWB chng ta to
cc dng xung c c tnh trc giao vi nhau. iu ch dng xung (PSM) s dng cc
dng xung khc nhau trc giao biu din bit 0 v 1. Gi s s dng hai dng
xung trc giao iu ch tn hiu. Xung pht i c th biu din l:
(3.6)
Trong
v v biu din hai dng xung khc nhau.Hai dng xung v c gi l trc giao
nu chng tho mn:
(3.7)
5.2.4 iu ch bin xung
iu ch bin xung (PAM) cho UWB c th biu din trong biu thc
(3.8)
Trong tham s dng xungc gi tr dng. V d chng ta t v c tp hp xung
nh phn s1=p(t), s2=2p(t).Ni chung, iu ch bin xung l phng php khng c
a dng trong truyn thng khong cch cc ngn. Ni chung, nhng l do ch yu
bao gm thc t l tn hiu iu ch bin c bin nh hn thng b nh hng bi tp m
nhiu hn so vi tn hiu c bin ln. Hn na, cng sut cn thit pht xung cng
ln vi xung c bin cng ln, trong khi cng sut pht rt gii hn trong h
thng UWB
.Hnh 3.5: PAM, PSM v OOK trong truyn thng UWB
5.2.5 Kho bt- tt
Kho bt tt (OOK) vi UWB c th coi thuc loi iu ch dng xung trong
tham s dng xung l 0 hoc 1, c ch ra trn biu thc (3.9)
(3.9)
V d, xung bt c to ra khi v xung tt c to ra khi ; qua , s1=p(t) v
s2=0. Kh khn ch yu ca OOK l s xut hin ca hin tng a ng, cc thnh phn
a ng n b thu gy kh khn cho vic xc nh s vng mt ca mt xung tng ng vi
bit 0. Kha bt tt thng l phng php iu ch nh phn, tng t nh BPSK, nhng
khng th m rng thnh iu ch M trng thi nh PPM, PAM, v PSM.5.3 Cc k
thut a truy nhp p dng trong UWB5.3.1 Nhy thi gian (TH) Nhy thi gian
(TH) thc hin a truy nhp bng cch chia knh thnh khe thi gian khng
chng ln nh hnh 3.10.
Hnh 3.10: Chia cc knh thnh cc khe thi gian khng chng ln
Nhng ngi dng c cho php s dng khe thi gian gn cho h bng m nhy thi
gian ring. Vic ng b yu cu nghim ngt. Ni cch khc, nu cc ngi dng khng
ng b, xung t c th xy ra, chng ln hai khe thi gian khi tham chiu thi
gian ca cc ngi dng l khc nhau. Truyn dn ch din ra trong 1/SF phn ca
thi gian v cng sut pht trong mi khe thi gian cho trc phi ln hn SF
ln cng sut pht trung bnh. Do y l bng chng v li ch so vi h thng khng
tri ph s dng mi xung trn mt bit, li x l l PG=Ns.SF. iu ny ng l v
cng sut tn hiu thu mch quyt nh tng ln mt lng SF so vi cng sut trung
bnh trn knh, trong khi cng sut tp m thu c l tng ng.
5.3.2 Tri ph trc tip (DS)
i lp vi TH trong truyn dn ch c thc hin trong 1/SF thi gian, cc
xung c pht lin tc trong h thng tri ph trc tip (DS). Cc tn hiu pht c
m ho bng cch nhn chng vi m lng cc c di , pha thu c th phn bit cc tn
hiu n t cc ngi dng khc nhau.
5.4 B pht v b thu UWB5.4.1 B pht S khi b pht UWB tng qut c ch ra
trn hnh 3.10. Th nht, d liu c to bi cc ng dng tch bit tng vt l ca b
pht. Cc ng dng c th l thu bao e-mail hoc duyt web trong mt my tnh c
nhn, ng dng lch trong thit b PDA, hoc d liu s t my chi DVD.
Hnh 3.9 S khi thu pht UWB chung
Phng php iu ch c s dng iu ch nh phn c th chuyn cc bit vo k hiu,
vi mi k hiu biu din nhiu bt. Cc k hiu ny c nh x ti mt dng xung. Cc
dng xung c to ra bi b to xung. Cn thit c mch nh thi chnh xc gi cc
xung ra tng khong yu cu. Nu PPM th cn nh thi cn chnh xc hn, thng nh
hn mt rng xung.
y l mu b thu pht cc k n gin, n b qua phng php sa li trc, n phc v
mc ch m t cc b thu pht tng i n gin. 5.4.2 B thu
Khi nim c bn
S khi b thu UWB tng qut c ch ra trong hnh 3.14. B thu thc hin
hot ng ngc li vi b pht khi phc d liu v a d liu n bt c ng dng back
end no cn n.C hai khc bit ch yu gia b thu v b pht. iu th nht l chc
chn phn ln b thu s cn mt b khuych i nng cng sut ca cc tn hiu rt yu
thu c. Th na l b thu phi thc hin chc nng d v bt tn hiu xc nh cc
xung cn thit trong s cc xung thu c v sau tip tc bm nhng xung ny iu
chnh s mt cn i xung ng h ca b thu v b pht.
Hnh 3.14: S khi chung ca b thu UWBCc my thu ci tin u thu thc hin
tch sng c th s dng cc b thu ci tin nh b thu ti u (optimal
receiver), b thu Rake, b thu gii tng quan (decorrelator receiver),
b thu tuyn tnh cc tiu li trung bnh bnh phng (LMMSE receiver) v b
thu LMMSE thch ng (adaptive LMMSE receiver). 8
_1191239424.unknown
_1191914241.unknown
_1192087469.vsdK thut da trn thi gian
iu ch v tr xung (PPM)
iu ch pha hai trng thi (BPSK)
Kho bt tt (OOK)
iu ch bin xung (PAM)
iu ch dng xung (PSM)
K thut da trn dng xung
_1192087999.vsdBit 0
Bit 1
Thi gian
_1192089980.unknown
_1192104784.vsd1
2
3
4
1
2
Tf
Tmono
t
Truyn dn
_1192087815.vsd
_1191930002.unknown
_1191930213.unknown
_1191929907.unknown
_1191260124.unknown
_1191260830.unknown
_1191630181.vsdD liu c ngha c to ra bi cc ng dng
a cc bit vo k hiu
Logic-nh x k hiu vo xung-Cc mch nh thi v,v
B to xung
Lung bit d liu
Back end
Front end
_1191758537.vsd
_1191630568.vsdD liu c ngha c to ra bi cc ng dng
Bin i k hiu thnh bt
Logic-Bt xung-Bm-nh x ngc xung vo k hiu
B to xung
Lung bit d liu
Back end
Front end
_1191260839.unknown
_1191260136.unknown
_1191239492.unknown
_1191239504.unknown
_1191239444.unknown
_1190356951.unknown
_1190357119.unknown
_1191222440.unknown
_1191222457.unknown
_1190382146.unknown
_1190402730.unknown
_1190357106.unknown
_1190316360.unknown
_1190356468.unknown
_1190316325.unknown
_1190274657.unknown
