OFDM_-_HVCNBCVT

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http://www.ebook.edu.vnChng 1: Gii thiu chung

Chng 1

Gii thiu chung

1.1. Nhng hn ch ca k thut hin hnh

K thut n sng mang

Cc k thut tri ph c s dng trong cc h thng thng tin di ng th h ba ckh nng chng li pha inh v nhiu, song tn ti nhng yu cu khng thc hin cchng hn: nu ngi dng cn c tc 20 Mbps giao din v tuyn v h s tri phl 128 (gi tr in hnh hin nay), dn n phi x l tc 2,56 Gbps theo thi gianthc v th cn c rng bng tn ln khng thc t. Mt khc, thy r

Do ti nguyn ph tn hn hp, v vy cn phi s dng hiu qu. Do nhng kh khn lin quan n hiu ng gn xa v c s tiu th cng sut

ln.

Ngoi ra, cc k thut n sng mang i ph km hiu qu i vi pha inh v truynlan a ng c bit trong trng hp tc bit rt cao. cc phng php iu chtruyn thng M-QAM, M-PSK, khi tc d liu truyn cao th ko theo rng khiu s gim, n mt gi tr m rng k hiu < tri tr cc i ca knh, khi knhs l knh la chn tn s v gy ISI cho tn hiu thu. y l mt nhc im chnhkhin cc h thng s dng cc phng php iu ch truyn thng khng th truyn dliu vi tc cao, hoc gi thnh rt cao i vi nhng dch v yu cu tc d liucao.

K thut a sng mang trc giao OFDM

OFDM l mt cng ngh cho php tng rng k hiu truyn dn do dung sai ang ln hn rt nhiu so vi cc k thut s dng trc y, cho php khc phcnhng nhc im cn bn ca k thut n sng mang.

1.2. Ghp knh phn chia theo tn s trc giao OFDM

Ghp knh phn chia theo tn s trc giao (OFDM) l mt k thut iu ch c th thayth cho CDMA. OFDM c u im vt tri so vi nhng h thng CDMA v cung cp

phng php truy cp khng dy cho h thng 4G.

tng ca OFDM l chia ton b bng tn truyn dn thnh nhiu sng mang con trcgiao nhau truyn cc tn hiu trong cc sng mang con ny song song. Theo , lungd liu tc cao c chia thnh nhiu lung tc thp hn lm cho chu k k hiutng theo s sng mang con.

u im

OFDM l gii php phn tp tn s. OFDM chia nh bng tn knh v tin hnh

truyn d liu c lp trn cc bng tn knh con ny.


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http://www.ebook.edu.vnChng 1: Gii thiu chung

OFDM t hiu qu s dng ph tn cao, do tnh trc giao ca cc thnh phnsng mang con.

OFDM l ng c vin ha hn cho truyn dn tc cao trong mi trng ding. S d OFDM lm c nh vy bi v, chu k k hiu tng cho nn dungsai tri tr ca h thng tng v hiu qu s dng ph tn cao ca cng ngh

OFDM. OFDM cho php gim c nh hng ca tr a ng v knh pha inh chn

lc tn s chuyn thnh knh pha inh phng. V vy, OFDM l gii php i vitnh chn lc tn s ca knh pha inh. Thun li ny ca OFDM cho php cn

bng knh d dng.

Do tri rng pha inh tn s trn nhiu k hiu, nn lm ngu nhin ho li cm(do pha inh Rayleigh gy ra), nn thay v mt s k hiu cnh nhau b mo honton l mt s k hiu cnh nhau b mo.

Tnh kh thi ca OFDM cao do ng dng trit cng ngh x l tn hiu s vcng ngh vi mch VLSI.

Nhc im

OFDM nhy cm vi dch Doppler cng nh lch tn gia cc b dao ng nipht v thu. Do tnh trc giao ca cc sng mang con rt nhy cm vi knhtruyn c dch Doppler ln.

Vn ng b thi gian. Ti my thu kh quyt nh thi im bt u ca khiu FFT.


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http://www.ebook.edu.vnChng 2: c tnh knh v tuyn di ng

Chng 2

c tnh knh v tuyn di ng

2.1. M u

Trong thng tin v tuyn di ng, cc c tnh knh v tuyn di ng c tm quan trngrt ln, v chng nh hng trc tip ln cht lng truyn dn v dung lng. Trongcc h thng v tuyn thng thng (khng phi cc h thng v tuyn thch ng), cctnh cht thng k di hn ca knh c o v nh gi trc khi thit k h thng.

Nhng trong cc h thng iu ch thch ng, vn ny phc tp hn. m bohot ng thch ng ng, cn phi lin tc nhn c thng tin v cc tnh cht thngk ngn hn thm ch tc thi ca knh.

Cc yu t chnh hn ch h thng thng tin di ng bt ngun t mi trng v tuyn.Cc yu t ny l:

Suy hao: cng trng gim theo khong cch. Thng thng suy hao nmtrong khong t 50 n 150 dB ty theo khong cch

Che ti: cc vt cn gia trm gc v my di ng lm suy gim thm tn hiu

Pha inh a ng v phn tn thi gian: phn x, nhiu x v tn x lm motn hiu thu bng cch tri rng chng theo thi gian. Ph thuc vo bng thngca h thng, yu t ny dn n thay i nhanh cng tn hiu v gy ranhiu giao thoa gia cc k hiu (ISI: Inter Symbol Interference).

Nhiu: cc my pht khc s dng cng tn s hay cc tn s ln cn khc gynhiu cho tn hiu mong mun. i khi nhiu c coi l tp m b sung.

C th phn cc knh v tuyn thnh hai loi: "pha inh phm vi rng" v "pha inhphm vi hp". Cc m hnh truyn sng truyn thng nh gi cng sut trung bnh thuc ti cc khong cch cho trc so vi my pht. i vi cc khong cch ln (vikm), cc m hnh truyn sng phm vi rng c s dng. Pha inh phm vi rng c

biu th bng tn hao do truyn sng khong cch xa. Pha inh phm vi hp m t sthng ging nhanh sng v tuyn theo bin , pha v tr a ng trong khong thigian ngn hay trn c ly di chuyn ngn. Pha inh trong trng hp ny gy ra dotruyn sng a ng.

Cc knh v tuyn l cc knh mang tnh ngu nhin, n c th thay i t cc ngtruyn thng n cc ng b che chn nghim trng i vi cc v tr khc nhau. Hnh2.1(a) cho thy rng trong min khng gian, mt knh c cc c trng khc nhau (bin chng hn) ti cc v tr khc nhau. Ta gi c tnh ny l tnh chn lc khng gian(hay phn tp khng gian) v pha inh tng ng vi n l pha inh chn lc khnggian. Hnh 2.1(b) cho thy trong min tn s, knh c cc c tnh khc nhau ti cc tns khc nhau. Ta gi c tnh ny l tnh chn lc tn s (hay phn tp tn s) v phainh tng ng vi n l pha inh chn lc tn s. Hnh 2.1(c) cho thy rng trong min

thi gian, knh c cc c tnh khc nhau ti cc thi im khc nhau. Ta gi c tnhny l tnh chn lc thi gian (hay phn tp thi gian) v pha inh do n gy ra l phainh phn tp thi gian. Da trn cc c tnh trn c th phn chia pha inh knh thnh:

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pha inh chn lc khng gian (pha inh phn tp khng gian), pha inh chn lc tn s(pha inh phn tp tn s), pha inh chn lc thi gian (phn tp thi gian ). Chngny s xt cc tnh cht knh trong min khng gian, thi gian v tn s.

2.2. Min khng gian

Cc thuc tnh trong min khng gian gm: tn hao ng truyn v chn lc khnggian. Tn hao ng truyn thuc loi pha inh phm vi rng cn chn lc khng gianthuc loi pha inh phm vi hp. Cc m hnh truyn sng truyn thng nh gi cngsut thu trung bnh ti mt khong cch cho trc so vi my pht, c gi l nh gitn hao ng truyn. Khi khong cch thay i trong phm vi mt bc sng, knh thhin r cc c tnh ngu nhin. iu ny c gi l tnh chn lc khng gian (hay

phn tp khng gian).

Tn hao ng truyn

M hnh tn hao ng truyn m t suy hao tn hiu gia anten pht v anten thu lmt hm ph thuc vo khong cch v cc thng s khc. Mt s m hnh xt chi titv a hnh nh gi suy hao tn hiu, trong khi mt s ch xt tn s v khongcch. Chiu cao anten l mt thng s quan trng. Tn hao do khong cch truyn dns tun theo quy lut hm m.

PL d-n (2.1)

trong n l m tn hao (n=2 cho khng gian t do, n2 cho cc vng thnh ph ngoi tri), d l khong cch t my thu n my pht.

T l thuyt v cc kt qu o lng cho thy cng sut thu trung bnh gim so vikhong cch theo hm log i vi mi trng ngoi tri v trong nh. Hn na ti mikhong cch d, tn hao ng truyn PL(d) ti mt v tr nht nh l qu trnh ngunhin v c phn b log chun xung quanh mt gi tr trung bnh (ph thuc vo khong

cch). Nu xt c s thay i theo v tr, c th biu din tn hao ng truyn PL(d) tikhong cch d nh sau:

Phan Minh c K54B

Hnh 2.1. Tnh cht knh trong min khng gian, min tn s vmin thi gian

(a) (b) (c)

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( ) [ ] ( ) ( ) +

+=+= X

d

dlgn10dPLXdPLdBdPL

0

_________

0

________

(2.2)

Trong ( )_______

dPL l tn hao ng truyn trung bnh phm v rng i vi khong cch

pht thu d; X l bin ngu nhin phn b Gauss trung bnh khng (o bng dB) vi lch

chun (cng o bng dB), d0 l khong cch tham chun gia my pht v my thu, nl m tn hao ng truyn.

Khi cc i tng trong knh v tuyn khng chuyn ng trong mt khong thi giancho trc v knh c c trng bi pha inh phng i vi mt rng bng tn chotrc, cc thuc tnh knh ch khc nhau ti cc v tr khc nhau. Ni mt cch khc, phainh ch n thun l mt hin tng trong min thi gian (mang tnh chn lc thigian).

T phng trnh 2.2 thy tn hao ng truyn ca knh c nh gi thng k phmvi rng cng vi nh hng ngu nhin. nh hng ngu nhin xy ra do pha inh

phm vi hp trong min thi gian v th hin cho tnh chn lc thi gian (phn tp thigian). nh hng ca chn lc khng gian c th c loi b bng cch s dng nhiuanten. MIMO (Multiple Input Multiple Output: Nhiu u vo nhiu u ra) l mt kthut cho php li dng tnh cht phn tp khng gian ny ci thin hiu nng vdung lng h thng.

2.3. Min tn s

Trong min tn s, knh b nh hng bi hai yu t: iu ch tn s v chn lc tn s.

2.3.1. iu ch tn siu ch tn s do hiu ng Doppler gy ra, khi c s chuyn ng tng i gia mythu v my pht dn n thay i tn s mt cch ngu nhin. Do chuyn ng tngi gia BTS v MS, cc thnh phn sng a ng b dch tn s. Dch tn s trong tns thu do chuyn ng tng i ny c gi l dch tn s Doppler, n t l vi tc chuyn ng, phng chuyn ng ca MS so vi phng sng ti ca thnh phn snga ng. Dch Doppler BD c th c biu din nh sau:

( ) ( )

=

= cosfc

cosB cD (2.3)

Trong l tc ca MS, l bc sng, l gc gia phng chuyn ng caMS v phng sng ti, c l tc nh sng v fc l tn s sng mang. T phng trnhcho trn thy nu MS di chuyn v pha sng ti dch Doppler l dng v tn s thu stng, ngc li nu MS di chuyn ri xa sng ti th dch Doppler l m v tn s thuc s gim. V th cc tn hiu a ng n MS t cc phng khc nhau s lmtng rng bng tn tn hiu. Khi v (hoc ) thay i dch Doppler thay i dn ntri Doppler.

2.3.2. Chn lc tn s

Sau y, chng ta s phn tch chn lc tn s cng vi mt thng s khc trong min

tn s: bng thng nht qun. Bng thng nht qun l mt s o thng k ca di tn strn mt knh pha inh c coi l knh pha inh "phng" (l knh trong tt c ccthnh phn ph c truyn qua c khuch i nh nhau v pha tuyn tnh). Bng thng

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nht qun cho ta di tn trong cc thnh phn tn s c bin tng quan. Bngthng nht qun xc nh kiu pha inh xy ra trong knh v v th c ngha c strong vic thch ng cc thng s iu ch. Bng thng nht qun t l nghch vi tritr (xem phn 2.5). Pha inh chn lc tn s rt khc vi pha inh phng. Trong cngmt knh pha inh phng, tt c cc thnh phn tn s truyn qua bng thng knh uchu nh hng ca pha inh. Ngc li pha inh chn lc tn s (cn gi l pha inh visai), mt s on ph ca tn hiu qua knh pha inh b nh hng nhiu hn cc phnkhc, th hin r tnh chn lc tn s ca knh ny. Nu bng thng nht qun knh nhhn rng bng tn ca tn hiu c truyn qua knh ny, th tn hiu ny chu nhhng ca pha inh chn lc ( phn tp tn s). Pha inh ny s lm mo tn hiu.

2.4. Min thi gian

S khc bit gia cc knh hu tuyn v cc knh v tuyn l knh v tuyn thay itheo thi gian, ngha l pha inh chn lc thi gian. C th m hnh ha knh v tuyndi ng nh l mt b lc tuyn tnh c p ng xung kim thay i theo thi gian. Mhnh knh truyn thng s dng m hnh p ng xung kim, y l mt m hnh trongmin thi gian. C th lin h qu trnh thay i tn hiu v tuyn phm vi hp trc tipvi p ng xung kim ca knh v tuyn di ng. Nu x(t) l tn hiu pht, y(t) l tnhiu thu v h(t,) l p ng xung kim ca knh v tuyn a ng ph thuc vo thigian, th tn hiu thu l tch chp ca tn hiu pht vi p ng xung kim ca knh nhsau:

( ) ( ) ( ) ( ) ( )==

,thtxd,thtxty (2.4)

Trong t l bin thi gian, l tr a ng ca knh i vi mt gi tr t c nh, *l k hiu tch chp.

nh hng a ng ca knh v tuyn thng c bit n dng phn tn thi gianhay tri tr. Phn tn thi gian (tn thi), hay tri tr xy ra khi mt tn hiu c truynt anten pht n anten thu qua hai hay nhiu ng c cc di khc nhau. Mt mttn hiu ny c truyn trc tip, mt khc n c truyn t cc ng phn x khcnhau c di khc nhau vi cc thi gian n my thu khc nhau. V vy tn hiu tianten thu chu nh hng ca tn thi ny s b mo dng. Khi thit k v ti u ha cch thng v tuyn s truyn s liu tc cao ta cn xt cc phn x ny.

Tn thi c th c c trng bi tr tri, tr tri trung bnh hay tr tri trung bnhqun phng.

2.4.1. Tr tri trung bnh qun phng

Thng s thi gian quan trng ca tn thi l tri tr trung bnh qun phng (RDS:Root Mean Square Delay Spread): cn bc hai mment trung tm ca l lch tr cngsut. RDS nh gi cho tri a ng ca knh. V th c s dng nh gi nhhng ca nhiu giao thoa gia cc k hiu (ISI).

2_____2 =

(2.5)

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( )

( )

=

k

k

k

kk

P

P

(2.6)

( )

( )

=k

k

k

2

kk____

2

P

P

(2.7)

trong P(k) l cng sut trung bnh a ng ti thi im k.

2.4.2. Tr tri cc i

Tr tri cc i (XdB) ca l lch tr cng sut c nh ngha l tr thi gian m nng lng a ng gim XdB so vi nng lng cc i.

2.4.3. Thi gian nht qun

Mt thng s khc trong min thi gian l thi gian nht qun. Thi gian nht qunxc nh c tnh "tnh" ca knh. Thi gian nht qun l thi gian m knhtng quan rt mnh vi bin ca tn hiu thu, c k hiu l Tc. Cc k hiukhc nhau truyn qua knh trong khong thi gian nht qun chu nh hng pha inhnh nhau. V th nhn c mt knh pha inh kh chm. Cc k hiu khc nhautruyn qua knh bn ngoi thi gian nht qun s bnh hng pha inh khc nhau.Khi ny knh pha inh kh nhanh. Nh vy di tc ng ca pha inh nhanh, mt s

phn ca k hiu tin s chu tc ng pha inh ln hn cc phn khc.

2.5.Quan h gia cc thng s trong cc min khc nhau

Trn y ch ra cc c tnh knh v cc thng s ca n trong cc min khng gian,tn s v thi gian. Cc c tnh ny khng tn ti c lp nhau m c quan h mt thitgia cc min xt. Mt s thng s trong min ny nh hng ln cc c tnh ca minkhc.

2.5.1. Bng thng nht qun v tri tr trung bnh qun phng

Thy r, l lch tr cng sut v p ng tn s bin ca knh v tuyn di ng quan hvi nhau qua bin i Fourrier. V th, c th trnh by knh trong min tn s bng cchs dng cc c tnh p ng tn s ca n. Tng t nh cc thng s tri tr trong

min thi gian, ta c th s dng bng thng nht qun c trng knh trong min tns. Tuy tri tr trung bnh qun phng t l nghch vi bng thng nht qun v ngcli, song quan h chnh xc ca chng l mt hm ph thuc vo cu trc a ng. Nuk hiu bng thng nht qun l BC v tri tr trung bnh qun phng l , th khi hmtng quan ng bao ln hn 90%, bng thng nht qun c quan h sau y vi tritr trung bnh qun phng:

50

1BC (2.8)

Cho thy hai thng s trn lin quan cht ch vi nhau, nn ch cn xt mt thng strong qu trnh thit k h thng.

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2.5.2. Thi gian nht qun v tri Doppler

Thi gian nht qun chu nh hng trc tip ca dch Doppler, l thng s knh trongmin thi gian v c tnh i ngu vi tri Doppler. Tri Doppler v thi gian nht qunl hai thng s t l nghch vi nhau. Ngha l:

DC B

1

T (2.9)

Khi thit k h thng ch cn xt mt trong hai thng s ni trn l .

2.6. Cc loi pha inh phm vi hp

Tu vo quan h gia cc thng s tn hiu ( rng bng tn, chu k k hiu,) v ccthng s knh (tri tr trung bnh qun phng, tri Doppler, ), m xc nh loi phainh phm vi hp da trn hai c tnh: Tri tr a ng v pha inh chn lc tn s.Tri tr a ng l mt thng s trong min thi gian, trong khi vic knh l pha

inh phng hay chn lc tn s li xt trong min tn s. V th thng s min thi gian,tri tr a ng, nh hng ln c tnh knh trong min tn s. Tri Doppler dn ntn tn v pha inh chn lc thi gian, v th da vo tri Doppler phn loi pha inh

phm vi hp thnh pha inh nhanh v pha inh chm. Tri Doppler l mt thng strong min tn s trong khi hin tng knh thay i nhanh hay chm li thuc minthi gian. Vy trong trng hp ny, tri Doppler, thng s trong min tn s, nhhng ln c tnh knh trong min thi gian. Bit c cc quan h ny s tr giptrong qu trnh thit k h thng. Bng 2.1 lit k cc loi pha inh phm vi hp.

Bng 2.1. Cc loi pha inh phm vi hp

C s phn loi Loi Pha inh iu kinTri tr a ng Pha inh phng BS


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hiu nng truyn dn, cn chn gi tr chu k k hiu trong gii thut iu ch thch ng t c knh pha inh phng thay v knh pha inh chn lc.

Da trn tri Doppler, phn loi knh thnh pha inh nhanh v pha inh chm. Nup ng xung kim knh (trong min thi gian) thay i nhanh trong chu k k hiu,ngha l nu thi gian nht qun knh nh hn chu k k hiu ca tn hiu pht, knh s

gy ra pha inh nhanh i vi tn hiu thu. iu ny s dn n mo dng tn hiu. Nup ng xung kim knh thay i vi tc chm hn nhiu so vi k hiu bng gcpht, knh s gy ra pha inh chm i vi tn hiu thu. Trong trng hp ny knh t ratnh i vi mt s chu k k hiu. Tt nhin ta mun c pha inh chm v n h trcht lng truyn dn n nh hn. Ta khng th xc dnh Doppler khi thit k h thng.V th, khi cho trc tri Doppler, ta cn chn rng bng tn tn hiu (bng thngsng mang con) trong gii thut iu ch thch ng nhn c knh pha inh chmthay v knh pha inh nhanh. Nh vy ta s t c cht lng truyn dn tt hn.

2.7. Phn b Rayleigh v Rice

Khi nghin cu cc knh v tuyn di ng, thng cc phn b Rayleigh v Rice cs dng m t tnh cht thng k thay i theo thi gian ca tn hiu pha inh phng.Sau y, chng ta s xt cc phn b ny v a ra cc c tnh c bn ca chng.

2.7.1. Phn b pha inh Rayleigh

C th coi phn b pha inh Rayleigh l phn b ng bao ca tng hai tn hiu phnb Gauss vung gc. Hm mt xc sut (PDF) ca phn b pha inh Rayleigh cbiu din nh sau:

( )


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cc thng s ca knh, cn phi bit bit nh hng ca cc thng s knh ni trn lnhiu nng knh.

Hnh 2.7 minh ho dng tn hiu OFDM trong min tn s, khi truyn qua mi trngknh pha ing Rice. Trn hnh v ta thy ti nhng im trng ca p ng knh th bin ph tn s ca tn hiu s b gim nhanh chng, v ti nhng im li ca p ngknh th bin ph ca tn hiu b ko ln theo ng p ng knh. Ti nhng imtrng hay li ca p ng knh u gy thu sai tn hiu sau b quyt nh ti pha thu.

Mc ch ca chng ta l tm gii php i ph nhng v tr m p ng knh b thngging bng cch s dng cc b cn bng, dng c ch thch ng chn lc sng mang v

iu ch M-QAM thch ng (AQAM) trong min tn s v thi.Hnh 2.8 v hnh 2.9 cho thy cc thuc tnh knh trong min tn s ph thuc vo tritr (RDS) v tha s K da trn cc kt qu m phng. C hai m hnh min tn s vmin thi gian u c m phng. Trn hnh 2.8, ta gi thit rng K bng 0dB cn trnhnh 2.9 ta gi thit rng RDS bng 42,1ns.

Phan Minh c K54B

Hnh 2.7 Ph tn hiu OFDM truyn qua m hnh knh pha inh Rice, vis sng mang = 100, kch thc FFT = 256, [plot_ofdm_spectrum.m]

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Hnh 2.8 cho thy rng tri tr cao dn n thay i bin hm truyn t knh trongmin tn s nhanh hn. iu ny cho thy rng cn phi n nh nhiu sng mang conhn cho h thng OFDM khi tri ph ln hn. T hnh 2.9 cho thy, khi tha s K gim,

bin hm truyn t knh trong min tn s b pha inh nhanh hn. Khi tha s Kln, bin hm truyn t knh trong min tn s t b pha inh hn nhiu. Ni mtcch khc khi tha s K ln, cho php n nh bng thng sng mang nh ngay c khitri tr ln. Tuy nhin ta cn bit ti tha s K no nh hng tri tr i vi thit k

bng thng sng mang con c th b qua. xc nh iu ny ta xt kt qu m phngtrn hnh 2.10.

Hnh 2.10 Biu th hm truyn t bin knh theo tn s i vi RDS bng 30ns vtha s K bng 0dB, 6dB v 15dB. Hnh ny cho thy rng tha s K nh dn n bin knh b pha inh nhanh hn trong min tn s. i vi K=0dB, pha inh bin cth ln ti 12 dB ti mt tn s no , i vi K=10dB, bin pha inh nh hn2,2dB trn ton bng tn v i vi K=15dB, pha inh ch gii hn 1dB trn ton bngtn. Vy c th kt lun rng Khi K ln hn 10dB bin hm truyn t knh trongmin tn s khng b pha inh nhiu v th khng cn t bng thng sng mang con

theo tri tr mc d bin ny pha inh nhanh hn khi tri tr ln.

Phan Minh c K54B

Hnh 2.8. Ph thuc bin hm truyn t knh min tn s vo tn sv RDS. a) nhn t trn xung, b) nhn t bn.

Hnh 2.9. Ph thuc bin hm truyn t knh vo K v tn s

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T cc phn tch trn c th kt lun nh hng ca tha s K v tri tr ln cc thuctnh knh trong min tn s nh sau:

Tri tr nh hng ln tc thay i bin trong hm truyn t knh trong mintn s. Tri tr cng cao th tc thay i bin trong min tn s cng ln.

Tha s K xc nh ln ca thay i bin hm truyn t knh min tn s. Kcng ln th thay i bin cng nh.

Khi tha s K nh hn 10 dB, chng pha inh chn lc tn s, cn n nhbng thng sng mang con ln hn cho OFDM khi tri tr ln hn.

Phan Minh c K54B

Hnh 2.10. Hm truyn t ca knh khi RDS=30ns vi cc gi tr K khc nhau

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http://www.ebook.edu.vn

Chng 3

Nguyn l hot ng ca OFDM

3.1. M u

Ghp knh theo tn s trc giao OFDM (Orthogonal Frequency Division Multiplexing)l mt phng php iu ch cho php gim thiu mo tuyn tnh do tnh phn tn caknh truyn dn v tuyn gy ra. Nguyn l ca OFDM l phn chia ton b bng thngcn truyn vo nhiu sng mang con v truyn ng thi trn cc sng mang ny. Theo, lung s tc cao c chia thnh nhiu lung tc thp hn. V th c th gimnh hng ca tr a ng v chuyn i knh pha inh chn lc thnh knh pha inh

phng. Nh vy OFDM l mt gii php cho tnh chn lc ca cc knh pha inh trong

min tn s. Vic chia tng bng thng thnh nhiu bng con vi cc sng mang con dnn gim rng bng con trong min tn s ng ngha vi tng di k hiu. Ssng mang con cng ln th di k hiu cng ln. iu ny c ngha l di k hiuln hn so vi thi gian tri rng tr ca knh pha inh phn tn theo thi gian, hay rng bng tn tn hiu nh hn rng bng tn nht qun ca knh.

3.2. Tnh trc giao

tng

tng OFDM l truyn dn song song (ng thi) nhiu bng con chng ln nhau trn

cng mt rng bng tn cp pht ca h thng. Vic xp chng ln cc bng tn contrn ton b bng tn c cp pht dn n ta khng nhng t c hiu qu s dng

ph tn c cp pht cao m cn c tc dng phn tn li cm khi truyn qua knh,nh tnh phn tn li m khi c kt hp vi cc k thut m ho knh kim sot lihiu nng h thng c ci thin ng k. So vi h thng ghp knh phn chia theotn s FDM truyn thng, FDM cng truyn theo c ch song song nhng cc bngcon khng nhng khng c php chng ln nhau m cn phi dnh khong bng tn

bo v ( gim thiu phc tp b lc thu).

Vy lm th no tch cc bng con t bng tng chng ln hay ni cch khc sau khi

c tch ra chng khng giao thoa vi nhau trong cc min tn s (ICI) v thi gian(ISI). Cu tr li v cng l vn mu cht ca truyn dn OFDM l nh tnh trc giaoca cc sng mang con. V vy ta kt lun rng nh m bo c tnh trc giao ca ccsng mang con cho php truyn dn ng thi nhiu bng tn con chng ln song phathu vn tch chng ra c, c bit l tnh kh thi v kinh t cao do s dng x l tnhiu s v tn dng ti a u vit ca VLSI.

Theo trc ht ta nh ngha tnh trc giao, sau ta p dng tnh trc giao ny voh thng truyn dn OFDM hay ni cch khc s dng tnh trc giao vo qu trnh tov thu tn hiu OFDM cng nh cc iu kin cn thit m bo tnh trc giao.

nh ngha


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Nu k hiu cc sng mang con c dng trong h thng OFDM l )t(s&)t(s ji . m bo trc giao cho OFDM, cc hm sin ca sng mang con phi tho mn iu kinsau

( ) ( )

==

+

ji,0

ji,1dtts.ts

T

1Tt

t

*

ji

s

s

(3.1)

Trong :

( )( )

=

=

,0

N,2,1k,ets

ftk2j

k

(3.2)

T1f= l khong cch tn s gia hai sng mang con, T l thi gian k hiu, N l s

cc sng mang con v N.f l bng thng truyn dn v ts l dch thi gian.

Minh hoOFDM t tnh trc giao trong min tn s bng cch phn phi mi tn hiu thng tinring bit vo cc sng mang con khc nhau. Cc tn hiu OFDM c to ra t tngca cc hm sin tng ng vi mi sng mang. Tn s bng tn c s ca mi sngmang con c chn l mt s nguyn ln ca tc k hiu, kt qu l ton b ccsng mang con s c tn s l s nguyn ln ca tc k hiu. Do cc sng mangcon l trc giao vi nhau.

Kin trc ca mt tn hiu OFDM vi 4 sng mang con c cho Hnh 3.1. Trong ,(3.1.1a), (3.1.2a), (3.1.3a) v (3.1.4a) th hin cc sng mang con ring l, vi tn s

tng ng 10, 20, 30, v 40 Hz. Pha ban u ca ton b cc sng mang con ny l 0.(3.1.5a) v (3.1.5b) th hin tn hiu OFDM tng hp ca 4 sng mang con trong minthi gian v min tn s.

Tnh trc giao trong min tn s ca tn hiu OFDM c th hin mt cch tng minh hnh 3.2. Thy r, trong min tn s mi sng mang con ca OFDM c mt p ngtn s dng sinc (sin(x)/x). Dng sinc c ng bao chnh hp, vi nh suy gim chmkhi bin ca tn s cch xa trung tm. Tnh trc giao c th hin l nh ca misng mang con tng ng vi gi tr 0 ca ton b cc sng mang con khc. Hnh 3.2cho ta thy vi cng rng bng tn cp pht cho h thng th hiu qu s dng phtn ca OFDM ln gp hai ln so vi c ch FDM truyn thng.

p ng tng hp 5 sng mang con ca mt tn hiu OFDM c minh ho ngmu en m trn hnh 3.3.

Phan Minh c K54B 20


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Phan Minh c K54B

Hnh 3.1 Dng sng ca mt tn hiu OFDM trong min thi gian v tns, [sim_ofdm_time_domain.m]

Hnh 3.2 Hnh dng ph ca tn hiu OFDM bng tn c s 5 sng mang,hiu qu ph tn ca OFDM so vi FDM, [sim_ofdm_mc.m]

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3.3. M hnh h thng truyn dn OFDM3.3.1. M t ton hc tn hiu OFDM

Tn hiu OFDM pht phc bng tn gc c xc nh nh sau:

( ) ( ),

==

kkTtsts k (3.3)

Trong sk(t-kT) l tn hiu OFDM pht phc bng gc th k c xc nh nh sau:

( )

( ) ( )

++

=

=

khcNu0,

TTkTtTTkT

kTtT

ij2expxkTtw

ktts winFFTGwin

12N

N/2i FFTki,

k (3.4)

Trong :

T l di k hiu OFDM

TFFT l thi gian FFT, phn hiu dng ca k hiu OFDM

TG l thi gian bo v, thi gian ca tin t chu trnh

Twin l thi gian m ca tin t v hu t to dng ph

f=1/TFFT l phn cch tn s gia hai sng mang

N l di FFT, s im FFT

k l ch s v k hiu c truyn

i l ch s v sng mang con, i{-N/2, -N/2+1, -1, 0, +1, ., -N/2}

xi,k l vect im chm tn hiu, l k hiu phc (s liu, hoa tiu,rng) c iu ch ln sng mang con i ca k hiu OFDM th k.

Phan Minh c K54B

Hnh 3.3 Ph tng hp ca tn hiu OFDM trong bng tn c s vi5 sng mang con, [sim_ofdm_mc.m]

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w(t) xung to dng c biu din nh sau:

( )

( )[ ]

( )[ ]

+

+

=

winFFTFFTwinFFT

FFTG

GGwinwinG

TTtT,TTtcos121

TtT1,

TtTT,TTtcos12

1

tw (3.5)

Phn tch (3.4) ta thy biu thc ny ging nh biu thc ca dy Fourier sau: j nf

n

0u(t) = c(nf )e 02

(3.6)

trong cc h s Fourier phc th hin cc vect ca chm tn hiu phc cn nf0 thhin cc sng mang con i/TFFT. Trong h thng s, dng sng ny c th c to ra bng

bin i Fourier ngc nhanh (IFFT). Chm s liu xi,k l u vo IFFT v k hiu

OFDM min thi gian l u ra.Tn hiu u ra ca b iu ch v tuyn c xc nh nh sau:

k

RF RF,ks (t)= s (t kT) (3.7)

trong sRF,k(t-kT) l tn hiu OFDM v tuyn th k c biu din nh sau:

( )

( ) ( )

++

+

=

=

khcNu,0

TTkTtTTkT

,kTtT

1f2jexpxkTtwRe

kTts winFFTGwin

12/N

2/Ni FFT

ck,i

k,RF (3.8)

Trong fc l tn s sng mang RF.

3.3.2. S h thng truyn dn OFDM

Hnh 3.4 trnh by s khi pht thu tn hiu OFDM in hnh. Theo , di ytrnh by vn tt chc nng cc khi.

My pht: Chuyn lung d liu s pht thnh pha v bin sng mang con. Cc sngmang con c ly mu trong min tn s, ph ca chng l cc im ri rc. Sau sdng bin i Fourier ri rc ngc (IDFT) chuyn ph ca cc sng mang con mangd liu vo min thi gian. Tuy nhin cc h thng trong thc t dng bin i Fourierngc nhanh (IFFT) v n tnh hiu ca n. Tn hiu OFDM trong min thi gian ctrn nng tn ln tn s truyn dn v tuyn.

My thu: Thc hin hot ng ngc li ca pha pht. Theo trc ht, trn h tntn hiu RF thnh tn hiu bng tn c s, sau s dng FFT phn tch tn hiu vomin tn s. Cui cng thng tin dng bin v pha ca cc sng mang con c

gii iu ch thnh cc lung s v chuyn tr li thnh d liu s ban u.

Phan Minh c K54B 23


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Chuyn iNi tip

thnh Song

song

IFFT

Khuch i cngsut v gii iuch sng mang

cao tn

iu chsng

mang con

Chnkhongbo v

Chn tng bkhung

iu chsng mang caotn v khuch

i cng sut

Loi bkhongbo v

Tchkhung

FFTGii iuch sngmang con

Chuyn iSong songhnh Ni

tip

.

.

.

.

.

.

D liu phtni tip

D liu thuni tip

Pha sngmang

Bin sng mang

I

Q

I

Q

I

Q

I

Q

I

Q

Sa li tn s

Tn hiu trong minthi gian

LO

Pha sngmang

Bin sngmang

My pht

My thu

Hnh 3.4 S khi h thng truyn dn OFDM

ng bnh thi

Anten

3.2.2.1. Tng chuyn i ni tip sang song song

Tng chuyn i ni tip sang song song chuyn lung bit u vo thnh d liu pht

trong mi k hiu OFDM, thng mi k hiu pht gm 40-4000 bit. Vic phn b dliu pht vo mi mi k hiu ph thuc vo phng php iu ch c dng v slng sng mang con. V d, i vi iu ch sng mang ca16-QAM th mi sngmang con mang 4 bit d liu, nu h thng truyn dn s dng 100 sng mang con th slng bit trn mi k hiu s l 400. Ti pha thu qu trnh c thc hin ngc li, khi d liu t cc sng mang con c chuyn ngc tr li l lung d liu ni tip banu.

Do tnh cht chn lc tn s ca knh pha inh (pha inh chn lc tn s) tc ng lnmt nhm cc sng mang con lm chng suy gim nhanh chng. Ti im p ng knhxp x 0, thng tin gi trn sng mang con gn im ny s b tn tht, hu qu l gycm li bit trong mi k hiu. Do c ch FEC l hiu qu cao nu cc li c phn tnrng (khng tp chung hay cm li), v vy ci thin hiu nng, a phn h thngdng ngu nhin ho nh l mt phn ca chuyn i ni tip thnh song song. Vn ny c thc hin bng cch ngu nhin ho vic phn b sng mang con ca mi mt

bit d liu ni tip. Ngu nhin ho lm phn tn cc cm bit li trong k hiu OFDMdo s tng hiu nng sa li ca FEC.

3.3.2.1. Tng iu ch sng mang con

Tng iu ch sng mang con lm nhim v phn phi cc bit d liu ngi dng lncc sng mang con, bng cch s dng mt s iu ch bin v pha. Vic xp xpiu ch sng mang con i vi 16-QAM c cho hnh 3.5, mi k hiu 16-QAM scha 4 bit d liu, mi t hp 4 bit d liu tng ng vi mt vector IQ duy nht.

Phan Minh c K54B 24


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nh hng ca tp m cng vo tn hiu pht 16-QAM (knh AWGN) c cho hnhhnh 3.5 (b) vi SNR thu = 18 dB.

3.3.2.3. Tng chuyn i t min tn s sang min thi gian

Sau tng iu ch sng mang con, tn hiu OFDM c dng l cc mu tn s, tn hiu

OFDM mun truyn trn knh phi c dng sng trong min thi gian. Php bin iFourier ngc nhanh (IFFT) s chuyn tn hiu OFDM trong min tn s sang min thigian. Tng ng vi mi mu ca tn hiu OFDM trong min thi gian (mi u ra caIFFT) cha tt c cc mu trong min tn s (u vo ca IFFT). Hu ht cc sng mangcon u mang d liu. Cc sng mang con vng ngoi khng mang d liu c t

bng 0.

3.3.2.4. Tng iu ch sng mang RF

u ra ca b iu ch OFDM l mt tn hiu bng tn c s, tn hiu ny c trnnng tn ln tn s truyn dn v tuyn. C th s dng mt trong hai hai k thut iu

ch sng mang cao tn l: "tng t" c cho hnh 3.7 v "s" c cho hnh 3.8.Tuy nhin hiu nng ca iu ch s s tt hn, do ng b pha chnh xc cho nn s cithin qu trnh ghp cc knh I v Q.

Phan Minh c K54B

Hnh 3.5. Tn hiu pht 16-QAM s dng m ho Gray, v tn hiu 16-QAMtruyn qua knh v tuyn, SNR = 18 dB, [sim_generate_qam.m]

Hnh 3.6. Tng IFFT, to tn hiu OFDM

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Hnh 3.9 m t dng sng trong min thi gian ca mt tn hiu OFDM. S lng sngmang = 500, kch thc FFT = 2000, khong thi gian bo v = 500. Sng mang iuch cao tn c tn s fc = 10 GHz.

Phan Minh c K54B

Hnh 3.7 iu ch cao tn tn hiu OFDM bng tn c sphc s dng k thut tng t

Hnh 3.8 iu ch cao tn tn hiu OFDM bng tn c s phcs dng k thut s

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Hu ht cc ng dng v tuyn, th tn hiu OFDM c to ra ti bng tn c s sdng cc mu phc, sau chuyn ph tn hiu bng tn c s ln ph RF bng cchdng mt b iu ch IQ, nh c cho hnh 3.7 v hnh 3.8. B iu ch IQ s dch

ph tn tn hiu OFDM t bng tn c s phc ln vng tn s v tuyn, v chuyn ttn hiu phc sang tn hiu thc (ly phn thc). Tn hiu RF pht lun l tn hiu thcv n ch bin i gi tr cng trng.

Mt tn hiu thc s tng ng vi mt tn hiu bng tn c s phc c tn s trungtm l 0 Hz trn vi tn s sng mang b iu ch IQ.

offc f2W

f += (3.9)

Trong cf l tn s sng mang dch tn hiu OFDM t bng tn c s phc ln tnhiu OFDM cao tn thc, W l rng bng tn tn hiu v offf l tn s dch t DC,xem hnh 3.10. Trong cc ng dng hu tuyn nh ADSL, hu ht cc sng mang conu c tng dch DC thp hn rng bng tn tn hiu. iu ny c ngha rng cth trc tip to tn hiu thc bng cch s dng tng IFFT thay v phi dng b iu

ch IQ chuyn dch tn s.

Phan Minh c K54B

Hnh 3.9. Dng sng tn hiu OFDM trong min thi gian,[sim_ofdm_signal.m]

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to ra mt tn hiu OFDM thc ch cn mt na cc sng mang con s dng cho iuch d liu, mt khc na gm cc lt tn s cao ca IFFT s c gi tr bin l linhp phc ca na cn li gm cc lt c tn s thp hn.

3.4. Cc thng s c trng v dung lng h thng truyndn OFDM

3.4.1. Cu trc tn hiu OFDM

Hnh 3.11 cho thy cu trc ca cc k hiu OFDM trong min thi gian. FFTT l thigian truyn d liu hiu qu, GT l thi gian bo v. Cng thy cc thng s khc,

winT

l thi gian ca s. Thy r quan h gia cc thng s l.winGFFTsym TTTT ++= (3.10)

Ca s c a vo nhm lm mn bin chuyn v khng ti cc ranh gii k hiu,v gim tnh nhy cm ca dch tn s. Loi ca s c dng ph bin l loi ca scosine tng c nh ngha bi.

Phan Minh c K54B

Hnh 3.10 Tn hiu OFDM dch DC, W l bng tn tn hiu,

f tn s dch t DC, f l tn s trung tm (sng mang)

Hnh 3.11 Cu trc tn hiu OFDM

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( )

( )

+

+

++

=

symsym

sym

sym

symsym

sym

sym

T1tT,T

Ttcos5.05.0

TtT,1

Tt0,T

tcos5.05.0

tw

(3.11)

trong l h s dc ca cosin tng v khong thi gian k hiu SymT , n ngn hnton b khong thi gian ca mt k hiu v ta cho php cc k hiu ln cn chng lnmt phn trong vng dc (roll-off region).

Mt k hiu OFDM bt u ti thi im symk kTtt == (bt u ca k hiu th k) cnh ngha bi cc phng trnh (3.12).

( )( )

( )[ ]

( )[ ]

++>

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