5-Chuong 1-Tong Quan Ve OFDM

Chng 1: Gii thiu tng quan v OFDM

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Chng1: GII THIU TNG QUAN V OFDM1.1 Gii thiu chng Chng ny s gii thiu v cc khi nim, nguyn l cng nh thut ton ca OFDM. Cc nguyn l c bn ca OFDM, m t ton hc, k thut n sng mang, a sng mang v cc k thut iu ch trong OFDM. Bn cnh cc ng dng v u nhc im ca h thng OFDM cng c a ra y. 1.2 Cc nguyn l c bn ca OFDM Nguyn l c bn ca OFDM l chia mt lung d liu tc cao thnh cc lung d liu tc thp hn v pht ng thi trn mt s cc sng mang con trc giao. V khong thi gian symbol tng ln cho cc sng mang con song song tc thp hn, cho nn lng nhiu gy ra do tri tr a ng c gim xung. Nhiu xuyn k t ISI c hn ch hu nh hon ton do vic a vo mt khong thi gian bo v trong mi symbol OFDM. Trong khong thi gian bo v, mi symbol OFDM c bo v theo chu k trnh nhiu gia cc sng mang ICI. Gia k thut iu ch a sng mang khng chng ph v k thut iu ch a sng mang chng ph c s khc nhau. Trong k thut a sng mang chng ph, ta c th tit kim c khong 50% bng thng. Tuy nhin, trong k thut a sng mang chng ph, ta cn trit xuyn nhiu gia cc sng mang, ngha l cc sng ny cn trc giao vi nhau. Trong OFDM, d liu trn mi sng mang chng ln d liu trn cc sng mang ln cn. S chng chp ny l nguyn nhn lm tng hiu qu s dng ph trong OFDM. Ta thy trong mt s iu kin c th, c th tng dung lng ng k cho h thng OFDM bng cch lm thch nghi tc d liu trn mi sng mang ty theo t s tn hiu trn tp m SNR ca sng mang .

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Ch.1

Ch.10

(a) Tit kim bng thng

Tn s

(b)

Tn s

Hnh 1.1: So snh k thut sng mang khng chng xung (a) v k thut sng mang chng xung (b). V bn cht, OFDM l mt trng hp c bit ca phng thc pht a sng mang theo nguyn l chia dng d liu tc cao thnh tc thp hn v pht ng thi trn mt s sng mang c phn b mt cch trc giao. Nh thc hin bin i chui d liu t ni tip sang song song nn thi gian symbol tng ln. Do , s phn tn theo thi gian gy bi tri rng tr do truyn dn a ng (multipath) gim xung. OFDM khc vi FDM nhiu im. Trong pht thanh thng thng mi i pht thanh truyn trn mt tn s khc nhau, s dng hiu qu FDM duy tr s ngn cch gia nhng i. Tuy nhin khng c s kt hp ng b gia mi trm vi cc trm khc. Vi cch truyn OFDM, nhng tn hiu thng tin t nhiu trm c kt hp trong mt dng d liu ghp knh n. Sau d liu ny c truyn khi s dng khi OFDM c to ra t gi dy c nhiu sng mang. Tt c cc sng mang th cp trong tn hiu OFDM c ng b thi gian v tn s vi nhau, cho php kim sot can nhiu gia nhng sng mang. Cc sng mang ny chng lp nhau trong min tn s, nhng khng gy can nhiu gia cc sng mang (ICI) do bn cht trc giao ca iu ch. Vi FDM nhng tn hiu truyn cn c

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khong bo v tn s ln gia nhng knh ngn nga can nhiu. iu ny lm gim hiu qu ph. Tuy nhin vi OFDM s ng gi trc giao nhng sng mang lm gim ng k khong bo v ci thin hiu qu ph.x(n) xf(n h(n)

D liu nh phn D liu ra

Sp xp Sp xp li

S/P

Chn pilotY(k)

IDFT

Chn di bo vy(n)

P/S

Knh

yf(n

P/S

c lng knh

DFT

Loi b di bo v

S/P

+

AWG N

Hnh 1.2: S h thng OFDM u tin, d liu vo tc cao c chia thnh nhiu dng d liu song song tc thp hn nh b chuyn i ni tip/song song (S/P: Serial/Parrallel). Mi dng d liu song song sau c m ha s dng thut ton sa li tin (FEC) v c sp xp theo mt trnh t hn hp. Nhng symbol hn hp c a n u vo ca khi IDFT. Khi ny s tnh ton cc mu thi gian tng ng vi cc knh nhnh trong min tn s. Sau , khong bo v c chn vo gim nhiu xuyn k t ISI do truyn trn cc knh di ng v tuyn a ng. Sau cng b lc pha pht nh dng tn hiu thi gian lin tc s chuyn i ln tn s cao truyn trn cc knh. Trong qu trnh truyn, trn cc knh s c cc ngun nhiu gy nh hng nh nhiu trng cng AWGN, pha thu, tn hiu c chuyn xung tn s thp v tn hiu ri rc t c ti b lc thu. Khong bo v c loi b v cc mu c chuyn t min thi gian sang min tn s bng php bin i DFT dng thut ton FFT. Sau , ty vo s iu ch c s dng, s dch chuyn v bin v pha ca cc sng mang nhnh s c cn bng bng b cn bng knh (Channel Equalization). Cc symbol hn hp thu c s c sp xp ngc tr li v c gii m. Cui cng chng ta s thu nhn c dng d liu ni tip ban u.

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S0Serial data streamS0, S1, , SN-1

Modulation at f0 Modulation at f1

Serial to Parallel convertor

Transmitter SN-1

Modulation at fN-1

Demodulation at f0 Demodulation at f1

S0 Parallel to serial convertor SN-1Output

Receiver

Demodulation at fN-1

Hnh 1.3: H thng OFDM c bnf0=1/T f1=2/T f2=3/T fN-1=N/T

f 2

f

N f=W

Hnh 1.4: Sp xp tn s trong h thng OFDM

Hnh 1.5: Symbol OFDM vi 4 subscriber

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Tt c cc h thng truyn thng v tuyn s dng s iu ch nh x tn hiu thng tin to thnh dng c th truyn hiu qu trn knh thng tin. Mt phm vi rng cc s iu ch c pht trin, ph thuc vo tn hiu thng tin l dng sng analog hoc digital. Mt s s iu ch tng t chung bao gm: iu ch tn s (FM), iu ch bin (AM), iu ch pha (PM), iu ch n bin (SSB), Vestigial side Band (VSB), Double Side Band Suppressed Carrier (DSBSC). Cc s iu ch sng mang n chung cho thng tin s bao gm kho dch bin (ASK), kho dch tn s (FSK), kho dch pha (PSK), iu ch QAM. K thut iu ch a sng mang trc giao da trn nguyn tc phn chia lung d liu c tc cao R (bit/s) thnh k lung d liu thnh phn c tc thp R/k (bit/s); mi lung d liu thnh phn c tri ph vi cc chui ngu nhin PN c tc Rc (bit/s). Sau iu ch vi sng mang thnh phn OFDM, truyn trn nhiu sng mang trc giao. Phng php ny cho php s dng hiu qu bng thng knh truyn, tng h s tri ph, gim tp m giao thoa k t ISI nhng tng kh nng giao thoa sng mang. Trong cng ngh FDM truyn thng, cc sng mang c lc ra ring bit bo m khng c s chng ph, do khng c hin tng giao thoa k t ISI gia nhng sng mang nhng ph li cha c s dng vi hiu qu cao nht. Vi k thut OFDM, nu khong cch sng mang c chn sao cho nhng sng mang trc giao trong chu k k t th nhng tn hiu c khi phc m khng giao thoa hay chng ph.

Hnh 1.6: Ph ca sng mang con OFDM .[2]

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Chng 1: Gii thiu tng quan v OFDM1.3 n sng mang (Single Carrier)

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H thng n sng mang l mt h thng c d liu c iu ch v truyn i ch trn mt sng mang.

Hnh 1.7: Truyn dn sng mang n.[9] Hnh 1.7 m t cu trc chung ca mt h thng truyn dn n sng mang. Cc k t pht i l cc xung c nh dng bng b lc pha pht. Sau khi truyn trn knh a ng. pha thu, mt b lc phi hp vi knh truyn c s dng nhm cc i t s tn hiu trn nhiu (SNR) thit b thu nhn d liu. i vi h thng n sng mang, vic loi b nhiu giao thoa bn thu cc k phc tp. y chnh l nguyn nhn cc h thng a sng mang chim u th hn cc h thng n sng mang. 1.4 a sng mang (Multi-Carrier) Nu truyn tn hiu khng phi bng mt sng mang m bng nhiu sng mang, mi sng mang ti mt phn d liu c ch v c tri u trn c bng thng th khi chu nh hng xu ca p tuyn knh s ch c mt phn d liu c ch b mt, trn c s d liu m cc sng mang khc mang ti c th khi phc d liu c ch.

Hnh 1. 8: Cu trc h thng truyn dn a sng mang.[9]

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Do vy, khi s dng nhiu sng mang c tc bit thp, cc d liu gc s thu c chnh xc. khi phc d liu mt, ngi ta s dng phng php sa li tin FFC. my thu, mi sng mang c tch ra khi dng b lc thng thng v gii iu ch. Tuy nhin, khng c can nhiu gia cc sng mang (ICI) phi c khong bo v khi hiu qu ph km. OFDM l mt k thut iu ch a sng mang, trong d liu c truyn song song nh v s sng mang ph mang cc bit thng tin. Bng cch ny ta c th tn dng bng thng tn hiu, chng li nhiu gia cc k t, lm c iu ny, mt sng mang ph cn mt my pht sng sin, mt b iu ch v gii iu ch ca ring n. Trong trng hp s sng mang ph l kh ln, iu ny l khng th chp nhn c. Nhm gii quyt vn ny, khi thc hin chc nng bin i IDFT/DFT c dng thay th hng lot cc b dao ng to sng sin, b iu ch, gii iu ch. Hn na, IFFT/FFT c xem l mt thut ton gip cho vic bin i IDFT/DFT nhanh v gn hn bng cch gim s php nhn phc khi thc hin php bin i IDFT/DFT v gip tit kim b nh bng cch tnh ti ch. Mi sng mang trong h thng OFDM u c th vit di dng [9]: Vi h thng a sng mang OFDM ta c th biu din tn hiu dng sau:S (t ) = 1 N

al k =0

N- 1

l,k e

j 2 k ( t lT s ( N + L ))

Trong ,

al,k N L

: l d liu u vo c iu ch trn sng mang nhnh th k : s sng mang nhnh : chiu di tin t lp (CP)1 1 = T NTs

trong symbol OFDM th l

Khong cch sng mang nhnh l

Gii php khc phc hiu qu ph km khi c khong bo v (Guard Period) l gim khong cch cc sng mang v cho php ph ca cc sng mang cnh nhau

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trng lp nhau. S trng lp ny c php nu khong cch gia cc sng mang c chn chnh xc. Khong cch ny c chn ng vi trng hp sng mang trc giao vi nhau. chnh l phng php ghp knh theo tn s trc giao. T gia nhng nm 1980, ngi ta c nhng tng v phng php ny nhng cn hn ch v mt cng ngh, v kh to ra cc b iu ch a sng mang gi thnh thp theo bin i nhanh Fuorier IFFT. Hin nay, nh ng dng cng ngh mch tch hp nn phng php ny c a vo ng dng trong thc tin. 1.5 S trc giao (Orthogonal) Orthogonal ch ra rng c mt mi quan h chnh xc gia cc tn s ca cc sng mang trong h thng OFDM. Trong h thng FDM thng thng, cc sng mang c cch nhau trong mt khong ph hp tn hiu thu c th nhn li bng cch s dng cc b lc v cc b gii iu ch thng thng. Trong cc my nh vy, cc khong bo v cn c d liu trc gia cc sng mang khc nhau. Vic a vo cc khong bo v ny lm gim hiu qu s dng ph ca h thng. i vi h thng a sng mang, tnh trc giao trong kha cnh khong cch gia cc tn hiu l khng hon ton ph thuc, m bo cho cc sng mang c nh v chnh xc ti im gc trong ph iu ch ca mi sng mang . Tuy nhin, c th sp xp cc sng mang trong OFDM sao cho cc di bin ca chng che ph ln nhau m cc tn hiu vn c th thu c chnh xc m khng c s can nhiu gia cc sng mang. c c kt qu nh vy, cc sng mang phi trc giao v mt ton hc. My thu hot ng gm cc b gii iu ch, dch tn mi sng mang xung mc DC, tn hiu nhn c ly tch phn trn mt chu k ca symbol phc hi d liu gc. Nu mi sng mang u dch xung tn s tch phn ca sng mang ny (trong mt chu k , kt qu tnh tch phn cc sng mang khc s l zero. Do , cc sng mang c lp tuyn tnh vi nhau (trc giao) nu khong cch gia cc sng l bi s ca 1/. Bt k s phi tuyn no gy ra bi s can nhiu ca cc sng mang ICI cng lm mt i tnh trc giao.

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Hnh 1.9: Cc sng mang trc giao Phn u ca tn hiu nhn bit tnh tun hon ca dng sng, nhng li d b nh hng bi nhiu xuyn k t (ISI). Do , phn ny c th c lp li, gi l tin t lp (CP: Cycle Prefix). Do tnh trc giao, cc sng mang con khng b xuyn nhiu bi cc sng mang con khc. Thm vo , nh k thut a sng mang da trn FFT v IFFT nn h thng OFDM t c hiu qu khng phi bng vic lc di thng m bng vic x l bng tn gc. 1.5.1 Trc giao min tn s Mt cch khc xem tnh trc giao ca nhng tn hiu OFDM l xem ph ca n. Trong min tn s, mi sng mang th cp OFDM c p tuyn tn s sinc (sin (x)/x). l kt qu thi gian symbol tng ng vi nghch o ca sng mang. Mi symbol ca OFDM c truyn trong mt thi gian c nh (TFFT). Thi gian symbol tng ng vi nghch o ca khong cch ti ph 1/TFFT Hz. Dng sng hnh ch nht ny trong min thi gian dn n p tuyn tn s sinc trong min tn s. Mi ti ph c mt nh ti tn s trung tm v mt s gi tr khng c t cn bng theo cc khong trng tn s bng khong cch sng mang. Bn

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cht trc giao ca vic truyn l kt qu ca nh mi ti ph. Tn hiu ny c pht hin nh bin i Fourier ri rc (DFT). 1.5.2 M t ton hc ca OFDM M t ton hc OFDM nhm trnh by cch to ra tn hiu, cch vn hnh ca my thu cng nh m t cc tc ng khng hon ho trong knh truyn. V mt ton hc, trc giao c ngha l cc sng mang c ly ra t nhm trc chuNn (Orthogonal basis). Phng php iu ch OFDM s dng rt nhiu sng mang, v vy tn hiu c th hin bi cng thc:

1 S s (t ) = N

N 1 n=0

A c ( t ). e

j [ n t + c ( t ) ]

(1.1)

Trong , = 0 + n. N u tn hiu c ly mu vi tn s ly mu l 1/T (vi T l chu k ly mu), th tn hiu hp thnh c th hin bi cng thc:

S s ( kT ) =

1 N

N 1 n=0

A n .e

j [(

0

+ n )kT + n

]

(1.2)

im ny khong thi gian tn hiu c phn thnh N mu c gii hn thun li cho vic ly mu mt chu k ca mt symbol d liu. Ta c mi quan h: = N .T Khi 0 = 0 th ta c:

1 S s ( kT ) = N

N 1 n=0

An e

j n

.e

j ( n ) kT

(1.3)

So snh (1.3) vi dng tng qut ca bin i Fourier ngc ta c:

g ( kT ) =

1 N

G NT n=0

N 1

n j 2 nk / N e

(1.4)

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Chng 1: Gii thiu tng quan v OFDMBiu thc (1.3) v (1.4) l tng ng nu:f = 1 NT = 1

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y l iu kin yu cu tnh trc giao. Do kt qu ca vic bo ton tnh trc giao l tn hiu OFDM c th xc nh bng php bin i Fourier. Cc thnh phn ca mt mng trc giao th c lp tuyn tnh vi nhau. C th xem tp hp cc sng mang pht i l mt mng trc giao cho bi cng thc:k (t ) = exp( j k t )

k = 0 = 2

k t

(1.5)

N u tp hp cc sng mang ny trc giao th mi quan h trc giao trong biu thc (1.1):j [2 ( p q ) t / ] dt = ( b a ) p (t ) q (t )dt = e a a b b

khi p = q

=

e j [2 ( pq)b / ]dt =0 j 2 ( p q) /

khi p =q v (b-a) =

(1.6)

( p,q l hai s nguyn) Cc sng mang thng tch ring ra tn s 1/, t n yu cu ca tnh trc giao th chng c tng quan trn mt thi on . N u tn hiu gi l trc giao nu chng c lp vi nhau. S trc giao cho php truyn tn hiu hon ho trn mt knh chung v pht hin chng m khng c can nhiu. N hng ti ph trong OFDM c t gn nhau, gn nht theo l thuyt trong khi duy tr tnh trc giao ca chng. OFDM t c trc giao bi vic sp xp mt trong cc tn hiu thng tin ring bit cho cc ti ph khc nhau. Cc tn hiu OFDM c to thnh t tng cc hiu hnh sin, mi hnh sin tng ng vi mt di ph. Di tn s c bn ca mt ti ph c chn l s nguyn ln thi gian symbol. Kt qu l cc ti ph c mt s nguyn cc chu k trong mt symbol v chng trc giao vi nhau.

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Di bo v ( CP) Tcp

Phn hu ch ca tn hiu Tg = N /WT

Hnh 1.10: Thm CP vo symbol OFDM V dng sng l tun hon v ch c m rng bng Tcp. Lc ny tn hiu c biu din trong khong m rng [0,T) l:N 1 k =0

s (t ) = xk . k (t ) y k(t) to thnh tp hp cc hm c s trc giao.

(1.7)

Lc ny,

k

(t ) = A k ew 1 = N T TCP

j 2 kf 1 t

f1 =

Mt s la chn hp l cho bin /pha:

Ak =Do ,

1 T T CP

e

j 2 kf 1 T CP

1 e j 2 kf 1 ( t T CP ) t [0, T ) k ( t ) = T T CP t [0, T ) 0

(1.8)

V tn hiu cui cng: S ( t ) =

l = k = 0

N 1

x k , l k ( t lT )

(1.9)

N h vy, trong ghp knh phn chia theo tn s trc giao, khong cch sng mang tng ng vi tc bit ca bn tin.

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Vic x l (iu ch v gii iu ch) tn hiu OFDM c thc hin trong min tn s, bng cch s dng cc thut ton x l tn hiu s DSP (Digital Signal Processing). N guyn tc ca tnh trc giao thng c s dng trong phm vi DSP. Trong ton hc, s hng trc giao c c t vic nghin cu cc vector. Theo nh ngha, hai vector c gi l trc giao vi nhau khi chng vung gc vi nhau (to thnh gc 90o) v tch ca hai vector l bng 0. im chnh y l nhn hai tn s vi nhau, tng hp cc tch cho kt qu bng 0.

Hnh 1.11: T ch ca hai vector trc giao bng 0 Hm s thng thng c gi tr bng 0. V d: Gi tr trung bnh ca hm sin sau:2 k

sin( t ) dt0

= 0

Qu trnh tch phn c th c xem xt khi tm ra din tch di dng ng cong. Do , din tch sng sin c th c vit nh sau:

Hnh 1.12: Gi tr ca sng sine bng 0 N u chng ta cng v nhn (tch phn) hai dng sng sin c tn s khc nhau, kt qu cng s bng 0.

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Hnh 1.13: Tch phn ca hai sng sine c tn s khc nhau. iu ny gi l tnh trc giao ca sng sine. N cho thy rng min l hai dng sng sin khng cng tn s, th tch phn ca chng s bng 0. y l c s hiu qu trnh iu ch OFDM.

Hnh 1.14: Tch hai sng sine cng tn s.

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N u hai sng sin c cng tn s nh nhau th dng sng hp thnh lun dng, gi tr trung bnh ca n lun khc khng. y l vn rt quan trng trong qu trnh iu ch OFDM. Cc my thu OFDM bin i tn hiu thu c t min tn s nh dng k thut x l tn hiu s gi l bin i nhanh Fourier (FFT). N hiu l thuyt chuyn i c thc hin bng chui trc giao. T phn tch trn, ta c th rt ra kt lun: khc phc hin tng khng bng phng ca p tuyn knh cn dng nhiu sng mang, mi sng mang ch chim mt phn nh bng thng, do vy nh hng khng ln ca p tuyn knh n d liu ni chung. S sng mang cng nhiu cng tt nhng phi c khong bo v trnh can nhiu gia cc sng mang. Tuy nhin, tn dng tt nht th dng cc sng mang trc giao, khi cc sng mang c th trng lp nhau vn khng gy can nhiu.

1.6 Cc k thut iu ch trong OFDMTrong h thng OFDM, tn hiu u vo l dng bit nhi phn. Do , iu ch trong OFDM l cc qu trnh iu ch s v c th la chn trn yu cu hoc hiu sut s dng bng thng knh. Dng iu ch c th qui nh bi s bit ng vo M v s phc dn = an + bn ng ra. Cc k t an, bn c th c chn l { 1,3} cho 16 QAM v {1} cho QPSK. M 2 4 16 64 Dng iu ch BPSK QPSK 16-QAM 64-QAM an, bn1 11 , 3 1 , 3 , 5 , 7

M hnh iu ch c s dng ty vo vic dng ha gia yu cu tc truyn dn v cht lng truyn dn.

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Chng 1: Gii thiu tng quan v OFDM1.6.1 iu ch BPSK

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Trong mt h thng iu ch BPSK, cp cc tn hiu s1(t), s2(t) c s dng biu din cc k hiu c s hai l "0" v "1" c nh ngha nh sau:[7]S i (t ) = 2 Eb cos[ 2 f c t + ( t ) + ] Tb

( t ) = ( i 1 ) ; 0 t T b ; i = 1, 2

(1.10)

Hay:

S1 ( t ) =

2 Eb cos[ 2 f c t + ] Tb

S 2 (t ) = Trong ,

2 Eb 2 Eb cos[ 2 f c t + + ] = S1 (t ) = cos[ 2 f c t + ] Tb Tb Tb Eb (t) : rng ca 1bit : N ng lng ca 1 bit : gc pha, thay i theo tn hiu iu ch

(1.11)

: gc pha ban u c gi tr khng i t 0 n 2 v khng

nh hng n qu trnh phn tch nn t bng 0 i = 1 : tng ng vi symbol 0 i = 2 : tng ng vi symbol 1 Mi cp sng mang hnh sine i pha 1800 nh trn c gi l cc tn hiu i cc. N u chn mt hm nng lng c s l:

(t ) =Khi ,S1 (t ) =

2 cos( 2f c t ); 0 t Tb TbE b (t ) E b (t )

S 2 (t ) =

(1.12)

Ta c th biu din BPSK bng mt khng gian tn hiu mt chiu (N =1) vi hai im bn tin (M=2) : S1 =Eb

, S2 = -

Eb

nh hnh sau:

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Hnh 1.15 : Biu khng gian tn hiu BPSK Khi tn hiu iu ch BPSK c truyn qua knh chu tc ng ca nhiu Gauss trng cng (AWGN ), xc sut li bit gii iu ch c xc nh theo cng thc sau: 2Eb Pe = Q N 0

(1.13)

Trong , Eb : N ng lng bit N0 : Mt nhiu trng cng1.6.2 iu ch QPSK

y l mt trong nhng phng php thng dng nht trong truyn dn. Cng thc cho sng mang c iu ch PSK 4 mc nh sau:[7] 2E S i ( t ) = T . cos( 2 t + ( t ) + ) 0 0tT t 0; t T

(1.14)

Vi pha ban u ta cho bng 0 (t ) = (2i 1) 4

(1.15)

Trong , i = 1,2,3,4 tng ng l cc k t c pht i l "00", "01", "11", "10" T = 2.Tb (Tb: Thi gian ca mt bit, T: thi gian ca mt k t) E : nng lng ca tn hiu pht trin trn mt k t. Khai trin s(t) ta c:

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Chng 1: Gii thiu tng quan v OFDM 2E S i (t ) = T cos[( 2 .i 1). ] cos( 2 f c t ) 4 0

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2E sin[( 2i 1)] . sin( 2f c t ) ( 0 t T ) T 4 T < t; t < 0

(1.16) Chn cc hm nng lng trc chuNn nh sau: 1 (t ) = 2 sin( 2 f c .t ) T 0 t T

(1.17a)

2 (t ) =Khi ,s i ( t ) = 1 ( t )

2 sin( 2 f c .t ) T4

0 t T

(1.17b)

E sin[( 2 i 1 )

] + 2 (t )

E cos[( 2 i 1 )

4

]

(1.18)

Vy, bn bn tin ng vi cc vector c xc nh nh sau:

E sin[( 2 i 1) 4 ] s i1 = si = si2 E cos[( 2 i 1) 4 trong tn hiu khng gian c cho trong bng sau:Cp bit vo Pha ca tn hiu QPSK4 3 4 5 47 4

( i = 1, 2 ,3, 4 )

(1.19)

Quan h ca cp bit iu ch v ta ca cc im tn hiu iu ch QPSK

im tn hiu Si

Ta cc im bn tin 1E 2 E 2 E 2

2E 2 E 2 E 2

00 01 11 10

S1 S2 S3 S4

E 2

E 2

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Ta thy mt tn hiu PSK 4 mc c c trng bi mt vector tn hiu hai chiu v bn bn tin nh hnh v.

Hnh 1. 16 : Biu tn hiu tn hiu QPSK Xem bng ta thy, mc '1' thay i vo E , cn logic '0' th bin i voE . V cng mt lc pht i mt symbol nn lung vo phi phn thnh hai tng

ng v c bin i mc ri nhn ri nhn vi hai hm trc giao tng ng.[7]1.6.3 iu ch QAM

Trong h thng PSK, cc thnh phn ng pha v vung pha c kt hp vi nhau to thnh mt tn hiu ng bao khng i. Tuy nhin, nu loi b loi ny v cho cc thnh phn ng pha v vung pha c th c lp vi nhau th ta c mt s iu mi gi l iu bin cu phng iu ch bin sng mang QAM (iu ch bin gc) . s iu ch ny, sng mang b iu ch c bin ln pha. iu ch QAM l c u im l tng dung lng truyn dn s.[7] Dng tng qut ca iu ch QAM, 14 mc (m-QAM) c xc nh nh sau:

S1 (t ) =

2 E0 ai cos( 2f c t ) T

2 E0 bi sin( 2f c t ); ( 0 t T ) T

(1.20)

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Chng 1: Gii thiu tng quan v OFDMTrong , E0

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: nng lng ca tn hiu c bin thp nht

ai , bi : cp s nguyn c lp c chn ty theo v tr bn tin. Tn hiu sng mang gm hai thnh phn vung gc c iu ch bi mt tp hp bn tin tn hiu ri rc. V th c tn l " iu ch tn hiu vung gc". C th phn tch Si(t) thnh cp hm c s:[7]

1 ( t ) = 2 (t ) =

2 bi sin( 2 f c.t ) T 2 a i sin( 2 f c.t ) T

0tT

0t T

(1.21)

Hnh 1.17: Chm tn hiu M-QAM

1.6.4 M Gray.

Gin IQ(Inphase Quadrature) cho s iu ch s ch ra vector truyn cho tt c cc lin hp t d liu. Mi lin hp t d liu phi c phn phi mt vector IQ duy nht. M Gray l mt phng php cho s phn phi ny, sao cho cc im canh nhau trong vm sao ch khc nhau mt bit n. M ny gip gim thiu t l li bit ton b v n gim c hi nhiu li bit xy ra t mt li symbol n.

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M Gray c th c s dng cho tt c cc s iu ch PSK ( QPSK, 8PSK, 16-PSK) v QAM(16-QAM,64-QAM,256-QAM...).

Bng M Gray

Hnh 1.18: Gin IQ ca 16-PSK khi dng m Gray. Mi v tr IQ lin tip ch thay i mt bit n.

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Hnh 1.19: Gin IQ cho cc dng iu ch s dng trong OFDM

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Chng 1: Gii thiu tng quan v OFDM1.7 Cc c tnh ca OFDM

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Qua bn cht ca OFDM, ta c th tm tt nhng u im v nhc im ca OFDM nh sau:1.7.1 u im

- OFDM tng hiu sut s dng bng cch cho php chng lp nhng sng mang con. - Bng cch chia knh thng tin ra thnh nhiu knh con fading phng bng hp, cc h thng OFDM chu ng fading la chn tn s tt hn nhng h thng sng mang n. - OFDM loi tr nhiu symbol (ISI) v xuyn nhiu gia cc sng mang (ICI) bng cch chn thm vo mt khong thi gian bo v trc mi symbol. - S dng vic chn knh v m knh thch hp, h thng OFDM c th khi phc li c cc symbol b mt do hin tng la chn tn s ca cc knh. - K thut cn bng knh tr nn n gin hn k thut cn bng knh thch ng c s dng trong nhng h thng n sng mang. - S dng k thut DFT b sung vo cc chc nng iu ch v gii iu ch lm gim chc nng phc tp ca OFDM. - Cc phng php iu ch vi sai (differental modulation) gip trnh yu cu vo b sung b gim st knh. - OFDM t b nh hng vi khong thi gian ly mu (sample timing offsets) hn so vi h thng n sng mang. - OFDM chu ng tt nhiu xung vi v nhiu xuyn knh kt hp. N goi nhng u im trn th OFDM cng c nhng hn ch.1.7.2 Nhc im

- Symbol OFDM b nhiu bin vi mt khong ng ln. V tt c cc h thng thng tin thc t u b gii hn cng sut, t s PARR cao l mt bt li nghim trng ca OFDM nu dng b khuch i cng sut hot ng min bo ha u khuch i tn hiu OFDM. N u tn hiu OFDM t s PARR ln hn th s

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gy nn nhiu xuyn iu ch. iu ny cng s tng phc tp ca cc b bin i t analog sang digital v t digital sang analog. Vic rt ngn (clipping) tn hiu cng s lm xut hin c mo nhiu (distortion) trong bng ln bc x ngoi bng. - OFDM nhy vi tn s offset v s trt ca sng mang hn cc h thng n sng mang. Vn ng b tn s trong h thng OFDM phc tp hn h thng n sng mang. Tn s offset ca sng mang gy nhiu cho cc sng mang con trc giao v gy nn nhiu lin knh lm gim hot ng ca cc b gii iu ch mt cch trm trng. V vy, ng b tn s l mt trong nhng nhim v thit yu cn phi t trong b thu OFDM .1.8 Kt lun

N i dung ca chng ch a ra cc khi nim c bn v mt s vn lin quan v OFDM. Trong thc t cn phi xt nh hng ca knh truyn v tuyn ln tn hiu trong qu trnh truyn i. V nh hng, tn hiu thu c th b suy gim bin , c th b mt thng tin mt s ch, mt mt cng sutChng sau s cp n cc c tnh knh truyn v mt s vn k thut trong OFDM.

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5-Chuong 1-Tong Quan Ve OFDM