LI NI U Trc y trong thi k chin tranh lnh, mn thng tin tri ph ch yu nm trong b mt, do phn ln thng tin v mn ny vo thi k ch c th tm thy trong cc ti liu mt. Ngy nay bc tranh thay i ng k. Sch bo cng khai v tri ph kh phong ph, cc ng dng ca k thut tri ph c m rng t lnh vc qun s sang lnh vc thng mi. V d nh cc h thng thng tin di ng t bo s dng a truy nhp tri ph (CDMA) ngy cng ph bin trn th gii, v ngay c h thng di ng th h 3 (3G) v cao hn cng chn tri ph lm phng php a truy nhp. Cng vi s hng dn ca c gio Phm Qunh Trang, nhm sinh vin chng ti i vo tm hiu ti K thut tri ph. Di y l mt s vn v k thut tri ph: 1. Tng quan v k thut tri ph 2. K thut tri ph trc tip DS/SS (Direct sequence spread spectrum) 3. K thut tri ph nhy tn FH/SS (Frequency hoping spread spectrum) 4. K thut tri ph nhy thi gian TH/SS (Time hoping spread spectrum) 5. Vn ng b trong tri ph 6. ng dng k thut tri ph trong thc t.

K THUT TRI PHCHNG I : TNG QUAN V K THUT TRI PH K thut tri ph l 1 cng ngh c s dng nhiu trong thc t v n c c tnh chng nhiu v bo mt rt cao. Ngy nay n l thnh phn tt yu trong cc h thng thng tin v tuyn ln nh: - CDMA s dng tri ph chui trc tip( DSSS) - GPS l h thng tri ph ln nht th gii , cc Wlan nh WIFI hay Bluetooth Mt s loi tri ph c ng dng rng ri nhiu trong thc t : Tri ph dy trc tip DS/SS: Qu trnh t c bng cch nhn ngun tn hiu vo vi tn hiu m gi ngu nhin mt cch trc tip tn hiu tri ph a ra c rng ph xp x tc ca m gi ngu nhin. Tri ph nhy tn FH/SS: Qu trnh tri ph t c bng cch nhy tn s sng mang trn mt tp ln cc tn s. S nhy tn ca tn s sng mang c quyt nh ca cc m nhy tn c dng gi ngu nhin c iu khin bi cc t m tri ph PN. Tri ph nhy thi gian TH/SS: l h thng m bit cn truyn c chia thnh cc khi k bit, mi khi c pht i mt cch ngu nhin trong cc cm ca cc khe thi gian. Khe thi gian c chn pht cho mi cm c nh ngha bng chui PN n c nhim v xc nh mu nhy khe thi gian

CHNG II : H THNG TRI PH CHUI TRC TIP

I. GII THIU CHUNG 1. M gi tp m s dng trong DSSS Nh bit ta dng m ngu nhin tri ph bn tin pha pht v gii tri ph tn hiu thu c pha thu. M "ngu nhin" ng vai tr trung tm trong cc h thng SS. Tuy nhin nu m ny thc s ngu nhin th thm ch my thu ch nh cng khng th ly ra bn tin v khng th bit c phng php ng b vi m thc s ngu nhin, dn n h thng tr nn v dng. V th phi thay th bng mt m gi ngu nhin. y l mt m tt nh bit trc i vi my thu ch nh. Nhng th hin ging tp m i vi cc my thu khng ch nh. M ny thng c gi l chui gi tp m (PN: Pseudo-Noise). Chui PN l mt chui cc s c lp li theo mt chu k nht nh. Ta s dng {c i , i= s nguyn} {...,c-1 , c0 , c1 ,...} biu th mt chui PN. Gi s N l chu k sao cho ci + N = ci . i khi ta gi N l di ca chui PN v mt chui tun hon ch l s m rng tun hon ca chui c di N. mt chui {ci} l mt chui gi tp m tt, gi tr ca ci phi c lp vi gi tr ca cj i vi mi i j. m bo iu ny l tng chui ni trn khng c lp li, ngha l chu k phi l . Trong thc t v chui PN phi tun hon nn chu k ca n phi ln t c thuc tnh ngu nhin tt. Trong mt h thng DSSS, mt tn hiu lin tc theo thi gian c gi l tn hiu PN c to ra t chui PN dng tri ph. Gi thit chui PN ny l c s hai, ngha l ci = 1, th tn hiu PN ny l:

c(t ) =

i =

c pi

Tc

(T iTc )

(1)

trong pTc(t) l xung ch nht n v c cho bi phng trnh (2.13), ci c gi l chip v khong thi gian Tc giy c gi l thi gian chip. Lu rng tn hiu PN c chu k l NTc. Mt th d ca chui ny c

cho hnh 3.1 i vi N = 15 v {ci , i = 0, 1, ..., 14} = {1, 1, 1, -1, 1, 1, -1, -1, 1, -1, 1, -1, -1, -1, -1}. Tn hiu (chui) PN cn c gi l tn hiu (chui) tri ph hay tn hiu (chui) ngu nhin.

Hnh 1:Th d v tn hiu PN c(t) c to ra t chui PN c chu k 15 tin cho vic kho st, ta s lp m hnh tn hiu PN nh l mt tn hiu c s hai gi ngu nhin, ngha l ta coi rng ci l +1 hay -1 vi xc sut nh nhau, ci v cj c lp vi nhau nu i j. Khi ny hm t tng quan s l :

| | ,| | TC 1 R C ( ) = TC ( ) = TC 0, #

(2)

Mt chui m c chu k N s c hm t tng quan chun ho c cho bi biu thc sau:

(3) Nu N cng ln th biu thc (3) cng gn biu thc (2). V vy n gin, ch yu chng ta s s dng hm tam gic ptr (2) l hm t tng quan cho mt tn hiu PN v biu thc (4) di y cho PSD:

2 ( f) T sin c ( fT) = c c c

(4)

2. S khi c trng mt h thng tri ph DSSS Cc phn t c s ca mt h thng thng tin tri ph c minh ha trn hnh di y:

Hnh: S khi h thng tri ph DSSS Chng ta thy rng b m ha v gii m, b iu ch v gii iu ch l cc phn t c s ca mt h thng thng tin s truyn thng. Ngoi cc phn t ny, mt h thng thng tin tri ph cn p dng 2 b to chui gi ngu nhin nh nhau, v c ng b vi nhau giao tip vi b iu ch v gii iu ch pha pht v pha thu nh trn hnh. Hai b to m gi ngu ny to

ra mt chui m nh phn gi tp ti b iu ch tri tn hiu c pht i v ph v gii tri ph tn hiu ti b gii iu ch. Di y trnh by v mt h thng tri ph kt hp iu ch sng mang BPSK. 3. H thng DSSS-BPSK a. My pht DSSS-BPSK S khi ca my pht DS/SS s dng BPSK c cho hnh 3. Ta c th biu din s liu hay bn tin nhn cc gi tr 1 nh sau:

d ( t) =

i=

i

d Tbp ( t ) iT b

(5)

Trong di = 1 l bit s liu th i v Tb l rng ca mt bit s liu (tc s liu l 1/Tb bps)

Hnh 3: S khi my pht DSSS-BPSK Tn hiu d(t) c tri ph bng tn hiu PN c(t) bng cch nhn hai tn hiu ny vi nhau. Tn hiu nhn c d(t)c(t) sau s iu ch cho sng mang s dng BPSK, kt qu cho ta tn hiu DSSS-BPSK xc nh theo cng thc sau:

S (t ) =

2 Eb d ( t ) c( t ) cos(2 c t ) f + Tb

(6)

Eb: Nng lng trn mt bit ca sng mang Tb : rng mt bt

Fc : Tn s sng mang : pha ban u

Vi Tb= NTc , th tn hiu ptr(6) c v trn hnh 3. b. S khi my thu DSSS-BPSK

DHKH: ng h k hiu; SM: Sng mang; th: tn hiu Hnh 4: S khi my thu DSSS-BPSK

Mc ch cu my thu ny l ly ra bn tin d(t) (s liu {di}) t tn hiu thu c bao gm tn hiu c pht cng vi tp m. Do tn ti tr truyn lan nn tn hiu thu l:

r ( t)= S ( t )

2E n( t) b r = + Tb

d( t

t ) c ( c ) co s 2' f ( t

(7))

Ebr : Nng lng trung bnh ca sng mang trn mt bit n(t): l tp m ca knh v u vo my thu Gi thit tn hiu thu c khng c tp m hay n(t)=0, th tn hiu thu c t pha pht l:

2 Ebr r1 ( t) = S ( t ) = Tb

d( t

)c( t ) cos 2 f c'( t ) (8)

Ta nhn tn hiu r1(t) vi vi tn hiu ng b PN c(t- ) c to ra my thu gii tri ph, ta c:

w( t ) =

2 Ebr ' d ( t ) c ( t ) cos 2 c f ( t ) + 2 Tb

(9)

V c(t- )= 1, nn c2(t- )=1 vi mi t, nn:

w( t) =

2 Eb r Tb

d ( t

(10) ) cos c f ( t ' ) + 2

Tn hiu ptr (10) s c gii iu ch sng mang khi bit tn s fc v pha ' , thu c:

z (t ) = E d (t ) br

(11)

Vy Zi =

Ebr , (v d (t ) =+1 hoc -1). Kt qu c a qua

b so snh ngng 0 ta c u ra c s hai 1 hay -1 theo thi gian ging nh tn hiu d(t) ban u. 3.Hnh nh m phng bng matlab

CHNG III : H THNG TRI PH NHY TN 1.Khi nim Trong tri ph nhy tn (FH), rng bng knh c sn W c chia nh thnh mt s ln cc khe tn khng ln ln nhau. Trong bt k khong thi gian truyn tin no, tn hiu truyn i u chim mt (hay nhiu hn mt) khe tn s ni trn. Vic chn khe (hay cc khe) tn s no trong mi mt khong thi gian truyn tn hiu c thc hin mt cch gi ngu nhin theo tn hiu li ra ca mt b to chui PN, chui m y ch c tc dng xc nh mu nhy tn.

2. H thng thu pht S khi ca mt h thng tri ph FH

Trong h thng ny th iu ch FSK thng c s dng, tn hiu FSK c to ra t lung s liu qua iu ch FSK. B iu ch chn mt trong hai tn s f0 hay f1 tng ng vi vic truyn mt con 0 hay mt con 1. Tn hiu FSK c trn vi tn hiu y(t) t b t hp tn s, m tn s ca y(t) thay i theo cc gi tr t m bit nhn c t b to chu PN, t m bit ny ta c th n nh c 2^m-1 tn s sng mang c to bi b tng hp tn s. Do tn hiu FSK c dch chuyn v tn s mt lng c quy nh bi chui li ra ly t b to chui PN. Ti my thu, mt b to chui PN y nh vy ( c ng b vi tn hiu thu c) c s dng d iu khin li ra ca b t hp tn s. Nh th, s chuyn dch tn s gi ngu nhin c a vo my pht s c loi b ti b gii iu ch nh vic trn tn hiu li ra ca b t hp tn s vi tn hiu thu c. tn hiu sn phm ca vic trn sau c gii iu ch bng b gii iu ch FSK. Mt tn hiu duy tr ng b b to chui PN vi tn hiu thu c thng c tch t tn hiu thu c. Trong h thng nhy tn th tc nhy tn c th nhanh hn hoc chm hn tc s liu. trong trng hp th nht gi la nhy tn nhanh, trng hp th hai gi l nhy tn chm

Hnh: Nhy tn nhanh v nhy tn chm CHNG IV: K THUT TRI PH NHY THI GIAN THSS 1. Khi nim v tri ph nhy thi gian THSS Cc tn hiu c truyn mt cch khng lin tc. Thay vo l cc tn hiu xut hin t ngt nhng thi gian c quyt nh bi tn hiu m gn cho ngi s dng. Trc thi gian c chia thnh cc khung v mi khung li c chia thnh K khe thi gian (time slot). Trong mi khung, ngi s dng s gi d liu trn mt trong K khe thi gian. Khe thi gian no s c s dng ph thuc vo tn hiu m gn cho thu bao. THSS s dng ton b ph bng rng cho cc khong thi gian ngn thay v ch s dng cc phn ca ph trong sut thi gian nh trong FH.

Hnh: Tri ph nhy thi gian ( THSS ) Trong : J l s khe thi gian T = T /J

S khi to v khi thu tn hiu TH-SS 2.Nguyn l hot dng ca h thng TH/SS Trong mt h thng tri ph nhy thi gian s liu c pht thnh cc cm. Mi cm gm k bit s liu v thi gian chnh xc pht mi cm c xc nh bi mt chui m PN. Ga s thang thi gian c chia thnh cc khung, mi khung T giy. Sau mi khung li c chia tip thnh J cc khe thi gian. V th mi khe thi gian chim rng T = T /J giy. Biu thi gian c th hin nh sau:

Biu thi gian cho mt h thng THSS Trong thi gian mi khung mt nhm k bit d liu c pht trong T giy tc l mt trong J khe thi gian. Cc khe thi gian s c s dng pht c xc nh bi chui PN. Mi bit ch chim T = T /K giy khi pht. Quan h gia T , T , T c m t trn hnh() . Ga s thi gian ca mt bit s liu l T, kp truyn dn s liu vo ta cn T = KT. Nu cc bit s liu vo l , i l s nguyn ta c th biu din tn hiu THSS nh sau: k 1

STH =Trong :

i= l 0 =

b

l ik

0

p(T t f iT

a iTs 0 lT )

pT0

l xung ch nht

n v rng l T0 giy

ai [0,1... j 1] l s ngu nhin c xc nh bi j bit ca chuiPN v J=2j

Vi i th hin khung th i,

ai

th hin s khe thi gian

l l s th t bit trong cm. S liu c truyn cc cm k bit mi ln vi mi bit c pht

trong khong T0 =

Tf jk

giy. V th tc bit trong cm l l/ T giy

truyn bng tn gc c rng bng tn l l/T Hz.Cn nu truyn bng thng th di thng s l 2/T0 Hz. V bn tin c rng l 1/T, rng bng tn c m rng l ( 1/T )/(1/T) = ( k T)J/ T = j khi truyn dn bng gc v 2j khi truyn dn bng thng. li x l l (2/T )/(2/T) =J khi truyn bng thng. 3. u nhc im ca h thng tri ph nhy thi gian THSS a. u im ca k thut tri ph nhy thi gian D thc hin hn so vi k thut FH-SS do khng cn to b tng hp tn s phc tp. Hiu qu s dng bng tn cao hn nhiu so vi vic s dng k thut TDM Trnh c hiu ng gn- xa v thi gian m mi ngi dng truyn l c lp.ngi dng trm gc khng gy nh hn n ngi dng xa. b. Khuyt im ca k thut tri ph nhy thi gian. Tri ph nhy thi gian l mt hnh thc nn tn hiu trn mim thi gian. Chnh v vy rng bit gim, tc bit tng v do i hi knh truyn phi truyn c tn hiu tc cao.

CHNG V: VN NG B TRONG TRI PH V NHNG NG DNG TRI PH I.VN NG B TRONG TRI PH Cc tn hiu tri ph u s dng m gi ngu nhin tri ph tn hiu hoc iu khin nhy tn s nn vn ng b c xem l yu t sng cn. Vi bt k k thut tri ph no chng ta cn phi c thng tin v thi gian ca tn hiu c pht nn tn hiu thu c v gii iu ch tn hiu va mi c nn. c bit i vi h thng DS-SS nu chng ta ch chch i 1 khong thi gian bng 1 chip th chng ta khng th nn c tn hiu tri ph thu c nn khng th tm ra c tn hiu d liu ban u. ng b chia lm 2 giai on Bt m( Acquisition) bc ny 2 m tri ph (m thu c v m t sinh ra bn nhn ) s ng chnh vi nhau, ng b gia my pht v my thu trong khong thi gian xac nh l Tc Bm m (tracking) bc ny nh s dng vng hi tip m m tri ph ti ch chnh xc nht lin tc c chn Mt s yu t nh hng n vic ng b : Khong cch gia my thu v my pht khng xc nh dn n tnh ton gi tr tr truyn dn khng chnh xc

Nhp tng i gi my thu v my pht khng c thit lp dn n s khc nhau v pha gia tn hiu tri ph ca my pht v my thu My pht v my thu khng c lp t cc b dao ng ging nhau dn n lch tn s gia 2 tn hiu c im ca hu ht chin lc ng b l trong khong thi gian bt m, ni thu tin hnh cho m thu v m to ra ti ch c tng quan vi nhau c c a lng nh gi s ging nhau gia chng. Sau i lng ny c so snh vi 1 mc chun nh trc a ra quuyt nh. Nu chng ng b th vic bt m kt thc, nu khng th th tc thu li a ra m c to ra ti ch c s thay i v tn s v pha v li so snh tip n bao gi chng ng b mi thi.Tn hiu thu BPF

Tch nng lng

Thit b quyt nh

B to tn hiu PN tham chiu

Pha d iu khin Logic

Bn thu chn 1 pha cho dy PN ti my thu nn ph tn hiu thu c.Tn hiu sau khi nn ph s cho qua b lc thng di. Nu pha ca chng ging nhau th BPF s nhn ton b cng sut ca tn hiu va thu c v thit b iu khin s cho kt thc chu trnh bm. Ngc li nu

pha th chn ny khng khp vi tn hiu thu c, th s xut hin tn hiu bng rng ti u vo ca BPF v n ch thu nhn c 1 phn cng sut rt nh. Da vo iu ny my thu quyt nh pha d khng ng v tip tc d pha khc. II. MT S NG DNG TRI PH TRONG THC TIN 1.Thng tin v tinh 2. o c li 3. H thng nh v ton cu (GPS) 4. V tuyn di ng a truy nhp nhy tn 5. Radar xung