Cc k thut iu ch sTc gi: Fukin XionDch gi: dungn-thnh vin vntelecom.orgLi ni uCc k thutiu ch s l iu thit yu trongmt s h thng thng tin s, cho d l h thng thoi, h thng thng tin di ng t bo, hay h thng thng tin v tinh.Trong khong 20 nm qua, nhng nghin cu v pht trin trong cc k thut iu ch s c nhiu hot ng v cho thy mt s kt qu ha hn. Tuy nhin, cc kt qu ny li phn tn trn rt nhiu tiliu.Kt qu l, cc k s v sinh vin trong lnh vc ny thng gp kh khn trong vic xc nh nhng k thut c th cho cc ng dng hay cc ti nghin cu. Cun sch ny cung cp cho bn c nhng thng tin hon chnh, cp nht ca tt c cc k thut iu ch s trong cc h thng thng tin s. Tn ti mt s lng khng l cc sch ti liu v thng tin s, mi trong s chng cha mt hay nhiu chng v cc k thut iu ch s bao gm c cc dng c bn ca iu ch, hay ch mt nguyn l ca cc k thut. Cng c mt s cun c t mt vi dng iu ch no . Cun sch ny s trnh by thng tin v cc nguyn l v cc ng dng ca tt c cc k thut iu ch s hin nay, cng nh cc k thut hin ang c nghin cu. vi mi trnh t iu ch, cc ch sau s c bao trm: nn tng lch s, cc nguyn l hot ng, t l li bit v li mu tn hiu (hiu qu dng ph), c tnh ph (hiu qu dng ph), cc s khi ca b iu ch, gii iu ch, khi phc sng mang (nu c), khi phc xung ng h, so snh vi cc trnh t iu ch khc, v cc ng dng. Sau khi ta c th hiu y v cc k thut iu ch cng nh cht lng ca chng trong knh AWGN, ta s tho lun v cht lng ca chng trong cc knh suy hao a ng.Cch sp xp ca schCun sch ny c chia thnh 10 chng. Chng 1 l li gii thiu cc kin thc c bn cn thit v cc h thng thng tin s, v cc cng c iu ch.Chng 2 ni v iu ch tn hiu bng gc m cha k ti sng mang. y cn c gi l nh dng tn hiu bng gc hay m ng (line coding). Mt cch truyn thng, tanhnghathutngiu ch l nn thng tin ln mt sng mang, tuy nhin,nu ta m rng nh ngha ny thnh nn thng tin ln mt trung gian truyn pht, th nh dng ny cng chnh l mt dng iu ch. iu ch bng gc rt quan trng khng ch bi n c s dng trong truyn d liu khong cch ngn, ghi t, ghi quang,(ND: nguyn bn ting anh l magnetic recording, optical recording)m cn v n chnh l u cui pha trc trong iu ch thng di. Cc chng 3 v 4 bao trm cc k thut kha dch tn s FSK v kha dch pha PSK c bn, bao gm c coherent v noncoherent. Cc k thut ny c s dng hin nay trong nhiu h thng thng tin s, nh cc h thng in thoi s dng t bo v cc h thng thng tin v tinh. Cc chng 5-7 ni v cc k thut iu ch pha nng cao trong ckha dch ti thiu (MinimumShift Keying-MSK), iu ch pha ni tip (Continues Phase Modulation-CPM), v multi-h phase modulation (MHSK). Cc k thut ny l cc kt qu nghin cu trong nhng nm gn y, v mt s trong chng ang c s dng trong cc h thng chuyn su, chng hn nh, MSK ang c NASA s dng trong Tn lacngnghtruynthngnngcao(ACTS-AdvancedCommunicationsTechnology Satellite) phng vo nm 1993, v cc tn la khc ang c hon thin cho cc ng dng tng lai.Chng8ni viuchbinvung(Quadratureamplitudemodulation-QAM). Cc trnh t QAM c s dng rng ri trong cc modem in thoi di ng. Chng hn, cc modem v.29 v v.33 c ngh bi CCITT (Consultative Committee for International Telephone and Telegraph)s dng 16- v 128-QAM, cho tc 9600 bps v 14400 bps, trn cc ng in thoi 4 dy.Chng 9 bao trm cc trnh t iu ch tit kim bng thng ng bao khng c nh. Chng ta s nghin cu tm trnh t, gi l QBL, QORC, SQORC, QOSRC, IJF-OQPSK, TIS-OQPSK, SQAM v Q2PSK. Cc trnh t ny ci tin mt ph vi rt t mt mt trong xc sut li. Chng c thit k trc tin cho thng tin v tinh.Chng 10 trc tin gii thiu s lc v cc c tnh ca cc knh c suy hao v phn hi a ng. Sau tt c cc dng iu ch c cp trong cc chng 2-8 c kim chng trong mi trng suy hao a ng.Cc ph chng A v B l cc kin thc c bn v ph tn hiu v l thuyt d v c lng tn hiu kinh in.Cun sch ny c th c s dng lm sch tham kho cho cc k s v cc nh nghincu. Ncngcthcsdnglmschgiokhoachoccsinhvintt nghip. Phn kin thc trong sch c th c hc trong kha hc na nm. Vi cc kha hc ngn, ngi hng dn c th la chn mt s chng thch hp.Li cm nTrc tin ti xin c cm n nh bin tp v kim lc ca Artech House. Ray Sperber, Mark Walsh, Barbara Lovenvirth v Judi Stone, nhng ngi c nhiu ph bnh v gp da trn nhng xem xt k lng ng gp cho s ci tin ca bn tho.Ti cng mun gi li cm n ti trng i hc Cleveland State v Fenn College of Engineering cho php ti ngh php trong sut nm 1997 khi ti vit phn ct yu ca cun sch. Ti cng rt bit n s h tr v ng vin ca nhiu cng s ti khoa K thut in v my tnh..Cui cng, l s ng h v gip t pha gia nh cng l mt ngun ng vin su sc.Fuqin XiongChng 1: gii thiuTrong chng ny chng ta tho lun s lc v vai tr ca iu ch trong mt h thng thng tin s in hnh, cc cng c iu ch c bn, v tiu chun la chn trnh t iu ch. Thm vo l phn m t s lc v cc knh thng tin khc nhau, phc v nh nn tng cho nhng tho lun sau ny v cc trnh t iu ch.1.1 cc h thng truyn thng sHnh 1.1 l s khi ca mt h thng thng tin s in hnh. Thng tin c th c gi i t mt ngun tng t (chng hn nh ging ni) hay t mt ngun s (d liu my tnh). B chuyn i tng t-s (A/D) ly mu v lng t ha tn hiu tng t v biu din cc mu di dng s (bit 1 hoc 0). B m ha ngun chp nhn tn hiu s v m ha n thnh dng tn hiu s ngn hn. y gi l m ha ngun, lm gim s d tha do cng gim tc truyn cn thit (ND: Ch ny t thm mt cht lm r ngha cu). iu ny lm gim bng thng yu cu ca h thng. b m ha knh nhn tn hiu ra ca b m ha ngun v thc hin m ha n thnh tn hiu s di hn. S d tha c thm vo mt cch c ch ch ln tn hiu s m ha nh vy mt s li do tp m hoc nhiu to ra trn sut ng truyn qua knh c th c hiu chnh li ti my thu. Ni chung th truyn pht thng tn s thng di cao, b iu ch do nn cc k hiu s m ha ln mt sng mang. i khi truyn pht thc hin bng c bn, b iu ch l iu ch bng gc, hay cng gi l b nh dng (formator), thc hin nh dng cc k hiu s m ha ln mt dng sng thch hp truyn. Thng thng, c mt b khuch i cng sut theo sau b iu ch. Vi truyn pht tn s cao, iu ch v gii iu ch thng c thc hin tn s trung gian (IF). Nu vo trng hp ny, mt b nng tn s c chn vo gia b iu ch v khuch i cng sut. Nu tn s trung gian l qu thp so vi tn s sng mang, mt s tng ca cc phin bn tn s sng mang c yu cu. Vi cc h thng khng dy, c mt anten tng cui ca my pht. Hnh1: S khi ca mt h thng thng tin s c bnTrung gian truyn pht thng c gi l knh, ti tp m cng vo tn hiu v cc nh hng ca fading vi suy hao xut hin nh h s nhn ln tn hiu. Thut ng tp m y l thut ng ngha rng, bao gm tt c nhng nhiu lon in ngu nhin t ngoi v trong h thng. Knh ni chung cng c bng thng tn s hu hn do n c th c xem nh mt b lc. Ti my thu, hu nh vic x l tn hiu ngc li xy ra. Trc tin, tn hiu nhn rt yu c khuch i (v h tn nu cn thit) sau c gii iu ch. Sau phn d tha c loi b bng b gii m knh v b gii m ngun thc hin khi phc tn hiu v dng nguyn gc trc khi c gi ti ngi s dng. Mt b bin i s-tng t (D/A) c s dng cho cc tn hiu tng t.S khi cho trongtrong hnh 1.1 ch l cu hnh mt h thng kinh in. Mt cu hnh h thng thc c th phc tp hn. Vi mt h thng nhiu ngi s dng, mt khi dn knh c chn vo trc khi iu ch. Vi h thng a trm, mt khi iu khin a truy nhp c chn vo trc my pht. Cc thit b khc nh tri tn v ma ha cng c th c thm vo h thng. Mt h thng thc cng c th n gin hn. M ha ngun v m ha knh cng c th khng cn thit trong mt h thng n gin. Trn thc t, ch c khi iu ch, knh, gii iu ch, v cc b khuch i l nht thit trong mi h thng truyn thng (vi cc anten cho cc h thng khng dy).Vi mc ch m t cc k thut iu ch v gii iu ch v phn tch cht lng ca chng, biu h thng n gin ha trong hnh 1.2 s c s dng thng xuyn.Hnh2: M hnh h thng thng tin s cho vic iu ch v gii iu ch.M hnh ny loi tr cc khi khng hp l vi quan im iu ch sao cho cc khi hp l c thy r rng. Tuy nhin, cc k thut modem (iu ch v gii iu ch) c pht trin gn y kt hp iu ch v m ha knh li vi nhau. Trong cc trng hp ny, cc b m ha knh l mt phn ca b iu ch v cc b gii m knh l mt phn ca b gii iu ch. T hnh 1.2, tn hiu nhn c ti u vo ca b gii iu ch c th c vit nh sau:[ ] ( ) ( ) ( ) * ( ) ( ) r t A t s t h t n t +(1.1)Trong * ch php chp. Trong hnh 1.2 knh c m t bi 3 yu t. Th nht l b lc knh. Do thc t l b lc s(t) t b iu ch phi qua my pht, knh (trung gian truyn pht) v my thu trc khi n c th ti b gii iu ch, b lc knh do l mt b lc hn hp vi hm truyn l:( ) ( ) ( ) ( )T C RH f H f H f H f (1.2)Trong ( )TH f,( )CH f,( )RH fl cc hm truyn ca my pht, knh v my thu. Cng nh vy, p ng xung ca b lc knh l:( ) ( ) * ( ) * ( )T C Rh t h t h t h t (1.3)Trong ( )Th t,( )Ch tv ( )Rh tl cc p ng xung ca my pht, knh v my thu. Nhn t th hai l h s A(t) m ni chung l phc. H s ny biu din fading trong mt s dng knh, nh l knh v tuyn di ng. Nhn t th ba l nhiu cng v s hng nhiu n(t). Chng ta s tho lun v fading v nhiu chi tit hn trong mc sau. M hnh knh trong hnh 1.2 l m hnh chung. N c th c n gin ha trong mt s tnh hung nh s thy trong mc tip theo.1.2 cc knh truyn thngc tnh knh ng mt vai tr quan trng trong nghin cu, la chn v thit k cc trnh t iu ch. Cc trnh t iu ch c nghin cu cho cc knh khc nhau bit cht lng ca chng trong cc knh ny. Cc trnh t iu ch c la chn hoc thit k tng ng vi c tnh knh ti u cht lng ca chng. Trong mc ny ta s tho lun mt s m hnh knh quan trng trong truyn thng.1.2.1 Knh nhiu trng Gauss cng (Additive White Gaussian Noise Channel-AWGN channel)Knh AWGN l mt m hnh ph bin phn tch cc trnh t iu ch. Trong m hnh ny, knh khng lm vic g ngoi cng thm mt nhiu Gauss trng vo tn hiu i qua n. iu ny nhn mnh rng p ng tn s bin ca knh l phng (d vi bng thng gii hn hay ko gii hn) v p ng tn s pha ca knh l tuyn tnh cho mi tn s sao cho cc tn hiu iu ch khi i qua n m khng mt bin v mo pha hay cc thnh phn tn s. Fading khng tn ti. Mo duy nht c to ra bi AWGN. Tn hiu nhn c trong 1.1 c n gin ha cn:( ) ( ) ( ) r t s t n t +(1.4)Vi n(t) l nhiu AWGN.Tnh trng ca n(t) nhn mnh rng c mt qu trnh ngu nhin tnh vi mt ph cng sut phng (PSD) cho tt c cc tn s. C mt quy c gi thit rng PSD ca n bng0( ) / 2 N f N f < < (1.6)iu ny nhn mnh rng mt qu trnh trng c cng sut hu hn. iu ny d nhin mang tnh l tng v mt ton hc. ng vi nh l Wiener-Khinchine, hm t tng quan ca nhiu AWGN l 2( ) { ( ) ( )} ( )j fR E n t n t N f e df (1.7)2 0 0( )2 2j fN Ne df Trong ( ) l hm delta Dirac. iu ny ch ra cc mu nhiu l khng t tng quan cho d hiu thi gian nh ti u chng na. Cc mu cng c lp do qu trnh l qu trnh Gauss.Ti mi im thi gian, bin ca n(t) tun theo hm mt xc sut Gauss cho bi:2221( ) exp{ }22p (1.7)Trong c dng biu din cc gi tr ca qu trnh ngu nhin n(t) v 2l lch ca qu trnh ngu nhin. C mt im th v cn lu l 2 vi qu trnh AWGN do 2l cng sut ca nhiu, l bt nh do tnh trng ca n.Tuy nhin, khi r(t) c ly tng quan vi hm trc giao (t) , th nhiu trong u ra c lch hu hn. Trn thc t:( ) ( ) r r t t dt s n +Trong ( ) ( ) s n t t dt V( ) ( ) n s t t dt lch ca n bng:22{ } ( ) ( ) E n E n t t dt 1 ' ;1 ] ( ) ( ) ( ) ( ) E n t t n dtd ' ; { ( ) ( )} ( ) ( ) E n t t n dtd 0( ) ( ) ( )2Nt t dtd 2 0 0( )2 2N Nt dt (1.18)Khi hm mt xc sut (PDF) ca n c th vit nh sau:20 01( ) exp{ }np nN N (1.19)Kt qu ny s c s dng thng xuyn trong cun sch.Ni mt cch khc, knh AWGN khng h tn ti do khng h c knh truyn no c th c bng thng l v nh. Tuy nhin, khi bng thng tn hiu l nh hn so vi bng thng knh, mt s knh thc t c th xp x vi knh AWGN. Chng hn, cc knh v tuyn thng tuyn LOS (line of sight), bao gm cc kt ni microwave(ND: sngccngn)mt tcnhvccktnivtinhcnh, xpxvi ccknh AWGN khi thi tit tt. Cc cp ng trc bng rng cng xp x knh AWGN do khng tn tinhiu no khc ngoi nhiu Gauss.Trong cun sch ny, tt c cc trnh t iu ch u c nghin cu trong knh AWGN. C hai l do cho vic ny. Th nht, mt s knh vn xp x knh AWGN, cc kt qu c th c s dng trc tip. Th hai, nhiu Gauss cng c biu din cho d c tn ti hay khng nhng nhn t lm suy yu khc ca knh nh bng thng hn ch, fading, a ng, v cc nhiu khc. Vy knh AWGN l knh tt nht m mt h thng c th c. Cht lng ca trnh t iu ch xc nh trong knh ny l bin trn ca cht lng. Khi c cc nhn t suy gim khc ca knh, cht lng h thng s gim. Cht lng trong AWGN c th s dng nh chun trong nh gi s suy gim cng nh tnh hiu qu ca cc k thut chng suy gim cht lng.1.2.2 Knh gii hn bng thngKhi bng thng knh nh hn bng thng tn hiu, knh gi l c bng thng hn ch. S gii hn bng thng phc v gy nn nhiu lin k hiu ISI (chng hn, cc xung s s m rng thi gian truyn (chu k k hiu Ts)) v gy nhiu ln k hiu tip theo, hay thm ch l c k hiu tip theo na. ISI gy tng xc sut li bit (Pb) hay t l li bit BER, nh n vn c gi. Khi vic tng bng thng knh truyn l iu khng th hoc khng hiu qu kinh t, cc k thut cn bng knh c s dng chng li ISI. Qua nhiu nm, mt s lng ln cc k thut cn bng c pht minh v s dng. Cc k thut cn bng mi xut hin lin tc. Chng ra s khng bao trm chng trong cun sch ny. V hng cc k thut cn bng knh, bn c c khuyn xem [1.chng 6] hay bt k nhng sch v cc h thng thng tin.1.2.3 Knh fadingFading l mt hin tng xy ra khi bin v pha ca tn hiu v tuyn bin i nhanh trong mt khong thi gian ngn hay khong lan truyn ngn. Fading c to nn bi nhiu gia hai hay nhiu phin bn ca tn hiu pht khi chng ti my thu nhng thi im khc nhau mt cht. Cc sng ny, gi l cc sng a ng, kt hp vi nhau ti anten cho mt tn hiu tng m c th bin i rt rng v c bin v tn s. Nu cc thi gian tr ca cc tn hiu a ng di hn chu k k hiu (symbol) (ND: hay cng c thgi lmu tnhiu),cctn hiu a ng phi c xem nh tn hiu khc. Trong trng hp ny, ta c cc tn hiu a ng c lp.Trong cc knh thng tin di ng, nh knh di ng mt t v knh di ng v tinh, nhiu fading v a ng c to nn bi nhng phn hi t cc cng trnh bao quanh v cc a hnh. Thm vo , s di chuyn tng i gia my pht v my thu cho kt qu l iu ch tn s ngu nhin trong tn hiu do mc dch tn Doppler khc nhau trn mi thnh phn a ng. S di ng ca cc i lng bao quanh, nh xe ti, cng to nn mt mc dch tn Doppler trn thnh phn a ng. Tuy nhin, nu cc i tng bao quanh di chuyn tc nh hn tc ca di ng, th hiu ng ca chng c th c b qua.Nhiu fading v a ng cng tn ti trong cc kt ni microwave LOS (ND: nh ni trn, LOS=tuyn thng) [3]. Trong nhng bui h tri trong v m, s hn lon p sut thng thng l nh. Tng i lu xp thnh tng vi nhng phn phi ca nhit v hi m khng ng u. S phn lp ca tng p sut thp hn to nn cc gradien ch s khc x t ngt to nn cc ng a tn hiu vi cc bin v cc tr kh khc nhau.Fading to nn cc bin i nhanh v bin v nhng lch pha trong cc tn hiu nhn. a ng to nn nhiu lin k hiu. Dch tn Doppler to nn s tri tn s sng mang v tri bng thng tn hiu. Tt c iu dn ti s suy hao cht lng ca iu ch. Vic phn tch cht lng iu ch trong cc knh fading s c tho lun chi tit hn.1.3 Cc cng c iu ch v bniu ch s l mt qu trnh nn mt k hiu s ln mt tn hiu thch hp truyn pht.Vi nhng truyn pht khong cch ngn, iu ch bng gc thng c s dng. iu ch bng gc thng c gi l m ng. Mt chui cc k hiu s thng c s dng to nn dng sng xung vung vi mt s c im no biu din mi dng k hiu m khng c s nhp nhng sao cho chng c th c khi phc trong lc thu. Hnh 1.3 cho mt s dng sng iu ch bng gc. Dng u tin l iu ch non-return zero-level (NRZ-L) thc hin biu din mt k hiu 1 bi mt xung vung dng vi di T v k hiu 0 bi mt xung vung m vi di T.Hnh3: Cc th d v iu ch s bng gc.Dng th hai l dng iu ch unipolar return to zero vi mt xung dng di T/2 biu din k hiu 1 v gi tr 0 biu din cho k hiu 0. Dng th 3 l dng mc 3 pha (biphase level) hay cn gi l Manchester, sau khi pht minh ra dng ny, vic iu ch s dng dng sng gm mt xung T/2 dng v xung T/2 m cho 1 v dng sng o ngc cho 0. Trnh t iu ch ny v cc trnh t khc s c tho lun chi tit hn trong chng 2. Vi cc truyn pht ng di v khng dy, iu ch thng di thng c s dng. iu ch thng di cng c gi iu ch sng mang. Mt chui cc k hiu s c s dng lm thay i cc thng s ca mt tn hiu hnh sin tn s cao gi l sng mang. Ni chung, mt tn hiu hnh sin c 3 thng s: bin , tn s v pha. Vy iu ch bin , iu ch tn s, v diu ch pha v l ba cng c iu ch c bn trong iu ch thng di. Hnh 1.4 cho ba dng iu ch sng mang nh phn c bn. l kha dch bin (ASK), kha dch tn s (FSK), v kha dch pha (PSK). Trong ASK, b iu ch y ra mt b sng mang cho mi k hiu 1, v khng tn hiu no cho mi k hiu0. Trnhtnycngcgi lkhabt-tt (OOK). TrongkhaASKthng thng, bin cho k hiu 0 khng thc s l 0. Trong FSK, vi k hiu 1, mt b sng mang tn s cao hn c pht ra v vi k hiu 0 mt b sng mang tn s thp hn c pht ra, hay ngc li. Hnh4: ba trnh t iu ch thng di c bn.Trong PSK, mt k hiu 1 c pht ra nh mt b sng mang vi 0 ln o pha trong khi k hiu 0 c pht ra nh mt b sng mang vi pha o 180.Da trn ba trnh t iu ch c bn , mt lot cc trnh t iu ch c th c tmthytcckt hpcachng. Chnghn, bngcchkt hphai tnhiuPSK (BPSK) vi cc sng mang trc giao, mt trnh t mi gi l kha dch pha cu phng (QPSK) c th c to ra. Bng cch iu ch c bin v pha ca sng mang, ta c th thu c mt trnh t gi l iu ch bin cu phng (QAM),v.v.1.4 Tiu chun la chn tiu chun iu chTinh thn ca vic thit k mt modem s l truyn mt cch hiu qu cc bit s v phc hi chng t cc nh hng ca nhiu v cc tc ng ca knh. C 3 tiu chun u tin khi la chn cc trnh t iu ch: hiu qu v cng sut, hiu qu ph v phc tp ca h thng.1.4.1 Hiu qu cng sutTllibit, hayxcsut libitcamttrnh tiuchl t l nghch vi 0/bE N, t l nng lng bit trn mt ph nhiu. Chng hn, bP ca ASK trong knh AWGN c cho bi:02bbEP QN _ ,(1.10)Trong bE l nng lng bit trung bnh, cn 0N l mt ph nhiu (PSD) v Q(r) l tch phn Gauss, i khi c bit ti l hm Q. Hm c nh ngha nh sau:2 1( )2uxQ x e du(1.11)Hm l hm gim n iu ca x. Do hiu qu cng sut ca mt trnh t iu ch c nh ngha mt cch thng thn nh t l 0/bE N cn thit vi mt xc sut li bit no (bP) tren mt knh AWGN.510bP thng c s dng nh t l li bit tham chiu.1.4.2 Hiu qu phVic xc nh hiu qu dng ph phc tp hn mt cht. Hiu qu ph c nh ngha nh s bit trn mt giy c th c truyn i trong mt Hert ca bng thng h thng. V d, mt ph cng sut mt di ca mt tn hiu ASK c iu ch bi mt chui bit ngu nhin c lp c xc sut ngang nhau c cho nh sau:2 22( ) sin [ ( )] ( )4 4s c cA T Af c T f f f f + v c cho trong hnh 1.5, trong T l di bit, A l bin sng mang, v cf l tn s sng mang. T hnh ta c th thy rng ph tn hiu tri t ti . Vy truyn i mt cch hon ho tn hiu, th cn mt bng thng h thng khng xc nh, bin thin da trn mt tiu chun khc. Chng hn, trong hnh 1.5, hu ht nng lng tn hiu tp trung trong di gia hai im 0, vy yu cu mt bng thng 0-0 c v nh Hnh5: mt ph cng sut ca ASKy . C 3 cch tnh hiu qu ph trong cc ti liu nh lit k sau y:Hiu qu ph Nyquist-gi thit h thng s dng b lc Nyquist (p ng xung ch nht l tng) ti bng gc, c bng thng yu cu ti thiu cho truyn pht nhiu ISI t do ca cc tn hiu s, th bng thng ti bng gc l 0.5sR, sR l tc k hiu, v bng thng ti tn s sng mang l WsR . Do 2/ logs bR R M , bR=tc bit, vi iu ch M-ary (ND: M y l s im c th thy khi xem biu chm im), hiu qu ph l 2/ W logbR M (1.12)Hiu qu ph null-null(ND: hay trn va dch l 0-0)-vi cc trnh t iu ch c cc im 0 ph mt cng sut nh ca ASK trong hnh 1.5, nh ngha bng thng nh rng ca bp sng chnh l cch thch hp nh ngha bng thng.Hiu qu ph phn trm-nu ph ca mt tn hiu iu ch khng c cc im khng, nh iu ch pha lin tip ni chung (CPM), bng thng null-null khng tn ti. Trong trng hp ny, bng thng phn trm c th c s dng. Thng thng 99% c s dng, cho d mt s s phn trm khc (nh 90%, 95%) cng c dng.1.4.3 phc tp h thng phc tp ca h thng ni ti tng s dy dn trong n v kh k thut ca h thng. Lin h vi phc tp ca h thng l gi thnh sn xut, d nhin l mi bn khon chnh trong vic la chn mt k thut iu ch. Thng thng, b gii iu ch phc tp hn b iu ch. B gii iu ch thch ng th phc tp hn nhiu so vi b gii iu ch khng thch ng do s khi phc sng mang c yu cu trong n. Vi mt s phng php gii iu ch, cc thut ton phc tp nh Viterbi cn s dng. Tt c l nn tng cho so snh phc tp hn.Do hiu qu v cng sut, hiu qu ph v phc tp h thng l tiu chun chnh khi la chn mt k thut iu ch, ta s lun ch ti chng khi phn tch cc k thut iu ch trong phn cn li ca cun sch.1.5 Tng quan cc trnh t iu ch s cung cp cho bn c mt ci nhn tng quan, chng ti lit k mt s vit tt v tn m t ca cc iu ch s khc nhau s c trong bng 1.1 v sp xp chng theo s cy nh trong hnh 1.6. Mt s trnh t c th thu c t nhiu hn mt trnh t cha. Cc trnh t trong m ha vi sai c th c s dng s c nh nhn D v nhng trnh t c th c gii iu ch khng thch ng s c nh nhn N. Tt c cc trnh t u c th c gii iu ch thch ng.Cc trnh t iu ch lit k trong bng v cy c phn loi thnh hai phn nhm ln: bin c nh v bin khng c nh. Trong lp bin c nh, c 3 lp con: FSK, PSK v CPM. Trong lp bin khng c nh, cng c 3 lp con: ASK, QAM v cc iu ch bin khng c nh khc na,Trong cc trnh t c lit k, ASK, PSK v FSK l cc iu ch c bn, cn MSK, GMSK, CPM, MHPM v QAM, v.v. l cc trnh t nng cao. Cc trnh t iu ch nng cao l cc bin th v kt hp ca cc trnh t c bn.Lpcbincnhnichung ph hp vi cc h thng thng tin c cc b khuch i cng sut hot ng trong vng phi tuyn ca c tuyn u vo-u ra thu c hiu qu khuch i ti a. Mt v d l TWTA (traveling wave tube amplifier)-b khuch i ng sng chy trong thng tin v tinh. Tuy nhin, cc trnh t FSK ni chung trong lp ny l khng thch hp vi ng dng v tinh do chng c hiu qu ph qu thp so vi cc trnh t PSK. FSK nh phn c s dng trong cc knh iu khin tc thp ca cc h thng t bo th h th nht, AMPS (advance mobile phone service of ) v ETACS (European total access communication system). Tc truyn c th l 10 Kbpsvi AMPSv8Kbpsvi ETATC. CctrnhtPSK, baogmBPSK, QPSK, OQPSK, v MSK c s dng trong cc h thng thng tin v tinh./4-QPSK c quan tm nht bi kh nng trnh di pha t ngt 180 v cho php gii iu ch vi sai. Trnh t ny c s dng trong cc h thng thng tin di ng s t bo, nh h thng s t bo M (USDC)(United State digital cellular).Tn vit tt Tn vit tt tng ngM tFrequency shift keyingBFSK FSK Binary frequency shift keyingMFSK M-ary frequency shift keyingPhase shift keyingBPSK PSK Binary phase shift keyingQPSK 4PSK Quadrature phase shift keyingOQPSK SQPSK Offset QPSK, Staggered QPSK/4-QPSK/4 Quadrature phase shift keyingMPSK M-ary Phase shift keyingContinuous phase modulation (CPM)SHPM Single-h (ch s iu ch) phase modulationMHPM Multi-h (ch s iu ch) phase modulationLREC Rectangular pulse of length LCPFSK Continuous phase shift keyingMSK FFSK Minimum shift keying, fast FSKSMSK Serial minimum shift keyingLRC Raised cosine pulse of length LLSRC Spectrally raised cosine pulse pff length LGMSK Gaussian minimum shift keyingTFM Tamed frequency modulationAmplitude and amplitude/phase modulationsASK Amplitude phase shift keying (tn thng thng)OOK ASK Binary on-off keyingMASK MAM M-ary ASK, M-ary amplitude modulationQAM Quadrature amplitude modulationCc iu ch bin khng c nhQORCQuadrature overlapped raised cosine modulationSQORC Staggered QORC QOSRCQuadrature overlapped squared raised cosine modulationQ2PSK Quadrature quadrature phase shift keyingIJF-OQPSK Intersymbol-interference/jitter-free OQPSKTSI-OQPSK Two-symbol-interval OQPSKSQAM Superposed-QAMXPSK Crosscorrelated QPSKBng 1 Cc trnh t iu ch sHnh6: cy iu ch sCc trnh t PSK c bin c nh nhng chuyn pha khng lin tc t k hiu ny sang k hiu khc. Cc trnh t CPM khng ch c bin c nh m cn c s chuyn pha lin tc. Vy chng c t nng lng bp bin trong ph hn so vi cc trnh t PSK. Lp CPM bao gm LREC, LRC, LSRC, GMSK v TFM. S khc nhau gia chng nm cc xung tn s khc nhau c th hin tn ca chng. Chng hn, LREC c ngha l xung tn s l mt xung hnh ch nht vi di L cc chu k iu ch. MSK v GMSK l hai trnh t quan trng trong lp PCM. MSK l mt trng hp ca CPFSK, nhng n cng c th nhn c t OQPSK vi dng xung hnh sin m rng, MSK c hiu qu v cng sut v hiu qu v ph rt tuyt. B iu ch v gii iu ch ca chng cng khng qu phc tp. MSK c s dng trong tn la cng ngh truyn thng chuyn su ca NASA (NASA's Advanced Communication Technology Satellite (ACTS)). GMSK c xung tn s Gauss. Vy n c th c hiu qu dng ph tt hn c MSK. GMSK ]cj s dng trong h thng d liu gi s dng t bo (CDPD) v h thng GSM chu u (global system for mobile communication).HMPM c ch c bit t khi c t l li tt hn so vi single-h CPM bng cch bin i c chu k h s iu ch h.Cc trnh t bin khng c nh ni chung, nh ASK v QAM, thng l khng ph hp vi cc h thng c cc b khuch i cng sut khng tuyn tnh. Tuy nhin QAM, vi biu chm sao tn hiu rng, c th thu c hiu qu s dng ph c bit cao. QAM c s dng mt cch rng ri trong cc MODEM s dng trong cc mng in thoi, nh cc modem my tnh. QAM cn c xem xt cho cc h thng v tin. Trong trng hp ny, tuy nhin, phn hi trong cng sut u vo v u ra ca TWTA phi c cung cp m bo tnh tuyn tnh ca khuch i cng sut.Chng 2: iu ch bng gc (hay cn c gi l line coding)iu ch bng gc c xc nh nh mt truyn pht trc tip m khng chuyn i tn s. y l cng ngh biu din cc chui s bng cc dng sng xung ph hp cho vic truyn pht bng gc. Mt lot cc dng sng c ra trong c gng tm ra nhng dng c cc c tnh mong mun, nh hiu qu dng ph v hiu qu v cng sut tt, v thng tin nh thi y . Cc dng sng iu ch bng gc cn c gi l m ng truyn, nh dng bng gc (hay nh dng dng xung, dng sng PSM (hay nh dng, m,...) (ND: nn bit thut ng ting anh: line codes, baseband formats, baseband waveforms, PCM waveforms). PCM (pulse code modulation) l qu trnh m mi chui nh phn biu din mt tn hiu tng t s ha c m ha vo mt dng sng xung. Vi tn hiu dng d liu th khng cn PCM. Do thut ng m ng truyn v nh dng bng gc (hay dng sng) thch hp hn v thng c s dng hn. Cc m ng truyn c pht trin ch yu bt u trong nhng nm 1960 bi cc k s ti AT&T, IBM hay RCA truyn pht s trn cc cp in thoi hay ghi s ln phng tin t [1-5]. Cc pht trin gn y trong m ng truyn ch yu tp trung vo cc h thng truyn pht li si quang [6-11].Trong chng ny trc tin ta gii thiu k thut m ha vi sai c s dng trong phn sau ca chng, trong cc m ng truyn cu trc. Sau ta gii thiu mt s cc m ng truyn c bn trong mc 2.2. Cc mt ph cng sut ca chng c tho lun trong mc 2.3. Phn gii iu ch ca cc dng sng l vn d tn hiu trong nhiu. Trong mc 2.4 trc tin ta m t b d thch ng ca cc tn hiu nh phn trong nhiu AWGN v sau ng dng cc cng thc kt qu ni chung thu cc biu thc v xc sut li hay t l li bit (BER) ca mt s m ng truyn. Cc kt qu ni chung c th c s dng cho bt k tn hiu nh phn no, bao gm c cc tn hiu thng di s c m t trong cc chng sau.