B GIO DC V O TOTP ON BCVT VIT NAM HC VIN CNG NGH BU CHNH VIN THNG ------- NGUYN QUANG HNG X L ANTEN MNG THEO KHNG GIAN-THI GIANTRONG THNG TIN V TUYN DI NG LUN N TIN S K THUT H NI - 2006 -i- B GIO DC V O TOTP ON BCVT VIT NAM HC VIN CNG NGH BU CHNH VIN THNG ------- NGUYN QUANG HNG X L ANTEN MNG THEO KHNG GIAN-THI GIANTRONG THNG TIN V TUYN DI NG Chuyn Ngnh: Mng v knh thng tin lin lc M s:2.07.14 LUN N TIN S K THUT NGI HNG DN KHOA HC: 1.TS. ng nh Lm 2.TS. Chu Ngc Anh H NI - 2006 -ii- Li Cam oan Tixincamoanylcngtrnh nghincucaringti.Ccsliu,kt qu nu trong bn lun n l trung thc v cha tng c ai cng b u v trong bt k cng trnh no khc. Tc gi Nguyn Quang Hng -iii- Li Cm n! TixinbytlibitnsusctiTS.ngnhLmvTS.Chu Ngc Anh tn tnh hng dn trong sut qu trnh lm lun n. c bit, s ch bo tn tnh v s to iu kin thun li trong cc hot ng nghin cu khoa hc ca TS. ng nh Lm c ngha v cng to ln ti c th honthnhclunnny.TicngxincmnPGS.TS.NguynMinh Dn v nhng ch dn, nh hng quan trng ngay t khi xy dng cng nghin cu.Cc kt qu mang tnh thc tin cao c c l nh s gip to iu kinnghincuticcphngthnghimHnQuccaTS.PhngVn Vn,TS.NguynKimLan,TSKH.NguynNgcSan.Ticngkhngth khngcmnTS.SeungChanBang,TS.ByungHanRyuvccbnng nghip Won Ik Kim, Il Guy Kim ti Phng th nghim thng tin di ng-Vin nghincuintVinthngHnQuc(ETRI)vnhnggipqubu trongthigiantithctptiy.XincmnWonokKwon-ngibn lunccmtnhcbitviVitNamvvnlintcgilinlcviti trongmynmquaquaviccungcptiliu,traoinhngthngtinv nhngphttrinkhoahccngnghminhttronglnhvclinquanti Vin ETRI.Cm n TS. Danie van Wyk-i hc Tng hp Nam Phi h tr ti c th pht trin phn mm m phng h thng W-CDMA t phin bn tun theo tiu chun c ca ng. Bn cnh , ssn sng trao i, gip ca GS.TS. Hak Lim Ho- i hc Tng hp Chon-An, Hn Quc cng gip ti nh hng mt cch r rng hn trong nghin cu. Cui cng, tixin cm n b m, tt c gia nh, bn b, ngi thn trc tip hay gin tip gip , chia s, ng vin ti rt nhiu c th hon thnh bn lun n ny. -iv- Mc Lc Ch Vit Tt .......................................................................................... vii Mc lc Hnh v.....................................................................................ix Mc lc Bng biu................................................................................xii M u.................................................................................................... 1 Chng 1. Tng quan vn nghin cu ............................................. 4 1.1.S lc v qu trnh pht trin k thut x l tn hiu mng ...... 4 1.1.1.S pht trin ca k thut anten: ...................................................................................4 1.1.2.Tn hiu trong min thi gian, khng gian....................................................................6 1.2.X l khng gian-thi gian trong thng tin di ng ...................... 9 1.2.1.M hnh h thng khng gian-thi gian........................................................................9 1.2.2.Mi trng thng tin di ng ......................................................................................14 1.2.3.M hnh v nh gi knh khng gian-thi gian.........................................................21 1.2.4.u, nhc im ca k thut x l khng gian-thi gian...........................................23 1.3.Phn loi anten ................................................................................ 25 1.4.t vn nghin cu..................................................................... 27 Chng 2. K thut x l i vi anten mng..................................... 31 2.1.K thut phn tp............................................................................ 31 2.1.1.Kt hp t l cc i ....................................................................................................36 2.1.2.Tng ch phn tp ........................................................................................................41 2.1.3.Tng ch anten .............................................................................................................42 2.1.4.nh hng ca tng quan nhnh ..............................................................................43 2.2.K thut to bp sng..................................................................... 47 2.2.1.Chuyn bp sng.........................................................................................................47 2.2.2.To bp sng thch nghi ..............................................................................................50 2.2.3.Cc thut ton thch nghi.............................................................................................55 -v- 2.3.Thuttontobpthchnghichtrcaknhhoatiucho ng ln DS-CDMA................................................................................ 59 2.3.1.Anten thng minh cho DS-CDMA..............................................................................59 2.3.2.M hnh tn hiu ..........................................................................................................61 2.3.3.Kt hp theo khng gian my thu trm gc .............................................................64 2.4.Tng kt chng.............................................................................. 67 Chng 3. Hiu qu v dung lng ca anten thng minh i vi h thng GSM............................................................................................ 68 3.1.nh gi hiu qu v dung lng khi s dng anten thng minh chuyn bp sng......................................................................................... 68 3.2.Kt qu tnh s................................................................................. 72 3.2.1.Hiu qu vdung lng vi h thng AMPS ............................................................72 3.2.2.Hiu qu vdung lng i vi h thng GSM........................................................74 3.2.3. xut mu ti s dng tn s cho mng GSM Vit Nam khi s dng anten thng minh.....................................................................................................................................76 3.3.nh hng ca pha-inh v che khut ti vic ti s dng tn s........................................................................................................... 77 3.3.1.nh hng ca s che khut .......................................................................................82 3.3.2.Cc vng nhiu............................................................................................................83 3.3.3.nh gi nh hng ca cc ngun nhiu ng knh trong thc t............................85 3.4.Hiuquvdunglngcaantenchuynbpsngvinh hng ca che khut v pha-inh............................................................ 90 3.5.Tng kt chng.............................................................................. 94 Chng4.PhihpkthuttobpvphntpchohthngW-CDMA.................................................................................................... 96 4.1.H thng W-CDMA......................................................................... 96 4.1.1.Cc c tnh ch yu ca W-CDMA...........................................................................97 -vi- 4.1.2.Knh vt l ng ln .................................................................................................98 4.1.3.Knh vt l ng xung..........................................................................................100 4.1.4.Mi trng m phng W-CDMA .............................................................................102 4.2.PhihpkthuttobpsngvphntpchohthngW-CDMA....................................................................................................... 107 4.2.1.Ch tiu k thut to bp sng...................................................................................107 4.2.2.Ch tiu k thut phn tp thu ...................................................................................112 4.2.3. xut phi hp k thut to bp v phn tp cho h thng W-CDMA.................115 4.3.Kt qu m phng ......................................................................... 117 4.4.okimhthngthnghimantenthngminhchoW-CDMA119 4.4.1.Gii thiu h thng th nghim.................................................................................119 4.4.2.Anten mng thng minh............................................................................................120 4.4.3.Cu hnh h thng v iu kin o............................................................................122 4.4.4.Kt qu o kim trn h thng th nghim...............................................................129 4.5.X l kt qu o kim v so snh vi kt qu m phng .......... 131 4.6.Tng kt chng............................................................................ 133 KT LUN V HNG PHT TRIN........................................... 134 Kt lun..................................................................................................... 134 Hng pht trin tip theo: .................................................................... 135 Bi bo, Cng trnh cng b.......................................................... 136 Ti liu tham kho.............................................................................. 138 Ting Vit.................................................................................................. 138 Ting Anh ................................................................................................. 139 -vii- Ch Vit Tt Ting AnhTing Vit ABF AMPS AWGN BER BLER BPSK cdf CIR CNR DIV DPCH DPCCH DPDCH DS FDD GSM LMS LOS MIMO MRC pdf RF rms SIRSIRtarget Adaptive beam-formingAdvanced Mobile Phone SystemAdditive White Gaussian NoiseBit Error RateBlock Error RateBinary Phase Shift KeyingCumulative Distribution FunctionCarrier-to-Interference Ratio Carrier-to-Noise Ratio Diversity Dedicated Physical ChannelDedicated Physical Control Channel Dedicated Physical Data ChannelDirect SequenceFrequency Division DuplexGlobal System for Mobile CommunicationsLeast Mean SquareLine Of SightMultiple-Input Multiple-OutputMaximum Ratio Combinerprobability density functionRadio FrequencyRoot Mean Square Signal-to-Interference Ratio Signal-to-Interference Ratio Target To bp sng thch nghi H thng in thoi di ng AMPS Tp Gauss Trng Cng T l Li Bt T l li khi Kho Chuyn Pha Nh phn Hm Phn b Tch lu T s cng sut sng mang trn nhiu T s cng sut sng mang trn tp Phn tp Knh vt l dnh ring Knh iu khin vt l dnh ring Knh d liu vt l dnh ring Chui tri ph trc tip Song cng phn tn H thng thng tin di ng ton cu GSM Trung bnh Bnh phng Nh nhtNhn thng Nhiu u vo Nhiu u ra B kt hp T l Cc i Hm mt xc sut Cao tn / Tn s v tuyn Cn Trung bnh Bnh phng (Cn qun phng) T s tn hiu trn nhiu T s tn hiu trn nhiu ch (c -viii- SNR TCP TDD TDMATDTD UE UMTS W-CDMA Signal-to-Noise RatioTrasmission Control ProtocolTime Division DuplexTime Division Multiple AccessTime Division Transmit Diversity User EquipmentUniversal Mobile Telecommunications SystemWideband Code Division Multiple Accesst trc trong php o) T s tn hiu trn tp Giao thc iu khin truyn Song cng phn thi a truy nhp phn thi Phn tp pht theo thi gian Thit b u cui H thng thng tin di ng UMTS 3G s dng W-CDMA CDMA bng rng -ix- Mc lc Hnh v HnhTrang Hnh 1.1. Tn hiu trong khng gian Hnh 1.2. M hnh h thng thng tin vi N phn t pht v M phn t thu trong mi trng tn x. Hnh 1.3. Phn loi k thut x l khng gian-thi gian v anten thng minh Hnh 1.4. Phn loi anten thng minh Hnh 2.1. Anten mng phn tp M phn t Hnh 2.2. Hm phn b tch lu ca s so vi s/ cho k thut kt hp t l cc i. Hnh 2.3. BER so vi = M khi M thay i Hnh 2.4. Hai phn t vi cc tn hiu tng quan Hnh 2.5. nh hng ca tng quan nhnh ln phn b cng sut u ra b kt hp t l cc i phn tp kp. Hnh 2.6. BER so vi (dB) ca b kt hp t l cc i 2 nhnh c pha-inh tng quan Hnh 2.7. Anten mng thch nghi Hnh 3.1. Mu ti s dng tn s trong thng tin di ngHnh 3.2. Tng dung lng bng anten chuyn bp sng cho nh khai thc AMPS c bng thng 12,5 MHz, h s ti s dng N=7. Hnh 3.3. Tng dung lng bng anten chuyn bp sng cho nh khai thc AMPS c bng thng 12,5 MHz, h s ti s dng N=4. Hnh 3.4. Tng dung lng bng anten chuyn bp sng cho nh khai thc GSM c bng thng 8 MHz, h s ti s dng N=4. Hnh 3.5. Tng dung lng bng anten chuyn bp sng cho nh khai thc GSM c bng thng 12,5 MHz, h s ti s dng N=4. 8111427344042454647536973747575-x- Hnh 3.6. Thay i CIR khi h s ti s dng tn s gim t 4 xung 1 (__: N=4, -x-: N=3, -o-: N=1) Hnh 3.7. Tng dung lng bng anten chuyn bp sngcho nh khai thc GSM c bng thng 8 MHz, h s ti s dng N=3. Hnh 3.8. Tng dung lng bng anten chuyn bp sng cho nh khai thc GSM c bng thng 12,5 MHz, h s ti s dng N=3. Hnh 3.9. Vng c nhiu v khng nhiu (a) khng c pha-inh (b) c pha-inh v che khut. Hnh 3.10. Xc sut mt lin lc khi c pha-inh v che khut Hnh 3.11. Ranh gii vng nhiu vi cc xc sut nhiu khc nhau khi c pha-inh v che khut Hnh 3.12. Xc sut nhiu ng knh, vi i cho trc, theo Zd. Hnh 3.13. Ch ra mt im ca xc sut rt cui gi vi su ng knh cho m=1,6 v 12 bp vd =6 v 12 dB. Hnh 3.14. th biu din Zd (hnh tri) v Ne theo m (hnh phi) (vi =0,7, n=4,5, Pout=1%, d=6dB, qd=22 dB) Hnh 3.15. Hm hiu sut ph tng i theo s bp sng (vi =0,7, n=4,5, Pout=1%, d=6dB, qd=22 dB) Hnh 4.1. Cu trc khung ca knh DPDCH/DPCCH ng ln Hnh 4.2. Cu trc khung ca knh DPCH ng xung Hnh 4.3. S khi tng th ng ln Hnh 4.4. S khi tng th ng xung Hnh 4.5. Giao din chnh ca phn mm m phng Hnh 4.6. Giao din thit lp cc tham s m phng Hnh 4.7. Kt qu m phng i vi phn tp MD = 4 anten, h thng to bp MB = 4 anten v h thng phi hp c phn tp v to bp mi trng khng nhn thng Hnh 4.8. Cu hnh h thng anten thng minh Hnh 4.9. Anten mng 7677778082848992929499102103104105106118120121-xi- Hnh4.10.HthngantenthngminhthnghimtiVinNghincu ETRI Hnh 4.11. Cu hnhh thng anten thng minh cho W-CDMA s dng trong o kim Hnh4.12. Ccknhcabtobpsngthchnghi(h tr3sc-tx8 anten) Hnh 4.13. Mu bp sng c nh ng xung Hnh4.14.Dngbpsngngxung(chuynmchbpsng)v ng ln (bp sng thch nghi) Hnh4.15.KtquokimSNRtrnTestbedtheogitrSIRtargett trc Hnh 4.16. Kt qu o kim BLER cho ABF 8-anten v DIV 2-antenHnh 4.17. T l li bt BER o c vi ABF 8-anten v DIV 2-anten123124125128129130130132 -xii- Mc lc Bng biu BngTrang Bng 4.1. Cc ch tiu k thut chnh ca W-CDMA Bng 4.2. Cc tham s u vo nh gi ch tiu BER 97117-1- M u Cc h thng thng tin di ng ang pht trin bng n trn th gii v cVitNam.Trcyucungycngcaocangisdngdchv thngtindingvchtlng,dunglngvtnhadng cadchvv cbitlccdchvtruyndliutccaovaphngtin,vic nghincu,ngdngcccngnghvkthuttintinpngnhucu ny lun l mt i hi cp thit.Mttrongscckthutcthgipcithinngkchtiuv dng lng ca h thng ang c tp trung nghin cu trn th gii trong thigiangnylkthutxlkhnggian-thigian.Kthutnycho phpsdngtiahiuquphtnchohthngthngtinvtuynni chungvhthngthngtindingtongniring.Nhsdngnhiu phntanten,kthutnychophptiuhoqutrnhthuhocphttn hiu bng cch x l theo c hai min khng gian v min thi gian ti my thu pht.[16,17,19, 28, 36] Vic tip tc nghin cu pht trin k thut ny tin ti c c cc snphmhudngcchtiuchtlngcao,ngthiphhpvikh nng x l, tnh ton ca cc thit b hin c cng nh ng dng n vo trong cchthngthngtindinghincmtcchhiuquthcslvn cp thit. Vic thc hin tt nhng nghin cu ny s mang li hiu qu rt to ln v dung lng cng nh hin thc hokh nng truyn d liu tc cao chocch thng thng tin dingnhGSMhayCDMAhinticng nh cc h thng thng tin di ng th h mi. Mc tiu ca lun n l nghin cu k thut x l khng-gian thi gian bng anten thng minh cho thng tin di ng vi cc trng hp c th anten thng minh cho mng GSM Vit Nam v cc h thng CDMA.-2- itngvphmvinghincucalunnltptrunggiiquyt nhng vn sau: -Nghincuthuttontobpthchnghicphctptnhtonthp nhng tc hi t cao ph hp vi kh nng ca thit b thc t. -nh gi hiu qu ca vic s dng anten thng minh trong h thng GSM c tnh n cc iu kin c th ca h thng GSM Vit Nam xut phngnngdng,trinkhainhmsdngtinguynmtcchhiu qu, c xem xt, nh gi nh hng ca pha-inh v che khut. -Nghin cu k thut nng cao ch tiu cho h thng anten thng minh cho W-CDMA, h thng thng tin di ng th h 3 IMT-2000. Phng php nghin cu c thc hin l nghin cu l thuyt kt hp vi m phng bng chng trnh my tnh nh gi kt qu: Vi h thng GSM, c tnh n cc tham s v iu kin c th ca mng li hin ang trin khai Vit Nam; Vi xut cho h thng W-CDMA, kt qu o kim thc hin trn h thng th nghim c s dng nh gi tin cy.Ni dung lun n bao gm 4 Chng. Sau phn M u, Chng 1 trnh by tng quan v k thut x l mng theo khng gian-thi gian v t vn nghincu.Chng2isuvophntchccantenmngnhiuphnt csdngtrongthngtindingvihaikthutphntpvtobp. Chngnycngxutsdngmtthuttontobpthchnghikt hpcknhhoatiuvlulngchohthngCDMAtriphtrctip. Chng 3 nh gi hiu qu ca vic s dng anten thng minh trong cc h thngthngtindingtong,xutsdngchohthngGSMVit Nam c xem xt n nh hng ca pha-inh v che khut. Trn c s nhn xtvnhnghnchcahthngantenthngminhthnghimchoW-CDMA, qua phn tch cc c tnh ca k thut phn tp v to bp trong mi trngpha-inhvnhiuatruynhp,Chng4xutsdngk -3- thut phi hp cho chp t c u im ca c hai k thut phn tp v to bp cho h thng W-CDMA. Kt qu o kim c thc hin trn h thng antenthngminhthnghimchoW-CDMAtiVinnghincuint Vin thng Hn Quc (ETRI) nh gi tin cy ca phng n xut. Cui cng l phn kt lun vhng pht trin tp trung vo nhng kt qu mi t c ca lun n. -4- Chng 1. Tng quan vn nghin cu 1.1.S lc v qu trnh pht trin k thut x l tn hiu mng 1.1.1. S pht trin ca k thut anten: Sngvtuyncphtminhravonm1861khiMaxell(ihc HongGiaLunn)aralthuytsngint.Hertz(ihc Karlsruhe) chng minh s tn ti ca sng ny bng thc nghim vo nm 1887bngsngng(tnh).Nm1890Branly(Paris)xydngmtb nht qun c th pht hin s c mt ca sng in t bng mt ci chai thu tinhchakimloi.Bnhtqunnysauctiptcphttrinbi Lodge (Anh). Ma h 1895, Marconi s dng my pht ca Hertz, b nht qun ca Lodge v lp thm anten to ra mt my pht v tuyn u tin... ng dng dn dng u tin ca k thut v tuyn l h thng in thoi vtuyn2MHzvonm1921trongngnhCnhst.Nhnghthngc phttrintipsau:FM(Armstrong-1933);HthngthngtincaBell tns150MHz,hthngIMTSsdngFMcaAT&T(1946);Khinim celllular(mngthngtindingtong)(PhngthnghimBell-1947);H thng AMPS (1970); Vo nhng nm 1990s: cc h thng thng tin i t ong GSM,IS-136(TDMA),CDMAIS-95,3Graivphttrinmtcchmnh m [34,36]. K thut anten c s dng cho cc h thng thng tin v tuyn cng c s pht trin nh sau: -1880-tinhngnm1890:Hertz,Marconi,Popovthitkccc anten c tn s hot ng v bng thng tt hn . -Nhngnm1900:antennhhngcsdngchophplinlc qua bin Atlantic -1905: s dng nhiu anten cho phn tp thu. -5- -Thp k 1920: Dn anten Yagi-Uda c pht minh em li tng ch v bng thng tt hn. -Chin tranh th gii th 2: Dn anten c s dng cho raa -Thp k 1970: ng dng x l tn hiu thch nghi my thu v tuyn cithinphntpthuvtritnhiubngccbxltnhiustrong qun s [29]. Vic s dng anten nhiu phn t my thu trong thng tin v tuyn m ra mt chiu mi trong x l tn hiu (chiu khng gian), cho phpcithinchtiuhthng.Tuynhin,ntrcnhngnm1990, vn cphttrin chyu viantenmngmich l kthutxl ring theo min khng gian (vd: xc nh hng ti) [16]. -Thp k 1990: K thut thu khng gian-thi gian (kt hp c min khng gian v thi gian) [29, 38] + 1996: Anten nhiu phn t c s dng trm gc h tr nhiu ngi dng trn cng knh + 1994: xut k thut tng dung lng knh v tuyn bng cch s dng anten nhiu phn t c my pht v my thu. tng ny tip tc c pht trin 1995, 1996, 1998 -> bt u mt cuc cch mng v l thuyt truyn thng [25, 28]. - T nhng nm 2000: K thut thu-pht khng gian-thi gian c tp trung nghin cu v pht trin [19, 20] Cththyrng,kthutxlkhnggian-thigianvimng(dn) antennhiuphntnhiucpphctpkhcnhaucngdng trong qun s t kh lu, nhng do tnh cht thay i lin tc ca mi trng truyn sng thng tin di ng trong khi kh nng x l theo thi gian thc ca my thu pht cn nhiu hn ch m k thut ny mi thc s c nghin cu ngdngtrongcchthngthngtindingtrongthigiangny[17, 29, 36, 38, 55]. Nh s dng nhiu phn t anten k thut ny cho php ti u -6- ho qu trnh thu hoc pht tn hiu bng cch dng c k thut x l tn hiu theominkhnggianvtheominthigiantimythupht,nhcho php s dng ti a hiu qu ph tn ca mng thng tin t ong [19].1.1.2. Tn hiu trong min thi gian, khng gian 1.1.2.1.Biu din tn hiu theo thi gian Tnhiu thcs(t)cbin iFourierlS(f).Php bin i Fourier ny tho mn biu thc i xng sau: S(f) = SH(-f)(1.1) Nu ni tn hiu l thc, ngha l ta ch xt cc tn s dng. Gi z(t) lng bao phc ca tn hiu thc s(t), v Z(f)l bin i Fourier ca z(t)[16]. ng bao phc cho tn s fc no (tn s sng mang) c xc nh trong min Fourier l: Z(f-fc) = 2u(f)S(f)(1.2) trong hm bc n v c nh ngha l:= H 0,8). Suy hao ng truyn Nukhngxcnhcccctnhtruynsngcamtknhv tuyn,ngitathng tnh suy hao tn hiutheokhongcchbngsuyhao -16- trongmitrngkhnggiantdo-mhnhcoivnggiaantenphtv anten thu l vng khng c bt k vt hp th hoc phn x nng lng sng vtuynno.Trongvngny,khquyncxemnhmitrngkhng hp th v ng nht hon ton. Ngoi ra, tri t c xem nh rt xa so vi tn hiu truyn sng. Trong m hnh khng gian t do, suy hao ca nng lng sng v tuyn t l nghch vi bnh phng khong cch. Cng sut thu c biu din theo cng sut pht vi h s suy hao Ls(R) - c gi l h s suyhaongtruynhocsuyhaokhnggiantdo.Trongccngdng thngtinvtuynding,suyhaongtruyntrungbnh,) (R Ls,lmt hm ph thuc khong cch R gia my di ng v trm gc, tng ng vi nl ln t s R trn khong cch tham chiu r0. Tc l: lnsrRR L||.|

\|=0) ((1.10) Khichintngdnsngmnhnhkhitnhiutruyndctheocc ng ph th, th nl c th nh hn 2. Khi xut hin cc vt che khut, nl s ln hn v nm trong khong gi tr t 2,5 n 5 [36, 51]. Cc o c thc nghim cho thy rng vi bt k gi tr no ca R, tng suy hao ng Lx(R) lmtbinngunhincphnblog-chunxungquanhgitrtrungbnh phthuckhongcch) (R Ls.Do,suyhaotngLx(R)cthcbiu dinbi) (R Lscngvimtbinngunhin X ,nhsau(tnhbngdB): [62] () ( ) ( )X r R n r L R Ll s x+ + =0 10 0/ log 10 (1.11) Trong X lbinngunhinGaussctrungbnhbngkhng(tnhbng dB)vphthucvokhongcchvvtrtrmgc.Vicchnlagitr cho X thngdatrnocthctvphthucvoloimitrng -17- thngtintong:macro-(ln),micro-(nh),hocpicro-(rtnh), v cc tham s knh khc. Gi tr thng c s dng nm trong khong t 6 n 10 dB. 1.2.2.2.Pha-inh v nh hng n vng ph sng Pha-inhcthcchiathnhhailoipha-inhchmv/hocpha-inh nhanh (c ti liu dng l pha-inh large-scale, v small-scale). Pha-inh chm (hay che khut) c suy hao nh c trnh by trong mc 1.2.2.1. Pha-inh nhanh biu hin hai c tnh l mo tn hiu (tri tr tn hiu) v s bin itheothigiancaknh.Doschuynnggiamyphtvmythu, knh truyn s bin i theo thi gian khi thay i ng truyn sng. Tc thayicacciukintruynsngnycxemnhtcbinthin nhanh ca pha-inh. Pha-inh nhanh thng c m t thng k bng phn bRayleigh,Rice[64]hocNakagami-m[65].Viclachnmhnhph hp ch yu ph thuc vo mi trng hot ng ca h thng thng tin. Nu sccngphnxanglnvkhngcthnhphntnhiutrong tm nhn thng, ng bao ca tn hiu thu thng c m t thng k bng hmmtxcsutRayleigh.Khicsxuthincathnhphntnhiu khng pha-inhvi cngmnh,vd nhtnhiu n t ng truyn trongtmnhnthng,ngbaopha-inhnhanhucmtbnghmmt xc sut Rice. Ngoi cc c tnh ton hc c bit ca m hnh pha-inh Nakagami-m, ngi ta thy rng m hnh ny c th m t chnh xc c tnh pha-inh ca cc tn hiu a ng v cc qu trnh tn x vt l khc. [66]Kchthccamttronghthngthngtindingtongcth c xc nh bng t l phn trm vng nm trong ng trn bn knh R m trong cng tn hiu thu c t trm gc ln hn mt ngng c th no.TatphnvngcdchvFulvngny(trongngtrnbn knh R, cng tn hiu thu c anten my di ng vt qu ngng xo -18- cho trc). Nu oxPl xc sut tn hiu thu c x ln hn xo trong mt vng dA th: =Rx udA PRFo021 (1.12) Gi s tr trung bnh ca cng tn hiuxph thuc vo khong cch r-n, trong r l khong cch t my pht v n l h s suy hao ng truyn, th cng sut tn hiu trung bnh (tnh bng dBm) l: Rrn x log 10 =(1.13) trong (dBm) l hng s ph thuc vo tng ch, cao anten, cng sut pht, v bng tn hiu trung bnh thu c ti r=R. Gistnhiutrungbnhccb(cngtnhiutrungbnhccb trnpha-inhRayleigh)tnhbngdBcbiudinbngmtbinngu nhin chun x c tr trung bnhx (dB) v lch chun (dB),xl tr trung bnhtheodonhococ.Tnhiux(dB)khicpha-inhlog-chun c vit l: x x x~ + = (1.14) trong x~ l tr trung bnh bng khng (zero mean) - phng sai n v ca bin ngu nhin Gauss. Vy, hm mt xc sut ca x l : 222) (21) ( x xe x p=(1.15) Phngtrnh(1.15)chothycngsuttnhiuthuctcthibin thin theo khong cch l n xr 10 /~10. Qu tch cc im c cng sut tn hiu bng nhau quanh my pht s khng cn l mt ng trong na m c dng cngbtthngkhicngtng.iukinpha-inhlog-chun,xcsut tn hiu thu c x ln hn xo l: -19- ) (r Pox= Xc sut (x xo) = oxdx x p ) ((1.16) |.|

\| = 22121 x xerfo(1.17) |.|

\|+ =2/ log 102121 R r n xerfo(1.18) |.|

\|+ = R r n xQo/ log 10(1.19) trong erf(.) l hm li kinh in bit : =xtdt e x erf02 2) ((1.20) = -erf(-x)(1.21) v c mi lin quan vi hm Q(.) : ==x ttdt e x Q2 /221) ((1.23) bng biu thc : |.|

\|=((

=221)2( 121) (xerfcxerf x Q (1.24) trong erfc(.) l hm li b.Ch l Q(-x) = 1 Q(x). Vi cc gi tr agumen ln (3), Hm Q c th c ly xp x bng bin di ca n [61] : 2 /221121~ ) (xexxx Q|.|

\| (1.25) t a = (xo-)/ 2v b = 10nlog(e)/ 2 , ta c |.|

\|+ =RudrRrb a erf rRF02log121(1.26) -20- (((

|.|

\|((

+ =baberf e a erfbab11 ) ( 121 22 1(1.27) |.|

\|+ =babQ e a Qbab12 ) 2 (22 1 (1.28) |.|

\|+ =babQ e R Pbabxo12 ) (22 1(1.29) Xcxut cng tn hiutrnchuvicangtrnln hnxol ) 2 ( )] ( 1 [ 5 , 0 ) ( a Q a erf R Pox= = . Phng trnh cui cho thy r rng l Fu ln hn) (R Pox. Cho v d, nu tn ti sao chox= xo ti r=R, th a = 0,) (R Pox = 0,5 v : )2(21 21bQ e Fbu+ =(1.30) Vy,nun=3v =9dB,th Fu=0,71.iunycth cgiithch nh sau: nu mt na s v tr trn chu vi ng trong bn knh R l nm trn ngngth71%vtrphatrongngtrnscmctnhiutrnmc ngng . Kchthccacthcxcnhdatrnvng/ngbinvng phc th,mcngng, hssuyhao,mccngsutkhong cchang xt hoc phn b chn ca phng trnh suy hao ng truyn trung bnh. V d, t mt vng ng bao cho trc) (R Pox= ta c : 2 12) ( =|.|

\|=oxerf a erf(1.31) Cthgiiphngtrnhnytheoari,ridatrnmccngsuttheo khong cch xc nh bn knh . -21- 1.2.3. M hnh v nh gi knh khng gian-thi gian Da trn cc m hnh knh thc t ca mt h thng thng tin di ng, cc c cu x l tn hiu hiu qu c th c s dng ci thin ch tiu h thng ;vicphntchhthngmtcchchnhxccngchophpdon dung lng v ch tiu h thng. Ni chung, cc m hnh m t tham s nh cngtnhiuthu,ctnhtrcngsutvphDopplerrtquantrng cho vic phn tch h thng dng anten ng hng. Nhng mt tham s rt quan trng trong cc h thng khng gian-thi gian l hng ti ca tn hiu thu li khng c trong cc m hnh truyn thng. Da trn cc khi nim c bn bit v pha-inh, tri Doppler, tng quan, cc m hnh knh mi cthcxydngcthcpcckhinimminhtritr, HNG TI v hnh dng ca anten mng thch nghi. Nhng cn ch l c tnh truyn sng ng ln v ng xung c th khc nhau (do s tri gc khcnhaumydingvtrmgc)-iunyrtquantrngtrongvic phn tch ch tiu h thng khng gian-thi gian. 1.2.3.1.M hnh knh c bn Trong mt h thng v tuyn di ng, mt tn hiu c th truyn t my pht ti my thu qua nhiu ng phn x - hin tng ny c gi l truyn sngang.Hiungnygyrasthayivbin,phavgcti ca tn hiu thu c, v c gi l pha-inh a ng. Gisrnghthngkhnggian-thigianbaogmKngisdng, mi ngi s dng pht mt tn hiu trn mt knh a ung ri rc c lp vingtruynLtimythu,mitnhiucmtbin,phavhng ti ring. Phn b ca cc tham s ny ph thuc vo loi mi trng thng tin di ng (macro-, micro-, hoc picro-).Hng ti ph thuc vo ba thnh phn khc nhau l: -22- Tn x ti u cui di ng (hin tng nhiu x ny cng thng b nh hng bi tc di ng); Tn x ti trm gc;Cc vt tn x xa. Loi tn x ny c th xut hin trong cc mi trng thnh th v nng thn do cc vt th c cu trc ln nh ni i, cc to nh, ... v c nh hng nht nh ti knh thng tin di ng k c khi cc vt tn x ny xa so vi trm gc v my di ng. Nu cc vt tn x ny nm trong tm nhn thng i vi c trm gc v my di ng th chng c th c vai tr ging nh cc vt phn x ri rc hoc vt phn x tp trung theo nhm. Khi cc vt phn x c nhm li, anten trm gc hoc my di ng c th xem nh cc thnh phn tn x nh im 1 v im 2 nu trn. Ta thy rng, m hnh phn b ca mi trng tn x chim mt vai tr quantrngtrongvicthitkhthng.Nhiumhnhphnbcami trng tn x khc nhau c xut, vi cc thuc tnh v chnh xc khc nhau. Chi tit v cc m hnh knh c nhng ng dng khc nhau trong vic phn tch h s dng anten thng minh c th tm thy trong ti liu [36]. Mt s m hnh c pht trin cho thc t, cn hu ht m hnh khc c xu hng phc v cho mc ch m phng. 1.2.3.2.nh gi c tnh knh khng gian Ngi ta thc hin nhiu o c thc nghim nh gi c tnh ca knhkhnggiantrongiu kinthct[36].Mtsktqunhging ch c tm tt sau y: -Phn ln nng lng tn hiu tp trung trong mt khong tr nh v trong phm vi hng ti nh mi trng vng nng thn, ngoi v thm ch trongnhiu mi trng thnh th. -Bng cnh s dng anten nh hng, c th gim c s tri tr. -23- -Tri gc tng khi cao anten trm gc gim. -Trigctngkhimitrngtruynsngthayitnngthnsangthnh th. -Mng cc phn t anten vi bp sng nh c th cung cp kh chnh xc thng tin v hng ti ca my di ng ang di chuyn vi tc cao. -Sthayitheothigiancakhiukhnggiancxuhngtngln theo tc di chuyn ca my di ng. Ngi ta cng thc hin php o tm hiu s bin i ca k hiu khng gian c v thi gian v tn s. Kt qu o cho thy rng khi my di ng v cc vt xung quanh khng chuyn ng, tn hiu khng gian thay i nh; khi mitrngvccvtxungquanhchuynngthsthayinymc trung bnh; v khi my di ng di chuyn th s thay i tn hiu khng gian mc ln nht. Cc kt qu o cng cho thy tn hiu khng gian thay i ng kkhitnssngmangthayi.Trongthct,sthayibintng i ca k hiu khng gian c th ln hn 10dB khi tn s thay i 10MHz. Do , k hiu khng gian ng ln khng th c p dng trc tip cho vic to bp sng ng xung trong hu ht cc h thng thng tin di ng tonghinnay(thngckhongcchvichcMHzgianglnv ng xung). 1.2.4. u, nhc im ca k thut x l khng gian-thi gian K thut khng gian-thi gian c kh nng ci thin ch tiu ca h thng thngtindingbngnhiucchkhcnhau.Ccuimnibtcak thut khng gian-thi gian l: Dunglngtng(hiuquphtntng)nhtngsngisdngtch cc i vi mt gi tr BER cho trc. -24- Gim nhiu ng knh ci thin cht lng dch v v/hoc tng h s ti s dng tn s. Gim tri tr v pha-inh. Bng cc k thut to bp sng v phn tp, c thcithintsSINRcahthngtrongmitrngpha-inh.Lin quan n vn ny l gim nh hng ca tri gc ca tn hiu thu c do cc vt tn x phn b xung quanh thu bao nh s dng cc bp sng hp thu tn hiu t thu bao. Gimxcsutmtlinlc.Bngcchgimnhiunhkthutkhng gian-thi gian, xc sut mt lin lc c th c gim xung. Tnghiuqutruyndn.Dotnhnhhngvtngchcahthng khng gian-thi gian cao, bn knh trm gc c th dc m rng v thu bao c th ch cn pht cng sut thp hn cho php ko di thi gian s dng pin. Gn knh ng. Khi dung lng ca h thng t ong vt qu gi tr gii hn,cththchinvicchiatchtoraccmi,mimis c phn b mt trm gc cng vi tn s mi, lm cho t l chuyn giao tng ln. iunycthkhc phc cbngccb x l khng gian-thi gian cho php to ra cc bp sng c lp. Ci thin chnh xc nh v bng cch s dng anten mng nhiu phn t. Gim chi ph, phc tp v cu trc mng. Do khng phi lo ngi v cc vnnhchuyngiaomm,gnknhng,tobpsngkhnglinh hot.Tuy nhin, vic s dng anten mng cng c mt s hn ch ng k v chi ph v phc tp sau: -25- Tngccyucuvphncngcngnhphnmmkhitngslng phn t anten ca mng. Trongcctrnghpthct,chtiucaantenmngcthgyracc nhhungbtlibicclimhnhknh,linhchun(calibration), lch pha v tp m tng quan gia cc phn t anten.1.3.Phn loi anten Cckthutantenthngminhcthcphnthnhbaloichnh [37]: To bp sng, phn tp, chia sc-t. Ty theo phng thc thc hin m cckthuttobpsngvphntpcthtiptccphnloicth hn na nh Hnh 1.4. Mt anten mng thch nghi bao gm mt nhm cc phn t pht x c phnbtheokhnggian,miphnttngngvibnsaoctng quan cao ca tn hiu c ch. u ra ca mi phn t c t trng s thch nghi v kt hp vi cc u ra khc tch ra tn hiu hu ch bng cch xp chng cc tn hiu thu c. Hnh 1.4. Phn loi anten thng minh Anten thng minhTo bp sng Phn tpChiasc-t Thch nghi Chuyn bpPht -26- Cc yu t nh mi trng tn x, phn b ngi dng, mi trng pha-inh.v.v. ng vai tr quan trng trong vic xc nh ch tiu ca h thng di ng khng gian-thi gian. Mt h thng thng tin di ng c thitk tt phisdngphntruyndnchungsaocho:tnglulngthngtinpht trung bnh cng ln cng tt; xc sut li trung bnh pha ngi dng cng nh cng tt; v tr trung bnh cng nh cng tt. Tuy nhin khng phi mi tiu ch ny u c th c tho mn ng thi; mt thit k tt i hi phi cn i c cc tiu ch ny. V d, k thut to bp sng c th c dng gimxcsutlicahthngbngcchgimnhiuCDMA.iuny c thc hin bng cch kt hp thng minh cc tn hiu thu c t nhiu phn t anten trm gc hoc my di ng. Trongmththngthngtindingdngantenmng,thnhphntn hiupha-inh nhanh torabin vphangunhinchotnhiuthu c trn mi phn t anten, lm nhiu lon vc-t quay camng. Trong trng hppha-inhRayleighhocNakagami,phacthlgitrbtktrong khong (0, 2], v khng th xc nh c hng ti ca sng nu ch gim st tn hiu trong mt thi gian ngn. Tng t, khi nim gin phng hng ca mng da trn gi thit sng phng ti cc phn t ca mng c bin khng i. Do , trong mi trng pha-inh, vic thc hin cc b to bp to v trit (null) bp tng ngvphanguntnhiumongmunvnhiucthlkhnghiuqu. Khi pha-inh nhanh c tng quan mnh gia cc phn t, n c th c coi nh mt nhn v hng vi vec-t quay, tc ng u ln cc phn t. Do c th thc hin vic khi phc li vc-t quay ny. Tuy nhin, s khng c ctngchphntpthudokthutnydatrnpha-inhkhngtng quan. Chnh v th pht sinh mu thun gia vic trnh lm nhiu bp sng v mong mun c c phn tp thu. -27- Tnhngphntchtrn,tathyrngcthloibbtcnh hng ca knh vi thit k my thu tt hn. Tuy nhin, do yu cu hn ch phc tp my u cui v do c tnh ca ng xung, gii php c s dng ci thin ng xung s l dng nhiu anten pht trm gc, nh khng lm tng phc tp ca my u cui. 1.4.t vn nghin cu Nhsdngnhiuphntanten,kthutxlkhnggian-thigian cho php ti u ho qu trnh thu hoc pht tn hiu bng cch x l theo c haiminkhnggianvminthigiantimythupht.Cckthutph bincbitlsdngantendqut(sc-t)(xltnhiutheokhng gian), phn tp (x l tn hiu theo khng gian-thi gian) v anten mng to bpsng(xltnhiutheokhnggian-thigian)cthcxemnh nhngvdinhnhcakthutxltheokhnggian-thigian. [16,17,19, 28, 36] Trong thc t, tt c cc h thng anten mng c th c xemnhbxlkhnggian-thigian.Ccbxlkhnggian-thigian tin tin hn bao gm c b tch a ngi s dng,v m ha khng gian-thi gian s to thnh mt h y v k thut x l khng gian-thi gian. [6, 16, 52] K thut MIMO tng qut dng nhiu anten c u thu v pht s lm chodunglngca hthngthngtinvtuyn lnhn ngksovi cc h thng thng thng. Tuy nhin, vn cn nhiu vn cn nghin cu cn tiptcphttrincthpdngkthutnyvothct[1,4,16,27]. LunnnghincukthutxlkhnggianthigianbngAntenthng minh. Anten thng minh nhiu cp phc tp khc nhau c ng dng trong qun s t kh lu, nhng mi thc s c nghin cu s dng trong -28- cchthngthngtindingtrongthigiangny[17,29,36,38,55]. Vic tng tnh nh hng ca bp sng c th lm tng dung lng (thng c p dng trong thnh ph) v m rng vng ph sng (p dng cho vng nngthn).Nhsdngnhiubpsng,antenthngminhgiptritnhiu tt hn do em li nhng u im sau cho h thng: [4, 17, 21, 22, 37] -Cho php s dng mu ti s dng tn s cht hn; -C th ci thin cht lng thng tin thoi trn t nh tng t s C/I; -C th gim hiu ng a ng, do gim yu cu d tr cng sut. Vic x l tn hiu trn anten mng c th da trn hai k thut chnh l phn tp hoc to bp. Vi k thut phn tp, tn hiu cc nhnh anten khc nhau c gi thit l khng tng quan, ngha lch tiu h thng cng tt khi mc tng quan ca tn hiu cc nhnh cng nh. Trong khi , k thut to bp li da trn gi thit l tn hiu cc nhnh tng quan vi nhau [1,25,34].Chnhvvykhongcchgiaccphnttrongantencah thng to bp sng nh hn so vi h thng phn tp. Trn l thuyt, c rt nhiu cc thut ton khc nhau c pht trin cho anten thng minh trong thng tin di ng [15, 19, 29, 36, 38]. Tuy nhin, dotnhchtthayilintccamitrngthngtinding,cngnh nhnghnchvkhnngxlcathitbthctmcchthngth nghim u ch s dng cc thut ton kinh in nh trung bnh bnh phng nhnht,bnhphngnhtithiuqui[27,34]...VihthngCDMA tri ph trc tip, lun n xut s dng mt thut ton to bp thc hin kt hp trn c knh hoa tiu v knh lu lng cho php t c tc hi t nhanh hn. Lun n cng nghin cu hiu qu ca vic s dng anten thng minh i vi cu hnh h thng GSM hin ang c trin khai [4, 5, 7], lm c s xutsdngmutisdngtnsmikhitrinkhaiantenthng -29- minh chomng GSM Vit Nam v xem xt nh gi nh hng ca pha-inhvchekhutvitrnghpnhiungknhtrongmngthngtindi ng t ong thc t. Phntchsuschnna,tathyrngkthutphntpthudatrn vic kt hp cc tn hiu khng tng quan (do pha-inh khng tng quan) thu c t cc phn t khc nhau ca anten mng, h thng phn tp khng lmtngchtiutrongmitrngtpGausstrngcng-AWGN[16,17]. KhimcnhiuatruynhptronghthngCDMAcaothnltcnhn chnh nh hng n ch tiu BER v knh s tin ti xp x knh Gauss nn hiu qu ca h thng phn tp gim [16, 52, 55]. Trong khi , h thng to bp(haythmchhthngsec-tho)slmgimmcnhiubngcch loi b nhiu ngi dng khi h thng, lm cho ch tiu BER tt hn [10]. Khi s ngi dng t v knh b pha-inh mnh (nh trng hp truyn sng iu kin khng nhn thng), h thng to bp khng ci thin c tn hiu thu do khng b sung c thng tin mi no vo tn hiu thuc, m ch hn ch c nhiu a truy nhp. Do , h thng to bp khng lm tng ch tiunhiu.Trili,hthngphntpkthpcmtstnhiubpha-inhmnhslmcithinchtiuhthng,cbitkhipha-inhctc ng mnh hn nh hng ca nhiu a truy nhp.[9] Mth thng phi hp ctobpvphntpsc cu imca vic gim nhiu bp sng m vn c c phn tp thu c bit l trong mi trng pha inh khi tn hiu ti cc phn t anten mng khng bao gi c th l tng quan hon ton. y cng l mt xut p dng cho WCDMA ca lun n, kt qu c thc hin bng m phng v so snh vi cc kt qu ocsdngringbitkthutphntp(DIV)hoctobptrnh thngthnghimantenthngminhtiVinNghincuintvVin thng Hn Quc (ETRI) nh gi tin cy ca phng n xut. -30- Ktqumphngchothyrngtrongmitrngnhiungidng, pha-inh mnh, k thut phi hp c to bp v phn tp cho ch tiu tt hn h thng to bp gi tr Eb/No ln hn 8dB, mc d di gi tr ny ch tiu cahthngtobpvnlnhn.Nhvy,cththyrngkthutphi hpclunnxutscbitcnghatrinkhaiccdchv truyn d liu tc cao, i hi c t s Eb/No ln. KtquokimtrnhthngantenthngminhchoW-CDMAIMT-2000 ti Vin nghin cu ETRI [27] cho trng hp anten-DIV v anten-ABF csdngnhgitincycaccktqumphng.Tathy rng kt qu o ch tiu cho trng hp anten-DIV rt ging vi kt qu m phng. Cn trng hp anten-ABF ch tiu o c tt hn kt qu m phng do h thng o kim s dng 8 anten to bp trong khi kt qu m phng cthchinchoch4anten.Nhvy,ktqumphnglphhpvi cc kt qu o kim v chng t c tin cy ca phng n xut. Tuy nhin, p dng vo thc t cn c nhng nghin cu tip theo v cu trc c th ca anten mng p ng c cho k thut ny.-31- Chng 2. K thut x l i vi anten mng2.1.K thut phn tp nhhngcapha-inhangtrongcchthngvtuyncth c gim bt bng cch s dng phn tp theo khng gian (anten my thu gmnhiuphnt).Trongmitrngpha-inh,cngsutsngmangcnphiphtcaohncngsuttrungbnhcthtcmttllibt (BER) mong mun no . Trong mt anten mng, tn hiu thu c bi cc phn t khc nhau c th c ly trng s ph hp to ra tn hiu kt hp binthinchmhntngtnhiuthnhphn.Antenmngnysyucu cng sut thp hn so vi trng hp ch s dng anten mt phn t, m vn t c BER mong mun. hthngphntpkhnggianhotngmtcchhiuqu,cctn hiuthuctccnhnhantenkhcnhauphikhnghocttngquan vi nhau nu tn hiu mt phn t b pha-inh su th vn c th phc hi c bng cch thu n phn t anten khc. iu ny c th thc hin c bng cch chn khong cch gia cc phn t mt cch ph hp.Khong cch yu cu gia cc phn t anten m bo khng tng quan(giitngquan)phthucvovicnighpcpgiaccphnt antenvvtrcaccvttnxgyratruyndnang.Vd,khi khng c nh hng ca vic ni ghp cp th khong cch cn thit gia cc phn t anten ca my di ng c cc vt tn x ng nht bao quanhphi l khong /2. Trong khi , khong cch ny phi l 10 hoc ln hn m bogitrgiitngquantngngtrmgc.iukinthhaicn thit cho k thut phn tp l cng tn hiu trung bnh ca cc ng truyn phn tp phi xp x bng nhau. Nhng nghin cu su v cc k thut kthptuyntnhcaratrong[32]v[53].Mtsphttringn -32- y ca vic ng dng phn tp anten trong thng tin di ng c phn tch trong [55]. Trongphnny,tasxemxtcscacckthutkthpphntp khng gian. Gi scc phn t anten c khong cch ph hp sao cho hon ton khng c tng quan gia cc nhnh khc nhau (mt nhnh c th c coi l 1 anten), ta hy xem xt kh nng ci thin ca anten mng vi cc k thut kt hp phn tp khc nhau; Sau mi phn tch n nh hng ca s tng quan nhnh gy ra do ni ghp gia cc phn t anten hoc tri gc ca tnhiun.ChtiuBERcaccccuiuchcbnviantenmng phn tp cng c nh gi. C 3 cch c bn kt hp tn hiu:-Chn lc: B chn lc l phng php n gin nht trong cc k thut phn tp: t mt tp hp M phn t anten, nhnh c t s tn hiu trn nhiu ln nht c chn ra v kt ni trc tip ti my thu. Nh vy, antenmngcMcnglnthkhnngcctltnhiutrn nhiu cng ln.-T l cc i: Phng php kt hp t l cc i tn dng tt nht kh nngcaccnhnhphntptronghthng.TtcMnhnhc nhn trng s vi cc t s tn hiu tc thi trn nhiu tng ng. Sau tn hiu t cc nhnh c ng pha trc khi ly tng tn hiu sao cho tt c cc nhnh c gp vo nhau theo pha sao cho tn hiu u ra c tng ch phn tp ln nht. Tn hiu tng chnh l tn hiu u ra thuccamng.PhngphpTlccicnhiuuimso vi phng php phn tp la chn nhng phc tp hn; do phi m bo tn hiu t cc nhnh l hon ton ng pha vi nhau v cc trng s phi c cp nht chnh xc.-33- -Tng ch u: L mt bin th ca k thut kt hp t l ti a; Trong trng hp ny tt c cc gi tr tng ch ca cc nhnh u bng nhau vkhngthayitrongqutrnhhotng.Gingvitrnghp trc, u ra s l tng ca cc tn hiu ng pha ca tt c cc nhnh. Hnh 2.1. Anten mng phn tp M phn t Tnhiuthucbiccphntckthptuyntnhnhtrong Hnh 2.1. Trng s kt hp c chn l *1w , *2w ,... *Mw , trong k hiu * lbiudinlinhpphc.Khiulinhpphccsdngtrongbiu din trng s ch nhm tin li v mt ton hc u ra kt hp c th c vitgnlwHs.Trongphnny,githitrngccnhnhlkhngtng quan. Tn hiu thu c mi phn t s khng phi l hng s, m dao ng theo h s pha-inh. H s pha-inh ph thuc vo tc ca my di ng v tn s tn hiu v tuyn v c chng minh l xp x bng bin i Doppler cc i. Bin i Doppler cc i fdM tng ng vi tn s fG (tnh bng GHz) v vi my di ng di chuyn tc v l: fdM = 1,4815 fG v(2.1) W1* W2* Wm*WM* 1 2 ... m M Phn t dSn(t) u ra kt hp U(t) -34- Theo c tnh ca knh v tuyn, thi gian nht qun ca knh v tuyn xp x bng nghch o ca h s pha-inh. Mi trng a ng gia anten pht v anten thu c coi nh mt b lc tuyn tnh thay i theo thi gian vminhnhMcctrnghobngmthmtruyntthngthp tng ng Tm(f;t), m = 1,...,M, vi bin s (agumen) t biu din nhng thay i theo thi gian ca p ng knh v tuyn v bin s fbiu din tnh cht chn tn ca knh. Gi s rng pha-inh mi nhnh phn tp l khng chn tn(haypha-inhphng),tacthbiudinhmtruyntbng Tm(f;t)=gm(t),tronggm(t)lmtilngthngkGaussphctrung bnh-bngkhng(zero-mean).Nhvycctnhiuthucccnhnh phn tp c th c biu din di dng: sm(t) ] ) ( ) ( [ ] ) ( [2 2 t f jmt f jmc ce t u t g e e t r e = =(2.2) trong : fc l tn s sng mang danh nh,u(t) l ng bao phc ca tn hiu pht, vrm(t) l ng bao phc ca tn hiu thu.Gis pha-inh l phng cho trng hp truyn dn bng hp, tr xut hintrongccthnhphnangunhhnnhiusovikhongthi gian ca mt k hiu. V gi s rng khong thi gian ca k hiu Ts nh hn nhiu so vi nghch o ca tc pha-inh sao cho mu pha-inh trn l khngthayi.thuntin,tachunhotnhiuphtsaochocngsut trung bnh l hng s 1 | ) ( |1) | ) ( (|2 /2 /2 2= =dt t uTt u EssTT sT (2.3) trong : ET l ton t k vng-thi gian hay trung bnh-theo thi gian.-35- ng bao phc ca tp cng trong nhnh my thu th m c gi thit l m(t) vi mt trung bnh trn mi khong thi gian k hiu hoc di hn bng mo m m TP P t u t E = = =2 2| ) ( |21) | ) ( (|21 (2.4) trong :- du ngoc nhn biu din trung bnh thng k, v- gi thit rng mi Pm u bng Pmo.Ta nh ngha t s sng mang-trn-tp (CNR) tc thi (m) v trung bnh (m) cho nhnh th m l: m moinhanh ptrungbinh congsuattamoinhanh gbinhcucbo ngmangtrun congsuatso= (2.5) mom TPt r E ) | ) ( |21(2= (2.6) momPt g2| ) ( |2, (do gm(t) hng s trn Ts ) (2.7) m moinhanh ptrungbinh congsuattakemoinhanh gbinhthong ngmangtrun congsuatso=(2.8) = mmooPP=(2.9) trong : Po l cng sut trung bnh thng k bng |gm|2/2 trn khong pha-inh (khong thi gian nht qun).Chrngdophpchunho(2.3),tasdngvngccbca ng bao tn hiu thu c trn mi nhnh l: 2 2 2 2| ) ( | | ) ( | | ) ( (| ) | ) ( (| t g t u t g E t r Em m T m T = (2.10) Nh vy, |gm(t)| c th ccoi l ng bao trung bnh cc b ca tn hiuthuc.Gispha-inhlpha-inhRayleigh,hmmtxcxut cho ng bao tn hiu thu c l: [27] -36- p(|gm|) = o mP gomePg2 / | |2 | |(2.11) v hm ca m l p(|m|) = mmem1(2.12) Trongphnsau,tachtptrungvophngphpkthptlccichonglntmydingntrmgc.Phngphpkthptlcc i hay c s dng trong m phng, c ch tiu tt nht, nhng cng phc tp nht khi thc hin.2.1.1. Kt hp t l cc i Trongkthutkthptlcci(MRC),tnhiuccnhnhc ly trng s v kt hp sao cho t c CNR tc thi cao nht c th vi cc k thut kt hp tuyn tnh. S dng phng trnh (2.2), ng bao phc tng nhnh th m c tp cng m(t) c th c vit l zm(t)= gm(t)u(t) + m(t) (2.13) Nu tn hiu thu c c ly trng s bng wm* th u ra kt hp U(t) ca mng l: U(t) = wHz = u(t)wHg+ wH, (2.14) trong : H k hiu lin hp Hermitian (lin hp phc, chuyn v),w = [w1,...,wm]T, = [1,...,m]T, g = [g1,...,gm]T.Gisrngmithnhphntplclpvinhau,thtngcngsut tp u ra Pmo(o/p) l: Pmo(o/p)=21|wH|2 = mMmmP w21| * |=(2.15) Do CNR u ra tc thi l: -37- ( )=====NnmmMmmmMmmmHTP wg wP wg w t u E12 *21*12 *2| |21| || ) ( |21(2.16) Cctrngstiucxcnhkhiccbinthintronglinquan n phn thc v phc ca wm bng khng. Ni cch khc, cc trng s ny c th thu c bng cch p dng bt ng thc Schwarz vo phng trnh (2.16). Vit wm= + jv tch hon ton theo hai tham s v v t bng khng ta c: wm* = mmPg*(2.17) vi ngha l cc tn hiu ny phi c kt hp vi trng s t l thun vi lin hp phc ca tn hiu cc nhnh v t l nghch vi cng sut tp trn cc nhnh . Nh vy, cc nhnh c CNR cao s c ly trng s ln hn cc nhnh c CNR thp. Cng cn ch rng cc tn hiu ly trng s u cng pha v l cng nht qun. CNR u ra vi cc trng s trn l: = == ====MmmMmmmP gP gPgMmm mMmm m1 12/ | * |/ | || |212112212 (2.18) i lng ny c coi l tng CNR ca tng nhnh. Vic thc hin b kt hp t l cc i s tn km do cc trng s cn bm c bin v pha ca p ng knh (gm(t)s). Hn na, cn c cc b chuyn pha v b khuych i tuyntnhtrnmtdidngrngcctnhiuuvo.Do,kthutkt hp t l cc i mang tnh l thuyt cao v thng c dng c dng so snh vi cc ch tiu ca cc k thut kt hp tuyn tnh khc. Dobiudintngtrong(2.18),ccphnbthngkcaCNRura trong trng hp ny c th c d dng rt ra t hm c trng ca n. Mt -38- ln na, trng hp m mi nhnh c CNR trung bnh bng nhau li tip tc cxemxt.DomiCNRulilngdng,vicdngbini LaplacexcnhhmctrngstinhnsovibiniFourier.Xt bin i Laplace F(s) == = =Mms s mrcMsm me e d P e s F10) ( ) ( S dng hm mt xc sut trong (2.12), ta c = +=Mm mss F111) ((2.19) NghchoLaplacecaphngtrnhtrnchotahmmtxcsut ) (mrcMp . Vy, hm mt xc sut cho b kt hp t l cc i M-nhnh l: + = +=j cj cMmmsmrcMdssejp1) 1 (21) ( ,c 0 = /1)! 1 (1eMMM,m = (2.20) Hm mt xc sut ny c phn b Erlang [48] do cng M phn b theo hm m c lp v ging nhau. CNR trung bnh u ra ca b kt hp l = = = = =MmMmnM1 1 (2.21) Hm phn b tch lu (c.d.f) tng ng l = = < =/010)! 1 (1) ( ) ( ob Pr ) (s sdx e xMd p Px M mrcM s smrcM == |.|

\|10/!11Mmmsmes(2.22) -39- == |.|

\|M mmsmes!1/(2.23) =1/1)! 1 () ( |.|

\| MssmrcMMePs(2.24) Hnh 2.2 trnh by hnh v hm phn b tch lu) (mrcMP ca k thut kt hp t s cc i vi M l tham s. Ta thy rng 99% d tin cy phn tp t l ccititkim12dBcngsutvihainhnhvtitkim19dBvibn nhnh. Cng cn ch rng hm mt xc sut b gii hn khi M -> . T (2.20)v(2.21),ddngthy rnghmmtxcsuttimcnhmdelta vi M ln ) ( ) ( lim ) ( = = > mrcMmmrcP P (2.25) Tcl,hmmtxcsutnygimnhmcatnhiuthuc trong mi trng khng gian t do khng c pha-inh. -40 -30 -20 -10 0 101E-41E-30.010.11-40 -30 -20 -10 0 101E-41E-30.010.11 PmrcM(gamas)10log(gamas/I_) M=1 M=2 M=3 M=4 Hnh 2.2. Hm phn b tch lu (c.d.f) ca s so vi s/ cho k thut kt hp t l cc i. -40- Chtiucami kthutiu chsiukinphntpcthc nhgibngcchxemxttllibt(BER)vikhngivlytrung bnh kt qu trn hm mt xc sut ca . V d, vi iu ch BPSK nht qun, BER cho k thut kt hp t l cc i vi gi thit tp ging nhau trn mi nhnh l: = =0/10)! 1 (1) (21) ( d eMerfc d p P PMMmrcM BPSKmrcBPSK =01) ()! 1 ( 21dx e x x erfcMx M ==|.|

\| + + |.|

\| Mmm Mmm MM021!)! 1 (21)! 1 (1 (2.26) ~ )! 1 ( !)! 1 2 (41|.|

\| M MMM vi >> 1 (2.27) trong += +=M 1(2.28) Ta thy rng BER gim theo t l 1/M vi M v ln. Ti gii hn khi M ->, BER bng nh khongphadiBPSKmrcBPSKP erfc P = = ) (21(2.29) Ktqunyngnhmongmunvuracabkthpstinti mtgitr bn(nnh)khiMln.Hnh 2.3trnh byhnhvBERsovi = M cho BPSK vi M l tham s. -41- -10 0 10 20 301E-61E-51E-41E-30.010.11-10 0 10 20 301E-61E-51E-41E-30.010.11 BER=(MI_) (dB) khong pha-dinh M=4 M=2 M=1 Hnh 2.3. BER so vi = M khi M thay i 2.1.2. Tng ch phn tp Tng ch phn tp ca mt mng M-phn t c xc nh bi phn ci thin nng lng ng truyn ng vi mt ch tiu k thut nht nh khi s dng k thut phn tp. Ch tiu k thut ny thng l t l li bt (BER). V d, trong Hnh 2.3, ta thy rng c BER bng 10-2 vi BPSK nht qun, t s tn hiu trn tp (CNR) trung bnh phi l 4,3 dB v 13,8 dB tng ng khi khngcvcpha-inhRayleigh.Nhvy,cngsuturatrungbnh trongtrnghpcPha-inhRayleighphicaohn9,5dB.Vicdng2 anten (tc l 2 nhnh phn tp) gim yu cu v cng sut xung 8,4 dB, v ta c th ni rng mng 2 phn t to ra tng ch phn tp l 5,4 dB (=13,8-8,4). R rng l tng ch phn tp cc i c th t c vi nhiu anten kt hp t l cc i l 9,5 dB mc BER ny, v gi tr ny s t tim cn vi M ln nh chng minh trong (2.29). Ta cng thy rng tc tng ca tng ch phn tp gim khi M tng. Do tng ch phn tp khi M di chuyn t M=10 n M=20thp hn nhiu khi M chy t 1 n 2. -42- 2.1.3. Tng ch antenTa cn phn bit r tng ch phn tp vi tng ch anten (c nh ngha ltscatssngmang-trn-tpuracamngtrntssngmang-trn-tpuracamtphntivicctnhiuuvoctnhtng quan cao- v d nh mt sng ti phng). Nh bit vi mt sng ti phng,p ng cc phn t khc nhau ch khc nhau bi mt h s exp(j), trong =kodcosphthucvokhongcchccphnt,tnscaotn,v gc ti ca sng phng so vi trc ca anten mng. Tn hiu u vo cho tng chantenscgithitlcdngu(t)oP 2 [1,exp(j),exp(j2),..., exp(j[M-1])]ToP t u 2 ) (= ,trongPolcngsuttrungbnhcami nhnh. Vi k thut kt hp chn lc, ch mt nhnh c kch hot ti mt thi im, do khng c tng ch anten. trng hp kt hp t l cc i v kt hp tng ch u cc trng s tng ng s bng hoc l mt phn ca w= oP 2 /Pmo,trongPmo lcngsuttpuvominhnh.Tnhiu kt hp cng vi in p tp cho mt sng phng ti l: U(t) =] ) ( 2 [2 H Homoot u PPP+= HmoomooPPt uPMP 2) (2+ (2.30) CngsutsngmangtrungbnhuralET(|[2MPou(t)/Pmo]2|)/2= 2M2(Po/Pmo)2, trong khi cng sut tp u ra l Po2/moH HP =2MPo/Pmo, gi s tp l khng tng quan cc nhnh v dng) (2tm =2moP . CNR u ra khi bng: mooPMP= (2.31) T ta thy r rng l tng ch anten mng bng M. Ch rng mc ci thinCNRtrungbnhcabkthptlccilnhnhaubtkcc -43- nhnh c tng quan vi nhau hay khng. Vn ny s c tip tc cp chi tit hn phn sau. Ktqutrncngcththuctlthuytantenchun.Lthuyt ny d on mc nh hng ca mt anten mng theo di ca n. S nh hng ca mt mng theo chiu rng, phn b u M phn t ph thuc vo d/ vi h s M. Vi d = n/2, n l s nguyn, nh hng ca mng bng M. Vi cc khong cch khc gia cc phn t, nh hng dao ng quanh M, tin ti tim cn M d/ ln. Vi d = n, trong n l s nguyn, nh hng b gim mnh do tc ng ca cc bp bn. 2.1.4. nh hng ca tng quan nhnh Chtiukthutphntpsgimkhiccnhnhkhnghontonbt tng quan vi nhau - vn ny cng c cp trong [32]hoc [53]. S tng quan nhnh c th b nh hng bi mt s yu t nh s gii-tri gc ca cc sng ti hay vic ni ghp cp gia cc phn t anten. V d, vi mtsngtiphng,hainhnhsluntngquanvinhaubtkkhong cch gia chng l bao nhiu. minh ho nh hng ny, ta xt trng hp hai nhnh dng k thut kt hp t l cc i, tn hiu Gauss phc cc nhnh l tng quan vi nhau vi h s tng quan phc l 12. H s ny xc nh mc tng quan gia cc tn hiu thu c hai im cch nhau mt khng cch d. Vi sng phng t mt phng xy, h s tng quan ny c nh ngha l: cos12d jkoe= (2.32) trong : -du ngoc nhn biu th trung bnh thng k i vi bin gc .-d l khong cch tng ng vi khong cch d gia cc phn t trong Hnh 2.4.-44- Hnh 2.4. Hai phn t vi cc tn hiu tng quan ng bao tng quan gia 2 nhnh l e |12|2. Trong trng hp c tng quan gia cc nhnh, hm phn b tch lu ca tn hiu kt hp trong (2.23) s c bin i thnh: [32] | | + + =|) | 1 /(12|) | 1 /(121212 12|) | 1 ( |) | 1 (| | 211 ) ( s se e Ps(2.33) v hm mt xc sut tng ng s tr thnh: | | + =|) | 1 /( |) | 1 /(1212 12| | 21) ( s se e p(2.34) CNRtrungbnhcatnhiukthpsduytr=2(clpvi |12|) v c th c kim tra d dng t (2.34). Tuy nhin, phn b ca s ph thuc vo |12| nh thy trong Hnh 2.5. BER cho c cu iu ch c bn nh BPSK nht qun c th c thc hin nh phn trc v bng =0) ( ) (21 d p erfc Pe ((((((

|||||.|

\| + +|||||.|

\| ++ + =|) | 1 (1111 |) | 1 (|) | 1 (1111 |) | 1 (| | 411212121212 (2.35) 12 d -45- -20 -10 0 101E-51E-41E-30.010.11-20 -10 0 101E-51E-41E-30.010.11 P(s)10log(s/I_) |ro12|2 =0.00 |ro12|2=0.50 |ro12|2=0.75 |ro12|2=1.00 Hnh 2.5. nh hng ca tng quan nhnh ln phn b cng sut u ra b kt hp t l cc i gm 2 nhnh phn tp. Vi trng hp c bit l tng quan hon ho, tc l 12=1, biu din trn c th rt gn tng ng vi trng hp M=1 (ch c 1 nhnh anten). H s tng quan ph thuc vo phn b ca sng ti theo gc t - ph thuc vo vtrphnbcaccvttnxnmgiaantenphtvantenthu.Nucc sngtiphnbngutheomtphngnmngangtmigc,nhi vitrnghpmythudingnmtrongmitrngcnhiuvttnx xungquanh,thcthpdngmhnh2DClarkecin.Ngcli,cc sng thu c trm gc trn cao s c hng ti nm trong mt gc tng i hp, v ta cn dng m hnh tn x trn m t trng hp ny. Trong m hnh tn x trn, cc vt tn x c gi thit l phn b u trong ng trn bn knh R quanh my di ng. Sng truyn t my di ng n trm gc ccoinhtruynquamtcnhtnxcavttnx.Dovngtnxl hu hn, cc sng pht ra t my di ng s ti trm gc trong phm vi mt gcnhquanhhngttrmgctimyding.Nukhongcchgia -46- my di ng v trm gc l D, th tng quan khng gian gia hai phn t vi m hnh tn x trn l: D dR kD dR k Joo/) / (2112 = (2.36) trong : Jn(.) l hm Bessel loi mt bc n.Gc tri ca sng ti s ph thuc vo kch thc tng i ca R v D. S dng d/ = 5, ch tiu BER c tnh t (2.35) cho cc gi tr tri gc khcnhauctrnhbytrongHnh2.6theo(tnhbngdB).Vim hnh chn, cn trung bnh bnh phng tri gc l 1o (xt t trm gc) lmtngngbaotngquanlnbng0,74.Rrngl,khitrigc tng,tnhiuccnhnhttngquanhnvngBERtinting phn tp 2-nhnh (12=0) l tng. So snh vi Hnh 2.3, ta thy rng khi gi tr cn trung bnh bnh phng ca tri gc bng 2o th hai tn hiu hu nh khng tng quan vi nhau. gi tr BER=10-2, tng ch phn tp cho trng hp phn tp 2-nhnh gim i khong 5 dB (= 14-9 dB) khi tn hiu thay i t khng tng quan sang tng quan hon ton. -10 -5 0 5 10 15 201E-41E-30.010.11-10 -5 0 5 10 15 201E-41E-30.010.11 Pe=2I_ phi rms=0 (ro=1) phi rms=0.5 (ro=0.93) phi rms=1 (ro=0.74) phi rms=1.5 (ro=0.49) phi rms=2 (ro=0.26) Hnh 2.6. BER so vi (dB) ca b kt hp t l cc i 2 nhnh c pha-inh tng quan -47- 2.2.K thut to bp sng 2.2.1. Chuyn bp sng Antenchuynbpsnggmnhiubpsngknhaumuraca chng c th thay i chiu ti mt hoc nhiu my thu nht nh. Do , mt trong h thng s c chia nh bimt nhm cc bp sng lin tc. Anten mng bm pha ng cng c th c coi l mt loi anten chuyn bp sng, nhng n s dng thm thng tin hng ti t ngi dng mong mun quayhngccibpsngvphanginncchtiutthn anten chuyn bp sng thng thng. Cn i vi anten thch nghi, cu trc bpsngcthbinithchnghivimitrngtnhiucaotn,nh hng bp sng ti ngi s dng mong mun, ng thi lm suy gim tng ch anten hng gy nhiu. Tuy c ch tiu tt hn, nhng anten mng thch nghithngphisdngnhngxlsphctpnncchiphtnkm hn h thng chuyn bp sng.Hthngchuynbpsngnginchbaogmmtmchtobp sng,mtchuynmchcaotn ciukhin logic chnbpsngmong mun.Mimythuphicmtcchlachnbpsngcthchn cbpsngmongmundavoccvectortrngsnh.Cch lachnbpsnghiuqulkhphctpvtuthucvophngphp truy nhp theo CDMA, TDMA hay FDMA.Antenchuynbpsngtoramttphpcnhccbpsngtng i hp. u ra cao tn ti cc bp sng ny c th l tn hiu cao tn hoc tn hiu qua x l bng gc s. Mi vng ph d qut (120o) c phc v bi mt mng cc chn t pht x ni vi nhau qua mch chuyn bp sng. Trong trnghpltng,ccbpsngctoralclpvinhau.Sbp sng c th thay i, v d: su bp sng c rng 20o hay bn bp sng c rng 30o cho mi vng ph d qut. -48- Viantenmngtuyntnh,rngbpsngtheophngnmngang c xc nh bi chiu di ca mng v bc sng. chiu thng ng, cc phn t anten c xp chng ln nhau gim rng bp sng theo chiu ny. iu cng lm tng tng ch anten v tng ch ny phc thuc vo c rng bp sng theo chiu nm ngang v thng ng. Tng ch c th tnh bng:G = Gd(,)(2.37) trong : l hiu sut anten,Gd l tng ch nh hng, v l rng bp sng tng ng theo phng nm ngang v thng ng, tnh bng [o]. Mchthngdngnhttolpbpsngtrongkthutchuynbp sng l ma trn Butler [49]. Ma trn to bp sng Butler ca h thng 8 bp sng lin tip c 8 cng vo v 8 cng ra. y l mt cu trc thun nghch, miucthlcuvohocuracaotn.Matrnnygm4b chuyn i hay b ni ghp cp theo hng v cc b chuyn pha c nh th ng. S lng mi loi phc thuc vo s bp sng pht ra. V d, vi anten mng tuyn tnh M phn t, s lng b ni ghp cp l: c = (M/2) log2M(2.38) trong : M l s bp sng.S b chuyn pha s nh l: s = M/2 log2(M-1)(2.39) Khi scng (bp sng) ln,thgitrtrnlkhln.Tuynhin,trong ccngdngthngtindingtong,gitrnymccthchpnhn c. -49- Cc mng Butler c th lp ln mi mu bp sng l bi s nhn ca 2: 2, 4, 8, 16, 32 .v.v. S bp sng s bng s phn t ca mng. Cc k thut to bp sng c th s dng trong cc mng hai chiu bng cch phi hp u ra cc ctphntantenthnhccmatrn,sau phi hpcc uracama trn ct ny thnh mt nhm ma trn hng. gia bng thng ca anten, khi khong cch gia cc phn t anten bng 1/2 di bc sng, v tr ca bp sng tnh bng: sin = 2k - 1 / M(2.40) trong : l gc lch, vk l s bp sng rngbpsngvkhongcchbpsngbiningcnhausovi tn s, nn c th duy tr mc giao ct khng i gia cc bp sng. Ngoixlbngkthuttngt(analog)nhtrn,antenchuynbp sng cng c th c thc hin bng k thut x l s. Khi , tn hiu cao tn c bin i xung trung tn, ri xung bng gc. Sau c chuyn i thnh tn hiu s trong b chuyn i tng t/s (A/D). Tn hiu ny tip tc c x l my thu s ri chuyn n mch to bp sng s.Cc h thng anten thng minh chuyn bp sng c u im l n gin v chi ph khng qu cao, nhng vn c mt s nhc im sau: -Th nhtlkhng trnhcnhiu caccthnhphn angn t cchnggnvihngcatnhiumongmun,dohthngdavo mchtobpsngcnhmthngnhycmvitnxgccacc thnh phn a ng hn l cc h thng da vo cc b x l mng thch nghi.-Thhailkhngckhnnglidngcuimcaphntpa ng bng cch kt hp cc thnh phn a ng.-50- -Th ba l cng sut nhn c t thu bao s b thng ging khi thu bao dichuynvngtrnquanhtrmgcdohintngvs(scalloping)- mtngngmccaginphnghngantenphthuchng tithayi theo ng knh camibpsngc torabimchto bp sng; Thng thng cc mch ny to ra cc bp sng an cho nhau, docngtnhiucathubaothayikhithubaodichuynt gia bp sng n bin vng ph ca mt bp sng no .Mc d c nhng nhc imnh trn song h thng chuyn bp sngvn c s dng ph bin v cc l do sau: -Ckhnngmrngphmviphsngtcchthngphctp.Tu theomitrngtruynsng,cchthngchuynbpsngcthlm gim tri tr, h tr mi trng thu bao tc cao... -Vvictobpsngcnhltrnghpnginnhtcakthut Anten thng minh nn chi ph thit k v vn s dng cc h thng ny s thp hn cc k thut phc tp khc. Khnngtngdunglngkhisdngantenthngminhchuynbp sng trong cc h thng thng tin di ng c nh gi c th trong Chng 3. Vi anten thch nghi, chc chn ch tiu h thng t c cn tt hn na. 2.2.2. To bp sng thch nghi K thut to bp sng thch nghi cho php hiu chnh mt cch mm do gin phng hng ca anten mng ti u mt s c tnh ca tn hiu thuc.Trongqutrnhquaybpsng,bpsngchnhcamngcth thay i hng mt cch lin tc hoc theo tng bc nh. Anten mng s dng k thut to bp sng thch nghi c th loi b tn hiu gy nhiu c hng ti khc hng tn hiu mong mun. Anten mng a phn cc cng c th loi b cc tn hiu gy nhiu c cc trng thi phn cc khc trng thi phn cc ca tn hiu mong mun, ngay c khi chng c cng -51- hng ti vi tn hiu mong mun. Nhng kh nng c bit ny c th c s dng ci thin dung lng ca h thng thng tin v tuyn.Dng hnh hccaantenmngvccyutkhcnhgin phng hng, hng, phn cc ca cc phn t u c nh hng trc tip n ch tiu cht lng ca anten mng. Cctrngsphccamiphnttrongantenmngcthctnh ton nhm ti u mt s c tnh ca tn hiu thu c.iu ny khng phi lun lun thc hin c; ngay c vi mt anten mng c mt bp sng nh hng ti a theo tn hiu mong mun vn c th khng to ra c tn hiu antenmnguratiu.Thngthng,victiuantenmngcthc hin bng cch to ra cc bp c gi tr bng khng (null) theo hng tn hiu gynhiu.Kthuttobpsngthchnghilmtphplpxpxcato bp sng ti u.Vimt anten mng tng qut, tn hiu u ra ca mng y(t) l tng c trngscacctnhiunhncsi(t)ccphntmngcgin phng hng gm(, ) (tng ch) v tp m nhit n(t) t cc my thu ni vi cc phn t (Hnh 2.7). Trong trng hp chng ta ang xt, s1(t) l tn hiu mong mun, v c L tn hiu khc c xem nh l ngun gy nhiu. Trong mt h thng thch nghi, trng s wm c xc nh theo phng php lp da trn tn hiu u ra y(t), mt tn hiu tham kho d(t) l tn hiu gn ng ca tn hiu mong mun, v cc trng s qu kh (c xc nh cc bc lp trc).Tnhiuthamkhocgithitlginghtvitnhiumong mun. Trong thc t gi thit ny c th t c hoc gn ng khi chng ta sdngmtchuihunluynhocchuingbhocmtmtriph CDMA m c my thu bit trc. y,tasxcnhccthnhphntrngstiuchophptithiu ho li bnh phng trung bnh (t) gia tn hiu u ra ca anten mng v tn -52- hiuthamkho.Tnhiumongmuns1(t),Ltnhiugynhiu,vtpm Gauss trng cng c coi l cng mt ngun. Gin phng hng khng nht thit phi ging nhau i vi mi phn t anten. Hnh 2.7. Anten mng thch nghi u ra ca mng c tnh bng: y(t)=wHx(t) (2.41) trong wH l bin i lin hp phc chuyn v ca vect trng s w. 2.2.2.1.Vect p ng ca mng Vect p ng ca mng i vi mt tn hiu c hng ti l) , ( v trng thi phn cc P c th c vit nh sau: ((((((

=) , , (:) , , () , , () , , (2121P g eP g eP g eP aMjjjM (2.42) iu khin-53- Trong : ml dch pha tng ng vi tr pha khng gian ca mt sng phng ca tn hiu ti theo hng) , ( , v) , , ( P gm l h s gin phng hng anten ca phn t th m. 2.2.2.2.Vt khng gian-phn cc Vt khng gian-phn cc l p ng tng ca mng i vi tn hiu c N thnh phn a ng v c biu din nh sau: ==Nnn n n nP a1) , , ( (2.43) Trong : nl bin v pha ca thnh phn th n. n n nP , , l gc ti v trng thi phn cc ca thnh phn th n. 2.2.2.3.Ma trn vt khng gian-phn cc p ng ca mng i vi nhiu tn hiu (trong trng hp mt tn hiu mongmun vLtnhiugynhiu)cth cbiudintheomatrnvt khng gian-phn cc. Cc ct ca ma trn l cc vt khng gian-phn cc ca bn thn cc tn hiu. Ma trn c vit nh sau: | || |i dLU UU|... |1 2 1==+ (2.44) Trong : Ud l p ng i vi tn hiu mong mun s1(t), Ui l p ng i vi cc tn hiu gy nhiu. u ra ca M my thu trc khi thc hin nhn trng s l: x(t)=U s(t)+n(t)(2.45) -54- 2.2.2.4.Tn hiu v tp m Cc tn hiu ti (bt k hng ti v phn cc) c th c vit l: 1 2 +1( ) =[ ( ), ( ),..., ( )] =[ ( ) | ( )]T TL d is t s t s t s t s t s t(2.46) Trong : sd(t)=s1(t) l tn hiu mong mun, vsi(t) l cc tn hiu cn li (tn hiu gy nhiu).Trongtrnghpttccctnhiucxemnhkhngtngquan vinhau vcdngsk(t)=ks uk(t)ejt trong ks lbin catnhiu v uk(t) l tn hiu iu ch bng gc chun. Tp m ti M my thu c tnh bng: n(t) = [n1(t), n2(t), ...,nM(t)]T(2.47) Tp m ti cc nhnh my thu khc nhau l khng tng quan. 2.2.2.5.Trng s ti u ti u cc trng s mi phn t, chng ta cn gim thiu li trung bnhbnhphnggiauracamngvtnhiuchund(t).Victiu ho SINR s lm cho cc trng s lch i mt i lng nhn v hng so vi cc trng s trnh by y. X l chnh lch ny nh i vi trng hp cc phn t ng hng, v nghim ca cc trng s ti u l [78]: -1=opt xx xdw R r(2.48) trong : ) ( ) ( t x t x RHxx =l ma trn hip bin (covariance) ca tn hiu, v ) ( ) (*t x t d rxd =l ma trn tng quan cho gia d(t) v x(t) Kt qu ny ging vi biu din cc trng s ti u i vi anten mng bao gm cc phn t ng hng [78]. Tuy nhin, trong trng hp ny, Rxx, rxd,vdo cwoptu phthucvogcti caL+1 tn hiu,vgin phng hng ca cc phn t. -55- Nh vy, vn l cn cp nht trng s ti u vi mt thut ton no c ch tiu tt nhng vn m bo c kh nng thc hin vi thit b thc t. 2.2.3. Cc thut ton thch nghi Cc thut ton tobpthch nghithchinccphplptinti xpx cctrngstiunitrn.Crtnhiuthuttonnhdngthchnghi cphttrin.unhcimcamtsthuttoncbnhaycs dng trong k thut to bp sng cho thng tin di ng c tm tt sau y. 2.2.3.1.Trung bnh bnh phng nh nht Thuttontrungbnhbnhphngnhnht(LMS)sdngphng phpcbcgimdnvtnhtonvecttrngsquysdngphng trnh: ] [ ] [ ] 1 [ ) (1*1 1n p n n w n wLMS p + =(2.49) trong : pl hng s tng ch v iu khin tc thch nghi.ThuttonLMSyucubittrcthngtinvtnhiumongmun. iunycththchinctrongmththngsbngcchphttheo chu k mt chui hun luyn c my thu bit trc, hoc s dng m tri ph trong trng hp h thng CDMA tri ph trc tip. Thut ton ny hi t chm nu di vc-t ring ca Rxx ln. u im: Lun lun hi t Nhc im: Yu cu tn hiu tham kho 2.2.3.2.Nghch o ma trn lin hplymu trc tip Cng thc cp nht trng s trong thut ton ny vn l cng thc (2.48), nhng Rxx v rxd c nh gi t d liu c ly mu trn mt khong thi gian xc nh. nh gi cc tham s ny l: -56- , ) ( ) (21==NN iHxxi x i x R (2.50) ==21) ( ) ( * NN ixdi x i d r (2.51) u im: Lun lun hi t; Tc hi t nhanh hn LMS.Nhc im: Yu cu tn hiu tham kho; tnh ton phc tp 2.2.3.3.Thut ton bnh phng ti thiu quy Thuttonbnhphngtithiuqui(RLS)clngRxxvrxds dng cc tng trng snh sau: (2.52) v (2.53) Nghch o ma trn hip bin c th thc hin bng cch quy, v iu ny dn n phng trnh cp nht trng s: )] ( ) 1 ( ) ( * )[ ( ) 1 ( ) ( n x n w n d n q n w n wH + = (2.54) trong : ) ( ) 1 ( ) ( 1) ( ) 1 () (1 11 1n x n R n xn x n Rn qxxHxx += (2.55) v )] 1 ( ) ( ) ( ) 1 ( [1 1 1 1 = n R n x n q n R Rxx xx xx (2.56) u im: Lun lun hi t; Tc hi t nhanhgp 10 ln so vi LMS Nhc im: Yu cu nh gi ban u v Rxx-1 v tn hiu tham kho. 2.2.3.4.Cc thut ton quyt nh trc tip Trong thut ton quyt nh trc tip, cc trng s c th c cp nht bngbtkthuttonnotrn,nhngtnhiuchunclyratqu -57- trnhthchingiiiuchtnhiuy(t),tclkhngyucuthngtin chun t bn ngoi. Tuy nhin, thut ton ny khng m bo s hi t v y(t) c th khc d(t). 2.2.3.5.Thut ton hng s theo khi Thut ton hng s theo khi l thut ton m dc xut bi Goddard, Treichler v Agee [17]. Thut ton ny khng yu cu bit trc thng tin v tn hiu mong mun. Thay vo , n tn dng cc thuc tnh bin khng ihocgnnhkhngicahuhtcckhundngiuchcs dngtrongthngtinvtuyn.Bngcchxemtnhiuthuccbin khngi,thuttonhngstheokhiskhiphcctnhiumong mun. Cng thc cp nht trng s c tnh bng: w(n+1) = w(n) + x(n)*(n) (2.57) trong : (n) = [1-|y(n)|2]y(n)x(n) (2.58) Khithuttonhngstheokhihitnshittinghimtiu, nhng s hi t ca thut ton ny khng c m bo bi v hm chi ph khng li v c th c cc gi tr cc tiu sai. Mt vn tim n khc l nu c nhiu hn mt tn hiu c cng mnh, thut ton c th a ra quyt nh nhm i vi tn hiu khng mong mun. C th khc phc vn ny nu c thm thng tin v tn hiu mong mun. Cc bin th hin ti ca thut ton hng s theo khi s dng cc hm chi ph khc nhau. Thuttonhngstheokhitrungbnhtithiulmtbinthca thuttonhngstheokhisdngphpnghchomatrntrctip. Trng s c th c tnh ton nh sau: -1=xx xdw R r(2.59) -58- TrongRxxvrxdcmtnh(2.48),ngoitrvicsdng c lng khi khng i ca tn hiu mong mun l | | yyd = . Cc phin bn nhiu ch ca thut ton hng s theo khi s dng qu trnh trc giao Graham-Schmidt to ra hai hoc nhiu hn tp cc trng s trcgiao.Thuttonhngstheokhinhiuchcthphntch(phn bit)stnhiubngsphntcaantenmng.Cthsdngtrcgiao mm hoc trc giao cng. Vi php trc giao cng, ban du, vi anten mng N phn t, N vc-t trng s trc giao c s dng. Mi vect trng s c cpnhtclpvinhausdngthuttonhngstheokhinhtrong (2.57) hoc (2.59). Tt c cc vect tr vect u tin c khi to li nh knhsaunhmtrnhtrnghpcnhiuhnmtvecthitticng mt gi tr. (2.60) u im: Khng yu cu tn hiu tham kho Nhc im: V mt l thuyt, c th khng hi t. 2.2.3.6.Cc k thut khc Cc phng php to bp sng thch nghi khc c th k n l k thut khiphcttngquanph(SCORE),lmtthuttonthchnghims dngctnhnnhtheochukcatnhiu.Bclungchuiging nhaunhtcngcthcsdngthchintobpsngthchnghi. Trongccmngthchnghitheokhnggian,chmtsphntcly trngsthchnghi.Kthutnycnhiuuimiviantenmngln. Hotngthchnghitheokhnggianchophpmngloibccctn hiu nhiu v yu cu mc tnh ton t hn so vi trng hp cp nht tt c cc trng s phn t. -59- 2.2.3.7.Nhn xt Ta thy rng mi thut ton khc u c nhng u nhc im ring. Do tnh cht thay i lin tc ca mi trng thng tin di ng, cng nh nhng hn ch v kh nng tnh ton tc thi khi thc hin trong thit b thc t m cc h thng th nghim u ch s dng cc thut ton kinh in khng i hiqutrnhtnhtonquphc tp.i litc hi tca thuntonc thchmhnhocthmchkhng cmbonhvitrnghpthut ton hng s theo khi. Phn sau s xut mt thut ton kt hp tn dng c knh hoa tiu vknh lulng gii quytvn trnmkhnglm tng phc tp tnh ton. 2.3.Thuttontobpthchnghichtrcaknhhoatiu cho ng ln DS-CDMA 2.3.1. Anten thng minh cho DS-CDMA Kh nng thc t ca h thng anten thng minh ph thuc vo vic ng dngcngnghvtuynxcnhbngphnmmsdngccthuttonphn tp v to bp thch nghi. H thng v tuyn xc nh bng phn mm thngthngchohthngantenthngminhgmcccbitnxung-bin i tn hiu thu c v trung tn. Ti , tn hiu tip tc c ly mu v s ho bi mt b bin i tng t/s (A/D) tc cao. Phn cn li ca qu trnh x l c thc hin bi phn mm. Mi phn t ca anten mng M phn t c b i tn xung v bin i tng t/s ring c ni vi mch to bp sng. Vic gii iu ch c thc hin bi phn mm v c dng chungchottcccphnt.Qutrnhtobpsngnycthcthc hinbngcchsdngcckhixls(DSP)hocphncng.Thng thng,ngitasdngccthuttonthchnghicpnhtccvc-t trngs.Ccvc-ttrngsthngctnhtonlinhhotsdnghoc -60- khng sdngchui hng dn.Ccthut ton phbinhay csdng trong thc t l cc thut ton n gin nh cp phn trc. Bng cch s dng anten mng thng minh trong h thng CDMA, nhiu ngisdnghncthngthidngchungmtbngtn;kthutto bpsnggipmingisdngccSINRcaohn.Ngoiccthut tonnginnutrn,nhiuthuttonphctpkhccxutcho cc h thng DS-CDMA m bo p ng yu cu dung lng cao. Phndiysxutmtthuntonmvndngccthutton kinh in ni trn khng lm tng phc tp tnh ton, nhng c s kt hpthmcaknhhoatiutcchtiutthn.Vimththng DS-CDMA 3G c th, ta thy rng ng ln ca h thng l nht qun [3]. tchtnhiunhtqun,phicmtknhhoatiungln.Do, phn ny s xut s dng mt thut ton to bp thch nghi cho ng ln DS-CDMAnhtqun.Thuttonnykhcccthuttonkhcchcs h tr thm ca knh hoa tiu. Ta xt ng ln ca mt h thng DS-CDMA gm c knh hoa tiu v knh lu lng v s dng tch nht qun trm gc bng anten nhiu phn t. b mo bin v pha ca tn hiu thu khi i qua cc knh truyn sng vtuyn,mttnhiuhoatiubittrcscsdngmythu.Hai phngphpphttnhiuhoatiuhaycsdngl:khiuhoatiu hoc knh hoa tiu. Phng php h tr knh hoa tiu c s dng phc hi mo pha v bin . Vi knh hoa tiu, nhiu k thut to bp da trn thut tontrung bnh bnh phng nh nht (LMS), bnh phng ti thiu qui c th c sdng.Trongxuty,cknhhoatiuvknhlulngcs dng cho k thut to bp sng. Ngoi thut ton LMS cp nht cc trng s ca b to bp sng cho knh hoa tiu, thut ton hng s theo khi cng -61- c s dng tn dng thng tin trn knh lu lng. Nh php cp nht kt hp ny, tc hi t ca b to bp thch nghi s nhanh hn. 2.3.2. M hnh tn hiu 2.3.2.1.Knh ng ln DS-CDMA nht qun Xt knh ng ln trong h thng DS-CDMA. Gi s rng c K ngi sdngtronghthng,mingisdngctnhthaiknhtruyn:mt knh lu lng v mt knh hoa tiu. Knh lu lng th k c th c biu din bng: ) ; ( ] [ ) ( n nT t d n s A t rknk k k = =(2.61) trong : T l di ca k hiu, kAl bin , S sk l k hiu th n, ) ; ( n t dk l dng sng tri ph k hiu th n v knh th k. y,SlkhiumS={1}ivikhochuynphanghphn BPSKvS={(1j)/ 2 }ivikhochuynphatphnQPSK.Dng sng tri ph c vit l: ) ( ] ; [ ) ; (10cNlk klT t n l c n t d ==(2.62) trong : cTl di chp, N=T/cTl tng ch x l, S n l ck ) ; (l phn t th l ca chui triph i vi k hiu th n ca knh th k, (t) l dng sng chp. -62- Ngoi knh lu lng, knh hoa tiu c th c biu din l: ) ; ( ) ( n nT t d s A t r knkk = =(2.63) trong : k Al bin , sl k hiu hoa tiu khng i, ) ; ( n t d kl dng sng tri ph ca knh hoa tiu th k. Trong , sng tri ph c dng: ) ( ] ; [ ) ; (10cNlk k lT t n l c n t d ==(2.64) trong : ) ; ( n l ckl phn t th l ca chui tri ph i vi knh hoa tiu th k. Thng thng, bin ca knh lu lng ln hn knh hoa tiu, tc l kA > k A .cthtchnhtqun,philycthngtinvphacaknh pha-inh qua knh hoa tiu. 2.3.2.2.Vc-t vt khng gian GisrngtrmgcctrangbmtantenmngcMphnt.K hiu a() l p ng ca mng i vi mt tn hiu c bin n v vi gc ti l . l gc tri ca gc ti danh nh . Cng vi knh ny, vt khng gian c th c xc nh bng: )~(1qQqqa v + == (2.65) trong : |q~| /2, Q l s tn hiu tn x cc b, v ql h s suy hao. -63- Nu cu to mng trn v khng ln, t d()=da()/d, ta c th ly xp x a(+~) bng: a(+ )~ a()+d() (2.66) Davocngthc(2.66)trn,vicnhgivtkhnggianc nghin cu trong [36]. Mt khc, nu nh, vt khng gian c th c vit l: v=a()(2.67) trong : ==Qqq1 Ni chung, c th c coi l mt bin ngu nhin. Cc m hnh cho vc-t khng gian v trn l cc m hnh tham s. s dng cc m hnh tham s ny, ta cn bit thng tin v hnh dng ca mng anten. Hn na, cc m hnh tham s ny ch c gi tr khi rt nh. Trong phn ny, ta khng s dng cc m hnh tham s ca vc-t vt khng gian xydngvc-ttobpsng(vc-ttobpsngkhngctnhtont ccthams,,v).Do,mhnhthamsskhngcxemxtv khng cn thng tin v hnh dng ca mng anten. 2.3.2.3.M hnh tn hiu vc-t K hiu vkl vt khng gian ca knh th k. Tn hiu thu c khng c tr truyn sng ca knh lu lng th k c th c vit l: ) ( ) (k k k kt r v t r =(2.68) trong : kl tr truyn sng. Cng thi gian ny, knh hoa tiu cng ti my thu. Knh hoa tiu th k thu c c th c vit l: ) ( ) (k k k kt p v t p = (2.69) Tn hiu thu c trm gc by gi l: -64- ) ( ) ) ( ) ( ( ) (1t w t p t r t yKkk k+ + ==(2.70) trong w(t) l tp nn. 2.3.3. Kt hp theo khng gian my thu trm gc 2.3.3.1.Cu trc ca b kt hp t y[n]biu din chui ly mu ca y(t) t b bin i tng t sang s (A/D)saublctrngkhtchodngsngchip(t).Gisrngtnhiu mong mun l tn hiu u tin. Khng lm mt tnh tng qut, ta gi s rng 1=0. Do , vi dng sng chp hnh ch nht, c th vit: [76] +=ccT nnTcdt t yTn y) 1 () (1] [(2.71) Trongphngphptriphthngthng,vimttpccbtng quan cho tn hiu truyn dnutin[r1(t) vp1(t)], vc-ttnhiu gii tri ph cho tn hiu knh truyn dn lu lng v hoa tiu th k tng ng bng: ] ; [ ] [1] [*1101n l c l nN yNn rNl+ ==(2.72) v] ; [ ] [1] [*1101n l c l nN yNn pNl+ == (2.73) trong :] ; [1n l cl phn t th l ca chui tri ph cho k hiu th n ca knh hoa tiu th 1 . T m hnh vc-t tn hiu, ta c: ] [ ] [ ] [1 , 1 1 1 1n i n s A v n rr+ =(2.74) v ] [ ] [1 , 1 1 1n i s A v n pp+ =(2.75) trong ] [1 ,n ir v] [1 ,n ip l chui vc-t nhiu-cng-tp tng ng trong knh lu lng v hoa tiu. Ch rng ] ; [ ] ; [*10n l c n l ck kNl= = N n l c n l ck kNl==] ; [ ] ; [*10 -65- 2.3.3.2.Thut ton kt hp cho to bp ccnhgikhiudliut] [1n r trong(2.74),cnphic mt b to bp. nh ngha vc-t trng s ca b to bp cho knh hoa tiu u tin l: TLw ] ... [1 , 1 , 2 1 , 1 1 = (2.76) nh gi k hiu c vit l: ] [ ] [ 1 1 1n r w n sH= (2.77) Trong my thu c b to bp, vc-t trng s 1wphi c tnh c th nhgittkhiu.Nubit] [1n s ,tiuchlitrungbnhbnhphng (MSE) c th c s dng tnh 1w : | |21 1 1 1 1| ] [ ] [ | min arg n r w n s A E wHw =(2.78) Nhngdokhngbit] [1n s ,cchtipcntrnlkhngththchin c. Tuy nhin, nh c knh hoa tiu, tiu ch li bnh phng nh nht tnh w1 c th tnh bng ) ( min arg1w C wpw=(2.79) trong | |21 1 1 1| ] [ ] [ | ) ( n r w n s A E w CHp = (2.80) Tlibnhphngnhnhttrong(2.79),mtthuttonLMStnh ton vc-t trng s thch nghi c th c rt ra. Do : ] [ ] [ ] 1 [ ) (1*1 1n p n n w n wLMS p + =(2.81) trong LMSl hm li tnh bng] [ ] 1 [ ] [1 1 1n p n w s A nHLMS = (2.82) v p l tng ch thch nghi ca thut ton LMS.Nhcptrn: 1A 1 no tc l rw>qri. Gi tr chnh xc ca h s bo v ny tu thuc vo phngphpiuchcsdng.Nhnchung,hsbovlthphn i vi cc phng php iu ch bng rng hn, chng hn iu ch FM c hsbovnhhnsoviccphngphpiuchcsnhAMhay SSB.Vd,truyndnthoisdngiuchFM25kHzcthmbo cht lng vi gi trCIR (hay l q) = 18 dB. Hin tng mt lin lc s xy ra khi mc nhiu qu cao v h s bo v khng c m bo. Xc sut mt lin lc c xc nh bng xc sut m rwqri. Khi khng c pha-inh v che khut, tn hiu ch b suy gim bi suy hao ng truyn, mi quan h gia cc ng bao tn hiu thu c t trm gc mong mun v trm gc gy nhiu c biu din nh sau: 2 /2 /11niinwwdrdr(3.12) -79- trong : n l h s m suy hao ng truyn, vdw , ditngnglkhongcchtmydingtitrmgcmong mun v trm gc gy nhiu. Gitrcahsbovtuthucvocngsutpht,tngchanten, chiu cao Ht so vi mt t... Gi thit cc thng s ca c hai trm gc ny l nh nhau, th qu tch nhng im tho mn rw=qri xc nh bi phng trnh sau: 2/ n iwdqd=(khng c pha-inh v che khut)(3.13) Nu anten ca trm mong mun c to (0,0,Ht) v ca trm gy nhiu l (D,0,Ht) trn mt phng nm ngang,th qu tch ny l mt ng trn tm ti(-D/(q4/n-1),0)vibnknhlDq2/n/(q4/n-1).Hnh3.9chothyvngb nhiu (b che khut) v khng nhiu (khng b che khut) gii hn bi ng trn.Nuchunhhngcapha-inhvchekhutcphnblog-chun, th ng bao tn hiu thu trong iu kin khng nhn thng c dng nh sau: [27, 49, 79] . 10 .120 /2 /dgndr (3.14) Trong : g l phng sai n v Gauss c trung bnh bng khng (zero-mean),l phng sai Rayleigh, vi tr trung bnh/ 2 , v d(dB) l lch chun ca phn b log-chun.Trong iu kin c pha-inh v che khut, qu tch tc thi ca cc im thomnrw=qriskhngcncdngngtrnna,mcxcnh bng phng trnh sau: [27] -80- n g gniwwi dw w di iqdd10 / ) (/ 210 ||.|

\|=(3.15) trong :gw, w, v dw : tng ng l phng sai phn b Gauss, Rayleigh v lch chun ca phn b log-chun ca tn hiu mong mungi, i, v di : tng ng l phng sai phn b Gauss, Rayleigh v lch chun ca phn b log-chun ca tn hiu (gy nhiu). Hnh 3.9. Vng c nhiu v khng nhiu (a) khng c pha-inh (b) c pha-inh v che khut. Hnh3.9(b)chothyvngnhiuvkhngnhiukhitnh n pha-inh vchekhut.Rrnglnhiucthxyrangayckhimydingrt gn v tr ca trm gc mong mun. Hm mt xc sut ca ng bao tn hiu thu khi c c pha-inh v che khut s c dng nh sau: [27] ( )22 2/10/10( ) exp exp104 10 8 2dddddrrd drr rp r dr + ( ( (= ( ( (3.16) trong : rd=20logr l ng bao trung bnh cc b tnh bng dB, vNhiu Khng nhiu Trm mong mun Trm gy nhiu dwdi Nhiu Khng nhiu Trmmong mun Trm gy nhiu dwdi -81- ddr =l trung bnh ca ng bao trung bnh cc b, tnh bng dB.Hin tng mt lin lc xy ra khi ri rw/q. Xcsutxy ra iu ny c th tnh c bng cch t rw=x, ri tm xc sut ri x/q: /( / ) ( )i i ix qP r x q p r dr = (3.17) v sau tch hp mi gi c th c ca x Hmmtxcsutnythomnbirihocrwcdngcngthc (3.16). Vy, ta c xc xut mt lin lc l: wwi w di di dw dw outr dqrr P r p q P =0) ( ) ( ) ; , , , ( (3.18) 2( 2 ) /101 exp( )1 10ddZ ttdt=+(3.19) trong , kt qu cui cng trn c tnh bng cch thay th cc ng bao hm mt xc sutc dng (3.16) vo (3.18) v (3.17) sau rt gn tch phn thu c.Trong (3.19) ta gi thit l: d w di d = = , v Z Zdlog 20= =d di dwq vi qd=20logq l h s bo v tnh bng dB.Vi0 d = dB,tchphntrncthtnhgnngvidng Pout=1/(1+Z2),trnghpnytngngvichcpha-inhmkhngc che khut. -82- Hnh 3.10. Xc sut mt lin lc khi c pha-inh v che khut Tchphn(3.19)ctnhbngphngphpsvktqucv trongHnh3.10.XcxutmtlinlctheoZd,vid lthams.Tathy rng,mcnhiucaoxyrathmchngaygnmypht,nicmctn hiu mong mun trung bnh cao hn nhiu so vi mc trung bnh ca nhiu. V d, nu di dw + = 40v h s bo v l 20 dB, th xc sut mt lin lc l khong1%,4%v15%tngngvid =0,6,v12dB.ChrngPout khngphthucvoccmctnhiutuyti,mchphthucvocc mc tng i c th hin trong d d dw diZ q = 3.3.1. nh hng ca s che khut Trong trng hp ch c nh hng ca hiu ng che khut, ng bao tn hiu s c dng: g rdrd d dgnd + = 20 /2 /10 .1(3.20) -83- Thchinphntchtngtnhphntrc,xcsutmtlinlckhi khng c pha-inh l: 02/ 2( , , , , ) ( ) ( )exp( )2( )2ddwdw diout w i w dw diZddrP q p r P r drqtdtZQ = = = (3.21) trong hm Q(.) l hm nh c nh ngha trong Chng 1. Hnh3.10cngvngcongbiudinxcsutmtlinlctheoZd vid lthamskhitnhiuchbtcngbischekhut.Vd,vi Zd=20 dB nh cp mc trc, xc sut mt lin lc khi ch c che khut ch cn 1% v 11% tng ng vid =6 v 12 dB. Ngc li, khng ch xcsutmtlinlcnhhn1%,thZdphitnhtl20dBtrongtrng hoc ch c pha-inh hoc ch c che khut md =6 dB. Gi tr ny tng ti 27 dB khi pha-inh c tnh trn nh ca che khut. Chnh lch v Zd l 7 dB ca27dB, tc l khong 26%. Cng xt tng t nh trn, Zd phi t nht l 39.5 dB khid =12 dB v c th tng ln ti 44 dB khi c thm pha-inh. Chnhlchsovitrnghpkhngcpha-inhl3.5dBtclkhong 10%. R rng nh hng ca pha-inh cng gim khi gi trd tng. V th khu vc a ng ( d ln) th hin tngmt lin lc ch yuldo s che khut. 3.3.2. Cc vng nhiu Cc vng nhiu v khng nhiu trong mi trng khng c pha-inh v che khut c ch ra trong Hnh 3.9(a). Khi c nh hng ca pha-inh v che khut th ranh gii gia cc vng nhiu v khng nhiu khng cn mt l ng n gin na v nhiu ng knh c th xy ra ngay cnh my pht cn -84- thu. Bin gii tm thi trong Hnh 3.11 s bin i bt thng v thay i khi tn hiu cn thu v tn hiu gy nhiu thay i. nghin cu mt cch nh lng,taxtvngphsngcxcxut1w iZ q = v / 2 niwi wdd| |= |\ ., ng gii hn vng nhiu vi mt xc sut c th no c v bng cch xc nh Z t Hnh 3.10 v tham chiu n theo khong cch bng cng thc: ( )2/ niwdqZd=(3.22) Qutchccimthomn(3.22)lnggiihnxcsutyucu, ngnycdngngtrntmti(-D/([qZ]4/n-1),0),bnknhl D[qZ]2/n/([qZ]4/n-1). Tt c nhng im ngoi ng trn ny s c xc xut nhiu ln hn cc im nm trong ng trn. Hn na, tm ca ng trn di s chuyn gn hn ti trm gc mong mun. Nh vy, xc sut nhiu thp ch xut hin trong mt khu vc nh xung quanh trm gc ny. Hnh 3.11 cho thy mt tp cc ng vin xc sut nhiu ng knh. Trng thi gy nhiu xu nht l khi my di ng nm mp . Hnh 3.11. Ranh gii vng nhiu vi cc xc sut nhiu khc nhau khi c pha-inh v che khut -85- Hstisdngknh(hoctns)QD,cnhnghaltsca khong cch gia cc ng knh trn bn knh ca ang xt: ( )2/1 1niDwd DQ qZR d= = + = +(3.23) trong,dw=Rngvitrnghpxunht.Vimhnhlcgic,kch thc cm N c quan h vi h s ti s dng tn s theo biu thc: ( ) | |3132/ 2 2 nDqZ QN+= = (3.24) Kch thc cm thc phi nm xung quanh gi tr xc nh trong (3.24), vi li hnh lc gic, kch thc ny c tnh bng i2+ij+j2 trong i v j l nhng s nguyn khng m. Cng thc (3.23) v (3.24) cng c th c s dng trong trng hp khng gian t do bng cch thay Z=1. Cng trnh [27] ch ra rng, vi yu cu nghim ngt hn v xc xut mt lin lc (Pout nh), pha-inh v che khut c th lm thay i mnh h s tisdng tn svkchthc cm (mu ti sdngtn s)khixuthin mt trm gy nhiu; kch thc cm ln hn so vi trng hp khng gian t do. 3.3.3. nh gi nh hng ca cc ngun nhiu ng knh trong thc t phntrc,tamigithitrngchcmtngunnhiungknh. Trong khi thc t l mt h thng thng tin di ng t ong chia lc gic c v s ngun nhiu. Tuy nhin, nhng ngun nhiu quan trng vn l nhng gn nht so vi ang xt. Lp utinca cc gy nhiu gmc6. Trong mc ny ta ch quan tm n 6 c cng khong D n ang xt ny vi gi s cc tn hiu ng knh l c lp v c tm quan trng nh nhau. Trong trng hp nhiu a ng, my di ng thu ng bao tn hiu rw t -86- trmgcmongmunvngbaork,k=1,...,NI=6tcctrmgynhiu. ng bao tng vi i ngun nhiu l: ==ikk ir R1(3.25) Nhiu ng knh s xy ra nu Ri rw/q. Do , xc sut mt lin lc vi tng nhiu ng knh l: =< =INki w outi p i qR r P P1) ( ) | ((3.26) trong : ( )w iP r qR i 1 phn b Suzuki hoc Rayleigh (xy ra vid =0) likhngcncdngnhvy.MuammarvGupta[29]gi thit rng tng ca i phn b Rayleigh c dng xp x phn b Gauss vi tr trung bnh:exp( )2diiiC = (3.27) v phng sai bng: 4( ) exp( ) 1diivar R iC (= ( (3.28) trong : C=10/ln10.Ngoi ra, Muammar v Gupta cng cho rng nu ta xt n tng (3.25), th mi phn b Suzuki c th xp x bng phn b log-chun khi ta cn chnh trtrungbnhvphngsaicanvigitrtngngcaphnbthc. Nh vy, hm mt xc sut trong (3.16) c th ly xp x bng: -87- | |222 1( ) exp( )2 2d dede derCp rr = (3.29) trong : ( ) / 4 ln Cdi de = (3.30) ( ) / 4 ln 42 2 2Cdi de+ = (3.31) l tr trung bnh v phng sai ca phn b log-chun tng ng.Lurnghs2C/rtrongcngthcp(r)phtsinhtphpbini JacobigiangbaothcrvbiudinbngdBcan(rd).Tngcai phnblog-chuncngxpxlmtphnblog-chunkhc[58]ctr trung bnh l: 222 ln4 4idede iC iC C = + + (3.32) v phng sai: 224 ln 1 ln2dei C exp i iC ( | | = + `(|\ . )(3.33) Xc sut c iu kin ca nhiu ng knh c th c tnh bng cch thay (3.29) vo (3.18), ri n gin ho [60]. Kt qu ta c: = < ) | ( i qR r Pi w =(((

||.|

\|++ +((

|.|

\|++212122 22121214 221212122KKerf eKKerfK (ch pha-inh) = dxdyy xy x K((

+ ||.|

\|((

2exp) , ( 4exp 12112 222(c pha-inh v che khut) (3.34) trong : = =4; log 20 ln21ii Z K Cd(3.35) -88- i dde idy xiCCZ K C + |.|

\|++ =22 2224ln4ln (3.36) d di dw dq Z = (3.37) trong d dwdi = = .Tch phn kp trong (3.34) c th vit di dng thch hp hn c th tnh s. Ta t: 04exp( ) 1 (41 164exp162223 222 2 2 2>((

+=||.|

\|((

+ =Ci ZiC CZ idde i d (3.38) cos , sin x t y t = = (3.39) cos , sind iCs Cs = = (3.40) th: 224d i x yexp exp expK C | |( | | = | (| |\ . \ .(3.41) | | ( ) exp exp stcos ( = + (3.42) Tch phn kp trn c rt gn thnh: 2/ 4/ 20 01 1( ) 12stcos tcost e ew iP r qR i te e e dtd (< = + (3.43) trong ta s dng tnh cht tun hon theo ca cos( + ) v gi tr ca hm cosbng 1 khi = .Sau khi tnh s phng trnh (3.43), ta v c th nh Hnh 3.12 vi i=1, 6 vd =6, 12 dB. Hnh v ny cng biu din xc sut nhiu ng knh vitrnghpchcpha-inh( d =0)theocngthc(3.34).Nhd on, nh hng kt hp ca pha-inh v che khut gy nhiu ln hn nhiu -89- so vi trng hp ch c pha-inh. Hn na, ng vi mt gi tr dxc nh, P(rw0dB.Tathyccngcongngviphnbgn nghunhtrngviccphnbchun,do cthsdngcngthc gn ng trong vic tnh ton nhiu ng knh. 0 10 20 30 40 50 601E-61E-51E-41E-30.010.110 10 20 30 40 50 601E-61E-51E-41E-30.010.11 P(rw