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I HC QUC GIA H NI TRNG I HC CNG NGH Nguyn Quang Vinh NGHIN CU H THNG MIMO-OFDM Ngnh:Cng ngh in t - Vin thng Chuyn ngnh: K thut in t M s:62.52.70 LUN VN THC S NGI HNG DN KHOA HC: GS.TSKH. HUNH HU TU H Ni - 2007

LI CM NTi xin by t s bit n n thy gio GS.TSKH. Hunh Hu Tu hngdn, ch bo tn tnh v dnh thi gian qu bu gip ti hon thnh lun vn ny.Ti cng by t s cm n n gia nh, c quan v bn b ng vin, h trv gip ti trong qu trnh hc tp v lm lun vn.i

MC LCLI CM N ..................................................................................................iMC LC ........................................................................................................iiDANH MC HNH .........................................................................................ivDANH MC CH VIT TT ......................................................................viM U ..........................................................................................................11. Tnh cp thit ca ti ..................................................................................2. Thc trng nghin cu MIMO-OFDM ...........................................................3. Mc tiu ca lun vn v hmng gii quyt ....................................................134CHNG 1: M HNH KNH MIMO .......................................................51.1. M hnh knh MIMO ...................................................................................1.2. Dung nng knh MIMO ...............................................................................1.2.1. Dung nng knh SISO, SIMO, MISO, MIMO .......................................1.2.2. Dung nng knh UT, IT .........................................................................1.3. M ha khng thi gian v ghp knh khng gian ......................................1.3.1. Cc k thut phn tp ............................................................................1.3.2. M ha khng gian - thi gian (STC) ...................................................1.3.3. Ghp knh khng gian (SM) ..................................................................1.3.4. So snh STC v SM ...............................................................................1.4. Mt s phmng php t hp u thu ............................................................1.4.1.T hp la chn (SC) ............................................................................1.4.2.T hp chuyn mch (SWC) ..................................................................1.4.3.T hp t s ti a (MRC) ......................................................................1.4.4.T hp khuch i ng u (EGC) ......................................................5881013131526292930303132ii

CHNG 2: H THNG MIMO-OFDM V C LNG KNHMIMO-OFDM ...........................................................................342.1. c trmng knh a mng .............................................................................2.2. H thng OFDM v nguyn l hot ng ....................................................2.3. Kt hp MIMO-OFDM ................................................................................2.4. lc lmng knh trong h thng MIMO-OFDM ..........................................2.4.1. Phng php c lng knh dng tn hiu hoa tiu (PACE) .............2.4.2.c lng knh trong h thng MIMO-OFDM ....................................343743454551CHNG 3: M PHNG ............................................................................. 553.1. Thit lp tham s m phng..........................................................................3.2. So snh cht lmng OFDM v MIMO-OFDM..............................................3.3. So snh LS v MMSE ..................................................................................3.4. nh hmng ca dch tn Doppler f ..........................................................d3.5. nh hmng ca tri tr rms ...........................................................................3.6. nh hmng ca s lmng mng truyn ........................................................3.7. nh hmng ca tri tr ti amax................................................................55565860626365KT LUN ...................................................................................................... 68TI LIU THAM KHO .............................................................................. 70PH LC A. L THUYT THNG TIN ................................................... 73PH LC B. MT S KHI NIM, NH NGHA V MA TRN ...... 78PH LC C. M NGUN CHNG TRNH M PHNG ................... 83iii

DANH MC HNHHnh 1-1.H thng MIMO c n ng-ten pht v n ng-ten thu ..........................TR5Hnh 1-2.Dung nng knh SISO, MISO, MISO ....................................................9Hnh 1-3.Dung nng knh MIMO ......................................................................... 10Hnh 1-4.Minh ha knh theo tr ring .................................................................. 11Hnh 1-5.Minh ha nh l y ..................................................................... 13Hnh 1-6.M ha STBC ......................................................................................... 16Hnh 1-7.S khi m ha Alamouti .................................................................. 19Hnh 1-8.M ha STTC ......................................................................................... 22Hnh 1-9.B m ha STTC vi trmng hp 2 ng-ten pht ................................... 23Hnh 1-10. S m lmi iu ch QPSK 4 trng thi vi 2 ng-ten ...................... 25Hnh 1-11. Cu trc D-BLAST ................................................................................. 26Hnh 1-12. Gii m D-BLAST ................................................................................. 27Hnh 1-13. Phmng php t hp la chn ................................................................. 30Hnh 1-14. Phmng php t hp chuyn mch ......................................................... 31Hnh 1-15. Phmng php t hp t s ti a .............................................................. 32Hnh 1-16. So snh cht lmng cc phmng php t hp u thu ............................. 33Hnh 1-17. T hp EGC vi s ng-ten thu khc nhau ............................................ 33Hnh 2-1.Phn b Rayleigh .................................................................................... 36Hnh 2-2.Phn b Ricean ....................................................................................... 36Hnh 2-3.Nguyn l iu ch OFDM .................................................................... 38Hnh 2-4.Dng sng ca mt k hiu OFDM ........................................................ 38Hnh 2-5.Nguyn l gii iu ch OFDM ............................................................. 39Hnh 2-6.Thm CP vo k hiu OFDM ................................................................. 40Hnh 2-7.Chn khong bo v trit tiu nhiu ISI ................................................. 41Hnh 2-8.S khi h thng thu-pht OFDM ..................................................... 42iv

Hnh 2-9.H thng MIMO-OFDM ......................................................................... 43Hnh 2-10. M hnh chn k hiu hoa tiu trong mt khung d liu OFDM ........... 46Hnh 2-11. lc lmng knh PACE 2x1D ................................................................. 49Hnh 2-12. H thng MIMO-OFDM ........................................................................ 52Hnh 2-13. Cu trc khung tn hiu MIMO-OFDM ................................................. 53Hnh 3-1.Phn b cng sut a mng theo hm e m .......................................... 56Hnh 3-2.Gin chm sao vi h thng OFDM ................................................. 57Hnh 3-3.Gin chm sao vi h thng MIMO-OFDM ..................................... 57Hnh 3-4.So snh cht lmng h thng OFDM v MIMO-OFDM ........................ 58Hnh 3-5.Gin chm sao vi mc lmng knh theo lut LS .............................. 59Hnh 3-6.Gin chm sao vi mc lmng knh theo lut MMSE ....................... 59Hnh 3-7.So snh cht lmng h thng khi mc lmng theo LS v MMSE ............ 60Hnh 3-8.Gin chm sao vi dch tn Doppler khc nhau .......................... 61Hnh 3-9.nh hmng ca dch tn Doppler ln cht lmng h thng ............... 61Hnh 3-10. Gin chm sao vi tri tr rms khc nhau ......................................... 62Hnh 3-11. nh hmng ca rms ln cht lmng h thng ......................................... 62Hnh 3-12. Gin chm sao khi s lmng mng truyn khc nhau ...................... 64Hnh 3-13. nh hmng ca s lmng a mng i vi cht lmng h thng ........... 64Hnh 3-14. Gin chm sao khi tri tr ti a thay i so vi CP ......................... 66Hnh 3-15. nh hmng ca tri tr ti a ln cht lmng h thng ........................... 66Hnh A-1. M hnh knh AWGN ............................................................................ 75v

DANH MC CC CH VIT TTAWGNAdditive White Gaussian NoiseTp m Gauss trng cng tnhBCE Blind/Semi-Blind ChannelEstimationlc lmng knh bng phmng phpmBERBit Error RateXc sut li bitBLASTBell Labs LAyered Space-TimePhn lp khng gian-thi gian dophng th nghim Bell xutCP Cyclic PrefixTin t vngD-BLAST Diagonal-BLASTBLAST phn lp choDDCEDecision-Directed ChannelEstimationlc lmng knh quyDFTDiscrete Fourier TransformBin i Fourier ri rcEGCEqual Gain CombiningT hp khuch i ng uFFTFast Fourier TransformBin i Fourier nhanhIDFTInverse Discrete Fourier Transform Bin i Fourier ri rc ngmcIFFTInverse Fast Fourier TransformBin i Fourier nhanh ngmcISI Intersymbol InterferenceNhiu xuyn k hiuLSLeast SquaredBnh phmng ti thiuMIMO Multi-Input Multi-OutputK thut x l nhiu u vo nhiuu raMLMaximum Likelihood(nh l) xc sut ging nhau cc iMMSEMinimum Mean Square ErrorTi thiu li trung bnh bnh phmngMRCMaximal Ratio CombiningT hp t s ti aOFDMOrthogonal Frequency DivisionMultiplexingGhp knh phn chia theo tn s trcgiaoPACEPilot-Aided Channel Estimationlc lmng knh dng tn hiu hoatiuRMSRoot Mean Squared(Tri tr) Cn trung bnh bnh phmngSCSelection CombiningT hp la chnvi

SMSpatial MultiplexingGhp knh khng gianSNRSignal to Noise RatioT s cng sut tn hiu trn tpnhiuSTBCSpace-Time Block CodeM khi khng gian thi gianSTCSpace-Time CodeM khng gian thi gianSTTCSpace-Time Trellis CodeM lmi khng gian thi gianSWCSwitched CombiningT hp chuyn mchV-BLAST Vertical-BLASTBLAST phn lp dcvii

1M U1. Tnh cp thit ca tiTrong s pht trin khng ngng ca ngnh thng tin v truyn thng, nht ltruyn thng khng dy, nhng dch v a phng tin l mt yu cu tt yu ca isng x hi. Tuy nhin, nhng thch thc ca cng ngh truyn thng khng phi lnh. N nh hng trc tip n cht lng cc dch v , ang v s c cung cpcho x hi. Khi m dch v gia tng c v mt s lng ln cht lng th nh hngy cng tr nn r rng, nghim trng hn.Th nht phi k n vn s dng tn s mt cch hiu qu. Nh bit, tns l ngun ti nguyn hn ch v c hoch nh v qun l rt cht ch. Mi hotng truyn thng khng dy d t hay nhiu u cn n mt di tn s nht nh thu-pht tn hiu. Nng cao hiu sut ph l vn nng khng ch ca ring ai,nay li cng tr nn cp thit hn bao gi ht.Th hai, vic tng tc truyn tin nhng khng th l l vic nng cao tincy thng tin nhm p ng li nhng yu cu ca dch v truyn thng a phngtin ngy cng pht trin mnh m. Mt khi tc v tin cy thng tin c cithin, cht lng dch v cng c nng cao.Th ba l thch thc n t hin tng pha-inh a ng gy ra. Trong mitrng truyn thng khng dy, tn hiu pht n c ni thu qua nhiu ng khcnhau do s phn x, khc x v tn x gy nn. Tn hiu thu c s gm nhiu tnhiu chng chp m mc thng ging cng v pha l khc nhau. Xt cho cngth khng phi do nhiu gy nn, m l mt dng ca tn hiu pht b bin dngv n cng mang thng tin ca tn hiu pht. Nu tn dng c nhng thng tin y nng cao cht lng bn thu th s tng ng k t s SNR.Mt u im chnh ca h thng truyn thng khng dy l kh nng di ng cacc thit b u cui thng tin. cng l mt thch thc cho qu trnh thu. Khi mythu v my pht chuyn ng tng i vi nhau bng vn tc khc khng, tn s timy thu s b x dch so vi tn s gc mt lng nht nh ty thuc vo vn tc dichuyn, ngi ta gi l hiu ng Doppler. gii quyt c vn tng tc v tin cy truyn tin, hin v angdng hai k thut chnh, l phn tp thi gian v phn tp tn s. Phn tp thi gianda vo nhng thng tin c pht lp li bn pht cn phn tp tn s da trn c

2s pht lp nhng di tn khc nhau. Tuy vy, c hai gii php u gy nn slng ph v tc truyn cng nh bng tn. Mt gii php k thut khc khcphc c phn no nhng khuyt im ca hai k thut trn, l phn tp khnggian hay cn c gi l phn tp ng-ten. Phng php ny s dng nhng thng tinpht nhiu ng-ten khc nhau m khng nh hng n s vi phm v di tn cngnh tc truyn thng tin. Tng t nh bn pht, ti bn thu cng c th s dngnhiu ng-ten gom li ti a nhng thng tin thu c quyt nh gii mnhng thng tin pht. H thng s dng nhiu ng-ten pht v nhiu ng-ten thuc gi l h thng MIMO - h thng nhiu li vo - nhiu li ra (Multi-Input Multi-Output).Thng qua b m ha khng gian-thi gian (STC), ghp knh thi gian (SM) bnpht v phng php t hp tn hiu u thu, h thng MIMO c th tng dung nngknh m khng cn tng di tn v cng sut pht. Bn cnh , vi vic m ha vgii m trc giao, thng tin phc hi ti bn thu c th tng tin cy, gim thiu liknh truyn. Cng vi vic tn dng thng tin knh truyn c c ti b thu, m mypht iu chnh cng sut pht hp l nng cao cht lng h thng.Nu nh nhng h truyn thng v tuyn mt ng-ten pht - mt ng-ten thu(SISO - Single Input Single Output) hin tng pha-inh a ng gy kh khn lncho b thu th vi h MIMO, n li c chuyn ha thnh tn hiu c ch, to nn mtli th nht nh. T , nng lng tn hiu thu c ci thin ng k t nhng tnhiu c ch, lm tng t s cng sut tn hiu trn tp nhiu v rt hu ch cho vicquyt nh tn hiu pht ban u.Tuy nhin, khi tn hiu c pht trong mi trng pha-inh a ng, tri tr doknh gy ra s rt ln. Khi mun tng tc truyn tin, ngha l phi gim chu k khiu ca thng tin pht. Lc , tri tr s ln hn chu k k hiu v gy ra knh suygim chn lc tn s, tn hiu s b mo dng v d gy ra nhiu xuyn k hiu ISI.Trong khi y l mt trong nhng u im ca k thut ghp knh phn chia tns trc giao (OFDM - Orthogonal Frequency Division Multiplexing). K thut nyphn chia di tn bng rng thnh nhng bng tn hp, pht tn hiu song song trnnhng bng con ny. Vi vic s dng tin t vng (CP - Cyclic Prefix), chu k tngk hiu pht trn cc bng con vn c m rng t l vi s bng con c chianay li cng ln hn. Chnh v vy, chu k k hiu pht s ln hn tri tr do knhtruyn to nn v knh truyn s tr nn phng hn, ngha l tn hiu truyn skhng b mo v trnh c nhiu xuyn k hiu ISI.

3Khng nhng vy, k thut OFDM chia di tn thnh nhng knh con c ditn hp hn, nhng cc di tn trc giao v c ghp xen k. Do vy hiu sut phtng ln ng k. Mt thun li khc phi k n trong k thut OFDM l vic cnbng knh c thc hin n gin hn v c thc hin cn bng trong min tn s.Nh vy, k thut MIMO tn dng thng tin d tha trong min khng gianlm tng dung nng knh v tin cy trong truyn tin. Nhng li vng phi mt vn v c tnh chn lc tn s ca knh trong mi trng a ng.Vi mt b sung hp l, k thut OFDM khc phc c nhc im y bngvic phn chia bng tn di rng thnh nhng bng hp m mi bng con l mt knhc c tnh phng, ngha l tn hiu truyn qua, knh ch c tc dng nh mt h snhn thng thng m khng h lm mo tn hiu. Bn cnh n cn c nhiu imu vit nh nng cao hiu sut ph do s b tr di tn an xen v lm gim phctp cn bng knh.S kt hp gia hai k thut MIMO v OFDM tr thnh mt ti rt cquan tm trong nhng nm tr li y v s hiu qu trong s dng ph cng nh tngdung nng, tc truyn tin v nng cao tin cy thng tin truyn dn.2. Thc trng nghin cu MIMO-OFDMMIMO l cng ngh mi c pht trin trong nhng nm gn y v n nhanhchng tr thnh mt ti rt c quan tm. c rt nhiu cng trnh khoa hc,lun vn cc cp nghin cu vn ny di nhiu gc .Vi cng ngh OFDM, nhng nghin cu cn lu, nhiu v y hn. iu th hin r v mt thi gian pht trin v ng dng vo i sng. Mt trong nhng ngdng ni bt nht l trong lnh vc pht thanh v truyn hnh s qung b.Vic kt hp hai cng ngh tin tin MIMO v OFDM thu ht s quan tm tcc nh khoa hc, cc cng ty, t chc trn khp th gii. N , ang v s cn phttrin nhiu hn na c v mt l thuyt ln trin khai ng dng mt cch rng ri. Tuyvy, do mi c pht trin trong vi nm tr li y, cng ngh kt hp MIMO-OFDM vn cn nhiu iu ng c quan tm, nghin cu.3. Mc tiu ca lun vn v hng gii quytMc tiu chnh ca lun vn l nghin cu k thut MIMO-OFDM mt cch ch thng. i t vic nghin cu h thng MIMO v nhng yu t lin quan ri n kthut OFDM v sau cng l kt hp MIMO-OFDM. Mt phn quan trng ca lun vnl vic nghin cu phng php c lng knh trong h thng MIMO-OFDM v

4dng matlab m phng cht lng phng php c lng knh dng tn hiu hoatiu (PACE). thc hin c nhng ni dung, mc ch ra nh trn, tin trnh lun vns c trnh by nh sau:Ban u lun vn phn tch u, nhc im ca k thut MIMO trong vic pli nhng thch thc trong truyn thng v tuyn hin ti v tng lai. OFDM l kthut c th khc phc c nhc im ca k thut MIMO v dn ti vic kt hpMIMO-OFDM l tt yu. Cng vi thc trng nghin cu ti ra nhng mcch thch hp m lun vn cn hng ti. l nhng g phi lm trong phn MU ny.Nhng vn lin quan n k thut MIMO nh dung nng knh SISO, SIMO,MISO, MIMO, m ha khng gian-thi gian, ghp knh khng gian, s c giiquyt trong chng 1, M HNH KNH MIMO.M u chng 2, H THNG MIMO-OFDM, s trnh by cc c trngknh a ng v nh hng ca n, tip n l nguyn l iu ch OFDM. Sau trnh by kt hp MIMO-OFDM trong cng mt h thng. Phn cui cng cng lphn rt quan trng. Phn ny s trnh by nhng phng php c lng knh cho hthng MIMO-OFDM m trng tm l phng php s dng k thut chn tn hiu hoatiu (PACE).T nhng phn tch l thuyt, tc gi s dng phn mm ng dng matlab mphng cht lng h thng dng phng php PACE trong chng 3, M PHNG.Cui cng l phn KT LUN tng kt nhng g thc hin v kt qu tc trong lun vn. ng thi xut mt s hng nghin cu ca tc gi trongtng lai.

5CHljNG 1. M HNH KNH MIMO1.1. M hnh knh MIMOGi s h thng MIMO ta xt gm n u vo tng ng vi n ng-ten pht vTTn u ra tng ng vi n ng-ten thu. H thng c th c m t trn hnh 1-1.RRTxRx 122,1h1,1hTnh,11,2h2,2hTnh,1TRnn ,h2,Rnh1,Rnh nTnR211y1x2y2xTnxRnyHnh 1-1. H thng MIMO c n ng-ten pht v n ng-ten thuTRKhi ch c mt ng-ten pht v mt ng-ten thu, h thng suy bin thnh h SISO(Single-Input Single-Output). Cn nu c n > 1 ng-ten pht v mt ng-ten thu th T l h MISO (Multi-Input Single-Output), ngc li nu ch c mt ng-ten pht v cn > 1 ng-ten thu th h thng suy bin thnh h SIMO (Single-Input Multi-Output).RVi m hnh trn, mi knh truyn dn gia mt cp ng-ten pht i v ng-ten thuj l mt knh v tuyn c p ng knh truyn l,ij,hRnj,,2,1,.Vy nn, h thng s cknh v tuyn thnh phn, ngha lknh SISO.Cc knh thnh phn ny c th c lp hoc tng quan vi nhau.Tni,,2,1RTnnRTnnTi mt thi im nht nh, cc tn hiuc pht trn tng ngtrn nTnxxx,,,21Tng-ten pht. Sau ti bn thu nhn c cc tn hiu. Khi, quan h gia tn hiu pht, tn hiu thu v p ng knh truyn c biu din nhsau:Rnyyy,,,21wHxy+(1.1)

6Trong ,,[]TnRyyyy21[]TnTxxxx21l vct tn hiu thu, pht. Khiu T pha trn biu th php chuyn v trong ma trn, c th y l chuyn tvct hng sang biu din dng vct ct.TRRTnnnnhhhhH,1,,11,1Ol ma trn tng ch knh;[TnRwwww21]l vc t tp m Gauss trng cng tnh, AWGN. Tng thnhphn,, c gi tr ngu nhin theo phn b Gauss vi tr trung bnhbng 0 v phng sai bng, cng c th k hiu dng.Rjnjw,,2,1,2),0(~2NwjNu N vctc pht lin tip, d liu thu c s l. Lc ny, mi quan h vo-ra c th c biu din di dng ma trnnh sau:NXXX,,,21NYYY,,,21WHXY+(1.2)Trong ,[]NYYY21Y,[]NXXX21X,[]NWWW21Wl ccthnh phn tn hiu thu, tn hiu pht v nhiu. biu din v thun tin trong qu trnh m phng tnh ton, ngi ta cngthng dng cch bin i qua li t ma trn (biu din bng mng 2 chiu) sanghng, ct d liu (biu din bng mng 1 chiu) v ngc li. Php bin i vct hac gii thiu trong ph lc B dng chuyn i t mng nhiu chiu sang mngmt chiu. Nu t,)(Yvecyvec)(Hvechvecv)(Wvecwvec, th biu thc (1.2)c th c vit li di dng nh sau:vecvecnTvecwhIXyR+(1.3)Trong ,l ma trn n v kch c nRnIRnRvl php nhn ma trnKronecker (xem ph lc B).Mt gi thuyt thng c s dng khi nghin cu v m hnh knh MIMO lcc phn t trong ma trn knh H c lp thng k vi nhau, cng c ngha l ccknh n c lp thng k vi nhau. Tuy nhin, trn thc t th cc thnh phn ykhng c lp vi nhau v nu 2 sng in t c ngun gc t 2 ng-ten khc nhau,c phn x bi cng mt vt th, cc h s lan truyn kt hp vi mi sng ny stng quan. Ni chung, nhng thnh phn ca H l tng quan bng mt lng ph

7thuc vo mi trng lan truyn cng nh s phn cc ha ca cc thnh phn ng-tenv khong cch gia chng.Mt m hnh kh thi vi H l tch s tng quan pha-inh h thng thnh 2thnh phn c lp, tng quan pht (transmit correlation) v tng quan thu (receivecorrelation), c m hnh nh sau [5,7]:(1.4)2/12/1twrRHRHTrong , Hwl ma trn m cc phn t ca n l bin Gauss v c lp cphng sai 1 v k hiul cn Hermitian bc 2 ca ma trn.2/1.Ma trn R xc nh s tng quan gia cc hng ca H, v c dng mrhnh s tng quan gia cc ng-ten nhn. Ma trn R c gi l ma trn tng quantpht v m hnh hip phng sai ca cc ct ca H tng ng.Mt iu quan trng khc ca knh v tuyn m khng th khng quan tm, lhin tng pha-inh a ng. Khi khng gian lan truyn sng m rng theo thi giancng nh khong cch, tn hiu pht khng ch truyn thng m mt phn b phn x,khc x, tn x bi mi trng. N lm cho tn hiu thu khng cn l duy nht mc tng hp, chng chp ca cc tn hiu n tr, c cng , pha l khc nhau. biu din c hin tng pha-inh a ng y vo trong mi lin h li vo-li ra bngma trn, trc ht ta biu din p ng knh truyn ng gia my pht th i v mythu th j bng:[])0()1(,,,ijijijhLhh(1.5)Trong , L l tr ln nht trong knh a ng,1,,1,0),(,Lkkhijl tng ch vi tng thnh phn a ng.T , biu thc lin h gia tn hiu thu v tn hiu pht c th c vit didng:(1.6))()()()(1,1,,11,1kkkkTTRRTnnnnnwxxhhhhy+OTrong ,(1.7)[]TTjjjnjkxLkxk,,2,1,)()1()(+x[]TnkwkwkwkR)()()()(21w(1.8)

81.2. Dung nng knh MIMO1.2.1. Dung nng knh SISO, SIMO, MISO, MIMONm 1948, Shannon ln u tin xut dung nng knh cho knh nhiu Gausstrng cng tnh (AWGN) c gi tr bng (ph lc A): +2021logPBCbits/s(1.9)Trong , B, P v02l di thng knh truyn, cng sut pht v cng sutnhiu.Bng vic thc hin phn tp khng gian, h thng MIMO c th ci thin ngk cht lng truyn thng (t l bt li BER) cng nh tc truyn dn (bits/s).Trc tin, chng ta s kho st dung nng knh MIMO vi s lng ng-ten pht, thukhc nhau v suy bin SISO, SIMO, MISO. Dung nng knh SISOVi1RTnn, h thng trn hnh 1-1 tr thnh h SISO. Lc ma trn knhsuy bin thnh h s nhn,. Vy nn dung nng h thng SISO tr thnh:hH +22021loghPCSISObps/Hz(1.10)T cng thc (1.10) d nhn thy dung nng knh SISO tng rt chm vi hmloga ca t s tn trn tp m. Hn na, suy hao pha-inh c th gy ra dao ng mnhv mc cng sut tn hiu, tng phng sai hay cng sut nhiu. y, m hnh mich x l tn hiu trong min thi gian v tn s m min khng gian cha c niti. Dung nng knh SIMOVi, h thng tr thnh h SIMO v trn knh suy bin thnhvct knh,. Lc ti bn thu c nhiu ng-ten thu cng mt tnhiu t ng-ten bn pht. Cha k n vic x l tn hiu ti bn thu, nng lng thuc tng ln ng k. Vy nn dung nng knh SIMO c tnh bng:2,1RTnn)1(TnhH +RnjjSIMOhPC122021logbps/Hz(1.11)

9trong , h l h s tng ch phc ca knh t ng-ten pht n ng-ten thu th j.jTrong trng hp ny, dung nng knh tng theo hm loga ca c t s SNR v slng ng-ten thu n . Do vy dung nng knh SIMO chc chn s ln hn trng hpRh SISO. Cht lng thc ca h ph thuc vo bn cht ca knh v tng quangia cc ng-ten. Dung nng knh MISOVi, h thng tr thnh h MISO v lc ny ma trn knh cngsuy bin thnh vct knh,. Dung nng h MISO c dng:1,2RTnn)(nRhH +TniiTMISOhnPC122021logbps/Hz(1.12)trong , h l h s tng ch phc ca knh t ng-ten pht th i n n ng-ten thu.iHnh 1-2. Dung nng knh SISO, MISO, MISONhn vo cng thc dung nng trng hp SIMO v MISO ta d nhn thy rngCMISO< CSIMOv iu ny d dng chng minh. Nu c 2 trng hp u c mc phntp hay s ng-ten bng nhau, ngha l n (MISO) = n (SIMO) = n th biu thc tngTR tng ch nm 1mh2u bng nhau, ch khc nhau t s tn trn tp nhiu hay ychnh l cng sut pht ca mi ng-ten. Do gii hn v tng cng sut pht, nn trong

10trng hp SIMO cng sut pht ch dnh ring cho 1 ng-ten v bng P cn trng0hp MISO, cng sut phi chia u cho n ng-ten v bng P /n. Vy nn dung nng0 knh MISO nh hn dung nng knh SIMO. Nhn xt ny cng d dng c minhha bng th m phng cc knh SISO, SIMO v MISO nh hnh 1-2. Dung nng knh MIMONh ch ra trong mc lc A, dng tng qut ca dung nng knh MIMO l:+HMIMOHPHIC221detlogbps/Hz(1.13)trong , I l ma trn ng nhtTRnn, P l ma trn hip phng sai ca vct tnhiu pht, HHl ma trn chuyn v, lin hp phc ca H. Hnh 1-3. Dung nng knh MIMO1.2.2. Dung nng knh UT, ITBn pht khng bit thng tin knh (Uninformed-Transmitter)Khi khng bit thng tin v knh ti bn pht, cng sut pht ti cc ng-ten sc pht u. Khi , dung nng knh MIMO c dng:+HToUTHHnPIC22/detlogbps/Hz(1.14)

11 tm hiu nhng c tnh ca H, chng ta c th thc hin phn hoch ma trnH theo gi tr k d (SVD) cho ha v tm gi tr ring. SVD ca mt ma trn H cbt k c th c vit nh sau:TRnn(1.15)HUDVH)(RRnnUvl ma trn tha mn iu kin:.l ma trn khng m, v ng cho c gi tr:)(TTnnVIVVUUHH)(TRnnD )0,,0,,,,(21mdiagD(1.16)vi diag(A) l vct bao gm cc phn t ng cho ca A,1,2, ...,ml cc trring khng m ca, m = min(nRn ) vT(1.17)RTHTRHnnHHnnHH,, Hnh 1-4. Minh ha knh theo tr ringCc ct ca U l vct ring ca HHHv cc ct ca V l vct ring caHHH. Php bin i SVD ch ra rng ma trn H c th cho ha thnh mt s lngknh con c lp trc giao, c cng sut ng vi knh th i li.Do , ta c th vit li nh sau:wxDy~~~+(1.18)vixVxyUyHH~,~vwUwH~(1.19)Tng t, phng trnh (1.14) c th vit li:+miiToUTnPC122/1logbps/Hz(1.20)vi1,2, ...,ml cc tr ring khc khng ca ma trn Wishart[5].

12Bn pht bit thng tin knh (Informed-Transmitter)Khi bit thng tin knh ti bn pht, ngi ta p dng phng php y(waterfilling) ti u ha cng sut tn hiu pht. nh l y ch ra rng cnphn chia tng cng sut pht vo tng knh con. Vi knh c tng ch cao th cngsut pht cn phi ln, vi nhng knh tng ch thp th cng sut pht thp i vthm ch l bng 0 v cng sut pht b hn ch:(1.21)01PPTniiVi P l cng sut pht ti ng-ten i v P l tng cng sut pht. Dung nngi0knh MIMO khi c xc nh nh sau: +TniiiITPC1221log(1.22)Theo phng php nhn Lagrange, ngi ta a ra hm [5]:++TTniiniiiPPLPZ101221log(1.23)Trong L l tham s nhn Lagrange vth hin mc sai lchcng sut pht,TniiPPL10il gi tr k d th i ca ma trn knh v2l cng sut nhiu. Cccng sut pht thnh phn c xc nh thng qua o hm ca Z: 0iPZ(1.24) 0/1/2ln122+LPPZiiii(1.25)Nu t 2ln12Lth cng sut thnh phn P c th xc nh bng:i12iiP(1.26)nh l y c minh ha trn hnh 1-5. [21]

13 Hnh 1-5. Minh ha nh l yD nhn thy rng, mt s thnh phn c nhiu qu cao nn cng sut pht bng0. th hin iu , biu thc (1.22) c vit li nh sau:(1.27)[]+miiITC12logTrong , ch nhng thnh phn dng v+)(1,2, ...,ml cc tr ringcavi m=min(N ,N ).TRSo vi m hnh pht cng sut ng u, pht tn hiu theo nh l y cli ch ng k, c bit ti vng c SNR thp. Tuy nhin li ch ny khng cn huhiu nhiu vi vng c SNR cao.1.3. M ha khng gian-thi gian v ghp knh khng gian1.3.1. Cc k thut phn tpTrong truyn thng di ng khng dy, cc k thut phn tp c dng rng ri gim tc ng ca pha-inh a ng v ci thin tin cy ca truyn dn mkhng tng cng sut pht hay bng thng. Phn tp nhm to ra cc bn sao tn hiupht ti ni thu, tt c u mang cng thng tin nhng s tng quan v thng k pha-inh l nh. tng ca phn tp l to ra hai hay nhiu mu c lp ca tn hiuc pht i v suy gim trn ng truyn vi mc suy gim khc nhau. iu nyc ngha l xc sut ton b cc mu ng thi thp hn ngng cho trc l nhhn xc sut ca mi mu ring bit nh hn ngng y. Do vy, mt t hp thch hpca nhng mu tn hiu y lm gim nh hng ca pha-inh v do vy ci thin c tin cy trong truyn dn.Da vo c tnh phn tp theo min m ngi ta phn loi cc k thut phn tpthnh 3 loi: Phn tp thi gian, phn tp tn s v phn tp khng gian.

14Phn tp thi gianPhn tp thi gian c th t c bng cch pht nhng mu tin ging nhautrong cc khe thi gian khc nhau, kt qu l c c cc tn hiu pha-inh khngtng quan ti u thu. Yu cu ca phng php ny l khong thi gian gia cc lnpht bn sao phi t nht bng thi gian kt hp ca knh. Trong truyn thng di ng,m sa sai c kt hp vi b xo trn t c phn tp thi gian. Trong trnghp ny, nhng bn sao ca tn hiu pht thng a ti bn thu di dng d thatrong min thi gian bng b m sa sai. Khong thi gian tch bit gia cc bn saoca tn hiu pht c to ra bng b xo trn thu c cc pha-inh c lp ti livo ca b gii m. V thi gian xo trn dn ti gii m tr, k thut ny thng rthiu qu vi mi trng pha-inh nhanh (tc di chuyn ln) khi m thi gian kthp l nh. Vi knh pha-inh chm, mt b xo trn ln c th dn ti tr rt ln vkhng th dng cho nhng ng dng thi gian thc nh video, m thanh, Chnh vvy, phn tp thi gian i khi cng khng th p dng trong mt s h thng di ng.V d, khi trm thu pht trng thi dng, phn tp thi gian khng th gip gimc suy hao pha-inh. Mt nhc im na l m hnh ny to ra d tha minthi gian, ngha l lm lng ph bng thng.Phn tp tn sTrong phn tp tn s, mt vi tn s c dng pht cng mt tn hiu. Cctn s cn cch nhau mt khong ln hn hoc bng bng thng kt hp to ra ccc pha-inh c lp. Bng thng kt hp s khc nhau vi cc mi trng khc nhau.Trong nhng h truyn thng di ng, nhng bn sao tn hiu pht c a ti nithu di dng d tha trong min tn s bng tn hiu tri ph. Cc k thut tri ph shiu qu khi bng thng kt hp ca knh l nh. Tuy nhin, khi bng thng kt hpca knh ln hn di thng tn hiu tri ph, tri tr a ng s nh so vi chu k khiu (knh phng). Trong trng hp ny, tri ph s khng hu hiu trong vic phntp tn s. Cng ging nh phn tp thi gian, phn tp tn s lm gim hiu sut phdo c d tha trong min tn s.Phn tp khng gianPhn tp khng gian l k thut ph bin trong truyn thng vi sng khng dyv cn c gi l phn tp ng-ten. K thut ny s dng nhiu ng-ten hay nhngdy ng-ten sp xp cng nhau trong khng gian truyn hay nhn tn hiu. Nhngng-ten ny c t cch nhau mt khong thch hp cc tn hiu trn tng ng-ten khng tng quan. Khong cch ny thay i theo cao ng-ten, mi trng lantruyn v tn s. Thng th khong cch ny bng khong mt vi bc sng l

15c c nhng tn hiu khng tng quan [5]. Trong phn tp khng gian, nhng bnsao ca tn hiu pht thng c gi ti my thu di dng d tha trong minkhng gian. Khng nh phn tp thi gian v phn tp tn s, phn tp khng giankhng lm suy gim hay mt mt v hiu sut ph. Tnh cht ny cho thy y l kthut thch hp vi s pht trin ca cng ngh truyn thng v tuyn tc d liucao trong tng lai.Phn tp phn cc v phn tp gc l 2 v d v phn tp khng gian. Trong phntp phn cc, cc tn hiu phn cc ngang v phn cc dc c pht bi 2 ng-tenphn cc khc nhau v nhn bi 2 ng-ten phn cc khc. Cc phn cc khc nhaum bo rng 2 tn hiu l khng tng quan m khng cn t ng-ten khong cchxa. Phn tp gc thng c dng cho truyn dn c tn s sng mang ln hn10GHz. Trong trng hp ny, v nhng tn hiu pht b phn tn nhiu trong khnggian, nhng tn hiu thu t cc hng khc nhau l c lp vi nhau. Do vy, 2 haynhiu ng-ten nh hng c th c t theo nhng hng khc nhau bn thu nhn c cc bn sao c lp ca tn hiu pht.Ty thuc vo nhng ng-ten c dng cho vic pht hay thu, chng ta c thphn loi phn tp khng gian thnh 2 loi: phn tp pht v phn tp thu. Trong phntp thu, nhiu ng-ten c dng ti bn thu thu gom cc bn sao ca tn hiupht. Nhng bn sao ny c t hp thch hp tng t s SNR v kh bt pha-inha ng. Trong phn tp pht, nhng ng-ten c dng ti bn pht. Nhng thngtin c x l v pht trn cc ng-ten. Phn tp pht s c ni chi tit hn phnsau.Trong h thng truyn thng thc t, t c nhng yu cu v cht lng hthng, c th phi kt hp 2 hay nhiu k thut phn tp gi l phn tp a chiu(multidimensional diversity). V d, trong h thng mng t ong GSM, nhiu ng-tenthu trm c s c dng kt hp vi vic xo trn v m iu khin li (errorcontrol coding) ng dng c 2 k thut phn tp khng gian v phn tp thi gian.C 2 k thut chnh thc hin c vic phn tp ng-ten trong h thngMIMO l m ha khng gian-thi gian (STC - Space-Time Coding) v ghp knhkhng gian (SM - Spatial Multiplexing).1.3.2. M ha khng gian - thi gian (STC)Chng ta xt mt h thng truyn thng m khng gian-thi gian c n ng-tenTpht v n ng-ten thu. D liu pht c m ha bng mt b m ha khng gian-thiRgian. Ti thi im t, mt khi m k hiu thng tin nh phn

16mtttcccc,,,211(1.28)c a vo b m ha khng gian thi gian. B m ha khng gian-thi gian nh xkhi d liu m bt nh phn vo n k hiu iu ch t mt tp MTm2im. Tc d liu m ha c chuyn i t ni tip sang song song, to ra chui n k hiuTsong song, c xp thnh mt vct ct nT1TntttTxxxx,,,211(1.29)k hiu T pha trn l chuyn v ma trn. Li ra song song n c pht ng thi bngTn ng-ten khc nhau, trong k hiuT, c pht ng-ten i v ton bcc k hiu pht cng thi gian pht TTitnix1,S. Vct cc k hiu m ha c pht ti thiim t trn cc ng-ten c ch ra trong biu thc (1.29) cn c gi l k hiukhng gian-thi gian.M khng gian-thi gian c phn ra lm 2 loi: m khi khng gian-thi gian(STBC) v m li khng gian-thi gian (STTC). STBCM ha STBCHnh 1-6 ch ra cu trc b m ha STBC. Ni chung, mt m STBC c nhngha bi ma trn truyn dn X, kch cpnT. y n l s ng-ten pht v p l sTchu k k hiu pht ca mt khi m. iu ch M khikhng gian-thi gian TnxTx nT1xTx 1 Ngun tinHnh 1-6. M ha STBCGi s rng chm sao tn hiu bao gm 2mim. Ti mi thi im m ha, mtkhi gm km bt thng tin c nh x vo chm sao tn hiu to ra k tn hiu iuch.k tn hiu c m ha bng b m ha STBC to ra nkxxx,,,21Tchui tnhiu song song c chiu di p theo ma trn truyn dn X. Cc chui tn hiu ny cpht ng thi qua n ng-ten trong p chu k k hiu.TTrong m STBC, s lng k hiu li vo ca STBC trong mt ln m ha l k.S chu k cn pht m khi trn nhng ng-ten pht l p. Ni cch khc, c p khiu khng gian-thi gian c pht t mi ng-ten cho mi khi gm k k hiu u

17vo. Tc ca m STBC c nh ngha bng t s gia s k hiu em m hav s chu k pht ht khi trn cc ng-ten: pkR(1.30)Hiu sut ph ca m STBC l: pkmrmRrBrssbbits/s/Hz(1.31)vivl tc bt v tc k hiu, v B l di thng.brsrLi vo ca ma trn truyn dn X l t hp tuyn tnh ca k k hiu iu chv lin hp phc. t c mc phn tp pht y nkxxx,,,21kxxx,,,,21T,ma trn truyn dn X c xy dng da trn tnh trc giao nh sau: TnkHIxxxcXX22221+++(1.32)vi c l hng s, XHl bin i Hermitian ca X vl ma trn n vTnITTnn.Hng th i ca X chnh l nhng k hiu pht t ng-ten th i mt cch lin tip trongp chu k, trong khi ct th j ca X ch th nhng k hiu pht ng thi qua n ng-tenTpht ti thi im th j. Phn t ca X trong hng th i v ct th j, k hiu l,(,) l tn hiu pht t ng-ten i ti thi im j.ji,xTni,,2,1pj,,2,1T ch ra rng tc ca m STBC c phn tp pht y nh hn hocbng 1, R1. M c tc R=1 th khng cn m rng bng tn, nhng khi m c R2) k hiu th phi mt p > n chu k.TTSTTC cng l m c li phn tp y v tc m ha cao. Bn cnh n cn c c li m ha, nhng thcCc phng php ghp knh khng gian lun mang li li v tc m ha,nhni phn tp, cc k hiu ca mt lp ch c pht i trn cng mt ng-ten. Hnth, n lun i hi phi c s ng-ten thu phi ln hoc bng s ng-ten pht. Cutrc D-BLAST tuy c hi phc tp trong vic m ha v lng ph i cht trong 2 ucui ca khi tin, nhng li khng yu cu v s lng ng-ten thu nhiu hn ng-tenpht v c bit l c c li phn tp.1.4. Mt s phmkng php t hp u thu [5]thi gian v ghp knh. hon thin qu trnhng php t hp ti bn thu c c tn hiu quyt nh trc khi gii m.Ni chung, cht lng ca h thng truyn thng vi cc k thut phn tp phthuc vo vic c bao nhiu bn sao tn hiu pht c t hp ti bn thu lmSNR. Do , nhng m hnh phn tp c th cng c phn loi theo ccphng php phn tp s dng ti ni thu. Theo phc tp trong thc hin v mc thng tin knh yu cu ti bn thu m c th phn loi thnh 4 loi k thut t hp

30combining - SWC), t hp khuch i ng u (equal-gain combining - EGC) v thp t s ti a (maximal ratio combining - MRC).1.4.1. T hp la chn (SC)T hp la chn l mt phng php t hp phn tp n gin. Xt mt h thngc nng-ten thu. Trong h thng ny, tn hiu c t s SNR cao nht ti mi chu ki ra, ngha l tn hiu ny ng vi tn hiu ti tt nht.TronHnh 1-1n1.4.2. T hp chuyn mch (SWC)Trong mt h th cc nhnh phn tpv la chn mt nhnh c th c SNR ln hn mt ngng cho trc. Tn hiu nyNR ca nhnh nh hn ngng. Khi , b thuli byu cu gim st cc nhnh lin tc v ng thi.iuRk hiu c la chn lm lg thc t th tn hiu n l tng ca tn hiu gc v nhiu, S+N, s c la chnv o c SNR l rt kh.Rx 1Rx 2Rx nR3.Phng php t hp la chng t hp chuyn mch , b thu qut ton bc chn lm li ra, cho n khi St u qut li v chuyn sang nhnh khc. M hnh ny cng cn c gi lphn tp qut (scanning diversity).So vi phn tp la chn, phn tp chuyn mch c cht lng km hn vphng php SWC khng chn lc tn hiu tt nht mt cch lin tc. Tuy nhin, nli thc hin n gin hn v khngVi c 2 m hnh phn tp la chn v phn tp chuyn mch, tn hiu li ra chbng mt trong nhng nhnh phn tp m thi. Thm na, chng li khng yu cu btk mt thng tin no v knh. Do , 2 m hnh ny c th s dng kt hp vich c lin kt cng nh iu ch khng c lin kt. LA CHN SNR TI A Tin x l& m Tin x l& m Tinl& m . . . Li ra1r2rRnrx

31Rx 1Rx 2 Tin x l& m Tin x l& m Tin x l& m QUT GIR VT NGTR SNNGRx nRRnr2r1r. . . Li raHnh 1-14. Phng php t hp chuyn mch1.4.3. T hp t s tT hp t s ti a l mt phng php t hp tuyn tnh. Trong mt b x l tn hiu li vo c nh trng s ring v cng vinhaui 1(1.73)ii a (MRC)hp tuyn tnh ni chung, cc t lm thnh tn hiu li ra. Nhng h s c th c la chn theo mt vi cch.Tn hiu li ra l mt t hp tuyn tnh ca cc bn sao ca tn hiu nhn c:Rniirrvir ,il tn hiu thu v h s nhn ca nhng h s nhn ca cc ng-ten thu c chn t l vi t s cng sut tn hiu trns tng-ten th i. Trong t hp t s ti a,cng unhiu. t A viil bin v pha ca tn hiu thu r . Gi s mi ng-tenic cng cng sut nhiu trung bnh, h s nhnic th biu din di dng:ijiieA(1.74)Phng php ny c gi l t hp ti SNR li ra ti a bng tng t s SNR ca tl l tng cc tnhiuu v n c th ti a ha SNR li ra. Tsng nhnh ring bit.Trong m hnh ny, mi tn hiu ring bit phi c lm ng pha v nhn mth s tng ng vi bin ca mi nhnh. Tn hiu sau khi xca cc nhnh c ng pha v nhn h s. Trong m hnh ny yu cu thngtin v pha v bin ca knh pha-inh. Vy nn, n c th phi kt hp vi vic dknh c lin kt, nhng khng th thc hin c vi d knh khng lin kt.

32 + Tin x l& m Tin x l& m Tin x l& m Rx 1Rx 2Rx nRRnr2r1r. . . 12RnLi raHnh 1-15. Phng phpt s ti a1.4.4. T hp khuch i ng u (EGC)qual Gain Combining) l mt phng phpt h(1.75)Theo phng php ny, ton b tn hih song hnh 1-16. Khich ththu tng ln th cht lng cng tng theo.Trt hpT hp khuch i ng u (EGC - Ep tuyn tnh ti u. N khng yu cu thng tin v bin pha-inh ca minhnh. H s nhn vi mi nhnh l 1.ijieu thu c lm ng pha v cng li vinhn bng nhau. Tuy cht lng khng bng so vi phng php t hp t s tia nhng phc tp trong vic thc hin li n gin hn nhiu.Cht lng cc phng php t hp u thu c so snh tru n l v ly trung bnh li bt cht lng s rt km. Cht lng tng dn lnkhi dng phng php t hp la chn, t hp khuch i u v t hp t s ti a.Phng php MRC cho cht lng tt nht, nhng ci gi phi tr l s phc tp trongkhi thit k b thu. Phng php EGC cho cht lng km hn cht t, nhng phctp li gim i do ch phi ng b v pha.Vi mi phng php, khi s ng-tenng hp s dng phng php EGC c minh ha trong hnh 1-17. Khi s lngng-ten tng t 2 ln 4 th cht lng tng ng k. Nhng khi tng t 6 ln 8 ng-tenth cht lng ci thin t hn. V n khi ng-ten tng n s lng nht nh chtlng s t n gii hn.

33 Hnh 1-16. So snh cht lng cc phng php t hp u thu Hnh 1-17. T hp EGC vi s ng-ten thu khc nhau

34CHlNG 2. H THNG MIMO-OFDM2.1. c trmng knh a mngTrong thng tin di ng v tuyn, ngoi vic gii quyt vn nhiu trn mngtruyn tin cn gii quyt nhiu vn pht sinh t mi trmng truyn nhm: mythu/pht di chuyn; mng truyn pht-thu b chn; tn hiu b phn x, khc x, tn xtrmc khi ti mc my thu. Do vy tn hiu nhn mc ti b thu l t hp ca nhiumng truyn, cc tn hiu c th mnh, yu khc nhau hay thi gian truyn nhanh,chm khng ng u hay thm ch tn s tn hiu b dch i i cht. c trmngcho nhng yu t tc ng y, ngmi ta ma ra hai thng s, tri tr v dch tnDoppler, gip cho vic nh gi knh mc st thc hn.Thng s tri tr l i lmng th hin mc phn no tr cng nhm cmng trung bnh ca knh truyn. i lmng cn trung bnh bnh phng tri tr (thmngmc gi l tri tr rms) mc tnh ton da vo bin v thi gian tr ca cc mngtruyn nhm sau [4]: 22)()()()(kkkkkkkkkkPPPPWWWWWWVW(2.1)Trong ,WVtri tr rms,)(kPWl cng sut ti thi gian trkW.Bn cnh , tr tri cc i (X dB) cng mc nhc n nhm l i lmng thhin mc nh hmng ca knh truyn. Tr tri cc i l thi gian tr m nnglmng a mng gim thp hn X dB. V do s phn x l v hn, nn khng th chthu thu ht tn hiu phn x ri gii m m cn chp nhn ct mt phn tn hiuphn x qu yu (nh hn X dB) m bo thi gian.Cc thng s tri tr mi ch l c trmng knh v mt thi gian, cn bng thngkt hp l c trmng cho knh v tn s. l di tn trn knh c th coi l bngphng (cho tn s i qua vi h s bng nhau v pha tuyn tnh). Ni cch khc, ldi tn m 2 tn s nm trong khong ny c tmng quan ln v bin , cn cchnhau ln hn s chu nh hmng khc nhau ca knh. Mi lin h gia bng thng kthp v tri tr rms c th xc nh bng biu thc:WVD1)( fc(2.2)

35Trong ,Dl h s t l ph thuc vo mi trmng, v nhm nu hm tmng quanln thD= 50, cn tmng quan 0,5 thD= 5. [4]Tri tr v bng thng kt hp l cc thng s m t bn cht phn tn thi gianca knh trong mt vng cc b, tuy nhin n vn khng cho thng tin v s thay itheo thi gian ca knh do s chuyn ng ca my thu i vi my pht. dch tnDoppler l thng s biu th s di chuyn y. Khi mt tn spht ti my thu, mmy thu chuyn ng tmng i vi my pht vn tc v, hmng di ng lch gcthtn s thu mc s dch i:cf coscos'cccdvfcvfff(2.3)Vi0, ngha l my thu di chuyn hmng thng v pha my pht th dchtn Doppler l ti a, vcdvfmax.Nu v l vn tc ti a ca my di ng th tn s thu mc s nm trong khong[)()(maxmaxdcdcffff]+. Cng vi , thi gian kt hp l mt i lmng biu thcho s tc ng ca dch tn Doppler v mt thi gian, t l nghch vi dch tnDoppler:max1)(dcftE(2.4)El h s t l, ty thuc vo tnh cht ca mi trmng truyn.Thi gian kt hp chnh l khong thi gian trong p ng xung ca knh cth coi l khng i. Ni cch khc, khi 2 tn hiu cch nhau mt khong nh hn thigian kt hp s c tmng quan bin ln. Nu nghch o rng ca tn hiu bngc s ln hn thi gian kt hp knh th knh s thay i nhanh trong thi gian bn tingy nn mo ti b thu, cn gi l knh suy gim nhanh. Ngmc li, khi nghch o rng bng c s nh hn thi gian kt hp knh th gi l knh suy gim chm.Mt iu quan trng khng km nhng thng s ca knh truyn l dngphn b xc sut ca knh truyn. C 2 m hnh knh truyn thmng mc xt n, l phn b Rayleigh v phn b Ricean.Phn b Rayleigh c hm mt xc sut l:

36 !000,2exp)(222rrrrrpVV(2.5) 02VV3V4V5V rV6065.0 p(r)Hnh 2-1. Phn b RayleighM hnh phn b Rayleigh mc p dng cho trmng hp trong cc thnh phnkhng c mng mnh tri. Cn khi c mt mng tri, v d nhm mng truyn thng(LOS), m hnh xem xt l phn b Ricean c hm mt xc sut nhm sau: !!000,0,2exp)(202222rrAArIArrrpVVV(2.6)p(r) dBK6dBKrHnh 2-2. Phn b Ricean

37 y, thng s A l bin nh ca thnh phn tri,)(0Il hm Bessel loi 1bc 0. Phn b Ricean thmng mc m t bi thng s K, nh ngha nhm l t s giacng sut tn hiu xc nh v phmng sai ca thnh phn a mng:222VAK(2.7a) dBAdBK222log10)(V(2.7b)Thng s K mc hiu nhm l nhn t Ricean, xc nh hon ton phn b Ricean.KhidBKA,0tc l khi thnh phn tri gim bin th phn b Riceantr thnh phn b Rayleigh. Trong m hnh ca chng ta chmng sau s dng phnb Rayleigh lm m hnh knh a mng.2.2. H thng OFDM v nguyn l hot ngOFDM l nm trong mt lp cc k thut iu ch a sng mang (MCM) trongthng tin v tuyn. Cn trong cc h thng thng tin hu tuyn chng hn nhm trongh thng ASDL, cc k thut ny thmng mc nhc n dmi ci tn: a tn (DMT).K thut OFDM ln u tin mc gii thiu trong bi bo ca R.W.Chang nm 1966v vn tng hp cc tn hiu c di tn hn ch khi thc hin truyn tn hiu quanhiu knh con. Tuy nhin, cho ti gn y, k thut OFDM mi mc quan tm nhc nhng tin b vmt bc trong lnh vc x l tn hiu v vi in t.iu ch v gii iu ch OFDM tmng chnh trong k thut OFDM l vic chia lung d liu trmc khi pht ithnh N lung d liu song song c tc thp hn v pht mi lung d liu trnCmt sng mang con khc nhau. Cc sng mang ny l trc giao vi nhau, iu nymc thc hin bng cch chn dn cch tn s gia chng mt cch hp l. Hnh2-3 m t nguyn l ca qu trnh to mt k hiu OFDM. Tt c cc thao tc trongmin mc ng khung u c th mc thay th bng php bin i IDFT.Cc sng mangn(t)l cc sng hnh sin c th mc biu din dmi dng lutha nhm sau: [[]TtTteTtntNWinC,00,01)(2](2.8)

38 xNc-1,m(t)x1,m(t)x0,m(t)S (t)m 0(t)1(t)Nc-1(t)IDFTHnh 2-3. Nguyn l iu ch OFDMTn s ca cc sng mang hn km nhau mt khong W/N Hz, trong W l Crng di tn. Mi sng mang mc nhn vi mt gi tr phc xn,mly t d liu uvo; ch s dmi n tmng ng vi ch s ca sng mang, v m l ch s ca ton b khiu OFDM (cn gi l khung OFDM). Mi tn hiu s (t) tmng ng vi mt immtrong khng gian Euclid N-chiu gi l khng gian tn hiu, mi im mc biu dinbi mt b cc gi tr (x , , x0m1,m, ..., xNc-1,m). Mt tp hp M im trong khng gian N -Cchiu ny mc gi l chm tn hiu (signal constellation). Cc im nm trong chmtn hiu ny c th l u ra sau khi thc hin php iu ch c s M bt k. Cc ktqu c mc sau khi thc hin php nhn s mc cng li v tn hiu cui cng s ldng sng mc truyn i qua knh.(2.9)10,)()(CNnnmnmmTtxtS Cc sng con trc giaoHnh 2-4. Dng sng ca mt k hiu OFDMNhm vy, chui v hn cc k hiu OFDM c th mc biu din:

39(2.10)mNnnmnmmCmTtXtStS10,)()()(Don(t)l mt xung vung mc iu ch ti tn s sng mang kW/N (Hz), nnCk thut OFDM thmng mc coi nhm l c NCsng mang, trn mi sng mang khiu mc truyn i vi tc thp hn ROFDM= R /N . Ch rng tc k hiu caSCmi knh con l tc truyn cc k hiu (hoc cc khung) OFDM.My thu OFDM c th mc coi l gm nhiu b gii iu ch, mi b s thchin chuyn tn hiu mi sng mang xung bng gc v tch phn trn mt chu kk hiu nhm khi phc li d liu ban u. S nguyn l ca qu trnh gii iuch mt k hiu trong k thut OFDM mc m t trong hnh (2-5). Chng ta c th ddng nhn thy, nu cc hmn(t)vi n = 0,1,...,N -1 l trc giao vi nhau tng iCmt th mi khi phc mc b (x0,m, x1,m, ..., xNc-1,m) ban u.Nc-1(t) T T T TTTxNc-1,m(t)x1,m(t)x0,m(t) S (t)m1(t)0(t)Hnh 2-5. Nguyn l gii iu ch OFDMTnh trc giao v di bo vim mu cht nhm c mc hiu qu s dng di tn cao l tnh trc giao cacc sng mang. Trong cc h thng ghp knh phn chia theo tn s thng thmng, ccsng mang mc phn tch bi mt di bo v nhm cho php thu v gii iu ch ccsng mang bng cc thao tc lc thng thmng. Tuy nhin, cc di bo v ny lm gim hiu qu s dng di tn. Nu cc sng mang l trc giao vi nhau, thchng c th mc sp xp sao cho cc di bng chng ln nhau sao cho vn c th thutt m khng c giao thoa vi cc sng mang ln cn (ICI). Tuy nhin, cc di bo vl cn thit duy tr tnh trc giao gia cc sng mang trong k thut OFDM, nhmngcch hot ng ca cc di bo v ny khc hn vi k thut FDM thng thmng.

40V mt ton hc, mt b cc hm mc coi l c lp tuyn tnh hoc trc giaonu:(2.11)qpqpAdttbaqp0)(*trong , * l k hiu ca lin hp phc. C nhiu b cc hm trc giao, ni ting nhtl cc hm lu tha phc to thnh c s ca php bin i Fourier: tketktikk2,)(0+(2.12)Nhm vy, nu p, q l s nguyn th cc hm ny s l c lp tuyn tnh. Tnh trcgiao ny gia chng gi v vic s dng php bin i Fourier ri rc (DFT)trong k thut OFDM.Nu tt c cc sng mang khng phi l sng mang mong mun b trn xungcc tn s bng mt s nguyn ln 1/T, trong T l chu k k hiu, th chng s ctch phn bng 0 trn mt chu k k hiu. Nhm vy, cc sng mang s l c lp tuyntnh, hoc trc giao vi nhau, nu dn cch gia cc sng mang l bi s ca 1/T.Tr ngi duy nht trong vic s dng DFT trong k thut OFDM l bn chtkhng tun hon ca tn hiu trong min thi gian. iu ny c th mc gii quytbng cch thm mt thi khong bo v T , on ny chnh l bn sao ca k hiu tchgcc trong T giy trmc (nhm trn hnh 2-6). on thm vo ny thmng mc gi lgtin t vng (CP - Cyclic Prefix) bi v n lm cho k hiu OFDM nhm l tun honi vi my thu. Tn hiu thu sau s mc xp x bng php chp tun hon gia tnhiu pht v p ng xung ca knh.Dibo vK hiu tch cc TgTsT Hnh 2-6. Thm CP vo k hiu OFDMChiu di ca di bo v b hn ch nhm m bo hiu sut s dng di tn, tuynhin, n phi di hn p ng xung ca knh nhm duy tr tnh trc giao gia ccsng mang con v loi b mc cc loi giao thoa ICI v ISI. Nhng li ch t mc

41nh chn thm di bo v ny thmng c gi tr hn nhng suy gim trong hiu sut sdng di tn v trong t s SNR.TgK hiu 1TgK hiu 2TgK hiu 3TgK hiu 4- - - - - - - - - - TgK hiu 1TgK hiu 2TgK hiu 3TgK hiu 4- - - - - TgK hiu 1TgK hiu 2TgK hiu 3TgK hiu 4- - - - - - - - - - TgK hiu 1TgK hiu 2TgK hiu 3TgK hiu 4- - - - - - - - Ttri trTgHnh 2-7. Chn khong bo v trit tiu nhiu ISIMt s u, nhc im ca k thut OFDM [1,2]S dng di tn rt hiu qu do cho php chng ph gia cc sng mang con.Hn ch mc nh hmng ca pha-inh v hiu ng a mng bng cch chia knhpha-inh chn lc tn s thnh cc knh con pha-inh phng tmng ng vi cc tn ssng mang OFDM khc nhau.Phmng php ny c mu im quan trng l loi b mc hu ht giao thoa giacc sng mang (ICI) v giao thoa gia cc k hiu (ISI) do s dng tin t vng.Nu s dng cc bin php xen r v m ho knh thch hp th s c th khcphc mc hin tmng suy gim xc sut li trn k hiu do cc hiu ng chn lc tns knh gy ra. C th s dng phmng php gii m ti mu vi phc tp gii m mc cho php. Qu trnh cn bng knh mc thc hin n gin hn so vi vic sdng cc k thut cn bng thch nghi trong cc h thng n tn.Kh khn ln nht i vi h thng s dng k thut OFDM l vn nhy cmvi dch tn hay ni cch khc l ng b tn hiu.H thng thu-pht OFDMHnh 2-8 l s khi ca mt h thng thu pht OFDM. Ban u, d liu uvo mc nh x t chui ni tip sang dng song song gm NCthnh phn nh bchuyn i S/P. Ngay sau , N tn hiu song song ny mc bin i IFFT to ra cCtrmng trc giao ca cc sng mang con. Tn hiu sau khi mc trc giao ha nh b

42IFFT s mc chuyn i tr v dng d liu ni tip bng b chuyn i song song -ni tip P/S. trit tiu mc nhiu xuyn k hiu ISI, v nhiu ICI, tn hiu mcchn thm CP ko di chu k k hiu v c khong bo v. Khong ny s c tcdng trit tiu hon ton nhiu ISI nu n ln hn thi gian tri tr do knh gy ra. Saukhi mc chn khong bo v, tn hiu dng s s mc chuyn i sang dng tnhiu tmng t (DAC) pht i. S/P IFFT P/S Chn CP DAC Knh iu ch OFDMx(n,1)x(n,Nc-1)s(n,0)s(n,1)s(n,N -1)csn P/S FFT S/P LoiCP ADC y(n,0)y(n,1)y(n,N -1)cr(n,0)r(n,1)r(n,N -1)crn ynxnGii iu ch OFDMHnh 2-8. S khi h thng thu-pht OFDMSau khi qua knh truyn c tc ng ca nhiu Gauss trng cng tnh AWGN, tnhiu mc ly mu v chuyn i v dng s nh b ADC. Tip n, phn CP mcloi b, chuyn t dng ni tip sang song song (S/P) chun b cho qu trnh bini FFT cho tr v trng thi ban u. Cc k hiu song song sau b FFT mc chuynv dng ni tip, chnh l d liu ban u.Trn y l nguyn l c bn, c th l tnh trc giao, di bo v v php bin iFourier trong k thut OFDM. y l nhng c tnh c bn nht ca k thut OFDM:tnh trc giao mc khng nh chc chn v hiu qu ca n trong l thuyt truyntin; di bo v cho php m bo tnh trc giao ng thi gip loi b mc ISI v ICI;cn php bin i Fourier gip cho k thut OFDM c th mc ng dng rng ritrn thc t. Cng vi nhng nghin cu su hn v l thuyt, OFDM ang chng t

43nhng mu im ca mnh trong cc h thng vin thng trn thc t c bit l trongpht thanh, truyn hnh s v c trong thng tin di ng.Vi nhng hiu qu do x l trong k thut OFDM mang li phn no khcphc mc nhmc im ca h thng MIMO. Do vy kt hp h thng MIMO-OFDMs l mt phmng php ha hn mang li nhiu li th.2.3. Kt hp MIMO-OFDMM haST Gii mST iu chOFDM iu chOFDM Gii iuch OFDM Gii iuch OFDM ),(1knx),(knxTn),(1kny),(knyRn),(kns),('knsLi ra Li voHnh 2-9. H thng MIMO-OFDMTi thi im th n, khi tn hiu u vo s(n) mc m ha ST thnh n k hiuTm hatmng ng vi cc nhnh ng-ten pht. Cc k hiutrn mi nhnh s mc ma qua b iu ch OFDM, sau ma ra ng-ten. Ti ming-ten thu, tn hiu thu s bng tng tn hiu t cc nhnh pht sau khi truyn quaknh tmng ng.)(,),(),(21nxnxnxTnNhm mc trnh by trong mc 2.2 tn hiu ra sau khi gii iu ch OFDM ti mtsng mang bt k bng tng ca tch tn hiu u vo b IFFT tmng ng v p ngtn s ca knh truyn gia 2 ng-ten v thnh phn tp m trng cng tnh. Do ,sau khi i qua b gii iu ch OFDM, tn hiu trn mi nhnh bt k mc xc nhnhm sau:(2.13)+TnijijiijknWknHknxkny1),(),(),(),(Vil p ng tn s vl thnh phn tp m ca knh giang-ten pht i v ng-ten thu j.),(knHji),(knWji

44Trong biu thc (2.13), k biu din thnh phn sng mang nhnh, n biu dinkhong thi gian mt k hiu OFDM,c gi tr phc ngu nhin vi k vngbng 0, phmng saiV),(knWji2. Trong lun vn ny, gi tr ca thnh phn tp mkhi thay i j, i, k, n mc gi thit l khng tmng quan vi nhau.),(knWjii vi h thng MIMO-OFDM nhm trn, t s tn hiu trn tp m ti mt mythu j bt k (0jn ) mc nh ngha nhm sau:R 212),(VTnijiknHESNR(2.14)trong ,El k hiu ca gi tr trung bnh.Vic gii m STC mc thc hin bng cch s dng thut ton Viterbi. ilmng mc s dng quyt nh tun theo nh lut ML: 2),(),(),(knxknHkny(2.15)Trong , l biu din ca khong cch Euclid,),(kny,,lnlmt l vct tn hiu thu mc, vct knh mc lmng v vct tn hiu pht mc lmngnhm dmi y:),(knH),(knx),(),(),(),(21knyknyknyknyRn),(),(),(),(),(,1,,11,1knHknHknHknHknHTRRTnnnnO(2.16)),(),(),(),(21knxknxknxknxTnT biu thc (2.15) suy ra vic mc lmng tham s knh ng vai tr quan trngi vi ch tiu cht lmng ca h thng MIMO-OFDM. Vai tr v nh hmng ca mc

45lmng knh trong h thng MIMO-OFDM s mc trnh by c th trong phn sau calun vn.2.4. lc lmng knh trong h thng MIMO-OFDMC 3 phmng php chnh thc hin mc lmng knh: lc lmng knh dng tn hiu hoa tiu (Pilot-Aided Channel Estimation -PACE): Ti bn pht, thc hin chn tn hiu bit (hoa tiu) vo khung tn hiuOFDM vi khong cch nht nh trong min thi gian, tn s. Ti bn thu, tn hiuly mu tmng ng vi cc im chn hoa tiu cho php nh gi knh truyn h thngti v tr y. Bng phmng php ni suy, t nhng im chn hoa tiu ta suy mcgi tr knh truyn ti cc im khc trong khung, gip cho vic quyt nh tn hiuchnh xc hn. lc lmng knh quy (Decision-Directed Channel Estimation - DDCE):Phmng php ny cng chn tn hiu dng hoa tiu nhmng t hn. Tn hiu bit (hoatiu hay chui hun luyn) ch chn ti u mi khung. Ti bn thu, da vo chuihun luyn bit thng tin knh tm thi. Ban u tn hiu mc quyt nh thngqua thng tin knh tm thi, sau c thng tin v tn hiu va mc x l ny litham gia vo vic quyt nh tn hiu sau... Thng tin ca c khung mc tn dng tia. Tuy nhin, cng l mt nhmc im ca phmng php ny. Khi tn hiu lp liqu nhiu, tc x l chung s gim i, kh c th p dng cho nhng ng dng ihi thi gian thc khi m khung d liu qu ln. lc lmng knh bng phmng php m (Blind/Semi-Blind Channel Estimation -BCE): y l phmng php khng s dng vic chn tn hiu bit ti bn pht.Quyt nh tn hiu ch da vo thng tin ca tn hiu thu. Phmng php ny tuy c tc truyn tin cao (do khng chn thm tn hiu hoa tiu) nhmng tc x l ti bn thuthp v cht lmng quyt nh khng cao bng 2 phmng php trn.2.4.1. Phng php c lng knh dng tn hiu hoa tiu (PACE)Nguyn l chung ca PACE l bn thu thc hin mc lmng knh da trn cc tnhiu pht bit (tn hiu hoa tiu) mc ghp trong d liu pht. Vic ghp tn hiuhoa tiu trong d liu pht mc thc hin ti mt s v tr xc nh trong th thigian - tn s ca mt khung d liu OFDM. n gin, ta s bt u tm hiu phmng php PACE i vi h iu chOFDM. V ri s m rng cho h MIMO-OFDM. Khi , tn hiu hoa tiu chn voli cn thm iu kin v tnh trc giao.

46Hnh 2-10 biu din trmng hp n gin khi cc k hiu hoa tiu mc chn votheo dng ch nht vi khong cch gia hai k hiu lin tip trn min thi gian l Dtv trn min tn s l D .f 0N -1C DtN -1S DfD liuPilotV tr mc lmng knhHnh 2-10. M hnh chn k hiu hoa tiu trong mt khung d liu OFDMNhm vy, vic mc lmng knh mc thc hin theo tng khung. Xt mt khungOFDM cha N k hiu mc nh s t 0,1,...,N -1, s dng N sng mang, k hiuSSC0,1,..., N -1.CTn hiu thu mc tmng ng vi mt khung OFDM l:),(),(),(),(knWknHknxkny+(2.17)Trong ,,ln lmt l tn hiu pht, p ng knhtruyn v tp m nn.),(knx),(knH),(knWGi s tn hiu hoa tiu mc chn theo mu hnh ch nht vo cc v tr tmngng n, k vi v tr u tin tmng ng vi n=0, k=0. Khi , k hiu hoa tiu smc chn cc v tr nhm sau: 1,,1,0,'1,,1,0,'tStfCfDNppDkDNppDn(2.18)trong , k hiu al ly gi tr nguyn nh nht khng b hn a.*c lng knh s dng b lc 2Dlc lmng knh s gm 2 bmc: mc lmng knh ti cc v tr chn tn hiu hoatiu v mc lmng knh trn ton b cc v tr cn li trong khung.

47Ti cc v tr chn k hiu hoa tiu, p ng tn s ca knh mc xc nh bngbiu thc: )','()','()','()','()','()','(~knxknWknHknxknyknH+(2.19)Ti cc v tr cn li ca knh, p ng tn s ca knh mc xc nh bng cchs dng cc gi tr mc mc lmng ca knh ti v tr tn hiu hoa tiu ma qua blc 2 chiu (2D):(2.20)knVknknHknkncknH,)','()','(~),,','(),(trong ,l p ng xung ca b lc 2 chiu,l tp hp cc v tr tnhiu hoa tiu mc s dng mc lmng p ng knh v tr (n,k). Trn thc t, gim lmng php tnh phi thc hin trong mc lmng, ngmi ta ch s dng mt phntrong ton b p ng knh tmng ng vi cc v tr chn k hiu hoa tiu. Gi s phnt trongl. Khi :),,','(knknckn,Vkn,VtapN tSfCgridtapDNDNNN(2.21)vil s v tr chn tn hiu hoa tiu trong mt khung OFDM.gridNVn t ra vi mc lmng knh l xc nh cc h s ca b lc sao cho xcsut sai lch mc lmng knh l nh nht. Xc sut sai lch mc lmng knh mc xcnh thng qua hm bnh phmng sai lch trung bnh: 2,),(),(knHknHEMSEkn(2.22)Cc h s b lc mc xc nh theo nguyn l trc giao:[]{}knVknknHknHknHE,)","(,0)","(~),(),((2.23)Th (2.20) vo (2.23) thu mc phmng trnh sau viknVkn,)","(:{}{}knVknknHknHEknkncknHknHE,)','(**)","(~)','(~),,','()","(~),((2.24)

48T gi thit tp mtrong biu thc (2.19) c gi tr ngu nhin vi kvng bng 0 v c lp vi v tr (n,k) suy ra:),(knW{}{})","()","(),()","(~),(**kknnknHknHEknHknHE(2.25)trong ,)","(kknnl hm tmng quan ri rc trn min thi gian - tn s cap ng knh. Mt khc:{} {} {}{}{} )"',"'()"',"'()","()','()","()','()","()','(~)","()','()","()','()","()','()","()","()","()','()','()','()","(~)','(~2**********kknnkknnknxknxEknWknWEknHknHEknxknxknWknWEknHknHEknxknWknHknxknWknHEknHknHE+++++V(2.26)vil phmng sai ca thnh phn tp m W(n,k),2V),(qpl hm delta 2 chiumc nh ngha nhm sau:(2.27)!+0,00,1),(22qpqpqpThay biu thc (2.25), (2.26) vo (2.24) v biu din cho tt c cc gi tr cav s dng biu din vct s thu mc:knVkn,)","((2.28)),(),(kncknTTtrong ,l vct kch c),(kn1tapNm cc thnh phn l)","(kknn,l vct kch c),(knc1tapNvi cc phn t lvl ma trnvung kch cgm cc phn t.),,','(knknctaptapNN)"',"'()"',"'(2kknnkknn+VT biu thc (2.28) ta suy ra h s b lc mc lmng knh ti mu tmng ng vimi v tr (n,k) mc xc nh bng:(2.29)1),(),(knkncTTNhn thy rng ma trns khng ph thuc vo v tr (n,k) nu ton b png knh trong khung OFDM u mc xc nh trn mt tp v tr k hiu hoa tiu.iu ch xy ra khi tt c cc k hiu hoa tiu u mc s dng ng thi kn,V

49mc lmng tt c cc v tr d liu khc, khi gridtapNN. Tuy ma trnch cnxc nh mt ln nhmng s lmng php tnh cn dng li rt ln. V vy, ngmi ta ch sdng mt vi tn hiu hoa tiu ln cn v tr d liu cn xc nh mc lmng m thi.*c lng knh s dng b lc 2x1D.Nhm trnh by trong phn trn, s lmng php tnh cn thc hin khi s dngb lc 2D trn min thi gian-tn s l rt ln. Nn, gim lmng tnh ton trong khivn gi mc chnh xc nht nh, ngmi ta s dng 2 b lc 1 chiu tmng ng vivic mc lmng theo 2 hmng: thi gian v tn s.0 tN -1S0N -1CfD liu PilotHnh 2-11. lc lmng knh PACE 2x1DThc cht, phmng php s dng 2 b lc 1 chiu gm 2 bmc mc lmng:Bc 1: S dng tham s b lc 1D theo min tn s)(~)( fkc mc lmng knh:)'(~)(~),'()(nhkcknHf(2.30)Vct h s b lc)(~)( fkcch ph thuc tn s k v mc xc nh theo biuthc (2.22), trong b qua thnh phn thi gian n. Vct)'(n~hc di khng lnhn s tn hiu hoa tiu trn mt ct, fCDN. Vic mc lmng theo biu thc (2.30)mc thc hin ti tSDNct c tn hiu hoa tiu trong mt khung OFDM.Bc 2: S dng tham s b lc 1D theo thi gian)(~)(tnc mc lmng knh:

50)(~)(~),(),()()()(khkcknHknHftt(2.31)Biu thc (2.31) cho thy vic mc lmng knh theo min thi gian da trnnhng p ng knh mc lmng theo min tn s. T y, cc d liu theo tnghng mc mc lmng da vo p ng knh bit.K thut mc lmng knh mt chiu (1D) cho h thng OFDM c th phn thnh2 loi c bn sau [13]:(a) Bnh phmng ti thiu (LS - Least-Squared)(b) Ti thiu sai li trung bnh bnh phmng (MMSE - Minimum Mean SquaredError)B mc lmng LS c phc tp thp, ch da vo tn hiu hoa tiu m khngtnh n tnh tmng quan gia cc knh truyn. Do vy phmng php ny b sai litrung bnh bnh phmng (MSE) cao, c bit trong vng c t l SNR thp. Trong khi, b mc lmng theo phmng php MMSE va da trn tn hiu hoa tiu va tnh nnhng thng k trn min thi gian ca knh, nn sai li MSE s nh hn nhiu.Nhmng phmng php MMSE li c phc tp cao.Phmng php mc lmng LS xc nh hm truyn nhm trong phmng trnh (2.19) bit trn. Sau y ta xem xt phmng php mc lmng MMSE v trong phn mphng ta s so snh 2 phmng php ny. gin tin trong cch vit, ta c th vit li (2.20) dmi dng:HCH~(2.32)Trong , H , H l mc lmng knh ti v tr tn hiu hoa tiu v ti cc v tr d~liu. C l p ng xung b lc.Nu p dng nh lut bnh phmng sai lch b nht (MMSE), tham s ma trn Cs mc xc sao cho hm trung bnh sai lch mc lmng sau y l nh nht: 2)(HHECMSE(2.33) gii bi ton MMSE, p dng nguyn l trc giao:{}0~HHHHE(2.34)Th biu thc (2.32) vo (2.34) ta thu mc:{}{}HHHHEHHCE~~~(2.35)

51 tnh mc C ta s khai trin cc i lmng{}HHHE~~v{}HHHE~{}{}{}fHHHHHEWHHEHHE+)~(~(2.36)vil ma trn tmng quan min tn s ca p ng knh)(nf++)0()2()1()2()0()1()1()1()0(fCfCfCfffCffffNNNNO(2.37)Trong , nu L l chiu di CP. T l tc ly mu ca h thng vsWVl tritr rms th: [13]{} csLTNkjTLmnkmnfNTkjeeHHEkscs/2111)(/2*,,WVVVWW+++(2.38)Mt khc:{}{}{} {CNfHHHHIWWEHHEWHWHEHHE++++~~)}~)(~(~~(2.39)Trong , {} 22~xWEWE.Th (2.36), (2.39) vo (2.35) ta thu mc:1+CNffIC(2.40)T , p ng knh s mc mc lmng bng:HIHCNff~1+(2.41)2.4.2. c lng knh trong h thng MIMO-OFDMXt h thng MIMO-OFDM s dng m ha Alamouti vi cu trc mc chotrn hnh 2-12. D liu ban u mc ma qua b m ha Alamouti theo tng khi 2NC

52k hiu. Qua b m ha Alamouti, tn hiu mc sp xp li thnh 2lung nhm sau:CNxxx221,,,Lung 1:*212*212*21,,,,,,,CCNNkkxxxxxxLung 2:*122*122*12,,,,,,,CCNNkkxxxxxx S/Pv chnPilot IFFT P/Sv chnCP Chnsngmang &khuchi *212*21,,,,CCNNxxxx S/Pv chnPilot IFFT P/Sv chnCP *122*12,,,,CCNNxxxx MhaSTBC CNxx21Chnsngmang &khuchi TxP/S FFT S/Pv bCP Tchsngmang CNyyy2,12,11,1,,, P/S FFT S/Pv bCP Tchsngmang CNxx21GiimSTBC CNyyy2,22,21,2,,, lc lmng knh RxHnh 2-12. H thng MIMO-OFDMSau tn hiu hoa tiu mc chn vo theo quy tc trc giao, ngha l ti thiim chn tn hiu hoa tiu nht nh ch c 1 nhnh c tn hiu truyn, cn nhnh kiath trng (Null). chu k k hiu tip theo th ngmc li. Vy l, ti mi ng-tenthu, tn hiu nhn mc ch do 1 ng-ten pht i. T , cc knh truyn s mc xcnh theo tn hiu hoa tiu. y, ta phi chn tn hiu hoa tiu trc giao v vic gii

53m STBC, c th l m Alamouti, cn bit mc cc knh truyn thnh phn t hpti b thu. Vic chn tn hiu hoa tiu mc th hin trong hnh 2-13 dmi y.ng-ten 1ng-ten 2 Tn hiu Pilot D liuV tr trngHnh 2-13. Cu trc khung tn hiu MIMO-OFDMVi tng cp knh truyn, mc lmng hm truyn mc xc nh nhm vi h thngOFDM. Sau ln ni suy theo min tn s, gi tr hm truyn 4 knh SISO mc xcnh. Tip tc mc lmng m rng theo min thi gian cho cc vng d liu. Lc ny,tt c cc v tr trong khung MIMO-OFDM u mc mc lmng knh truyn.Nhim v tip theo l gii m Alamouti thu mc tn hiu trung thc. Vic giim mc thc hin cng lc vi 4 tn hiu 2 chu k lin tip ti 2 ng-ten thu. Ktqu t hp tn hiu thu mc th hin trong (1.45) v (1.46):(2.42)++21*22,1*1,211212,1||~jjjjjijijwhwhxhx(2.43)+21*21,1*2,212212,2||~jjjjjijijwhwhxhxThnh phn tp m Gauss,, do c gi tr ngu nhin v tr trung bnh bng 0nn thnh phntrong (2.42) haytrong(2.43) c thtb trphn no cho nhau. Trong khi , thnh phnjiw+21*22,1*1,jjjjjwhwh21*21,1*2,jjjjjwhwh

542,1,||21212,kxhkijijmc tng thc s do tnh chng chp ca tn hiu khngngu nhin. Chnh v vy, phmng php t hp ny lm tng tn hiu c ch.

55CHlNG 3. M PHNG3.1. Thit lp tham s m phngTheo nguyn l ch ra trong s 2-12, ta s thit lp cc tham s mphng cht lng ca h thng khi dng phng php PACE c lng knh. Bngthng tn hiu gc l 1MHz, tng ng vi chu kly mu k hiu. S lng sng mang con c s dng cng chnh l bc ca bbin i Fourier nhanh, NssTS1106C= 128. Khi , mi k hiu OFDM c di bngsTS128128.M hnh knh pha-inh Rayleigh a ng c s dng m phng gm 8ng, c tr ln lt l [0, 1, 2, 3, 4, 8, 12, 15]T . Nng lng hay cng sut caScc ng suy gim theo hm m vi tr ca tng ng [9]. Tri tr cn trungbnh bnh phng, rms, l mt i lng c ly lm chun cho vic thit lp cngsut cc ng tr. C th l cng sut cc ng c tnh theo cng thc:rmsitdelayieP(3.1)trong , delay v tirmsl tr v tri tr cn trung bnh bnh phng.Nh cp, trnh c nhiu ISI v ICI th cn chn mt khong bo v c di lun ln hn tri tr cc i. Trong trng hp ny, di bo v cng l phntin t vng CP, s phi c di ln hn tri tr cc i v ta ly CP = 16Ts. Vy nnchiu di ca mt k hiu OFDM khi c tin t vng CP l 144s.Trong h thng truyn thng di ng, mt im khng th b qua l dchtn Doppler f , do s di chuyn tng i gia bn thu v pht gy ra. Tn s ny phdthuc vo tn s sng mang v vn tc tng i gia bn thu v pht. Vi cng vntc V, khi tn s sng mang l f th gy ra dch tn Doppler l f . Nhng cng vi0dvn tc , khi tn s sng mang l 10 ln f th dch tn cng tng ln 10 ln so vi0f (theo cng thc 2.3). Mt khc khi dch tn l f th n tc ng trc tip ln tnddhiu di gc (1MHz). Trong m phng [12], nh hng ca f ln tn hiu theo tchd. Ngha l khi fSdTfdv l do no tng ln (vn tc hay tn s sng mang tng) thu gy tc ng mnh hn i vi tn hiu. Bi vy, y ta ch xt n thun tn sDoppler l 50Hz, 100Hz, 200Hz,... m khng quan tm n tn s sng mang l baonhiu, hay vn tc chuyn ng tng i gia bn thu v pht l bao nhiu.

56M STBC c s dng trong h m phng l m Alamouti, ngha l c 2 ng-ten pht v 2 ng-ten thu. Do vy, cc tn hiu hoa tiu c chn theo nguyn tc trcgiao tng cp 2 k hiu mt. Ti mi ng-ten, 2 k hiu hoa tiu lin tip c gi tr nh nu ra trong hnh (3-13). Khi gii m, tn hiu c kt hp trong 2 chu k k hiulin tip gii m Alamouti. Hnh 3-1. Phn b cng sut a ng theo hm e m3.2. So snh cht lmng OFDM v MIMO-OFDMNh ta bit, h thng kt hp MIMO-OFDM ngoi nhng u im ca OFDMcn c thm li phn tp do MIMO mang li. nh gi c cht lng caMIMO-OFDM so vi khi ch dng OFDM, ta s m phng, nh gi cht lng ca 2h thng trn thng qua gin chm sao ca tn hiu 16-QAM v ng lin h t lbt li BER - t s tn trn tp m SNR.

57 Hnh 3-2. Gin chm sao vi h thng OFDM Hnh 3-3. Gin chm sao vi h thng MIMO-OFDM

58 Hnh 3-4. So snh cht lng h thng OFDM v MIMO-OFDMQua hnh (3-2), (3-3) ta cng cha nhn r c s thit hn v cht lng ca 2h thng. Nhng vi hnh (3-4) ta d dng nh gi cht lng ca 2 h thng so vinhau. chnh lch gia 2 ng BER ca 2 h thng th hin li phn tp cam Alamouti. Vi m hnh ny, li phn tp trung bnh c 5dB. Ngha l, vi cngcht lng BER=10-2th h OFDM cn c SNR = 21dB trong khi vi h MIMO-OFDM ch cn SNR = 16dB.3.3. So snh LS v MMSENh trnh by trong mc (3.4), c lng knh theo lut LS cha s dngc thng tin v tng quan min tn s, thi gian (cng thc 2.38). Trong khi ,MMSE da vo thng tin tng quan ni suy knh truyn. Hnh (3-5), (3-6) v (3-7) cho thy s khc bit gia c lng knh truyn theo lut LS v MMSE. Trongcc phn sau, chng ta ch quan tm n phng php c lng knh dng chunMMSE.

59 Hnh 3-5. Gin chm sao vi c lng knh theo lut LS Hnh 3-6. Gin chm sao vi c lng knh theo lut MMSE

60 Hnh 3-7. So snh cht lng h thng khi c lng theo LS v MMSE3.4. nh hmng ca dch tn Doppler fdKhi c s di chuyn tng i gia my thu v my pht, dch tn Dopplerxut hin. Lc , tn s thu s lch so vi tn s pht mt lng t l thun vi vntc v tn s sng mang. dch tn ny, t nhiu lm nh hng ti s ng b bnthu. lch ny cng tng s lm cht lng h thng cng xu i. Nhn hnh (3-8)ta thy r s khc nhau trong gin chm sao khi dch tn thay i t 50Hz n200Hz. Khi f cng ln, cc tn hiu trn gin chm sao 16-QAM cng tri rng ra,dngha l sai li cng ln. Trong hnh (3-9) ch r c t l bt li vi dch tn khcnhau v ti SNR khc nhau. Khi dch tn l 50Hz v 100Hz, cht lng h thngkhng my thay i. Thm ch khi SNR = 30 dB, t l bt li ca 2 trng hp l xpx nhau. Nhng khi tng dch tn ln, cht lng bt u thay i r rt. Vi dch tn 150Hz, 200Hz cht lng dn tin ti bo ha khi SNR t 30dB. Do vy, khim phng so snh nh hng ca cc tham s, ta cn t dch tn khng qu100Hz m bo cht lng khng qu ti.

61 Hnh 3-8. Gin chm sao vi dch tn Doppler khc nhau Hnh 3-9. nh hng ca dch tn Doppler ln cht lng h thng

623.5. nh hmng ca tri tr rmsHnh 3-10. Gin chm sao vi tri tr rms khc nhau Hnh 3-11. nh hng ca rms ln cht lng h thng

63Tri tr cn trung bnh bnh phng rms l i lng th hin c c v trln cng trung bnh cng nh mc phn tn ca cc ng tr trong m phngknh a ng. Do vy, vic nh gi nh hng ca rms ln cht lng h thng lmt phn quan trng. Nh trnh by v m hnh xy dng ban u, cc ng tr[0, 1, 2, 3, 4, 8, 12, 15] ln T v cng sut ph thuc vo tr tng ng v tri trSrms. Khi rms tng ngha l cc mc nng lng ca cc tr bc cao cng tng ln.iu lm cho mc sai li cng nghim trng. Trong m phng ny, ta thay irms vi mt vi gi tr, [0.5 1.0 1.5 2.0]T .SHnh (3-10) biu th gin chm sao ti vng SNR=30dB. Khi rms cng tng,cc chm sao phn tn cng mnh, sai li cng ln. Mc sai li th hin thng quat l bt li c ch ra trong hnh (3-11). Khi rms nh, cht lng h thng rt tt,nhng khi rms tng ln cht lng h thng gim dn. iu ny cng d dng giithch. Khi rms ln, nng lng ca tn hiu trc tip hay tn hiu n sm nht (trbng 0) gim i so vi tng nng lng cc thnh phn tr cn li. Ti ni thu, cc tnhiu tr vi h s bin (cn bc hai ca cng sut hay nng lng) tng ng sc cng chng chp vi tr tng ng. Khi c th xy ra trng hp, tn hiuchnh (tr bng 0) l tn hiu 16-QAM c bin nh nht bng 21122+(khi khiu pht l (-1,-1), (-1,1), (1,-1) hoc (1,1)) trong khi tn hiu tr c th bng 183322+(khi k hiu pht l (-3,-3), (-3,3), (3,-3) hoc (3,3)), vil ts bin ca tn hiu tr trn tn hiu chnh. Nu tn hiu tr c nng lng ln,n c th ln t tn hiu chnh lm sai li trong khi quyt nh. V vy, trong m phngny, ta ch dng vi rms khng qu ln.3.6. nh hmng ca s lmng mng truynNng lng sau khi pht ra t ng-ten pht c truyn i theo nhiu hng, cnhng ng truyn ti c ng-ten thu, c nhng ng truyn khng n ni. Dovy, vic nh gi nh hng s lng ng truyn ln cht lng h thng l cnthit. M hnh gi thit nng lng c phn phi theo hm e m cho 8 ng tr. Tas m phng, nh gi cht lng khi s ng truyn ln lt l, 4, 6, 8 ng. Khi sng truyn bng 1, ngha l ch c tn hiu trc tip (tr bng 0), khi s ng bng4, cc ng c tr bng [0, 1, 2, 3]T c xt n, Tuy vi cch gii quyt ny,Sta khng thc s nh gi ng c thc t v c th khi s ng bng 4 nhng lil cc ng tr [0, 1, 4, 12]T chng hn, nhng n cng nh gi c phn no nhShng ca s ng tr ln qu trnh gii m, quyt nh.

64 Hnh 3-12. Gin chm sao khi s lng ng truyn khc nhau Hnh 3-13. nh hng ca s lng a ng i vi cht lng h thng

65Vi cch xy dng m phng nu trn, kt qu so snh v gin chm sao cv trn hnh (3-12). Nhn thy chm sao vi trng hp ch c 1 ng truyn rtgn, v khng c nh hng ca a ng m ch c nh hng ca pha-inh do tns Doppler gy ra v nhiu AWGN. Nhng khi s ng tng ln 4, 6 v 8 chm saoc phn tn khng chnh lch nhau l my. iu chng t khi ln qu 4 ngth cht lng gn t ngng di. Nh gii thch phn rms bn trn, nnglng khi tr cng nhiu th cng nh, vy nn mc nh hng ca n lm saili cng gim i. iu ny c th so snh d dng bng hnh (3-13).3.7. nh hmng ca tri tr ti amaxNh trnh by trong mc (2.2) v nguyn l chn tin t vng CP gim nhhng knh a ng. Khi CP c di nh hn tri tr ti amaxs gy ra nhiu ISIv ICI. Trong m hnh ny, nh gi nh hng ca tri tr ti a i vi chtlng h thng ta thay v tng tri tr ti a bng vic thay i di CP. Ban u tas , trong khi tri tr cc i l. Sau , tng CP ln 8TSTCP4ST 15S, 16T , 32TSSv xem xt cht lng h thng thng qua gin chm sao v mi lin h gia t lbt li v t s SNR.Nhn hnh (3-14) biu th gin chm sao vi cc di CP khc nhau ta ddng nhn thy cht lng h thng rt km khi di CP qu nh. KhiSTCP4chm sao gn nh khng cn ranh gii trong khi SNR = 40dB. Nhng khi di CPtng ln, chm sao bt u tch ra. n lcSTCP16, cht lng h thng khngtng khi di CP tip tc tng. iu ny rt d nhn ra trong hnh (3-15) khi m tcht lng h thng qua mi lin h BER-SNR. ViSTCP16v, t lbt li ging ht nhau, thm ch c nhng im nh SNR = 25dB cht lng vicn tt hn. l do nh hng ca nhiu AWGN v khi s k hiu mphng cha ln. iu ny c cng c gii thch trong phn gii thiu kthut OFDM. Khi chiu di CP vt qu tri tr cc i th trnh c can nhiuISI, ICI. Lc , c tng chiu di CP ln na th cng khng c tc dng. Do vy,cng vic nh gi c tri tr cc i hay tr tri cc i (mc 2.1) chnh xc thc hin chn CP cho hiu qu nht, trnh lng ph dung lng truyn.STCP32STCP16

66 Hnh 3-14. Gin chm sao khi tri tr ti a thay i so vi CP Hnh 3-15. nh hng ca tri tr ti a ln cht lng h thng

67Thc cht trong m phng ny vn cha gii quyt c vic s dng thng tina ng nh l tn hiu c ch, m ch coi nh l mt tn hiu gy nhiu trong qutrnh quyt nh. Khi s dng k thut chn CP, chng ta thc hin c vic tchcc k hiu ti ni thu. Ch c iu vi cc ng tr, tn hiu c trt t hi khc mtcht. Khi , phn cui (c chiu di ng bng tr ca nhnh) ca k hiu ct ln pha u. Nh vy, nu ghp vng trn, ngha l phn cui tn hiu ni viphn u, th tn hiu chnh v tn hiu tr ch khc nhau thi gian tr. Khi ta xoayvng ng bng thi gian tr, tn hiu mi thu c chnh l tn hiu vi tr tngng. gii quyt c bi ton ny ngi ta c th dng cc mch lc phi hp(matching filter) hay phng php thu Rake [4].

68KT LUNTrong lun vn ny, chng ti gii thiu m hnh h thng MIMO cng nhmOFDM cng cc k thut x l ca tng h thng. c bit vi h thng MIMO, lunvn trnh by cc k thut s dng c bn pht v bn thu tng dung lmngcng nhm cht lmng h thng.T vic ch ra cc mu, khuyt im ca tng h thng dn ti vic kt hpMIMO-OFDM l tt yu vi s i hi ngy cng ln ca x hi. S kt hp s tndng mc mu im ng thi s gii quyt mt s tn ti trong mi h thng ring l. pht huy ti a cc mu th ca MIMO-OFDM cn c cc gii php x l ccvn k thut ca tng loi cng ngh cng nhm ca c h thng. Mt trong nhngvn quan trng l mc lmng knh. lc lmng knh cho php my thu bit mcthng tin v cc tham s ca knh. Vi h thng OFDM, knh truyn l n nht - ch1 ng-ten pht, 1 ng-ten thu nn vic mc lmng knh cng n gin hn so vi hthng MIMO-OFDM trong c vic chn tn hiu hoa tiu bn pht v x l tn hiuti bn thu. Nhmng b li , MIMO-OFDM li c mc li phn tp - mu im cacng ngh MIMO.lc lmng knh c th thc hin theo mt vi phmng php nhm PACE, DDCEhay BCE. Ty theo yu cu ca tng ng dng, h thng m dng phmng php nocho thch hp. Bng vic phn tch l thuyt v thut ton PACE, chng ti dngchmng trnh lp trnh ng dng Matlab m phng cht lmng ca h thng MIMO-OFDM theo cc nh hmng ca mt s tham s nhm dch tn Doppler, tri tr cntrung bnh bnh phmng (rms), s lmng mng truyn (pha-inh a mng) v tr tia (max).Tuy nhin, qua m phng, thut ton PACE cng cn nhng hn ch nht nh.Vic dng tn hiu hoa tiu u n lm gim dung lmng thng tin thc v chma tndng mc nhng thng tin t nhng tn hiu mc gii m. t mc hiu suttruyn tin cao, ta cn phi tng t s SNR ca h thng. Nhmng nu tng trc tip bngcch tng cng sut pht c th gy ra nhiu gia cc thit b pht v tuyn khc nhau.Vy nn, cch tt nht tng mc t s SNR l s dng cc k thut nhm t mc li m ha, c th dng thm m ha FEC hay c th thay m STBC bng mSTTC,...

69 tip tc nghin cu su hn ti v h thng MIMO-OFDM, hmng tiptheo trong thi gian ti l: Nghin cu h thng MIMO-OFDM c s dng m ngoi FEC hoc thay thvic dng m STBC bng dng m STTC c mc li m ha nhm cithin cht lmng cng nhm tin cy mng truyn. Nghin cu phmng php mc lmng knh quy (DDCE) cho h thngMIMO-OFDM.70TI LIU THAM KHOTing Vit[1].ng Quang Hiu (2000), Nguyn l ca k thut OFDM, Tp ch in t tr,S 1, H Ni.[2]. Phan Hng (2006), Cng ngh OFDM trong truyn dn bng rng im-a imtc cao, Tp ch Bu chnh vin thng & Cng ngh thng tin, H Ni.[3].Nguyn Huy Qun (2007), Nghin cu nh hng ca sai li trong c lngknh ln ch tiu cht lng h thng MIMO-OFDM, Lun vn thc s, Hv Cngngh BCVT, H Ni.[4]. Trnh Anh V (2006), Thng tin di ng, Nxb. HQGHN, H Ni.Ting Anh[5].Branka Vucetic and Jinhong Yuan (2003), Space-Time Coding, Wiley Press,Autralia.[6].Daniel Larsson (2006), Analysic of channel estimation methods for OFDMA,Master thesis, Royal Insitute of Technology (KTH), Stockholm, Sweden.[7].Erik G.Larsson and Petre Stoica (2003), Space-Time Block Coding for WirelessCommunications, Cambridge University Press, NewYork, USA.[8].Ezio Biglieri and Giorgio Taricco (2004), Transmission and Reception withMultiple Antennas: Theorectical Foudations, NOW Press, Torino, Italy.[9].Gunther Auer (2003), Channel Estimation in two dimensions for OFDM Systemswith Multiple Transmit Antennas, IEEE Global Telecommunications Conference,San Francisco, USA.[10]. Hamid Jafarkhani (2005), Space-Time Coding: Theory and Practice, CambridgeUniversity Press, New York, USA.[11]. Helmut Bolcskei (2003), Principles of MIMO-OFDM Wireless Systems, SwissFederal Insitute of Technology (ETH), Zurich.

71[12]. Hiroshi Harada and Ramjee Prasad (2002), Simulation and software radio formobile communications, Artech House Press, London, England.[13]. Igor Tolochko and Mike Faulkner (2002), Real Time LMMSE ChannelEstimation for Wireless OFDM Systems with Transmitter Diversity, Tele andMicro-Electronics Center, Victoria University, Victoria, Australia.[14]. Ivan Cosovic and Gunther Auer (2006), Capacity of MIMO-OFDM with PilotAided Channel Estimation, DoCoMo Euro-Labs, Munchen, Germany.[15]. Jang-Wook Moon et al., (2006), Pilot-Assisted and Blind Joint Data Detectionand Channel Estimation in Partial-Time Jamming, IEEE Trans, Commun, Vol.54,No.11, pp. 2092-2102.[16]. King F.Lee and Douglas B.Williams, (2002), Pilot-Symbol-Assited ChannelEstimation for Space-Time Coded OFDM Systems, EURASIP Journal onApplied Signal Processing, No.5, pp. 507-516.[17]. Michele Morelli and Umberto Mengali (2001), A Comparision of Pilot AidedChannel Estimation Methods for OFDM Systems, IEEE, Transactions on signalprocessing, Vol. 49, No. 12, pp. 3065-3073.[18]. Rappaport T.S. (1996), Wireless Communication: Principle and Practice, PrenticeHall Press, USA.[19]. Rose Trepkowski (2004), Channel Estimation Strategies for Coded MIMOSystems, Master thesis, Virginia Polytechnic Institute and State Uninversity,Virginia.[20]. S. Thomas Alexander (1986), Adaptive Signal Processing, Springer-Verlag Press,New York, USA.[21]. Shuo Pan (2006), Capacity of MIMO Systems for Spatial Channel ModelScenaiors, Master thesis, Australian National University, Canberra, Australia.[22]. Siavash M.Alamouti (1998), A simple transmit diversity technique for wirelesscommunications, IEEE J. SAC, Vol. 16, No.8, pp. 1451-1458.[23]. Timo Roman (2006), Advanced Receiver structures for mobile mimo multicarriercommunication systems, Ph.D. Thesis, Helsinki University of Technology, Espoo,Finland.

72[24]. Vahid Tarokh et al., (1999), Space-Time Block Codes from Orthogonal Designs,IEEE Transactions on information theory, V. 45, No.5, pp. 1456-1467.[25]. Van-Duc Nguyen et al.(2007), Channel Estimation and Interference Cancellationfor MIMO-OFDM Systems, IEICE Trans, Commun, Vol.E90-B, No.2, pp. 277-290.73PH LC A. L THUYT THNG TINA-1. EntropyEntropyTrc ht chng ta cn bit khi nim v entropy, l i lng o sng stca ngun tin - bin ngu nhin.nh ngha : Gi x l vct ngu nhin ri rc c tp gi tr l X v hm trngkhi xc sutXxpX),Pr()(. Entropy H(x) ca vct ngu nhin x cnh ngha bngXXXppxH)(1log)()((A.1)Nu php logarit l c s 2, th entropy c tnh theo bits. Cn nu l c s eth entropy c tnh theo n v nats. Nu x khng phi l ri rc, ta c th nhngha entropy cho bin lin tc nh sau.nh ngha : Entropy ca bin ngu nhin lin tc x c hm mt xc sut)(xfc th c xc nh bng biu thcdffxHXX)(1log)()((A.2)nh ngha : Entropy chung ca cp vct ngu nhin- Trng hp ri rc:XYYXYXppyxH),(1log),(),(,,(A.3)- Trng hp lin tc:ddffyxHYXYX),(1log),(),(,,(A.4)nh ngha : (Entropy c iu kin)- Trng hp ri rc:XYXYYXXXppxyHpxyH)|(1log),()|()()|(|,(A.5)

74- Trng hp lin tc:ddffxyHXYYX)|(log1),()|(|,(A.6)A-2. Thng tin tng h v dung nng knhnh ngha : Thng tin tng h l i lng cho bit lng thng tin m mtngun tin ny c v mt ngun thng tin khc.Entropy H(x) chnh l lng thng tin trung bnh ca mt tin, cn H(x|y) chnh llng mt thng tin trung bnh khi pht mt tin. Lng thng tin thc t thu cchnh l lng thng tin tng h I(x;y) c tnh nh sau: XYXYXYXXYXYYXXYXYXXYXYYXXXXpppppppppppyxHxHyxI)()|(log),()|(1log),()(1log),()|(1log),()(1log)()|()();(|,|,,|,(A.7)V)(),()|(,|YYXYXppp, nn thng tin tng h I(x;y) c th vit li l:XYYXYXYXppppyxI)()(),(log),();(,,(A.8)T y ta thy r, vai tr ca x v y c th hon v cho nhau, th hin tnh tngh ca x v y.Tng t vi trng hp lin tc, thng tin tng h l:ddffffyxIYXYXYX)()(),(log),();(,,(A.9)Dung nng ring ca knh chnh l lng thng tin tng h cc i:);(maxyxICS(A.10)Dung nng ca knh l tch ca tc truyn k (mu/s) vi dung nng ring caknh.SCkC.(bits/s)(A.11)

75A-3. Dung nng knh AWGNp dng l thuyt trn vo vic tnh ton dung nng ca knh AWGN hay chnhl dung nng Shannon.Bi ton t ra l ngun pht tn hiu x c rng bng thng l B(Hz), cngsut l S. Knh truyn AWGN c cng sut nhiu l N. C mt gi thit l ngun tin xv nhiu l c lp thng k vi nhau. AWGN yxB(Hz)Hnh A-1. M hnh knh AWGN)()()(tntxty+(A.12)Ta cn tnh thng tin tng h)|()();(xyHyHyxI(A.13)Trong , dffdffdfddfffddffxyHXYXYXYXYXXYXYXXYYX)|(1log)|(1)|(1log)|()()|(1log)|()()|(1log),()|(|||||||,(A.14)Khi x bit, y = x + n, th y s c phn b ging nh nhiu n. Ngha l,)(Yfv)|(|XYfcng c phn b Gauss. Vy nn ta c:)()|(1log)|(||nHdffXYXY(A.15)T ,constnHnHxyH)()(1)|(.Thng tin tng h:ConstyHyxI)();(, v t cc i khi H(y) t cc i.Trong khi , H(y) t cc i khi v ch khi y c phn b Gauss.

76 )2(2log21)(2log21)(2log21)(2222maxxnnxenxeyeyH+++(A.16)Do x v n c lp thng k vi nhau, nn0nx. Do vyNSy+2, ta c: )(2log21)(maxNSeyH+(A.17) +++NSNNSeNNSenHyHyxI1log21log212log21)(2log21)()();(maxmax(A.18)T , dung nng knh AWGN l: +NSByxIBCBCS1log.);(22max(A.19)A-4. Dung nng knh MIMONh ta bit, entropy ca bin ngu nhin lin tc x c tnh bng:[])(log)()(log)(22xpxpdxxpExHxxxx(A.20)Nu x l vct ngu nhin Gauss gi tr thc c chiu di m, tr trung bnhvma trn hip phng sai P:),(~PNx(A.21)th PemPxPxpPxPxpdxxHmTmT222/12/12122/12/121log21)2(log2)2(log)2()(+(A.22)Cng nh vy, nu x l mt vct ngu nhin Gauss trn c chiu di n:),(~PNxC(A.23)th PenxH22log)(log)(+(A.24)Nhng vect Gauss c tnh cht rt c bit: trong tt c cc vect ngu nhin xc tr trung bnh bng 0 v ma trn hip phng sai P, entropy H(x) l ti a nu x lbin Gauss.

77),|()|()|,(HxyHHyHHxyIreal(A.25)Sau p dng (A.14) v (A.22): IemHxyH222log21)2(log2),|(=(A.26)R rng, nu x c tr trung bnh bng 0 v ma trn hip phng sai P, th y c trtrung bnh 0 v ma trn hip phng sai l.IHPHT2+x c tr trung bnh bng 0 v ma trn hip phng sai P, th thng tin tng htrong (A.25) t ti a khi y l bin Gauss, khi x l bin Gauss. Vi la chn binGauss cho x nh trn, chng ta c: THPHIemHyH++222log21)2(log2)|((A.27)v do vy, TTrealHPHIIHPHIHxyI222222max1log21log21log21)|,(++(A.28)Nu thay ton b cc i lng l cc gi tr phc, v n l bin Gauss trn:(A.29)),0(~2INnCth HcomplexHPHIHxyI22max1log)|,(+(A.30)

78PH LC B. MT S KHI NIM, NH NGHA V MA TRNB-1. Ma trnMa trn X kch c mn c biu din nh sau:(B.1)nmmmnnxxxxxxxxxX,2,1,,22,21,2,12,11,1OTrong ma trn trn mi phn ttng ng l phn t thuc hng th i v ctth j. Cc phn t ca ma trn c th nhn gi tr thc hoc phc. X c gi l matrn vung nuji,xnm.B-2. Chuyn v ca ma trnChuyn v ca ma trn X, k hiu X , l ma trn c cc hng tng ng bng ccTct cng ch s ca ma trn X:(B.2)nmnnmmTxxxxxxxxxX,,2,12,2,22,11,1,21,1OBin i Hermit ca ma trn X, k hiu X , l ma trn c cc phn t hngHtng ng bng lin hp phc ca phn t ct cng ch s ca ma trn X:(B.3)*,*,2*,1*2,*2,2*2,1*1,*1,2*1,1nmnnmmTHxxxxxxxxxXconjXOA-3. ng cho chnh ca ma trnng cho chnh ca ma trn vung X c nh ngha bng vct vi cc phnt thuc ma trn X c ch s hng bng ch s ct:[]TmmxxxXdiagXdiag,2,21,1,)()((B.4)

79A-4. Vt ca ma trnVt (Trace) ca ma trn vung, k hiu Tr{X}, bng tng cc phn t ngcho ca ma trn X:(B.5)mpppxXTr1,}{Tnh cht:}{}{YXTrXYTr(B.6)A-5. nh thc ca ma trnnh thc ca ma trn vung X, k hiu X hoc , c xc nh quynh sau:)det( XMa trn vung cp 1, nh thc l 11xX;Ma trn vung cp 2, nh thc l 21122211xxxxX;Ma trn vung cp m, nh thc c tnh bng cng thc: ++mqpqqppqmppqqppqMxMxX11)1()1((B.7)Trong ,l nh thc con, c tnh bng nh thc ca ma trn cha bi hng thc p v ct th q.pqMTnh cht: YXXY(B.8)A-6. Gi tr ring, vct ring ca ma trnGi tr (v hng)v vct u tng ng c gi l gi tr ring v vct ringca ma trn X nu:uXu(B.9)Tnh cht: Xkk(B.10)}{XTrkk(B.11)

80A-7. Hng ca ma trnHng theo hng ca ma trn X l s cc hng c lp tuyn tnh ca ma trn.Hng theo ct ca ma trn X l s cc ct c lp tuyn tnh ca ma trn. Vi mt matrn bt k, hng theo hng bng hng theo ct c gi chung l hng ca ma trn, khiu l rank(X).A-8. Mt s loi ma trn in hnh- Ma trn khngX c gi l ma trn khng nu tt c cc phn t ca ma trn X c gi tr bng 0.- Ma trn n vMa trn vung X c gi l ma trn n v nu cc phn t trn ng cho chnhbng 1 v cc phn t cn li c gi tr bng 0. Ma trn n v c k hiu bngI .m- Ma trn ng choMa trn vung c gi ma trn ng cho nu cc phn t khng nm trnng cho chnh c gi tr bng 0. Ma trn ng cho c k hiu bngm.Nu gi cc phn t trn ng cho chnh l, c th biu din matrnnh sau:mddd,,,21m),,,(21mmddddiag(B.12)- Ma trn nghch oMa trn nghch o ca ma trn vung X, k hiu X , l ma trn tha mn:-1(B.13)mIXXXX11- Ma trn HermitMa trn X c gi l ma trn Hermit nuHXX.- Ma trn unitaMa trn vung X c gi l ma trn unita nu:(B.14)mHHIXXXXNh vy, ma trn unita l ma trn c nghch o bng bin i Hermit ca ma trn,ngha l:1XXH(B.15)

81Ma trn unita c k hiu.mUA-9. Phn hoch ma trn theo gi tr ring (EVD)Gi s X l ma trn Hermit. Khi , tn ti ma trn ng chov ma trnunita sao cho:m(B.16)HmmmUUXCp ma trn (,) gi l phn hoch theo gi tr ring (EigenValueDecomposition - EVD) ca ma trn X. ng cho ca ma trnbao gm cc gitr ring (gi tr thc) ca X.mUmmA-10. Phn hoch ma trn theo gi tr k d (SVD)Gi s X l ma trn kch c mn c hng bng r. Khi , tn ti ma trn ngchokch c rr vi cc phn t trn ng cho chnh c gi tr dng v hai matrn U, V sao cho:r(B.17)HrHrHVUXIVVIUUB ma trn (U,V,) gi l phn hoch ma trn theo gi tr k d (Singular ValueDecomposition - SVD) ca ma trn X.rA-11. Vect ha ma trnGi s X l ma trn kch c mn. Vct ha ma trn X c thc hin bng cchxp chng lin tip cc ct ca ma trn X:(B.18)[]TnmmmxxxxxxxXvec,2,2,22,11,1,21,1)(Tnh cht:[])()(}{YvecXvecYXTrHH(B.19)A-12. Tch KroneckerTch Kronecker ca ma trn X kch c mn v ma trn Y kch c m1n , k hiu1YX, l ma trn kch c mm1nn xc nh nh sau:1

82(B.20)YxYxYxYxYXnmmn,1,,11,1OTnh cht:)()()(YvecXZXYZvecT}{}{}{YTrXTrYXTr111)(YXYX(B.21)HHHYXYX)()()())((BYAXYXBA

83PH LC C. M NGUN CHNG TRNH M PHNG%===============================Chuong trinh chinh =============================%================================MIMO_OFDM.m==============================clear allclcformat longpilot_interval=10;% Khoang cach chen hoa tieupilot=[0 1 0 1];% Ky hieu hoa tieu (dang bit)cp_len=16;% Chieu dai CPSNR_dB=[0:5:40];ls_ber=zeros(1,length(SNR_dB));% Khoi tao gia tri BERmmse_ber=zeros(1,length(SNR_dB));for i=1:length(SNR_dB)ls_err_bit=0;mmse_err_bit=0;total_bit=0;packets=2;% 2 goi thong tinfor l=1:packetsofdm_symbol=200;% 200 ky hieu OFDM/1 goidata_source=input_bit(128,ofdm_symbol); % 128 ky hieu 16-QAM/1 ky hieu OFDM[nbit,mbit]=size(data_source);total_bit=total_bit+nbit*mbit;qam_symbol=bit_to_qam(data_source); % Anh xa tu day bit sang 16-QAM[data_ant1 data_ant2]=ST_Coding(qam_symbol);% Thuc hien ma hoa STC [s1 -s2*;s2 s1*]pilot_symbol=mod_qam16(pilot);%%%% chen hoa tieu[piloted_ant1,piloted_ant2,pilot_num]=add_pilot(pilot_interval,pilot_symbol,data_ant1,data_ant2);%%%% bien doi IFFTmod_out1=ifft(piloted_ant1,128);mod_out2=ifft(piloted_ant2,128);%%%% chen tien to vong CPafter_cp1=add_cp(mod_out1,cp_len);after_cp2=add_cp(mod_out2,cp_len);nline=8;% so da duongdelay=[0 1e-6 2e-6 3e-6 4e-6 8e-6 12e-6 15e-6];trms=.5*1e-6; % trai tre rmspow=exp(-delay/trms);fd=100; % do dich tan Dopplert_res=1e-6; % khoang thoi gian phan giaicount11=10000; count21=20000;count12=30000; count22=40000;start_count11=(l-1)*(5*count11);start_count21=(l-1)*(5*count21);start_count12=(l-1)*(5*count12);start_count22=(l-1)*(5*count22);tau_max=16e-6/t_res; % tre troi cuc dai%%% Truyen qua moi truong da duongsig_over_H11=multi_path(after_cp1,nline,pow,delay,fd,t_res,count11,start_count11);

84sig_over_H12=multi_path(after_cp2,nline,pow,delay,fd,t_res,count12,start_count12);sig_over_H21=multi_path(after_cp1,nline,pow,delay,fd,t_res,count21,start_count21);sig_over_H22=multi_path(after_cp2,nline,pow,delay,fd,t_res,count22,start_count22);%%% Tin hieu thu duoc tai cac angten thusig_over_H1=sig_over_H11 + sig_over_H12;sig_over_H2=sig_over_H22 + sig_over_H21;%%% Them nhieu AWGNsnr=10^(SNR_dB(i)/10);[nnl,mml]=size(sig_over_H1);power1=0;power2=0;for k=1:nnlfor b=1:mmlpower1=power1+real(sig_over_H1(k,b))^2+imag(sig_over_H1(k,b))^2;power2=power2+real(sig_over_H2(k,b))^2+imag(sig_over_H2(k,b))^2;endendaverage_power1=power1/(2*nnl*mml);average_power2=power2/(2*nnl*mml);sigma1=sqrt(average_power1/(2*snr));sigma2=sqrt(average_power2/(2*snr));receive_sig1=add_AWGN(sigma1,sig_over_H1);receive_sig2=add_AWGN(sigma2,sig_over_H2);%%% Loai bo CPremove_cp1=remove_cp(receive_sig1,cp_len);remove_cp2=remove_cp(receive_sig2,cp_len);%%% Bien doi FFTdemod_out1=fft(remove_cp1,128);demod_out2=fft(remove_cp2,128);%%% Uoc luong kenh theo luat LS va MMSEls_sig=ls(demod_out1,demod_out2,pilot_symbol,pilot_interval,pilot_num);[mmse_sig]=mmse(demod_out1,demod_out2,pilot_symbol,pilot_interval,...pilot_num,t_res,trms,tau_max,snr);%%% Anh xa ket qua sau xu ly PACE vao 16-QAMls_sig_bit=qam_to_bit(ls_sig);mmse_sig_bit= qam_to_bit(mmse_sig);%%% Dem loi bitls_err=count_error(data_source,ls_sig_bit);mmse_err=count_error(data_source,mmse_sig_bit);ls_err_bit=ls_err_bit+ls_err;mmse_err_bit=mmse_err_bit+mmse_err;end%%% Tinh ti le loi bit (BER)ls_ber(i)=ls_err_bit/total_bit;mmse_ber(i)=mmse_err_bit/total_bit;end%%% Ve gian do chom sao sau khi da xu ly LS va MMSEfigure;plot(ls_sig,'.');figure;

85plot(mmse_sig,'.');%%% Ve lien he BER-SNRfigure;semilogy(SNR_dB,ls_ber,'b-*')hold onsemilogy(SNR_dB,mmse_ber,'r-o')hold off%*******************************************Ket thuc chuong trinh chinh****************************************%=================================CC HM===================================%******************************* input_bit( ) ************************function x=input_bit(N,NL)for i=1:NLinput_0=rand(1,4*N);for j=1:4*Nif input_0(j)>0.5input(j,i)=1;elseinput(j,i)=0;endendendx=input;%**********************************************************************%**************************** bit_to_qam( ) *************************function output=bit_to_qam(input)[N,NL]=size(input);N=N/4;output=zeros(N,NL);for j=1:NLfor n=1:Nfor ic=1:4qam_input(ic)=input((n-1)*4+ic,j);endoutput(n,j)=mod_qam16(qam_input);endend%***********************************************************************%*************************** qam_to_bit( ) ****************************function output=qam_to_bit(input)[m,n]=size(input);output=zeros(4*m,n);for j=1:nfor i=1:my=demod_qam16(input(i,j));for ic=1:4output(4*(i-1)+ic,j)=y(ic);endendend%************************************************************************

86%*************************** mod_qam16( ) ***************************function x=mod_qam16(y)ify==[0 0 0 0]x=1+j;elseif y==[0 0 1 0]x=1-j;elseif y==[1 0 0 0]x=-1+j;elseif y==[1 0 1 0]x=-1-j;elseif y==[0 1 0 0]x=3+j;elseif y==[0 0 0 1]x=1+3*j;elseif y==[0 1 1 0]x=3-j;elseif y==[0 0 1 1]x=1-3*j;elseif y==[1 0 0 1]x=-1+3*j;elseif y==[1 1 0 0]x=-3+j;elseif y==[1 1 1 0]x=-3-j;elseif y==[1 0 1 1]x=-1-3*j;elseif y==[0 1 0 1]x=3+3*j;elseif y==[1 1 0 1]x=-3+3*j;elseif y==[1 1 1 1]x=-3-3*j;elseif y==[0 1 1 1]x=3-3*j;end%************************************************************************%*************************demod_qam16( ) ***************************function y=demod_qam16(x)y=real(x);y1=imag(x);if (y>=0)&(y2)y=3;elseif (y=0)&(y12)y1=3;elseif (y1