Moe.vn DANH MC T VIT TT T vit tt Ngha ting AnhNgha ting Vit ACIAdjacent Channel InterferenceNhiu knh ln cn AWGNAdditive White Gausse NoiseNhiu Gausse trng cng BERBit Error RateT l li bit CDMACode Division Multiple Accessa truy nhp phn chia theo m COFDMCodeOrthogonal Frequency Division Multiplexing Ghpknhphnchiatheotns trc giao m ha CPCyclic PrefixTin t vng DFTDiscrete Fourier TransformBin i Fourier ri rc thun FFTFast Fourier TransformBin i Fourier nhanh thun FSKFrequency Shift Keyingiu ch kha dch tn GIGuard IntervalKhong bo v HPAHigh Power AmplifierB khuch i cng sut ln IBOInput Back Off li u vo ICIIntercarrier InterferenceNhiu lin sng mang IDFTInverse Discrete Fourier Transform Bin i Fourier ri rc ngc IFFTInverse Fast Fourier TransformBin i Fourier nhanh ngc ISIIntersymbol InterferenceNhiu lin symbol LANLocal Area NetworkMng cc b LSLeast SquareBnh phng ti thiu MMSEMinimum mean square errorLibnhphngtrungbnhti thiu OBOOutput Back Off li u ra OFDMOrthogonalFrequencyDivision Multiplexing Ghp knh phn chia theo tn s trc giao PAPRPeak to Average Power RatioT s cng sut nh trn trung bnh PSKPhase Shift Keyingiu ch kha dch pha QAMQuadrature Amplitude Modulation iu ch bin vung gc TDMATime Division Multiple Accessa truy nhp phn chia theo thi gian Moe.vn LI NI U ........................................................................................................... 1 CHNG I TNG QUAN V OFDM .................................................................... 3 1.1CC KNH V TUYN ............................................................................. 3 1.1.1Truyn dn a ng .............................................................................. 3 1.1.2Hiu ng Doppler .................................................................................... 6 1.1.3Nhiu lin sng mang ICI (Intercarrier Interference) ............................ 8 1.2C S TRUYN DN A SNG MANG................................................. 8 1.2.1Nhc im truyn dn n sng mang (Single Carrier) ...................... 8 1.2.2Truyn dn a sng mang (Multicarrier) ............................................... 9 1.3OFDM .......................................................................................................... 11 1.3.1S trc giao gia cc sng mang .......................................................... 11 1.3.2S c bn ca OFDM ....................................................................... 13 1.4KT LUN CHNG ............................................................................... 15 CHNG II CC VN TRONG OFDM ........................................................ 16 2.1CC DNG IU CH TRONG OFDM ................................................. 16 2.1.1iu ch kha dch pha PSK (Phase Shift Keying) .............................. 16 2.1.2iu ch bin vung gc QAM (Quadrature Amplitude Modulation)18 2.1.3M ha Gray .......................................................................................... 19 2.1.4iu ch thch nghi trong OFDM ......................................................... 19 2.2KHONG BO V GI (GUARD INTERVAL) V TIN T VNG CP (CYCLIC PREFIX) ................................................................................................ 21 2.2.1Khong bo v GI ( Guard Interval) .................................................... 21 2.2.2Tin t vng CP (Cyclic Prefix)............................................................ 22 2.3C LNG KNH ................................................................................. 23 2.3.1Cu trc symbol pilot trong khung tn hiu OFDM .............................. 23 2.3.2Cc phng php c lng knh ........................................................ 25 Moe.vn 2.4T S CNG SUT NH TRN TRUNG BNH PAPR (PEAK TO AVERAGE POWER RATIO) ................................................................................. 28 2.4.1nh ngha PAPR .................................................................................. 29 2.4.2Cc phng php gim PAPR .............................................................. 30 2.4.3Bc x ph khng mong mun ............................................................. 34 2.5M HNH H THNG OFDM .................................................................. 35 2.6KT LUN CHNG ............................................................................... 36 CHNG III CC PHNG PHP NG B TRONG OFDM ...................... 38 3.1TNG QUAN V NG B TRONG OFDM ........................................ 38 3.1.1nh hng ca mt ng b thi gian .................................................. 40 3.1.2nh hng ca mt ng b tn s ....................................................... 44 3.2CC PHNG PHP NG B TRONG OFDM ................................. 49 3.2.1Cc phng php ng b thi gian ..................................................... 49 3.2.2Cc phng php ng b tn s .......................................................... 57 3.3KT QU M PHNG ............................................................................. 63 3.3.1Tin trnh m phng .............................................................................. 63 3.3.2nh hng ca mt ng b ti tn hiu OFDM .................................. 65 3.3.3Phng php c lng CFO trn min tn s Moose ......................... 66 3.4KT LUN CHNG ............................................................................... 70 KT LUN .............................................................................................................. 71 1 moe.vn Moe.vn LI NI U Cng vi s pht trin nhanh chng ca cc thit b cng ngh, cc dch v s cng ngy cng tr nn phong ph, p ng nhu cu ca con ngi. T mt thit b mthoichcchcnnglinlccbn,chicinthoigiycnhiu tnh nng vt tri nh: truyn hnh di ng, khng dy tc cao, c c nhng thnh tu , con ngi khng ngng Internet tm cch ci tin, nng cao kh nng truyn ti ca cc h thng v tuyn. Nh chng ta bit, mi trng v tuyn lun tn ti nhng hn ch nh c tnh bin i nhanh theo thi gian, kh ontrc,dbcannhiu,nhtlkhibngthngknhln.Vvy,tmra phng php truyn dn va m bo tc , va chng li nh hng xu ca mi trng lun l thch thc ln. Phng php ghp knh phn chia theo tn s trc giaoOFDM(OrthogonalFrequencyDivisionMultiplexing)raigiiquyt c nhng vn trn.OFDM l k thut iu ch cho cc ng dng a dch v tc cao nh chun 802.11a,chundnhchomngLANvtuynhiusutcaoHiPERLAN/2(High Performance Radio LAN/2) v pht thanh truyn hnh s ca ngy nay v tng lai nh truyn hnh s mt t DVB-T (Digital Video Broadcasting Terrestrial), pht thanh s DAB (Digital Audio Broadcasting) do cc u im ni tri so vi cc k thutiuchtruynthngnhchngcnhiulinsymbolISI(Intersymbol Interference), tri tr, fading a ng, hiu sut s dng ph cao. Nhng OFDM chainhcimln.ThnhttnhiuOFDMtrnminthigianltng hp ca tn hiuccsngmangcon,nnxut hin cc nh bin gymolncho cc b khuch i. Th hai , OFDM rt nhy vi lch thi gian v tn s.Vn ng b thi gian v tn s v ang c cc nh khoa hc nghin cu trong qu trnh pht trin OFDM. Xut pht t , em chn ti: Tm hiu 2 moe.vn Moe.vn cc phng php ng b trong OFDMlm n tt nghip. Mc tiu ca n l nghin cu, tm hiu cc nguyn tc, k thut c bn trong vic ng b thi gian v tn s trong h thng OFDM.Ni dung n gm tt nghip bao gm phn m u, ni dung ba chng v kt lun Phnmu: Gii thiu ti, xc nh tnh cp thit v ngha khoa hc, thc tin, mc ch nghin cu v phm vi thc hin ca n tt nghip. Chng I: Tng quan v k thut OFDM: Gii thiu s lc mc ch ra i, nguyn l c bn ca OFDM Chng II: Trnh by cc vn c bn trong h thng OFDM Chng III: Tm hiu cc phng php, nguyn l ng b trong OFDM Phnktlun:Khngnhlinhngktqutccanttnghip cng nhng mt cn hn ch 3 moe.vn Moe.vn CHNG ITNG QUAN V OFDM ChngIcanstrnhbynhngkhinimcbnnhttrongOFDM nh nhu cu dn n s ra i OFDM, tnh trc giao, s c bn hnh thnh h thng OFDM 1.1 CC KNH V TUYNHiu c cc c tnh ca knh truyn l v cng cn thit. N gip ta c ci nhn tng qut v knh, ng thi l nn tng ta chn gii php truyn dn hp l, m bo c tc , hiu qu bng thng 1.1.1 Truyn dn a ng 1.1.1.1Tri tr Trong lin lc v tuyn, s lan truyn ca sng c da trn cch di chuyn ca n t bn pht ti bn thu. Trn ng i, chng chu tc ng ch yu ca ba hin tng: phn x, khc x, tn x. Hnh 1.1 - ng i ca tia sng t bn pht n bn thu 4 moe.vn Moe.vn Phn x sng l hin tng vt l xut hin khi sng in t lan truyn p vo b mt vt th c kch thc rt ln so vi bc sng, v d nh b mt tri t hay ta nh cao tng. Nhng vt th ny khin cho tn hiu b phn xKhc x sng l hin tng xut hin khi ng i ca sng gia my thu v my pht b chn bi b mt vt cn khng ng nht hoc l nh. iu ny ging nh vic chng ta b cong tia sng quanh vt th v dn tri sng qua cc l.Tn x l hin tng vt l xut hin khi s bc x sng in t b nh lch hngbiccvtthrtnhsovibcsng.Hnh1.1mtcctrnghp truyn sng nh vy trong mi trng.Do chu cc tc ng trn m sng t trng thi mt tia u pht b tch ra thnh nhiu tia khc nhau i theo nhiu ng khc nhau ti u thu. Qung ng i cng di, cng tin hiu n cng yu. Ta gi hin tng ny l truyn dn a ng, c trng ca truyn dn trn knh v tuyn. Hin tng a ng cn dn n h qu tn hiu b tri tr. Trong khong thi gian sau thi im tia ti u tin mta gi l thigian tritr

,cctia tivn theoccng khc nhaun u thu. Hnh 1.2 m t p ng xung ca knh truyn khi c tri tr. Hnh 1.2- Hin tng tri tr 5 moe.vn Moe.vn 1.1.1.2 Nhiu lin symbol ISI (Intersymbol Interference) Tronghthngbngrng,khitccngcaothkhongcchgiacc symbol cng ngn li. Khong cch gia p ng xung ca cc symbol lin tip v vy cng hp hn. Do c a ng, phn tn hiu tri tr ca symbol trc s ln phn tn hiu ti ca symbol tip theo n. Cc symbol lc ny gy nhiu ln nhau, tagiylhintngISI(IntersymbolInterference).Hnh1.3minhhamt trng hp ISI in hnh. Ti thi im ly mu ca mt symbol, ui symbol trc nh hng n symbol sau. Hnh 1.3- ISI trn min thi gian ISIphthucvokhongcchgiaccsymbolcngnhkhongcchgia ccpngxung.Nicchkhc,tnhiuctccngcao(bngthngcng ln) th nh hng ca ISI cng ln. 1.1.1.3Fading Docctiasngchunh hngcatruyndn angnntnhiu u thu l tng hp ca cc tia ti. Nu cc tia ti ng pha nhau, chng s cng vo nhaulmlntnhiutngln.Nucctiatingcpha,chngslmgim mc tn hiu. Qu trnh ny l ngu nhin v lun thay i theo thi gian. V vy, fading l s thng ging ngu nhin ca cng tn hiu ti im thu. 6 moe.vn Moe.vn

Hnh 1.4- Tia ti cng pha v ngc pha ti im thu Hnh 1.5- Phn loi fading Fadingxttheocctiuchkhcnhauscnhiuloikhcnhaunhtrn hnh 1.5. Xt trn min tn s, ta c fading phng (flat fading) v fading chn lc tns(frequency-selectivefading).Trongfadingphng,bngthngcaknh truyn rt ln hn bng thng ca tn hiu. Do , tt c cc thnh phn tn s u thng ging mt lng nh nhau. Trong fading chn lc tn s, bng thng ca tn hiu ln so vi bng thng knh truyn. V vy, cc thnh phn tn s trong bng tn hiu c mc thng ging khc nhau, c th gy t lin lc. 1.1.2 Hiu ng DopplerHiungDopplergyrabischuynngtngicamythusovi my pht. Khi my pht pht sng vi tn s fc khng iu ch, sng ny i ti my thu theo nhiu tia. Xt tia th i 7 moe.vn Moe.vn Hnh 1.6- Hiu ng Doppler Khi tn hiu thu c theo tia sng th i c tn s b dch i mt lng dch tn Doppler:i m Df f o cos . = vi cv ffcm.=(1.1) Tn s tn hiu thu c l:|.|

\| + = +i c i m ccvf f f o o cos 1 cos . (1.2) trong vl vn tc chuyn ng ca my thu. NhvylngdchtnDopplerlnnhtkhi0 =io hay 0180tclkhimy thu chuyn ng dc theo phng pht sng. Ph Doppler ca sng mang c dng nh hnh 1.7 HiungDoppler khin cho sngmang bdchchuyn, dn nbng thng tnhiubdchchuyn,gynnthusai.Tathyrngcctnhiucbngthng cng hp th s cng nhy cm vi hiu ng Doppler. Hnh 1.7- Ph Doppler ca sng mang Tia sng th i v oi My thu 8 moe.vn Moe.vn 1.1.3 Nhiu lin sng mang ICI (Intercarrier Interference) Bng tn ca cc knh ln cn nhau lun c sp xp sao cho chng khng lm nhiu ln nhau. Nhng do hin tng Doppler hay mt ng b, cc knh ny c th b dch tn, m rng ph khin chng chng ln gy nhiu ln nhau. y l hintngnhiulinsngmangICI(IntercarrierInterference).ICIcthlm hiu sut h thng gim mnh, bt buc ta phi dng cc bin php khc phc thch hp u thu. 1.2 C S TRUYN DN A SNG MANG Tnhngtrngicaknhtruynvtuyntcnglntnhiu,cngvi nhu cu truyn ti d liu ngy cng tng cao, ngi ta lun tm cch m rng bng thng ng thi m bo tin cy, cht lng truyn tin, gi thnh hp l. y l mtththchkhngnhchoccnhsnxutcngnhqunlmng.Phng phptruyndntrnmtbngthngtruynthngkhngcnpngcnhu cu na. V khi nim truyn dn a sng mang ra i, to nn tng cho nhiu k thut mi c p dng.1.2.1 Nhc im truyn dn n sng mang (Single Carrier) Nhc im u tin truyn dn n sng mang l nhy cm vi fading. Khi truyntnhiutcthp,khongthigiantnticamtsymbolngn,ph chimcatnhiuthp.Tctruyncngcao,disymbolcnghp,ph chim cng ln. Bng thng tng s dn ti fading trn knh v tuyn biu hin l fadingchnlctns.Ccthnhphntnstrongbngtnhiuthngging khnggingnhau,dnnhmtruyntngcngcaknhkhngcdngcosin nng, gy ra ISI.Mtnhcimnacatruyndnn sngmanglbhn chtcdo tri tr. Khi tc truyn tng, nh hng ca tri tr cng s tng. Nh ta bit, 9 moe.vn Moe.vn khcphcnhhngcaISI,tritr,uthungitathngdngcc Equalizer.Nhngtctruyncngln,Equalizercngphctpvchcth p ng v mt k thut mt gii hn nht nh. V vy, vi truyn dn n sng mang, tc truyn lun b gii hn.Trong cc h thng a truy nhp phn chia theo m CDMA, khc phc hin tng tri tr, ngi ta s dng cc my thu Rake. My thu Rake c tc dng tng hp tt c cc thnh phn tn hiu a ng c c mc tn hiu tt nht. Tc truyn cng tng th phc tp ca my thuRake cng tng. Do , trong h thng CDMA, tc truyn cng b gii han.Nhng hn ch ny ca truyn dn n sng mang c gii quyt trong k thut truyn dn a sng mang (Multi carrier) 1.2.2 Truyn dn a sng mang (Multicarrier) Nhng hnh thc ban u ca truyn dn a sng mang ra i t rt sm. didngtruyndnaknh(Multichannel).Hnh1.8mtvictruyndna knh Hnh 1.8 - S truyn dn a knh LungtnhiucntruynxcphntchthnhNlungconnhhnnh cc b lc

. u thu, gii iu ch cc thao tc c thc hin theo th t ngc li. Nh th, t tn hiu bng tn rng phi chu fading a ng, nay h thngch phichufading phng doccsngmangconc thcoi kh hp so 10 moe.vn Moe.vn vi hm truyn knh. iu ny cn gip gim phc tp ca bEqualizer u thu.Mtkhc,phtnhptngngvivicrngvkhongcchcc symboltnhiuln, nhhngcaISIgimnh.trnhICIgiaccphtn, ngitachnvocckhongGuardband.Chnhvvymphngphpnyc hiu qu s dng ph thp. Mi knh trong h thng yu cu phi c b thu pht ring v vy dn n tn thit b.Truyn dn a sng mang l mt trng hp c bit ca truyn dn a knh, trong d liu c chia thnh cc lung d liu con, c iu ch song song nhau ri truyn i trn cc sng mang con (Hnh 1.9a) Hnh 1.9a- S truyn dn a sng mang Hnh 1.9b- Ph tn truyn dn a sng mang Phngphpnyhntruyndnaknhchcclungsngmangcon c xp st nhau, khng tnGuardband (Hnh 1.9b). Cng mt rng ph tn vi truyn dn n sng mang, truyn dn a sng mang c th mang mt lung d liuctctngng.Hnnafadingtrnknhlcnychcnlfading 11 moe.vn Moe.vn phng, nh hng ca ISI c hn ch. Nhc im tn ti y vn lvic tn N cp my thu pht cho N sng mang.1.3 OFDM Nm1966,R.WChangphtminhrakthutOFDM(Orthogonal FrequencyDivisionMultiplexing)lmttrnghpcbitcaphngphp truyn dn a sng mang, trong cc sng mang con trc giao vi nhau.1.3.1 S trc giao gia cc sng mang tmhiutnh trcgiaoca sngmang,taxt haitnhius1(t) vs2(t) c dng sng thi gian, chu k Ts. Hai tn hiu ny trc giao khi hm tng quan ca chng bng khng R =

= 0(1.3) Trnmintn s, khicc sngmangtrcgiao vinhau thtns trungtm ca sng mang ny ri vo im khng (null) ca cc sng mang khc (hnh 1.10). Hnh 1.10- Hnh nh cc sng mang trc giao trn min tn s Nugi

ltnscaknhconknhconthi,Nlsknhcon,l khong cch gia cc knh con, W l rng bng thng, Wi l rng knh con, T l rng symbol OFDM, Ti l rng symbol ca knh con th

= i.

=; W =

; (1.4) 12 moe.vn Moe.vn Biu din ton hc ca sng mang th i s l

. Xt tng quan ca hai sng mang th i v th j R =

=

=

(1.5) Vi i = j th R 0 ;i j th R = 0 Hnh 1.11 m t qu trnh truyn tn hiu OFDM trn cc sng mang trc giao. Cnggingnhtruynasngmang,dliucchiathnhNlungconiu ch song song. iu khc l ch N sng mang con u ra trc giao, chng ln ln nhau, cho php truyn cng tc nhng vi ph tn nh hn. Hnh 1.11- S m t truyn dn OFDM khc phc nhc im l tn thit b thu pht cho N sng mang con, nm 1971, Weistein v Ebert p dng bin i Fourier ri rc DFT (Discrete Fourier Transformation) v bin i Fourier ri rc nghch IDFT (Inverse Discrete Fourier Transformation)vo qu trnh iu ch v gii iu ch tn hiu OFDM.Php bin i Fourier ri rc l php bin i hai chiu DFT :

(1.6a) IDFT:

(1.6b) 13 moe.vn Moe.vn Trn thc t, iu ch/gii iu ch tn hiu OFDM, ta s dng cc cp khi tnhtonbiniFouriernhanhFFT(FastFourierTransformation)vbini Fourier nhanh nghch IFFT (Inverse Fast Fourier Transformation) thay th cho N cpmythupht.ngthivicpdngbiniFouriernhanhFFTgiptit kim thi gian tnh ton so vi bin i DFT. Hnh 1.12- Dng tn hiu OFDM trn min thi gian 1.3.2 S c bn ca OFDM Hnh 1.13- S c bn ca OFDM 14 moe.vn Moe.vn S c bn ca h thng OFDM c th hin trn hnh 1.13. Lung d liu cntruyndidngbitcchiathnhNlungconnhnhbchianitip songsongS/P(SerialtoParallel).urakhi nyciu ch bnggc song song.Nsymboliuchca vokhibiniIFFTNim,ctng hp li thnh symbol OFDM nh khi bin i song song ni tip P/S (Parallel to Serial) ri truyn i trn knh. Nh vy, mt symbol OFDM bao gm tn hiu ca N symbol bng gc trn N sng mang.Gi

l symbol th l ti sng mang th k, l = 0,1,2 , k= 0,1, N1. Nh b bin i S/P, thi gian truyn symbol tng ln l NTs. Symbol OFDM s c di l Tsym = NTs. Sng mang th k ti symbol th l c dng

{

(1.7) Tn hiu sau bin i IFFT trn min thi gian c dng

) = Re{

{

}

}(1.8a) =

Tn hiu lin tc ny c th c ly mu tn s

=

=

(1.8b) Gi s tn hiu nhn c c dng

=

vi l

< tl

+ n

(1.9a) Tn hiu ny c ly mu, c dng {

}

s i qua b FFT gii iu ch

=

15 moe.vn Moe.vn = {

}

=

=

(1.9b) Davonhngphntchtrn,tacthkhiqutqutrnhiuch/gii iu ch bng b IFFT/FFT vi s hnh 1.14 Hnh 1.14- S m t iu ch OFDM 1.4 KT LUN CHNG Trong nhiu nm qua, k thut OFDM ni ring v truyn dn a sng mang ni chung c s dng rng ri, nht l trong truyn dn v tuyn, gii quyt tt nhng vn t ra trong truyn dn n sng mang. OFDM hot ng rt hiu qu trong cc mi trng c tri tr, a ng, fading. OFDM kt hp cng vi m hahpl,ghpxenlcngcmnhmchngliccsuygimtrnng truyn.Cchthngdingthhmi,ccchunLANtccaonh4G, Wimax cng ng dng OFDM. Kthut ny c k vng s s dng nhiu hn na trong cc h thng vin thng tng lai. 16 moe.vn Moe.vn CHNG IICC VN TRONG OFDM VicpdnglthuytvOFDMvothctkhngphingin.Cctc ngcaknhtruyn,cccimringcaOFDMlmchovicamhnh OFDM vo thc tin tr nn kh khn. Chng II s trnh by v cc vn c bn gp phi trong OFDM 2.1 CC DNG IU CH TRONG OFDM Trong h thng OFDM, tn hiu u vo l tn hiu nh phn. V vy, iu ch trongOFDMliuchs.Dngbituvocchiathnhccnhnh,mi nhnh gm N dng bit khc nhau tng ng vi phng thc iu ch s. Hay ni cch khc, phng thc iu ch bng gc s quyt nh s bit u vo b iu ch.Trong truyn dn OFDM, vic m bo s trc giao gia cc sng mang con rt quan trng. Do , ch c nhng phng thc iu ch tuyn tnh c s dng. y l iu ch kha dch pha M-PSK (Phase Shift Keying) v iu ch bin vung gc M-QAM (Quadrature Amplitude Modulation) kt hp vi m ha Gray. PhngthciuchkhadchtnFSK(FrequencyShiftKeying)khngc dngvsgyphtxph,phdnghmBesselkhngcccimkhngca ph khin cho cc sng mang con mt tnh trc giao 2.1.1 iu ch kha dch pha PSK (Phase Shift Keying) PhngthciuchkhadchphaM-PSKsdngctnhphacasng mang iu ch tin tc. M l s trng thi c th c ca symbol sau iu ch. Do diphasngmangl0,ccphacasngmangcsdngtruyn thng tin s thng qua iu ch pha s l

= 2 m/M vi m = 0,1M1. Nh th i vi iu ch pha nh phn (M = 2), hai gc pha ca sng mang l

= 0 v 17 moe.vn Moe.vn

= . i vi iu ch pha s M mc, M =

trong k l s bit thng tin trn mt symbol truyn i. Chm tn hiu 8-PSK c m t trn hnh 2.1. Dng tn hiu PSK tng qut l

vi m = 0,1, M1(2.1) trong

l dng xung b lc pht. A l bin tn hiu Hnh 2.1- Chm sao tn hiu 8PSK Khi mun tng tc iu ch, ta tng s bit c iu ch trn mt symbol. iunylmtngchsM.Mcngtng,vicngmttstntrntpthcc im tn hiu cng li gn nhau. Do , xc sut li bit cng ln theo. Ph ca tn hiu PSK ch cha thnh phn mang tin tc v cc hi bc 3, 5 m khng cha thnh phn sng mang 18 moe.vn Moe.vn Hnh 2.2- Dng tn hiu PSK trn min thi gian PSK c s dng kh ph bin trong iu ch s, nht l 4-PSK v 8-PSK.2.1.2 iu ch bin vung gc QAM (Quadrature Amplitude Modulation) Phng php iu ch vung gc QAM s dng hai sng mang vung gc l

v

,mimttrongchngciuchbichuiclpcc bitthngtin.Hnh2.3biudindngtnhiutrnminthigian.Hnh2.4biu din chm sao tn hiu 16-QAM, trong hai thnh phn sng mang c iu ch bi cp chui hai bit c lp nhau. Tng qut hn, QAM c th xem nh mt dng hn hp ca iu ch pha v iu ch bin .Dng tng qut ca tn hiu QAM l

vi m = 1,2...M1, n = 1,2M2(2.2)

Hnh 2.3- Tn hiu 8-QAM trn min thi gian

19 moe.vn Moe.vn Hnh 2.4- Chm sao tn hiu 16-QAM QAMthchhpviccngdngtccao.Ngitathngsdng16-QAM v 64-QAM. 2.1.3 M ha Gray M Gray l mt h thng k s nh phn, trong hai gi tr lin tip ch khc nhau mt ch s. Trong OFDM, m Gray c s dng gn cc t hp bit cho cc symbol sao cho cc symbol ln cn nhau ch khc nhau mt bit. Trong qu trnh truyn v nhn tn hiu, hu nh cc symbol b sai li thnh cc symbol ln cn. Do , s dngmGray,nu xyralith hunhchlimtbit,t l li bit BER (Bit Error Rate) s gim. Bng 2.1 minh ha trng hp m Gray vi M = 16 Bng 2.1-Bng m Gray DecimalM GrayDecimalM Gray 0000081100 1000191101 20011101111 30010111110 40110121010 50111131011 60101141001 70100151000 2.1.4 iu ch thch nghi trong OFDM Cc k thut iu ch truyn thng thng c mt s nhc im ging nhau. l s dng cc tham s iu ch c nh m bo cht lng truyn dn ti 20 moe.vn Moe.vn trngthixunhtcaknhtruynnnscthnglngnhnhaumithi im dn n lng ph hiu qu ph tn.iuchthchnghitrongOFDMcsdngthchnghingthams iu ch ng vi cc thng s tc thi ca knh, nhm m bo cht lng truyn dn yu cu. C nhiu phng php, thut ton thch nghi khc nhau, nhng c bn c ba thut ton chnh: -Thch nghi theot s tn trn tpSNR (Signal to Noise Ratio) pht trn mi sng mang -Thch nghi theo c ch chuyn mc iu ch -Thch nghi theo c ch chn lc sng mang M hnh thut ton thch ng iu ch c minh trn hnh 2.5. Thng tin v cht lng knh trn tng sng mang con truyn c h thng nhn bit qua c lng knh. Sau , da vo thng tin nhn c, h thng chn cc tham s ph hp truyn. Cc tham s bao gm kiu iu ch, t l m, kiu m ha. Do iu ch thch nghi nn dng iu ch, m ha trn mi sng mang con c th khc nhau. Nu knh tt, h thng s tng bc iu ch, gim t l bit m trn bit tin nhm tng tc knh thng tin. Ngc li, nu knh xu, bc iu ch s gim,sbitmtngnhmtngkhnngsalichotnhiu.Bnthusc thng bo v cc tham s thch nghi tin hnh gii iu ch.

c lng knh Chn tham s cn thit theo thng tin v knh cho ln truyn tip theo Thng bo cc tham s cho bn nhn 21 moe.vn Moe.vn Hnh 2.5- S chung thut ton thch nghi2.2 KHONGBOVGI(GUARDINTERVAL)VTINTVNGCP (CYCLIC PREFIX) 2.2.1 Khong bo v GI ( Guard Interval) Mt trong nhng u im quan trng ca OFDM l vic i ph hiu qu vi tri tr a ng. Bng cch chia lung tn hiu ra thnh N lung con, rng gia cc symbol tng ln, nh hng ca tri tr gim ng k. Mc d vy, vic truyn tnhiuOFDMvnkhngtrnhkhinhhngcaISI.Nhntrnhnh2.6ta thy, ti mi sng mang con, cc symbol u b nh hng ca tri tr. Tia ti u tin b ln ln bi tia ti tr, gy ISI, BER ca h thng tng. Hnh 2.6- nh hng ca ISI trong OFDM gii quyt vn ny, khi truyn tin ngi ta thm vo khong bo v GI. GIlmtkhong thigianmtrong khngmangtintc. GIct xen k gia cc symbol (hnh 2.7). Khi ny, chiu di symbol OFDM s bao gm chiu di symbol tin tc cng vi chiu di khong GI. Nh c GI nn khi xut hin tri tr, cc tia ti sau do tri tr s ln ln phn GI khng mang tin ca tn hiu symbol ti trc, trnh c ISI. 22 moe.vn Moe.vn Mt khc, chiu di GI phi hp l. Nu ngn qu s khng th chng c tri trlncaknh.Nulnquslmgimhiusuttruyntin.Hnh2.8chra trnghpkhongGIngnhnkhongtritrtia,phnuicasymbol OFDM (phn hnh qut) ln ln phn u ca symbol OFDM tip theo, gy ISI Hnh 2.7- V tr ca GI trong khung tn hiu OFDM

Hnh 2.8- Trng hp GI ngn hn tri tr ca knh 2.2.2 Tin t vng CP (Cyclic Prefix) Nhc im ca GI l ch, do khng mang tin nn ti phn giao nhau gia symbol v GI, tn hiu b gin on, lm ph tn hiu m rng, v mt tnh trc giao gia cc sng mang, gy ICI. trnh ICI, ta s dngtin t vng CP. Mc ch ca CP l lm cho tn hiu lin tc tsymbol OFDM ti GI bng cch copy phn cuisymbolOFDMlnu.VicthmCPvo khungtnhiuOFDMcthc 23 moe.vn Moe.vn hin sau khi IFFT bn pht v g b CP c thc hin trc khi FFT u thu. Hnh 2.9- M t CP trn min thi gian/tn s 2.3 C LNG KNH Trong h thng OFDM, bn pht iu ch cc chuibit tin thnh ccsymbol PSK/QAM, s dng bin i IFFT chuyn chng thnh tn hiu trn min thi gianritruyntrnknhvtuyn.Tnhiuthucthngbmodoctnh caknh.khiphcliccbittruyn,tcngcaknhtruyncnphi c c lng v b u thu.Mt khc, mi sng mang con trong OFDM c th coi nh mt knh v tuyn c lp vi iu kin khng c ICI. Do vy, tn hiu ti mi sng mang con s chu tcngcaknhtruynkchnhau.ngthi,tnhiutiuthucthc khi phc bng vic c lng knh ti tng sng mang con. Vic c lng knh truyn c thc hin nh cc symbol pilot hay mo u c bit trc u pht v u thu, kt hp vi cc thut ton c lng. Ngoi ra, cc kha cnh khc nh hiu sut, phc tp tnh ton cng phi c tnh n 2.3.1 Cu trc symbol pilot trong khung tn hiu OFDM Davocchspxpccpilot,tacbaloipilot:kiukhi(block),kiu rng lc (comb) v kiu li (lattice) 2.3.1.1Kiu khi (block) 24 moe.vn Moe.vn Vi cch sp xp cc pilot theo kiu khi, tt c cc sng mang con u cha pilot. Chng c truyn nh k theo thi gian. Nh vy, knh truyn s c c lngtrnminthigian.GiStlkhongthigiangiuhailntruynsymbol pilot. c th nm bt kp nhng thay i ca c tnh knh truyn, khong cch giahaipilotphikhnglnhnnghchotnsDoppler,haychnhlthi gian kt hp ca knh (coherence time). St

(2.3)Cch sp xp ny ph hp cho vic c lng fading chn lc tn s do tt c cc sng mang con u c pilot Hnh 2.10- Sp xp cc pilot theo kiu block 2.3.1.2Kiu rng lc (comb) Cchspxppilotkiurnglccmtnhhnhdi.Vikiuny, misymbolOFDMchapilottrongnhngsngmangconnhtnh.Nhvy, knh truyn s c c lng trn min tn s. Gi Sf l chu k lp li ca pilot trn min tn s. c th theo kp cc thay i ca knh truyn dn a ng, tn s truyn pilot phi ngang bng bng thng kthpca knh.Bngthng kthpchnhbng nghcho ca trln nht ca mi trng v tuyn

. Ta c25 moe.vn Moe.vn Sf =

(2.4) Tri ngc vi kiu block, sp xp pilot kiu rng lc thch hp vi cc knh c fading nhanh hn l knh c fading chn lc tn s. Hnh 2.11- Sp xp cc pilot theo kiu comb 2.3.1.3Kiu li (Lattice) Cch sp xp pilot kiu li l s kt hp ca hai kiu trn. Knh truyn c c lng trn c min tn s v thi gian. Hnh 2.12- Sp xp cc pilot theo kiu Lattice 2.3.2 Cc phng php c lng knh26 moe.vn Moe.vn 2.3.2.1 c lng da vo symbol hun luyn (training symbol) Ccsymbolhunluynhaychnhlccpilotlmtcchhiuquc lngknh.Tuynhin,hiuqutruyndnsbgimdoccpilotnykhng mang tin. Vi phng php c lng ny, hai k thut c s dng ph bin l LS (Least Square) v MMSE (Minimum Mean Square Error).Gi s cc sng mang con trc giao, khng c ICI, pilot ca N sng mang c th c biu din bng ma trn X = [

] (2.5) trong X[k] l pilot ca sng mang th k. Symbol hun luyn u thu c biu din di dng ma trn nh sau Y[

] = [

] [

] [

] = XH + Z(2.6) Z l vector nhiu tp Z =

Tagi lvectorclngcaknhtruynH.ViphngphpLeast Square, ctnhtonmtcchngnnhtvnginnht.Thngquaqu trnh bin i, ta c

=

vi k = 0,1,2 N1; (2.7) Vi phng php MMSE, , c tnh nh s 2.13, trong = W. Ta gi

l gi tr t tng quan ca ,

l gi tr tng quan cho gia vector H v

= E{

} = E{

} 27 moe.vn Moe.vn = E{

} = E{

} = E{

} + E{

} = E{

} +

(2.8) Ta tnh c , = W=

(2.9) Hnh 2.13- S phng php c lng MMSE 2.3.2.2c lng da trn php bin i DFT Phng php c lng da trn DFT c s dng nng cao hiu qu ca phng php LS hay MMSE, gim s nh hng ca nhiu tp. Tagi lvectorccdoclngknhtruynbngphngphp LS hay MMSE. Nhn vo s 2.12 ta thy

= IDFT {}= h[n] + z[n] vi n = 0,1, N-1; 28 moe.vn Moe.vn Hnh 2.14- S hot ng trong phng php DFT B qua thnh phn ch cha nhiu tp, coi tri tr ca knh truyn c di L. Xt c cha tn hiu bao gm L mu

= { (2.10) Nh vy

= DTF{

} 2.3.2.3c lng knh DD (Decision- Directed) Trong phng php c lng knh DD, ta ch cn gi cc pilot vo thi im banucaqutrnhtruyn.Sau,ccthngtinvknhscgivtrn knh DD (Decision- Directed) m khng cn symbol pilot na. S ca phng php ny c ch ra trn hnh 2.13. Hnh 2.15- S phng php c lng Decision Directed Ngoinhngphngphpnutrn,vicclngknhcncthc hinbngnhiukthuttintinkhcnhclngdatrncctnhiuxp chng, c lng bng thut ton k vng cc i 2.4 TSCNGSUTNHTRNTRUNGBNHPAPR(PEAKTO AVERAGE POWER RATIO) TnhiutruyntrongOFDMcthcnhnggitrnhrtlntrnmin thigiandocrtnhiuthnhphnsngmangccngvinhau.Vvy,h thngOFDMcnhcimltnhiucPAPRln.HintngPAPRlm 29 moe.vn Moe.vn gimhiuqubkhuchitrongmypht.PAPRgynhhnglnnht ng ln v hiu qu khuch i my pht lc ny ph thuc ch yu vo ngun pin c hn trong cc thit b di ng 2.4.1 nh ngha PAPR V mt tng qut, k c nhng b khuch i tuyn tnh cng c nhng vng bo ha u ra do mc tn hiu u vo ln qu mc bnh thng. ng vi mc cngsutuvomax

scmccngsutura

mbochotn hiu c khuch i tuyn tnh. V vy, tn hiu u vo cn c iu chnh li (Backoff)lmvictrongvngtuyntnh.Vngphituyncabkhuchi cng sut ln (HPA- High power amplifier) c biu th qua thng s IBO (Input Back off) hay OBO (Output Back off).IBO = 10

; (2.11) OBO = 10

(2.12) PAPR l t s gia cng sut ln nht v cng sut trung bnh ca tn hiu phc s(t)PAPR {s(t)}=

{

} =

{

} (2.13) KhisdngiuchPSKviNsngmangcon,cngsutsngmangt nh khi tt c N thnh phn sng mang cng pha. Khi , cng sut nh s bng N ln cng sut trung bnh. Xc sut xy ra PAPR s cng gim khi N cng tng. Mt khc, xc sut c PAPR trong M-QAM s ln hn M-PSK vi M>4.30 moe.vn Moe.vn Hnh 2.17- c tnh vo ra ca b khuch i cng sut ln HPA 2.4.2 Cc phng php gim PAPR CcbinphpgimPAPRcchiarathnhhainhm:nhmgymotn hiu nh ct nh (clipping) v lc, gy mo trc, v nhm khng gy mo tn hiu nh m ha, tri DFT Phng php ct nh ct i cc nh cng sut ln trnh a b khuch i vovngboha.Phngphpnyngintrongthchin,nhngnsgy nhiu trong bng v ngoi bng, do ph hng s trc giao gia cc sng mang.Phng php s dng m s chn ra cc t m c kh nng gim nh PAPR. Vic khng gy nn mo, khng gy pht x ngoi bng. Nhc im ca phng php ny l vic s dng m s lm gim hiu qu bng thng. ng thi n kh phc tp trong vic tm ra nhng m ti u nht v ta phi xy dng bng m ha, gii m kh ln, c bit l khi s sng mang con tng.PhngphpxotrnsxotrndliuuvocasymbolOFDMv truyn i symbol c PAPR nh nht. T xc sut xut hin PAPR c th c gim xung. Khi s sng mang con cng tng, k thut ny cng tr nn phc tp.Phngphpsdngmotrcthchnghicthbnhngnhhngphi tuyn t b khuch i cng sut trong h thng OFDM.31 moe.vn Moe.vn Phng php tri DFT x l tn hiu u vo bng bin i DFT, hoc nhanh hnbngIFFT.VicnycthgimPAPRcaOFDMxungtngngvi truyn dn n sng mang. K thut ny kh hiu qu vi cc thit b ucui di ng ng ln, nht l vi chun 3GPP LTE.2.4.2.1 Ct nh (Clipping) v lc y l k thut gim PAPR n gin nht. Ct nh c ngha phng php ny sgiihncngsutcatnhiuphtphtvmtmcchotrcno.Tuy nhin, phng php ny c mt s nhc im: -Ctnhgyramotnhiu,dolmBERcahthngtng.Xcsut phn b ca nh tn hiu ln cng cao th mo tn hiu cng tng -Ct nh cn gy pht x ngoi bng, gy nhiu knh ln cn ACI (Adjacent ChannelInterference)choccknhlncn.Vnnycthkhcphc bngviclc.Tuynhin,viclcphlilmchotnhiusaulcvt ngng ct cho trc.Biudintonhccatnhiusaukhicctccnhtnhiuvtqu ngng nh sau, vi A l gi tr ngng ct,

(m) l tn hiu trn min thi gian.

= {

(2.14) Ni chung, hiu qu ca phng php ny c th c nh gi thng qua mt s tiu ch -gnsngtrongbngvphtxngoibng,cthquanstc thng qua th mt ph cng sut -PhnbcahsnhcaPAPR,ltsgianhtnhiungunhin Rayleigh v gi tr trung bnh ca tn hiu.-T s ct nhClipping Ratio l t s gia gi tr nh ngng cho trc v gi tr trung bnh ton phng ca tn hiu cn ct 32 moe.vn Moe.vn -Gi tr BER ca tn hiu c m ha v khng c m ha Hnh 2.18a- Tn hiu trc khi thc hin clipping Hnh 2.18b-Tn hiu sau ct nhTn hiu sau lc Ta xt tn hiu trn min thi c hm mt xc sut v mt ph cng sut dng nh hnh 2.18a. Nhn hnh 2.18b ta thy tn hiu sau khi c ct nh ph s m rng. Ph ny s c thu nh sau khi lc. Nhng sau lc, nh tn hiu li tng ln 2.4.2.2S dng m ha gim PAPR 33 moe.vn Moe.vn Trongphngphpny,ccbitdliuscmhathnhccmkhc nhau. Ly v d s dng iu ch tn hiu 16-PSK. Bn bt d liu s c m ha, ngha l thm mt sbit m to thnh t m hon chnh. Sau , cc t m c PAPRtrnminthigiannhnhtscchn.Tctruynscgim xung gim t s PAPR. Hnh 2. 19- Tn hiu trc v sau khi m ha vi s sng mang N = 16 Mt phng php tng t nh phng php trn l phng php nh x chnla(selectivemapping).Tnhiuuvoscnhnvimtchuic pha khc nhau sau c a qua bin i IFFT. Tn hiu OFDM u ra c a vo b chn. B chn ny s chn pht nhng tn hiu c PAPR thp nht.Ngoi hai phng php c bn nu trn, cn rt nhiu phng php khc gim PAPR nh tri DFT, t trc tn s, Hnh 2.20- Phng php nh x chn la 34 moe.vn Moe.vn 2.4.3 Bc x ph khng mong mun TnhiusauiuchbaogmccthnhphnsngmangconQAM/PSK chaclc.Phcachngcdngnngimkhchm.iunyc th gy nn ACI Hnh 2.21- Mt ph cng sut ca tn hiu OFDM gim thnh phn pht x ngoi bng, trnh gy nhiu ACI cho cc knh ln cn, ta c th s dng b lc thng di. Nhng c c hiu qu vi cc b lc ta cn qu trnh tnh ton phc tp. Thay vo , ta s dng mt ca s trn min thi gian c dng cosin nng vi h s un lc . Hm truyn ca s c dng

= {

(2.15) vi Tg l di khong CP, Tsub l di symbol OFDM.Tn hiu sau khi c x l bng hm ca s s c dng

=

(t - lTsym).

(2.16)35 moe.vn Moe.vn Hnh 2.22- Dnh hm ca s cosin nng trn min thi gian Hnh2.22mtcasdngxltnhiuOFDM.Khihsunlc tng, phn bin gim dn ti hai bin ca s s thoi hn, nh hng ca ACI s gim xung.2.5 M HNH H THNG OFDM Nh vy, t s c bn ca OFDM, c th ng dng l thuyt vo thc t, ta phi b sung thm nhiu thnh phn nhm gii quyt nhngvn tn ti trong OFDMnhISI,tcngcaknhtruyn,phtnhiunhthhintrnhnh 2.23.KhiPilotinsertioncnhimvchnccpilotvokhungtnhiuOFDM phc v c lng knh. u thu, khi Channel correction s da vo cc pilot tm ra cc thng s knh truyn hin ti. Khi GI insertion and windowing u phtvRemoveGIuthucnhimvchn/gGIcngnhxltnhiu OFDMtheocascosinnng.CckhiDAC,ADC,Upconverter,Down converter s x l tn hiu pht/thu tn hiu cao tn ra/t knh truyn. 36 moe.vn Moe.vn Hnh 2.23- M hnh h thng OFDM 2.6 KT LUN CHNG Nhvy,quaqutrnhtmhiuccvnvOFDM,tanhnthyphng php ny c nhng u im v nhc im ring KthutOFDMcnhiulichmcckthutghpknhkhckhngc c.OFDMchophptruynthngtintccaobngcchchiabngtnra thnh nhiu sng mang con trc giao, fading chn lc tn s trn knh nh m thnh fading phng. Hin tng ICI do cng c loi b. Nh s dng khong bovGI/CPnndtrnknhctritr,hintngISIcngbhnch.Cng nh CP m h thng OFDM ch cn b cn bng min tn s n gin. Vic truyn dn c iu chnh thch nghi theo iu kin knh v tuyn nh c ch c lng knh.Do OFDM l phng php truyn dn a sng mang nn nhc im chnh ytscngsutnhtrncngsuttrungbnhPAPRln,dnnvickhc 37 moe.vn Moe.vn phc phc tp. Mt khc, tn hiu OFDM l tng hp ca nhiu sn mang con nn ng bao bin khng bng phng, gy mo phi tuyn trong cc b khuch i.Mt nhc im na trong phng php ny l vic s dng nhiupilot, cc khongGI/CPkhngmangtinkhinhiuqutruyntingim.rngccsng mang con nh nn h thng OFDM nhy cm vi hiu ng Doppler cng nh i hi mt qu trnh ng b nhanh chng v chnh xc.Ngy nay, k thut OFDM cn c kt hp vi phng php m knh trong thngtinvtuyn.CchthngnycncgilCOFDM(codeOFDM). Trong h thng ny tn hiu trc khi c iu ch OFDM s c m knh vi ccloimkhc nhaunhmmcchchng licc li ngtruyn.Tuynhin n khng tp trung tm hiu v COFDMm s tin hnh tm hiu cc phng php ng b trong OFDM. 38 moe.vn Moe.vn CHNG IIICC PHNG PHP NG B TRONG OFDM C mt s mt khin cho ng b trong h thng OFDM khc bit so vi ng b trong truyn dn n sng mang. OFDM chia lung d liu thnh mt c s cc lung con. Mi lung con li c tc truyn nh hn v khong thi gian symbol Ts di hn, qua m nh hng ca tri tr gim xung. Hn na, vic ny cn khin cho h thng chu li do mt ng b thi gian tt nh cc khong CP khng mang d liu. Tuy nhin, trong thc t lun c nhng trng hp c bit, sai lch mt ng b vt qu gii hn kim sot ca h thng. Mt khc, do rng cc sng mang con rt nh so vi tng rng bng thng nn vic ng b tn s tr nn kh khn. V d nh vi h thng truyn dn OFDM lm vic tn s trung tm l fc = 1500MHz vi rng symbol T = 500ms th t s gia khong cch cc sng mang v tn s trung tm s l

= 1,33.

, i hi chnh xc rt cao trongngb.Trongchngny,tasxemxtccphngphpngbtrong OFDM 3.1 TNG QUAN V NG B TRONG OFDM ivihthngOFDM,ngblthaotccbn,nhtlkhitruyndn trn knh vi tc ngy cng cao v nh hng nh fading, tri tr ngy cng ln. ng b trong OFDM bao gm ng b v thi gian v tn s.V mt tn s, khi n my thu, tn hiu dng analog cao tn trc tin c trn tn xung bng gc. Tn s a vo trn tn phi chnh xc trnh thu li. Do , b to dao ng ti my thu c lin tc iu chnh cho chnh xc vi tn s bn pht. mo pha v bin ca mi sng mang con cng c c lng b ti u thu.39 moe.vn Moe.vn V mt thi gian, b to xung ng h phi c ng b my thu c th nhn bit im bt u khung, symbol tn hiu, trnh ISI. Tn hiu xung ng h ly mu ti khi chuyn i tng t/s cng cn c iu chnh chnh xc. Hiu sut ca cc phng php ng b c quyt nh bi mt s thng s: -T s tn trn tp SNR ti thiu c cu ng b hot ng -Thi gian iu chnh v khong iu chnh -Lng thng tin mo u cn thit cho qu trnh iu chnh - phc tp trong qu trnh hin thc ha thit k - chnh xc v kh nng chu ng khi c fading v tri tr c th phn tch nhng nh hng ca lch tn s v lch thi gian tivictruyndnOFDM,tagivlnltbiudingitrlchtns sngmangCFO(CarrierFrequencyOffset)vlchthigiansymbolSTO (Symbol Timing Offset) chun ha. Tn hiu nhn c u thu sau bin i FFT c dngYl(k)

= {

}

= {

{

}}

+

=

{{

}

}

+

=

(3.1) trong

,Yl(k),

,

lkhiuchccthnhphncasngmang con th k ca symbol OFDM th l ln lt bao gm: tn hiu phtX, tn hiu thu 40 moe.vn Moe.vn cY,hmtruynknhHvnhiutptrnknhZ.Ttccbiudintrn min tn s. Tn hiu chu s mt ng b v thi gian v tn s trn knh c dng

= IFFT{ Yl(k)} = IFFT{

} =

)

(3.2) vi

= IFFT(

) 3.1.1 nh hng ca mt ng b thi gian3.1.1.1nh hng ca mt ng b thi gian symbol STO IFFT v FFT l nhng khi chc nng c bn cn thit cho vic iu ch v gii iu chti u pht v thuOFDM. Vic sdng bFFTNimu thu ihidisymbolOFDM,khongcchccmucaNsngmangconphi chnh xc nh u pht. Hay ni cch khc, ta cn phi c c ch ng b thi gian pht hin im bt u ca mi symbol OFDM. Nu tn hiu pht c dng x(n) trn min thi gian, X(k) trn min tn s th tn hiu nhn c chu nh hng ca mt b s c dng-Trn min thi gian: y(n) ~ x(n+ ) -Trn min tn s:Y(k) ~

X(k) Hnh 3.1- Cc trng hp lch im bt u symbol OFDM 41 moe.vn Moe.vn lch thi gian STO trn min thi gian to ra mt di pha trn min tn stng ng vi sng mang th k. Da vo v tr m im bt u symbol OFDM b xc nh lch, nh hng ca STO s khc nhau -Trnghpthnht:imbtuclngsymbolOFDMcauthu trng khp vi im bt u symbol OFDM nh u pht. Trng hp ny tnhiucboton,khngphichussailchdoSTO.Strcgiao gia cc sng mang khng b vi phm. -Trnghpthhai:imbtuclngsymbolOFDMxyratrc imbtu thcsca symbolOFDM, nhngvn cha ln ln phn tri trdinhtcasymboltrc.Khiny,tacoinhsymbolOFDMvn cha ln ln symbol trc n ng ngha vi vic khng xut hin ISI, nhng phn tn hiu symbol ta cn thu s b mt mt phn. xem xt nh hng, ta s dng bin i FFT cho tn hiu nhn c dng:

= yl(n)

=

)

=

{

}

=

=

(3.3) trong :

=

.

= { Nhnvobiuthctrn,tanhnthytnhtrcgiaogiaccthnhphnsng mang c bo ton. Tuy nhin, STO to ra mt lng dch pha tng ng vi 42 moe.vn Moe.vn lng trn sng mang th k, khin cho chm sao tn hiu b quay i so vi ban u Hnh 3.2- Chm sao tn hiu khi c STO trng hp 1 v 2 -Trnghpthba:imbtuclngsymbolOFDMxyratrc imbtuthcsvlnlnphntritrcasymbolOFDMtrc. Khi ny ISI xut hin. ng thi ISI ny cn gy ra ICI gia cc thnh phn sng mang -Trng hp th t: lc ny, im bt u c lng symbol OFDM ca u thu xy ra sau im bt u thc s . iu ny c ngha tn hiu OFDM nhn cbaogmphn lnphnsymbolOFDMcnthung vmtphn tn hiu ca symbol OFDM tip sau .

= {

(3.4) vi Ng l di khong CP.

= FFT{

}

+

=

+

43 moe.vn Moe.vn =

+

=

+

(3.5) trong

=

= { Hnh 3.3- Chm sao tn hiu khi c STO trng hp 3 v 4 Thnh phn th hai trong biu thc 3.5 biu din nh hng ca ICI, c ngha s trc giao gia cc sng mang b vi phm. Cn thnh phn th ba ca biu thc biu din nh hng ca ISI ti symbol ang xt. 3.1.1.2nh hng ca mt ng b ng h trong vic ly mu tn hiu S lch pha trn min thi gian i vi xung ng h ly mu c th coi tng ng vi lch thi giansymbol STO. Vic lch pha ny xy ra khi tn s ly musdng u thu v phtkhng ging nhau.Hnh 3.4biu din nh hng 44 moe.vn Moe.vn caviclchphatnslymu.Ccmutngnglchnhaumtgitrc nh.CnggingnhlchthigiansymbolSTO,vicmtngbtnsly mutrnminthigiancndnnsquayphatnhiutrnmintns.Tuy nhin gi tr lch ny nh nn ta c th coi n nh mt phn ca STO m khng cn phi c khi b ring. Hnh 3.4- Minh ha s lch pha tn s ly mu trn min thi gian 3.1.2 nh hng ca mt ng b tn s Tn hiu bng gc c iu ch sng mang u pht v sau u thu, n c a tr li bng gc nh tn s ging ht bn pht c to ra t b dao ng ni. V c bn, c hai loi mo tn hiu lin quan n tn s. Mt l nhiu pha gy ra do s khng n nh ca b to dao ng hai bn pht v thu. Hai l lch tn s sng mang CFO gy ra do lch tn gia hai b to dao ng ti hai u thu phtvhintngDopplervidchtnfd.Cutocaccbtodaong khng th hon ton ging ht nhau nn s sai lch tn s l khng th trnh khi.Gi ft l tn s sng mang pht, fr l tn s sng mang nhn c. lch tn s foffset c tnh bngfoffset = ft - fr (3.6) 45 moe.vn Moe.vn dch tn Doppler vi v l tc di chuyn ca my thu, c l tc nh sng l fd =

(3.7) lch tn s chun ha lc ny s l =

(3.8) vil khong cch gia cc sng mang con Trn min tn s, tn hiu thu c chu nh hng ca CFO c dng: Y(k) ~ X(k - ) Trn min thi gian, lch tn gy ra mt s dch pha c gi tr 2n ng vi mu tn hiu th n: y(n) ~

x(n) Hnh 3.5- nh hng ca s lch tn s Gisbquanhhngcanhiupha,tnhiunhnctrnminthi gian l

=

(3.9) 46 moe.vn Moe.vn Trnhnh3.6,tac th thy,tnhiutrnminthigianbquayphanhanh hn khi gi tr tng ln, tng ng vi ng th dc hn. Khi > 0.5, gi tr sai pha bin i khng cn theo ng tuyn tnh na. Hnh 3.6- S sai pha ca tn hiu trn min thi gian khi c CFO lchtnschunhabaogmhaithnhphn:phnnguyn

vphnthp phn

: =

+

3.1.2.1 nh hng ca lch tn s phn nguyn IFO (Integer Carrier Frequency Offset) Hnh 3.7- nh hng ca phn nguyn CFO n tn hiu thu 47 moe.vn Moe.vn Tn hiu trn min tn s u pht c dng X(k), khi chu tc ng ca IFO scdngX(k-

).Tnhiutrnsngmangthkbdchimtsnguyn

sng mang. Nu phn sai lch ny khng c b, n s khin BER ca h thng tng mnh. Tuynhin, s trc giao gia cc sng mang vn c gi nguyn. V vy ICI khng xut hin. Hnh 3.7 minh ha s dch tn ca tn hiu do IFO 3.1.2.2nhhngcalchtnsphnthpphnFFO(FractionCarrier Frequency Offset) Xt tn hiu nhn c u thu trn min tn s

= FFT{

} =

=

+

=

+

=

+

+

=

+

+

=

+

+

=

.

+

+

48 moe.vn Moe.vn =

+

+

(3.10) vi

=

Thnhphnthnhttrongbiuthc3.10biudintnhiutrnsngmang th k vi bin v pha b mo do FFO.

biu din nh hng ca ICI t cc sng mang bn cnh. Phn thp phn ca lch tn s chnh l nguyn nhn gy nn ICI gia cc sng mang. nh hng ca FFO c th nhn thy trn hnh 3.8 Hnh 3.8- Chm sao tn hiu vi nh hng ca FFO 3.1.2.3nh hng ca lch tn s ly mu (Sampling Frequency Offset) lch tn s ly mu SFO (Sampling Frequency Offset) gia u thu v u pht xut hin do s khc nhau gia b to dao nga vo khi ly mu ti u thu v u pht Hnh 3.9 m t nh hng ca lch tn s ly mu. Gi tr lch thay i theothigian.TnhiunhncchunhhngcaSFOtrnmintnsc dng

= Y(k)

+

(3.11) 49 moe.vn Moe.vn vi =

; Thnh phn u tin trong biu thc biu din tn hiu trn sngmang th k vi bin v pha b mo.

biu din thnh phn ICI do SFO gy nn. SFO khin symbol trn min thi gian b thm/ bt cc mu tn hiu theo mt chu k nht nh. V vy, lch ny cn c x l khi n u thu. Hnh 3.9- nh hng ca lch tn s ly mu 3.2CC PHNG PHP NG B TRONG OFDM Cc phng php ng b trong OFDM c th phn loi thnh hai nhm: -Data aided: s dng cc symbol c bit vi cu trc tn hiu bit trc -Non data aided: s dng c tnh thng k ca cc tn hiu d liu thu c 3.2.1 Cc phng php ng b thi gian Nh phn tch, lch thi gian symbol STO khng ch gy nn mo pha mcnlmxuthinISItronghthng.khcphcnhngnhhngtrn, im bt u symbol phi c nhn bit mt cch chnh xc bng cch c lng gi tr STO. Vic c lng ny c th thc hin trn c min thi gian v tn s.3.2.1.1c lng STO trn min thi gianGi thit tn hiu thu c l cc symbol OFDM vi khong CP c di Ng mu tng ng Tg giy. Chiu di phn tn hiu cha d liu l Nsub tng ng Tsub 50 moe.vn Moe.vn giy. Trn min thi gian, STO c c lng qua hai phng phpData aided v Non data aided. 3.2.1.1.1 Phng php Non data aided Trong phng php ny, ta s dng khong CP ca symbol OFDM. CP l bn sao mt on tn hiu cha d liu trong symbol OFDM. iu c ngha ta c th s dng s tng ng gia CP v on d liu kia cho vic c lng STO.V mt tng qut, nu gi tn hiu nhn c l y(t) = x(t) + n(t) trong x (t) l tn hiu u pht, n(t) l nhiu trn knh v tuyn th, dng ton hc ca hm tng quan gia hai ca s l R(t) =

Hnh 3.10- S phng php c lng STO bng khong CP Tn hiuthuc aqua bto tr, sau clytchphn.Bso snh Findmaximumcorrelationstmragitrtngquancci.Tlch thi gian c tm ra. Tuy nhin, trn min thi gian ri rc, c nhiu phng khc nhau xc nh gi tr tng quan cc i gia hai ca s. Thc t, thao tc trn symbol, ta k hiu khon CP l B cn khong d liu tng ng l B. Hai khi B v

cch nhau mt khong Nsub mu tn hiu. u thu, ta s dng hai ca s W1 vW2 cng c di Tg giy, tng ng khong 51 moe.vn Moe.vn Ng.Chngcthtrttrntrcthigianostngnggiaccmutn hiu cha trong chng. S tng ng ny cc i khi khong CP v on d liu tng ng ri hon ton vo hai ca s W1 v W2. T gi tr cc i ny ta c th tm ra im bt u ca symbol. Hnh 3.11- c lng STO s dng khong CP trn min thi gian Phng php tnh ton n gin nht l ng dng biu thc (3.12). im bt u thc ca symbol l v tr lm cho sai khc gia hai ca s nh nht.

=

{

} (3.12) V n gin nn phng php ny c hn ch ln l s chnh xc gim mnh khi c CFO. Mt k thut khc c th khc phc hn ch ny l tm gi tr cc tiu ca bnh phng hiu gia Ng mu ca s W1 v lin hp phc ca Ng mu ca s W2.

=

{

}(3.13) Ngoi ra,ta c th tnh ton gi tr tng quan gia hai ca s theo thi gian. Gi tr tng quan ny t cc i khi hai ca s nm vo v tr ca khong CP v on d liu tng ng 52 moe.vn Moe.vn

=

{

} (3.14) Hnh 3.12a- Gi tr tng quan ca hai ca s theo thi gian vi tm symbol/48 sng mang con Hnh 3.12b- Gi tr tng quan ca hai ca s theo thi gian vi tm symbol/192 sng mang con Hnh 3.12 m t bin hm tng quan gia hai ca s trn min thi gian trong vng tm symbol v 48/192 sng mang. Ta nhn thy ng vi tm symbol c tm nh gi tr tng quan. Mi nh tng ng vi mt im bt u ca symbol. Tuy nhin, cc nh gi tr ny c bin khng bng nhau. L do l mc d nng 53 moe.vn Moe.vn lng trung bnh trn mi symbol khng i, nng lng trn phn GI/CP bin i kh nhiu. Mt hin tng khc ta c th thy l vic gia cc nh tng quan vncnhngbintngquankhngmongmun.Nhngbingitrnyhnh thnh khi hai ca s trt qua nhng vng tn hiu thuc hai symbol khc nhau. Ta bitrnghaisymbolkhcnhauthngchadliukhcnhau,hmtngquan u ra s l mt hm ngu nhin. Gi tr ca n i khi cn cao hn c nh tng quan m ta mong mun. Khi mt mu tn hiu cng dy, tng ng vi s sng mang cng nhiu, cc gi tr bin cng gim.Nh vy phng php c lng STO s dng khong CP n gin nhng tin cy km do khong CP lun chu nh hng ca tri tr. Phng php ny ch thc schiu qu khi s sngmangsdngln (thng ltrn100).khc phc nhc im ny, ta c th thay cc symbol ngu nhin bng cc symbol hun luyn vi cu trc c nh bit trc 3.2.1.1.2 Phng php Data aided Ccsymbolhunluyncthcsdngchongbuthu.Tri ngc vi k thut s dng CP, y ta s dng ton b phn symbol hun luyn chongb.Phngphpnykhcphccnhhngcatruyndna ng. Hnh 3.13- Cu trc symbol hun luyn 54 moe.vn Moe.vn H thng OFDM c th s dng hai symbol hun luyn ging nhau hoc mt symbolhunluynchaihaybnphnginghtnhau.Hnh3.13minhhacu trc mt symbol hun luyn. Trong cc khong k hiu bng C trn min thi gian, ta c th chn dy bit 0 vo. u thu khi nhn c symbol s c gng tnh tng quangiacckhonglpC,ttmraimbtu.Gingnhphngphp trc, tnh tng quan, ta s dng hai ca s trt trn min thi gian W1 v W2.

=

{ |

|

} (3.15) STO c c lng bng cch tm ra cc tiu ca bnh phng lch gia hai khi mu tn hiu trong hai ca s. Nhc im ca phng php l khi c tri tr, symbolangxtsbnhhngbiccsymboltrcvsau.Do,gitrcc tiu lch c th xut hin sau thi im chnh xc cn nhn bit. trnh iu ny,tacthtthitlpmtngng.VdgitrSTOcnxcnhstng ng vi cc tiu u tin ca hm tng quan m ti bnh phng lch di ngng chn. Mt nhc im khc l bi chiu di ca s l N/2 nn hm gi tr lch s c mt vng phng trong cc khong CP. Khong ny khng c ch trong vic c lng STO. khc phc ta tnh trung bnh gi tr tng quan trn di mt khong CP. Gi tr lch s t min khi ly trung bnh ti v tr khong B. 55 moe.vn Moe.vn Hnh 3.14- Cu trc symbol hun luyn vi 4 khong lp

=

(

) (3.16) vi

= |

|

(

|

|

)

Nhmnngcaotnhchnhxcchophngphpny,cutrcsymbolhun luyn cchiathnhbn khongvimu tn hiu trn haikhongC sau l o ca cc gi tr trong hai khong C u.Mt cch c lng STO khc l s dng gi tr tng quan cho gia symbol hun luyn v tn hiu nhn c, v symbol hun luyn c bit trc u thu. Lc ny ta ch cn s dng mt ca s trt c chiu di Tsub/2 tm gi tr tng quan. 3.2.1.2c lng STO trn min tn s theo phng php Data aided Nh phn tch, lch thi gian symbol khin tn hiu quay pha trn min tn s. Do ln quay pha t l vi ln STO nn ta c th c lng STO bng lchphagiaccsngmanglintip.Trnmintnstagiiccsymbol hunluynccutrcgingnhauvbittrcuthu,binbngnhau

=

v p ng knh trn min tn s tng ng ca chng xp x nhau

vi. u thu ta nhn c

(3.17) 56 moe.vn Moe.vn Hnh 3.15- c lng STO da vo p ng knh Gi tr STO c th c xc nh nh sau =

arg

(3.18)Ngoi ra ta c th s dng k thut c lng khc nh h qu ca vic quay pha (hnh 3.15). Ta xc nh gi tr STO thng qua p ng knh truyntr.

=

(

) (3.19) trong

= IFFT{

} =

=

=

=

(3.20) vi nng lng symbol hun luyn gi s cho bng 1:

=

= 1 Hnh 3.16- p ng xung ca tn hiu Hnh3.16minh ha png xungca haitnhiu,ng linntlca tn hiu khng c STO, ng t nt l ca tn hiu c STO. Tn hiu chu nh hng 57 moe.vn Moe.vn ca STO s c p ng xung tr hn so vi tn hiu bnh thng ng bng gi tr STO.3.2.2 Cc phng php ng b tn s Cng ging nh ng b thi gian, vic ng b tn s cng c thc hin trn c min thi gian v tn s bng phng php Data aided v Non data aided 3.2.2.1c lng CFO trn min thi gian 3.2.2.1.13.2.2.1.1 Phng php Non data aided Nh ta bit, khi ng b thi gian trong h thng l hon ho, lch tn ssngmangvimtlngskhintnhiutrnminthigianbquaypha 2n/N. Gi s tn hiu khng phi chu tc ng ca knh truyn th lch pha gia khong CP v on symbol gc tng ng do CFO s l 2N/N = 2. CFO c th c xc nh bng gc ca tch chp gia CP v on d liu tng ng.=

arg{

}vi n = -1, -2, , -Ng(3.21) gim nhiu, ta ly trung bnh trn khong CP =

arg{

}(3.22) Gitr lktqucahm

nndigitrcabiuthc(3.21)nm trong khong [-,+) /2 = [-0.5, 0.5). V vy phng php ny khng th xc nh ccgitrCFOnguyn.Mtkhc,tch

chchorasthckhi khngcCFO.iunycnghakhiCFOkhc0,

chogitr phc. Tuy nhin, phn o ca tch ny vn c th s dng. Sai s lc ny c tnh bng cng thc

=

{

}

(3.23) 58 moe.vn Moe.vn trong L l s mu dng ly trung bnh. K vng ca hm

l E{

} =

sin(

) |

|

(3.24) vi k l s sng mang,

l nng lng tn hiu pht,

l p ng knh truyn vi sngmang th k. K vng hm

c dng sng sin dao ng quanh gc 0, l gi tr m ta mong mun khi thc hin ng b. Nh vy, phng php c lng CFO bng CP n gin vi di gi tr hp v lun tn ti sai s. Hnh 3.17- c tnh bin hm E{

} 3.2.2.1.2 Phng php Data aided Khi mi bt u tin hnh ng b, gi tr CFO c th ln, phng php c lng bng khong CP khng th p ng. Ta phi tm mt phng php khc c diclngrnghn.Digitrnycthtnglnbngcchgimkhong cch gia hai khi tn hiu dng ly tng quan. Trong h thng OFDM, ta thc hin bng cch s dng cc symbol hun luyn vi chu k lp li ngn.Gi D l s nguyn biu din t s gia chiu disymbol OFDM hun luyn vi chiu di khong lp li, c ngha D l s on mu lp li trn mt symbol. 59 moe.vn Moe.vn

= {

(3.25) trong

l tn hiu symbol hun luyn trn min tn s,

l bin ca symboliuchbcM,N/Dlsnguyn.Do

v

ccutrc ging nhau nn.

=

(3.26) Gi tr CFO c th c c lng nh sau =

arg{

}(3.27) Hnh 3.18a- S tng qut phng php Data aided trn min thi gian 60 moe.vn Moe.vn Hnh 3.18b- c tnh quan h gia CFO v chnh xc khi D = 1 v D = 4 Di gi tr CFO trong trng hp ny s l || D/2. Khi D tng, di c lng s tng theo, ng thi khi lng mu tn hiu dng tnh hm tng quan s gim. iu ny c ngha rng di gi tr CFO c lng t l nghch vi chnh xc (Hnh 3.18b). khc phc, ta s dng c lng trung bnh =

arg{

}(3.28) 3.2.2.2 c lng CFO trn min tn s bng phng php Data aided Phng php u tin c tn l Moose. Nu hai symbol hun luyn truyn lin tip nhau, ta c th biu din chng nh sau

=

=

(3.29) Gi tr CFO c th c tnh nh biu thc =

{

}(3.30) 61 moe.vn Moe.vn Biuthc3.30lytrungbnhtrnccsngmangkhckhngnnsaiss tng khi s sng mang khc khng gim. Mc d di CFO c lng ch nm trong di , ta c th tng ln D ln bng cch tng khong lp nh lm vi phngphpclngtrnminthigian.Phngphpnyihitronggiai on u ca vic lin lc phi c mt s symbol hun luyn truyn lin tip tnh ton. Khi , vic truyn d liu khng th din ra.Mt phng php na ng b tn s l s dng cc pilot hay cn c tn l Classen. Cc pilot c chn vo trn min tn s trong nhng sng mang con (hay tone) nht nh. iu ny c ngha mi symbol d liu OFDM truyn i u cha cc pilot phc v cho ng b nhng tn s sng mang con nht nh. Gi tr CFO s c tm ra nh lch ca hai pilot trong hai symbol OFDM lin tip.S3.19mtmtsclngCFOClassencth.Haisymbol OFDM

v

sau khi ng b thi gian s c lu trong b nh. Tip ,tnhiucbiniFFTthnh {

}

v {

}

trongbao gm c pilot. Sau khi c lng CFO t pilot, tn hiu s c b CFO trn min thi gian. Qu trnh ny din ra theo hai ch : ch kha v ch bm. Trong ch kha, CFO c tnh ton trn di rng, bao gm phn nguyn CFO. Cn trong ch bm, CFO phn thp phn c c lng. CFO s c b theo tng phn nguyn v phn thp phn 62 moe.vn Moe.vn Hnh 3.19- S ng b tn s s dng cc pilot(Classen) 3.2.2.3 c lng lch tn s ly mu Viclymutn hiutrnminthi gianu thul khngththiu trong qu trnh x l tn hiu. Ty thuc vo c ch iu khin xung ng h ly mu, k thutb lchtnslymucthchialmhailoi:lymungbvly mu khng ng b.Viphngphplymungb,xungnghckimsottrctip thng qua VCXO (Voltage Controlled Crystal Oscillator) v DPLL (Digital Phase LockedLoop).BTED(TimingErrorDetection)cnhimvtnhtonquay pha v iu chnh tn s ra ca VCXO thng qua DPLL.Viphngphplymukhngngb,tnslymuccnhbi thch anh. Lng sai pha c b cc khi pha sau, c th l cc b lc FIR ni suy. 63 moe.vn Moe.vn Hnh 3.20a- My thu OFDM vi s ly mu ng b Hnh 3.20b- My thu OFDM vi s ly mu khng ng b V phng php ny khng yu cu tn hiu phn hi nn d trin khai trong thc t, ng thi h thng s nhy cm hn vi vic lch tn. Khi Skip/duplicate Interpolate c tc dng chng li vic thm/bt symbol OFDM khi c mt ng btnslymu.TnhiuscbphatikhiFEQ(Frequency-domain Equalizer). 3.3 KT QU M PHNG 3.3.1 Tin trnh m phng Ta tin hnh m phng trn phn mm Matlab phng php c lng CFO Moose. Cc bc tin hnh nh sau: 64 moe.vn Moe.vn -Chnccthngschohthngnh:sFFT,kiuiuch,gitrCFO, gi tr SNR -To ngu nhin lung d liu -Chn pilot-iu ch bng gc, thc hin bin i FFT -Chn CP -To vng lp tng ng vi s gi tr SNR -To lch tn s CFO cho tn hiu -To vng lp phc v tnh gi tr MSE trung bnh v sau -Truyn tn hiu qua knhnhiu Gausse trng cng AWGN (Additive White Gausse Noise) -Thu v c lng CFO s dng phng php Moose -Tnh ton gi tr li bnh phng trung bnh MSE -V th da trn s liu c -Kt thc Lu tin trnh m phng c th hin nh s 3.1 To d liu u vo Chn pilot iu ch bng gc Bin i IFFT Chn CP i = 1 n N. N l s gi tr SNR Iter = 1 n max Tnh gi tr MSE Knh AWGN To CFO 65 moe.vn Moe.vn no yes no yes S 3.1-Lu m phng 3.3.2 nh hng ca mt ng b ti tn hiu OFDM tm hiu nh hng ca ng b, ta tin hnh m phng vi cc thng s c th. Tn hiu c truyn i l tn hiu OFDM c iu ch 16-QAM, c kch thc FFT (Nfft) l 256. Chiu di khong CP chn l Nfft/4. Ta thc hin truyn tn hiutrnknhnhiuGaussetrngchunAWGN(AdditiveWhiteGausseNoise) khng c tri tr vi t s tn trn tp SNR = 30 dB. Khi xut hin lch tn s, thi gian, chm sao tn hiu b mo ng thi t l li trong h thng OFDM tng nh c th thy trn hnh 3.21. V th MSE Tnh gi tr trung bnh cng MSE i = N iter = max 66 moe.vn Moe.vn Hnh 3.21- nh hng ca CFO, STO ti chm sao tn hiu 3.3.3 Phng php c lng CFO trn min tn s Moose Tathchinmphnghiusut phngphpclngCFOMoosetrong gitrlchtnsctnhtondatrnssaiphagiahaisymbolhun luyntheocngthc(3.30).TnhiuOFDMctruynitrnknhAWGN, iu ch 16-QAM, kch thc FFT l Nfft =256. Hnh 3.22 cho thy chnh xc cavicclngtlthunvitsSNRvigitrCFOtrongkhong(-0.5,0.5). Vi cc gi tr nm ngoi khong ny, gi tr c lng c khng cn chnh xc (hnh 3.23) 67 moe.vn Moe.vn Hnh 3.22- th c lng gi tr CFO theo SNR vi CFO = 0.01 v CFO = 0.4, Nfft = 256 Hnh 3.23-Vic c lng khng chnh xc khi CFO nm ngoi khong (-0.5,0.5) nh gi hiu sut ca phng php c lng, ngi ta cn da vo gi tr sai s bnh phng trung bnhMSE.68 moe.vn Moe.vn Hnh 3.24- th MSE vi cc gi tr CFO khc nhau Hnh 3.25- th MSE vi CFO ngoi khong cho php Da vo hnh 3.24, ta thy, trong khong gi tr CFO (-0.5,0.5) MSE gn nh khngthayi.Haynicchkhc,hmMSEkhngphthucvogitrCFO 69 moe.vn Moe.vn trong khong (-0.5,0.5). Ngc li, vi cc gi tr CFO nm ngoi khong ny, gi tr hm MSE s tin v 1, vic c lng khng cn chnh xc na. iu ny c th hin trn hnh 3.25. Hnh 3.26- th MSE vi cc gi tr Nfft khc nhau ti CFO = 0.15 Hnh 3.26 m t hm MSE vi cc gi tr khc nhau ca kch thc FFT. Ta thygitrMSEtlthunvikchthcFFT.iunyccxutphtt cng thc 3.29, gi tr CFO c c lng t cc mu trn cc sng mang khc khng trn min tn s ri ly trung bnh cc sng mang. S im ca b bin i FFT cng cao, s mu khc khng trn min tn s cng ln, vic c lng cng chnh xc.Hnh 3.27 th so snh MSE ca phng php Moose so vi phng php dng khong CP v Classen. Ta c th thy vi cng gi tr Nfft l 256, CFO = 0.25, hiu sut phng php Moose ln hn so vi phng php s dng khong CP v nh hn so vi phng php Classen.70 moe.vn Moe.vn Hnh 3.27- th MSE ca ba phng php c lng CFO khc nhau 3.4 KT LUN CHNG Nh vy, vic ng b c th thc hin trn c min tn s v thi gian, bng nhiuphngphpkhcnhausdngchnhccsymboldliuhaysymbolc bitdnhring.Miphngphplicunhcimkhcnhau.Ccphng phpNon data aided tuy n gin nhng chu nh hng ca tc ng mi trng nhiu.CcphngphpDataaidedsdngccsymbolhunluyncchnh xc v tin cy cao hn Khu ng b trong OFDM v cng quan trng, quyt nh hiu sut lm vic ca h thng. Tuy nhin, mi trng truyn thng v tuyn l rt phc tp v mi mi trng khc nhau th tn hiu s chu nh hng khc nhau. V th nu nghin cuvngdngccbinphpngbkhcnhauvothcttastoch thng c mc p ng nhu cu tt hn.71 moe.vn Moe.vn KT LUN S ra i ca OFDM to tin cho nhiu cng ngh bng rng pht trin nhuimvttrisoviccphngphptruynthngkhc.Ngoivicc khnngchngISI,fadingang,OFDMcnchophptruyntitccao vi hiu qu bng thng ln. m bo hiu sut lm vic cao ca h thng, vai trcaccungblrtln.Trongkhunkhnny,emtrnhby nhngphngphpcbnvngbtrongOFDM.Qutrnhngbcth thc hin c trn min thi gian v tn s. K thut ng b v c bn c hai loi: s dng cc symbol mang d liu c sn hoc s dng cc symbol hun luyn dnh ringchongb.Victrnhbytuyvyvnchaisuvoccckthut phc tp m ch dng li mc tng quan l thuyt. V vy, vi kin thc cn hn ch, n ca em vn cn nhiu sai st, hn ch, mong nhn c s b sung, ng gp, gip ca thy c v cc bn.Qua qu trnh tm hiu v nghin cu n ca mnh em cng thu c rt nhiu nhng kin thc b ch cho bn thn, cng c thm c nhng kin thc v chuyn ngnh hc cng nh cch gii quyt, nghin cu ti liu khi ng trc mt vn m mnh quan tm. cng l mt bi hc qu gi, gip ch rt nhiu cho em trong cng vic v cuc sng . Em xin chn thnh cm n s ging dy nhit tnh ca cc thy, c gio trong Moe.vn TI LIU THAM KHO [1]PGS-TSNguynQucBnh,Mphngcchthngthngtins,NXB Qun i nhn dn 2002. [2] PGS-TS Trn Xun Nam, Bi ging Tng quan v OFDM.[3]HenrikSchulze,ChristianLuders,TheoryandapplicationsofOFDMand CDMA Wideband wireless communications, John Wiley & Sons Ltd 2005. [4]K.Fasel, S.Kaiser, Multi-Carrier and Spread Spectrum Systems From OFDM andMC-CDMAtoLTEandWimax,AJohnWileyandSons,Ltd,Publication 2008. [5]RichardvanNee,RamjeePrasad,OFDMforWirelessMultimedia Communications, Artech House Boston-London 2000. [6] Yong Soo Cho, Jaekwon Kim, Won Young Yang and Chung G.Kang, MIMO-OFDMWirelessCommunitcationswithMatlab,JohnWiley&Sons(Asia)Pte Ltd 2010. Moe.vn PH LC CHNG TRNH M PHNG 1CHNGTRNHMPHNGNHHNGCAMTNGB N TN HIU OFDM % OFDM_basic.m clc clear all %Initial NgType=1; % NgType=1/2 for cyclic prefix/zero padding if NgType==1, nt='CP';elseif NgType==2, nt='ZP'; end Ch=0;% Ch=0/1 for AWGN/multipath channel ifCh==0,chType='AWGN';Target_neb=1000;elsechType='CH'; Target_neb=500; end %figure(Ch+1), clf PowerdB=[0 -8 -17 -21 -25]; % Channel tap power profile 'dB' Delay=[1 2 3 4 5];% Channel delay 'sample' Power=10.^(PowerdB/10); % Channel tap power profile 'linear scale' Ntap=length(PowerdB); % Chanel tap number Lch=Delay(end)+1; %Channel length Nbps=4; M=2^Nbps; % Modulation order=2/4/6 for QPSK/16QAM/64QAM Nfft=64;% FFT size %Ng=0; %Nfft/4; % Ng=0: Guard interval length Ng=Nfft/4; Nsym=Nfft+Ng;% Symbol duration %Nvc=Nfft/4; %Nvc=0; Moe.vn Nused=Nfft-Nvc; EbN0=[0:5:30]; % EbN0 N_iter=500; % Number of iterations for each EbN0 Nframe=4; % Number of symbols per frame sigPow=0; % Signal power initialization file_name=['OFDM_BER_' chType '_' nt '_' 'GL' num2str(Ng) '.dat']; fid=fopen(file_name, 'w+'); norms=[1 sqrt(2) 0 sqrt(10) 0 sqrt(42)]; % BPSK 4-QAM 16-QAM

%Begin for i=0:length(EbN0) est = []; %randn('state',0); rand('state',0);% BER initialization Neb=0; Ntb=0; % Initialize the number of error/total bits for m=1:N_iter % Tx X= randint(1,Nused*Nframe,M); % bit: integer vector Xmod= qammod(X,M,0,'gray')/norms(Nbps); %Tx constellation if m == 1 && i==0 scatterplot(Xmod) end if NgType~=2, x_GI=zeros(1,Nframe*Nsym); elseifNgType==2,x_GI=zeros(1,Nframe*Nsym+Ng);%ExtendanOFDM symbol by Ng zerosend kk1=[1:Nused/2]; kk2=[Nused/2+1:Nused]; kk3=1:Nfft; kk4=1:Nsym; Moe.vn for k=1:Nframe if Nvc~=0, X_shift= [0 Xmod(kk2) zeros(1,Nvc-1) Xmod(kk1)]; elseX_shift= [Xmod(kk2) Xmod(kk1)]; end x= ifft(X_shift); %x_GI(kk4)= guard_interval(Ng,Nfft,NgType,x); x_GI(kk4)= add_CP(x,Ng); kk1=kk1+Nused; kk2= kk2+Nused; kk3=kk3+Nfft; kk4=kk4+Nsym; end %CFO&STO if Ch==0 y= add_CFO(x_GI,0.02,Nfft); %y = add_STO(x_GI,1); %y = x_GI else% Multipath fading channel channel=(randn(1,Ntap)+j*randn(1,Ntap)).*sqrt(Power/2); h=zeros(1,Lch); h(Delay+1)=channel;% cir: channel impulse response y = conv(x_GI,h);end if i==0 % Only to measure the signal power for adding AWGN noise y1=y(1:Nframe*Nsym); sigPow = sigPow + y1*y1'; continue; end % Add AWGN noise snr = EbN0(i)+10*log10(Nbps*(Nused/Nfft)); % SNR vs. Eb/N0 Moe.vn noise_mag = sqrt((10.^(-snr/10))*sigPow/2); y_GI = y + noise_mag*(randn(size(y))+j*randn(size(y))); %y_GI = y; % Rx kk1=(NgType==2)*Ng+[1:Nsym]; kk2=1:Nfft; kk3=1:Nused; kk4=Nused/2+Nvc+1:Nfft; kk5=(Nvc~=0)+[1:Nused/2]; if Ch==1 H= fft([h zeros(1,Nfft-Lch)]); % Channel frequency response H_shift(kk3)= [H(kk4) H(kk5)];end for k=1:Nframe est1 = CFO_CP(y_GI,Nfft,Ng); est = [est est1]; %Y(kk2)= fft(remove_GI(Ng,Nsym,NgType,y_GI(kk1))); Y(kk2)= fft(remove_CP(y_GI(kk1),Ng)); Y_shift=[Y(kk4) Y(kk5)]; if Ch==0,Xmod_r(kk3) = Y_shift; else Xmod_r(kk3)= Y_shift./H_shift; % Equalizer - channel compensation end kk1=kk1+Nsym;kk2=kk2+Nfft;kk3=kk3+Nused;kk4=kk4+Nfft; kk5=kk5+Nfft; end %Rx constellation if m == 1 EbN0(i) Moe.vn scatterplot(Xmod_r) end

X_r=qamdemod(Xmod_r*norms(Nbps),M,0,'gray'); Neb=Neb+sum(sum(de2bi(X_r,Nbps)~=de2bi(X,Nbps))); Ntb=Ntb+Nused*Nframe*Nbps;%[Ber,Neb,Ntb]=ber(bit_Rx,bit,Nbps);if Neb>Target_neb, break; end end if i==0 sigPow= sigPow/Nsym/Nframe/N_iter; fprintf('Signal power= %11.3e\n', sigPow); fprintf(fid,'%%Signal power= %11.3e\n%%EbN0[dB] BER\n', sigPow); else Ber = Neb/Ntb; fprintf('EbN0=%3d[dB], BER=%4d/%8d =%11.3e\n', EbN0(i), Neb,Ntb,Ber) fprintf(fid, '%d\t%11.3e\n', EbN0(i), Ber); if Ber