ky thuat ofdm

8/2/2019 ky thuat OFDM

1/13

ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vn

Chmmg 2 KY THU~ T OFDM2.1 GiOi thifu chiroug

Ghep kenh phan chia theo tAn s6 true giao OFDM (Orthogonal FrequencyDivision Multiplexing) la Ic y thuat di~u chB da song mang dtroc su dung rong raitrong cac irng dung vo tuyen ldn hiru tuyen, OFDM duoc chon lam chuan cho h~thong phat am thanh s6 DAB, h~ th6ng phat hinh s6 DVB va mang LAN khongday ... Uu di~m cua OFDM la kha nang truyen dfr li~u t6c d(l cao qua kenh truyenfading co tinh chon IQc tAn s6 va su dung bang thong hieu qua. Ngoai ra, qua trinhdi~u chB va giai di~u chB da song mang co th~ duoc thuc hien dB dang nho phepbiBn d6i Fourier thuan va nghich, Trong chuang nay chung ta se di sau vao tim hieutung di;ic di~m cua OFDM: khai niem, di~u chB da song mang, h~ thong OFDMbang co sa , ky thuat xu li tin hieu OFDM, chen Pilot, ti~n t6 li;ipCP ...2.2 Hf th6ng OFDM2.2.1 SO'C l 6 kh6i

IHinh 2.1 Sa d6 kh6i h~ thong OFDM

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http://www.4tech.com.vn/http://www.4tech.com.vn/


2/13

ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vnNguyen If lam vi~c: Ddu tien, dong dfr lieu vao t6c dQ cao ducc chia thanh nhieu dong dfr li~usong song t6c dQ thAp han nho bQ chuyen d6i S/P(SeriallParallel). M6i dong dfrli~u song song sau do duoc rna hoa su dung thuat toan FEC(F orward ErrorCorrecting) va diroc s~p x~p theo met trinh tu h6n hop, Nhfrng ky tu h6n hop duocdua d~n ddu vao cua kh6i IFFT. Kh6i nay se tinh toan cac rn~u thai gian tuong irngvoi cac kenh nhanh trong mien tdn s6 Sau do, khoang bao v~ duoc chen vao d~ giam nhiSu xuyen ky ur lSI dotruyen tren cac kenh vo tuyen di dQng da duong. Cu6i cling bQ loc phia phat dinhdang tin hi~u thai gian lien tuc se chuyen d6i len tk s6 cao d~ truyen tren cac kenh, Trong qua trinh truyen, tren cac kenh se co cac ngu6n nhiSu gay anh huangnhir nhiSu Gausian trang cong AWGN. dphi a thu, tin hieu thu duoc chuyen xuong tdn s6 thAp va tin hieu roi rae datduoc tai bQ loc thu. Khoang bao v~ duoc loai bo va cac rn~u duoc chuyen d6i ti rmien thai gian sang mien tdn s6 bang phep bien d6i DFT dung thuat toan FFT. Saudo, my vao sa d6 di~u ch~ diroc su dung, Sl,I dich chuyen v~ bien dQ va pha cuasong rnang nhanh se duoc can bang bang bQ can bang kenh/Channel Equalization).Cac ky tir h6n hop thu duoc se duoc s~p x~p nguoc tnr lai va duoc giai rna. Cu6icling, chung ta nhan duoc dong dfr li~u n6i ti~p ban ddu.2.3 K y thu~t xu If tin hi~u OFDM2.3.1 Mi hoa sua sai trutrc FEe

Trong h~ th6ng thong tin s6 noi chung, rna hoa sua sai tnnrc FEC (ForwardError Correcting) diroc su dung d~ nang cao chAt hrong thong tin, cu th~ la dam baoty s6 l6i trong gioi han cho phep rna khong phai nang cao gia tri cua ty s6 Eb/N 0(hoac SNR), di~u nay cang th~ hien ro a kenh truyen bi tac dQng cua AWGN. Mahoa FEC diroc chia thanh 2 loai rna chinh:

.:. Ma kh6i (Block coding)

.:. Ma chap (Convolutional coding).Ngoai ra, ngiroi ta con dung rna hoa Trellis: la rnQt dang cua rna ch~p nhimg

co them phdn rna hoa, Ben thu co th~ su dung thuat toan Viterbi.

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3/13

C hU 'O 'D g 2: K Y THU~T OFDM www.4tech.com.vn2.3.2 Phan tan ki tv

Do fading lua chon tAn s6 cua cac kenh vo tuyen di~n hinh lam cho nhirngnhom song mang phu it tin c~y han nhfmg song mang khac, Vi v~y tao ra cac chuml6i bit IOn han duoc phan tan mQt each ngdu nhien, HAu h~t cac rna sua l6i khongduoc thi~t k~ d~ sua l6i chum. Do do, bQ phan tan ki ttr duoc tao ra nham ngdunhien hoa S\I xuAt hien cua nhirng bit l6i tnnrc khi giai rna. Tai bQ phat, b~ng eachnao do ngiroi ta hoan vi nhimg bit da rna hoa sao cho nhfrng bit kB nhau bi eachnhau nhieu bit. Tai bQthu, viec hoan vi ngiroc lai dtroc thuc hien tnnrc khi giai rna.2.3.3 sip x~p

VB nguyen t~c, co th~ ap dung bAt ky phirong phap diBu ch~ nao cho m6isong mango Dang diBu ch~ diroc quy dinh boi s6 bit 0 ngo vao va c~p gia tri (I, Q) 0ngf ra. Tire la dong bit tren m6i nhanh duoc s~p x~p thanh cac nhom co Nbs (1,2,4,8) bit khac nhau nrong irng voi cac phuong phap diBu ch~ BPSK, QPSK, 16-QAM,64-QAM.

Nbs Dang diBu ch~ 8 . n , bn1 BPSK [1]2 QPSK (4-QAM) [1]4 16 QAM [1][3]8 64 QAM [1][3][5][7]Hinh 2.2 Bang cac gia tri 8 . n , bn theo dang diBu ch~

Noi chung, mo hinh diBu ch~ tuy thuoc vao viec dung hoa giira yeu cAut6c dQtruyen ddn va chAt luong truyen ddn. MQt uu di~m d~c biet lura hen cho cac irngdung da phirong ti~n sau nay la mo hinh diBu ch~ khac nhau co th~ duoc ap dungcho cac kenh (song mang phu) khac nhau, chang han cho cac lap dich vu khac nhau.2.3.4 Sfr d1}.ngIFF TIFFT trong OFDM

OFDM la Ic y thuat diBu ch~ da song mang, trong do du lieu duoc truyen songsong nho rAt nhieu song mang phu, D~ lam duoc diBu nay, cir m6i kenh phu, ta cAnmQt may phat song sin, mQt bQ diBu ch~ va met bQ giai diBu ch~. Trong tnrong hops6 kenh phu la kha IOn thi each lam tren khong hieu qua, nhieu khi la khong th~

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C hU 'O 'D g 2: K Y THU~T OFDM www.4tech.com.vnthirc hien duoc, Nham giai quyet vAn d~ nay, kh5i thuc hien clnrc nang bien d6iDFT/IDFT duoc dung dB thay th~ toan bQ cac bQ tao dao dQng song sin, bQ di~uch~, giai di~u ch~ dung trong m6i kenh phu,

FFTIIFFT duoc xem la mQt thuat toan giup cho viec thuc hien phep bien d6iDFT/IDFT nhanh velgon hon.2.3.4.1 Phep bi~n d6i

DFT la phep bi~n d6i Fourier roi rae (Discrete Fourier Transform), thuc hienchuyen d6i tin hieu x(n) trong mien thai gian sang tin hieu trong mien tAn s5 X(k).Phep bien d6i IDFT la qua trinh nguoc lai, thirc hien chuyen d6i ph6 tin hieu X(k)thanh tin hieu x(n) trong mien thai gian.

Gia su tin hieu x(n) co chieu deli la N (n = 0,1, 2, ... , N-l). Cong tlnrc cua phepbi~n d6i DFT la

N-lX(k) = L x(n)W;n= O

,k=O, 1, ... ,N-l (3.9)_j27lTrong do WN duoc xac dinh la W N = e N (3.10)

Do vay, W : co gia tri la2 sknw : = e - J - - Y

Cong thirc cua phep bien d6i IDFT la(3.11)

s-ix(n) = N 1 " X(k)WN-nk~ ,n=O, 1, ... ,N-lk= O

(3.12) Chuyen d6i Fourier nhanh(FFT) la thuat toan giup cho viec tinh toan DFTnhanh velgon hon.Tir cong thirc (3.9), (3.12) ta thAy thai gian tinh DFT bao gam

.:. Thai gian thirc hien phep nhan plnrc .:. Thai gian thirc hien phep cong plnrc .:. Thai gian dQCcac h~ s5 WN.:. Thai gian truyen s5 lieu,

Trong do chu y~u la thai gian thuc hien phep nhan phirc. Vi v~y, muon giamthoi gian tinh toan DFT thi ngiroi ta t~p trung chu y~u vao viec giam thai gian thuc

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5/13

C hU 'O 'D g 2: K Y THU~T OFDM www.4tech.com.vnhien phep nhan phirc. Ma thai gian thuc hien phep nhan phirc ti l~ voi s6 phep nhan,Do do dB giam thai gian tinh DFT thi ngiroi ta phai giam duoc s6 hrong phep tinhnhanh bang each su dung thuat toan FFT. DB tinh tnrc tiSp dn N2 phep nhan. Khitinh bang FFT s6 phep nhan chi con ~ log, N. Vi v~y t6c dQ tinh bang FFT nhanh

h r h .J. l' 2Nan tin truc tiep a ---. log, NNgoai fa FFT con co iru diBm giup tiSt kiem bQnho bang each tinh tai ch6.

2.3.4.2 U ng d1}.ng FFT/IFFT trong OFDM Sa d6 kh6i cua h~ thong OFDM su dung FFT hinh 2.3

Hinh 2.3 Sa d6 kh6i cua h~ th6ng OFDM dung FFT Tai may phat, tin hieu diroc dinh nghia trong mien tAn s6, la tin hieu s6 daduoc lAym~u, va duoc dinh nghia nhu phil Fourier roi rae t6n tai chi tai tAn s6 roirae, M6i song mang OFDM wang img voi mQt phAn ill cua phil Fourier rei rae.Bien dQva pha cua cac song mang phu thuoc data duoc truyen, Sl,Tchuyen tiSp dataduoc d6ng bQtai cac song mang.va co thB xu ly cung nhau, symbol by symbol.

Xet mQt chu6i datatd., d., d2, ... ,dN-1), trong do dn=ctn+jbn (ctn,bn=1 voiQPSK,ctn,bn= 1,3 voi l6QAM, ... )

N-l N-l _ .21fnD ='" d e-J (2mmlN) ="'de J N ,.m L . . J n L . . J n Val k=O,1,2, ... ,N-l

n= O n = O(3.13)

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6/13

ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vntrong do I n = n/(I1T), tk=kAt vel M la khoang thai gian ky nr duoc hra chon mQteach my y cua chu6i dn. Phan thuc cua vector D co thanh phAn

N-lYm = Re{DJ= L:[an c os (2 ifn tm )+ hn s in (2 ifn tm )] k = O,1,..,N-1n= O (3.14)

NBu thanh phAn nay qua bQ loc thong thAp trong khoang thai gian I1t, tin hieu datduoc gAndung voi tin hieu FDM

N-lyet) = Lancos(2ifn tm) + b ; s i n( 2i fn tm ) 0 ~ t ~ NI1tn= O

(3.15) Hinh (3.9) minh hoa qua trinh FFT cua h~ thong OFDM co s a . DAutien, datavao duoc chuyen ti r n5i tiBp sang song song velduoc nhom thanh x bits duoi dangmet s5 phirc, S5 x xac dinh chom sao tin hieu cua song mang tuong irng, nhu16QAM hoac 32QAM. S5 phirc duoc diBu chB trong bang g5c bang thuat toanIFFT vel duoc chuyen tro lai thanh data n5i tren duong truyen, Khoang bao v~duoc chen giira cac k Y tu d~ tranh lSI. Cac k Y ttr rei rae diroc chuyen thanh analogvelLPF d5i voi tren tAn s5 RF. May thu thuc hien qua trinh nguoc lai cua may phat, MQt bQ tap-equalizerduoc su dung, H~ s5 tap(tap-coefficents) cua bQ loc duoc tinh toan dira tren thongtin kenh,2.4Cac vin d~ k y thn,t trong OFDM

OFDM la giai phap ky thuat rAt thich hop cho truyen ddn vo tuyen t5c dQ cao.Tuy nhien, d~ co th~ dem ap dung vao cac h~ thong, co ba vAn dB cAn phai giaiquyet khi thirc hien h~ th5ng su dung OFDM:

.:. Uoc hrong tham s5 kenh .:. D6ng bQ song mangvAn dB thir nhAt lien quan tnrc tiBp dBn chi tieu chAt hrong h~ thong OFDM

nBu dung phirong phap giai diBu chB lien kBt, con hai vAn dB sau lien quan dBn viecxu ly cac nhuoc di~m cua OFDM. Ngoai ra, d~ nang cao chi tieu chAt hrong h~th5ng, ngiroi ta sir dung rna hoa tin hieu OFDM.

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7/13

ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vn2.4.1 U6'c IU'Q'Ilgtham s a kenh

Uac hrong kenh (Channel estimation) trong h~ thong OFDM la xac dinh hamtruyen dat cua cac kenh con va thai gian dS thirc hien giai di~u chS ben thu khi benphat sir dung kiSu di~u chS kSt hop (coherent modulation). DS uoc hrong kenh,phuong phap ph6 biSn hien nay la dung tin hieu ddn duong (PSAM-Pilot signalassisted Modulation). Trong phuong phap nay, tin hi~u pilot ben phat sir dung la tinhieu da dtroc ben thu biSt truce v~ pha va bien dQ.Tai ben thu, so sanh tin hieu thuduoc voi tin hieu pilot nguyen thuy se cho biSt anh huang cua cac kenh truyen ddndSn tin hieu phat, Uoc hrong kenh co thS diroc phan tich trong mien thai gian vatrong mien tAns6. Trong mien thai gian thi cac dap irng xung hen) cua cac kenh conduoc uoc hrong, Trong mien tAns6 thi cac dap irng tAns6 H(k) cua cac kenh conduoc uec hrong, Co hai vAnd~ chinh duoc quan tam khi sir dung PSAM : vAn d~ tlnr nhAt la Iva chon tin hieu pilot : phai dam bao yeu cAuchongnhiBu, han chS t6n hao v~ nang hrong va bang thong khi sir dung tin hieunay. Voi h~ th6ng OFDM, viec Iva chon tin hieu pilot co thS diroc thuc hientren gian da thai gian-tan s6, vi v~y ky thuat OFDM cho kha nang Iva choncao han so voi h~ th6ng dan song mangoVi~c Iva chon tin hieu pilot anhhuang rAtIan dSn cac chi tieu h~ thong,

VAnd~ tlnr hai la vi~c thiSt kS bQuoc hrong kenh: phai giam duoc dQphirc~~~~~~~~~~~~ili~~~~v~ t6c dQthong tin cao (nrc la thai gian xir ly giam) va cac chi tieu h~ thongla hai yeu du nguoc nhau. Chang han, bQuoc hrong kenh tuyen tinh t6i Uti(theo nguyen ly binh phuong l6i nho nhAt-MSE) la bQ loc Wiener hai chieu(2D-Wiener filter) co chi tieu ky thuat rAtcao nhung cling rAtphirc tap, Vivay, khi thiet kS dn phai dung hoa hai yeu du tren,

2.4.2 D6ng bQ trong OFDMDang bQ la mQt trong nhirng vAnd~ dang rAtduoc quan tam trong ky thuat

OFDM boi no co y nghia quyet dinh dSn kha nang cai thien cac nhiroc diSm cuaOFDM. Chang han, nSu khong dam bao sir dang bQv~ tAns6 song mang thi se ddndSn nguy co mAttinh tnrc giao gitra cac song mang nhanh, khien h~ th6ng OFDM

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8/13

ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vnmAt di cac Uti diBm d~c trung nho S l , T tnrc giao nay, Trong h~ thong OFDM, nguoi taxet d~n ba 10l;1idang bQ khac nhau la : dang bQ ky tv (symbol synchronization),dang bQ tAn s6 song mang (carrier frequency synchronization), va dang bQ tAn s6lAym~u (sampling frequency synchronization).2.4.2.1 D6ng bQ k y tv

Dang bQ ky tv nham xac dinh chinh xac thai diBm b~t dAumQt ky tv OFDM.Hien nay, voi ky thuat sir dung ti~n t6 l~p (CP) thi dang bQ ky tv dil duoc thirc hienmQt each d~ dang han. Hai y~u t6 dn duoc chu y khi thuc hien dang bQ ky tv la 16ithai gian (timing error) va nhi~u pha song mang (carrier phase noise) .

:. L6i thM gianL6i thai gian gay ra S l , T sai l~ch thai diBm b~t dAu mQt ky tv OFDM. N~u 16i

thai gian du nho sao cho dap irng xung cua kenh v~n con n~m trong chieu daikhoang ti~n t6 l~p (CP) thi h~ thong v~n dam bao S l , T true giao gifra cac song mangoTrong tnrong hQ"Pnay thi thai gian tr~ cua mQt ky tv duoc xem nhir la dQ dich phacua kenh truyen va dQ dich pha nay duoc xac dinh nho ky thuat uoc luong kenh,Trong tnrong hQ"Pnguoc lai, n~u chieu dai cua CP nho han 16i thai gian thi h~ th6ngse xuAt hien 16i lSI. Co hai phuong phap dB thuc hien dang bQ thai gian, do la :dang bQthai gian dira vao tin hieu pilot va dang bQthai gian dira vao ti~n t6 l~p .

:. Nhi~u pha song mangNhi~u pha song mang la hien tirong khong 6n dinh v~ pha cua cac song mang

do S l , T khong 6n dinh cua bQtao dao dQng ben phat va ben thu.2.4.2.2 D6ng bQ tin s6 song mang

Trong dang bQ tAn s6 song mang, hai vAn d~ chinh duoc quan tam d~n la : 16itAn s6 (frequency error) va thuc hien uoc luong tAn s6.

:. L6i tin s6L6i tAn s6 duoc tao ra do S l , T khac biet v~ tAn s6 giira hai bQ tao dao dQng ben

phat va ben thu, do dQ dich tAn Doppler, hoac do nhi~u pha xuat hien khi kenhtruyen khong tuyen tinh. Hai anh huang do 16i tAn s6 gay ra la : suy giam bien dQ

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9/13

ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vntin hieu thu diroc (vi tin hieu khong duoc lAym~u tai dinh cua m6i song mang hinhsin) va tao ra nhi~u xuyen kenh ICI (vi cac song mang bi mAt tinh true giao) .

:. U'C IU'Q'Ilgtin s6Tirong tu nhir ky thuat d6ng bQ ky nr, d~ thuc hien d6ng bQ tAn s6, co th~ sir

dung tin hieu pilot hojic sir dung tiSn t6 l~p. Trong ky thuat sir dung tin hieu pilot,mQt s6 song mang duoc sir dung d~ truyen nhirng tin hieu pilot (thuong la cac chu6igia nhi~u). Sir dung nhirng ky tl,I dff biSt truce vS pha va bien dQ se giup ta uochrong duoc dQ quay pha do l6i tAn s6 gay ra. D~ tang dQ chinh xac cho bQ uochrong, nguoi ta sir dung them cac vong khoa pha (Phase Lock Loop-PLL).Nh,n xct : MQt vAn dS dn duoc quan tam dSn la m6i quan h~ gifra d6ng bQ ky nrva d6ng bQ tAn s6 song mango D~ giam anh huang cua Sl,ImAt d6ng bQ tAn s6 songmang thi co th~ giam s6 hrong song mang, tang khoang each gitra hai song mangcanh nhau. Nhung khi giam s6 song mang thi phai giam chu ky cua m6i ky tir trenm6i song mang, d~n dSn vi~c d6ng bQ ky tl,I rAtkho khan va phai ch~t che han. DiSudo chirng to hai vAn dS d6ng bQ tren co quan h~ chat che l~n nhau, dn phai co Sl,Idung hoa hQ'Ply d~ h~ th6ng dat duoc cac chi tieu ky thuat dS ra.2.4.2.3 D6ng bQ tin s6 liy mluTai ben thu, tin hieu lien tuc thea thai gian thu duoc lAym~u thea d6ng h6 ben

thu, vi v~y se xuAt hien Sl,IbAt d6ng bQ gifra d6ng h6 ben phat va ben thu. Nguoi tadira ra hai phirong phap d~ khac phuc Sl,IbAt d6ng bQ nay. Phuong phap thir nhAt lasir dung bQ dao dQng diSu khien b&ng di~n ap (Voltage Controlled Oscillator-VCO).Phuong phap thir hai duoc goi la : lAym~u khong d6ng bQ; trong phirong phap nay,cac tAn s6 lAy m~u v~n ducc giu nguyen nhung tin hi~u ducc xir ly s6 sau khi lAym~u d~ dam bao Sl,Id6ng bQ.2.5 D~c tinh kenh truyen trong k y thu,t OFDM2.5.1 Sv suy hao

Suy hao la sir suy giam cong suAt tin hi~u khi truyen illdi~m nay dSn di~mkhac, No la kSt qua cua chieu dai duong truyen, chuang ngai v~t va hieu irng daduong, D~ giai quyet vAn dS nay, phia phat thirong duoc dua len cang cao cang t6t

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ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vnd~ t5i thieu s5 IUQ11gv~t can. Cac vung tao bong thuong rAt rQng, t5e dQ thay d6icong suAt tin hieu cham, Vi th~, no con duoc goi Iafading chdm.

amplitude

time 011 position

trequemcy

Hinh 2.4 Dap irng tAns5 cua kenh truyen da duong2.5.2 Tap a m tring Gaussian

Tap am trang Gaussian co m~t dQ ph6 cong suAt la dang d~u trong ca bangthong va tuan thea phan b5 Gaussian. Theo phirong thirc tac dQng thi nhiSuGaussian la nhiSu cong, NhiSu nhiet-sinh ra do Sl,I chuyen dQng nhiet cua cac hatmang di~n gay ra-la loai nhiSu tieu bieu eho nhiSu Gaussian trang cong tac dQngd~n kenh truyen ddn. D~e biet, trong h~ thong OFDM, khi s5 song mang phu la rAtIOn thi hAu h~t cac thanh phAn nhiSu khac cling co th~ dtroc eoi la nhiSu Gaussiantrang cong tac dQng tren tung kenh con vi xet tren tirng kenh con rieng le thi d~edi~m cua cac loai nhiSu nay thoa man cac di~u kien cua nhiSu Gaussian trang cong,2.5.3 Fading Rayleigh

Fading Rayleigh la loai Fading (Fading phang) sinh ra do hien tuong daduong (Multipath Signal) va xac suAt mire tin hieu thu duoc suy giam so voi rmrctin hieu phat di tuan thea phan b5 Rayleigh. Loai fading nay con duoc goi la fadingnhanh vi Sl,I suy giam cong suAt tin hieu ro r~t tren khoang each ng~n (tai cac mrabuoc song) ill 10-3OdB.

Trong moi tnrong da duong tin hieu thu duoc suy giam thea khoang each dosu thay d6i pha cua cac thanh phan da duong (thay d6i pha la do cac thanh phan tinhieu d~n may thu vao cac thai di~m khac nhau d~n trS Ian truyen, TrS Ian truyen segay ra Sl,I xoay pha cua tin hieu),

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ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vn

Hinh 2.5 Cac tin hieu da duongFading Rayleigh gay ra do S l , T giao thoa (tang hoac giam) boi S l , T kBt hQ'Pcua

cac song thu duoc. Khi bQ thu di chuyen trong khong gian pha giira cac thanh phAnda duong khac nhau thay d6i gay ra giao thoa ciing thay d6i, ill do ddn dBn S l , T suyhao cong sudt tin hi~u thu duoc, Phan b6 Rayleigh thuong duoc su dung d~ mo ti ltrang thai thay d6i thea thai gian cua cong sudt tin hieu nhan duoc,2.5.4Fading Iva chon tin s a

Trong truyen ddn vo tuyen dap irng ph6 cua kenh la khong bang phang, no bid6c va suy giam do phan xa ddn dBn tinh trang co mQt vai tAn s6 bi triet tieu tai dAuthu. Phan xa ti r cac v~t gAnnhu m~t ddt, cong trinh xay dung, cay c6i co th~ ddn dBncac tin hieu da duong co cong sudt tirong tv nhu tin hieu nhin thang, Di~u nay se taora cac diem "O"(nulls) trong cong sudt tin hieu nhan duoc do giao thoa.2.5.5Tr~iitr~

Trai tr~ (Delay spread) la khoang chenh l~ch thai gian gifra tin hieu thu tnrctiBp va tin hieu phan xa thu duoc cu6i cling tai bQ thu do hieu irng da duong, Trongthong tin vo tuyen, trai tr~ co th~ gay nen nhi~u xuyen ky tv lSI. Di~u nay la do tinhieu sau khi trai tr~ co th~ chong ldn dBn cac ki tv Ian c~n. Nhi~u xuyen ki tv setang khi t6c dQ tin hieu tang. Di~m b~t dAu cua hieu irng tang dang k~ khi trai tr~IOnhan khoang 50% chu ky bit. Trong Ic y thuat OFDM, t6c dQtin hieu giam sau khiqua bQ S IP lam cho chu ky tin hieu tang. Tu do lam giam nhi~u lSI do trai tr~.

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ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vnD!t'"E:I'c::::t Signa "\ r=>\ ! \~I \ ""_"'""_:_~Renecbed 8" Deayed Sigi"laJ[1

I~ / -\ ~\ Time

Hinh 2.6 Trai trS da duong2.5.6 Djeh Doppler

Khi bQ phat va bQ thu chuyen dQng nrong d6i voi nhau thi t~n s6 cua tin hieutai bQ thu khong giong voi t~n s6 tin hieu tai bQphat, Cu th~ la : khi nguon phat vanguon thu chuyen dQng huang vao nhau thi t~n s6 thu duoc se Ian han t~n s6 phatdi, khi nguon phat va nguon thu chuyen dQng ra xa nhau thi t~n s6 thu duoc se giamdi. Hieu irng nay duoc goi la hi~u irng Doppler.2.6 Die di~m v a -ung d1}-ngcua ky thu,t OFDM2.6.1 Ifu di~m cua ky thu,t OFDMDiroi day la cac Utidi~m chinh cua ky thuat OFDM:.:. Kha nang chong nhiSu lSI, ICI nho ky thuat giam t6c dQ tin hieu b~ng bQS IP , su dung tiBn t6 HipCP, cac song mang phu tnrc giao voi nhau .:. Hieu sudt su dung ph6 cao han so voi FDM do ph6 cua cac song mang phuco th~ chong phu len nhau r n a vfut dam bao chdt Iuong tin hi~u sau khi tach song.00000000 ITeq LIIe-nc)!.. .

'fj-eqLlle-lllJO~)10.

Hinh 2.7 So sanh viec sir dung bang t~n cua FDM va OFDM.:. Cac kenh con co th~ coi la cac kenh fading phang nen co th~ dung cac bQcanb~g dan gian trong sU6tq u a trinh nh~ thong t in, giam dQphirc tap cua may thu.

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ChU'O'Dg2: KY THU~T OFDM www.4tech.com.vn.:. Di~u chB tin hieu dan gian, hieu qua nho su dung thuat toan FFT va cac bQADC, DAC dan gian,

2.6.2 NhU'f}'cdi~m ciia k y thu~t OFDMBen canh nhfrng uu di~m thi h~ thong OFDM con tan tai nhieu nhiroc di~m:.:.H~ th6ng OFDM tao ra tin hieu tren nhieu song mang, dai dQng cua tin hieuIOn nen cong sudt tuang d6i Cl,IC dai PAPR Ion, han chB hoat dQng cua bQkhuech dai cong sudt..:.D~ bi anh huang cua dich tAnva pha han so voi h~ th6ng mQt song mango Viv~y phai thuc hien t6t dang bQtAn s6 trong h~ thong,

Cling voi cac nhiroc di~m tren, it co nhu du OFDM trong thong tin c6 dinh do cach~ thong hien tai v~n dang hoat dQng t6t va hieu qua, la nguyen nhan viec trien khaisan phNn moi dat rmrc khiem t6n trong khi uu di~m cua h~ thong su dung Ic y thuatnay rdt ra rang.2.6.3 irng dy.ng cua k y thu~t OFDM

Hien nay, OFDM dff diroc khuyen nghi su dung trong cac h~ th6ng thong tins6 t6c dQ cao nhu phat thanh va truyen hinh s6 va se duoc img dung trong h~ th6ngthong tin di dQng tuang lai nhu h~ th6ng LAN vo tuyen, cac cong nghe truyen d~ns6 t6c dQ cao: ADSL, VDSL. .. OFDM cling la mQt giai phap dAy lura hen d~ thuchien h~ th6ng thong tin di dQng da phuang ti~n (G4).2.7 K~t lu~n chmrng

Trong chuang nay, chung ta dff tim hi~u t6ng quat v~ h~ th6ng OFDM. Nocho thdy r~ng day la mQt giai phap cong nghe dAy lura hen, Ky thuat OFDM khongphai la mQt ky thuat da truy nhap vi tdt ca cac song mang duoc di~u chB bang dfrli~u cua cling mQt thue bao. D~ h6 tro nhieu thue bao, OFDM phai duoc kBt hop voimQt ky thuat da truy nhap. Cong nghe MC-CDMA la Sl,I kBt hop gifra OFDM vaCDMA. Vi thB,a chuang tiBpthea chung ta se cling tim hi~u v~ cong nghe MC-CDMA.

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ky thuat ofdm

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