BC hoàn chỉnh . OFDM

8/3/2019 BC hon chnh . OFDM

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Tng Quan OFDM

MC LC

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i Hc Cng Ngh Thng Tin & Truyn Thng


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Tng Quan OFDM

LI NI U

Trong nhng nm gn y, cc dch v vin thng pht trin ht scnhanh chng to ra nhu cu to ln cho cc h thng truyn dn thng tin.Mc d cc yu cu k thut cho cc dch v ny l rt cao song cn c cc gii

php thch hp thc hin.Orthogonal Frequency Division Multiplexing (OFDM) l mt phng

php iu ch cho php truyn d liu tc cao trong cc knh truyn chtlng thp. OFDM c s dng trong pht thanh truyn hnh s, ng dythu bao s khng i xng, mng cc b khng dy. Vi cc u im camnh, OFDM ang tip tc c nghin cu v ng dng trong cc lnh vckhc nh truyn thng qua ng dy ti in, thng tin di ng, WirelessATM ...

OFDM nm trong lp cc k thut iu ch a sng mang. K thut nyphn chia di tn cho php thnh rt nhiu di tn con vi cc sng mang khcnhau, mi sng mang ny c iu ch truyn mt dng d liu tc thp. Tp hp ca cc dng d liu tc thp ny chnh l dng d liu tc cao cn truyn ti. Cc sng mang trong k thut iu ch a sng mang l hsng mang trc giao. iu ny cho php ghp chng ph gia cc sng mang,do s dng di thng mt cch c hiu qu. Ngoi ra s dng h sng mangtrc giao cn mang li nhiu li th k thut khc, do cc h thng iu cha sng mang u s dng h sng mang trc giao v c gi chung l ghpknh theo tn s trc giao OFDM.

Vit Nam hin nay ang trin khai mt s ng dng s dng k thutiu ch a sng mang OFDM nh truyn hnh s DVB-T, ng dy thu baokhng i xng ADSL v truyn thng qua ng dy ti in PLC.

iu ch a sng mang l mt k thut tng i mi m v phc tp.Vi thi gian v kin thc cn hn hp nn chc chn bo co ny cn nhiuthiu st. V vy,em mong nhn c s ng gp ca cc thy c.

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Chng 1. Gii Thiu Tng

Quan V OFDM

0.1 Gii thiu chung

K thut ghp knh theo tn s FDM (Frequency Division Multiplexing)

c s dng t hn mt th k nay truyn nhiu tn hiu tc chm. Vd nh in bo qua mt knh c bng thng rng bng cch s dng cc sngmang c tn s khc nhau cho mi tn hiu. pha thu c th tch c cc tnhiu bng cch s dng cc b lc th phi c khong cch gia ph ca ccsng mang. Ph ca cc tn hiu khng st nhau gy nn lng ph bng thng vdo hiu sut s dng bng thng ca FDM l kh thp.

iu ch a sng mang tng t nh FDM, song thay v truyn cc bntin ring r, cc sng mang s c iu ch bi cc bit khc nhau ca mt bntin tc cao. Bn tin ny c th dng song song hoc ni tip sau cchuyn i ni tip - song song truyn i trn cc sng mang.

C th so snh iu ch a sng mang vi iu ch n sng mang s

dng cng mt knh nh sau: iu ch a sng mang nu s dng nhiu b thupht th s phc tp v gi thnh cao. Mi sng mang s truyn mt bn tin con,tng ca cc bn tin con ny cho bn tin cn truyn i c tc nh hn bn tinc truyn bi mt sng mang duy nht cng s dng knh bi v h thnga sng mang cn cc khong bo v trnh nhiu gia cc sng mang con.Mt khc h thng n sng mang d b giao thoa gia cc k hiu intersymbolinterference (nhiu ISI) bi v khong thi gian ca cc symbol l ngn v moln sinh ra trn bng tn rng, so vi khong thi gian di ca symbol v bngtn hp ca h thng a sng mang. Trc khi pht trin k thut cn bng, iuch a sng mang c s dng truyn dn tc cao mc d gi thnh caov hiu sut s dng bng thng thp.

ng gp c bn cho s pht trin ca OFDM l vic ng dng bini Fourier (FT) vo iu ch v gii iu ch tn hiu. K thut ny phn chiatn hiu ra thnh tng khi N s phc. S dng bin i Fourier ngc IFFT(Inverse Fast Fourier Transform) cho mi khi v truyn ni tip. Ti pha thu,

bn tin gi i c phc hi li nh bin i FFT (Fast Fourier Transform) cckhi tn hiu ly mu thu c. Phng php iu ch OFDM ny c cpn vi ci tn iu ch a tn ri rc DMT (Discrete Multi-Tone). Ph ca tnhiu DMT trn ng truyn ging ht ph ca N tn hiu iu ch QAM vikhong cch ca N tn s sng mang bng tc tn hiu nh cp trn.Trong mi sng mang c iu ch QAM vi mt s phc. Ph ca mi tn

hiu QAM c dng sin(kf)/f nh ca h thng OFDM ban u.

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Hnh 1.1 : H thng OFDM s dng FFT

Hnh 1.2 : Chng ph trong OFDM

Tuy nhin bin i IDFT vi N s phc s cho gi tr phc c c phnthc v phn o. M khi truyn ta ch truyn phn thc. iu ny c th thchin bng cch thm N s phc lin hp vo khi N s phc ban u. Bin iIDFT cho khi 2N s phc lin hp s cho 2N s thc truyn i i din chomi khi, chng tng ng vi N s phc.

u im ni bt nht ca iu ch a tn ri rc l tnh hiu qu cabin i Fourier nhanh FFT. Mt php bin i FFT cho N im ch cnNlog2N php nhn so vi N2 php nhn trong bin i Fourier thng thng.Hiu qu ca bin i FFT c bit tt khi N l lu tha ca 2, tuy nhin iu

ny khng phi l bt buc. Bi v s dng bin i FFT nn h thng DMTyu cu t php tnh trn mt n v thi gian hn h thng iu ch n sngmang tng ng c s dng b cn bng.

K thut OFDM c bit thnh cng trong cc ng dng v tuyn, nim OFDM th hin c nhiu nht cc u im ca mnh. l tnh chng linh hng ca nhiu do phn x nhiu ng Multipath, chng li pha inh lachn tn s SF (selective fading). K thut iu ch OFDM kt hp vi cc

phng php m ha v xo trn (interleaving) thch hp cho php truyn dliu tc cao qua knh v tuyn vi tin cy cao. Mt ng dng khc caOFDM l truyn d liu tc cao trong mng LAN khng dy ( WirelessLAN ). Trong Wireless LAN, tr truyn dn l nh nhng vi tc cao ti

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hng chc Mbps th khong thi gian tr l ln so vi chu k symbol. Trongtrng hp ny, k thut iu ch a sng mang OFDM c s dng.

Nguyn l c bn ca OFDM

OFDM bt ngun t k thut phn knh theo tn s (FDM), mt kthut c bit ti v s dng rng ri. FDM cho php nhiu bn tin ctruyn i trn mt knh truyn v tuyn. Do vy FDM c xp vo phngthc truyn dn n sng mang. Mt v d n gin ca FDM l vic s dngtn s khc nhau cho cc trm v tuyn bin iu tn s. Tt c cc trm phtng thi nhng khng gy nhiu ln nhau do cc trm ny pht i cc sngmang c tn s khc nhau. Di thng cc tn hiu ny c t cch nhau mtkhong tn s sao cho ti pha thu b lc thng di phn bit c tn hiu cnthu, lc b tn hiu ca cc sng mang khc. iu ny c ngha l gia cc sngmang c mt khong tn s khng c s dng truyn tin tc. Sau khi qua

b lc, tn hiu thu c s c gii iu ch nhn c tin tc cn thu.Nh vy c th thy khng c s chng ph ca cc tn hiu trong min tn s.

Khc vi FDM, trong k thut OFDM mt bn tin c truyn i trnmt s Nn sng mang con ( Nn c th iu chnh c tu theo ln ca bntin), thay v mt sng mang duy nht nh k thut FDM. Khi nim sng mangcon hon ton ging vi khi nim sng mang m ta cp, im khc bitduy nht l cc sng mang con ny c di thng nh hn nhiu so vi cc sngmang s dng trong FDM. Nn sng mang con ny to thnh mt nhm, ta tmgi l tn hiu OFDM. Di ph ca ton h thng s bao gm rt nhiu ccnhm nh vy, s sng mang con trong mi nhm c th tu bin.

Cc sng mang con trong mt nhm c ng b c v thi gian v tns, lm cho vic kim sot nhiu gia chng c thc hin rt cht ch. Ccsng mang con ny c ph chng ln ln nhau trong min tn s m khng gyra ICI do tnh trc giao gia chng c bo m. Vic chng ph ny lm tngng k hiu qu s dng di tn.

Trong k thut FDM, khng c s ng b gia cc sng mang vi nhaunn cc sng mang c th c iu ch theo c 2 phng thc: tng t v s.Trong OFDM, cc sng mang con c ng b vi nhau nn ch s dng

phng thc iu ch s. Mt k t (symbol) OFDM c hiu l mt nhmcc bit c truyn mt cch song song. Trong min tn s, cc symbol nytn ti di dng cc khi ph ring r. Trong tng khi c s chng ph gia

cc sng mang v tnh trc giao trong tng khi lun lun c m bo.

1.1. Trc giao trong OFDM

Tn hiu c gi l trc giao vi nhau nu chng c lp vi nhau.Trc giao l mt c tnh cho php nhiu tn hiu mang tin c truyn i trnknh truyn thng thng m khng c nhiu gia chng. Mt tnh trc giaogia cc tn hiu s gy ra s ri lon gia cc tn hiu, lm gim cht lngthng tin. C rt nhiu k thut phn knh lin quan n vn trc giao. Kthut phn knh theo thi gian (TDM) truyn mt lc nhiu bn tin trn mtknh bng cch cp cho mi bn tin mt khe thi gian. Trong sut thi gian

truyn mt khe thi gian, ch c mt bn tin duy nht c truyn. Bng cch

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truyn khng ng thi cc bn tin nh vy ta trnh c nhiu gia chng.Cc bn tin c th c xem nh l trc giao vi nhau, trc giao v mt thigian. K thut FDM t ti s trc giao gia cc tn hiu trong min tn s

bng cch cp cho mi tn hiu mt tn s khc nhau v c mt khong trng

tn s gia di thng ca 2 tn hiu.OFDM t c s trc giao bng cch iu ch tn hiu vo mt tp

cc sng mang trc giao.Tn s gc ca tng sng mang con s bng mt snguyn ln nghch o thi gian tn ti symbol. Nh vy, trong thi gian tn tisymbol, mi sng mang s c mt s nguyn ln chu k khc nhau. Nh vy,mi sng mang con s c mt tn s khc nhau, mc d ph ca chng chngln ln nhau nhng chng vn khng gy nhiu cho nhau .

Trong k thut in t, tn hiu truyn i c biu din bi mt dngsng in p hoc dng in theo thi gian, ta gi chung l sng mang. Sngmang ny thng c dng hnh sin. Sau khi c iu ch tin tc, trong sngmang khng ch tn ti duy nht mt tn s m l mt t hp gm: tn s trung

tm ca sng mang v cc hi. Mc tng i ca mt tn s khi so snh vimt tn s khc c cho bi ph in p hoc dng in. Ph ny c c

bng php bin i Fourier dng sng mang trong min thi gian. V mt lthuyt, t c gi tr ph chnh xc th phi quan st dng sng mang trnton b min thi gian , tc l phi thc hin php bin i Fourier trn ton bmin thi gian, ti v hn im. Khng mt h thng k thut no c th lmc iu ny. Thc t cho thy ch cn thc hin php bin i. Fourier ti mts hu hn im l c th khi phc c dng sng mang m khng lm mti bn cht ca tin tc. Php bin i Fourier ti mt s hu hn im c gil php bin i Fourier ri rc ( DFT DiscreteFourier Transform). Qu trnhkhi phc dng sng mang t ph ca n c gi l php bin i Fourierngc.

1.2. Thu pht tn hiu OFDM

Hnh 2.1 : S khi thu pht OFDM

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c th ca tn hiu OFDM l n hon ton c to ra trong min s,do rt kh ch to cc my thu pht kha pha di rng trong min tng t.Ti khi pht, d liu s sau khi c iu ch vo cc sng mang c em

i thc hin php bin i Fourier to s trc giao gia cc sng mang.Trong thc t ngi ta dng php bin i Fourier nhanh (FFT) cho

bc ny. FFT l mt dng bin i Fourier ri rc (DFT) nhng cho hiu qutnh ton cao hn nn c dng trong cc h thng thc t. Sau khi toc s trc giao gia cc sng mang, cc sng mang ny li c chuyn vmin thi gian bng IFFT truyn i. Lc ny ta to c mt tn hiuOFDM gm mt nhm cc sng mang trc giao vi nhau trong min thi gian.Lu , tn hiu OFDM mi ch bng tn c s, cn c chuyn ln ti tns c la chn truyn i.

Khi thu thc hin qu trnh ngc li khi pht. Tn hiu OFDM thu tanten c chuyn v bng tn c s x l. Tn hiu ny sau c qua

FFT phn tch tn hiu trong min tn s. Pha v bin ca cc sng mangcon c nhn din v c chuyn thnh d liu s cn thu.

1.3. Chuyn i ni tip - song song

D liu s thng dng mt chui cc bit lin tip. Trong h thngOFDM, mi symbol thng mang t 40 4000 bits, do bc chuyn i nitip song song l cn thit t cc bit thng tin ln OFDM symbol. S bitthng tin trn mt symbol ph thuc vo phng thc iu ch v s sng mangcon.

V d, ta s dng phng thc iu ch 16-QAM, nh vy mi sngmang s mang 4 bits thng tin, v s sng mang con s dng l 100 th s bitthng tin trn mt symbol s l 4x100 = 400 (bits). Ch rng nu ta dng

phng thc iu ch thch nghi (Adaptive Modulation) th s bit thng tin trntng sng mang con c th khng ging nhau. Ti pha thu qu trnh ngc li,chuyn i song song ni tip, s c thc hin chuyn d liu v dng nitip nh ban u.Khi tn hiu OFDM truyn trong mi trng a ng, do phainh chn la tn s s xut hin nhng nhm sng mang con b suy gimnghim trng ti mc gy ra li bit ti pha thu. Cc im trng trong p ngtn s ca knh truyn c th lm cho thng tin trn mt s sng mang ln cnnhau b ph hu, kt qu l c mt cm cc bit lin nhau b li. Nu nh cm bitli ny khng qu ln, nm trong tm kim sot ca b sa li pha thu th

vn s chng ng ngi. Nhng thc t, cc cm bit li ny li thng khln, trong khi kh nng kim sot ca b sa li li rt hn ch, v li vic cithin kh nng sa li thng rt tn km. Mt tng n gin v d thchin gii quyt vn ny l: nu nh cc cm bit li ny gm cc bitkhng ln cn nhau th khi chuyn i song song sang ni tip pha thu, cc

bit li ny s nm ri rc, v nh vy ta trnh c cc cm bit li ln. Do hu ht cc h thng thc t, ngi ta u s dng mt b xo trn bit haycn gi l ci xen (interleaving) nh l mt phn ca qu trnh chuyn i nitip song song. Thay v truyn cc bit tun t theo v tr ca chng trong chui

bit thng tin u vo, ta truyn chng khng theo th t, ri sau li sp xpchng ng th t pha thu.

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1.4. iu ch sng mang ph

Cc sng mang ph sau khi c cp pht cc bit thng tin truyn i,chng s c iu ch pha v bin bng cc phng thc iu ch thch

hp. Lc ny sng mang c biu din bng vector IQ. Qu trnh iu ch vocc sng mang con thc cht l qu trnh nh x cc bit thng tin theo mt s iu ch (Constellation) c th. Do qu trnh ny cn gi l Mapping.Ti my thu, thc hin vic gii m vect IQ thnh t m ban u. Trong qutrnh truyn, nhiu v mo ca knh truyn lm cho cc vect IQ thu nhn ckhng r nt, do c th gy li nhn din t m. Do vi mi phng thciu ch s cn mt t s tn hiu trn tp m nht nh. V d vi phng thciu ch 16-QAM, khi t s tn hiu trn tp m cho php l S/N = 18dB.

1.5. Chuyn i t min tn s sang min thi gian

Sau giai on iu ch sng mang con, ta n nh c cho mi sngmang con mt bin v pha da trn cc bit thng tin c truyn i v

phng thc iu ch sng mang c s dng, nhng sng mang con khngtruyn tin s c bin bng 0. y l bc xy dng tn hiu OFDM trongmin tn s. truyn c th tn hiu OFDM phi c chuyn v min thigian bng IFFT. Trong min tn s, mi im ri rc m ti ta thc hinIFFT tng ng vi mt sng mang con. Cc sng mang con c bin bngkhng s c s dng nh di bo v.

1.6. iu ch tn s v tuyn

Tn hiu OFDM c to ra sau giai on IFFT mi ch tn s c s,tn hiu ny cn phi c nng ln tn s cao hn phc v cho vic truyndn. Bc ny c th p dng k thut tng t hoc k thut chuyn i s. C2 k thut u c cc thao tc ging nhau, tuy nhin iu ch s c xu hngchnh xc hn do chnh xc trong vic phi ghp 2 knh I&Q , mt khc kthut iu ch s cho gi tr pha chnh xc hn.

1.7. Khong bo v GI ( Guard Interval )

Vi mt di thng cho trc, tc symbol ca mt tn hiu OFDM nh

hn nhiu so vi tc symbol ca mt sng mang trong h thng n sngmang. Nu s dng phng thc iu ch BPSK th tc symbol s bng vitc bit. Nh ta bit, di thng ca mt tn hiu OFDM s bng di thngcho trc trn chia cho N sng mang con. Do vy tc bit ca mt tn hiuOFDM s nh hn N ln tc bit trn mt sng mang trong h thng n sngmang. Tc symbol trn sng mang con thp to cho OFDM c kh nng chuISI rt tt.

Tuy nhin, cn c th ci thin hn na kh nng chu ISI ca h thngOFDM bng cch chn thm cc di bo v vo trc mi symbol . Di bo vca mi symbol l mt phn bn sao ca chnh symbol , c th l phn uhoc phn cui hoc c 2 phn ca chnh symbol . Thng th ngi ta hay

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dng phn cui ca symbol lm di bo v cho symbol . Khi khong bov GI c gi l CP (Cyclic Prefix). Chn thm di bo v lm thi giantruyn ca symbol tng ln, do lm tng kh nng chu ISI. Nh cp trn, mi sng mang con mang mt phn tin tc ca 1 symbol, dng mt phn

symbol lm di bo v cn to cho vic truyn dn c lin tc, khng c sngt qung gia cc symbol. Hn na, di bo v cn cho php gim li do sx dch thi gian my thu.

1.8. Chng li do dch thi gian

gii m tn hiu OFDM, my thu phi thc hin FFT vi tngsymbol ly ra c bin v pha ca sng mang con. Vi cc h thngOFDM c tc ly mu nh nhau cho c my pht v thu, th kch thc FFT

phi nh nhau cho c tn hiu pht v tn hiu thu nhm duy tr c tnh trcgiao gia cc sng mang con. Do chn thm di bo v mi symbol thu c c

thi gian ly mu l TG + TFFT, trong khi my thu ch cn gii m tn hiutrong khong thi gian TFFT. Do khong thi gian TG l tha. Vi mtknh truyn l tng khng c tr truyn dn, my thu s khng gp phi bt ks x dch no v mt thi gian v vn ly mu chnh xc m khng cn bt kmt khong ngn cch no gia cc symbol. Tuy nhin, trong thc t khng cknh truyn no l l tng, trn mi knh truyn lun lun c tr truyn dn.Di bo v s chuyn i cc x dch v mt thi gian ny thnh s quay phaca cc sng mang con trong tn hiu thu c. Lng quay pha ny t l vitn s ca sng mang con. Gi s lng thi gian x dch l nh nhau vi ccsymbol khc nhau, khi lng di pha do s x dch thi gian d dng cloi b bi bc cn bng knh truyn. Trong mi trng a ng, di bo vcng ln th ISI cng c loi b nhiu, li do s x dch thi gian cng cgim thiu.

1.9. Chng nhiu gia cc symbol

Trong mt tn hiu OFDM, bin v pha ca mt sng mang con phic gi khng i trong sut thi gian truyn mt symbol nhm duy tr cs trc giao gia cc sng mang con. Nu bin v pha ca sng mang con b

bin i trong thi gian truyn ca symbol th dng ph ca sng mang conkhng cn l dng sinc na, dn n cc im 0 trong dng ph s khng xut

hin ti cc tn s trung tm ca cc sng mang con, gy ra nhiu gia cc sngmang con (ICI). Ti bin gii gia cc symbol, bin v pha thay i t ngtti gi tr mi tng ng vi symbol mi. Trong mi trng a ng, ISI sgy ra s phn tn nng lng gia cc symbol vi nhau, do s c s thayi nht thi ca pha v bin sng mang con ti thi im bt u casymbol. C ngha l bin v pha ca sng mang con ti thi im bt usymbol s nh hn hoc ln hn bin v pha thc s ca n.

Bin v pha ny s lin tc thay i di s tc ng ca cc thnhphn a ng. Thi gian tn ti ca s thay i nht thi ny t l vi trtruyn dn ca knh truyn. Nu tr truyn dn khng vt qu di bo v thkhi thc hin FFT bin v pha ca sng mang i vo n nh, do

khng gy ra li nhn din pha v bin sng mang. Cc nh hng khc ca

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hin tng a ng nh : s quay pha ca cc sng mang, s gim bin sng mang u c th c hiu chnh bi bc cn bng knh truyn.

Vic chn thm di bo v gii quyt c phn ln cc nh hngdo ISI gy ra vi tn hiu thu, nhng di bo v ch pht huy hiu qu khi tr

truyn dn khng vt qu phm vi ca n. Trong thc t, cc thnh phn ang suy gim rt chm theo thi gian, trong khi di bo v li khng th lnmt cch tu (di bo v cng ln th hiu sut s dng ph tn s cng thp),do khng th loi b trit nh hng ca ISI ln tn hiu thu .

1.10. Mo u v phn cch sng mang

Chn di bo v s lm chm tc symbol nhng khng nh hngn s phn cch gia cc sng mang ti my thu.Khong cch gia cc sngmang quyt nh bi tc ly mu v s im thc hin FFT ti my thu :

Trong : l khong cch tn s gia cc sng mang con (Hz)l tc ly mu (Hz)

l s im thc hin FFT

1.11. Hn di v to ca s tn hiu cho OFDM

Tn hiu OFDM trong min thi gian l tp hp ca mt nhm sngmang con dng sin c qua iu ch. Mi sng mang con c t trong

mt ca s thi gian dng ch nht. Ca s ny t gii hn cho tng OFDMsymbol, v quyt nh p ng tn s ca tn hiu OFDM c to ra. Hnhdi y l mt v d v dng sng ca mt sng mang con OFDM s dng

phng thc iu ch PSK. Bin ca sng mang l khng i, nhng phathay i theo symbol. Kt qu l ti bin gii gia cc symbol c s thay i

pha t ca sng mang. Kt qu ca s i pha t ngt trong min thi gian ls phn tn nng lng gia cc symbol trong min tn s.

Hnh 2.2. Dng sng trong min thi gian ca sng mang con

1.12. Lc thng di

Khi tn hiu s c chuyn sang dng tng t truyn dn th b lcc dng trnh tp (aliasing). Tp l hin tng tn hiu sai xut hin khi

tn hiu tng t c s ho. S dng b lc thng di s loi b c cc

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bp sng ph ca tn hiu OFDM. Lng bp sng ph c lc b ph thucvo nhn ca b lc c s dng. Nhn chung cc b lc s cho chnhxc, dc c tuyn lc cng nh tnh thch nghi cao hn cc b lc tng t.Do trong h thng OFDM s dng cc b lc s s rt hiu qu trong vic

hn di tn hiu. V nh ngha mt h thng s dng lm bin dng s phnb ph tn s ca cc thnh phn ca mt tn hiu theo cc ch tiu cho cgi l b lc s.

Hnh 2.3. p ng tn s ca tn hiu OFDM khng qua b lc.

Hnh 2.4. p ng tn s ca tn hiu OFDM s dng b lc FIR vichiu di ca s l 3 .

Thc t l b lc c th lc b hon ton cc bp sng ph, nhng ngngha vi n l chi ph tnh ton tng ln, v lm gim SNR hiu dng ca hthng. Hn na, vic lc tn hiu cng ct b mt phn ng k nng lng cacc sng mang con pha ngoi, lm mo dng cc sng mang con ny v gyra ICI. Nh trn hnh 2.4 ta c th thy c ti 8 sng mang con b ct b mt

phn nng lng bp sng chnh.

1.13. S dng bo v dng cos nng

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Tng Quan OFDM

Mt trong nhng phng php n gin nht loi b cc bp sngph (cc bp th cp) l t di bo v cho tn hiu OFDM, gim bin symbol dn v 0 trc khi chuyn sang symbol khc. iu ny to ra mt s

chuyn i mm do gia cc symbol, do gim c nng lng ca cc bpsng ph. Di bo v cos nng ca cc symbol c th chng ln nhau m chgy ra mt lng ISI khng ng k, my thu khng nhn bit c. u imca vic chng ph ny cho php tng gp i chiu di di bo v m khnglm tng thm yu cu v di thng cho symbol.

2. Cc k thut ca OFDM

2.1. ng bDch thi gian v tn s trong OFDM

Gi s nhiu gia cc tn hiu clock l khng ng k, mt tn hiuOFDM c truyn i c th c biu din nh sau:

Trong : k1 l di khong bo v u symbol,k2 l di khong bo v cui symbol ,w(t) l hm ca s min thi gian .

Tn hiu thu c pha thu r(t) s c lc v ly mu vi tn s lymu l bi s ca 1/T.

Tn hiu u ra ca my thu FFT vi gi thit knh truyn l tngc biu din nh sau:

Trong : Xc(f) l bin i Fourier ca tn hiu analog c lp li mt

cch nh k tng ng vi tn hiu pht ra bi my pht IFFT. Do , Xc(f)c ph vch ti tn s k/T v W(f) l bin i Fourier ca hm ca s w(t).Gi thit tn hiu ly mu theo thi gian c dch pha tng i v

dch pha ny khng i sut trong mt symbol, khi tn hiu pha thu c ly mu c th n gin ha nh sau:

Trong : biu din mo pha ca ng bao .

Sau khi bin i Fourier tn hiu c dng sau :

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Nh vy, dch pha theo thi gian hay mo pha ca ng baokhng nh hng n tnh trc giao ca cc sng mang con .

1.1. ng b trong h thng OFDM

Trc ht, cn ng b khung (hoc gi) cung cp thng tin iu chnhv OFDM symbol. Tip , cn hiu chnh tn s trc khi thc hin bin iFFT gim nh hng ca nhiu ICI (interchannel interference). Sau thmmt giai on iu chnh mn v b tn s c thc hin. Ni chung iu chnhkhung v sa tn s l cng vic rt phc tp, i hi cu hnh phn cng cngnh phn mm cao. K thut iu chnh v sa tn s trong OFDM c th chiathnh hai loi: phng tin d liu (data aided) v k thut phi tuyn ( non-linear techniques ). K thut phng tin d liu s dng mt mu bit bithoc tn hiu pilot nh gi tr v dch tn s. K thut phi tuyn s dngtnh chu k ca tn hiu rt ra thnh phn hi mong mun bng cch s dngton t phi tuyn. K thut phng tin d liu data aided c p dng trong

nhiu h thng thng tin s OFDM nh HDTV, Wireless LAN.

1.1.1. ng b thi gian v ng b khung

Hnh 3.1. ng b khung

1.1.2. c lng dch tn s

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Hnh 3.2. c lng dch thi gian

1.2. Cn bngMt trong nhng u im ca k thut iu ch a sng mang l chng

li c giao thoa gia cc symbol ISI ( inter - symbol interference). Khongthi gian tn ti di ca OFDM symbol cho php chng li tr do phn x nhiung (multipath) v ISI, min l tr ca knh khng vt qu khong bov GI (guard interval) ca symbol. Nh vy h thng OFDM khng b nhhng ca nhiu ISI, do vic cn bng min thi gian khng phi lc nocng cn n. Tuy nhin trong trng hp cn truyn d liu vi tc caotrong knh truyn c s phn tn thi gian ln th phi dng n b cn bng liu khng th trnh khi. Cu trc ca b cn bng trong OFDM khc vi

trong h thng n sng mang thng thng. Mc ch ca b cn bng khngphi l loi tr hon ton m l hn ch ISI. Cn bng trong min tn s, khikhng c giao thoa gia cc sng mang ICI c dng b cho h s khuychi ca knh ti cc tn s sng mang khc nhau.

1.2.1. Cn bng trong min thi gian

H thng a sng mang chng li c nhiu ISI nu nh tnh trc giaogia cc symbol k nhau c bo m trong min tn s. Ni cch khc, thigian tn ti ca tn hiu OFDM c m rng vt qu chu k T. Tuy nhin,nu p ng xung ca knh b tri ra ln hn khong bo v ca symbol, khi

nhiu ISI s xut hin lm gim hiu nng ca h thng.C mt vi khc bit c bn ca b cn bng trong min thi gian cho

h thng OFDM l: B cn bng trong h thng OFDM khng cn kh ht nhiu ISI,nhng

cn gii hn di ca n. Khng ging nh trong h thng n sngmang, b cn bng ch cn gim thiu ISI ti nh hn khong bo v.

B cn bng khng hot ng theo kiu DFE ( Decision FeedbackEqualizers ) khi truyn d liu tr khi c tuyn truyn t ca knhthay i nhanh. B cn bng c iu chnh trong khi truyn chuihun luyn (gi l training mode) v gi nguyn cc tham s trong khitruyn d liu (gi l data mode).

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B cn bng trong OFDM l b cn bng c nh li. Trong b cn bngDFE, li nhn c t tch v hng ca u vo v u ra ca mch ct(slicer). Cn trong OFDM li nhn c t u ra ca b lc xung v blc chun.

V tr ca ca s p ng xung c nh hng ln n hiu qu ca bcn bng .

1.2.2. Cn bng trong min tn s

Hnh 3.3. H thng OFDM vi b cn bng trong min tn s

1.3. Kh ting vng

Hnh 3.4. H thng s dng k thut echo cancel

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Hnh 3.5. B kh ting vng echo canceller

1.4. M ha knh

Hu ht cc ng dng ca OFDM u phi s dng cc phng thc mha t c hiu qu mong mun. Trong cc h thng v tuyn, do nhhng ca pha inh nn cn t s S/N rt cao c th t c xc sut li bitmong mun. Thm vo l nhiu t cc knh radio khc RFI (radio frequencyinterference) cng nh hng rt ln ti t s S/N. Trong cc h thng hutuyn, thng dng cc b iu ch nhiu mc t c tc bit cao.Trong trng hp ny, m ha l bt buc t c tc bit cao nht c thtrong mi trng c nhiu xuyn m, gin xung v cc nhiu khc.

1.4.1.M ha khi trong OFDM

Trong m ha khi, d liu u vo c chia thnh tng khi k bit vu ra l cc khi n bit (vi n > k). y, n - k bit thm vo gi l cc bit kimtra c tnh ton t k bit u vo theo mt thut ton nht nh. Phng thcm ha ny lm tng bng thng ln R ln. Vi R = n/k gi l t s m, R chnhl tc u ra trn tc u vo b lp m.

Hnh 3.6. Cu trc b lp m khi

Vi khi n bit u ra, ch c 2kkh nng hp l trong tng s 2n khnng c th xy ra. T m c chn vi khong cch Hamming ti thiu.Khong cch Hamming l s bit khc nhau ca hai t m X v Y, k hiu l

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d(X, Y). Nh vy, t m c c trng bi b tham s [n, k, d]. pha thu,khi n bit c phc hi li vi mt xc sut li nht nh. B gii m s tmmt t m hp l v c khong cch Hamming vi khi nhn c l nh nht.Sau n - k bit kim tra c th c xa v kt qu thu c l bn sao ca t

m k bit u vo. Nu d = 2t + 1 th m khi c th sa sai vi mt lng licho php. Trong OFDM, nu n ph hp vi s bit trong mt symbol th misymbol c th c m ha c lp v khng cn nh cc symbol trc trongqu trnh gii m.

Phng php gii m ni n trn l phng php da trn quytnh cng (hard decision). Vi phng php ny, b gii m thc hin vi cc

bit u ra ca b gii iu ch. Do cc li xy ra vi tn hiu analog s bmt b iu ch. S tt hn nu c mt s nh gi cc li trong khi giim. y l phng php gii m ha mm (soft decoding). Trong phng

php ny, b gii iu ch khng ch gii iu ch cho ra cc bit m ha nhthng thng, m cn kim tra tin cy khi a ra cc bit . Khi m tin

cy thp hn ngng cho php, b gii iu ch s cho ra mt k t c bit gil k t trng (erasure symbol) thay cho mt bit. Nh vy b gii m s lmvic vi 3 mc tn hiu u vo. Bt k t hp ca t li v e k t trng c thc sa sai nu d = 2t + e + 1. Phng php gii m ha quyn nh mm nyc th cho hiu qu sa li cao nu chn c ngng thch hp cho k ttrng.

M ha khi, c bit l m Reed-Solomon rt hiu qu trong vicchng li li chm. Trong h thng n sng mang nhng li chm c th xyra do nhiu xung hoc do pha inh. Trong h thng a sng mang cng thnggp phi li chm. Nhiu xung c ph rng nn c th nh hng ti vi sngmang. Cn pha inh la chn tn s vi di tn chu nh hng ca n rng hndi tn ca mt sng mang nn cng c th nh hng n mt vi sng manglin nhau.

Mt dng m khi rt hay c s dng pht hin li (khng sa li)l m vng CRC (Cyclic Redundancy Check). y mt s c nh cc bitkim tra c thm vo t m. Pha thu s s dng cc bit ny kim tra, nu

pht hin ra li th s yu cu truyn li. Nguyn l to m vng l chia t mcho a thc sinh g(x) chn trc v thm phn d vo t m ban u. Nh vyt m truyn i s chia ht cho g(x), v pha thu s s dng c im ny ca tm pht hin li. Di y l mt s a thc sinh hay c s dng tngng vi 8 bit kim tra (a thc u) v 16 bit kim tra (cc a thc sau):

M vng CRC c u im l b lp m v gii m n gin dng thanhghi dch v cng modul-2. M vng CRC vi n bit kim tra cho php pht hinmi li c di nh hn n v vi xc sut 1 - 2n cho cc li khc.

1.4.2.M ha vng xon

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M vng xon hay cn gi l m chp l mt phng php m ha rtquan trng trong OFDM. Khc vi m khi, m vng xon khng thc hin mha cho tng khi m thc hin m ha lin tc vi dng bit. Do m vngxon l mt loi m lin tc v c nh ngha l khng ch ph thuc vo t m

u vo m cn ph thuc vo cc t m trc .B m ha bao gm mt thanh ghi dch m bit, n b cng modul-2 v

mt b chuyn mch ly n bit u ra t n b cng modul-2. Nu trong michu k chuyn mch c k bit u vo thanh ghi dch ( k < m ) th tc m has l k/n, tc l vi k bit u vo b m ha vng xon s cho n bit u ra. Nhvy m vng xon c c trng bi b 3 thng s [n, k, m]. Cu trc ca bm ha vng xon nh sau:

Hnh 3.7. B m ha vng xon tng qut

M vng xon c th c biu din bng gin trng thi vi s trngthi l s = 2m-1. Ngoi ra cng c th biu din m vng xon di dng athc, cy m hoc li. Di y l gin trng thi ca b m ha vng xon[3,1,3] tc l vi mi bit u vo cho 3 bit u ra, v di ca thanh ghi dchl 3 bit.

Hnh 3.8. B m ha [3,1,3] v gin trng thi

Vic gii m vng xon thc cht l xc nh ng c khong cch

Hamming nh nht vi t m nhn c trong s 2r ng c th ca s cy

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m, vi r l s k hiu k bit u vo (thng th k = 1). Khong cch Hamminggia hai chui k hiu chnh l s k hiu khc nhau gia chng. Vy s con s1 ca kt qu cng molul-2 cc bit (k hiu) tng ng chnh l khong cchHamming. V nguyn tc ta c th ln lt thc hin 2r php cng modul-2 ca

tng b m c th vi chui bit nhn c tm ra b m c khong ccHamming nh nht. B m ny c xc sut cao nht chnh l bn tin pht.Tuy nhin trong thc t vi mt n v bn tin c s k hiu rt ln th khngth thc hin 2r php cng modul-2 trong thi gian c th chp nhn c.

Thut ton Viterbi ch ra quy tc loi b tt c cc b m c xc sutthp, do ta ch phi xt vi 4r b m thay v mt khi lng khng l l 2r

b m. Ni dung thut ton Viterbi nh sau:Gi chui k hiu u vo b gii m l x.Gi chui k hiu u ra b gii m l y.Hm tng quan (likelihook function) gia chng l:

Metric nhnh th j c k hiu l:

B gii m thc hin tm cc i hm tng quan tc l tm trn s cy m sao cho tng metric trn ng l cc i. ng vi mi k hiu u

vo l mt trng thi, mt nhnh no ca cy m. Cn thc hin php cng -so snh - chn ACS (Add - Compare - Select) sau: Cng mi metric hin ti vihai nhnh c th, so snh hai metric mi thu c v chn ra nhnh no cmetric ln hn.

C th m ha vng xon lin tc cho dng s liu m ch cn khi ub lp m khi khi ng h thng. Tuy nhin, gim su ca cy m ngita thng khi u v kt thc b lp m mt cch nh k. Trong OFDMkhong nh k c th l chu k symbol.

Kt hp gia m vng xon v m ha khi trong mt chui m ha lmt k thut m ha rt mnh. M ha khi l m ha ngoi, c thc hinu tin my pht v cui cng my thu. M vng xon l m ha trong, rt

hiu qu trong vic gim xc sut li c bit l khi s dng gii m quyt nhmm. Nu b m vng b li th s to ra li chm. Li xy ra khi thut tonViterbi chn nhm b m. B m khi ngoi, c bit l m Reed - Solomon cci xen (interleaving) rt hiu qu trong vic sa cc li chm.

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Hnh 3.9. Chui m ha v ci xen

1.4.3. M ha mt li

M ha mt li ( Trellis Coding ) tng t nh m ha vng xon,nhng khc im c bn l m ha mt li gn cht vi qu trnh iu ch.Qu trnh gii m quyt nh mm da trn khong cch Euclid ti thiu v lmt phn ca qu trnh gii iu ch.

Trong m ha mt li, phn d khng phi l cc bit hay k t cthm vo trc khi iu ch m c to ra bi vic s dng s iu chnhiu im hn so vi yu cu nu khng m ha. Tc k t n b iu chkhng i, do bng thng yu cu cng khng i. Bi v c nhiu im chomt k t nn c v l xc sut li vi t s S/N cho trc s tng. Tuy nhincng ging nh trong m vng xon, gia cc symbol c s ph thuc,do chc mt s nht nh cc chui im l hp l. Bng cch s dng hp l ccrng buc pha thu, xc sut li bit thc ra l gim.

Bc u tin trong thit k m ha mt li l to ra s iu ch mrng v chia n thnh cc tp con. Cc im trong mi tp con c sp xp saocho c khong cch Euclid ln v s tng ng vi cc bit cha c m ha.Phn cn li (cc bit c m ha) s xc nh tp con. Ch c nhng chuitp con nht nh l c php, cc chui ny c xc nh bi mt m vngxon n gin. Cng ging nh trong m ha vng xon, chui ny c th c

biu din bng gin trng thi hay mt li (trellis). cho cc chui cphp cch xa nhau, cc s iu ch con c chn sao cho ph hp vi ccnhnh c khong cch ln nht.

Chng ta s minh ha qu trnh ny bng b m ha mt li n gins dng s iu ch 16-QAM iu ch 3 bit cho mi k t. Hnh di yl s iu ch c chia thnh 4 tp con, mi tp con c 4 im vi cc nhntng ng biu hin chng cng tp con, v s b m ha mt li:

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Hnh 3.10. B m ha mt li

B m ha mt li c 3 bit u vo trong cc bit b 2 v b3 l cc bitkhng c m ha s xc nh im trong tp con, cn bit b1 s ti b m vngxon tc 1/2 to ra 2 bit xc nh tp con c s dng. Di y l s trng thi ca m ha trellis :

Hnh 3.11. S trng thi ca m ha mt liTrong s trn, cc trng thi c xc nh theo cc bit S1S2 ca

thanh ghi dch trong b m vng xon, cc nhn dch chuyn trng thi xc nhtp con c s dng khi xy ra dch chuyn trng thi .Ti pha thu, thut ton Viterbi c s dng kt hp gii iu ch v giim. Vi mi k hiu nhn c, s xc nh khong cch t ti im gnnht ca mi tp con. Bnh phng khong cch ny l metric c s dngtrong thut ton Viterbi. Vi mi trng thi, khng ch bit c m ha v tngmetric c lu li m c bit khng c m ha tng ng vi im trong tpcon m gn nht vi mi symbol.

Hnh 3.12. Metric s dng cho gii m Viterbi

Trong OFDM thng thc hin m ha trellis cho cc sng mang vimi OFDM symbol. Ti u ca mi symbol, b m ha c khi to vi mttrng thi bit trc. V ti cui ca mi symbol b m cng c a v mt

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trng thi bit trc. Vi cc sng mang trng (Null sub-carriers) th c bqua. S m ha c th thay i bng cch gi c nh s bit m ha trongmi sng mang v thay i s bit khng m ha.

1.4.4.M ha turbo trong OFDM

M ha turbo gn y c s dng thnh cng trong rt nhiu hthng thng tin. M ha turbo tng qut bao gm hai hay nhiu m chui hocm song song. Mt b m turbo in hnh gm M ha mt li (TrellisCoding) tng t nh m ha vng xon, nhng khc im c bn l m hamt li gn cht vi qu trnh iu ch. Qu trnh gii m quyt nh mm atrn khong cch Euclid ti thiu v l mt phn ca qu trnh gii iu ch.

Vic thit k cu trc v phc tp ca m ha turbo b gii hn bicc thng s h thng khc. Tr gii m nh hng rt ln ti thit k my thu.Bi v cu trc b gii m turbo l lp v bao gm khi gii ci xen,gii ci xen

cho mi ln lp li, nu tr qu ln s nh hng ti cht lng ton b hthng. Mt vn quan trng khc m h thng yu cu l tng ch m ha. Khim tc li bit BER ca m ha turbo c ci thin r rt th h thng c thlm vic t s tn hiu trn tp m S/N nh. Tuy nhin nhiu chc nng khcca my thu li yu cu mt t s S/N ti thiu no , iu ny cng cn ctnh ton k lng khi thit k h thng

Hnh 3.13. Cu trc b gii m TurboC 3 dng khc nhau ca u vo mm (Soft input) cho mi b gii

m l k t thng tin khng m ha, thng tin d tha t m i xng quyRSC (Recursive Symmetric Code) u tin, v cc thng tin ngoi lai. u ra lcc trng s thng tin khng m ha, thng tin u tin (priori information) vthng tin ngoi lai tng ng.

2. Cc ng dng ca OFDM

2.1. ng dng ca OFDM trong thng tin v tuyn2.1.1. Pht thanh s DAB

Vi nhng c im u vit ca mnh, iu ch a sng mang c bitthch hp cho truyn thng qung b tn hiu s. C quan tiu chun Chu u chp nhn OFDM l chun iu ch cho pht thanh v truyn hnh s cho cmt t v v tinh.

Chun pht thanh s DAB (Digital Audio Broadcast) c ban hnh cho

3 dng sau:

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Dng 1 : p dng cho mng n tn SFN ( Single Frequency Networks)mt t.

Dng 2 : p dng cho pht thanh s mt t thng thng cc aphng.

Dng 3 : p dng cho pht thanh qung b qua v tinh.

Vi cc thng s ca cc dng nh sau:

Trong ni bt nht l dng 1 dng cho mng n tn SFN. Mng n

tn ch c th thit lp khi s dng k thut iu ch a sng mang. Mng ntn bao gm nhiu trm pht phn b trn vng cn ph sng pht cng mt tnhiu cng mt tn s v ng b v thi gian. N cho php ph sng mt

vng rng ln v khc phc nhng im cht do b chn ti nhng v tr cbit. Nh vy mt my thu s nhn c tn hiu t nhiu my pht trng khpv tn s, nh dng tn hiu v thi gian, ngoi tr tr truyn dn l khc nhau.Hnh di y s minh ha mt my thu trong mng n tn nhn c nhiutn hiu vi tr truyn dn khc nhau:

Hnh 4.1. Cc tn hiu thu c trong mng n tn

Trong trng hp khong thi gian ti ca cc tn hiu bao gm cc tnhiu phn x nhiu ng v cc tn hiu ti t cc trm pht khc nh hnkhong bo v gia cc symbol, khi tnh trc giao gia cc sng mang c

bo m. Nh vy cng sut thu c s tng ln. Trong trng hp ngc lith s xy ra giao thoa gia cc sng mang. Giao thoa ny tng ng vinhiu Gaussian v t l vi khong m nhiu ISI vt qu khong bo v. Vd my thu nhn c tn hiu trc tip t hai my pht cch nhau 40 Km, khi

khong thi gian hai tn hiu ti my thu cch nhau c 133s. Ngoi ra cn

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c tn hiu phn x nhiu ng t cc trm pht ny. Do yu cu khongbo v ln hn 133s. Trong thc t khng th tng khong bo v ln qu caov khi s lm gim hiu sut s dng bng thng. Do khong cch giacc trm pht phi nh loi b nhiu ISI.

H thng pht thanh s DAB theo chun Chu u :

Cu trc ca mt my pht thanh DAB s s dng k thut iu chOFDM nh sau:

Hnh 4.2. S khi my pht DAB

B m ha m thanh thc hin ly mu, lng t ha, s ha v m ha

tn hiu m thanh u vo. Tc ly mu l 48kHz v s ha 16 bit trn mtmu cho ra tc 768 kbps cho mt knh. Sau tc ny c m haxung tc thp hn ty thuc vo cht lng yu cu. Tn hiu s u rac ngu nhin ha bng cch cng modul-2 vi dy gi ngu nhin (pseudo-random sequence) cn bng lng cc bit 1 v cc bit 0. Ngu nhin hacn nhm mc ch cho ph tn s pht i c phn tn thch hp. Saungu nhin ha, tn hiu c m ha vng xon vi tc 1/2. Nh vy dng

bit ra khi b m ha vng xon c tc gp i tc u vo. Tip dngbt c xo trn (interleaving) trc khi a ti b ghp knh.

Nh vy mi khung c 77 symbol, mi symbol l 3072 bit (1536 sngmang x 2 bit trn mi sng mang). Nh ni di mi symbol l 1246s do di ca khung l 77 * 1,246 = 96 ms.

Hnh 4.3. Cu trc khung DAB

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K hiu ng b u tin l null tc l khng c tn hiu, do vicng b khung th c th c thc hin n gin bng cch tch sng ng

bao. K hiu ng b th hai c mu c nh, dng cho ng b khung v cungcp pha chun tch sng vi sai cc symbol tip theo. Cc thng tin overhead

c dng mang cc thng tin v knh, cc tham s sng mang dng chogii m thng tin.

Nh vy tc bit thng tin c truyn qua 1 knh DAB l:

2.1.2. Truyn hnh s DVB

Truyn hnh s qung b DVB (Digital Video Broadcasting) l mt khinim rng bao gm truyn hnh,cc ng dng a phng tin di ng(Multimedia mobile application ) v cc dch v cung cp d liu khng dy.DVB c th kt hp vi cc mng khng dy khc nh mng cellular cungcp mt h thng truy nhp d liu bt i xng khng dy dng truy nhpinternet khng dy.

Truyn hnh s l bc pht trin tip theo ca truyn hnh mu tngt. truyn vi cht lng tng ng cn tc bit l 216 Mbps cho hnhnh v 1536 kbps cho m thanh. Do m ha ngun l cc k quan trngtrong h thng DVB. Cc h thng truyn hnh s hin nay s dng k thutnn MPEG-2 cho php nn xung tc 3,5Mbps cho c hnh nh v m thanh.

Hin nay ang tn ti 3 chun truyn hnh s l ATSC ( Advanced

Television Systems Committee ) ca M, chun DVB ca Chu u, v chunISDB ( Intergrated Service Digital Broadcasting ) ca Nht. Trong c haichun s dng k thut iu ch a sng mang OFDM l chun DVB v ISDB.Cc chun ny c ban hnh thnh cc chun c th cho truyn hnh mt t,v tinh v truyn hnh cp. Trong chun truyn hnh s mt t DVB-T tr u th ca mnh v c nhiu nc la chn trong c nc ta. Do di y s tp trung vo DVB-T nh l mt ng dng ca OFDM.

Truyn hnh s mt t DBV-T ca Chu u chn k thut COFDM(Coded Orthogonal Division Frequency), l k thut iu ch a sng mangOFDM kt hp vi k thut m ha knh truyn, v nhng l do sau:

Loi b nhiu: S tn ti ng thi ca truyn hnh s v truyn

hnh tng t yu cu h thng tuyt i khng gy nhiu vitruyn hnh tng t v cc nhiu bng hp khc, bi v truynhnh tng t rt nhy cm vi nhiu. Trc khi s dng kthut iu ch a sng mang th vn ny c gii quyt bngcch gim cng sut pht v s dng cc k thut m ha tc tc li bit yu cu.

Hiu ng a ng (multipath effect): Sng truyn theo nhiung ti my thu vi tr truyn dn khc nhau l nguyn nhnchnh lm suy gim cht lng cc knh truyn hnh mt t.Kin trc mng n tn SFN ca DVB to ra nhiu tn hiu cngti my thu t cc trm pht khc nhau v do phn x thngthng. S tn ti ca nhiu tn hiu vi tr khc nhau ti

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my thu yu cu mt k thut iu ch mnh chng li hiung ny. COFDM cho php loi b hon ton hiu ng a ngmin l di ca khong bo v GI c thit k mt cch hpl. di ca khong bo v cng ln th khong cch gia cc

trm pht trong mng n tn cng xa, tuy nhin s gim hiusut s dng bng thng.

Truyn hnh s chun Chu u DVB-T :

Chun truyn hnh s DVB-T (Digital Video Broadcasting - Terrestrial)c ban hnh bi ETSI (European Telecommunication Standards Institute)da trn cc tng sau:

1) a ra m hnh h thng pht sng truyn hnh s mt t.2) Xc nh yu cu v cht lng tn hiu v kh nng tng thch cho cc

loi dich v khc nhau.3) Quan tm ti vn x l tn hiu my thu m rng cc gii php

thc hin khc nhau.4) Nhm vo mc ch chng hiu ng nhiu ng, chng pha inh la

chn tn s v chng hiu ng Doppler.5) To kh nng thit lp mng n tn.6) To kh nng a phng tin.7) To kh nng di ng.

DVB-T c hai ch l 2k v 8k tng ng vi s im bin i

IFFT/FFT. Mode 2k c 1705 sng mang, mode 8k c 6817 sng mang. Vi mimode u c th truyn ti hai ch phn cp v khng phn cp. i vich khng phn cp, cc chng trnh c pht vi cng mc tn hiu dovy ch cc vng c cng trng ln hn mc ngng mi c th thu c.Cn ch phn cp th cc chng trnh c pht vi mc u tin vmc tn hiu khc nhau, do c nhng vng ch thu c cc chng trnh utin. DVB-T s dng cc k thut QPSK, 16-QAM, 64-QAM iu ch chocc sng mang, nhng ch phn cp ch thc hin c khi s dng k thutiu ch 16-QAM v 64-QAM.

Di y l s khi h thng pht truyn hnh s theo chun DVB-T:

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Hnh 4.4. S khi h thng pht sng DVB-T

u vo my pht DVB-T l dng truyn ti MPEG-2 a chng trnhc tc bit t 4,98 Mbps n 31,67 Mbps. Khi hot ng ch phn cpth s c thm b splitter chia dng truyn ti thnh hai dng u tin cao vu tin thp. Hai dng truyn ti ny c m ha khc nhau v a ti b iuch phn cp.

u tin dng MPEG-2 c ngu nhin ha thch ng vi c tnhknh truyn v ch lm vic ca my pht. Qu trnh ngu nhin ha cthc hin bng cch cng modul-2 vi chui gi ngu nhin PRBS cho bi athc sinh 1 + x14 + x15 vi chui khi u l 100 1010 1000 0000.

OFDM thng s dng chui m ha gm m ha khi, m vng xon

v ci xen. M ha ngoi s dng m ha khi Reed Solomon (204, 188) tc lkhi u vo l 188 byte v u ra l 204 byte. B m ny c kh nng sa sai 8

byte. Tip l b ci xen ngoi tng kh nng sa sai ca m khi RS (204,188) trong trng hp xy ra li chm. M ha trong s dng m vng xon vitc c bn l 1/2. Nu truyn ch phn cp th hai dng truyn ti cm ha vi tc khc nhau. Do ngoi tc c bn l 1/2 cn c cc tc m ha vng xon khc l 2/3, 3/4, 5/6 v 7/8. Tip theo l b ci xen trong

bao gm ci xen bit v ci xen symbol. ch khng phn cp dng bit saum ha ti b ci xen bit c chia thnh 2,4,6 lung tng ng vi cc phng

php iu ch QPSK, 16-QAM v 64-QAM. ch phn cp hai dng bit saukhi c m ha ring s cng c a ti b ci xen trong v mi dng chia

lm hai lung cho iu ch 16-QAM, vi iu ch 64-QAM th dng u tincao c chia lm 2 lung cn dng u tin thp c chia lm 4 lung. Cixen symbol (symbol interleaving) c thc hin sau ci xen bit. Mc ch caci xen symbol l xo trn cc symbol trc khi iu ch vo cc sng mang.

Khi iu ch OFDM bao gm iu ch (Mapper), thch ng khung(Frame adaptation ), bin i IFFT, chn khong bo v, iu ch s I/Q, v

bin i D/A.

Di y l mt s ca khi iu ch OFDM dng cho truyn hnhs DVB-T:

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Hnh 4.5. S iu ch OFDM cho DVB-T

Tiu chun DVB-T c thit k truyn dn trong mi trng chunhiu nh hng ca nhiu, pha inh ... v c kh nng chng li phn x nhiung, hiu ng Doppler cho php thit lp mng n tn, thu di ng. Trongtiu chun li c nhiu phng n khc nhau nh mode truyn (mode 2k hay8k), ch truyn (u tin hay khng u tin), phng php iu ch, khong

bo v, t s m ha nn s linh hot v hiu qu. Do DVB-T t r u thca mnh so vi cc chun truyn hnh s khc v ang c s dng nhiunc trn th gii.

2.1.3. Mng LAN khng dy

Cng vi nhng tin b vt bc ca my tnh c nhn v cc thit btruyn thng, cc ng dng chuyn mch gi khng dy cng ngy cng cquan tm dn ti s ra i ca mng ni b khng dy Wiless LAN. WilessLAN c IEEE ban hnh thnh chun 802.11 vo nm 1999 v ngy cngc hon thin vi cc version khc nhau.

c tnh ca Wiless LAN l tc cao, tnh di ng thp, tri trtruyn dn thp v thng c s dng mi trng trong nh. Do WilessLAN chn phng thc iu ch OFDM, v :

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OFDM gii quyt c kh khn ca Wiless LAN l b phn xa ng ln do cc vt th mi trng trong nh gy nn.Mc d tri tr do phn x a ng l nh nhng do truyndn tc cao, chy k symbol nh nn nh hng ca tr a

ng ti h thng Wiless LAN l rt ln. Tng i hiu qu trong vic s dng bng thng. Thch hp vi tc truyn thay i. Chng nhiu bng hp tt. Nhiu bng hp gy ra do nhiu ngi

s dng v do cc thit b khc gy ra. Hiu qu tnh ton cao: s dng bin i Fourier nhanh FFT cho

php gim phc tp xung do ch phi thc hin Nlog2Nphp tnh cho 1 symbol. Vi N l s sng mang.

ng b tt: my thu OFDM t chu nh hng ca tr thi gianhn so vi dng k thut tri ph.

Theo chun IEEE 802.11a, h thng WLAN lm vic tn s 5 GHz sdng cc s iu ch khc nhau vi tc m ha khc nhau cho phptruyn d liu vi tc t 6 Mbps n 54 Mbps. S kt hp gia cc s iu ch vi tc m ha vng xon khc nhau cho php truyn ti tc bittng ng nh sau:

B iu ch OFDM trong WLAN c tng cng 64 sng mang bao gm12 sng mang trng (zero), 48 sng mang d liu v 4 sng mang dn ng(pilot). rng bng thng ca WLAN l 20 MHz, do khong cch gia ccsng mang l 20MHz/64 = 312.5 kHz. Chu k ca mt OFDM symbol l 4 strong khong bo v GI l 0,8 s. Khong bo v y l CP (cyclic prefix).B iu ch OFDM s dng bin i IFFT 64 im (64 point IFFT).

Dng bit t phn lp MAC ca ca lp lin kt d liu c a ti lpvt l truyn dn. Dng bit c m ha vng xon vi tc ph hp t c t s li bit cho php. Dng bit m ha dng ni tip c bini 48 dng bit song song c tc thp iu ch vo 48 sng mang d liu.Qu trnh iu ch y thc cht l nh x (Mapping) tng t hp bit ca 48

lung d liu song song theo s iu ch (Constellation) to thnh 48 s

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phc v c bin i IFFT 64 im v c bin i song song sang ni tipto thnh chy k hu ch ca symbol. Khong bo v dng cyclic prefix vi tl 1/5 s c chn vo bng cch copy 16 im cui ca chu k hu ch tvo u chu k to thnh 1 OFDM symbol hon chnh c chu k l 4 s. Tip

cc symbol c iu ch vung gc, nng ln di tn 5 GHz, khuych icng sut cao tn v dn ti anten pht truyn i. Pha thu thc hin qu trnhngc li vi pha pht.

Di y l s b thu pht (Transceiver) ca WLAN:

Hnh 4.6. S khi b thu pht trong WLAN

Phn lp MAC (Medium Access Control) trong mng WLAN c thitk hot ng trong mi trng c nhiu v c kh nng di ng vi tc

thp. Do c nhng khc bit so vi cc mng LAN hu tuyn thng thng.Hiu ng u cui n (Hidden terminal effect) xy ra khi mt thit b u cuikhng pht hin ra qu trnh truyn dn ca thit b u cui khc v c gngchim dng ng truyn l mt nguyn nhn gy ra xung t. Khng gingnh trong mi trng hu tuyn cc xung t xy ra u do tht thot cc gi bxung t. Trong mi trng v tuyn, my thu c th pht hin chnh xc mtgi xung t nh cng sut pht ca cc trm khc v c tnh ca knh truyn.S pht hin sai cc gi nhn c do pha inh nhiu ng cng l mtnguyn nhn khc gy ra xung t.

Chun IEEE 802.11 cung cp hai mode hot ng cho phn lp MAC.Mode th nht da trn hm iu phi phn b DCF (Distributed coordination

function) tng t nh k thut phn pht gi ti u. Mode th hai da trnhm iu phi im PCF (Point coordination function) l phng php b phiuc iu khin tp trung c qun l bi im truy nhp AP (access point) vc dng cho cc dch v d liu nhy cm vi tr. im truy nhp c niti cc h thng phn tn c th l Ethernet, FDDI hay cc mng backbonekhc. H tr mode DCF l bt buc cho mng WLAN. Nu mng ch h trmode DCF th c gi l mng khng theo th thc (ad-hoc network). Khi tt c cc user u c c hi s dng ti nguyn mng nh nhau.

Theo chun IEEE 802.11, WLAN s dng phng thc a truy nhpsng mang cm ng vi trnh xung t CSMA/CA (Carrier sense multipleaccess with collision avoidance) tng t nh CSMA/CD ca Ethernet. Trong

CSMA/CA mt u cui ch c th nghe knh truyn trc khi truyn d liu

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m khng th nghe c trong khi truyn nh trong CSMA/CD. Nu knhtruyn ri trong mt khong thi gian ln hn khong cch gia cc frameDIFS (Distributed inter-frame spacing) th s tin hnh truyn. Ngc li thit

b u cui s tip tc nghe ng truyn ti khi knh ri hn mt DIFS. Cn

tnh nng trnh xung t Collision Avoidance yu cu thit b u cui s chthm mt khong thi gian ngu nhin sai khi knh ri mt DIFS. Trong thigian ch th trm vn tip tc nghe ng truyn. iu ny trnh c xung tkhi c nhiu hn mt trm ng thi nghe ng truyn v cng truyn khing truyn ri. Trong trng hp xy ra xung t, cc trm ny s phi thchin li vi thi gian ch thm di hn. Hin nhin l khng c s m bo vthi gian tr nh nht trong mode DCF. Vi cc ng dng nhy cm vi tr nhVoIP th s dng mode PCF. Mode PCF s dng mt iu phi vin da vo

phng thc b phiu iu phi vic truyn dn m khng xy ra xung t.

2.2. ng dng ca OFDM trong thng tin v tuyn

2.2.1. ng dy thu bao ADSL

K thut ng dy thu bao s DSL (Digital Subscriber Line) xut hint u nhng nm 1980. Thc ra y l mt h cc cng ngh thng c gil cc cng ngh xDSL, ch x th hin cho cc cng ngh DSL khc nhau nh :ADSL, HDSL, VDSL... y l cc k thut truy nhp im ti im kt nigia thu bao v tng i trung tm cho php truyn ti nhiu dng thng tin sliu m thanh, hnh nh qua i dy ng. Gii php ca xDSL l s dng ditn ln hn pha trn di tn m dch v thoi s dng, v vy bng thng truyndn cao hn. Trn , ngi ta s dng cc phng php m ho khc nhau c th truyn c tc d liu rt cao. Tc ca ng dy xDSL tuthuc thit b s dng, khong cch t tng i ti thu bao, cht lng tuyncp, k thut m ho ... Thng thng k thut ny cho php hu ht khchhng truyn t tc 128 kbit/s ti 1,5 Mbit/s. Vi k thut mi nht VDSLcho php truyn s liu vi tc ln ti 52 Mbit/s theo hng t tng ixung thu bao.

im ni bt ca k thut xDSL l tn dng c c s h tng cpng ph bin trn th gii nn n mau chng chuyn t giai on thnghim sang th trng thng mi rng ln p ng nhu cu phn phi ccdch v bng rng ti ngi s dng. Ngoi ra, khi vn u t xy dng

mng truy nhp s dng cp quang qu tn km th cng ngh ny thu ht sch ca nhiu nh sn xut thit b vin thng, cc c quan qung b phtthanh truyn hnh, cc nh khai thc dch v, cc cng ty in thoi ni ht tonn s cnh tranh lm gim chi ph thit b v gi c dch v. Mt yu t quantrng gp phn thc y s pht trin v hon thin ca cng ngh ny l s rai cc tiu chun chung cho hot ng ca xDSL do t chc vin thng quct ITU v nhiu t chc tiu chun khc a ra.

c tnh ca knh truyn :

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ADSL s dng i dy ng truyn thng ca in thoi tng t. Tnhiu truyn i trn ng dy ng chu tc ng ca mi trng tp m ca

bn thn mch vng dy ng v cc can nhiu in t khc t pha bn ngoi: Tp m trng: Nhn chung c rt nhiu ngun tp m v khi khng th

xt ring tng loi ta c th coi chng to ra mt tn hiu ngu nhin duynht vi phn b cng sut u mi tn s. Tn hiu ny c gi ltp m trng. Tp m nhit gy ra do chuyn ng ca cc electron trongng dy c th coi nh tp m trng c phn b Gaussian c gi ltp m Gausian trng cng AWGN (Additive White Gaussian Noise).Tp m ny nh hng c lp ln tng k hiu c truyn hay nicch khc chng c cng vi tn hiu bn tin.

Xuyn m: Xuyn m xy ra khi tn hiu t cc i dy k cn gy nhiuvi nhau. Thnh phn xuyn m tip tc truyn theo hng ban u gil xuyn m u xa FEXT (Far End Crosstalk). Thnh phn xuyn mtruyn ngc li ti u pht gi l xuyn m u gn NEXT (Near EndCrosstalk). NEXT c nh hng ln hn FEXT i vi truyn dn haichiu i xng v FEXT b suy hao trong sut chiu di truyn dn trongkhi NEXT ch i qua mt khong cch nh ri quay tr li u pht.Mt dng c bit ca NEXT l nhiu trong mt i dy k cn c cngdng tn hiu truyn dn, c gi l self-NEXT (t xuyn m). V idy ng thng nm trong mt b cp nhiu i vi chiu di mchvng ngn nn nh hng ca xuyn m u gn rt ln.

Hnh 4.7. Nhiu xuyn m u gn v u xa

Nhiu tn s v tuyn : Cc ng dy xon i cn bng ch c thitk truyn thoi nn ch chng c nh hng ca cc tn hiu tn sv tuyn tn s lm vic thp. Cn h thng ADSL lmvic vi tn s

cao th s cn bng b gim nn b cc tn hiu tn s v tuyn c thxm nhp. Mc nhiu ph thuc vo khong cch ngun nhiu timch vng. Nhng ngun nhiu chnh thuc loi ny l cc h thng vtuyn qung b iu bin AM v cc h thng v tuyn nghip d. Cctrm v tuyn AM pht qung b trong di tn t 560-1600 KHz. Tuynhin do tn s lm vic ca cc trm ny l c nh nn nhiu do chnggy ra c th d on c. Ngc li, nhiu v tuyn nghip d likhng on trc c v tn s lm vic thay i v c nhiu mc cngsut pht.

Tp m xung: sinh ra do giao thoa in t tc thi.

H thng ADSL :

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D liu sau khi c ng khung c a vo m ha pht hin vsa li pha thu. ADSL cng s dng chui m ha gm m khi, m vngxon v ci xen nh cp. Tip tn hiu s c kt hp vi Pilot ri

qua b bin i d liu ni tip thnh nhng dng s song song c tc thphn. Cc dng s song song ny s c iu ch QAM (QAM mapping) vthc hin thut ton IFFT iu ch DMT. Sau cc tn hiu iu ch sc bin i thnh ni tip v chn thm khong bo v CP (cyclic prefix) chng ISI, ng b khung. Cui cng tn hiu s c qua b DAC bin ithnh tn hiu tng t truyn i.

Hnh 4.8. S khi b iu ch DTM

Hnh 4.9. S khi b gii iu ch DTM

Trong ADSL tc truyn hng ln v hng xung l khc nhau do cc tham s ca cc b iu ch v gii iu ch DMT cng khc nhau:

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ADSL thc hin song cng bng phng thc trit ting vng nh trnh by phn trc. Vic thc hin song cng cng mt bng tn s timt thi im cho php s dng bng tn mt cch c hiu qu, nhng b li

phng thc ny kh phc tp v t tin.

TI LIU THAM KHO

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1. K thut iu ch a sng mang Nguyn l v ng dng ca OFDM .

Lun vn tt nghip i hc K Thut Cng Ngh TP.HCM .

2. Multi-Carrier Digital Communications Theory and Applications of

OFDM. Ahmad R.S.Bahai and Burton R.Saltzberg-Kluwer Academic /

Plenum Publishers .

3. Cng ngh OFDM v ng dng trong truyn hnh s mt t. Lun vn

tt nghip Phm Quc Hng .i hc K Thut Cng Ngh TP.HCM .

4. Basics & History of Orthogonal Frequency Division Multiplexing

(OFDM) - Richard Van Nee .

BC hoàn chỉnh . OFDM

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