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(OFDM) ang c coi l mt k thut tim nng cho phptng tc truyn cho WLAN nhm t c tc camng LAN c dy. D bo trong tng lai khng xa, vi OFDM,cc WLAN s t v thm ch c th vt xa tc ca mngLAN c dy truyn thng.

Nhng tin b v p dng k thut OFDM trong lnhvc mng WLAN c thc t ho trong cc chun cng nghip c IEEE thng qua nh IEEE 802.11a v IEEE 802.11g vi

tc truyn d liu cc i t c l 54 Mbps. Hin nay,nhm cng tc IEEE 802.11n ang tp trung nghin cu pdng MIMO-OFDM vo WLAN t c tc d liu ti500 Mbps hoc hn na.

Thy c tm quan trng ca cc vn trn, lun vnny s thc hin ti nghin cu tng quan v ng dng

ca k thut OFDM trong WLAN. Lun vn gm bn chng,c cu trc nh sau:Chng 1: Tng quan v mng WLAN

Chng ny t vn ti sao cn c WLAN; cc kthut ang c p dng t c tc truyn trong ccWLAN hin nay; cc yu cu i vi mt WLAN .

Chng 2: Tng quan v OFDMChng ny gii thiu k thut OFDM; ng dng ca

OFDM trong cc chun WLAN hin c; v nhng xu hngnghin cu pht trin ng dng OFDM trong WLANChng 3: Nhng nghin cu ng dng OFDM trong

chun WLAN mi: chun IEEE 802.11n

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Chng ny tip tc a ra nhng gii php k thut ci thin tc truyn d liu trong WLAN; gii thiu cngngh MIMO OFDMChng 4: Nhng nghin cu tip theo v OFDM trong

WLANChng ny tp trung gii thiu cc nghin cu v k

thut thch ng OFDM a ngi dng c d bo s l cngngh tng lai.

Cui cng l kt lun v ti liu tham kho. Trong khun kh thi gian cng nh trnh bn thn

c hn, lun vn s khng trnh khi nhng thiu st. V vy,ti knh mong c s ch bo tn tnh ca cc thy nidung lun vn c hon thin hn.

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Chng 1Tng quan v mng cc b khng dy (WLAN)

1.1 Gii thiu khi qut v mng WLAN1.1.1 Mng cc b khng dy ( Wireless Local Area

Network-WLAN) l g?WLAN l mt h thng cc thit b hp thnh mt nhm

trong phm vi nh c th trao i thng tin vi nhau bngsng v tuyn in thay v dng dy dn. WLAN c mi khnng nh cc mng LAN c dy truyn thng nhng khng bhn ch v cp, dy dn. Hin nay, WLAN ang ngy cngpht trin, c th thay th dn cc mng my tnh hutuyn ti cc khu vc cc b. Vi WLAN, c s h tng mng(cp ni) khng cn thit na m thay vo l mi trng

truyn v tuyn. Do vy, c s h tng c th d dng thayi c khi cn theo yu cu cng vic.

WLAN ch ging mng LAN truyn thng ch chngu ph thuc vo mt mi trng vt l truyn tn hiu iqua. Thay v phi dng mt i dy xon hoc cp quangnh trong LAN, WLAN s dng tia hng ngoi (IR) hoc cc tn

s v tuyn (RF), trong vic s dng RF ph bin hnbi v di thng rng hn v vng ph sng rng hn. Trongthc t, hin nay cc WLAN s dng di tn 2,4 GHz v 5GHz. Phn chia nh vy ca ph tn s v tuyn c dnhring cho cc thit b khng c ng k tn s trn ton thgii. WLAN to nn mt s m rng t do v tnh mm do cho

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thit k trong phm vi kin trc ca mt to nh hoc mtkhu vc cc b no .1.1.2 Cu trc c bn ca mt mng WLAN

Trc ht phi hiu r rng: cc h thng khng dykhng c ngha l hon ton khng s dng dy dn. Ccthit b khng dy ch l mt phn ca mt mng LAN cdy truyn thng, cc h thng khng dy c thit k vxy dng sao cho c th kt ni c vi cc h thng LAN c

dy truyn thng. Mt khc, cc thit b khng dy phic vn hnh vi kh nng m ha v gii m; nn v giinn; tri ph v gii tri ph; pht v thu tn hiu khngdy nh a s cc thit b thng tin v tuyn khc.

Di y l s ca mt WLAN c xy dng nh mtphn ca mt mng LAN c dy:

Cu hnh mt WLAN in hnh bao gm c mt b thupht hay im truy cp (Access Point- AP) ni vo mng cdy t mt im c nh s dng cp mng tiu chun. APnhn d liu, lu tr vo b m, pht d liu gia ccthit b u cui trong mng. Cc thit b u cui ny c

th l my tnh xch tay, my in, cc thit b cm tay, haycc thit b khng dy khc v mng hu tuyn. Mt APn l c th h tr mt nhm nh ngi s dng trong phmvi khong 30 mt. Cc thit b khng dy ca ngi s dngtrao i d liu vi mng nh cc card mng khng dy(wireless adapter) ca my tnh c nhn hay cc thit b t-

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Hnh 1.1 : S mt mng WLAN

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ng t ca in thoi i ng, my h tr c nhn s(Personal Digital Assistant-PDA).

Cc thit b WLAN u tin c tc truyn d liuthp, thiu cc chun quc t nn khng c s dng phbin. Sau ny, cc h thng hin i c chun ho quct nh hin nay mi c th truyn d liu vi tc cao, cchp nhn tn ton th gii v c tnh tng thch gia ccsn phm ca cc nh sn xut khc nhau.

Trong nhng nm gn y, cc t chc quc t v phttrin cng ngh mng khng dy nh IEEE 802.11 v Linminh Wi-Fi cng nhau tin hnh chun ho cc chun vmng khng dy. Cng ngh khng dy hin nay h tr chocc tc d liu khc nhau v c cc giao thc cn thit c th cng hot ng vi mng WLAN truyn thng. Hn

na, gi thnh cc thit b khng dy mi gim nhiu sovi trc kia. WLAN ang l mt s la chn rt c trin vngv mng cc b v vic a nhng h thng ny vo sdng khng cn s cho php c bit no ca cc c quanqun l tn s.

1.1.3 Cu hnh mng cc b khng dy WLANMi mng khng dy cn c mt b thu pht v tuyn

v an ten. Cc thnh phn cu thnh nn mng khng dygm: cc trm (Station - STA) v cc im truy cp (AP).

Trong , mt trm dch v c s (Basic Service Set - BSS)c to nn bi hai hay nhiu STA lin lc c vi nhau v thit

lp thnh mt mng. Mt trm dch v m rng (Extention

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Service Set - ESS) c to nn bi nhiu BSS c kt ni vinhau (mi BSS c mt AP). Mt mng cc b khng dy hotng theo mt hoc hai ch : ch mng ngang hng(Ad-hoc Mode) v ch c h tng c s (InfrastructureMode).

Ch Ad-hoc : mi my tnh trong mng giao tip trctip vi nhau thng qua cc card mng khng dy m khngdng n cc thit b nh tuyn hay cc im truy cp

khng dy.Ch Infrastructure : cc my tnh trong h thng

mng s dng mt hoc nhiu cc thit b nh tuyn (haythit b thu pht), cn c gi l cc thit b im truy cp(AP), thc hin cc hot ng trao i d liu vi nhau.

Trong ch Ad-hoc, mi STA thng tin trc tip vi cc

STA khc bn trong mng. Ch ny ch hot ng c khicc STA trong phm vi truyn sng ca nhau v c ththng tin c vi nhau. Nu mt trm trong mng Ad-hocmun thng tin vi mng bn ngoi phm vi ph sng camng th cn c mt trong cc trm ng vai tr hot ngnh mt cng (gateway) hay b nh tuyn (router). Trong

ch Infrastructure, mi trm gi thng tin ca mnh timt im trung tm (AP). Khi AP hot ng vi vai trnh l mt cu ni v tip tc gi thng tin vo mng (khngdy hoc c dy).

Trc khi cc STA v cc AP c th trao i thng tin vinhau, chng phi thit lp mt mi quan h hay mt lin

kt. Qa trnh lin kt gm ba trng thi: Cha nhn thc v

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cha lin kt; nhn thc v cha lin kt; nhn thc v lin kt.

chuyn i gia cc trng thi, cc bn trao i vinhau cc thng ip c gi l cc khung bo hiu . Cc APc thit k pht nh k vo khng gian khung bohiu. lin kt vi mt AP v tham gia vo BSS, mt STAno phi nghe cc tn hiu bo hiu nhn dng ccAP trong phm vi ph sng ca mnh. Sau khi STA nhn c

mt khung bo hiu, STA chn BSS gia nhp. Tn camng (network name) hay cn gi l s nhn dng b dch v(Service Set Indentifier - SSID) c cha trong khung bohiu. Mt trm cng c th gi mt khung yu cu thm d tm AP lin kt c vi SSID mong mun. Sau khi STAnhn dng c AP, STA v AP thc hin mt qu trnh nhn

thc bng cch trao i mt s khung qun l.Hin nay, c hai c cu nhn thc: nhn thc kho m

(Open Key Authentication) v nhn thc kho chia s (SharedKey Authentication). Mc d nhn thc kho chia s l an tonhn nhng phn ln cc mng hin nay li s dng c chnhn thc kho m l c ch nhn thc n gin v d thc

hin nht - chnh l mt l do dn n nguy c mt anton cho mng.

Nu vic nhn thc tin trin tt, trm chuyn ti giaion th hai - nhn thc m cha lin kt d liu c. Khi trm s gi mt khung yu cu lin kt v AP p li bngmt khung chp nhn lin kt. Nu hai giai on ny hon

thnh, trm s tip tc giai on th ba - nhn thc v

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lin kt c. Khi , trm s tr thnh mt nt mng v cth truyn pht cc khung d liu vi cc nt mng khccng nh vi cc mng khc chng hn nh Internet.

1.1.4 Mi trng mng khng dy 1.1.4.1 Mi trng vt l

Trong lnh vc mng, lun cn mt nn tng cho cmng. Nn tng - cho c mng LAN c dy ln WLAN, cp

n lp th nht (lp vt l) ca m hnh tham chiu lin ktcc h thng m (OSI). Lp vt l l lp xc nh cc tham sk thut v chc nng, th tc, c hc v in, duy tr vhu b lin kt vt l gia cc h thng.

Hin nay, cc mi trng mng thng c s dng lp vtl bao gm:

- Cp xon c v khng c v bc chng nhiu- Cp ng trc- Cp quang- Sng v tuyn

Trong sng v tuyn in l mi trng c s dngbi cc k thut v tuyn trong c WLAN.

1.1.4.2 Cc dng sng v tuyn

Khi qut chungCc sng v tuyn in truyn d liu khng nhn

thy c qua khng trung, thng trn mt khong cch xa vitriu kilmt. Tt c cc thit b khng dy u s dng

sng v tuyn in truyn thng tin.

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Mt s dng sng v tuyn chnh nh: Sng v tuyn(Radio waves); sng viba (Micro waves); nh sng hng ngoi:s dng mt tia sng hi t trong di tn s hng ngoi. Rtthng dng vi cc thit b iu khin t xa, nhng cng cdng truyn t thit b n thit b, chng hn giaPDA v my tnh; nh sng nhn thy c (Visible light); nhsng cc tm (Ultra voilet light); Tia X (X rays); Tia Gamma.Cc dng sng ny lan truyn vi tc nh sng trong chn

khng vi mt bc sng v tn s f c trng c tnh bng:c f = vi c 3.10 8 m/s bng vi tc nh sng.

S phn chia tn s v tuynPh sng in t c chia thnh 8 vng truyn

sng v tuyn, mi vng u c s qun l ca nh nc. Ccvng ny c xp t vng tn s thp (VLF) n vng tn s

c bit cao (EHF).

10

V tuyn viba v v tinh

3 KHz 300GHz

Truyn thng vtuyn

VLF LF MF HF VHF UHF SHF EHF3KHz 30 KHz 300 KHz

3MHz

30MHz

3GHz

30GHz

300MHz 300 GHz

Bmt i l u in ly

B mtv

truynthng

Khongkhng

Hnh 1.2: S phn chia tn s v tuyn

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VLF: tn s rt thp VHF: tn s rt caoLF: tn s thp UHF: tn s cc caoMF: tn s trung SHF: tn s siu caoHF: tn s cao EHF: tn s cbit cao

C 5 kiu truyn v tuyn khc nhau l b mt,i lu, in ly, truyn thng, khong khng (v tr). Kiutruyn thng c dng trong v tuyn ph thuc vo tn s

ca tn hiu. Mi tn s truyn qua mt lp xc nh cakh quyn v c truyn v nhn mt cch hiu qu nhtbng k thut thch hp ca lp ny.

Sng VLF truyn nh sng b mt qua khng kh v nc.Sng ny c dng nhiu nht cho hng hi v truyn thngngm di bin. Sng LF: ging VLF, n cng truyn theo b

mt. LF c dng cho v tuyn tm xa ca hng hi, vtuyn dn ng hoc b nh v hng hi. Sng MF: ccsng tn s trung bnh truyn trong tng di lu, n b hpth bi tng in ly. MF c dng trong sng v tuyn AM,v tuyn hng hi, v tuyn cc tn s bo ng Sng HF :cc tn hiu tn s cao truyn thng tng in ly. dng

trong truyn thng quc t, truyn thng qun s, inthoi, in bo Sng VHF: truyn theo ng thng vtuyn VHF, dng trong v tuyn hng khng AM, cu h hngkhng hng hi v.v Sng UHF: cc sng tn s siu cao sdng truyn thng nhn thng v tuyn UHF, dng trongin thoi di ng, v tuyn t bo, truyn viba v.vSng

SHF: sng c bit cao truyn theo kiu nhn thng v

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truyn v tr, dng trong viba mt t, viba v tinh vtruyn thng raa. Sng EHF: sng cc k cao s dngtruyn v tr, ng dng chnh l cho mc ch khoa hc nhtruyn thng raa v tinh, th nghim.

Ph tn s v tuyn (RF) chng ta quan tm bao gmmt s cc bng tn nh vi sng, tn s UHF v cc tn sVHF. Cc tn s v tuyn c s dng rt nhiu trong thngtin, mt s di phi c cp php, mt s t th khng cn

c cp php. Ph tn khng cp php ny li rt c achung trong cc ng dng nh cc mng WLAN. N l cc bngtn trong cng nghip, khoa hc, y t (hnh 1.3) [19].

1.2 Xu hng pht trin ca mng WLANK thut WLAN ban u c nh ngha bng chun

gc IEEE 802.11 cho tc truyn d liu thp, khong 1Mbps n 2 Mbps. Mc d chm so vi LAN hu tuyn (ctc truyn t nht l 10 Mbps), nhng s t do v tnhmm do cho php cc mng ny d dng tm c mtv tr ngy cng vng chc trong cng ngh thng tin. ngdng u tin ca WLAN l trong qun l kho tng. ,

nhng ngi lm vic di ng thng s dng cc thit b cmtay qun l thng k v tp hp cc d liu bn hnghoc kho bi. Sau , cc bnh vin ng dng WLAN thu thp v phn pht thng tin ca cc bnh nhn. Tiptheo, cc trng hc bt u c lp t mng khngdy gim cc chi ph v cp, trong khi vn m bo

chia s kt ni Internet. Nhn thy WLAN cn phi c chun

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Hnh 1.3: Cc bng tn khng cp php trong cngnghip, khoa hc, y t

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ging nh chun mng c dy (chng hn nh chun Ethernet).Nm 1991, cc nh sn xut thit b khng dy bt taynhau thnh lp Lin minh tng thch Ethernet khngdy gi tt l WECA. WECA xut v xy dngchun cho WLAN da trn s ng gp cc cng ngh mi.WECA sau c i tn thnh Lin minh Wi-Fi. Thng6/1997, IEEE thng qua chun gc 802.11 cho mng cc bkhng dy. T , khi nim mng cc b khng dy

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(WLAN) mi chnh thc xut hin. Hnh 1.3 minh ha qutrnh pht trin ca WLAN.

Cc cng ngh WLAN hin nay h tr tc d liungy cng cao, tin cy ngy cng tt hn v chi phngy cng gim. Tc d liu tng t 1Mbps ban uln 54 Mbps hin nay.

Trong tng lai, s c nhiu ci tin c a ra ci

thin cht lng WLAN. V d, cc nhc im v bo mt s -

14

TcMng

Tn svtuyn

860Kbps

900 MHz 2.4 GHz

c quyn

1 to 2Mbps

11Mbps

54Mbps

5 GHzIEEE802.11bt u c xut

802.11 cphchun

802.11a,b c

phchun

802.11g c

xut1990 1992 1994 1996 1998 2000 2002 20032004 2006

Hnh 1.4: S pht trin caWLAN

802.11g c

phchun

T 100 Mbps

Da vo chun

802.11nd kin

phchun

802.11n cxut

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c khc phc trong cc cng ngh tng lai. Tc truynd liu s tng ti 500 Mbps hoc thm ch hn na chophp d dng truyn ti qua WLAN nhng d liu a ph-ng tin nh hnh nh ng, m thanh cht lng cao v.v...1.3 Cc k thut ang c p dng t c tc truyn d liu trong cc mng WLAN hin nay.

Phn ny s trnh by tm tt v nhng k thut cbn c s dng t c tc truyn d liu hin nay

ca cc mng WLAN. Cc k thut ny bao gm: tri phnhy tn (FHSS), tri ph chui trc tip (DSSS), ghp knhphn chia theo tn s trc giao (OFDM).

1.3.1 Tri ph nhy tn (FHSS) Tri ph nhy tn l mt trong hai k thut c bn s

dng trong WLAN theo chun gc IEEE 802.11 t ctc d liu t 1 Mbps n 2 Mbps.

Tri ph nhy tn l mt k thut tri ph s dng linhng tn s tri d liu ra khong trn 83 MHz ca phtn. S linh ng tn s chnh l mt sng to trong thngtin v tuyn thay i tn s mt cch nhanh chng

trong di tn v tuyn cho php. Cn c trn tiu chun thit lp, tri ph nhy tn trong mng WLAN s dng 83MHz xung quanh di 2,4 GHz ISM.

15

To mnhytn

T hptn s

To mnhytn

T hptn s

Chs

Lctr c

Tchdliu

Dliu

Dliu

Hnh1.5: S khi h thngFHSS

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trong trng PSF thuc PLCP header c s dng xc nhtc d liu ca PSDU [13].

Hnh 1.6 FHSS PHY PLCP preamble, Header, v PSDU

GFSK l mt k thut iu ch c s dng bi FHSSPMD, lm lch tn s sang mt trong hai pha ca bc nhysng mang ph thuc vo vic k hiu nh phn t PSDU cphi l 1 hay 0. Chu k bit ca bng thng thng s dng l0.5. Nhng thay i trong tn s biu din nhng k hiuc cha thng tin trong PSDU. i vi GFSK 2 mc, th bit 1 t-ng ng lm lch tn s ln pha trn t bc nhy sng mangv bit 0 tng ng lm lch tn s xung pha di t bc nhysng mang. Tn s lm lch (fd) cn ln hn 110 KHz i vimi v tuyn trong IEEE 802.11. lch tn s sng mangc tnh bng:

S nh phn 1 = Fc + fdS nh phn 0 = Fc fd

GFSK 4 mc cng tng t nh GFSK 2 mc v c sdng t tc d liu l 2 Mbps trong cng bng thngb chim. B iu ch kt hp 2 bit nh phn t PSDU

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c lm trng v m ho chng thnh cc cp k hiu(10,11, 01, 00). Cc cp k hiu ny s to ra 4 lch tns t tn s bc nhy sng mang, hai trn v hai di. Cc cpk hiu c truyn vi tc l 1 Mbps v i vi mi bit cgi i, cho kt qu tc d liu l 2 Mbps. Nhy knh FHSS

Mt tp cc chui nhy c nh ngha trong IEEE802.11 s dng trong di tn 2,4 GHz. Cc knh c tcch u nhau qua mi bng tn trn khu 83,5 MHz. Ccbc nhy knh c s khc bit t nc ny sang nc khc.

Bc nhy knh do FHSS PMD iu khin. FHSS PMDtruyn ti PSDU c lm trng bng cch nhy t knhny qua knh khc theo kiu gi ngu nhin s dng mttrong cc chui nhy.

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1.3.2 Tri ph chui trc tip (DSSS) Tri ph chui trc tip c s dng trong chun gc

IEEE 802.11 v tip tc c s dng trong cc chun m rng802.11b v 802.11g t c tc d liu ti a 11Mbps.

DSSS s dng di tn 2,4 GHz lm mi trng truyn dnv tuyn RF. Vic truyn d liu qua mi trng truyn dnc iu khin bi lp con DSSS PMD di s hng dn ca lp

con DSSS PLCP.DSSS PMD tip nhn cc bt thng tin nh phn t n

v d liu giao thc PLCP (PPDU) ri bin i chng thnhnhng tn hiu RF cho mi trng truyn khng dy thngqua cc k thut iu ch sng mang v tri ph DSSS.

DSSS nh ngha mt knh l mt di cc tn s lintip c rng ca bng l 22 MHz, mi knh s ly mttrong 11 tn s c xc nh trc lm trung tm v trira 11 MHz trn hai hng (trn v di tn s trung tm). V d,knh 1 trong chun IEEE 802.11b hot ng trong di t2,401 GHz n 2,423 GHz. Knh ny s dng tn s trung

tm 2,412 GHz, cng hoc tr i 11 MHz. Knh 2 s dngtn s 2,417 GHz, cng hoc tr 11 MHz. Tng t, iu nyc p dng cho cc knh khc [4].

19

To mtri ph

To m tri

ph cc b

Dliu

Blctr c

Tchdliu

Dliu

Sngmang

Sng mang ccb

Hnh 1.7: S khi ca h thng DSSStrong 802.11b

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iu ch DSSSDSSS PMD truyn chui bit u khi tin PLCP

preamble v PLCP header vi tc 1 Mbps s dng iuch kho dch pha nh phn vi sai (DBPSK). Tu thuc voni dung trng tn hiu ca PLCP header, n v d liu

giao thc MPDU c gi i hoc vi tc 1 Mbps s dng

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Hnh 1.8: Hnh dng knhtruyn

Khong cch knh tithiu gia cc tn strung tm

Hnh 1.9: Khong cch knh ti thiu cho mng DSSS

Bc M

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iu ch DBPSK, hoc vi tc 2 Mbps s dng iu chkho dch pha cu phng vi sai (DQPSK).

DQPSK c kh nng d tr sa sai tt hn vi xuynnhiu gia cc k hiu (ISI) gy nn bi tp m v a ngtrong mi trng truyn n, do DBPSK c s dng iu ch PLCP preamble. Phng php tri ph Barker

Lp DSSS PHY l mt trong hai lp PHY RF 2,4 GHz c lachn t chun gc IEEE 802.11. Ti my pht:

T Barker (11-bit) +1, -1,+1,+1, -1, +1, +1, +1, -1, -1,-1

Ti my pht, t Barker 11 bit c cng m dun 2 (chcnng php ton Ex-Or) vi mi bit thng tin ca PPDU. PPDUc to nhp tc thng tin, chng hn vi tc 1

Mbps, v t Barker tc 11 Mbps (to khi chip). Kt quu ra ca b cng m dun 2 l mt tn hiu vi tc dliu cao hn 10 ln so vi tc thng tin. Ti my thu, tnhiu DSSS c xon (tch chp) vi t Barker 11 bit v ly t-ng quan vi nhau. Qu trnh tng quan s khi phc cc bitthng tin PPDU theo tc thng tin c truyn v cc tn

hiu gy nhiu khng mong mun trong bng s b tri rngra khi bng. Vic tri ph v gii tri ph mt tn hiubng hp thnh mt tn hiu bng rng thng dn n tng

ch x l c o bng n v xi ben (dB). Tng ch xl l t s gia tc tn hiu DSSS vi tc thng tinPPDU. FCC v MKK qui nh tng ch x l ti thiu ti Bc

M v Nht Bn l 10 dB [13].

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Cc knh hot ng v cc yu cu v cng sut pht ca DSSS.

Mi knh PHY DSSS chim 22 MHz rng ca bngthng, v hnh dng ph knh biu din hm Sin X/X c lc.Mt n pht knh DS trong IEEE 802.11 qui nh rng ccsn phm ph c lc n 30 dBr t tn s trung tm vmi sn phm khc c lc n 50 dBr. iu ny cho phpt 3 knh khng nhiu cch nhau 25 MHz trong bng tn2.4 GHz.

Ngoi vic phn nh tn s v bng thng, cng sutpht l mt tham s chnh c qui nh trn ton th gii.Bc x v tuyn cho php ti a ca PHY DSSS thay i tkhu vc ny sang khu vc khc. Ngy nay trn th trng ccsn phm khng dy chn 100 mW lm mc cng sut

pht RF danh nh. c ci nhn tng qut hn v hai phng php tri

ph c bn trong chun gc IEEE 802.11, ta hy so snhchng:

DSSS FHSS Thng lng cao

hn t b nh hng ca nhiu hn

C li xa hn Chng ting diC th nng cpln cc tc cao tn s 2,4 GHz

Chi ph r hnD dng lp t

Nhiu phng n la chn sn phm,

nhiu nh cung cp hn

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Bng 1.1: So snh gia DSSS v FHSS

1.3.3 Ghp knh phn chia theo tn s trc giao(OFDM)

Ghp knh phn chia theo tn s trc giao (OFDM) cs dng trong cc chun m rng IEEE 802.11a v IEEE802.11g. Ngoi ra, k thut ny cn dng cho chun mng

khng dy ca chu u (HyperLAN2). K thut ny chophp t tc d liu 54 Mbps.

OFDM chia mt sng mang d liu tc cao thnh mts sng mang con (knh con) c tc thp hn c rng20 MHz. C 52 knh con, mi knh con rng khong 300 KHz,OFDM s dng 48 trong s 52 knh con cho truyn d liu v

4 knh cn li cho sa li. OFDM s dng ph hiu qu hnnh khong cch gia cc knh rt kht nhau. Mt khc, viOFDM hiu qu s dng ph cao hn rt nhiu v tt ccc sng mang u trc giao vi nhau [1]. Nhng nghin cusu hn v phng php ny s c trnh by chi tit trongchng 2.

1.4 Nhng hn ch ca mng WLAN v nhng yu cut ra

1.4.1 Nhng hn ch hin nay ca WLANHin nay, WLAN c hai hn ch c bn l tc d

liu thp hn so vi mng LAN hu tuyn v tnh bo mt

cn cha cao.

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V tc d liu, hot ng ca WLAN ph thuc voloi thit b, cu hnh mng, s ngi s dng, khong cchgia cc thnh phn ca mng, chun WLAN ang s dngv nh hng ca cc thnh phn kt ni c dy ca mng.Cc LAN hu tuyn truyn thng hot ng tc t 10Mbps ti 100 Mbps lp truy nhp, cn cc WLAN hot ng tc t 11 Mbps n 54 Mbps. Cc WLAN theo chun802.11a v 802.11g c tc t ti 54 Mbps, gp khong

nm ln so vi chun 802.11b c tc 11 Mbps. Do c cctrng khng phi d liu thc s, chng hn nh trng mou, trng ng b, trng dch v... (gi chung loverhead ), nn thng lng thc ca WLAN thp hn nhiu sovi tc d liu l thuyt. Chng hn, thng lng thc caWLAN 802.11a ch t 34 Mbps. C li lin lc cng nh hng

ln n tc d liu. C li lin lc cng xa th tc dliu ca WLAN cng gim. iu ny rt t xy ra i vi mngLAN c dy. Nh vy, r rng l WLAN vn c tc thphn cc LAN c truyn thng.

V bo mt, tnh qung b ca phng thc truyn bngsng v tuyn lm cho vic bo mt ca WLAN thc hin kh

khn hn nhiu so vi mng LAN hu tuyn. Cc nguy c bxm nhp bt hp php v b chn bt d liu trong khitruyn vi WLAN ln hn nhiu so vi mng LAN c dy.1.4.2 Cc yu cu t ra

Tnh tin cyCc h thng khng dy mi phi c tin cy nh cc

h thng c dy hin c. Truyn thng qua mng LAN phi

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l mt lin kt khng c li (xc xut li 10 9). Khi mt li - c pht hin trong mt gi d liu, thng tin s khng chnhxc. Thng thng, gi d liu s c yu cu gi li. Vimng LAN hu tuyn, cc h thng cp ng trc hoc cpquang c t s tn/ tp ln trnh sai lch thng tin.Khi xy dng cc WLAN vic c gng gi t l li mc t-ng ng nh trong cc h thng LAN hu tuyn l mt cngvic kh khn hn nhiu. Tnh trong sut:

Cc h thng WLAN s khng thay th cc LAN c dytrong cc mi trng trong nh. Chng s chia s mi trngtruyn vi nhau, v vy m cc phn mm mng phi lmvic c vi c hai kiu mng. Nu p dng m hnh 7 lp(OSI) th WLAN v LAN ch khc nhau lp th nht v lpth hai (lp vt l v lp lin kt). Gi s rng chng ta cnthay i mt h thng LAN c dy thnh mt WLAN m vnmun gi li cc card mng v ch g b cp i. Cc giao dinca thit b phi c s dng bin i cc tn hiuin trong card truyn thng thnh tn hiu khng dy(hoc RF). Cc giao din ny phi lm vic hon ton trong

sut i vi ngi s dng.i vi mi trng bn ngoi, ni m cc chun v cc sn

phm ring bit va mi c pht trin (tc d liu thphn cc h thng LAN hin nay) th tnh trong sut phi lyu cu bt buc). Tnh bo mt:

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Tnh bo mt ca mng LAN hu tuyn ch b vi phmkhi cp b mc trm. Thng thng, trong mng LAN hutuyn t khi cn m ho d liu truyn. Vi WLAN, ta hu nhkhng th kim sot c vic truyn sng v tuyn ra xungquanh do tnh cht qung b ca sng. Do vy, m ho dliu l yu cu bt buc i vi cc WLAN. t nh hng tihiu sut tryn d liu, m ho d liu phi c thc hinbng phn cng hoc bng cc bin php truyn tng t (v

d nh s dng k thut phi ph). Ngoi ra, cn c c chnhn thc v xc nhn cc thit b u cui trong mng.Nu khng s dng cc bin php bo mt th WLAN s cnguy c cao vi cc i tng truy cp bt hp php, dn nhiu qu nh nghn mng, virus, d liu b nghe, xem trmv.v.. Cc hnh ng ny khng th gim st c cc lp

thp nh lp vt l v lp lin kt trong m hnh 7 lp OSI mch c th gim st c cc lp cao hn nh lp vn chuyn vcc lp trn na. Tnh di ng:

C hai biu hin khc nhau ca tnh di ng. Biu hinth nht l di ng hon ton: mng c kh nng gi v nhn

thng tin trong khi thit b di ng ang di chuyn trongmt khu vc c ph kn bi WLAN. Biu hin ny kh tc bi v cc mi trng khc nhau c s nh hng khcnhau v bi v khong cch gia cc nt mng l khc nhau.V d v thit b di ng hon ton l thit b u cuicm tay cho nhn vin an ninh. Biu hin th hai, tnh di

ng va phi: mng to ra kh nng kt ni bng cc thit

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b u cui trong phm vi ca khu vc c bao ph caWLAN. V d, mt ngi c th dng mt my tnh xch taytrong phng hp trao i d liu qua mt WLAN.

Tnh di ng l mt u im ng k vi mt h thngthng tin ln v khng ch c truyn gi d liu mcn c c truyn thoi, fax, nhn tin Trong hon cnh nyc th khng cn gim st thng tin ca cc thit b ucui. Khi cng sut pht l mt rng buc ng k th cc vi

t bo (ng knh 200 mt hoc nh hn) l mt gii phphp dn cho cc khu vc c mt ngi s dng cao.

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Linh hot trong thit kV d, khi mun thay i v tr ca ngi s dng trong

mt vn phng hay trong nh ga, sn bayth ch cn thayi v tr ca im truy cp m khng cn phi lp tli ton b h thng dy cp mng. Gi thnh hp l

Quy trnh lp t v gi thnh chi ph lu di ca ccthit b thp hn so vi mng c dy c bit l trong mitrng hay c di chuyn hoc sa i.

Trong chng ny lun vn gii thiu tng quan vmt h thng mng cc b khng dy vi cc u ngc im vphm vi ng dng ca n. Qua , gip cho ta thy c tmquan trng ca vic p dng WLAN trong cc mng gia nh,cng ty, trng hc Chng tip theo s trnh by v kthut iu ch a sng mang OFDM, mt cng ngh thencht trong WLAN.

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Chng IITng quan v ofdm

2.1 Gii thiu khi qut v OFDM2.1.1 Lch s pht trin

Hin nay, truyn dn thng tin s dng k thut iuch a sng mang (MultiCarrier Modulation - MCM) ang -

c quan tm rt nhiu trn th gii. Chng ny s im qua

lch s pht trin v ng dng ca k thut iu ch ghpknh phn chia tn s trc giao - OFDM. y l mt dngiu ch a sng mang ph bin nht s dng trong WLANhin nay v cng s l mt k thut cn bn cho tng lai camng WLAN.

Cng nh k thut tri ph, cc h thng OFDM c

a vo s dng trong qun s vo cui nhng nm 1950.Mt s h thng OFDM nh KINEPLEX, ANDEFT v KATHRYN sdng AN/GSC 10 modem d liu tc thay i cho thngtin v tuyn tn s cao [15]. Cc tn s trc giao c s dngvi khong cch knh l 82 Hz cung cp khong thi gianphng v gia cc tn hiu k tip nhau [6].

Nm1966, ln u tin k thut OFDM c cng b rngri vi bi bo ca Robert W.Chang. ng a ra phng phpl thuyt truyn ng thi lung d liu tuyn tnh cdi tn knh truyn b hn ch m khng c xuyn nhiugia cc symbol (ISI) v xuyn nhiu gia cc sng mang(ICI). Bn quyn pht minh u tin v OFDM da trn bi

bo ny c ng k vo nm 1970 [11].

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Mt bc t ph quan trng trong lch s OFDM l vonm 1971 khi Weinstein v Ebert s dng bin i Fourierri rc thc hin iu ch v gii iu ch bng gc.Vic s dng bin i Fourier ri rc loi b i s i hi vdi tn cho cc b dao ng sng mang con, v vy m victhc hin tr nn d dng hn.

Mt s kin quan trng khc trong lch s OFDM l khiPeled v Ruiz a ra tin t lp (Cyclic Prefix - CP) hoc

Cyclic Extension (khong m rng tun hon) vo nm 1980[14]. iu ny gii quyt c vn duy tr c tnhtrc giao ca tn hiu c truyn i trong cc iu kintruyn dn kht khe. Vo nhng nm 1990, OFDM ckhai thc s dng cho thng tin d liu bng thng rng trncc knh v tuyn FM di ng, cho cc ng thu bao s tc

bit cao, cho cc ng thu bao s khng i xng (ADSL- tc ln ti 6 Mbps), v cho cc ng thu bao s tc rt cao (VDSL). Qung b Audio s (DAB) l mt thng phmu tin s dng cng ngh OFDM. DAB c pht trin vonm 1987, nm 1992 c xut v nm 1994 cc chunca n c a ra. Sau l s xut hin ca qung b video

s (DVB). Qung b Video s pht trin vo nm 1993. Thng 8/2000, Philips Semiconductors ch to thnh

cng chip u tin v OFDM bng rng (WOFDM), cng nghh tr truyn dn v tuyn tc cao [18]. y l chip utin c tc thng tin v tuyn hai chiu ln hn 2 Mbps vitc thit k t 45 Mbps theo chun 802.11a.

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Cu hi t ra i vi s pht trin ca OFDM l ti saovic pht trin sn phm mi t mc khim tn trong khiu im ca h thng s dng k thut ny rt r rng.Nguyn nhn c a ra l t c nhu cu OFDM trong thngtin c nh do cc h thng hin ti vn ang hot ng ttv hiu qu. Mt nguyn nhn khc l do h thng OFDM sto ra cc tn hiu c cng sut tng i cc i ln, lmgim hiu sut ca b khuych i cng sut.

Nh vy, c th ni iu ch a sng mang l mt kthut ang c quan tm nghin cu rng ri trn th gii.V vy, nm bt c nguyn l, cc c tnh c bn cngnh kh nng p dng ca OFDM l mt yu cu cn thit.Phn ny tp trung gii thiu OFDM trong mng cc b vtuyn WLAN.

2.1.2. Nguyn l c bn ca OFDMGhp knh theo tn s trc giao (Orthogonal Frequency

Division Multiplexing - OFDM) s dng nguyn l ghp knhtheo tn s (Frequency Division Multiplexing - FDM) truynthng cho php nhiu tin tc c gi qua mt knh vtuyn. Tuy nhin, OFDM cho hiu qu ph tt hn [7].

OFDM khc vi FDM nhiu im. Trong pht thanhthng thng, mi i pht thanh truyn trn mt tn s khcnhau, s dng hiu qu FDM duy tr s ngn cch giacc i. Tuy nhin, khng c s kt hp ng b gia cctrm vi nhau. Vi cch truyn OFDM trong DAB hoc DVB,nhng tn hiu thng tin t nhiu trm c kt hp trong mt

dng d liu ghp knh n. Mi sng mang con trong tn

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hiu OFDM c ng b thi gian v tn s vi nhau. Ccsng mang ny chng lp nhau trong min tn s, nhngkhng gy can nhiu gia cc sng mang (Inter CarrierInterference - ICI) do bn cht trc giao ca iu ch. ViFDM, cc tn hiu truyn cn c khong bo v tn s lngia cc knh ngn nga can nhiu. iu ny lm gimhiu qu ph. Tuy nhin, vi OFDM vic thc hin trc giaocc sng mang lm gim ng k khong bo v ci thin

hiu qu ph.OFDM cn c tn gi khc l iu ch a sng mang

trc giao - OMCM - da trn nguyn tc phn chia lung dliu tc cao thnh cc lung d liu tc thp, truyntrn nhiu sng mang trc giao nhau. Cng ngh OFDM trid liu cn truyn trn rt nhiu sng mang, mi sng mang

c iu ch ring bit vi tc bit thp. Trong cng nghFDM truyn thng, cc sng mang c lc ra ring bit bo m rng khng c chng ph, bi vy khng c hin t-

ng giao thoa k hiu ISI gia cc sng mang nhng ph li ch-a uc s dng vi hiu qu cao nht. Vi OFDM, nu khongcch sng mang c chn sao cho nhng sng mang trc

giao trong chu k k hiu, th nhng tn hiu c th - c khi phc m khng b giao thoa hay chng ph. Ph camt sng mang con OFDM c dng nh hnh 2.1a.

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Ph ca sng mang tn hiu OFDM c dng nh hnh2.1b. Hai sng mang nhnh cnh nhau s c mt phn phtrng ln nhau. Tuy nhin ng vi tn s ph mt sng

mang t cc i th ph ca sng mang cn li trit tiunn tnh trc giao gia cc sng mang c m bo. S btr, sp xp sng mang nh hnh 2.1b nng cao hiu sutph nhng cng nng cao yu cu v ng b tn s giacc sng mang.

2.1.3. a sng mang (Multicarrier)

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Hnh 2.1b: Ph ca tn hiu OFDM

Hnh 2.2: Mi tr ng c asng mang

Ding

Vt cn ng

truyntnhiu

Tnhiu bnhiu

Tn hiub phnx

Tonh Tn hiu

n t trmgc

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Nu truyn tnh hiu khng phi bng mt sng mangm bng nhiu sng mang, mi sng mang ti mt phn dliu c ch v c tri u trn c bng thng th khi png knh l xu s ch c mt phn d liu c ch b mt.Khi dng nhiu sng mang c tc bit thp, nhiu d liu

gc s c thu chnh xc. hi phc d liu mt, ngi tadng phng php sa li trc (Forward Error Correction - FEC). my thu mi sng mang c tch ra khi dng cc b lcthng thng v gii iu ch. Tuy nhin khng c cannhiu gia cc sng mang (ICI) cn phi c khong bo v(guard period) khi hiu qu ph km [4].

Gii php khc phc vic hiu qu ph km khi ckhong bo v l gim khong cch gia cc sng mang vcho php ph ca cc sng mang cnh nhau trng lp nhau.S trng lp ny l c php nu khong cch gia cc sngmang c chn chnh xc. Khong cch ny c chn ng vitrng hp cc sng mang trc giao vi nhau. l phng php

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ghp knh theo tn s trc giao. Phng php ny mi c avo thc tin .

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2.1.4. S trc giaoTrc giao ch ra rng c mt mi quan h ton hc

chnh xc gia cc tn s ca cc sng mang trong h thngOFDM. Trong h thng FDM thng thng, cc sng mang cchnhau mt khong ph hp tn hiu thu c th nhn bitbng cch s dng cc b lc v cc b gii iu ch thngthng. Cn d liu trc mt khong bo v gia cc sngmang khc nhau. Vic a vo cc khong bo v ny lm

gim hiu qu s dng ph ca h thng [10] .OFDM c th sp xp cc sng mang sao cho cc di

bin ca chng che ph ln nhau m vn c th thu cchnh xc cc tn hiu, khng c s can nhiu gia cc sngmang. Mun c nh vy cc sng mang phi trc giao vmt ton hc. My thu OFDM bao gm cc b gii iu ch,

dch tn mi sng mang xung mc DC. Sau tn hiunhn c ly tch phn trn mt chu k ca symbol phchi d liu gc. Nu tt c cc sng mang u c dchxung tn s tch phn, th kt qu tnh tch phn chocc sng mang khc s l bng khng. Bt k s phi tuynno gy ra bi can nhiu gia cc sng mang cng s lm

mt i tnh trc giao.V mt ton hc, trc giao c ngha l cc sng mang

c ly ra t nhm trc chun (Orthonomal basis)

( ){ }/ 0,1,...i t i = c tnh cht sau [4]:

( ) ( )2

1

1

0

T

i k ik T

i k t t dt

i k

== =

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(2-

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Trong : ( )i t l cc hm sng mang c s.Nh vy:

( ){ } ( ) ( ){ }sin 2 / , cos 2 /i t n t T m t =Ngoi ra c th biu din trc giao theo hm phc

( ) [ ]{ }2 / / 0,1,... j N t ji t T i = = =

c tnh cht :

( )2

0

sin 0k

t dt

= Khong cch gia 2 sng mang trc giao cnh nhau s l

f =1/ u T. V d, nu tn hiu l sin(mx) vi m =1,2 th n

trc giao trong khong t - n . Vic x l (iu ch v

gii iu ch) tn hiu OFDM c thc hin trong min tns, s dng cc thut ton x l tn hiu s (Digital SignalProcessing - DSP). Nguyn tc ca tnh trc giao thng cs dng trong phm vi DSP. Trong ton hc, s hng trc giaoc c t vic nghin cu cc vect. Theo nh ngha, haivect c gi l trc giao vi nhau khi chng vung gc vinhau (to nhau mt gc 90 0) v tch ca hai vect l bngkhng. im chnh y l tng nhn hai hm s vinhau, tng hp cc tch v nhn c kt qu l 0.

u tin ta ch n hm s thng thng c gi trtrung bnh bng khng. (V d gi tr trung bnh ca hmsin di y). Nu cng na chu k dng v chu k m ca

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(2-

Hnh 2.3: Tch trc giao ca haivect bng 0

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dng sng sin nh trong hnh 2.4 chng ta s c kt qu l 0.Qu trnh tch phn c th c xem xt khi tm ra dintch di dng ng cong. Do , din tch ca 1 sng sin cth c vit nh sau:

( ) ( )1

0i k ik i k

t t dt i k

== =

Nu nhn v cng (tch phn) hai dng sng sin c tns khc nhau kt qu cho thy qu trnh ny cng bng 0(hnh 2.5).

iu ny gi l tnh trc giao ca dng sng sin. N chothy rng min l hai dng sng sin khng c cng tn s, thtch phn ca chng s bng khng. Thng tin ny l immu cht hiu qu trnh iu ch OFDM.

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(2-5)

Hnh 2.4: Tch ca hai dng snghnh sin

Hnh 2.5: Tch phn ca hai sng sin khct n s

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Nu hai sng sin c cng tn s nh nhau th dng snghp thnh lun dng, gi tr trung bnh ca n lun khckhng (hnh 2.6). y l c ch rt quan trng cho qutrnh gii iu ch OFDM. Cc my thu OFDM bin i tnhiu thu uc t min tn s nh dng k thut x l tnhiu s gi l bin i Fourier nhanh (FFT). Vic gii iuch cht ch c thc hin k tip nhau trong min tn s(digital domain) bng cch nhn mt sng mang c to ra

trong mt my thu vi mt sng mang uc to ra trong mythu khc c cng tn s v pha. Sau , php tch phn cthc hin, tt c cc sng mang s v khng ngoi tr sngmang c nhn, n c dch ln trc x, c tch ra hiu quv gi tr symbol ca n khi c xc nh. Ton bqu trnh ny c lp li kh nhanh chng cho mi sng

mang, cho n khi tt c cc sng mang c gii iu ch.Nhiu l thuyt chuyn i c thc hin bng chui trcgiao .

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T phn tch trn ta c th rt ra kt lun : khc phc hin tng khng bng phng ca c tnh

knh cn dng nhiu sng mang, mi sng mang chchim mt phn nh bng thng, do vy t b nh hngca c tnh knh n d liu ni chung .

S sng mang cng nhiu cng tt nhng cn phi ckhong bo v trnh can nhiu gia cc sng mang.

Tuy nhin, tn dng tt nht th cc sng mang phitrc giao vi nhau. Khi cc sng mang c th trng lpnhau m khng gy can nhiu .

2.1.5 M t ton hc ca OFDM

M t ton hc OFDM l trnh by tn hiu c to ranh th no, my thu vn hnh nh th no v cng cung cpmt cng c hiu r nhng tc ng xu trong knhtruyn. Phng php iu ch OFDM truyn mt s ln sngmang c di thng hp c t cch nhau chnh xc trongmin tn s. trnh vic s dng mt s lng ln b iu

ch v b lc my pht cng nh mt s lng ln b lc v bgii iu ch b sung my thu th phng php ny phis dng cng ngh x l tn hiu s hin i.

Trong ton hc, mi sng mang c m t nh mt sngphc [4]:

( ) ( )( )

.c c j t

c c s t A t e +

=

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Tn hiu thc l phn thc ca S c(t). C Ac(t) v c(t)(bin v pha tng ng ca sng mang) c th thay itrn mi symbol thng qua symbol c bn. i vi iu chQPSK, bin ca sng mang thng bng 1 v pha s ly mttrong bn gc phn t ca h thng iu ch QPSK thng th-ng. i vi symbol th p, trn khong thi gian (p-1)

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thi im ny, khong thi gian tn hiu c phnthnh N mu c gii hn. l thun li ly mu trongmt chu k ca mt symbol d liu. V th c mi lin h:=NT

Nu n gin biu thc (2-9) m khng lm mt i

tnh tng qut bng cch cho 0=0, th tn hiu tr thnh :

( ) ( )1

0

1n

N j n kT j

s nn

S kT Ae e N

=

=

Tip theo ta c th so snh biu thc (2-10) vi dngtng qut ca bin i Fourier ngc :

( )1

2 /

0

1 N j nk N n

n g kT G e

N NT

=

=

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(2-

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Biu thc (2.9) v (2-10) l tng ng nu :1 1

f NT = =

y cng l iu kin yu cu cho tnh trc giao. Dovy, kt qu ca vic bo ton tnh trc giao l tn hiuOFDM c th c xc nh bng cch th tc bin iFourier. Cc thnh phn ca mt mng trc giao c lptuyn tnh vi nhau. C th xem tp hp cc sng mang pht

i m l mt mng trc giao c cho bi cng thc :( ) ( )k k t exp j t =

0 2k k t

= =

Nu tp hp cc sng mang ny thc s trc giao thmi quan h trc giao trong biu thc (2-13) s c biu din

nh sau :

(vi p v q l hai s nguyn )Nu tch phn uc m rng ra c pha ca mi sng

mang th biu thc (2-10) c sa li nh biu thc (2-14).Cc tn hiu l trc giao nu chng c lp vi nhau. S trc

giao cho php truyn tn hiu mt cch tt nht trn mtknh chung v pht hin chng m khng c can nhiu.Ghp knh theo thi gian (TDM) cho php truyn nhiu tnhiu thng tin trn mt knh n bi vic gn khe thi gianng nht cho mi tn hiu thng tin ring bit. Trong mikhe thi gian ch c mt tn hiu t mt ngun n c

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truyn ngn nga s can nhiu t nhiu ngun thng tinkhc nhau. Do vy, TDM l trc giao v bn cht.

Trong min tn s, a s cc h thng FDM trc giaov mi tn hiu truyn ring bit c cch ly nhau theotn s ngn nga can nhiu. Mc d cc phng php nyl trc giao, nhng thut ng OFDM c dnh ring chomt dng c bit ca FDM. Cc sng mang con trong OFDMc t gn nhau, trong khi vn duy tr tnh trc giao gia

chng. Cc tn hiu OFDM c to thnh t tng cc tn hiuhn sin, mi hnh sin tng ng vi mt sng mang con. Dytn s c bn ca mi sng mang con c chn l s nguynln thi gian symbol. Kt qu l tt c cc sng mang con cmt s nguyn cc chu k trong mt symbol, v chng trcgiao vi nhau.

2.1.6 H thng my thu pht OFDMH thng my thu pht OFDM hon chnh c m t

trn hnh 2.9. Trong m hnh ny, sa li trc (FEC) c thmvo chng li li cm. Mt h thng OFDM c thm m hoknh v ghp xen knh c gi l OFDM c m ho (COFDM).

Trong min tn s, d liu nh phn u vo c tphp li v c m ho FEC (dng m ho xon). Chui bit m cxo trn lm cho phc tp thm. Sau , mt nhm ccbit m ho c nhm li vi nhau v c nh x thnh ccim tn hiu tng ng. Khi , d liu c biu dindi dng s phc theo chui. Mt b chuyn i ni tip-song

song (S/P) c s dng v thut ton chuyn i Fourier

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nhanh bt bin theo thi gian (IFFT) c thc hin trn dliu phc song song. D liu bin i c nhm li vinhau mt ln na theo s cc sng mang con qui nh.CP chn vo u mi khi d liu theo c im k thutca h thng v d liu c dn li thnh dng ni tip. Tithi im ny, d liu c iu ch OFDM v sn sng -

c truyn i. Mt b chuyn i s-tng t (DAC) c sdng chuyn i d liu s trn min thi gian sang d

liu tng t trn min thi gian. Sau , qu trnh iuch tn s v tuyn c thc hin a tn hiu bng gcln min tn s cao truyn i.

Sau khi truyn tn hiu OFDM t ng ten ca my pht,tn hiu s i qua tt c cc vt th v cc chng ngi vt caknh truyn khng dy. Gi s knh truyn dn l knh c

p ng xung bin i theo thi gian h( , t) vi tri tr < TCP. Ngoi ra, knh truyn cn to ra thnh phn nhiu nnn(t) l nhiu trng dng phc.

Sau khi thu c tn hiu, my thu bin i tn hiu t-ng t sang min tn hiu s bng cch s dng b chuyni tng t - s (ADC). Ti thi im tm li tn hiu thu

c, ng b sng mang c thc hin. Sau chuyn iADC, ng b thi gian s c hon tt. Khi IFFT c sdng gii iu ch tn hiu OFDM. Sau qu trnh clng knh c thc hin s dng cc symbol hoa tiu giiiu ch. Trong qu trnh c lng knh, d liu phc thuc tm li v c gii nh x theo biu tn hiu truyn

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dn. Sau , gii iu ch FEC v gii xo trn c s dng tm li lung bt gc c truyn i.

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47Knh v tuyn a ng

iu ch a tnhiu v min tns thp

RF

My thu

iu ch a tnhiu bng gc

ln min tn scao

Bin i s-tng t (DAC)

RF

D liu vo nhphn

My pht

Tnhiu

truynbnggc

iu ch OFDMqua IFFT

CP

Bin i nitip - song song

M ho sa li vxo trn

Tp tn hiud liu phc

Chn symbol hoatiu

nh x symbol(iu ch d

liu)

ng bsngmang

Bin i t ng t s (ADC)

Tn hiuthu bnggc

ngb thigian

CP

Gii m sa li vgii xo trn

D liu ra nhphn

Bin i songsong ni tip

Gii iu chOFDM qua IFFT

Tp tn hiud liu mc

thuGii nh x symbol(gii iu ch d

liu)

c l ng knh datrn cc symbol hoa

tiu

Tn hius

Tn hiu tng tHnh 2.7: S my pht v my thu

OFDM

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2.1.7 Hiu sut s dng ph ca OFDMVi vic to ra tn hiu OFDM nh trn, hai sng mang

nhnh lin nhau s cch nhau khong thi gian bng f c=1/T s.Di y l tnh ton hiu sut ph ca OFDM:Bng thng tng cng ca tn hiu iu ch OFDM l:

( )2

1c

S S

N W N f

T T +

= +

Nu s dng iu ch mt sng mang nh bnh thngth bng thng cn thit truyn dn thng tin vi tc tng ng nh h thng a sng mang l:

2'

S

N W

T =

So snh gia hai cng thc trn ta thy hiu sut sdng ph ca iu ch OFDM so vi iu ch mt sngmang thng thng tng khi s sng mang N tng. Trong trng

hp gii hn N , bng thng cn thit truyn dn OFDMch bng mt na so vi truyn dn mt sng mang thng th-ng.

Hnh 2.9 ch ra s khc nhau gia h thng FDM thngv OFDM. Vi k thut iu ch a sng mang OFDM chngta tit kim c 50% di thng [13].

Tnh trc giao c s dng trong OFDM sp xp ccsng mang con theo cch m cc bng tn ph ca cc sngmang ring bit chng lp ln nhau v tn hiu vn thu c my thu m khng c ICI.

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2.1.8 Khong bo v (GUARD PERIOD)

Vi mt bng thng h thng cho, tc symbol catn hiu OFDM thp hn nhiu tc symbol ca s

truyn sng mang n. V d, iu ch n sng mangBPSK, tc symbol tng ng vi tc bit truyn i. Tuynhin, vi OFDM bng thng h thng c chia cho N c sngmang con, to thnh tc symbol nh hn N c ln so vitruyn sng mang n. Tc symbol ny lm cho OFDMchu ng tt vi can nhiu gy ra bi truyn lan a ng

[4].

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Hnh 2.9: Di bo v ca tnhiu OFDM

Hnh 2.10: Thm mt khong bo v vo tnhiu OFDM

Hnh 2.8: im khc nhau gia h thng FDM thng v OFDM

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C th gim nh hng ISI trong tn hiu OFDM bngcch thm vo mt khong bo v trc mi symbol. Khongbo v ny l bn sao chp tun hon theo chu k, lm mrng chiu di ca dng sng symbol. Mi sng mang con,trong phn d liu ca mi symbol, c mt s nguyn ln ccchu k. Do vy vic a vo cc bn sao chp ca symbol lintip nhau s to thnh mt tn hiu lin tc, khng c s ginon ch ni. Nh vy, vic sao chp u cui ca symbol

to ra mt khong thi gian symbol di hn. Hnh 2.10ch ra cch thc thm vo mt khong bo v.

Tng chiu di ca cc symbol l T S = TG + TFFT, trong T S l tng chiu di ca symbol trong cc mu. TG lchiu di ca khong phng v, T FFT l kch c ca IFFT sdng pht tn hiu OFDM. Ngoi ra, khong bo v cng

c a ra chng li cc li dch thi gian trn my thu.2.1.8.1 Bo v chng li dch thi gian (time offset)

gii m tn hiu OFDM, my thu phi nhn uc FFTca mi symbol thu c tm ra bin v pha ca ccsng mang con. i vi h thng OFDM dng vi cng mt tns ly mu cho c my pht v my thu, h thng phi dng

cng mt kch thc FFT cho c my thu v pht duy tr strc giao ca cc sng mang con. Mi symbol thu c c ccmu di T G + TFFT do b sung khong bo v. My thuch cn cc mu TFFT ca symbol thu c gii m tnhiu. Cc mu T G cn li l khng cn thit. i vi knh l t-ng khng c m rng tr my thu, c th d tm c

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dch thi gian bt k (ln nht l bng khong bo v T G)m vn t c s cc mu ng.

Do bn cht tun hon, khong bo v dch thi gianch dn n s quay pha ca tt c cc sng mang controng tn hiu. Gi tr quay pha t l vi tn s sng mangcon. Vi cc sng mang con tn s Nyquist, th s thay il 180 0 cho mi lch thi gian mu. chng minh rng lch thi gian c duy tr khng i t symbol ny ti

symbol khc, nn s quay pha cho lch thi gian c th cloi b nh mt phn ca cn bng knh trong mi trng ang ISI gim di ca khong bo v, dn n li lchthi gian cho php .

2.1.8.2 Bo v chng li ISI

Trong tn hiu OFDM bin v pha ca sng mangcon phi c duy tr khng i trong chu k symbol bo m tnh trc giao cho mi sng mang. Nu chng bthay i c ngha l dng ph ca cc sng mang con skhng ng dng sin v nh vy im khng (Null) s khng tn s ng, dn n can nhiu gia cc sng mang. bin

ca symbol, bin v pha thay i t ngt ti gi tr mi,cn thit cho symbol d liu tip theo.

Trong mi trng a ng, ISI gy ra s tri rng nng l- ng gia cc symbol, dn n s thay i nhanh bin vpha ca cc sng mang con im u symbol. di canhng nh hng thay i nhanh tng ng vi s m rng

tr ca knh v tuyn. Tn hiu thay i nhanh l kt

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qu ca mi thnh phn a ng cc thi im khc nhaumt t, lm thay i vect sng mang con thu c. Hnh2.11 ch ra nh hng ny. Vic a vo cc khong bo vcho php c thi gian phn tn hiu thay i nhanhny b suy hao tr li trng thi ban u, do vy FFT ly li -

c phn trng thi ng ca symbol. iu ny loi b nh h-ng ca ISI. khc phc ISI th khong bo v phi dihn tr m rng ca knh v tuyn. Cc nh hng cn li

m a ng gy ra nh thay i bin v quay pha s - c sa bi cn bng knh .

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Hnh 2.11a: Chc nng ca khong bo vchng ISI

Khng c a ng

Hnh 2.11b: Chc nng ca khong bo vchng ISI

Knh c a ng

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Khong bo v chng li cc nh hng thay i nhanhdo a ng loi b cc nh hng ca ISI. Tuy nhin trongthc t cc thnh phn a ng c khuynh hng suy gimchm theo thi gian, dn n vn cn ISI ngay c khi khong

bo v tng i di c s dng .

2.1.9 gii hn bng thng ca OFDM v ca s OFDM trong min thi gian tng ng vi tng cc

sng mang hnh sin c iu ch. Mi symbol nm trongmt khong thi gian xc nh vi hm ca s hnh ch

nht. Ca s ny xc nh bin ca mi symbol OFDM v xcnh c tuyn tn s c to ra. Hnh 2.12 l mt v ddng sng thi gian truyn OFDM khi dng kho dch pha(Phase Shift Keying-PSK) [4], bin sng mang con l cnh v pha thay i t symbol ny ti symbol kia truyn d liu. Pha ca sng mang con khng i i vi

ton b symbol, dn n nhy bc pha gia cc symbol.

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Nhng thay i t bin ny gia cc symbol dn n sm rng trong min tn s.

2.1.9.1 Lc bng thngLc bng thng c s dng khi tn hiu c bin i

t min s sang dng min tng t v ngc li, ngn ngas chng ph. Trong OFDM hiu qu ca lc bng thng l loi b mt s bp sng trn OFDM. Gi tr loi b bpsng bin ph thuc vo dng b lc c s dng. Nhn

chung lc s to nn linh hot, chnh xc v tc ct (cut off rate) ln hn nhiu so vi lc tng t. Nh vy,chng c bit c ch trong vic hn ch bng thng tnhiu OFDM.

2.1.9.2 phc tp tnh ton bng lc bng thng FIR

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Hnh 2.12: Ph tn hiu OFDM gm 52 sng mangcon khng c hn ch bng thng

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Vic dng lc bng thng s l phng php rt hiu qu loi b cc bp sng bin do tn hiu OFDM to ra. Khkhn l chi ph tnh ton cao. thc hin b lc bng thngFIR s tap cn thit tng ng vi [4]:

.T taps

T

W IFFT N ceil

F

=

y N taps l s tap trong b lc FIR; W t l rng qu ca hm ca s c dng to b lc FIR; IFFT l kchthc FFT c s dng to tn hiu v F t l rng qu ca b lc chun ha cho khong cch ti ph. Ceil l phplm trn v pha ln hn. V d ceil (1.1) = 2 .

Cng sut bp sng bin ca tn hiu OFDM khng clc l -20 dBc v sau khi lc l -109 dBc. rng qu cahm ca s c s dng l 3.0 nn s tap cn thit l :

3.0 6424

8taps N ceil

= Mi tap ca b lc FIR yu cu hai thut ton nhn v

tch ly MAC (Multiply and Accumulate) nh kt qu ca ccmu phc, v nh vy i vi tn s ly mu 20 MHz s phptnh s l 20x106x24x2=960 triu MAC. Trong cc ng dngm s tap cn thit trong b lc l ln (>100), vic thc hinb lc FIR nh dng FFT c th hiu qu hn.

2.1.9.3 nh hng ca lc bng thng ti ch tiu k thut OFDM Trong min thi gian, mt symbol OFDM c dng hnh

ch nht, tng ng vi suy gim dng sin trong min tn s.

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Nu lc bng thng mt tn hiu OFDM vi b lc kiu tnggch (brick wall), th tn hiu s c dng hnh ch nht ctrong min tn s, lm cho dng sng min thi gian c suygim dng sin gia cc symbol. iu ny dn n ISI, lmgim ch tiu k thut. C th loi b ISI do vic lc gy rabng cch dng khong bo v c di v bng vicchn lc lch thi gian ng b gia cc khong bov, do vy hu ht nng lng ISI s b loi b.

Hnh 2.13 m t ch tiu k thut m phng ca tnhiu OFDM c lc bng thng vi cc rng qu khcnhau cho b lc knh khng c nhiu. Hnh v ny ch rach tiu ca truyn OFDM khi lch v ng b thi gianb thay i. Khong bo v c s dng trong m phngny c cng di nh phn IFFT ca symbol. Vic s dng

khong bo v rt di ny lm cho h thng chu c nh h-ng ca lch thi gian trong mt khong rt rng caSNR hiu dng tnh bng cch trung bnh ha SNR hiudng trn tt c cc sng mang con. lch thi gian bng 0tng ng vi vic my thu nhn c FFT phn IFFT ca tnhiu pht. lch thi gian m tng ng vi vic my thu

nhn c FFT phn IFFT ng v mt phn ca khong bov symbol.

ISI l thp nht khi lch thi gian l m v bngmt na di khong bo v. B lc c c tuyn cngdc bao nhiu (trong hnh v b lc dc nht loi b cc bpsng bin xung thp hn 100 dBc trong gii hn hai khong

cch sng mang) th ISI cng di by nhiu. Nu khong bo

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v bng 50% thi gian symbol ton phn, th di khongbo v s bng thi gian symbol c ch. SNR hiu dng catn hiu OFDM c lc bng thng ph thuc vo nh hngca c ISI c ICI [4].

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Hnh 2.13: Hiu qu SNR l mt hm ca dch thigian cho lc bng thng 52 sng mang con tn hiu

OFDMca tn hiuFDM gm 52 ti ph c lc bng thng

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2.1.10 Khong bo v cosin nng (RAISED COSINEGUARD PERIOD )

Mt trong nhng phng php n gin nht tritcc bp sng bin ca tn hiu OFDM l un trn khong bov ca tn hiu OFDM v gim t t n ti 0 trc symbol tiptheo. Vic gim t t ny lm chuyn dch trn tru gia ccsymbol dn n gim cng sut cc bp sng bin.

Hnh 2.14 m t mt symbol OFDM n vi khong bo

v c dng hnh cosin nng RC (Raised Cosine). Phn cosinenng ca khong bo v c th chng lp vi cc symbol trcv sau, v n ch to nn s bo v ti thiu chng li a -ng v li thi gian v b my thu lm ng. V phn ny lmgim dn ti 0, nn n dn ti 0 v dn ti ISI b sung tithiu. u im chnh ca vic chng lp l di ca

phn cosin nng. C th l gp i m khng t gy raoverhead thi gian b sung.

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2.1.11 nh hng ca nhiu trng cng tnh Gaussian(Aditive White Gaussian Noise ) n OFDM

Nhiu tn ti trong tt c cc h thng thng tin trnknh vt l tng t, chng hn nh knh v tuyn. Cc ngun

nhiu chnh l nhiu nhit, nhiu in trong cc b khuchi my thu v cc can nhiu gia cc t bo thng tin.Ngoi ra, nhiu cn c th c to ra bn trong cc h thngthng tin nh l kt qu ca can nhiu gia cc symbol ISI,can nhiu gia cc sng mang ICI v mo xuyn iu ch(Inter-Modulation Distortion - IMD). Cc ngun nhiu ny lm

gim t s tn hiu trn nhiu, v gii hn ng k hiu quph ca h thng .

Nhiu l nguyn nhn c hi chnh trong hu ht cch thng thng tin v tuyn. Do vy vic nghin cu cc nhhng ca nhiu n t l li thng tin v mt s bin phpdung ha gia mc nhiu v hiu qu ph h thng l rt

quan trng. Hu ht cc dng nhiu trong h thng thng tinv tuyn c th c m hnh ha chnh xc nh dng dliu nhiu trng chun cng tnh, nhiu ny c mt ph u (cn gi l nhiu trng) v c phn b Gauss vbin (c xem nh phn b chun hoc ng cong hnhvung). Nhiu nhit v nhiu in do s khuch i, ch

yu c tnh cht ca nhiu Gauss trng, do vy c th m

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hnh ho chng chnh xc theo AWGN. Hu ht cc ngunnhiu khc c tnh cht AWGN v s truyn l OFDM. Cctn hiu OFDM c mt ph phng v phn b bin Gauss v s sng mang l ln nn can nhiu gia cc t bot h thng OFDM khc cng c cc tnh cht AWGN. Cng vimt l do nh vy, ICI, ISI v IMD cng c cc tnh cht AWGNcho cc tn hiu OFDM.

2.1.12 nh hng ca mo ti OFDM Tn hiu OFDM c cng sut nh cao hn so vi cng

sut trung bnh ca n. Khi sng mang RF c iu ch vitn hiu OFDM th iu ny s dn ti s thay i tng tca ng bao sng mang. T , dn ti yu cu l tn hiuphi c khuych i v truyn i mt cch tuyn tnh.

Vic duy tr tuyn tnh mc cng sut cao l rt khkhn, do vy hu ht mo trong truyn v tuyn thng xyra trong b khuych i cng sut ca my pht. Ngoi racn c th c mo b sung trong my thu nu my thu cthit k khng hp l. Tuy nhin nhn chung vic duy trmc mo trong my thu mc thp nht th d hn l duy

tr n trong my pht. Mo trong my pht gy ra mi vn trong chui truyn dn, v n c th dn n m rngph, gy can nhiu cho cc h thng bn cnh tn s RF. Dol do ny ch cn xem xt nh hng ca mo trong mypht.

Tnh phi tuyn trong truyn dn dn n hai sn phm

mo ch yu, mo iu ch tng h (Inter- Modulation

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Distortion - IMD) v cc hi. Cc hi l cc thnh phn tn s X ln tn s sng mang RF vi X l s nguyn. V d nutn s sng mang l 900 MHz th cc hi s xy ra 1800MHz, 2.7 GHz v.vC th d dng loi b cc hi nh b lcbng thp tng i n gin u ra my pht .

IMD gy nhiu vn hn v n dn n cc thnhphn mo, c trong bng tn v ngoi bng tn nhng gnvi tn s truyn dn chnh. Cc thnh phn ny l kt qu

ca vic trn cc thnh phn hi ca h thng, v trn ktip gia cc sn phm IMD. Cc thnh phn trong bng tnto thnh nhiu cng tnh vi tn hiu OFDM my thu, lmgim SNR ca h thng, thm ch ngay c khi khng cngun nhiu khc. Cc thnh phn ngoi bng tri rng tnhiu theo tn s, gy can nhiu vi cc tn hiu thng tin v

tuyn khc trong cc bng tn bn cnh. Thm ch nu tnhiu c hn ch bng thng hon thin trc khi a ti bkhuych i cng sut my pht, m rng ph s xy ra nukhuych i cng sut gim, tuy nhin s gim ny l khngnhiu v cc b lc bng thng hot ng tn s RF thngc c tuyn khng tht tt. gim mo phi tuyn phi

chn im lm vic ph hp trong c tuyn vo ra ca bkhuych i cng sut back off (Output power back off -OBO). Trong truyn dn OFDM, dng iu ch QPSK OBOkhong 2-3 dB v QPSK l s iu ch rt kho, chng lic nh hng ca mo. Cc s iu ch c hiu sutbng thng cao hn (v d 16 QAM, 256-QAM..) nhy cm

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hn vi mo v chng yu cu SNR hiu dng cao hn. Vd 16-QAM OBO l 16 dB, vi 64-QAM l khong 10dB.

2.1.13 nh hng ca sai li ng b n ch tiu cht lng h thng OFDM .

Mt trong nhng hn ch h thng s dng OFDM l khnng d b nh hng bi li do ng b, c bit l ngb tn s do lm mt tnh trc giao gia cc sng mang

nhnh. Trong h thng OFDM, thng xt n ba loi ngb: ng b k t, ng b tn s sng mang v ng btn s ly mu. nh gi nh hng ca sai li ng b thngda trn vic xc nh suy gim ca t s cng sut tnhiu trn nhiu SNR. Bng 2.1 di y trnh by cc ktqu [18]:

Loi/lng li ng b suy gim SNR(Db)Li tn s sng mang 1 , knhAWGN

( )20

103ln10

S E D N

Li tn s sng mang 1 , knhpha inh

( )202

1 0,5974 sin10log

sin

S E N

Dc

+

Nhiu pha sng mang, rng 2

( )0

11 46ln10

S E D N

Li ng b tn s ly mu3

S f ti sng mang con th n( )

26

010log 1 10

3S

S

E D n f

N

+

Li thi gian (Khng ng k nu nh)

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Ch : 1: chun ho theo khong cch tn s gia hai knh

1S T

2: chun ho theo tn s ly mu, tnh theo phntriu 3: chun ho theo thi khong mt k t tn hiupht

Bng 2.1: Suy gim SNR theo li ng bDa vo bng, ta c mt s nhn xt sau:- ng b tn s sng mang gia my thu v my pht

nh hng n ch tiu cht lng h thng nhiunht. Suy hao SNR t l thun vi bnh phng sailnh tn s sng mang

- Suy hao SNR theo li ng b tn s ly mu phthuc vo bnh phng ca ch s sng mang vbnh phng dch tn s ly mu tng i.

- nh hng ca li thi gian s b trit tiu nu dchthi gian nh sao cho n khng lm p ng xungca knh vt ra ngoi khong thi gian ca khongphng v.

2.1.14 u im v hn ch ca h thng OFDMNgoi hai c im v kh nng chng nhiu ISI, ICI,

nng cao hiu sut s dng ph, vic s dng OFDM cn ccc u im v hn ch sau y: u im:

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- Thc hin iu ch tn hiu n gin nh s dng kthut FFT

- Khng ging nh CDMA, cc my thu OFDM tp hp nng l- ng ca tn hiu trong min tn s, do n cho phpchng tn hao nng lng trn min tn s

- OFDM cho php thng tin tc cao c truyn songsong vi tc thp trn cc knh bng hp. Cc knh conc th coi l cc knh pha inh khng la chn tn s nn

c th dng cc b cn bng n gin trong sut qutrnh nhn thng tin. H thng OFDM chng c nh h-ng ca pha inh chn lc tn s.

- Trong mt knh truyn bin i chm theo thi gian mtcch va phi, OFDM c th lm tng ng k dung lngbng cch lm tng thch tc truyn d liu trn sng

mang con theo t s SNR ca sng mang con ring .- My pht OFDM lm n gin ho hiu qu knh truyn,

do ch cn mt my thu c cu trc n gin l cth khi phc li d liu c truyn i. Nu chng ta sdng kiu iu ch kt hp th qu trnh c lng knh srt n gin, mt khc, chng ta s khng cn c lng knh

nu s dng kiu iu ch vi phn.- Quy trnh duy tr tnh trc giao trong OFDM n gin

hn nhiu so vi cc k thut CDMA, TDMA ngay c trongiu kin truyn dn a ng kht khe nht.

- OFDM c th c s dng cho cc ng dng a phngtin tc cao vi chi ph dch v thp.

- OFDM c th h tr cho truy cp gi ng.

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- Cc mng n tn s c th thc hin c trong OFDM,m c bit hp dn l cho cc ng dng qung b.

Hn ch- Tn hiu OFDM l tp hp ca nhiu tn hiu trn hiu

sng mang, di ng ca tn hiu ln nn cng sut tngdi cc i ln, hn ch hot ng ca b khuych i

- D b nh hng ca dch tn v pha hn so vi h thng

mt sng mang. V vy, yu cu phi ng b tt tn strong hot ng.

2.2 ng dng ca OFDM trong cc chun WLAN hin cHin nay OFDM c s dng trong cc chun WLAN sau

y: IEEE 802.11a, IEEE 802.11g, HyperLAN 2 t c

tc ln ti 54 Mbps. Trong phn ny ta s i nghin cusu hn v ba chun ny.

2.2.1 Chun IEEE 802.11aChun IEEE 802.11a l s m rng lp vt l ca chun

gc cho mng cc b khng dy IEEE 802.11. Chun gc IEEE

802.11 nh ngha mt tp hp cc yu cu cho lp vt lPHY v lp iu khin truy cp MAC. Vi d liu tc cao,chun ny a ra hai lp vt l m rng l IEEE 802.11b hotng di tn 2,4 Ghz v IEEE 802.11a hot ng di tn5 Ghz.

Chun IEEE 802.11a h tr cc ng dng WLAN yu cu

tc d liu cao hn 11 Mbps trong di tn 5 GHz. OFDM

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c s dng trong chun 802.11a nhm t c hiu sutcao trong vic chng li pha inh a ng.

Trong 802.11a, thi gian truyn mi bit tng ln t-ng ng vi s lng sng mang, lm cho h thng t nhy cm

hn i vi xuyn nhiu a ng. hon chnh OFDM ,IEEE 802.11a cn a ra mt s phng php iu ch v mho khc nhau la chn cho nhng tc khc nhau Tac bng 2.2 mt s phng php iu ch v m ho dngtrong chun IEEE 802.11a:

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Hnh 2.15: nh dng khung IEEE 802.11a vi bng tn5 Ghz

Bng 2.2: Mt s ph ng php iu ch v m ho dngtrong 802.11a

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IEEE 802.11a ti u lp vt l cho vic truyn dn nidung a phng tin nh lung video. Lp vt l OFDM c khnng truyn cc khung d liu dch v vi tc d liu lnti 54 Mbps nhng ni cn truyn d liu a phng tin.

2.2.2 Chun IEEE 802.11gChun IEEE 802.11g l chun WLAN mi c pht hnh

thng 9/2003. Chun ny cng dng OFDM c th h tr tc d liu ti 54 Mbps nh 802.11a. Nhng khc vi802.11a, 802.11g hot ng di tn 2.4 GHz ging nh

802.11b. V th, IEEE 802.11g tng thch ngc tr li vi802.11b. Chun 802.11g bao gm nhng c im ttnht ca hai chun 802.11a v 802.11b m c th thch hpvi nhng cng ngh mi trong tng lai.

Vi kh nng hot ng tng thch vi cc tc hinc (802.11b), ton b mng WLAN khng phi nng cp khi

chuyn ln cc tc cao hn ca chun 802.11g. Cng vibng tn v k thut iu ch s dng, cc tc dliu ang hot ng m 802.11g c cho rng s tr thnhchun c la chn s dng nhiu hn 802.11b l chunthng dng trn ton th gii hin nay [13]. Bng 2.1 biudin cc tc d liu khc nhau t c khi s dng OFDM

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vi cc knh con kt hp vi cc k thut iu ch khc nhau lp vt l (trong cc chun 802.11a v 802.11g).

K thut iuch vi cc knh

ph

Tc d liucho mi knh ph

(Kbps)

Tng tc dliu

(Mbps)BPSK 125 6BPSK 187.5 9

QBPSK 375 18QBPSK 250 1216QAM 500 2416QAM 750 3664QAM 1000 4864QAM 1125 54

Bng 2.3: Cc k thut iu ch vi cc tc t-ng ng

2.2.3 Chun HyperLAN2Chun mng cc b khng dy HyperLAN2 (High

Performance radio LAN 2) l chun mng khng dy cnh ngha bi Vin chun vin thng chu u ( European

Telecommunications Standards Institute - ETSI) vo thng 9

nm 1999. Chun HyperLAN2 l chun s dng OFDM thchng cho cng ngh v tuyn bng rng trong di tn 5 GHzv h tr tc d liu n 54 Mbps. Chun ny kt ni ccthit b di ng trong cc mi trng trong nh v cng cng.Mt s c im ni bt ca HyperLAN2 bao gm: c lp v mng v cc ng dng: Kin trc giao thc

HyperLAN2 linh hot, d thch nghi v ph hp vi nhiu

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dng ca mng c nh. HyperLAN2 c th kt ni tinhiu c s h tng mng nh mng Internet, IP, ATM, PPP,v c mng di ng t bo 3G.

Qun l tn s mt cch t ng: Ging nh mng i ngt bo - GSM, HyperLAN2 khng cn ln k hoch qun ltn s. Cc AP thit lp mt h tr t ng la chnknh truyn v tuyn thch hp bn trong mi vng phsng ca AP. Do , vic trin khai mng c n ginho i rt nhiu.

Cht lng dch v: Kt ni nh hng, d liu truyn trnnhng kt ni gia cc trm u cui di ng v AP phic thit lp trc khi truyn. c bit chun ny rt thchhp vi cc ng dng nh video v thoi.

Bo mt: bao gm nhn thc v m ho. Cc AP v cc trmu cui di ng c th nhn thc ln nhau mbo quyn truy cp mng. S nhn thc ny cn h tr ccchc nng nh dch v th mc, dch v nh x cc volumev.v... Ngoi ra, trong bo mt HyperLAN2 cn c c ch mho, bo v lu lng ngi s dng trn mt kt ni thitlp chng nghe trm v ly cp d liu.

Tng t nh cc chun WLAN khc, HyperLan2 ch nhngha lp vt l v lp iu khin lin kt d liu. Ngoi ra,chun ny cn nh ngha mt lp mi c gi l lp con hit (Convergene Sublayer). Lp con ny quan tm ti cc chcnng dch v c lp gia cc lp iu khin lin kt d

liu v lp mng. Lp con hi t c chc nng nhn cc gi v

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cc t h thng mng c dy, ri nh dng chng chuyn qua mi trng khng dy. Nh cc chun WLAN khc,HyperLan2 c th thc hin kt ni gia cc trm ti ccim truy cp v cho php cc nt mng di ng truynthng trc tip vi nhau. Lp con hi t cng c th s dngtrn lp vt l kt ni cc mng IP, ATM v UMTS (mng ding 3G ca chu u). Trn lp vt l, HyperLAN2 dng ccphng thc iu ch BPSK , QPSK , 16QAMor 64 QAM kt hp

vi OFDM. V bo mt, d liu HyperLAN2 c bo mt bngcc thut ton DES v 3DES. Cc AP v my di ng c thnhn thc ln nhau.

2.3 Mt s hn ch ca cc h thng mng WLAN sdng OFDM hin nay

Cc mng WLAN s dng OFDM hin nay vn cn c mts hn ch, l: Tc truyn d liu cn thp, cc imi ch t n 54 Mbps, thp hn nhiu so vi tc 100Mbps ca mng c dy thng thng. Tc ny cha chocc ng dng a phng tin ang ngy cng ph bin trnmng, chng hn nh truyn thoi IP qua WiFi (VoWiFi), truyn

video, cc ng dng game, truyn hnh cht lng cao (HDTV).Cc chun WLAN c tc cao nht hin nay l chun

IEEE 802.11a ch trong phm vi nh (khong 25 mt) v ccthit b ch to c gi thnh kh cao, chun IEEE 802.11g ctc th hn ch v s knh truy cp. Khi c nhiu thitb khng dy dng chung mi trng truyn, bng thng mng

s b chia s dn ti gim tc d liu. Hn na, khi

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khong cch lin lc tng ln, cc mng ny u phi gim tc d liu. Do vy, tc thc t cn thp hn nhiu so vitc nu ln trong cc ti liu v chun.

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2.4. Xu hng nghin cu pht trin ng dng OFDMtrong WLAN

Hin nay cng ngh OFDM v ang c nghin cung dng rt mnh m trong lnh vc thng tin v tuyn.

ng dng ca OFDM s dnh cho mch vng v tuyn niht (Wireless Local Link - WLL), mng cc b khng dy(WLAN), mng din rng khng dy (WMAN), mng v tuync nhn t bo (Cellular Wireless PAN). Cc h thng a

truy nhp da trn OFDM nh OFDM-TDMA, v MC-CDMA angc xem xt ti nh mt th h tip theo ca h thng vtuyn nhiu ngi s dng (Multi-user OFDM).

OFDM s tip tc l mt cng ngh then cht trong phttrin WLAN (vi vai tr va l phng thc iu ch va l cch a truy cp) v OFDM s vn l gii php cho vn

lin lc khng dy a ngi dng trong WLAN. Bc tin mitrong OFDM c d on s l s pht trin ca cng nghOFDM nhiu u vo - nhiu u ra (MIMO-OFDM) trong ds dng nhiu anten pht v thu ci thin tc truyntrong WLAN. Mt s nghin cu mi p dng MIMO vo OFDM cho thy c th tng tc d liu ng k.

Nh vy, chng hai ca lun vn gii thiu s lc vcc nguyn l c bn ca OFDM bao gm tnh trc giao vdi bo v v p dng bin i Fourier nhanh trong OFDM.y l nhng c tnh c bn nht ca k thut OFDM,tnh trc giao c khng nh chc chn v hiu quca n trong l thuyt truyn tin. Di bo v cho php m

bo tnh trc giao ng thi gip loi b c ISI v ICI. Php

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bin i Fourier nhanh gip cho k thut OFDM c th cng dng rng ri trong thc t v.v

Vi nhng nghin cu su hn v l thuyt, OFDM schng t nhng u im ca mnh trong cc h thng vinthng, nh trong cc mng di ng khng dy. Do khun khc hn nn lun vn ch trnh by s lc m cha trnh byvo cc vn cng rt c bn khc ca OFDM trong WLAN,chng hn nh vn loi b giao thoa ISI v ICI, vn

nng cao hiu sut s dng di tn v.v...Phn tip theo ca lun vn s tip tc trnh by v

mt k thut mi bt u c p dng trong WLAN s dngOFDM - l MIMO OFDM.

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Chng 3NHNG NGHIN CU ng dng OFDM trong chun

wlan mi: chun IEEE 802.11n

Cc chun WLAN hin i nht hin nay (IEEE802.11a,IEEE 802.11g v HyperLAN2) ch thch hp cho cc ng dngmng thng thng, chng hn nh th in t, duyt Webhoc truyn file d liu. Tuy nhin, khi yu cu v tc

d liu cao hn, chng hn nh vi cc ng dng mi nh thoiqua WLAN, truyn video, truyn hnh qua WLAN, cc trchi in t (game) a phng tin, th cc chun chath p ng c. Tc truyn d liu cc i 54 Mbps(m thc t do nhiu nguyn nhn khng th t c) cacc chun cha p ng c yu cu ca cc ng dng nh

vy. p ng c cc ng dng mi, cn c mt chunWLAN mi c tc d liu cao hn. Do vy, chun IEEE802.11n c xut v hin nay ang c pht trin.Chun ny, d kin s c thng qua vo khong cui nm2006 - u nm 2007, tip tc s dng OFDM lm cng nghnn tng, s cho tc d liu vt qu 100 Mbps p

ng cc yu cu mi.i tng chnh ca chun 802.11n l nhng sa i lp

vt l v lp iu khin truy cp mi trng (PHY/MAC) so vichun gc IEEE 802.11 c th h tr tc truyn dliu thp nht l 100 Mbps. Yu cu v tc ti thiu nhvy cho thy 802.11n c tc cao hn gn hai ln so vi

cc mng WLAN hin nay.

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3.1 Nhng gii php k thut ci thin tc truyn d liu trong mng WLAN

C nhiu gii php c a ra ci thin tc truyn d liu trong mng WLAN. Phn ny ch nu ra haitrong nhiu cc phng php ang c xut s dngtrong chun IEEE 802.11n. l ci thin tc truyn dliu vi h thng nhiu anten (MIMO) v Ci thin tc

truyn bng cch s dng k thut MIMO kt hp vi cc knhbng thng rng.

3.1.1 Ci thin tc truyn d liu vi h thngnhiu anten (MIMO)

Mt vn t ra vi vic nghin cu tng tc

truyn d liu ca cc h thng mng khng dy 802.11 ls dng cc h thng nhiu anten cho c my pht v mythu. Cng ngh c cp y c gi l cng nghnhiu u vo nhiu u ra (Multiple-input multiple-output- MIMO)

MIMO s dng phng thc pht nhiu tn hiu vo mi

trng v tuyn ri thu nhiu tn hiu t mi trng v tuynnhm ci thin hot ng v tuyn. Vi vic s dng nhiuanten, kt hp vi k thut chng nhiu u vo v chngnhiu u ra, MIMO c th tng hiu qu s dng ph mkhng lm mt i tnh n nh ca h thng. MIMO sdng nhiu anten khc nhau d tm cc knh ging nhau,

mi ln pht i vi cc c tnh khng gian khc nhau. Mi

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b phn s nghe tn hiu t mi trm pht ri tn dng tnha ng nng cao cht lng thu.

Mt c im khc ca cng ngh MIMO l s dng kthut ghp knh phn chia theo khng gian (Spatical DivisionMultiplering - SDM). SDM ghp v mt khng gian nhiulung d liu c lp mt cch ng thi (to thnh cc ccknh o) bn trong mt knh ph ca bng thng. K thutMIMO SDM c th lm tng ng k tc truyn d liu

do s lng cc lung d liu khng gian x l c tng ln. Milung khng gian i hi phi c cp anten thu/pht ti ucui truyn dn. iu quan trng y l k thut MIMOyu cu mt chui cc tn s v tuyn ring bit v mt bbin i tng t - s (ADC) cho mi anten MIMO. Cn cnhiu hn hai chui anten RF c thit k k lng c th

gim gi thnh v vn m bo c cc yu cu v hotng.3.I.2 Ci thin tc truyn bng cch kt hp MIMOvi cc knh bng thng rng

Mt phng php quan trng khc tng tc truyn vt l l s dng cc knh bng thng rng. Vic s

dng mt knh bng thng rng hn kt hp vi k thutOFDM s ci thin ng k cht lng hot ng ca mng,c bit khi c ng dng mng i hi bng thng ln,chng hn nh truyn video hoc game a phng tin. Ccknh bng thng rng c hiu qu cao hn trong qu trnhx l tn hiu s. Cc knh bng thng rng t c bng

mt k thut c gi l lin kt knh (Channel Bonding). t-

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ng v lin kt knh xut pht t c s nh lut Shannon,trong c th tng dung lng knh bng cch tng bngthng ca n. K thut ny lin kt hai knh ln cn thnhmt knh c bng thng rng hn. Gi s hai knh 20 MHzln cn trong mt h thng WLAN c kt hp li, bng thngmng IEEE 802.11 s tng ln gp hai ln, ngha l t 40MHz. Cu trc kt hp MIMO vi cc knh bng thng rngem li hiu qu truyn cao hn.

C hai cng ngh, MIMO v lin kt knh tng bngthng c s dng tho mn cc nhu cu thng lng caohn ca cc ng dng mi nh thoi v video. iu ny cng c ngha hn vi cc mi trng mng trong cc ng dngthng lng cao ng thi hot ng, chng hn nh va cng dng thoi, va c video hoc truyn hnh cht lng cao

qua WLAN.

3.2 Mc ch nghin cu ca IEEE 802.11n

Mt chun WLAN mi ang c IEEE nghin cu phttrin, vi mc tiu a kt ni khng dy bng thng rng lnmt tm cao mi. Cng ngh ny ha hn s tng ng k

tc ca cc mng cc b khng dy WLAN. l chunIEEE 802.11n ang c nhm cng tc 802.11n ca Vin kthut in t (IEEE) pht trin. Chun WLAN mi xutny da vo cng ngh MIMO-OFDM, h tr tc cao hnbng cch s dng hai anten mi u ca tn hiu (mt pht, mt thu), thay v mt anten mi u nhhin nay. V d v mt s thit b mng nh vy c th

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nghim, bao gm cc AP v cc card mng, c th thy trnhnh 3.1. Nhng th nghim ban u cho thy cc thit bcng ngh mi ny c th t tc ti thiu 135 Mbps vtc ti a ti 540 Mbps vi bng thng knh truyn l 40MHz.

Hnh 3.1: Hai anten mi u thitb gip chun 802.11n tng tc khng dy ln gp bn ln

xut chun 802.11n da trn cng ngh MIMO-OFDM

c thit k c th trin khai trn ton cu v tngthch ngc vi cc chun Wi-Fi hin ti nh 802.11b, 802.11av 802.11g. Bng cch s dng cng nh dng knh 20 MHzm hng triu thit b WLAN hin nay ang s dng, chunmi 802.11n c kh nng tng thch vi hu ht cc nn tngmng ci t trc, ng thi nng cao hiu sut ca cc

mng WLAN trong di tn s RF c phn b.Vi MIMO-OFDM, 802.11n cho phm vi ph sng WLAN

n nh hn vi tc truyn d liu siu nhanh (c tht ti 500 Mbps hoc hn na). Ngoi ra, chun 802.11n scho php ngi dng thc hin nhiu cng vic hn vi WLAN,c bit trong cc ng dng a phng tin.

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3.3 MIMO-OFDM3.3.1 Gii thiu

Truyn dn a ng l c im ca tt c cc mitrng truyn khng dy. Thng thng, c mt ng dn trctip t my pht ti my thu. Nhng trong thc t, c nhiu tnhiu truyn t my pht i theo cc ng khc nhau ti mythu, tn hiu c th n thng my thu hoc cng c thgp phi vt chn i hng, hoc b nh hng ca nhiu

v.v Tn hiu truyn theo nhng ng khng trc tip, timy thu mun hn tn hiu i theo ng trc tip v chngthng b suy yu hn so vi tn hiu truyn thng. Giiphp no c a ra gii quyt vn a ng - chnh l MIMO. MIMO khng loi b a ng, m ngc li, ntn dng hin tng ny hot ng (Hnh 3.2).

u tin, MIMO trong WLAN c thit lp da trn ccchun 802.11g v 802.11b, cho php truyn nhiu hn haitn hiu mng qua cng mt knh v tuyn 802.11 ti mtthi im m khng b nhiu sng [6].

Mt h thng vi nhiu anten my pht v my thu

chnh mt l h thng nhiu u vo nhiu u ra (MIMO).H thng MIMO c tnh a dng khng gian do c nhiuanten ring r trong mt mi trng nhiu ng phn tn. Hthng MIMO c th c thc hin theo mt s k thut no tn dng hin tng pha inh v tng hiu qu cngsut. C ba vn cn cp trong k thut MIMO. Th

nht, ci thin hiu qu cng sut lm tng tnh a dng

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Hnh 3.2: Mng MIMO c bn c thit lp trong mi trng a ng

My pht My thu

Vt cn

Vt cn

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khng gian. Th hai, s dng mt lp tip cn tng cngsut. Th ba, khai thc ni dung ca knh ti my pht. Khaithc ni dung knh my pht l phn tch c tnh knhang s dng t c cng sut pht hp l nht.

Trong cng ngh mng khng dy, MIMO c kt hp viOFDM to thnh cng ngh MIMO-OFDM. Vi cng ngh tnghp ny, OFDM c th kt hp vi cc dn anten ti my pht

v my thu tng tnh a dng v tng cng kh nng hthng bin thin theo thi gian v cc knh la chn tn s.Hnh 3.3 m t cu trc ca mt h thng MIMO-OFDM.Phn my pht ca h thng ny gm mt ngun d liu(Data Source), mt b m ho knh (Channel Encoder), mt bm ho MIMO (MIMO Encoder) v nhiu b iu ch OFDM

(OFDM Modulator)- mi b phc v cho mt an ten pht. Phnmy thu bao gm nhiu b gii iu ch OFDM (OFDMDemoulator) - mi b ly tn hiu t mt anten thu, mt bgii m MIMO (MIMO Demodulator), mt b gii m knh(Channel Decoder) v mt b tch d liu (Data Sink). Ngoira, trong my thu cn c b phn thc hin x l tn hiu cho

my thu.OFDM c chp nhn trong cc chun WLAN hin

nay l 802.11a, 802.11g ca IEEE (Hoa K) cng nh chunHyperLAN2 ca ETSI (Vin chun ca chu u). OFDM cng c s dng trong chun mng din rng khng dy IEEE802.16a v ang c xem xt trong chun IEEE802.20a, mt

chun nhm duy tr kt ni bng thng tc cao ti ngi s

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dng, tc c th ln ti 60 Mbps. Chun IEEE 802.11a LANhot ng tc d liu ti 54 Mbps (mt knh iu kincho php) vi khong cch knh l 20 MHz. Nh vy hiu qubng thng l 2,7 b/s/Hz. Thng lng thc t ph thuc nhiuvo giao thc iu khin truy cp mi trng truyn. Gingnh vy, IEEE802.16a hot ng trn nhiu ch ph thucvo iu kin knh trong phm vi d liu t 4,20 ti 22,91Mbps vi di thng in hnh l 6MHz, iu c ngha l

hiu qu di thng t 0.7 n3.82 bit/s/Hz.Nhng s pht trin gn y trong k thut MIMO ha

hn mt bc tin ln trong vic pht trin k thut OFDM. Hthng MIMO OFDM bng rng vi hiu qa di thng khong10b/s/Hz l kh thi vi mi trng LAN/MAN.

3.3.2 M hnh h thng MIMO-OFDMMt h thng a sng mang c th c thc hin hiu

qu bng cch dng IFFT nh mt b iu ch v FFT nh mtb gii iu ch. D liu c truyn l cc h s trongmin tn s v mu ti u ra l mu trong min thigian. Cu trc ca mt h thng MIMO-OFDM c thhin trn hnh 3.3.

Gi s t X={X 0,X1XN-1} l chiu di ca khisymbol d liu. Bin i Fourier tn s ri rc ngc ca X, tac dy min tn s l [3]:

{ } ( )n N k x IFFT X n=

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gim bt nhng tc ng ca knh tri tr, cn cmt khong bo v bao gm mt CP hoc mt hu t cni vi dy X. Trong trng hp l mt CP, dy c truyn vikhong bo v l:

( ) , ,... 1,0,1,... 1 g n N x x n n G N = =

trong , G l chiu di khong bo v trong mu, (n) N lphn cn li ca php ton n Modulo N.

Ngi ta thc hin a dy x g qua mt cp bin i t-ng t - s (ADC) thc hin phn thc v phn o vimt l l mu 1/Ts v tn hiu tng t I,Q c chuyn ithnh tn s sng mang RF. trnh ISI, chiu di khong CPphi bng hoc ln hn chiu di ca knh p ng xungthi gian ring bit M. Thi gian truyn mt symbolOFDM l: TS=N T+G T c gi l symbol thi gian OFDM. Tnhiu OFDM c truyn qua knh thi gian RF, sau khi nhnc n li c chuyn i ngc xung bng gc. Vi CP, onxon ring bit ca dy truyn vi nhng xung knh hi ps tr thnh mt vng xon. Do vy, ti my thu, nhng muG u tin nhn c s c b i, tip theo l N: im bini DFT trn dy kt qu.

Cu trc khung ca mt h thng MIMO OFDM tiu biuc trnh by trn hnh 3.4.

Phn preamble gm nhng chui symbol Q c chiu diNI+G trong G

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Trc tin ta xem xt n phn preamble ca khungOFDM. Chiu di NI+G ca dy ng b u tin thu cbng vic kch tt c cc h s th I ca on N- vectmin tn s vi mt chui symbol khc khng t mt bngch ci c chn. Dy tn hiu dn ng min - tn s ctruyn t anten th i l:

( ){ }1

N q

k k

S =

Trong : q = (c-1)Q+i v c =1,2,Q.Gi h s thi gian-min u tin v loi b mi ci cn

li. Mt CP c ni ti mi on N I dy min thi gian. t H ij lvect ca cc knh con c h s gia vic pht ca anten

th i v thu ca anten th j v t ( ){ }1

0

I N l k

k R

=l dy mu thu c

ti an ten thu th l. Sau khi loi b khong phng v, mu thu

c { }1

0

I N l k k

R

=c nhc li I ln v iu ch s dng N-im

FFT cho kt qu nh sau:

( ) ( ){ }( ) ( ) ( ) ( ),1Q

l l q l q l k N k k k

q R FFT r k H S W

== = +

trong k = 0,1,N-1Ma trn gii iu ch OFDM Rk c kch thc (Q L) ca

sng mang th k c th c dng trong vic truyn ma trnmu Sk c kch thc (QxQ), ma trn h s knh H k kch thc

(QxL) v ma trn tp m trng chun W K kch thc (QxL) l:

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Hnh 3.4: cu trc khung h thng QxLOFDM

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, , , ,.k Q L k Q Q k Q L k Q L R S H W = +

Trong R,H,W c th c coi nh l mt tp hp ca matrn N c kch thc (QxL) hoc l mt tp cc vect caon N.

3.3.3. c lng knh MIMO-OFDM Thng tin trng thi knh c i hi trong MIMO-OFDM

vi m khng gian-thi gian ti my pht v nhn dng tn hiuti my thu. chnh xc ca n nh hng trc tip ti victhc hin ca ton b h thng MIMO-OFDM. Phn ny s giithiu mt vi cch tip cn vi c lng knh OFDM.

c lng knh cho OFDM c th khai thc mi tng quangia thi gian v tn s ca nhng tham s knh. V mt hthng MIMO c Q anten truyn, tn hiu t mi anten thu tisng mang con th k ca khi OFDM th n c th c biuth [11]:

( ) ( ), , , ,

1

Qq q

n k n k n k n k q

R H X W =

= +

Trong H (q)n,k l s hi p li tn s knh ti sng

mang con th k ca khi OFDM th n tng quan tng ng vivic truyn anten th q, v W n,k l tp m tng chun cthm vo. Thch thc vi c lng knh MIMO l mi tn hiu thuc tng ng vi vi tham s knh.

Khi p ng knh ti cc tn s khc nhau c tngquan, tham s knh ti cc sng mang con khc nhau c th

c biu th:

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( ) ( )01

, ,0

N q q km

nk nm N m

H h W

=

=

Vi k = 0,1,n-1 v q = 1,,Q. Tham s N 0 ph thucvo qung tr ca knh khng dy v khong thi gian

symbol OFDM, v ( )( )2 / N W exp j N = .

Do , tn ti H (q)n,k chng ta ch cn c lng H (q)n,m.Nu tn hiu truyn X(q)n,k t anten truyn th q c bit viq = 1,,Q 2, v h (q)n,k, mt c lng thi gian ca h (q)n,m, c th -

c tm thy bng vic ti gin hm gi tr sau:

( ) ( )0

211

, , ,0 1 0

N Q N q qkm

n k n m K n k k q m

R h W x

= = = %

Tnh ton trc tip a li kt qu sau:( ) ( )

( ) ( )

( )

( )

( )

( )

11 1 1 1

1

Qn n n n

Q QQ Q Qn n n n

A A h b

A A h b

=

%L

M L L M M%L

Trong ( )qnh% l c lng tm thi ca vc t tham s knh

( ) ( ) ( )( )0,0 , 1,...,T q q q

n n n N h h h =% % %

V ( ) ( ) ( ) ( ), ,, , ,ij ij i in m n n m na A b b c nh ngha nh sau:

( ) ( ) ( )1

, , ,0

N ij i j km

n m n k n k N k

a x x W

==

( ) ( )( ) 01 21 2

1

,, 0

N ij ij

n n m mm m

A a

==

( ) ( )1

, , ,

0

N i i km

n m n k n k N

k

b r x W

=

=

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Hnh 3.5 minh ho s khi c lng knh c bn choh thng MIMO-OFDM vi hai anten truyn. tnh ton c l-

ng knh tm thi, cn c mt ma trn nghch o 2N 0x2N0

ca ( )1,n mh v ( )2,n mh .

Ni chung, mt ma trn nghch o QN 0xQN0 l cnthit vi h thng MIMO-OFDM c Q anten truyn, iu cng c ngha l qu trnh tnh ton rt phc tp.

Mc ch thit k c lng knh l nhm gim phctp ca thit b trong khi vn m bo c chnh xctheo yu cu.

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Hnh 3.5: c l ng tham s knh c bn cho h thng MIMOvi hai anten truyn

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3.3.4 Mt s ng dng th nghim ca MIMO-OFDM trong WLAN

Hin nay c kh nhiu th nghim ng dng cngngh MIMO-OFDM, di y l mt s ng dng in hnh.

Cng ngh MIMO cho my tnh xch tay My tnh xch tay kt ni khng dy tr nn

thng dng trong thi gian gn y, nhng y l ln utin cng ngh MIMO, c tch hp trong sn phm gip tng

phm vi v tc truy cp.Hin nay, xut hin nhng chip MIMO th nghim c

th tng tc truyn d liu trong mng khng dy ln ti200 Mbps v phm vi hot ng l 180 n 275 mt, trongkhi phm vi trung bnh ca chun 802.11g (Wi-Fi) ch t t45 n 90 mt vi tc ti a 54 Mbps.

V d v dng chip mi ny c th k n chip s dngcng ngh MIMO ca hng Airgo (M) dng th nghim trongcc laptop. Chp ny cho php s dng hai b thu pht vtuyn, trong khi a phn cc my tnh xch tay kt ni Wi-Fihin nay (tch hp dng chip Centrino ca Intel) ch chophp s dng mt b thu pht v tuyn. S dng nhng chip

MIMO c th trnh c hin tng rt tn hiu thng xy ratrong cc mng WiFi hin nay.

True-MIMO True MIMO l th h mi ca cng ngh MIMO. True

MIMO c nhiu hng sn xut thit b khng dy sdng, trong c Belkin, Netgear, Sohoware. Phin bn mi

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nht hot ng vi tc 240 Mbps, d dng vt qua chunWi- Fi (54 Mbps) v Ethernet (100 Mbps).

Mng c dy truyn thng kt ni cc thit b vi nhauv ti Internet vn lun c coi l phng php nhanh v ngtin cy nht hin nay. Ngay c cc mng khng dy tc cao, da trn chun Wi-Fi, d tr nn ph bin trongnhng nm gn y vn b nh gi l chm v khng anton so vi mng c dy truyn thng. Tuy nhin, phin bn

cng ngh mi vi MIMO ph v ro cn v tc v thayi li suy ngh so mn ch tin tng vo h thng dydn. Nhng sn phm u tin s dng chip True MIMO nhmy tnh xch tay, th khng dy, thit b nh tuyn, myin, cng ngoi v.v... c mt trn th trng vo quIV/2005.

Trong bui trnh din cng ngh u nm 2006, hthng True MIMO ca hng Airgo truyn khng dy mtb phim DVD ti gia cc my tnh xch tay, t cch nhauvi phng vi nhng bc tng lm bng m khng h bmt, nhiu hay gim tn hiu. Vi phin bn mi True MIMO,kh nng truyn cn cao hn th v cho php tng thch vi

mi thit b gia dng Wi-Fi hin nay nh camera s, thit bc nhn v h thng loa v.vBn cnh , vic ci tincng ngh cng gip gim chi ph sn xut. V d, thit bnh tuyn khng dy s dng MIMO hin nay ch c gi d-i 200 USD.

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Tm li, trong chng ny ta cp n mt s kthut MIMO-OFDM, trong c m hnh h thng MIMO-OFDM, c lng knh OFDM Ngoi ra, mt s ng dng thnghim ca MIMO-OFDM trong cc h thng mng WLANcng c trnh by trong chng ny.

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Chng 4

Nhng nghin cu tip theo v OFDM trong WLAN

4.1 Nhng yu cu nghin cu mi v OFDM trong mngkhng dy

OFDM c s dng thnh cng trong mt s h thngtruyn thng nh Qung b audio s (DAB), Qung b video s(DVB) v trong truyn thng t im ti im. OFDM cngc coi l mt cng ngh c bn trong thng tin di ng thh th t (4G). OFDM cung cp mt giao din rt linh hot,thm vo , bn cht a sng mang ca OFDM cho phpcc knh v tuyn c m t v kim sot nhanh chng ddng, to ra cc c hi ti u ho thc s ton b hthng. Vic ti u ho h thng bao gm:

- Vic cp pht sng mang con gim thiunhng hiu ng tiu cc ca pha inh chn lc tn s.

- To iu kin cho vic cp pht nhng s iu ch trn mt sng mang con ring l ph hp vi iu kin hin thi.

- Thay i bng thng ca mi ngi dng davo cht lng ca lin kt. iu ny cho php bngthng ca nhng ngi s dng yu c gim i, bivy nng lng cn li ln duy tr truyn thng.

Mc d c nhiu u im nh vy, nhng s dng OFDM chotruyn thng hai chiu hoc cho nhng ng dng a ngidng vn cn nhiu tr ngi. Nhng ng dng ny bao gm:

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mng cc b khng dy, mch vng cc b khng dy(Wireless Local Loop - WLL), in thoi di ng v Internet ding tc cao. Cn xem xt p dng OFDM vi nhng hthng nh th no, c lng nh gi nhng li th v ktqu t c nh th no. Chng ny s gii thiu mt s kthut mi c th c s dng ci thin h thng OFDMa ngi dng. Cc k thut ny gp phn ci thin hiu quph v cht lng dch v (QoS). Cc k thut ny bao gm:

k thut iu ch thch nghi a ngi dng v k thut nhv sng mang con a ngi dng.

4.2 Cc k thut thch ng cho OFDM nhiu ngi dng

4.2.1 K thut iu ch thch nghi iu ch thch nghi l mt k thut mnh trong vic

lm cc i thng lng d liu ca cc sng mang con ccp pht cho mt ngi s dng. iu ch thch nghi thchin o t s tn hiu trn tp (SNR) ca mi sng mang controng khi truyn, sau la chn mt s iu ch lm cc i hiu qu ph trong khi vn duy tr t t li bit(BER) mc c th chp nhn c. K thut ny c s

dng trong cng ngh bng thng rng ADSL (AsymmetricalDigital Subscriber Line - ng thu bao s bt i xng) lm cc i lu lng h thng. ADSL s dng ng truyn dnOFDM qua nhng cp in thoi ng. p ng tn s knhca cp ng l mt hng s tng i v nh vy s phn bli ca s iu ch l khng cn thit [4]. Hnh 4.1 lmt v d v ng dng ca iu ch thch nghi trong mt

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sng mang khi t s tn trn tp ca knh bin i theo thigian.

Vic s dng iu ch thch nghi trong mi trngkhng dy gp nhiu kh khn do p ng knh v t stn hiu trn tp (SNR) c th thay i nhanh chng, ihi phi thng xuyn cp nht theo di s bin i ny.

iu ch thch nghi cha c s dng rng ri trongnhng ng dng khng dy bi v rt kh theo di c hiu

qu knh v tuyn. Do vy, ngi ta mi ch dng li mcnghin cu vic s dng iu ch thch nghi trong nhngh thng n sng mang.

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Phn ny a ra mt phc tho v iu ch thch nghiv nh v ngi dng thch nghi, m trng tm l lm cci tc d liu s dng v cht lng dch v. ngin h thng, hu ht cc h thng OFDM hin nay s dngmt s iu ch c nh cho tt c cc sng mang con.

Tuy nhin, mi sng mang con trong h thng OFDM a ngidng c th s dng nhiu s iu ch khc nhau ph

thuc vo iu kin knh. Trong mt knh v tuyn a ng, pha inh chn lc

tn s c th dn n mt s bin i ln trong cng sutnhn c ca mi sng mang con. Vi mt knh c ng dntn hiu khng trc tip, s bin i ny c th xp x 30dB kt qu cng sut nhn c. Hn na, giao thoa t

nhng ln cn c th l nguyn nhn gy ra thay ing k trong h thng bng thng. i ph vi s bini ln v t s tn hiu trn nhiu (SNA) qua h thngsng mang con, ngi ta c th nh v thch nghi s iu ch sng mang con. Do vy, hiu qu s dng ph cth c lm cc i trong khi vn duy tr mt t l li bit

(BER) c th chp nhn c.SNR phi ln hn ngng c chn trong bng di y

duy tr mt t l BER ln nht. T hnh 4.1 trn cho thyrng cc s iu ch c cp nht lin tc.

Bng 4.1: Yu cu i vi SNR duy tr mt t l BER di

mt mc ngng a ra

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Hnh 4.1: iu ch thch nghi. S iu ch c t c bn trn SNR

ca knh

S iuch

T l SNR(dB)

Hiu quph

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Trong nhng h thng s dng mt s iu ch cnh, vic iu ch sng mang con phi c t chc saocho c th t c mt t l BER c th chp nhn ctrong nhng iu kin knh xu nht. iu ny c thchin trong hu ht cc h thng s dng BPSK hoc QPSK.

Vi nhng trm t xa, c th s dng mt s iuch cao hn khi iu kin knh v tuyn l tt. Nu mt

trm t xa kt ni ti mt trm gc th vic s dng iuch thch nghi c th tng ln s iu ch c th tng t 1b/s/Hz (i vi s BPSK), t 4 n 8 b/s/Hz (i vi s 16-QAM, 256-QAM), v lm tng thm mt cch ng khiu qu ph cho ton h thng. S dng iu ch thchnghi c th kim sot c hiu qu BER ca knh truyn.

Nhng hn ch ca iu ch thch nghi iu ch thch nghi vn cn tn ti mt s hn ch cn

nghin cu khc phc, bao gm: Th nht l c s cn bnggia vic iu khin cng sut v iu ch thch nghi.Nu mt trm t xa c knh truyn tt th cng sut truyn

c th c duy tr v s iu ch mc cao c s dng(64-QAM) ngc li cng sut c th b gim v s iuch cng gim mc i. Th hai l cp n s bindng: li tn s v s bin i cng sut cc i cho phpgia nhng ngi s dng gii hn s iu ch cc i cth c dng. Cng sut nhn c vi cc sng mang ln cn

phi c s bin i khng ln hn 20 n 30 dB ti trm

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