Damh1 Mc Cdma

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Tim hieu He thong Thong tin di dong dung ky thuat MC CDMA

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LI NI UTrong x hi hin i ngy nay, nhu cu trao i thng tin l mt nhu cu thit yu. Cc h thng thng tin di ng ra i to cho con ngi kh nng thng tin mi lc, mi ni. Nhu cu ny ngy cng ln nn s lng khch hng s dng thng tin di ng ngy cng tng, cc mng thng tin di ng v th c m rng ngy cng nhanh. Chnh v vy, cn phi c cc bin php tng dung lng cho cc h thng thng tin di ng hin c. H thng CDMA ra i v chng t c kh nng h tr nhiu user hn so vi cc h thng trc . Hn na, so vi hai phng php a truy nhp truyn thng l phn chia theo tn s FDMA v phn chia theo thi gian TDMA th phng php truy nhp phn chia theo m CDMA c nhng c im ni tri: chng nhiu a ng, c tnh bo mt cao, h tr truyn d liu vi tc khc nhau Tuy nhin, trong tng lai, nhu cu v cc dch v s liu s ngy cng tng, mng thng tin di ng khng ch p ng nhu cu va i va ni chuyn m cn phi cung cp cho ngi s dng cc dch v a dng khc nh truyn d liu, hnh nh v video Chnh v vy, vn dung lng v tc cn phi c quan tm. Trong nhng nm gn y, k thut ghp knh theo tn s trc giao OFDM (Orthogonal Frequency Division Multiplexing), mt k thut iu ch a sng mang, c s dng rng ri trong cc ng dng v tuyn cng nh hu tuyn. u im ca OFDM l kh nng truyn d liu tc cao qua knh truyn chn lc tn s, tit kim bng thng, h thng t phc tp do vic iu ch v gii iu ch a sng mang bng gii thut IFFT v FFT. p ng nhu cu ngy cng cao ca ngi s dng, tng v k thut MC-CDMA ra i, da trn s kt hp ca CDMA v OFDM. MC-CDMA k tha tt c nhng u im ca CDMA v OFDM: tc truyn cao, tnh bn vng vi fading chn lc tn s, s dng bng thng hiu qu, tnh bo mt cao v gim phc tp ca h thng. Chnh v vy, MC-CDMA l mt ng c vin sng gi cho h thng thng tin di ng trong tng lai.

Nhim v ca ti: Tm hiu mng thng tin di ng s dng k thut MC-CDMA (Multicarrier Code Division Multiple Access).

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TM HIU H THNG THNG TIN DI NG DNG K THUT MC CDMALi ni u Chng 1: GII THIU CHUNG 1.1 Khi qut v H thng di ng t bo 1.2 S pht trin ca H thng thng tin di ng Th h th nht (1G) Th h th hai (2G) Th h th 3 (3G) Con ng i ln 3G ca Vit Nam 1.3 Knh truyn v tuyn v cc hin tng nh hng n cht lng knh truyn 1.3.1 Cc hin tng nh hng n cht lng knh truyn a. Hiu ng a ng (Multipath) b. Hiu ng Dopler c. Hiu ng bng rm (Shadowing) 1 1 3 3 4 4 4 5 5 5 6 6

1.3.2 Cc dng knh truyn 6 a. Knh truyn chn lc tn s v Knh truyn Fading phng 6 b. Knh truyn chn lc thi gian v Knh truyn khng chn lc thi gian 8 1.4 Cc k thut a truy nhp 1.4.1 Gii thiu chung 1.4.2 FDMA : a truy nhp phn chia theo tn s 1.4.3 TDMA : a truy nhp phn chia theo thi gian 1.4.4 CDMA : a truy nhp phn chia theo m Chng 2: K THUT A TRUY NHP PHN CHIA THEO M (CDMA) 2.1 Gii thiu 2.2 K thut tri ph 2.2.1 Gii thiu chung 2.2.2 Chui nh phn gi ngu nhin (PRBS) 2.2.3 K thut tri ph bng cch phn tn ph trc tip (DS SS) 2.2.4 K thut tri ph bng phng php nhy tn s (FH SS) 2.2.5 K thut tri ph bng phng php nhy thi gian (TH SS) 2.2.6 Cc chui tri ph 2.3 Nguyn l CDMA 2.4 My thu RAKE 9 9 10 11 12 13 13 13 15 16 19 22 23 24 25

Tm hiu H thng TTD dng k thut MC-CDMA

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2.5 iu khin cng sut 2.5.1 Hiu ng Gn Xa 2.5.2 iu khin cng sut 2.6 Chuyn giao (Handoff) 2.6.1 Chuyn giao mm 2.6.2 Chuyn giao cng 2.7 H hng DS CDMA 2.8 W CDMA (Wideband Code Division Multiple Access) Chng 3: MC CDMA (Multi Carrier Code Division Multiplexing Access) 3.1 OFDM (Orthogonal Frequency Division Multiplexing) 3.1.1 Gii thiu 3.1.2 Nguyn tc c bn ca OFDM 3.1.3 Tnh trc giao 3.1.4 H thng OFDM 3.1.5 Chng nhiu lin k hiu (ISI) bng cch s dng khong bo v 3.1.6 u, khuyt im ca OFDM 3.1.7 Kt lun 3.2 MC CDMA 3.2.1 Gii thiu 3.2.2 Nguyn l chung ca k thut MC CDMA 3.2.3 My pht v my thu MC CDMA theo m hnh 1 3.3 u, khuyt im ca h thng TTD dng k thut MC CDMA 3.3.1 u im 3.3.2 Khuyt im 3.4 S khc nhau gia MC CDMA v OFDMA Kt lun v hng pht trin ti Cc ch vit tt Ti liu tham kho

26 26 27 27 28 28 29 29 31 31 31 32 33 36 38 40 40 41 41 42 43 45 45 45 46 47 48 50

Tm hiu H thng TTD dng k thut MC-CDMA

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CHNG 1:

GII THIU CHUNG

1.1 KHI QUT V H THNG DI NG T BO Thay v dng cc my pht sng v tuyn cng sut cc ln cung cp cc dch v thng tin di ng cho mt lng ln users trong mt vng rng ln th thng tin t bo chia cc vng a l thnh cc (cell), mi s dng my pht sng cng sut nh hn v cc b iu khin c gi l trm gc (BS Base Station). Cc ny kt hp li to thnh mng t bo nh hnh 1.1.

Cc trm gc (BS) c iu khin bi Trung tm chuyn mch di ng (MSC Mobile Switching Center), cc MSC ny li c iu khin bi Mng in thoi chuyn mch cng cng (PSTN Public Switched Telephone Network).

Hnh 1.1. H thng di ng t bo Tm hiu H thng TTD dng k thut MC-CDMA Trang 1

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Trong h thng thng tin di ng t bo, tn s m cc my di ng s dng l khng c nh mt knh no m thay i ty theo v tr ca user. Mi c gn cho mt nhm tn s. V vy, cc k cn nhau s dng cc nhm tn s khc nhau trnh nhiu ng knh. Cn cc cch xa mt khong cch nht nh th c th s dng li nhm tn s (Frequency Reuse). Nh vy m dung lng ca h thng c th c tng ln m khng cn tng thm di tn.

iu ny c minh ho trong hnh 1.2. Ta nhn thy c mt nhm 7 cells tn s c s dng li. Nhm 7 cells ny c gi l 1 cluster. V d : cell th nht (c nh s 1) ca cluster ny s s dng cng tn s vi cell 1 ca cluster khc.

Hnh 1.2. S dng li tn s vi cluster gm 7 cells. Khi user di chuyn t cell ny qua cell khc trong khi ang thc hin cuc gi th MSC s thc hin nh tuyn li cuc gi khng lm gin on cuc gi. Qu trnh ny gi l chuyn vng (Handoff). Nhng u im ca H thng thng tin di ng t bo: + Do s dng hiu qu di tn s m dung lng ca h thng tng rt nhiu. + Cht lng ca h thng c ci thin do kh nng chng nhiu ng knh (CCI Co-Channel Interference). + Tng cht lng truy cp v chuyn vng gia cc cell.

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1.2 S PHT TRIN CA H THNG THNG TIN DI NG Thng tin di ng pht trin qua cc th h khc nhau.

3th Generation (3G) (2000s) Digital Voice, Internet, Media 2nd Generation (2G) (1990s) Digital 1stGeneration (1G) (1980s) Analog Voice AMPS, NMT Voice & Data GSM, IS-95, PDC W-CDMA , CDMA2000, IMT-2000

Th h th nht 1G : + Cc h thng thng tin di ng 1G c xy dng t nhng nm 80 V d: NMT (Nordic Mobile Telephone) ca cng ty Ericsion, Thy in. AMPS (American Mobile Phone System) ca cng ty AT&T, M + Da trn cng ngh analog, dng k thut a truy cp phn chia theo tn s (FDMA Frequency Division Multiple Access). + Cung cp nhng dch v ch yu l thoi. + Cc h thng di ng 1G c pht trin trong phm vi Quc gia, do khng c kh nng tng thch ln nhau. Do yu cu thng tin di ng ngy cng cao, hn th na l nhu cu phi c mt h thng thng tin di ng ton cu. V vy, h thng thng tin di ng th h th 2 (2G) ra i.

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Th h th hai 2G : + c pht trin ngay trong thp nin 90. + Da trn cng ngh s, dng k thut a truy cp phn chia theo thi gian (TDMA Time Division Multiple Access) v a truy cp phn chia theo m (CDMA Code Division Multiple Access). + Theo quan im ngi s dng, h thng 2G hp dn hn h thng 1G bi v ngoi dch v thoi truyn thng, h thng 2G cn cung cp thm mt s dch v truyn d liu, tuy tc cn thp. + Cha thc hin c h thng thng tin di ng ton cu, do trn th trng tn ti mt s h thng di ng 2G nh GSM (Global System for Mobile Communication), IS-95 (Interim Standard 95), PDC (Personal Digital Celular) Trong , GSM c s dng rng ri nht. n nhng nm 2000, h thng thng tin di ng th h th 3 (3G) ra i vi mc tiu hnh thnh mt h thng thng tin di ng duy nht trn ton th gii. + Da trn cng ngh s vi s khng nh u th vt tri ca CDMA . + C kh nng cung cp nhng dch v c tc khc nhau nh thoi, Internet tc cao, truyn hnh nh cht lng cao, nhn tin a phng tin (MMS) + Cc chun cho 3G: IMT-2000, CDMA2000, W-CDMA H thng di ng 3G cha c p dng rng ri, nhng cc nghin cu v h thng 4G, m cng ngh ch yu l cc k thut a sng mang, c tin hnh v MC-CDMA l mt ng c vin sng gi. V vy, vic tm hiu v H thng thng tin di ng dng k thut MC-CDMA l cn thit v mang ngha thc t. Con ng i ln 3G ca Vit Nam: Hin nay, cc nh cung cp dch v di ng chnh Vit Nam l Mobifone, Vinaphone, S-Fone v Viettel. Con ng i ln 3G t cc cng ngh khc nhau u c: cc nh khai thc GSM s i ln W-CDMA, cn cc nh cung cp s dng cng ngh CDMA s tin ln CDMA2000. By gi ch cn vic xc nh thi im trin khai cho ph hp. Nm 2004, Ericsson cng Mobifone th nghim thnh cng dch v di ng 3G. Vo cui nm nay, nh cung cp ny d nh s a ra dch v 2,5G hay cn gi l EDGE. Trong khi , Vinaphone v Viettel vn ang s dng cng ngh thuc th h th 2 (2G). Tm hiu H thng TTD dng k thut MC-CDMA Trang 4

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Tnh n nay, S-Fone l nh cung cp u tin v duy nht s dng cng ngh CDMA. Chun m S-Fone ang s dng l CDMA2000 1X, chun ny ch cch chun 3G CDMA 2000 1X Evdo mt khong khng xa. V vy, S-Fone s c kh nng tin nhanh hn trn con ng hng ti 3G. Mc d hin nay a s thu bao di ng nc ta cha c nhiu nhu cu g khc hn ngoi nhu cu m thoi di ng, nhng theo tuyn b ca cc nh u t th u nm sau (2006), mng 3G ca Vit Nam s c trin khai.(theo www.vtv.vn/vi-vn/VTV1/cuocsongso/2005/12/73939.vtv)

1.3 KNH TRUYN V TUYN Cht lng ca cc h thng thng tin ph thuc nhiu vo knh truyn, ni m tn hiu c truyn t my pht n my thu. Khng ging nh knh truyn hu tuyn l n nh v c th d on c, knh truyn v tuyn l hon ton ngu nhin v khng h d dng trong vic phn tch. Tn hiu c pht i, qua knh truyn v tuyn, b cn tr bi cc to nh, ni non, cy ci , b phn x (reflection), tn x (scattering), nhiu x (diffraction), cc hin tng ny c gi chung l Fading. V kt qu l my thu, ta thu c rt nhiu phin bn khc nhau ca tn hiu pht. iu ny nh hng n cht lng ca h thng thng tin v tuyn. 1.3.1 Cc hin tng nh hng n cht lng knh truyn: a. Hiu ng a ng (Multipath) Nhiu a ng l kt qu ca s phn x, tn x, nhiu x ca tn hiu trn knh truyn v tuyn. Cc tn hiu c truyn theo cc ng khc nhau ny u l bn sao ca tn hiu pht i nhng b suy hao v bin v b tr so vi tn hiu c truyn thng (Line of Sight). Tn hiu thu c ti my thu l tng ca cc thn