Tim Hieu He Thong Mimo Ofdm

7/29/2019 Tim Hieu He Thong Mimo Ofdm

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i hc quc gia TP. H Ch Minh- i hc Khoa hc t nhinKhoa in t vin thng

TIMN HC: THNG TIN DI NG

GVHD: Thy Trng Tn Quang

Nhom thc hien:

1. Nguyen Huynh Thanh Thao 06200722. Nguyen Th Thu Thuy 0620070

3. Nguyen Thanh Thoai Van 0620116

Thanh pho Ho Ch Minh, ngay 10 thang 01 nam 2010

Mc lc

Chng 1: H thng MIMO 21.1. Cc m hnh h thng thng tin khng dy 21.2. Khi nim v h thng MIMO 31.3. K thut phn tp 41.4. Cc li trong h thng MIMO 6


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Chng 2: K thut OFDM 82.1. Tng quan v k thut OFDM 82.2. u im v nhc im ca k thut OFDM 92.3. M hnh h thng OFDM 9

Chng 3: H thng MIMO-OFDM 14

3.1. Gii thiu 153.2. M hnh h thng MIMO-OFDM 15

Chng 1: H th ng MIMO

1.1. Cac mo hnh he thong thong tin khong day:

Cac mo hnh he thong thong tin khong day co the c phan loai thanh bon he thong

c ban la SISO, SIMO, MISO, va MIMO nh hnh 1.1

Tm hiu h thng MIMO-OFDM Trang 2


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Hnh 1. Phan loai he thong thong tin khong day

1.1.1. He thong SISO:He thong SISO la he thong thong tin khong day truyen thong ch s dung mot anten

phat va mot anten thu. May phat va may thu ch co mot bo cao tan va mot bo ieu che, giaiieu che. He thong SISO thng dung trong phat thanh va phat hnh, va cac ky thuat truyen

dan vo tuyen ca nhan nh Wi-Fi hay Bluetooth. Dung lng he thong phu thuoc vao t so tn

hieu tren nhieu c xac nh theo cong thc Shanon:C = log2(1+SNR) bit/s/Hz

1.1.2. He thong SIMO:

Nham cai thien chat lng he thong, mot pha s dung mot anten, pha con lai s

dung a anten. He thong s dung mot anten phat va nhieu anten thu c goi la he thong

SIMO. Trong he thong nay may thu co the la chon hoac ket hp tn hieu t cac anten thunham toi a ty so tn hieu tren nhieu thong qua cac giai thuat beamforming hoac MMRC

( Maximal- Ratio Receive Combining). Khi may thu biet thong tin kenh truyen, dung lng

he thong tang theo ham logarit cua so anten thu, co the xap x theo bieu thc:

C = log2(1+N.SNR)

1.1.3. He thong MISO:

He thong s dung nhieu anten phat va mot anten thu c goi la he thong MISO. He

thong nay co the cung cap phan tap phat thong qua ky thuat Alamouti t o cai thien lng

tn hieu hoac s dung Beamforming e tang hieu suat phat va vung bao phu. Khi may phatbiet c thong ti kenh truyen, dung lng he thong tang theo ham logarit cua so anten phat

va co the c xac nh gan ung theo cong thc :

C = log2(1+N.SNR)1.1.4. He thong MIMO:



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He thong MIMO la he thong s dung a anten ca ni phat va ni thu. He thong co the

cung cap phan tap phat nh a anten phat, cung cap phan tap thu nh vao a anten thu nham

tang chat lng he thong hoac thc hien Beamforming tai ni phat va ni thu e tang hieu

suat s dung cong suat, triet can nhieu. Ngoai ra dung lng he thong co the cai thien ang

ke nh vao o li ghep kenh cung cap bi ky thuat ma hoa khong gian_thi gian V-BLAST.

Khi thong tin kenh truyen c biet tai ca ni phat va thu, he thong co the cung cap o liphan tap cc cao va o li ghep kenh cc ai, dung lng he thong trong trng hp phan

tap cc ai co the xac nh theo:

C = log2(1+NT.NR.SNR)

Dung lng he thong trong trng hp at o li ghep kenh cc ai co the xac nh

theo bieu thc:

C = min(NT,NR). log2(1+SNR)

Vi cac u iem ve hieu suat, triet can nhieu, dung lng va chat lng he thong

MIMO ang c nghien cu e ng dung vao cac he thong thong tin tng lai. Tuy nhienhe thong MIMO khong co kha nang chong lai fading chon loc tan so, v vay ky thuat ket hp

gia MIMO va OFDM cung ang c nghien cu trong cac chuan khong day nh chuan

IEEE 802.11n (WLAN), IEEE 802.16e (WIMAX).

1.2. Khai niem ve he thong MIMO:

He thong MIMO (Multiple Input Multiple Output) c nh ngha la tuyen thong tin

iem-iem vi a anten tai pha phat va pha thu. Nhng nghien cu gan ay cho thay he

thong MIMO co the tang ang ke toc o truyen d lieu, giam BER, tang vung bao phu hethong vo tuyen ma khong can tang cong suat hay bang thong he thong. Chi ph phai tra e

tang toc o truyen d lieu chnh la viec tang chi ph trien khai he thong anten, khong gian

can thiet cho he thong cung tang len, o phc tap cua he thong x ly so tn hieu nhieu chieu

cung tang len.

1.3. Ky thuat phan tap:

Trong truyen thong khong day di dong, ky thuat phan tap c s dung rong rai e

lam giam anh hng cua fading a ng va cai tien o tin cay cua kenh truyen ma khongyeu cau tang cong suat phat hoac tang bang tan can thiet. Ky thuat phan tap yeu cau nhieu

ban sao tn hieu phat tai ni thu, tat ca mang cung mot thong tin nhng co s tng quan rat

nho trong moi trng fading. Y tng c ban cua phan tap la neu ni thu nhan hai hay nhieu

ban sao cua tn hieu mot cach oc lap th nhng mau nay b suy giam cung oc lap vi nhau.



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ieu nay co ngha la khi mot ng tn hieu cu the b suy giam th ng tn hieu khac co

the khong b suy giam. V vay, s ket hp hp ly cua cac phien ban khac nhau se lam giam

anh hng cua fading va cai thien o tin cay cua ng truyen.

Co nhieu cach e at c phan tap. Phan tap thi gian co the thu c qua ma hoa

(Coding) va xen kenh (Interleaving), phan tap tan so neu ac tnh cua kenh truyen la chon

loc tan so, phan tap khong gian s dung nhieu anten phat hoac thu at cach nhau vi khoangcach u ln.

1.3.1 Phan tap khong gian :

Phan tap khong gian con goi la phan tap anten. Phan tap khong gian c s dung

pho bien trong truyen thong khong day dung song viba. Phan tap khong gian s dung nhieu

anten hoac chuoi array c sap xep trong khong gian tai pha phat hoac pha thu. Cac anten

c phan chia nhng khoang cach u ln sao cho tn hieu khong tng quan vi nhau.

Yeu cau ve khoang cach gia cac anten tuy thuoc vao o cao cua anten, moi trng lan

truyen va tan so lam viec. Khoang cach ien hnh khoang vai bc song la u e cac tnhieu khong tng quan vi nhau. Trong phan tap khong gian, cac phien ban cua tn hieu phat

c truyen en ni thu tao nen s d tha trong mien khong gian. Khong giong nh phan

tap thi gian va tan so, phan tap khong gian khong lam giam hieu suat bang thong. ac tnh

nay rat quan trong trong truyen thong khong day toc o cao trong tng lai.

Trong phan tap anten thu, nhieu anten c s dung ni thu e nhan cac phien ban

cua tn hieu phat mot cach oc lap. Cac phien ban cua tn hieu phat c ket hp mot cach

hoan hao e tang SNR cua tn hieu thu va lam giam bt fading a ng.

Trong he thong thc te, e at c BER cua he thong theo yeu cau, ta ket hp haihay nhieu he thong phan tap thong thng e cung cap s phan tap nhieu chieu (multi-

demnsional diversity).

1.3.2. Phan tap tan so:

Trong phan tap tan so, s dung cac thanh phan tan so khac nhau e phat cung mot

thong tin. Cac tan so can c phan chia e am bao b anh hng cua fading mot cach oc

lap. Khoang cach gia cac tan so phai ln hn vai lan bang thong nhat quan e am bao

rang fading tren cac tan so khac nhau la khong tng quan vi nhau. Trong truyen thong di

ong, cac phien ban cua tn hieu phat thng c cung cap cho ni thu dang d tha trong

mien tan so con c goi la trai pho, v du nh trai pho trc tiep, ieu che a song mang va

nhay tan. Ky thuat trai pho rat hieu qua khi bang thong nhat quan cua kenh truyen nho. Tuy

nhien, khi bang thong nhat quan cua kenh truyen ln hn bang thong trai pho, trai tre a

ng se nho hn chu ky cua tn hieu. Trong trng hp nay, trai pho la khong hieu qua e



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cung cap phan tap tan so. Phan tap tan so gay ra s ton hao hieu suat bang thong tuy thuoc

vao s d tha thong tin trong cung bang tan so.

1.3.3. Phan tap thi gian:

Phan tap theo thi gian co the thu c qua ma hoa va xen kenh. Sau ay ta se so

sanh hai trng hp: truyen ky t lien tiep va dung xen kenh khi o li kenh truyen rat nho

Hnh 2. Phan tap theo thi gian

Phan tap thi gian co the at c bang cach truyen d lieu giong nhau qua nhng

khe thi gian khac nhau, tai ni thu cac tn hieu fading khong tng quan vi nhau. Khoang

cach thi gian yeu cau t nhat bang thi gian nhat quan cua kenh truyen hoac nghch ao cua

toc o fading . Ma ieu khien loi thng c s dung trong he thong truyen thong

e cung cap o li ma (coding gain) so vi he thong khong ma hoa. Trong truyen thong di

ong, ma ieu khien loi ket hp vi xen kenh e at c s phan tap thi gian. Trong

trng hp nay, cac phien ban cua tn hieu phat en ni thu di dang d tha trong mien

thi gian. Khoang thi gian lap lai cac phien ban cua tn hieu phat c quy nh bi thi

gian xen kenh e thu c fading oc lap ngo vao bo giai ma. V ton thi gian cho bo xenkenh dan en tr hoan viec giai ma, ky thuat nay thng hieu qua trong moi trng fading

nhanh, o thi gian nhat quan cua kenh truyen nho. oi vi kenh truyen fading cham neu

xen kenh qua nhieu th co the dan en tr hoan ang ke.

1.4. Cac o li trong he thong MIMO :


cd fv

c

f .

1=


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He thong MIMO s dung a anten phat va thu co the cung cap 3 o li la: o li

Beamforming, o li ghep kenh khong gian va o li phan tap khong gian .

1.4.1. o li Beamforming:

Beamforming giup he thong tap trung nang lng bc xa theo hng mong muon giup

tang hieu qua cong suat, giam can nhieu va tranh c can nhieu ti t cac hng khong

mong muon, t o giup cai thien chat lng kenh truyen va tang o bao phu cua he thong.e co the thc hien Beamforming, khoang cach gia cac anten trong he thong MIMO

thng nho hn bc song (thong thng la ), Beamforming thng c thc hien

trong moi trng t tan xa. Khi moi trng tan xa manh he thong MIMO co the cung cap o

li ghep kenh khong gian va o li phan tap.

1.4.2. o li ghep kenh khong gian:

Tan dung cac kenh truyen song song co c t a anten tai pha phat va pha thu

trong he thong MIMO, cac tn hieu se c phat oc lap va ong thi ra cac anten (hnh 3.5)

, nham tang dung lng kenh truyen ma khong can tang cong suat phat hay tang bang thong

he thong. Dung lng he thong se tang tuyen tnh theo so cac kenh truyen song song trong

he thong. e cc ai o li ghep kenh qua o cc ai dung lng kenh truyen thuat toan V-

Blast (Vertical- Bell Laboratories Layered Space-Time) c ap dung .

1.4.3. o li phan tap khong gian:

Trong truyen dan vo tuyen, mc tn hieu luon thay oi, b fadding lien tuc theo khong

gian thi gian va tan so, khien cho tn hieu tai ni thu khong on nh, viec phan tap cung cap

cho cac bo thu cac ban sao tn hieu giong nhau qua cac kenh truyen fadding khac nhau (hinh

3.6), bo thu co the la chon hay ket hp hay ket hp cac ban sao tn hieu nay e giam thieu

toc o sai bit BER, chong Fadding qua o tang o tin cay cua he thong. e cc ai o li

phan tap, giam BER va chong lai fadding, thuat toan STBC ( Space-Time Block Code) va

STTC (Space-Time Trellis Code) c ap dung .

Thc te, e he thong co dung lng cao, BER thap, chong c fadding, ta phai co s

tng nhng gia o li phan tap va o li ghep kenh trong viec thiet ke he thong.


2/


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Chng 2: Ky thuat OFDM

2.1. Tong quan ve ky thuat OFDM:

Truyen dan a song mang MC ( Multicarrier Communication) la mot dang FDM

nhng c dung cho mot luong d lieu phat va mot luong d lieu thu tng ng. MC khong

c dung e ghep kenh cac tn hieu khac nhau nh FDM, ma dung e chia nho luong d

lieu thanh cac luong d lieu song song. Dang MC n gian nhat chia luong d lieu vao thanh



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N luong tn hieu nho e truyen qua N kenh truyen con trc giao. Sau o, N luong nay ieu

che tai N tan so song mang khac nhau roi c ghep kenh roi a len kenh truyen. pha

thu th lam ngc lai phan kenh, giai ieu che, va ghep cac luong d lieu song song thanh

mot luong duy nhat nh ban au. N c chon u ln sao cho o rong mot symbol ln hn

nhieu trai tre cua kenh truyen hoac bang thong tng luong nho hn coherence bandwidth

cua kenh truyen, nham am bao cac luong con khong b ISI nghiem trong.

MC la c s cua OFDM, iem khac biet o la OFDM s dung tap cac song mang trc

giao nhau. Tnh trc giao co ngha la cac tn hieu c ieu che se hoan toan oc lap vi

nhau. Tnh trc giao nhau at c do cac song mang c at chnh xac tai cac v tr null

cua cac pho tn hieu a ieu che, ieu nay cho phep pho cua cac tn hieu co the chong lan

len nhau tc la hoan toan khong can dai bao ve, nen tiet kiem bang thong ang ke so vi

FDM truyen thong.

Hnh . Bang thong c s dung hieu qua trong OFDM

2.2. u iem va nhc iem cua OFDM:

2.2.1. u iem:

Ky thuat OFDM co nhieu u iem ma cac ky thuat ghep kenh khac khong co c.

OFDM cho phep thong tin toc o cao bang cach chia kenh truyen fading chon loc tan so

thanh cac kenh truyen con ch chu fading phang. Nh viec s dung tan so song mang trc

giao nen hien tng nhieu lien song mang ICI co the loai bo, do cac song mang phu trc

giao nen cac song mang nay co the chong lan len nhau ma pha thu van co the tach ra c



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dan en hieu qua s dung bang thong he thong rat hieu qua. Khi s dung khoang bao ve co

tnh chat cylic prefix ln hn trai tre ln nhat cua kenh truyen a ng th hien tng nhieu

lien ky t ISI se c loai bo hoan toan. Nh vao khoang bao ve co tnh chat cylic prefix

nen he thong s dung ky thuat OFDM ch can bo can bang mien tan so kha n gian. IFFT

va FFT giup giam thieu so bo dao ong cung nh giam so bo ieu che va giai ieu che giup

he thong giam o phc tap va chi ph thc hien, hn na tn hieu c ieu che va giai ieuche n gian, hieu qua nh vao IFFT va FFT.

2.2.2. Khuyet iem:

OFDM la mot ky thuat truyen a song mang nen nhc iem chnh cua ky thuat nay

la ty so cong suat nh tren cong suat trung bnh PAPR ( Peak- to-Average Power Ratio) ln.

Tn hieu OFDM la tong hp tn hieu t cac song mang phu, nen khi cac song mang phu ong

pha, tn hieu OFDM se xuat hien nh rat ln. ieu nay khien cho viec s dung khong hieu

qua bo khuyech ai cong suat ln HPA ( High-Power Amplifier). Mot nhc iem khac cua

OFDM la rat nhay vi lech tan so, khi hieu ng dch tan Doppler xay ra tan so song mangtrung tam se b lech, dan en bo FFT khong lay mau ung tai nh cac song mang, dan ti

sai loi khi giai ieu che cac symbol.

2.3. Mo hnh he thong OFDM:

Hnh 4. S o khoi he thong OFDM



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Bo chuyen oi noi tiep-song song Serial/Parallel va Parallel/Serial:

Theo Shanon toc o d lieu cao nhat cho kenh truyen ch co nhieu trang AWGN

(khong co fading):

Cmax = B.log2 (1 + S/N) [ bps] (2.5)

Vi B la bang thong cua kenh truyen [Hz]

S/N la t so tn hieu tren nhieu cua kenh truyen.V vay muon truyen d lieu vi toc o cao hn C max ta phai chia nho luong d lieu toc

o cao thanh cac luong d lieu toc o thap hn C max bang cach s dung bo chuyen oi noi

tiep sang song song Serial/Parallel.

Tc la chia luong d lieu vao thanh tng frame nho co chieu dai k b bit kN, vi b la

so bit trong mo hnh ieu che so, N so song mang. k, N se c chon sao cho cac luong d

lieu song song co toc o u thap, e bang thong tng ng u hep, sao cho ham truyen trong

khoang bang thong o co the xem la phang. Bang cach s dung bo S/P ta a chuyen kenh

truyen fading chon loc tan so thanh kenh truyen fading phang.Ngc lai pha phat, pha thu se dung bo Parallel/Serial e ghep N luong d lieu toc

o thap thanh mot luong d lieu toc o cao duy nhat.

Bo Mapper va Demapper:

Tng symbol b bit se c a vao bo mapper muc ch la nang cao dung lng kenh

truyen. Mot symbol b bit se tng ng mot trong M = 2b trang thai hay mot v tr trong

constellation ( gian o chom sao)

BPSK s dung symbol 1 bit, bit 0 hoac bit 1 se xac nh trang thai pha 00 hoac 1800 ,

toc o baud hay toc o chuoi se bang toc o bit Baud = Rb . QPSK s dung symbol 2 bit (Dibit) , Baud = Rb/ 2 .

8-PSK hay 8-QAM s dung symbol 3 bit (Tribit), Baud = Rb/ 3 .

16-PSK hay 16-QAM s dung symbol 4 bit (Quabit), Baud = Rb/ 4 .

So bit truyen trong mot symbol tang len ( M tang len ), th hieu qua bang thong

Beffeciency = Rb/ BT = log2M = b [bps/Hz] tang len, tuy nhien sai so BER cung tang len. Nyquist

a a ra cong thc tnh dung lng kenh truyen toi a trong moi trng khong nhieu: C =

2Blog2M trong o B la bang thong kenh truyen. Do o ta khong the tang M len tuy y c,

cong thc tren cho phep ta xac nh M ln nhat, so bit ln nhat co the truyen trong mot

symbol.

Mot so phng thc ieu che so thng dung trong bo Mapper

M-PSK ( Phase Shift Keying)

M-DPSK (Differential Phase Shift Keying)



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M-QAM (Quarature Amplitude Modulation)

M-PSK(M-Phase Shift Keying):

Trong PSK Xn co bien o khong oi va pha phu thuoc b bit vao dn. M-PSK co M trang

thai pha phu thuoc vao b = log2M bit vao, pha cua Xn gia tr la 1 trong M goc pha :

. Mot so dang M-PSK thng gap:

BPSK hay PRK co 2 trang thai pha phu thuoc 1 bit vao .

QPSK co 4 trang thai pha phu thuoc 2 bit vao.

8-PSK co 8 trang thai pha phu thuoc 3 bit vao.

16-PSK co 16 trang thai pha phu thuoc 4 bit vao.

DPSK (Differential Phase Shift Keying):

ay la mot dang cua M-PSK, trc khi i vao bo M-PSK tn hieu se c x ly sai

biet, ky t ra khoi bo nay cha ng thong tin ve s khac nhau gia hai ky t lien tiep. Bo

giai ieu che se so sanh s khac biet ve pha gia 2 k t lien tiep e xac nh ky t thu c.

Thong thng nhieu tac ong len 2 ky t lien tiep gan nh nhau, sai biet gia 2 ky t lien

tiep se giong nhau trong trng hp co nhieu va khong co nhieu.

M-QAM:

Trong s o M-QAM Xn co bien o pha phu thuoc b bit vao d n, QAM c s dung rat

pho bien trong cac ng truyen vo tuyen so toc o cao. Sau ay la bieu thc tong quat cua

tn hieu M-QAM:

Xn = ai + jbi i = 1, .., M

Vi ai, bi = a,3a, 5a,., (log2M-1) , ai va bi phu thuoc b bit vao dn .

Bo IFFT va FFT:

Phep bien oi IDFT ( Inverse Discrete Fourier Transform) cho phep ta tao tn hieu

OFDM de dang, tc la ieu che N luong tn hieu song song len N tan so trc giao mot cach

chnh xac va n gian. Phep bien oi DFT (Discrete Fourier Transform) cho phep ta giai

ieu che lay lai thong tin t tn hieu OFDM. Nh s dung phep bien oi IDFT va DFT ma ta

tinh gian c bo tong hp tan so phc tap pha phat va phia thu. Neu khong s dung IDFTva DFT bo tong hp tan so phai tao ra tap tan so cach eu nhau chnh xac va ong pha,

nham tao ra tap tan so trc giao hoan hao

Bo Guard Interval Insertion va Guard Interval Removal[1]

Hai nguon nhieu giao thoa (interference) thng thay trong cac he thong vo tuyen so,

cung nh trong he thong OFDM la ISI va ICI


MiMi

i ,....,2,12 ==


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ISI ( Intersymbol Interference) : nhieu giao thoa lien ky t, c nh ngha la xuyen

nhieu gia cac symbol trong khoang thi gian T symbol cua cac frame FFT lien tiep

(trong mien thi gian).

ICI ( Inter-carrier interference): nhieu giao thao lien song mang, c nh ngha la

xuyen nhieu gia cac kenh song mang phu (subchannels) cua cung mot frame FFT

(trong mien tan so)Nhieu ICI c loai bo hoan toan nh s dung tap song mang trc giao lam tap tan so

cua cac kenh phu. Nhieu ISI se gan nh c loai bo hoan toan neu ta s dung so lng

song mang N u ln, khi o bang thong cua moi kenh u nho so vi coherence bandwith, tc

la o rong mot symbol ts se ln hn trai tre cua kenh truyen.

Bang thong OFDM:

Toc o symbol cua he thong OFDm s dung N song mang

Bang thong tn hieu OFDM c xac nh theo bieu thc:

Vi : he so khoang bao ve

Khi so song mang ln N>>1 ta co

Bo bien oi D/A va A/D :


sTNR =

sGs

OFDMT

N

TB

)1(

12

+=

s

GG

T

=

GGs

GssGssGs

OFDM

RT

N

NT

N

T

N

TT

N

TB

+

+

+=

1

1.

1

1.

)1

12(

)1(

2

)1(

12


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Chuoi symbol xn sau khi c chen khoang bao ve cho chuoi sn ,se c a vao

bo bien oi t so sang tng t D/A va bo loc thong thap tao ra tn hieu lien tuc s(t) e a

ra kenh truyen vo tuyen.

pha thu, bo A/D lam ong tac ngc lai bo D/A, bo A/D selay mau tn hieu OFDM

thu c r(t), cho ra tn hieu so ri rac r n, sau o rn se c cho qua bo Guard Interval

Removal e loai bo khoang bao ve

Bo Up-Converter va Down-Converter:

Cac bo Up-Converter va Down-Converter chnh la cac bo oi tan so can bang

(Balance Modulator). Sau khi qua bo bien oi D/A va loc thong thap, tn hieu s(t) len tan so

cao tao thanh tn hieu sRF(t) e anten phat co the de dang bc xa tn hieu ra khong gian.

pha thu, tn hieu rRF(t) thu c t anten phat se c oi tan xuong thanh tn hieu r(t) nh

bo Down-Converter.

Bo Equalizer

Do cac kenh song mang phu co bang thong hep ch chu fading phang, nen mot bo

Equalizer n gian c s dung nham toi u tn hieu ri rac trc khi cho qua bo giai ieu

che so demoulation hay de-mapper, e giam bt t so bit loi BER cua he thong

Viec chen CP vao moi symbol tai pha phat a lam cho phep chap tuyen tnh ke hp

tr thanh phep chap vong trong khoang thi gian ts.

Ngoai ra e cai thien BER ngi ta con s dung them cac khoi FEC (Forward Error

Correction) e sa loi n, s dung them khoi Interleavers e hoan oi v tr bien cac bit loi

dang chum thanh cac bit n e FEC co the sa c.

Chng 3: H thng MIMO-OFDM

3.1. Gii thiu:

Cac he thong thong tin khong day luon c nghien cu nham cai thien chat lng

dung lng cung nh kha nang chong lai hien tng a ng. oi vi cac he thong thong

tin thong chat lng tn hieu co the cai thien bang cach tang cong suat, dung truyen lng he


G

G


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thong co the tang khi tang bang thong. Tuy nhien cong suat cung ch co the tang ti mot

mc gii han nao o v cong suat phat cang tang th he thong cang gay nhieu cho cac he

thong thong tin xung quanh, bang thong he thong cua he thong cung khong the tang mai len

v viec phan bo bang thong a c nh chuan san.

He thong MIMO co the tang dung lng kenh truyen, s dung bang thong rat hieu

qua nh ghep kenh khong gian, cai thien chat lng cua he thong ang ke nh vao phan taptai pha phat va pha thu ma khong can tang cong suat phat cung nh tang bang thong cua he

thong. Ky thuat OFDM la mot phng thc truyen dan toc o cao vi cau truc n gian

nhng co the chong fading chon loc tan so, bang cach chia luong d lieu toc o cao thanh N

luong d lieu toc o thap truyen qua N kenh truyen con s dung tap tan so trc giao. Kenh

truyen chu fading chon loc tan so c chia thanh N kenh truyen con co bang thong nho

hn, khi N u ln cac kenh truyen con chu fading phang. OFDM con loai bo c hieu ng

ISI khi s dung khoang bao ve u ln. Ngoai ra viec s dung ky thuat OFDM con giam o

phc tap cua bo Equalizer ang ke bang cach cho phep can bang tn hieu trong mien tan so.T nhng u iem noi bat cua he thong MIMO va ky thuat OFDM, viec ket hp he thong

MIMO va ky thuat OFDM la mot giai phap ha hen cho he thong thong tin khong day bang

rong tng lai.

Hnh 5. Cac chuan thong tin khong day cua IEEE



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Hnh 5 mo ta cac chuan thong tin khong day cua IEEE tng ng toc o bit va vung

bao phu, trong o cac chuan mau sam se c ng dung he thong MIMO-OFDM trong tng

lai, ieu nay cho thay tam ng dung cua he thong MIMO-OFDM rat rong.

3.2. He thong MIMO-OFDM:

3.2.1. Mo hnh he thong MIMO-OFDM:Cau truc may thu va phat cua he thong MIMO-OFDM bao gom he MIMO NT anten

phat va NR anten thu, ky thuat OFDM s dung N song mang phu c mo ta nh hnh 6:

a) S o khoi he thong MIMO-OFDM

b) S o khoi bo phat OFDM

c) S o khoi bo thu OFDM

Hnh 6. Mo hnh he thong MIMO-OFDM

Symbol thu c t anten thu th i, tai song mang phu th k cua symbol OFDM co the

bieu dien nh sau:



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Vi:

Xj (k) la symbol phat tren song mang th k trong symbol OFDM Vi(k) la nhieu Gauss tai anten thu th i trong mien tan so, tc la N-FFT cua nhieu

trong mien thi gian vi(t) la o li kenh truyen t anten phat th j ti anten thu th i tai song mang

phu th n. chnh la N-FFT cua ap ng xung cua kenh truyen t anten

phat th j ti anten thu th i. Neu may thu co the c lng chnh xac trang thai

kenh truyen th se c biet chnh xac ng vi moi symbol OFDM.

Kenh truyen he thong MIMO-OFDM co the mo ta thong qua ma tran H nh sau

Hnh 7 mo ta ro hn ma tran H, ky thuat OFDM co tac dung chia kenh truyen chon

loc tan so thanh N kenh truyen con fading phang. He thong MIMO-OFDM tng ng vi

he thong MIMO.


Nk

kVkXkkXkkXkkY

kVkXkkXkkXkkY

kVkXkkXkkXkkY

RTTRRRR

TT

TT

NNNNNNN

NN

NN

,,3,2,1

)()()()()()()()(

)()()()()()()()(

)()()()()()()()(

2211

222221212

112121111

=

++++=

++++=

++++=

)(kij

)(kij )(tcij

)(kij

=

)()()(

)()()(

)()()(

)(

21

22221

11211

kkk

kkk

kkk

kH

TRRR

T

T

NNNN

N

N


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Hnh 7. Ma tran kenh truyen

Co hai loai he thong MIMO-OFDM la: he thong MIMO-OFDM Alamouti vi muc

ch at o li phan tap toi a nham toi u chat lng he thong va he thong MIMO-OFDM

V-BlAST vi muc ch at o li ln nhat nham tang toi a dung lng he thong thong tin

khong day trong moi trng fading chon loc tan so .

3.2.2. Mo hnh he thong MIMO-OFDM Alamouti:

Hnh 7 va 8 la s o he thong MIMO-OFDM Alamouti vi cac khoi c ban nhat. S

o Alamouti c ap dung nham at c o li phan tap ln nhat trong moi trng fading

chon loc tan so vi cau truc phan cng kha n gian.

Hnh 7. May phat MIMOOFDM Alamouti



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Hnh 8. May thu MIMO-OFDM Alamouti

3.2.3 Mo hnh he thong MIMO-OFDM V-Blast:

Hnh 9. May phat MIMO-OFDM VBLAST



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Hnh 10. May thu MIMO-OFDM VBLAST

3.2.4. Ty so cong suat nh tren cong suat trung bnh PARR:

He thong MIMO-OFDM la he thong a song mang, nen cung phai oi mat vi mot

van e quan trong la s meo phi tuyen t bo khuyech ai cao tan. Nguyen nhan cua van e

nay la do ty so cong suat nh tren cong suat trung bnh PARR kha ln dan en tam ong

cua tn hieu kha ln khi he thong a song mang s dung so song mang phu ln.

Mot so ky thuat nham muc ch giam thieu PARR trong he thong MIMO-OFDM

a c e ngh, chung co the c chia thanh 2 hng tiep can. Trong hng th nhat, ta se

chen them cac thong tin d tha nham giam bien o nh trong tng symbol phat t o giam

PARR cua he thong. Hng th hai la cho tn hieu i qua mot bo sa oi nham loai bo cac

nh tn hieu co bien o ln. Hng tiep can th nhat co u iem la khong gay xuyen nhieu

nhng lam tang tnh phc tap va lam giam toc o truyen cua he thong, hng th hai kha

n gian t lam tang tnh phc tap cua he thong nhng lai gay ra xuyen nhieu cho he thong.



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Tim Hieu He Thong Mimo Ofdm

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