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Chng 4:M PHNG H THNG MIMO-OFDM

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CHNG 4 M PHNG H THNG MIMO - OFDM

4.1 Gii thiu chngTrong cc chng trc phn tch k thut OFDM cng nh tm hiu cc

k thut phn tp nng cao tin cy v ci thin cht lng tn hiu trn knh

truyn. Trong chng ny, s trc quan ha nhng phn l thuyt chng trc

bng cch s dng phn mm Matlab m phng k thut Alamouti cho h thng

OFDM. Mc tiu m phng l so snh BER ca h thng s dng k thut

Alamouti cc thng s anten khc nhau, sau so snh vi cc k thut khc. Qua

, a ra nhng nhn xt v hiu qu ca k thut Alamouti-OFDM cng nh kh

nng p dng thc t ca k thut ny.

4.2 M hnh h thng Alamouti-OFDM s m phng

Hnh 4.1: M hnh h thng Alamouti- OFDM (2TX 1RX)

Chng ny s tin hnh m phng BER ca h thng Alamouti-OFDM nh hnh

4.1. Cc bit c to ra ngu nhin v iu ch Alamouti-OFDM bn pht, sau

c nhn vi p ng knh truyn h v cng thm nhiu to thnh tn hiu bn

thu. Tn hiu bn thu c to ra t cng thc y = S*h + n.

Bn thu s thc hin gii iu ch Alamouti-OFDM vi gi thit bit

c chnh xc p ng knh truyn h ti to li tn hiu ban u. Gi chui bit

pht l x v chui bit m bn thu ti to c l x1. Ta thc hin so snh x vi x1

tnh ra s bit li ri v th BER ca h thng.

tuc la qua trinh han di ri,qua trinh mo la han se nhan cai dap ung kenh ,ngang doan mo c

nhieu ?


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4.3 Lc tin hnh m phng h thng MISO v MIMO OFDMAlamouti.

Hnh 4.2 S khi m phng h thng

Bt u

Khi to cc thng sTo vecto d liu v pilotTo p ng knh truyn h

iu ch s MQAM

Bin i IFFT

To tn hiu thu : y = S*h + n

To tn hiu trn 2 anten pht

Bin i FFT d liu

Gii iu ch

Tnh BER

Hin th

Kt thc

Data?

To tn hiu kt hp

N

Y


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4.4. M phngu tin ta so snh k thut phn tp pht Alamouti v phn tp thu MRRC

trong cng mt h thng OFDM rt ra mt s nhn xt. Cc phn tip theo s tp

trung m phng k thut phn tp pht vi s lng anten thay i c bn pht v

bn thu.

4.4.1. H thng MIMO OFDM s dng k thut phn tp pht v thu

4.4.1.1. Kt qu

0 2 4 6 8 10 12 14 16 18 2010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

SNR (dB)

Biterrorrate(BER)

Compare BER of MRRC-OFDM and Alamouti-OFDM system

OFDM (No diversity)

MRRC-OFDM (1TX, 2RX)


Alamouti-OFDM (2TX, 1RX)


Hnh 4.3 th so snh BER trong h thng OFDM k thut phn tp pht v thu.

Kt qu :

0dB 2dB 4dB 6dB 8dB 10dB 12dB 14dB 16dB 18dB 20dB

1x1 0.1181 0.0917 0.0632 0.0436 0.0285 0.0190 0.0112 0.0080 0.0047 0.0031 0.0020

2x1 0.0885 0.0601 0.0343 0.0186 0.0095 0.0043 0.0018 0.0008 0.0004 0.0001 3.6e-5

1x2 0.0433 0.0249 0.0126 0.0072 0.0027 0.0013 5e-04 2.2e-4 7.8e-5 1.8e-5 1.2e-5

2x2 0.0295 0.0136 0.0052 0.0018 4.2e-4 1.3e-4 9.1e-6 1.3e-6 0 0 0

1x4 0.0083 0.0029 7.2e-4 2.2e-4 3.8e-5 2.4e-5 1.3e-6 0 0 0 0

Bng 4.1. Kt qu BER trong h thng OFDM ca k thut phn tp pht v thu

1 ben phan tap

1 ben phan tap psao ma so sanh d


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Nhn xt:

- Cng sut tn hiu trn nhiu SNR cng ln th t l li bit BER cng gim.V

cng sut pht cng ln so vi cng sut nhiu th tc ng ca nhiu s khng ng

k, cht lng ca h thng s cng tt.

- Khi tng s lng anten th cht lng h thng cng tt. iu ny u ng

cho phn tp pht v phn tp thu.

- Vi cng bc phn tp, k thut phn tp thu MRRC cho kt qu tt hn k

thut phn tp pht Alamouti. Xt vi SNR = 10dB:

o Trng hp Alamouti 2x1 c BER=5.10 -3ln hn trng hp MRRC

1x2 c BER=2.10-3.

o Trng hp Alamouti 2x2 c BER=7.10 -5ln hn trng hp MRRC

1x4 c BER=7.10-6.

- K thut phn tp thu MRRC cho cht lng tn hiu tt hn k thut phn tp

pht Alamouti v s lng anten thu nhiu hn th s lng tn hiu thu nhiu hn

v vic khi phc li tn hiu s m bo hn. Tuy nhin trong thc t, ng

downlink, s dng phn tp thu th thit b di ng phi tch hp nhiu anten thu,

dn n nhiu kh khn v vn kch thc, phc tp v chi ph sn xut. Thay

vo , c th s dng k thut phn tp pht trm BTS khc phc cc nhcim trn.

4.5 M phng cc h thng khi kt hp OFDM

H thng MIMO-OFDM s c m phng vi cc thng s nh sau:

Thng s ca h thng thu pht OFDM:

- S OFDM symbols trong mt khung truyn l : 10

- S OFDM pilot symbol trong mt khung truyn l : 1

- Kiu iu ch s : M-QAM, vi M= 16 ( M=4,16, 64, 128, 256)

- Kch thc FFT/IFFT : 32, 64, 128,

- di CP l: 10

- S anten pht l: 2


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- Cng sut tn hiu trn nhiu SNR cng ln th t l li bit BER cng gim.

V cng sut pht cng ln so vi cng sut nhiu th tc ng ca nhiu s

khng ng k, cht lng h thng s cng tt.

- Vi cng s lng anten pht theo thut ton Alamouti, khi ta tng s lng

anten thu th t l li bit BER gim. Cng s dng nhiu anten th cht lng

ca tn hiu gii m cng tt.

- iu ny chng t rng k thut Alamouti p ng c yu cu khi cn m

rng h thng khng dy.

Trng hp 2: M phng h thng Alamouti vi s lng cc anten thu khc

nhau. S anten thu ln lt l N_rx = 2, 4, 6, 8

Kt qu m phng:

0 2 4 6 8 10 12 14 16 1810

-7

10-6

10-5

10-4

10-3

10-2

10-1

SNR (dB)

Biterrorrate

(BER)

Alamouti-OFDM system with 2xM antenna diversity





Hnh 4.6 BER v SNR ca Alamouti (2Tx * N_rx) vi N_rx = 2,4,6,8

Kt qu :


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2x2 0.0302 0.014 0.005 0.00176 0.0004 0.1e-3 1.97e-5 3.7e-6 1.9e-7 0 0

2x4 0.0045 9.6e-4 1.3e-4 1.72e-5 0.4e-6 0 0 0 0 0 0

2x6 7e-4 8.94e-5 2.7e-6 5.56e-6 0 0 0 0 7 0 02x8 1.35e-4 6.0e-6 1.95e-7 0 0 0 0 0 0 0 0

Nhn xt : S dng s anten thu cng nhiu th BER cng thp. Performance h

thng tt hn

4.5.2 H thng MRRC (1Tx * 2Rx) v MRRC (1Tx * 4Rx) kt hp OFDM

Trng hp 1 : M phng h thng MRRC ( 1Tx * 2Rx) v MRRC ( 1Tx * 4Rx )

0 2 4 6 8 10 12 14 16 18 2010

-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

SNR (dB)

Bit

errorrate(BER)

MRRC-OFDM system with 1xM antenna diversity



Hnh 4.7 BER v SNR ca h thng MRRC

Kt qu :


1x2 0.04528 0.02493 0.0123 0.00602 0.0029 0.00113 5.4e-4 2.1e-4 9.7e-5 4.7e-5 1.4e-5

1x4 0.00869 0.00274 7.5e-4 1.74e-4 3.41e-5 7.32e-6 1.1e-6 1.9e-7 9.7e-8 0 0

Nhn xt:


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Tng t nh h thng Almouti, khi phn tp thu MRRC 1Tx * 2Rx v

MRRC 1Tx* 4Rx th cht lng tn hiu c ci thin rt tt, BER thu c cng

gim v khi ta cng tng lng anten thu th cht lng BER thu c cng gim.

Tuy nhin,trong thc t, ng downlink, s dng phn tp thu th thit b di

ng cm tay phi tch hp nhiu anten thu, dn n nhiu kh khn v vn kch

thc, phc tp v chi phi sn xut. Thay vo s dng k thut phn tp pht

trm BTS khc phc c nhc im trn.

Trng hp 2: M phng h thng MRRC vi s lng cc anten thu khc

nhau, s anten thu ln lt l N_rx = 2, 4, 6

0 2 4 6 8 10 12 14 16 18 2010

-7

10-6

10-5

10-4

10-3

10-2

10-1

SNR (dB)

Biterrorrate(BER)

MRRC-OFDM system with 1xM antenna diversity

OFDM (No diversity)




Hnh 4.8 BER v SNR ca h thng MRRC-OFDM vi s anten khc nhau

Nhn xt:


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Khi SNR tng s lm cho BER ca tn hiu gim

Cng mt mc SNR nhng h thng MRRC-OFDM c BER thp hn so vi

h thng OFDM khng phn tp. Do , ta thy rng h thng MRRC-

OFDM cng c kh nng ci thin cht lng ca tn hiu rt tt

Khi s antenna thu ca h thng MRRC-OFDM cng tng th cht lng

ca tn hiu s cng tt do BER gim ng k.

4.5.3 H thng Alamouti(1Rx*2Tx) v h thng Mrrc(2Rx*1Tx)

0 2 4 6 8 10 12 14 16 18 2010

-5

10-4

10-3

10-2

10-1

SNR (dB)

Biterrorrate(BER)


Alamouti-OFDM (2TX,1RX)

MRRC-OFDM (1TX,2RX)

Hnh 4.9. BER v SNR ca Alamouti(2Rx*1Tx) v h thng Mrrc(1Rx*2Tx)

Nhn xt:

Mc d cng bc phn tp l 2 phng php MRRC cho t l BER v SNR

tt hn Alamouti (2Rx*1Tx) vi cng mc cng sut tn hiu .Tuy nhin

do cng sut pht ca h thng Alamouti (1Rx*2Tx) ch bng mt na

cng sut m b pht m h thng MRRC yu cu.


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K thut ny cng khng yu cu bt k thng tin phn hi t b thu n

b pht v khng tnh ton phc tp tng t nh MRRC

4.5.4 H thng Alamouti(2Tx*2Rx) v h thng Mrrc(1Tx*4Rx)

0 2 4 6 8 10 12 14 16 1810

-7

10-6

10-5

10-4

10-3

10-2

10-1

SNR (dB)

Biterrorrate(BER)


Alamouti-OFDM 2x2

MRRC-OFDM 1x4

Hnh 4.10. BER v SNR ca alamouti(2Tx*2Rx) v h thng Mrrc(1Tx*4Rx)

Nhn xt:

Tuy cng bc phn tp l 4, h thng MRRC (1Tx*4Rx) ci thin BER

nhiu so vi h thng Alamouti(2Tx*2Rx) do Mrrc s dng n 5 anten

trong khi h thng Alamouti ch s dng 4 anten v s anten thu ca Mrrc

gp i so vi Almouti.

Bn cnh , tng cng sut pht ca h thng Alamouti mi bng cng

sut pht ca MRRC.iu ny rt c ngha trong thc t khi cn gim chiph v ch to thit b cm tay.


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4.5.5 H thng Ofdm v h thng Alamouti.

0 2 4 6 8 10 12 14 16 18 2010

-4

10-3

10-2

10-1

100

Signal to Noise Ratio (SNR),dB

BitErrorRate(BER)

Transmit vs. Receive Diversity

SISO OFDMALAMOUTI-OFDM 2x1

Hnh 4.11. BER v SNR ca Ofdm v h thng Alamouti.

Nhn xt:

T th ta nhn thy phng php Alamouti ci thin r rt t l li bit

BER so vi khi khng s dng phn tp.bn cnh ta thy k thut

alamouti cng nng cao SNR.

Mt trong nhng li th ca k thut phn tp alamouti l tin cy.Nu

mt trong nhng chui b mt,th s c chui khc c nhn. Ni cch

khc, cc tn hiu c th vn cn pht hin, nhng vi cht lng km hn.


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4.6 Kt lun chng:

Chng ny thc hin chng chng trnh m phng h thng truyn

dn OFDM s dng k thut phn tp pht. M khi khng gian thi gian m in

hnh l thut ton Alamouti. Cc m phng trn lm r hn nhng l thuyt m

ta cp mi chng trc.Mi h thng u c u v nhc im ring ca

n,ty vo mc ch v iu kin s dng m ta c th p dng tng h thng.Tuy

nhin, ta vn c th thy rng k thut Alamouti vn c nhng th mnh ring m

cc h thng khc khng th c c khi p dng thc t nh tnh n gin v hiu

qu kinh t.


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KT LUN TI

K thut phn tp anten l mt ci tin cc k quan trng trong vic nng cao

cht lng ca knh truyn. Trong qu trnh nghin cu k thut phn tp trong hthng MIMO-OFDM, n tp trung vo cc k thut phn tp pht c p

dng ph bin hin nay. Bn cnh , n cng tm hiu nhng vn c bn

v k thut OFDM v knh truyn v tuyn nhm c mt ci nhn ton din v h

thng MIMO-OFDM.

K thut alamouti c xut trong ti ny l mt k thut thu pht tn

hiu n gin ci thin cht lng tn hiu ti ni nhn bng cch x l n gin

qua hai anten pht. Bc ca phn tp l 2 bng vi MRRC ( b t hp thu t l ti

a) vi mt anten pht vi 2 anten ti ni thu. K thut ny l d dng c pht

bi 2 anten pht v M anten thu tha mn bc phn tp l 2M. iu ny c

thc hin m khng c phn hi no t ni thu n ni pht v ch cn mt vi tnh

ton nh. K thut ny khng yu cu m rng bng thng, khng cn quan tm

thi gian hoc tn s.

K thut phn tp anten pht c th ci thin hiu sut li, tc d liu,

hoc nng lc ca h thng khng dy.c bit n c th gim thiu fading v s

dng k thut iu ch mc cao hn tng hiu qu tc d liu, hoc cc yu

t ti s dng nh hn trong mt mi trng multicell (a t bo) tng cng

nng lc h thng.


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HNG PHT TRIN TI

Do thi gian c hn nn n ch kho st h thng MIMO c 2 anten pht

v M anten thu. V vy hng pht trin tip theo ca n l m rng s lnganten pht bng cc m khng gian thi gian khc.

Sau khi kt thc n ny,em s nghim cu h thng tng qut STBC nh

p dng k thut tarok vo h thng OFDM v a h thng Alamouti Ofdm trong

mi trng wifi ra mi trng wimax. ng thi em s tm hiu thm v cc loi

m ha khng gian-thi gian khc nh m ha li STTC, m ha lp BLAST.T

s c ci nhn tng qut hn v h thng thng tin di ng hin nay.


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TI LIU THAM KHO

[1]. DAVID TSE, Fundamentals of Wireless Communication, University of California,

Berkeley& Pramod Viswanath, University of Illinois, Urbana-Champaign, September 10,

2004.

[2]. Siavash M.Alamouti, A Simple Transmit Diversity Technique for Wireless

Communications.

[3]. Erik G.Larsson, Space-time Block Coding for Wireless Communications,

Cambridge University, 2003.

[4]. Nguyn Vn c, L thuyt v cc ng dng ca k thut OFDM, i hc Bch

khoa H Ni, 2006

[5]. Hc vin Bu chnh vin thng,L thuyt tri ph v a truy nhp v tuyn.

[6]. Matlab Simulink

[7]. www.dsplog.com

[8]. Nguyn Thy Trinh-Nguyn Thnh Tho, So snh t s cng sut nh trung bnh

ca h thng Fourier OFDM v Wavelet OFDM, i hc Nng, 2008.

[9]. Ts. inh c Anh V, Bin i Fourier ri rc (DFT), Bch khoa thnh ph H

Ch Minh.

[10]. Ts. Phan Hng Phng, Ts. Lm Chi Thng, K thut phn tp anten trong ci

thin dung lng h thng MIMO.

[11]. Ks. Mai Hng Anh, Phn tp pht s dng m khi khng gian thi gian cu trctrc giao.

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