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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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Chng 4:M PHNG H THNG MIMO-OFDM
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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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Chng 4:M PHNG H THNG MIMO-OFDM
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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)
MRRC-OFDM (1TX, 4RX)
Alamouti-OFDM (2TX, 1RX)
Alamouti-OFDM (2TX, 2RX)
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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Chng 4:M PHNG H THNG MIMO-OFDM
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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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Chng 4:M PHNG H THNG MIMO-OFDM
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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
Alamouti-OFDM (2TX, 2RX)
Alamouti-OFDM (2TX, 4RX)
Alamouti-OFDM (2TX, 6RX)
Alamouti-OFDM (2TX, 8RX)
Hnh 4.6 BER v SNR ca Alamouti (2Tx * N_rx) vi N_rx = 2,4,6,8
Kt qu :
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Chng 4:M PHNG H THNG MIMO-OFDM
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0dB 2dB 4dB 6dB 8dB 10dB 12dB 14dB 16dB 18dB 20dB
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
MRRC-OFDM (1TX, 2RX)
MRRC-OFDM (1TX, 4RX)
Hnh 4.7 BER v SNR ca h thng MRRC
Kt qu :
0dB 2dB 4dB 6dB 8dB 10dB 12dB 14dB 16dB 18dB 20dB
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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Chng 4:M PHNG H THNG MIMO-OFDM
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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)
MRRC-OFDM (1TX, 2RX)
MRRC-OFDM (1TX, 4RX)
MRRC-OFDM (1TX, 6RX)
Hnh 4.8 BER v SNR ca h thng MRRC-OFDM vi s anten khc nhau
Nhn xt:
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Chng 4:M PHNG H THNG MIMO-OFDM
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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)
Compare BER of MRRC-OFDM and Alamouti-OFDM system
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)
Compare BER of MRRC-OFDM and Alamouti-OFDM system
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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Chng 4:M PHNG H THNG MIMO-OFDM
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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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Chng 4:M PHNG H THNG MIMO-OFDM
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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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Chng 4:M PHNG H THNG MIMO-OFDM
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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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