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Performance Evaluation of Weight-Based ICI-Cancellation Scheme in OFDM Systems. Jyh-Horng Wen a , Jia-Wei Liu b , Gwo-Ruey Lee c and Cheng-Yi Hsieh d. 指導教授:溫志宏 教授 報告者 : 謝承毅. Outline. Introduction ICI Mechanism of Standard OFDM Systems ICI Self-Cancellation Scheme - PowerPoint PPT Presentation
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Performance Evaluation of Performance Evaluation of Weight-Based ICI-Weight-Based ICI-Cancellation Scheme in Cancellation Scheme in OFDM SystemsOFDM Systems
Jyh-Horng Wena, Jia-Wei Liub, Gwo-Ruey Leec and Cheng-Yi Hsiehd
指導教授:溫志宏 教授報告者 : 謝承毅
1
OutlineOutlineIntroductionICI Mechanism of Standard OFDM
SystemsICI Self-Cancellation SchemeThe Proposed Weight-Based ICI
Self-Cancellation SchemeSimulation ResultsConclusion
2
IntroductionIntroduction
For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference(ICI).
The ICI self-cancellation scheme is a simple way for ICI reduction.
3
ICI Mechanism of Standard ICI Mechanism of Standard OFDM SystemsOFDM Systems
Fig.1 Block diagram of the FFT-based OFDM systems ICI canceling mapping:
(1) (0)
(3) (2)
( 1) ( 2)
( 1) ( ) , 0, 2,4,..., 2
i i
i i
i i
i i
X X
X X
X N X N
X k X k k N
1 1
2 1 2
3 2 3
1
1
Differential coding:
( ) ( )
( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( )
( ) ( ) , =1
( ) ( ) ( ) , =2,3,...
i i i
i i
i i i
X k D k
X k X k D k
X k X k D k
X k X k D k
X k D k i
X k X k D k i
Serial
Data
InputS/P
SignalMapping
IFFT P/SAdding Cyclic Prefix
D/AUp
Converter
Channel
DownConverter
A/DRemoving
Cyclic Prefix
S/PFFTSignal
De-mappingP/S
Serial
Data
Output
Differential Coding
ICI Canceling Mapping
ICI Canceling
Demapping
Differential Decoding
ICI Self-Cancellation
Scheme
DifferentialEncoding
4
The transmitted signal in time domain can be written as1
2 /
0
1( ) ( )
Nj kn N
i ik
x n X k eN
and the received signal in time domain can be written as 2 /( ) ( ) ( ) ( ) j n N
i i i iy n x n h n w n e The corresponding frequency domain response could be
obtained by FFT, which gives1
0
1
0,
( ) ( ) ( ) ( ) ( ), 0,1,2,... 1
( ) ( ) (0) ( ) ( ) ( ) ( )
N
i i i ik
N
i i i i ik k m
Y m X k H k C k m W m m N
X m H m C X k H k C k m W m
and C(k – m) denotes the ICI coefficient between the mth and the kth subcarriers, which could be expressed as
1(1 )( )sin( ( ))
( )sin( ( ))
j k mNk m
C k m eN k m
N
, /offsetf f
Inter-carrier interference
Desired signal
The discrete-time channel response of slow fading channel could be expressed as1
,0
( ) ( )L
i l il
h n h n l
5
We assume that X(k) is zero mean and statistically independent with H(k).We further assume E[| H(k)|2]=1 .Therefore, the CIR can be derived as
2
12
0,
2
2
(0)
( )
.N
k k m
m
CCIR
C k m
E S m
E I m
The desired received signal power can be represented as
2 2 2 2( ) ( ) ( ) (0)i iE S m E X m E H m C
and the ICI power is
1
2 2 2 2
0,
( ) ( ) ( )N
i ik k m
E I m E X k E H k C k m
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Assume the transmitted symbols are constrained so that X(1) =-X(0), X(3) = -X(2),…,X(N-1)= -X(N-2), then the received signal on subcarrier m becomes
1
0
2
0
2
0
2
0
( ) ( ) ( ) ( ) ( )
( ) ( ) ( ) ( 1) ( 1 ) ( )
( ) ( ) ( ) ( 1 ) ( )
( ) ( ) (0) (1) ( ) ( ) ( ) ( 1 ) ( )
N
i i i ik
N
i i i ikeven
N
i i ikeven
N
i i i i ikevenk m
m k k C k m m
k k C k m k C k m m
k k C k m C k m m
m m C C k k C k m C k m m
WY X H
WX H H
WX H
WX H X H
2
0
( 1) ( ) ( ) ( 1) (0) ( ) ( ) ( 1) ( ) ( 1)N
i i i i i ikevenk m
m m m C N C k k C k m C k m mWY X H X H
Similarly, the m+1-th subcarrier signal is expressed as
In such a case, the ICI coefficient is denoted as
'( ) ( ) ( 1 )C k m C k m C k m
ICI Self-Cancellation ICI Self-Cancellation SchemeScheme
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1
0,
'( ) ( ) ( 1),
( ) 2 ( ) ( 1 ) ( 1)
( ) ( 1).
i i i
N
ik k even
i i
Y m Y m Y m
X k C k m C k m C k m
W m W m
The demodulation for self-cancellation is suggested to work in such a way that each signal at the (m + 1)-th subcarrier (m is even) is multiplied by −1 and then summed with the one at the m-th subcarrier. Then the resultant data sequence is used for making symbol decision. It can be represented as
The corresponding ICI coefficients then becomes
''( ) 2 ( ) ( 1 ) ( 1).C k m C k m C k m C k m
0 10 20 30 40 50 6010
-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
ε = 0.2
Subcarrier
Am
plitu
de o
f IC
I coeff
icie
nt
C(k-m)
C(k-m) - C(k+1-m)
2C(k-m) - C(k+1-m) - C(k-m-1)
Fig. 2 Comparison of C(k – m), C’(k – m) and C”(k – m)
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The Proposed Weight-Based ICI Self-The Proposed Weight-Based ICI Self-Cancellation SchemeCancellation Scheme
The received signal Yi’(m) could be represented as
'
1
0,
0, ,
( ) ( ) ( 1),
( ) ( )[ ( ) ( 1 ) ( 1) ( ) ]
( ) ( 1),
( ) ( ) (0) (1) ( 1) (0)
( ) ( )[ ( ) ( 1 ) ( 1) ( ) ]
i i i
N
i ik k even
i i
i i
N
i ik k even k m
Y m Y m Y m
X k H k C k m C k m C k m C k m
W m W m
X m H m C C C N C
X k H k C k m C k m C k m C k m
1
( ) ( 1), (9)i iW m W m
In (9), the weight, λ, is a real value between 0 and 1. Also, the summation of these weights is assumed to be 2, i.e.,
2
9
The CIR in the proposed scheme could be derived
2
12
0,,
2
2
(0) (1) 2 ( 1) (0)
( ) ( 1 ) 2 ( 1) ( )
,
m N
kk evenk m
X
I
C C C N CCIR
C k m C k m C k m C k m
The weighting values, λ and ρ, could be determined based on maximum CIR.
2 2 2 2
2 2
2 2 2 2
( / ) ( / )0
( )
( / ) ( / ) 0
m X I I X
I
X I I X
dCIR d d d d
d
d d d d
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90
0.5
1
abs(W
eig
hting v
alu
e)
λ
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91
1.5
2
Normalize frequency offset
abs(W
eig
hting v
alu
e)
ρ
Fig. 3 The values of weight for the different frequency offsets 10
The mean and variance of the collected symbols could be derived.
The mean and variance of Mi ,SNR=1 11
1
2
1
2
0,
1
1
is numbers of OFDM symbol
is OFDM symbol index
( )
( 1)
S
ii
S
ii
Ni
im m even i
T MS
V M TS
S
i
Y mM
Y m
Simulation ResultsSimulation Results
Table I The simulation parameters for the proposed scheme
Total Number of Simulation 10240000
Number of Subcarrier 512
Samples in Cyclic Prefix 128
Modulation Type QPSK
Carrier Frequency 2.4GHz
Channel Model Six-Ray and AWGN
12
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-20
-10
0
10
20
30
40
50
60
Normalize frequency offset
CIR
Conventional OFDM
Zhao's schemeProposed scheme
Fig. 5 Performance comparison with Zhao’s scheme 13
0 1 2 3 4 5 6 7 8 910
-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
SNR(dB)
BE
R
AWGN ε = 0
Proposed scheme ε = 0.05
Zhao's scheme ε = 0.05Proposed scheme ε = 0.5
Zhao's scheme ε = 0.5
Fig. 6 Performance on the BER under the AWGN channel
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0 5 10 15 20 2510
-4
10-3
10-2
10-1
100
SNR(dB)
BE
R
Proposed scheme ε = 0.05
Zhao's scheme ε = 0.05Proposed scheme ε = 0.5
Zhao's scheme ε = 0.5
Fig. 7 Performance on the BER under the frequency selective fading channel
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ConclusionConclusionThe performances of CIR and BER
on the proposed schemes with a large frequency offset are better than that of Zhao’s scheme.
16