Post on 20-Dec-2015
12/7/06
Interference Cancellationin MIMO-OFDM Systemwith Outdated Channel State Information
COST 289
Etienne Auger, Carlo Mutti, Marc Kuhn and Armin Wittneben
© ETH Zürich | WCG
212/7/06 augere@ee.ethz.ch
Content
Introduction
System model Multi-stream interference precoder Outdated channel state information
Signal detection
Interference cancellation
Conclusion
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Introduction Multiple-input multiple-output (MIMO) systems are facing
multi-stream interferences (MSI)
In a downlink scenario, no/little data exchange is
assumed between users
MSI suppression can be performed at the transmitter side
if channel state information (CSI) is available [Auger-
2005]
If only partial CSI is available: interferences not fully
suppressed
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System model MIMO-OFDM system
Precoder for multi-stream interference in time domain
with instant. power constraint
Synchronous transmission
Detection based on maximum a posteriori criterium
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Channel model
Channel matrix H in time domain L taps subchannel: Hi,j[n]
entries ~ i.i.d. CN(0,1/L)
Perfect CSI at the receiver, outdated CSI at the
transmitter
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Outdated channel state information [Goeckel]
Delay in perfect feedback link due to dedicated feedback channel or TDD perfect: no error occurs during feedback precoder matches an outdated channel E[ Hi,j[n] . i,j[n] ] = c (0≤c≤1/L)
receiver knows H and
F H*H
H
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Equivalent channel
time domain perfect CSIT
outdated CSIT with
frequency domain
adj( ) det( ) H H E H I
( ) ( ) ' '' ( )
( ) ( ) ' '
FFT FFTFFT
FFT FFT
1,1 1,2 1,1 1,2
2,1 2,2 2,1 2,2
E E E EE E
E E E E
adj( )H H E
1,1 1,2
2,1 2,2
E EE
E E
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Interference energy
equivalent channel matrix
energy in signal subchannel
energy in interference subchannel
2 2
1,1 2,2 1F F
E E E E
2 2 21,2 2,1 1
F FE E E E
1,1 1,2
2,1 2,2
E EE
E E
(0≤ρ≤1)
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Interference cancellation
received vector in frequency domain
transmitted vector in frequency domain
detection of s[k] when is known s1,s2 from 4-QAM constellation
AWGN noise
1 1,1 1 1,2 2 1
2 2,1 1 2,2 2 2
y k k s k k s k n k
y k k s k k s k n k
E E
E E
1
2
y ky k
y k
1
2
s ks k
s k
E
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Vector detection with single observation
joint MAP detection look for and equiprobable inputs: maximize and
2 pairs candidates: at receiver 1
at receiver 2
choose element with largest estimated SINR (minimal
estimated noise and interf. realization)
implies information exchange between receivers
1 2 1(( , ) | )p s s y
1 11 22 21 2
ˆ ˆ( , )
ˆ ˆ( , )
s s
s s
1 2 2(( , ) | )p s s y
1 1 2( | ( , ))p y s s 2 1 2( | ( , ))p y s s
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User detection with single obervation
no information exchange between receivers
single user MAP detection
minimize (ln is concave)
1 2 3 4
1 2 3 4
1 2 3 4
1 2 3 4
, 1,2
, , ,
4
, , ,1
4
, , ,1
24
22, , ,1
ˆ
arg max ( | )
arg max (( , ) | )
arg max ( | ( , ))
( )1arg max exp( )
22
p
p
p
p
p p
p ps c c c c
p i ps c c c c
i
p p is c c c c
i
p p i
s c c c ci
s
p s y
p s c y
p y s c
y s c
24
1
( )p p ii
y s c
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Interference cancellation
step 1 :
parallel
successive cancellation
1 1 2 3 4
2 1 2 3 4
1 1 1, , ,
2 2 2, , ,
1
2ˆ̂ arg max ( | , )
ˆ̂ arg max ( | , )
ˆ
ˆ
s c c c c
s c c c c
s p s y
s
s
p s y s
1 2ˆ , ˆs s
2 2
1 21 22 2
1 1 2 2
ˆ ˆ,
ˆ ˆ
As DsSINR SINR
y As y Ds
1 2
1,2arg max( , )SINR SINRp
ˆarg max ( | , )
ˆ̂ arg max ( |
ˆ̂
ˆ, )ˆ
q
p
q qs
p p p qs
q ps p s y s
s p s y s
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ZF vs. MSI for imperfect CSI
ZF based on
no IC, just detection
ρ≠1: MSI more robust than ZF
user detection
H
0 5 10 15 20 25 30 3510
-4
10-3
10-2
10-1
100
2x2 L=3 OFDM without IC
Es/2N
0 [dB]
SE
R
=1 TD precoder=0.99 TD precoder=0.98 TD precoder=0.9 TD precoder=1 ZF precoder=0.99 ZF precoder=0.98 ZF precoder=0.9 ZF precoder
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Detection step (joint vs. user detection) same results with y2
user detection
performs badly when
low correlation
joint detection on
interf. terms for low
correlation
joint detection on
signal terms best 10-1
100
10-2
10-1
100
user
SE
R
s1 from y1 with joint detection
s2 from y1 with joint detection
s1 from y1 with user detection
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Interference cancellation
user detection with
several iterations
does not improve
much SER
joint detection
remains best for any
# iterations
0 5 10 15 20 25 30 35 4010
-4
10-3
10-2
10-1
100
= 0.99
Es/2N
0 [dB]
user
SE
R
=1 reference
user detection, IC 2 steps
user detection, IC 1 stepuser detection, IC 3 steps
joint detection, IC 1 step
joint detection, IC 2 steps
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Conclusion
MSI precoder performance with outdated CSIT and
perfect CSIR
signal detection with single observation joint or for each user
interference cancellation SINR based iterative
joint detection achieves better user SER
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Thank you !!
Questions
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Simulation results for user detection
0 5 10 15 20 25 30 35 4010
-4
10-3
10-2
10-1
100
Es/2N
0
SE
R
2x2, 3 taps, user optimal detection, =0.99
user optimal detection with parallel IC
no IC
reference =1user optimal detection IC 1 iteration
user optimal detection IC 2 iterations
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Results for joint detection
0 5 10 15 20 25 30 35 4010
-4
10-3
10-2
10-1
100
2x2, 3 taps, joint optimal detection, =0.99
Es/2N
0
SE
R
reference =1
no IC
successive detection 1 iteration
successive detection 2 iterations
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Joint vs. User detection
From 2 iterations, joint and user detection have similar performance
0 5 10 15 20 25 30 35 4010
-4
10-3
10-2
10-1
100
2x2, 3 taps, optimal detection, =0.99
Es/2N
0
SE
R
reference =1
joint optimal detection 2 iterations
user optimal detection IC 2 iterationsjoint detection 1 iteration
user detection 1 iteration
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Conclusion
MIMO-OFDM with MSI precoder
perfect CSIR and outdated CSIT IC required optimal user and joint detection multistage IC for both detections
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Multistage
parallel:
successive: same scheme as for multistage
detection for user
1 1 2 3 4
2 1 2 3 4
1 1 1 2, , ,
2 2 2 1, , ,
ˆ̂ ˆarg max ( | , )
ˆ̂ ˆarg max ( | , )
s c c c c
s c c c c
s p s y s
s p s y s