12/7/06 Interference Cancellation in MIMO-OFDM System with Outdated Channel State Information COST...

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12/7/06

Interference Cancellationin MIMO-OFDM Systemwith Outdated Channel State Information

COST 289

Etienne Auger, Carlo Mutti, Marc Kuhn and Armin Wittneben

212/7/06 augere@ee.ethz.ch

Content

Introduction

System model Multi-stream interference precoder Outdated channel state information

Signal detection

Interference cancellation

Conclusion

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-

If only partial CSI is available: interferences not fully

suppressed

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

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Perfect CSI at the receiver, outdated CSI at the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

Outdated channel state information [Goeckel]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feedback link due to dedicated feedback channel or TDD perfect: no error occurs during feedback precoder matches an outdated channel E[ Hi,j[n] .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

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outdated CSIT with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

Interference energy

equivalent channel matrix

energy in signal subchannel

energy in interference subchannel

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

(0≤ρ≤1)

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 ky k

s ks k

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( , )

1 2 2(( , ) | )p s s y

1 1 2( | ( , ))p y s s 2 1 2( | ( , ))p y s s

User detection with single obervation

no information exchange between receivers

single user MAP detection

minimize (ln is concave)

1 2 3 4

, , ,1

22, , ,1

arg max ( | )

arg max (( , ) | )

arg max ( | ( , ))

( )1arg max exp( )

p ps c c c c

p i ps c c c c

p p is c c c c

s c c c ci

p s c y

p y s c

( )p p ii

step 1 :

parallel

successive cancellation

1 1 2 3 4

2 1 2 3 4

1 1 1, , ,

2 2 2, , ,

2ˆ̂ arg max ( | , )

ˆ̂ arg max ( | , )

s c c c c

s p s y

p s y s

1 2ˆ , ˆs s

1 21 22 2

1 1 2 2

ˆ ˆ,

As DsSINR SINR

y As y Ds

1,2arg max( , )SINR SINRp

ˆarg max ( | , )

ˆ̂ arg max ( |

ˆ, )ˆ

p p p qs

q ps p s y s

s p s y s

ZF vs. MSI for imperfect CSI

ZF based on

no IC, just detection

ρ≠1: MSI more robust than ZF

user detection

0 5 10 15 20 25 30 3510

2x2 L=3 OFDM without IC

0 [dB]

=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

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

s1 from y1 with joint detection

s2 from y1 with joint detection

s1 from y1 with user detection

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

= 0.99

0 [dB]

=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

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

Thank you !!

Questions

Simulation results for user detection

0 5 10 15 20 25 30 35 4010

2x2, 3 taps, user optimal detection, =0.99

user optimal detection with parallel IC

reference =1user optimal detection IC 1 iteration

user optimal detection IC 2 iterations

Results for joint detection

0 5 10 15 20 25 30 35 4010

2x2, 3 taps, joint optimal detection, =0.99

reference =1

successive detection 1 iteration

successive detection 2 iterations

Joint vs. User detection

From 2 iterations, joint and user detection have similar performance

0 5 10 15 20 25 30 35 4010

2x2, 3 taps, optimal detection, =0.99

reference =1

joint optimal detection 2 iterations

user optimal detection IC 2 iterationsjoint detection 1 iteration

user detection 1 iteration

Conclusion

MIMO-OFDM with MSI precoder

perfect CSIR and outdated CSIT IC required optimal user and joint detection multistage IC for both detections

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 ( | , )

s c c c c

s p s y s

12/7/06 Interference Cancellation in MIMO-OFDM System with Outdated Channel State Information COST...

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