12/7/06 Interference Cancellation in MIMO-OFDM System with Outdated Channel State Information COST 289 Etienne Auger, Carlo Mutti, Marc Kuhn and Armin

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Interference Cancellationin MIMO-OFDM Systemwith Outdated Channel State Information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 [email protected]

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System model Multi-stream interference precoder Outdated channel state information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-

2005]

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Perfect CSI at the receiver, outdated CSI at the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Outdated channel state information [Goeckel]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feedback link due to dedicated feedback channel or TDD perfect: no error occurs during feedback precoder matches an outdated channel E[ Hi,j[n] .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

F H*H

H


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outdated CSIT with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


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)



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


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


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



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

ârg 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


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


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



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


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

-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


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


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


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 ârg max ( | , )

ˆ̂ ârg max ( | , )

s c c c c

s c c c c

s p s y s

s p s y s

Documents

12/7/06 Interference Cancellation in MIMO-OFDM System with Outdated Channel State Information COST 289 Etienne Auger, Carlo Mutti, Marc Kuhn and Armin