1. COST 289 Etienne Auger, Carlo Mutti, Marc Kuhn and Armin Wittneben
Interference Cancellation in MIMO-OFDM System with Outdated Channel State Information
COST 289
Etienne Auger, Carlo Mutti, Marc Kuhn and Armin Wittneben
© ETH Zürich | WCG

2. Content Introduction System model Signal detection
Multi-stream interference precoder
Outdated channel state information
Signal detection
Interference cancellation
Conclusion

3. 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

4. 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

5. 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Perfect CSI at the receiver, outdated CSI at the transmitter

6. Outdated channel state information [Goeckel]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precoder matches an outdated channel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

7. Equivalent channel time domain frequency domain perfect CSIT
outdated CSIT with
frequency domain

8. Interference energy equivalent channel matrix
energy in signal subchannel
energy in interference subchannel
(0≤ρ≤1)

9. 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

10. 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

11. User detection with single obervation
no information exchange between receivers
single user MAP detection
minimize (ln is concave)

12. Interference cancellation
step 1 :
parallel
successive cancellation

13. ZF vs. MSI for imperfect CSI
ZF based on
no IC, just detection
ρ≠1: MSI more robust than ZF
user detection

14. 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

15. Interference cancellation
user detection with several iterations does not improve much SER
joint detection remains best for any # iterations

16. 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

17. Thank you !! Questions

18. Simulation results for user detection

19. Results for joint detection

20. Joint vs. User detection
From 2 iterations, joint and user detection have similar performance

21. Conclusion MIMO-OFDM with MSI precoder perfect CSIR and outdated CSIT
IC required
optimal user and joint detection
multistage IC for both detections

22. Multistage
parallel:
successive: same scheme as for multistage detection for user