This document discusses various MIMO techniques including single-user and multi-user MIMO. It begins with an overview of STBC, which is used in 802.11ac and provides transmit diversity with low cost. Spatial multiplexing allows multiple parallel channels to improve throughput. For single-user MIMO, transmit beamforming enhances signal reception through precoding techniques like SVD that establish parallel channels. Multi-user MIMO further increases capacity but introduces interference that must be managed through precoding and receiver techniques like zero-forcing. Channel feedback is also required to implement beamforming and precoding.

1. 1. Overview on basic MIMO techniques
2. Single User Beamforming
3. Multi User MIMO

2. Mohamed Seif

3. STBC
 WiFi 802.11ac uses STBC (Alamouti Scheme).
 STBC is a transmit diversity scheme that comes with a
robust performance achieved with low cost.
Figure: Alamouti’s Scheme for 2*1

4. STBC (cont’d)

5. STBC (cont’d)
 In 802.11ac, four STBC modes were defined (2x1, 4x2,
6x3 or 8x4).
Figure: Alamouti’s Scheme for 4*2.

6. Spatial Multiplexing
Figure: 2*2 MIMO system.

8. Motivation
 Transmit beamforming is used to enhance the reception
of signals.
Figure: Depicts the difference between two
schemes.

9. Motivation (cont’d)
 In broader sense, higher SINR.
 In Transmit beamforming, parallel channel paths are
established such that each symbol is transmitted.
Figure: Conventional MIMO
system equivalent channel.
Figure: TX beamforming
equivalent channel (Ideal).

10. SVD approach
Where X is either U or V

11. SU-Beamforming Pre-coding
Figure: Equivalent channel model after pre-coding.
U

12. SU-Beamforming Pre-coding

13. Receiver design
 The received signal is then
 ZF or MMSE technique is applied to estimate the
symbols.

14. Figure: Tx Beamforming gain over conventional MIMO
in terms of un coded BER.
Results

15. Channel Correlation and Interference
Figure: Channel correlation can degrade the
beamforming gain.

16. Figure: Effect of channel correlation.
Results

17. Channel Feedback
 The Access Point (AP) does not have any Channel State
Information about the channel (CSI).
 Transmit Beamforming requires CSI in order to apply the
weighting matrix V.
 Channel feedback techniques are used.

18. Channel Feedback (cont’d)
 Communication system is a closed loop system.
 TDD is commonly used.
 Compressed feedback technique is used to reduce
overhead.
Figure: Closed loop system.

19. Compressed Channel Feedback

20. Compressed Channel Feedback (cont’d)

21. Compressed Channel Feedback (cont’d)

22. Histogram and Optimization
 Variable feedback rate may be investigated using
source coding.
Ref. W.H. Chin, C. Yuen, Design of differential quantization for low bitrate channel state
information feedback in MIMO-OFDM systems. IEEE VTC 2008 Spring.

23. Results

25. Multi-User MIMO (MU-MIMO)
 Capacity increasing.
 Interference Problem.
Figure: MU-MIMO scheme.

26. MU-MIMO modeling

27. MU-MIMO Pre-coding
 ZF Pre-coding (channel inversion)
 Regularizing the inversion
Figure: Block diagram for MU-MIMO pre-coding process.
Ref. An Introduction to the Multi-User MIMO Downlink, IEEE comsoc.

28. Receiver design

29. Receiver design (cont’d)
 The estimated symbol is then
Where,

30. Figure: Comparison between the two different
pre-coding techniques for MU-MIMO
Performance of the two schemes will converge at higher
SNR.
Results

31. Figure: Effect of feedback delay in MU-MIMO.
Results

32. THANK YOU!