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
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).
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.
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.
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.