Spatial techniques in WiFi 802.11ac
-
Upload
mohamed-seif -
Category
Engineering
-
view
269 -
download
10
Transcript of Spatial techniques in WiFi 802.11ac
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
STBC (cont’d)
In 802.11ac, four STBC modes were defined (2x1, 4x2,
6x3 or 8x4).
Figure: Alamouti’s Scheme for 4*2.
Motivation
Transmit beamforming is used to enhance the reception
of signals.
Figure: Depicts the difference between two
schemes.
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).
Receiver design
The received signal is then
ZF or MMSE technique is applied to estimate the
symbols.
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.
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.
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.
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.
Figure: Comparison between the two different
pre-coding techniques for MU-MIMO
Performance of the two schemes will converge at higher
SNR.
Results