1. Preambles, Beamforming, and the WWiSE Proposal
January 2005
doc.: IEEE /1645r0
January 2005
Preambles, Beamforming, and the WWiSE Proposal
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C. Hansen, Broadcom
C. Hansen, Broadcom

2. January 2005
doc.: IEEE /1645r0
January 2005
Abstract
A discussion of closed loop techniques and the WWiSE proposal for n.
C. Hansen, Broadcom
C. Hansen, Broadcom

3. Outline Background Closed loop in 802.11n Beamforming
January 2005
Outline
Background
Closed loop in n
Beamforming
Simulation Results with the WWiSE Preambles
Conclusions
C. Hansen, Broadcom

4. Background Initial WWiSE proposal did not address closed loop
January 2005
Background
Initial WWiSE proposal did not address closed loop
Not needed for high performance
Other means, such as Space-Time Block Codes (STBC) are effective
Cost / Complexity
Mode explosion
Interoperability
WWiSE has added provisions for closed loop (04/866r6)
Mode feedback
Channel estimate feedback
C. Hansen, Broadcom

5. Closed Loop for 802.11n There are many ways to use feedback
January 2005
Closed Loop for n
There are many ways to use feedback
Rate selection
Mode selection
Beamforming
Rate and Mode selection require simple feedback
Easy to implement
Easy to standardize
Beamforming requires channel state feedback
More complicated
C. Hansen, Broadcom

6. Beamforming P Streams M Transmit Antennas N Receive Antennas
January 2005
Beamforming
P Streams
M Transmit Antennas
N Receive Antennas
MIMO Channel Matrix, H is N by M
TX can apply a M by P Beamforming (Precoding) Matrix V
Channel at RX looks like a N by P matrix
C. Hansen, Broadcom

7. January 2005
SVD Beamforming
One proposed techniques is to compute V with a Singular Value Decomposition (SVD)
U and V are unitary matricies, S is a diagonal matrix.
If H is known at TX, V can be applied.
RX stations sees a new channel that looks like US.
C. Hansen, Broadcom

8. Reception of SVD Beamformed Frames
January 2005
Reception of SVD Beamformed Frames
Is it feasible to apply SVD beamforming to an n frame defined in the WWiSE proposal?
For example
2 TX streams
4 TX antennas
Compute 4 by 2 V matrix for each sub-carrier and apply at TX
Use the 2 stream frame defined in 04/866r6.
C. Hansen, Broadcom

9. Reception of SVD Beamformed Frames
January 2005
Reception of SVD Beamformed Frames
Contrary to speculative arguments in 05/1635, you can apply SVD beamforming to WWiSE preambles
Receiver processing works the same way
Excellent channel estimation performance
Example Simulation Results
1000 independent 4 TX, 2 RX Model D Channels
SVD beamform 2 TX Streams
25 db SNR
Compute channel estimate and show error cumulative distribution function (CDF)
C. Hansen, Broadcom

10. Channel Estimation Results
January 2005
Channel Estimation Results
1000 channels
Channel Model D
SNR = 25 dB
4 Transmit Antennas
2 Transmit Streams
2 Receive Antennas
C. Hansen, Broadcom

11. Channel Estimation Results
January 2005
Channel Estimation Results
Preamble processing with or without beamforming is the same
Same benefits of the WWiSE preamble carry forward
Good performance
Low overhead
Long preambles from other proposals are unnecessary
C. Hansen, Broadcom

12. January 2005
Conclusions
WWiSE has added provisions for closed loop to its proposal (04/866r6)
WWiSE is not advocating a specific beamforming method
Some members of n have claimed WWiSE preambles are not compatible with beamforming
WWiSE preambles are fully compatible with beamforming.
C. Hansen, Broadcom