1. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 1 WWiSE Preambles and MIMO Beamforming? Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at.http:// ieee802.org/guides/bylaws/sb-bylaws.pdfstuart.kerry@philips.compatcom@ieee.org Date: 2005-01-15 Authors:

2. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 2 Abstract Interest in beamforming for WLAN products is growing. This is evidenced by the recent launch of several products that provide enhanced rate at range performance via beamforming - using 802.11a/b/g waveforms! It is expected that this trend will continue as MIMO is introduced. The WWiSE proposal for 802.11n High Throughput WLAN has a preamble structure that can not support of advanced beamforming techniques. This presentation itemizes the problems associated MIMO BF (e.g., SVD BF) with the WWiSE preambles.

3. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 3 Summary Problem 1 –WWiSE structure does not allow omni-directional transmission of SIG-N –Result: Hidden node problems Problem 2 –WWiSE preambles are designed for “per antenna training” –Low overhead BF (beamforming) requires “per spatial stream training” Eliminates explicit feedback of BF steering matrices by exploiting reciprocity –Conjecture: WWiSE could apply ½ symbol cyclic shift training to spatial streams Problem 2b –WWiSE channel estimation requires smoothing algorithms –Channel smoothing cannot be applied with MIMO BF (e.g., SVD)

4. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 4 Problem 1: SIG-N is not omni-directional WWiSE ½ symbol cyclic shift training applied to 2 spatial streams to support MIMO BF SIG-N must then be transmitted in BF NOT omni-directional This introduces hidden node problems Also, how are short symbols transmitted – do 400 ns cs, 1600 ns cs and spatially multiplexed BF data all yield the same Rx power?

5. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 5 Problem 1 MAC Protection? –MAC protection mechanisms assume omni-directional transmission of control packets PHY Protection –Omni-directional part provides PHY protection –However, the shift to BF part  large increase in Rx power –Can drive receiver into saturation –Solution: 2 nd AGC is needed (as in TGn Sync)

6. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 6 Problem 2: Per Antenna Training Per Spatial Stream Training –Receiver directly estimates combined channel –Low-overhead: no explicit transmission of BF matrices –Transparency: no prior knowledge of packet BF structure at the receiver Rx acquisition and equalizer processing is identical for both BF and non-BF packets Per Antenna Training –Requires high overhead of explicit communication of BF matrices –Complex Rx signal processing Exploitation of channel reciprocity with per spatial stream training is the preferred BF solution

7. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 7 Problem 2b: WWiSE Requires Ch. Est. Smoothing 0.25 0.5 0.25 recovers Smoothing Window -0.25 0.5 -0.25 recovers 1600 ns cs produces introduces a  1 factor on H 2 (f k ), -1 for odd k

8. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 8 Why is channel est. smoothing bad for MIMO BF? Smoothing requires high adj. tone coherence However, we must estimate the combined channel –BF matrix has poor adjacent tone coherence Why? –Eigen-channel rank reversals For each tone, eigen-channels are ranked by singular values Eigen-channels can reverse ranks on adjacent tones – resulting in an adjacent tone swap of corresponding columns of BF matrix Result – very low adjacent tone coherence –Singular value multiplicity (nearly equal singular values) Common eigen space - blurs distinction between eigen-channels Numerical precision issues

9. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 9 SVD Background diagonal matrix of singular values matrix of right singular vectors = BF matrix U and V are orthonormal matrices; columns are orthonormal Columns of U and V are left and right singular vectors V is the optimal BF matrix Uniqueness:  is unique U and V are unique up to per column phase factor

10. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 10 BF Adjacent Tone Coherence = BF matrix for subcarrier = i th column of Definition: Adjacent Tone Coherence Properties: is a complex number

11. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 11 What about the arbitrary singular vectors phase? Columns of V are unique up to an arbitrary phase. Select these phases to maximize phase coherence between adjacent frequencies. Problems? YES! This is an additional non-linear processing step in a time-critical operation. Result: is an optimistic measure of adjacent tone coherence.

12. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 12 Example: 2 x 2, Model B Eigen-channel rank reversal

13. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 13 Example: 2 x 2, Model D Nearly equal singular values  coherence breakdown

14. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 14 Example: 2 x 2, Model D Eigen-channel rank reversal

15. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 15 Example: 4 x 2, Model D Loss of coherence in 2 nd eigen channel only

16. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 16 Example: 4 x 2, Model D Eigen-channel rank reversal Loss of coherence in 2 nd eigen channel only Joint loss of coherence due to rank reversal

17. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 17 Example: 4 x 4, Model D

18. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 18 Example: 4 x 4, Model D

19. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 19 Conclusions The WWiSE preamble is a barrier for MIMO BF –Non-omni-directional SIG-N –Required smoothing in channel estimation –WWiSE is a dead end for Tx MIMO BF MIMO BF is important for future extensibility of the 802.11n standard –Recently launched BF products have demonstrated enhanced rate-at- range performance in standards based (a/b/g) solutions –BF allows concentration of cost and power consumption to the AP for downlink intensive applications (e.g., video) The 11n MIMO standard should provide seamless support MIMO Tx beamforming

20. doc.: IEEE 802.11-05/1635r0 Submission January 2005 John S. Sadowsky, IntelSlide 20 References IEEE 802.11-04/0886r6, “WWiSE Proposal: High throughput extension to the 802.11 Standard”