January 2005

C. Hansen, Broadcom

Slide 1

doc.: IEEE 802.11-05/1645r2

Submission

Preambles, Beamforming, and the WWiSE Proposal

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.

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Date: 2005-01-17

Name Company Address Phone email Christopher J. Hansen

Broadcom 190 Mathilda Place Sunnyvale, CA 94096

+1 408 543 3378

chansen@broadcom.com

Carlos Aldana Broadcom 190 Mathilda Place Sunnyvale, CA 94096

+1 408 922 5907

caldana@broadcom.com

Joonsuk Kim Broadcom 190 Mathilda Place Sunnyvale, CA 94096

+1 408 543 4455

joonsuk@broadcom.com

Authors:

January 2005

C. Hansen, Broadcom

Slide 2

doc.: IEEE 802.11-05/1645r2

Submission

Abstract

A discussion of preambles, closed loop techniques, and the WWiSE proposal for 802.11n.

January 2005

C. Hansen, Broadcom

Slide 3

doc.: IEEE 802.11-05/1645r2

Submission

Outline

• Background

• Closed loop in 802.11n

• Beamforming

• Simulation Results with the WWiSE Preambles

• Conclusions

January 2005

C. Hansen, Broadcom

Slide 4

doc.: IEEE 802.11-05/1645r2

Submission

Background

• Initial WWiSE proposal did not address closed loop– We preferred the simplicity of Open Loop

• Space-Time Block Codes (STBC) are very effective

– Cost / complexity concerns

– Mode explosion

– Interoperability

• WWiSE has added provisions for closed loop (04/866r6)– Mode feedback

– Channel estimate feedback

January 2005

C. Hansen, Broadcom

Slide 5

doc.: IEEE 802.11-05/1645r2

Submission

Closed Loop for 802.11n

• 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

January 2005

C. Hansen, Broadcom

Slide 6

doc.: IEEE 802.11-05/1645r2

Submission

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

nxHVy )(

January 2005

C. Hansen, Broadcom

Slide 7

doc.: IEEE 802.11-05/1645r2

Submission

SVD Beamforming

• Qualcomm and TGnSync have proposed to compute V with a Singular Value Decomposition (SVD)

HUSVH • U and V are unitary matrices, S is a diagonal matrix. • If H is known at TX, V can be computed and applied to the TX

signal.• RX stations sees a new channel that looks like US.

January 2005

C. Hansen, Broadcom

Slide 8

doc.: IEEE 802.11-05/1645r2

Submission

Reception of SVD Beamformed Frames

• Arguments from Qualcomm/TGnsync (05/1635r0)– “The WWiSE preamble is a barrier for MIMO BF”

– “WWiSE is a dead end for Tx MIMO BF”

• In Reality

– WWiSE preambles are preferable for beamforming

January 2005

C. Hansen, Broadcom

Slide 9

doc.: IEEE 802.11-05/1645r2

Submission

Reception of SVD Beamformed Frames

• How could one apply SVD beamforming to an 802.11n 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.

Time

TransmitAntennas

1

10 x 0.8 = 8s

2

1.6 + 2 x 3.2 = 8s 4s

SS40GF

SS40GF(400 ns cs)

LS40GF

LS40GF(1600 ns cs)

GI2

GI2

SIG-N

SIG-N(1600 ns cs)GI GI

GIGI Data

Short sequence Long sequence Signal Data payload

Data

January 2005

C. Hansen, Broadcom

Slide 10

doc.: IEEE 802.11-05/1645r2

Submission

Reception of Beamformed frames

V Matrix802.11n

RX

ChannelH

TX Streams

Beamforming Transmitter

January 2005

C. Hansen, Broadcom

Slide 11

doc.: IEEE 802.11-05/1645r2

Submission

Reception of SVD Beamformed Frames

• Receiver processing for beamformed frames– Same as for frames that are not beamformed

– Initial acquistion is the same

– Channel estimation is the same

– Processing and decoding is the same

January 2005

C. Hansen, Broadcom

Slide 12

doc.: IEEE 802.11-05/1645r2

Submission

Simulation of SVD Beamformed Frames

• Simulation Setup– Same receiver as we used for FRCC simulations

– 1000 independent channels

– SVD beamform 2 TX Streams• 2 and 4 TX antennas

– 25 db SNR

– Compute channel estimate and show cumulative distribution function (CDF) of the mean square error

January 2005

C. Hansen, Broadcom

Slide 13

doc.: IEEE 802.11-05/1645r2

Submission

Channel Estimation Results

1000 channels

Channel Model D

SNR = 25 dB

4 Transmit Antennas

2 Transmit Streams

2 Receive Antennas

-32 -30 -28 -26 -24 -22 -20 -18 -160

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Mean Square Channel Estimation Error (dB)

Pro

bab

ility

of

Mea

n S

qu

are

Ch

ann

el E

stim

atio

n E

rro

r <

X

January 2005

C. Hansen, Broadcom

Slide 14

doc.: IEEE 802.11-05/1645r2

Submission

Channel Estimation Results

• 1000 channels

• Channel Model B

• SNR = 25 dB

• 2 Transmit Antennas

• 2 Transmit Streams

• 2 Receive Antennas

-29 -28 -27 -26 -25 -24 -23 -22 -210

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Mean Square Channel Estimation Error (dB)

Pro

bab

ility

of

Mea

n S

qu

are

Ch

ann

el E

stim

atio

n E

rro

r <

X

SVD Beamforming

No Beamforming

January 2005

C. Hansen, Broadcom

Slide 15

doc.: IEEE 802.11-05/1645r2

Submission

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 TGnsync are unnecessary

January 2005

C. Hansen, Broadcom

Slide 16

doc.: IEEE 802.11-05/1645r2

Submission

Why is the WWiSE Preamble Design Better?

• It is flexible– Legacy compatible when necessary

– Shorter for highest throughput

– Always shorter than the TGnsync proposal

• It works with open loop

• It works with beamforming

• As phy rate increases, the throughput gain increases

January 2005

C. Hansen, Broadcom

Slide 17

doc.: IEEE 802.11-05/1645r2

Submission

Conclusions

• WWiSE has added provisions for closed loop to its proposal (04/866r6)

• WWiSE is not advocating a specific beamforming method

• Some members of 802.11n have claimed WWiSE preambles are not compatible with beamforming

• WWiSE preambles are superior for beamforming.