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January 2008 Hart et al (Cis Slide 1 doc.: IEEE 802.11-07/3001r0 Submission Legacy Coexistence – A Better Way? Date: 2007-11-26 N am e C om pany A ddress Phone em ail Brian H art Cisco System s,Inc. 170 W estTasm an D rive, San Jose, CA 95134 408-526-3346 [email protected] Raghuram Rangarajan Cisco System s 170 W estTasm an D rive, San Jose, CA 95134 408-5258143 [email protected] Authors:

Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

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Page 1: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 1

doc.: IEEE 802.11-07/3001r0

Submission

Legacy Coexistence – A Better Way?

Date: 2007-11-26

Name Company Address Phone email Brian Hart Cisco Systems, Inc. 170 West Tasman Drive,

San Jose, CA 95134 408-526-3346 [email protected]

Raghuram Rangarajan Cisco Systems 170 West Tasman Drive, San Jose, CA 95134

408-5258143 [email protected]

Authors:

Page 2: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 2

doc.: IEEE 802.11-07/3001r0

Submission

Summary

• Some say VHT should steer clear of 5 GHz because of the coexistence problems

• 11n has taken a long time to resolve just 20/40 coexistence

• We argue that this is mainly because of limitations in early implementations leading to standards compromise

• If we start with solid PHY coexistence via additional modest RX requirements, many coexistence problems disappear

Page 3: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 3

doc.: IEEE 802.11-07/3001r0

Submission

Coexistence of 11n with 11abg is a standards compromise

• A collection of protections

− some for sound technical reasons

− others based on what vendors had (or had not) implemented some time in the past

Page 4: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 4

doc.: IEEE 802.11-07/3001r0

Submission

Some coexistence mechanisms are disconnected from legacy behavior

• In 20/40, CCA protection via ED at -62 dBm on the secondary

− More hidden nodes on the secondary

• With GF, CCA protection via ED at -72 dBm

− More hidden nodes with GF

• In 20/40, no virtual carrier sense on the secondary

− RTS, CTS, CTS2self on secondary not respected

− Duration/ID field on secondary not respected

− Duration/ID field in 40 MHz frames not respected by legacy and non-40MHz devices on either channel (weak ACK protection, should start a TXOP with a 20 MHz frame)

• In 20/40 exponential backoff using medium busy measured on the primary only, with a brief (PIFS) CCA inspection on the secondary

− Less responsive to congestion on the secondary channel

Page 5: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 5

doc.: IEEE 802.11-07/3001r0

Submission

Can We Devise An Improved CCA?

• ED at -82 dBm is challenging:

− Requires either a low noise floor or causes a higher false-busy rate

• Preamble detection at -82 dBm is well understood for 20 MHz

− For 40/80/160 MHz, this requires 2/4/8 parallel filters for each 20 MHz sub-channel, and parallel short symbol detectors

• Preamble detection is ineffective after a transmission, for in-progress frames on other 20 MHz channels

− We need a mid-packet CCA

Page 6: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 6

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA

• Packet detection without a preamble

− No carrier frequency recovery

− No timing recovery

− No channel estimation

• OFDM looks like Gaussian noise yet can be identified by its regular cyclic extension

• Note: mid-packet CCA is possible for DSSS and CCK also

− DSSS cross-correlation

− CCK is composed of QPSK chips, so x4/|x|3 looks like DC

• Obscured by carrier frequency offsets and delay spread

Page 7: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 7

doc.: IEEE 802.11-07/3001r0

Submission

Example Scheme for Mid-Packet CCA for OFDM

• Many further improvements are possible (e.g. short GI)

CCA

Moving Average

0.8us

Max(Re(in)) / RMS(in) > thresh AND > -82dBm

Existing CCA

OR

+

Delay 3.2us and

conjugate

X

Delay 3.2us and

conjugate

X

Delay 4us and

conjugate

X

Autocorrelation “Singly differential”

Cyclic extension

“matched filter”

Autocorrelation“Doubly differential”

for easy carrier frequency immunity

Average over 16 or

32us at 4us-

spacing

Page 8: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 8

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA Performance – Channel Type & Detection Duration

Page 9: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 9

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA Performance – SNR

Page 10: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 10

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA Performance – Carrier Frequency Offset

Page 11: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 11

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA Performance – Number of RX Antennas

Page 12: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 12

doc.: IEEE 802.11-07/3001r0

Submission

How to Interop Test for Mid-Packet CCA Compliance

• Test 1: 40 MHz BSS enabled. 20 MHz BSS on primary alternates (a) periods of near-100% duty cycle long-packet SIFS-spaced traffic with (b) periods of no traffic. Record PER.

• Test 2: 40 MHz BSS enabled. 20 MHz BSS on secondary alternates (a) periods of near-100% duty cycle long-packet SIFS-spaced traffic with (b) periods of no traffic. Verify 40 MHz frames are transmitted from B to A (e.g. via MAC stats). Record PER.

• Any excess PER from test @ to test 1 is due to disallowed transmissions from DUT B

A AP

B DUT

C AP

D

Attenuation matrix65dB between AB, CD

95dB between AC, AD, BC, BD

20/40 MHz BSS 20 MHz BSS on primary then secondary

Page 13: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 13

doc.: IEEE 802.11-07/3001r0

Submission

Summary of Coexistence Mechanisms

• Parallel start-of-packet detection on each 20 MHz channel is feasible

− Parallel filters and short symbol detectors

• Parallel mid-packet detection on each 20 MHz channel is feasible

− Parallel filters and cyclic extension detectors

− Behavior at packet end is TBD: EIFS backoff? Something else?

• Parallel PLCP decoding on each 20 MHz channel is tougher but is not so hard it can be ruled out yet

− Parallel receivers, but for the brief BPSK PLCP only

• Parallel virtual carrier sense on each 20 and 40 MHz channel is infeasible

− Fully parallel receivers

− Ability to receive on one channel even when transmitting on a nearby channel

Page 14: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 14

doc.: IEEE 802.11-07/3001r0

Submission

Questions?

?

Page 15: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 15

doc.: IEEE 802.11-07/3001r0

Submission

Strawpoll

If VHT produced a PAR for 5 GHz operation, would you support further investigation into improved legacy coexistence methods such as are described on slide 13?

Yes

No

Abstain

Page 16: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 16

doc.: IEEE 802.11-07/3001r0

Submission

Backup Slides

Page 17: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 17

doc.: IEEE 802.11-07/3001r0

Submission

Open Problems

• What about false alarms from other wireless systems?

• What about false alarms from adjacent-channel WiFi?

• How well does this work with short-GI?

Page 18: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 18

doc.: IEEE 802.11-07/3001r0

Submission

False Alarms from Non-WiFi Signals

• Previous scheme was optimized for OFDM vs noise

• What about false alarms from other wireless systems?

− Sinusoids that positively autocorrelate at any delay

− Narrowband signal that approximate sinusoids

− etc

• Target the cyclic extension specifically:

− We have a positive autocorrelation for 800ns then “noise” for 3.2us

− Replace 800ns moving average impulse response with an impulse response with zero mean

vs

0.8us 0.8us3.2us

1

0-0.25

Page 19: Doc.: IEEE 802.11-07/3001r0 Submission January 2008 Hart et al (Cisco)Slide 1 Legacy Coexistence – A Better Way? Date: 2007-11-26 Authors:

January 2008

Hart et al (Cisco)

Slide 19

doc.: IEEE 802.11-07/3001r0

Submission

False Alarms from Non-WiFi Signals – Still OK against noise