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 brianh@cisco.com

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

408-5258143 ragranga@cisco.com

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

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

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

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

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

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

January 2008

Hart et al (Cisco)

Slide 8

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA Performance – Channel Type & Detection Duration

January 2008

Hart et al (Cisco)

Slide 9

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA Performance – SNR

January 2008

Hart et al (Cisco)

Slide 10

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA Performance – Carrier Frequency Offset

January 2008

Hart et al (Cisco)

Slide 11

doc.: IEEE 802.11-07/3001r0

Submission

Mid-Packet CCA Performance – Number of RX Antennas

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

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

January 2008

Hart et al (Cisco)

Slide 14

doc.: IEEE 802.11-07/3001r0

Submission

Questions?

?

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

January 2008

Hart et al (Cisco)

Slide 16

doc.: IEEE 802.11-07/3001r0

Submission

Backup Slides

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?

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

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