July 2006

David Tung, Ralink

Slide 1

doc.: IEEE 802.11-06/0909r0

Submission

On L-SIG TXOP Protection

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: 2006-07-11

Name Company Address Phone email David Tung Ralink

Technology +1-408-725-

8070 Ext. 16 dtung@ralinktech.com

Authors:

July 2006

David Tung, Ralink

Slide 2

doc.: IEEE 802.11-06/0909r0

Submission

Summary of L-SIG TXOP Protection Related Comments

• L-SIG TXOP protection related comments ID (CID 3765, 8277, 7258, 6788) : – C1: L-SIG TXOP shouldn’t be used under mixed legacy-HT BSS.

– C2: L-SIG TXOP shouldn’t be used under overlapping BSS,

– C3: NAV be set by L-SIG only when HT-SIG CRC pass.

– C4: Applying HT-SIG CRC to cover L-SIG so to enhance L-SIG TXOP protection.

July 2006

David Tung, Ralink

Slide 3

doc.: IEEE 802.11-06/0909r0

Submission

Outline of this Presentation

• Provide an analytical framework for the problem.

• Discussions and conclusions.

July 2006

David Tung, Ralink

Slide 4

doc.: IEEE 802.11-06/0909r0

Submission

Assumptions of the Analytical Framework

• CCA detection: there are three types of CCA detection, namely positive CCA (packets exist with positive detection), miss CCA (packets exist but miss detection) and false CCA (packets not exist but positive detection). We assume miss CCA will always incur false CCA (most likely in reality).

• Assume bit error rate of L-SIG and HT-SIG are all Pb_sig and independent bit-wise. We will study Pb_sig = from ~0.5 to 10^-6. Independent comes from interleaver. (That is iid)

• Assume 1T1R AWGN.• 8 bits HT-SIG CRC: 1. Assume all <= 8 bit errors can be detected

by CRC. 2. Assume 1/256 for > 8 bits errors can not be detected by CRC.

• Assume coding gain of 64 state rate ½ BCC is 7dB (without deducting rate reduction loss.)

July 2006

David Tung, Ralink

Slide 5

doc.: IEEE 802.11-06/0909r0

Submission

Definition of Key Parameters

• Pb_sig: Bit error rate of L-SIG and HT-SIG• Ps_lsig: Symbol error rate of L-SIG.• Pfa_lsig: False alarm rate of L-SIG. (related to C1)• Ps_sig: Symbol error rate of HT-SIG.• Pfa_lsig2: False alarm rate of L-SIG with HT-SIG CRC pass.

(related to C3)• Pfa_htsig: False alarm rate of HT-SIG.• Ps_sig2: Symbol error rate of HT-SIG with CRC cover L-SIG.• Pfa_htsig2: False alarm rate of HT-SIG with CRC cover L-SIG.

(related to C4)• Pe_1000B: Packet error rate of 1000 byte packets with the same

modulation and coding as L-SIG.

July 2006

David Tung, Ralink

Slide 6

doc.: IEEE 802.11-06/0909r0

Submission

Equations of Key Parameters

• Pb_sig = from ~0.5 to 10^-6

• Ps_lsig = 1- Pb_sig^24

• Pfa_lsig = n =2:2:24} Cn24 *Pb_sig^n

• Ps_sig = 1- Pb_sig^48

• Pfa_htsig = 1/256*n =9:48} Cn48 * Pb_sig^n

• Pfa_lsig2 = Pfa_lsig * (Pb_sig^48+ Pfa_htsig);

• Ps_sig2 = 1- Pb_sig^72

• Pfa_htsig2 =1/256* n =9:72} Cn72 * Pb_sig^n

• Pe_1000B = 1- Pb_sig^8000.

July 2006

David Tung, Ralink

Slide 7

doc.: IEEE 802.11-06/0909r0

Submission

Error Probabilities vs. BER

July 2006

David Tung, Ralink

Slide 8

doc.: IEEE 802.11-06/0909r0

Submission

Converting BER to SNR

• SNR_uncoded = 20*log10(erf-1(1-Pb_sig)) (dB)

• SNR = SNR_uncoded + 7 (dB)

July 2006

David Tung, Ralink

Slide 9

doc.: IEEE 802.11-06/0909r0

Submission

Error Probabilities vs. SNR

July 2006

David Tung, Ralink

Slide 10

doc.: IEEE 802.11-06/0909r0

Submission

General Discussions

• The analytical curves show us the first time of whole picture, instead of just one particular point.

• The figures cover two important regions, The higher SNR region dictates the performance of positive CCA . The lower SNR region dictates the performance of false CCA.

• Red curve is the performance of legacy device and 11n draft.• Magenta curve is the performance suggested by C3.• Green curve is the performance suggested by C4.• Red Curve has the worse performance.• There is a crossing over point between magenta and green curves.• Green curve has the best performance under positive CCA.• Magenta curve has the best performance under false CCA.

July 2006

David Tung, Ralink

Slide 11

doc.: IEEE 802.11-06/0909r0

Submission

L-SIG TXOP under Positive CCA

• Even L-SIG protection along, false alarm rate is low under usable SNR: < 10^-3 with SNR 0dB.

• C3 doesn’t really improve this region.

• C4 is doing very well in this region.

• Doesn’t really show problem here! This is not where the real problem happens!

July 2006

David Tung, Ralink

Slide 12

doc.: IEEE 802.11-06/0909r0

Submission

L-SIG TXOP under False CCA

• According to 802.11g standard positive CCA rate > 90% within starting 4us. The others are miss CCA and false CCA. Assume miss CCA will introduce false CCA, False CCA rate < 10%.

• False CCA can also be caused by co-channel (OBSS) or adjacent channel interference.

• Note that BER is approach 50%.

• False SIG rate for L-SIG protection is 50%!

• False SIG rate for C3 is 0.2%

• False SIG rate for C4 is 0.39%

• This is where the real problem with L-SIG protection!

July 2006

David Tung, Ralink

Slide 13

doc.: IEEE 802.11-06/0909r0

Submission

C1 Discussion

• C1: L-SIG TXOP shouldn’t be used under mixed legacy-HT BSS.– Under False CCA, L-SIG protection has 50% false SIG rate, this

mechanism will kill legacy device under environment with significant co-channel or adjacent channel interference.

– Even under clean environment with 10% false CCA rate, overall 5% (50%*10%) false SIG rate with random LENGTH field could be a problem!

– It is important that we accept this comment, so that millions deployed legacy device wouldn’t be at risk with this feature.

– Suggest to accept it.

July 2006

David Tung, Ralink

Slide 14

doc.: IEEE 802.11-06/0909r0

Submission

C3 Discussion

• C3: NAV be set by L-SIG only when HT-SIG CRC pass.– Best performance of proposed method under false CCA. It is also

important that it requires change only inside L-SIG TXOP, not other parts of spec.

– Need to improve the region between positive CCA and false CCA.

– Suggest to accept.

– This conclusion is inline with Intel’s analysis.

July 2006

David Tung, Ralink

Slide 15

doc.: IEEE 802.11-06/0909r0

Submission

C4 Discussion

• C4: Applying HT-SIG CRC to cover L-SIG so to enhance L-SIG TXOP protection. – Improve performance at both false CCA and positive CCA

regions.

– However compared to C3, this method require change that will affect design outside L-SIG TXOP which is neither necessary nor desirable.

– Suggest to reject it.

– This conclusion is inline with Intel’s analysis.

July 2006

David Tung, Ralink

Slide 16

doc.: IEEE 802.11-06/0909r0

Submission

Summaries of Suggested Resolution

• C1: L-SIG TXOP shouldn’t be used under mixed legacy-HT BSS.– Accept. See slide 13 for detail.

• C2: L-SIG TXOP shouldn’t be used under overlapping BSS, – Accept. Similar to C1.

• C3: NAV be set by L-SIG only when HT-SIG CRC pass.– Accept. See slide 14 for detail.

• C4: Applying HT-SIG CRC to cover L-SIG so to enhance L-SIG TXOP protection.– Reject. Major change with similar performance under key false CCA

condition as compared to C3. See slide 15 for detail.

July 2006

David Tung, Ralink

Slide 17

doc.: IEEE 802.11-06/0909r0

Submission

Four Possible Approaches

• Remove L-SIG TXOP protection completely– Given the minor potential benefit of L-SIG TXOP protection with

several known induced difficulties, the potential benefit is doubtable. This is most clean way to resolve all sort of L-SIG TXOP issues.

• Extended HT-SIG CRC to cover L-SIG and forbid L-SIG TXOP under mixed legacy HT BSS.– This is a clean solution too, but will need to change core spec. (even

though a little one). Shall we change a core mandatory spec because of an optional feature?

• Honor L-SIG only when HT-SIG CRC pass. with RSSI threshold gating and forbid L-SIG TXOP under mixed legacy HT BSS.– This will works. The best compromise.

• Honor L-SIG only when HT-SIG CRC pass.– This approach along is not enough.

July 2006

David Tung, Ralink

Slide 18

doc.: IEEE 802.11-06/0909r0

Submission

Straw Poll

• Remove L-SIG TXOP protection completely

• Extended HT-SIG CRC to cover L-SIG and forbid L-SIG TXOP under mixed legacy HT BSS.

• Honor L-SIG only when HT-SIG CRC pass. with RSSI threshold gating and forbid L-SIG TXOP under mixed legacy HT BSS.

• Honor L-SIG only when HT-SIG CRC pass.