July 2006
doc.: IEEE 802.11-06/0909r0
On L-SIG TXOP Protection
Date: 2006-07-11
Authors:
Name
David Tung
Company
Address
Ralink
Technology
Phone
email
+1-408-7258070 Ext. 16
[email protected]
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Submission
Slide 1
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
Summary of L-SIG TXOP Protection
Related Comments
• L-SIG TXOP protection related comments ID (CID
3765, 8277, 7258, 6788) :
–
–
–
–
Submission
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.
Slide 2
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
Outline of this Presentation
• Provide an analytical framework for the problem.
• Discussions and conclusions.
Submission
Slide 3
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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.)
Submission
Slide 4
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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.
Submission
Slide 5
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
Equations of Key Parameters
•
•
•
•
•
•
•
•
•
Pb_sig = from ~0.5 to 10^-6
Ps_lsig = 1- Pb_sig^24
Pfa_lsig = S{n =2:2:24} Cn24 *Pb_sig^n
Ps_sig = 1- Pb_sig^48
Pfa_htsig = 1/256*S{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* S{n =9:72} Cn72 * Pb_sig^n
Pe_1000B = 1- Pb_sig^8000.
Submission
Slide 6
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
Error Probabilities vs. BER
Submission
Slide 7
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
Converting BER to SNR
• SNR_uncoded = 20*log10(erf-1(1-Pb_sig))
• SNR = SNR_uncoded + 7
(dB)
Submission
Slide 8
(dB)
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
Error Probabilities vs. SNR
Submission
Slide 9
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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.
Submission
Slide 10
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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!
Submission
Slide 11
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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!
Submission
Slide 12
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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.
Submission
Slide 13
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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.
Submission
Slide 14
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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.
Submission
Slide 15
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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.
Submission
Slide 16
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
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.
Submission
Slide 17
David Tung, Ralink
July 2006
doc.: IEEE 802.11-06/0909r0
Straw Poll
• Remove L-SIG TXOP protection completely
• Extended HT-SIG CRC to cover L-SIG and forbid LSIG 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.
Submission
Slide 18
David Tung, Ralink