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Doc.: IEEE 802.11-10/0534r1 Submission Duration in L-SIG Date: 2010-05-17 Youhan Kim, et al.Slide 1 Authors: May 2010.

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Presentation on theme: "Doc.: IEEE 802.11-10/0534r1 Submission Duration in L-SIG Date: 2010-05-17 Youhan Kim, et al.Slide 1 Authors: May 2010."— Presentation transcript:

1 doc.: IEEE /0534r1 Submission Duration in L-SIG Date: Youhan Kim, et al.Slide 1 Authors: May 2010

2 doc.: IEEE /0534r1 Submission Introduction Bits in VHT-SIG are very precious resource to signal important system parameters Length field in L-SIG is shown to already contain sufficient information to convey the duration of a VHT packet Relying on L-SIG length field to convey the duration of a VHT packet is not compatible with L-SIG TXOP and GF preamble –However, both L-SIG TXOP and GF preamble are shown to have limited benefit –Reducing the number of bits in VHT-SIG (improve efficiency of every VHT packet) has greater benefit than L-SIG TXOP and GF preamble Several options to protect the integrity of duration information in L-SIG are presented Youhan Kim, et al.Slide 2 May 2010

3 doc.: IEEE /0534r1 Submission L-SIG Length Conveys Number of Symbols (1) Similar to 11n, use L-SIG spoof rate of 6 Mbps for 11ac packets –3 bytes / symbol Long GI packet –4 us / symbol –Legacy spoof symbols = L-SIG length / 3 bytes per symbol –VHT payload symbols = Legacy spoof symbols – VHT preamble symbols Youhan Kim, et al.Slide 3 VHT Payload legacy spoof symbols = L-SIG length / 3 bytes per symbol L preamble VHT preamble L-SIG spoof rate is fixed at 6 Mbps (3 bytes / symbol) 20 usec VHT payload symbols = legacy spoof symbols – VHT preamble symbols May 2010

4 doc.: IEEE /0534r1 Submission L-SIG Length Conveys Number of Symbols (2) Short GI packet –3.6 us / VHT symbol –End of frame may not be aligned to a 4 us boundary –Legacy devices using L-SIG may find the end of the packet to occur up to 3.6 usec after the energy on the air has disappeared But this is existing problem in 11n Youhan Kim, et al.Slide 4 VHT Payload 3.6 * VHT symbols Legacy spoof time = 4 usec per symbol * legacy spoof symbols Legacy spoof symbols = L-SIG length / 3 Short GI symbol time= 3.6 usec L-SIG symbol time = 4.0 usec Remainder <= 3.6 usec L preamble VHT preamble May 2010

5 doc.: IEEE /0534r1 Submission Ambiguous End of Short GI Packets L-SIG can only indicate time in units of 4 us Two 3.6 us short GI boundaries may map to the same 4 us normal GI boundary used by L-SIG –Option A: Use L-SIG length % 3 == 1 to select between the two –Option B: Use extra bit in the VHT-SIG to select between the two –Option C: Pad to the next 3.6 usec symbol if there is ambiguity Youhan Kim, et al.Slide Short GI packet with N symbols Short GI packet with N+1 symbols L-SIG spoof with M symbols May 2010

6 doc.: IEEE /0534r1 Submission L-SIG TXOP Optional feature in 11n –L-SIG length used to signal a duration that is longer than the actual frame duration –Starts with initial handshake RTS/CTS using L-SIG TXOP is less efficient than legacy RTS/CTS –RTS/CTS using L-SIG TXOP must be sent using HT PPDU –Legacy RTS/CTS does not have HT preamble (16 us) –Legacy RTS/CTS can be heard by legacy devices are well –EIFS always triggered in legacy devices Legacy devices at disadvantage in gaining channel access L-SIG TXOP initiator should transmit CF-END frame using legacy rate after L-SIG TXOP protected sequence Youhan Kim, et al.Slide 6 May 2010

7 doc.: IEEE /0534r1 Submission Usefulness of L-SIG TXOP (1/3) Hidden node at receiver –L-SIG TXOP does not help Youhan Kim, et al.Slide 7 AB C A B C To B Collision at B L-SIG duration L-SIG To B May 2010

8 doc.: IEEE /0534r1 Submission Start EIFS Usefulness of L-SIG TXOP (2/3) Hidden node at transmitter –EIFS can protect normal transmit frame with response ACK or SIFS-based transmit bursting Youhan Kim, et al.Slide 8 A B C Start EIFS Data to B L-SIG duration Data to B L-SIG duration Clear EIFSStart EIFS ACK A B C Data to B Data to B Clear EIFSStart EIFS ACK With L-SIG TXOP Without L-SIG TXOP (Without RTS/CTS) AB C Legacy PLCP HT PLCP A B C RTS Data to B CTSACK Without L-SIG TXOP (With RTS/CTS) MAC duration May 2010

9 doc.: IEEE /0534r1 Submission Usefulness of L-SIG TXOP (3/3) Hidden node at transmitter (contd) –Legacy RTS/CTS could be used to protect cases when EIFS is not sufficient to protect the response frame (e.g. RDG) Youhan Kim, et al.Slide 9 A B C Data to B L-SIG duration Data to B L-SIG duration ACK A B C RTS CTS With L-SIG TXOP Without L-SIG TXOP Data to A Data to B Data to A Start EIFS MAC duration Legacy PLCP HT PLCP AB C May 2010

10 doc.: IEEE /0534r1 Submission 11ac L-SIG TXOP Viewed by 11n Devices 11ac L-SIG TXOP, if defined, is useful for 11ac devices only –11ac packets are detected as 11a packets by 11n devices –Even L-SIG TXOP capable 11n devices will not be able to understand 11ac L-SIG TXOP packets –11ac L-SIG TXOP now triggers EIFS for both 11a and 11n devices We believe 11ac networks will be mostly heterogeneous –Do not see great benefit in a 11ac L-SIG TXOP mode addressing only 11ac devices Youhan Kim, et al.Slide 10 May 2010

11 doc.: IEEE /0534r1 Submission L-SIG TXOP and Efficiency Improvement for 11ac Between improving the efficiency of VHT packets and supporting L-SIG TXOP, we feel it is a better tradeoff to improve efficiency of every VHT packet –If VHT duration is not signaled again in VHT-SIG, then efficiency of every VHT packet is increased 12 extra bits in VHT-SIG to signal other valuable system parameters or reduce VHT-SIG length –L-SIG TXOP has limited benefit RTS/CTS or self-CTS is a better mechanism for cases relying heavily on NAV –Even L-SIG TXOP capable 11n devices not able to understand 11ac L-SIG TXOP L-SIG TXOP is not a widely used feature –Not aware of any silicon vendor who has implemented and deployed L-SIG TXOP –Not aware of any customer who has enabled L-SIG TXOP Not defining 11ac L-SIG TXOP does not prevent usage of 11n L-SIG TXOP –Devices may still choose to use 11n L-SIG TXOP for HT packets if desired Youhan Kim, et al.Slide 11 May 2010

12 doc.: IEEE /0534r1 Submission 11ac Green-Field Preamble An 11ac GF preamble, if defined, may not have L-SIG 11ac GF preamble will have limited usage because most 11ac networks will be heterogeneous networks –11ac excludes operation in 2.4 GHz band. Mainly intended for operation in 5 GHz band Widespread usage of 5 GHz band by 11n devices important for success of 11ac –Allows smooth transition from 11n to 11ac –5 GHz band is indeed becoming more popular with 11n deployment 11n GF preamble had limited usage in the field so far Prefer to have single 11ac preamble type –Having separate GF preamble just for select few cases does not justify the effort and cost to support two different preamble types Youhan Kim, et al.Slide 12 May 2010

13 doc.: IEEE /0534r1 Submission Robustness of Duration in L-SIG Validity of L-SIG can be checked by –Parity (1 bit) –Rate = 6 Mbps (4 bits) –Reserved bit (1 bit) If further improvement on robustness is desired –Option 1 Accept VHT packet only if both VHT-SIG CRC and L-SIG checks pass –Option 2 Include L-SIG length field (or the entire L-SIG) in the VHT-SIG CRC Youhan Kim, et al.Slide 13 May 2010

14 doc.: IEEE /0534r1 Submission Simulation Results: Ch D Definition –L-SIG pass: Parity pass, Rsvd bit = 1 Rate = 6 Mbps –VHT-SIG pass: VHT-SIG CRC pass Both L-SIG and VHT-SIG passed: Can I trust duration to demodulate? –Green circle Prob. of incorrect duration if signaled in VHT-SIG –Red circle Prob. of incorrect duration if signaled in L-SIG: Option 1 (VHT CRC only covers VHT-SIG) –Blue circle Prob. of incorrect duration if signaled in L-SIG: Option 2 (VHT CRC also covers L-SIG length) L-SIG passed but VHT-SIG failed: Can I trust duration in L-SIG to defer TX? –Red star Prob. of incorrect duration if VHT CRC only covers VHT-SIG –Blue star Prob. of incorrect duration if VHT CRC only covers VHT-SIG Youhan Kim, et al.Slide 14 May 2010

15 doc.: IEEE /0534r1 Submission Simulation Results: Ch B Definition –L-SIG pass: Parity pass, Rsvd bit = 1 Rate = 6 Mbps –VHT-SIG pass: VHT-SIG CRC pass Both L-SIG and VHT-SIG passed: Can I trust duration to demodulate? –Green circle Prob. of incorrect duration if signaled in VHT-SIG –Red circle Prob. of incorrect duration if signaled in L-SIG: Option 1 (VHT CRC only covers VHT-SIG) –Blue circle Prob. of incorrect duration if signaled in L-SIG: Option 2 (VHT CRC also covers L-SIG length) L-SIG passed but VHT-SIG failed: Can I trust duration in L-SIG to defer TX? –Red star Prob. of incorrect duration if VHT CRC only covers VHT-SIG –Blue star Prob. of incorrect duration if VHT CRC only covers VHT-SIG Youhan Kim, et al.Slide 15 May 2010

16 doc.: IEEE /0534r1 Submission Observations Option 1: VHT-SIG CRC covers only VHT-SIG –Improved L-SIG robustness compared to relying on L-SIG checks only (parity, rate, reserved bit) –When VHT-SIG CRC fails, lower probability of error in L-SIG length than option 2 More reliable for deferring transmission when VHT-SIG CRC fails –Does not require change to CRC processing compared to 11n Option 2: VHT-SIG CRC covers L-SIG length –Further improvement on the L-SIG length protection if needed Youhan Kim, et al.Slide 16 May 2010

17 doc.: IEEE /0534r1 Submission Conclusions Do not need to indicate VHT packet duration again in VHT-SIG –Length field in L-SIG already has sufficient information to signal the duration of a VHT packet L-SIG TXOP and GF preamble not supported in 11ac –Both have limited benefit –Reducing the number of bits in VHT-SIG (improve efficiency of every VHT packet) has greater benefit –Does not prevent devices from using 11n L-SIG TXOP on HT packets if desired Several options may be considered to protect the integrity of the duration information in L-SIG –Option 1 Accept VHT packet only if both VHT-SIG CRC and L-SIG checks pass –Option 2 Include L-SIG length field (or the entire L-SIG) in the VHT-SIG CRC computation Youhan Kim, et al.Slide 17 May 2010

18 doc.: IEEE /0534r1 Submission Strawpoll Do you support adding the following item into the specification framework document, 11-09/0992? –R3.2.X: The number of OFDM symbols in a VHT packet shall be computed using the length field in L-SIG. –Yes: –No: –Abstain: Youhan Kim, et al.Slide 18 May 2010


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