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Duration in L-SIG

Duration in L-SIG. Authors:. Date: 2010-05-17. 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

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Duration in L-SIG

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  1. Duration in L-SIG Authors: Date: 2010-05-17 Youhan Kim, et al.

  2. 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.

  3. 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 legacy spoof symbols = L-SIG length / 3 bytes per symbol 20 usec L preamble VHT preamble VHT Payload VHT payload symbols = legacy spoof symbols – VHT preamble symbols L-SIG spoof rate is fixed at 6 Mbps (3 bytes / symbol) Youhan Kim, et al.

  4. 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 Short GI symbol time= 3.6 usec 3.6 * VHT symbols L preamble VHT preamble VHT Payload Remainder <= 3.6 usec Legacy spoof time = 4 usec per symbol * legacy spoof symbols L-SIG symbol time = 4.0 usec Legacy spoof symbols = L-SIG length / 3 Youhan Kim, et al.

  5. 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 same4 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 Short GI packet with N symbols 3.6 3.6 3.6 Short GI packet with N+1 symbols 3.6 3.6 3.6 3.6 L-SIG spoof with M symbols 4 4 4 Youhan Kim, et al.

  6. 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.

  7. Usefulness of L-SIG TXOP (1/3) • Hidden node at receiver • L-SIG TXOP does not help L-SIG duration To B A L-SIG B Collision at B C To B C A B Youhan Kim, et al.

  8. 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 A Data to B Data to B With L-SIG TXOP B ACK ACK L-SIG duration L-SIG duration C Start EIFS Clear EIFS Start EIFS A Data to B Data to B Without L-SIG TXOP (Without RTS/CTS) C B ACK ACK A B Start EIFS Clear EIFS Start EIFS C A RTS Data to B Legacy PLCP Without L-SIG TXOP (With RTS/CTS) B CTS ACK HT PLCP MAC duration C Youhan Kim, et al.

  9. Usefulness of L-SIG TXOP (3/3) • Hidden node at transmitter (cont’d) • Legacy RTS/CTS could be used to protect cases when EIFS is not sufficient to protect the response frame (e.g. RDG) With L-SIG TXOP A Data to B Data to B ACK Data to A B L-SIG duration L-SIG duration C C A B Start EIFS A RTS Data to B Legacy PLCP Without L-SIG TXOP CTS Data to A B HT PLCP MAC duration C Youhan Kim, et al.

  10. 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.

  11. L-SIG TXOP andEfficiency 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

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