1. Duration in L-SIG Date: 2010-05-17 Authors: May 2010 Month Year
doc.: IEEE yy/xxxxr0
May 2010
Duration in L-SIG
Date:
Authors:
Youhan Kim, et al.
John Doe, Some Company

2. May 2010
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)
May 2010
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)
May 2010
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
May 2010
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
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 May 2010
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)
May 2010
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 C
To B A B
Youhan Kim, et al.

8. Usefulness of L-SIG TXOP (2/3)
May 2010
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 A B C
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) B
ACK
ACK C
Start EIFS
Clear EIFS
Start EIFS 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)
May 2010
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) A B C
With L-SIG TXOP A
Data to B
Data
to B B
ACK
Data
to A C
L-SIG duration
L-SIG duration
Start EIFS A
RTS
Data
to B
Legacy PLCP
Without
L-SIG TXOP B
CTS
Data
to A
HT PLCP C
MAC duration
Youhan Kim, et al.

10. 11ac L-SIG TXOP Viewed by 11n Devices
May 2010
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 and Efficiency Improvement for 11ac
May 2010
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.

12. 11ac Green-Field Preamble
May 2010
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
May 2010
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 parity passes
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
May 2010
Simulation Results: Ch D
Definition
L-SIG pass: Parity pass, Rsvd bit = 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
Youhan Kim, et al.

15. Simulation Results: Ch B
May 2010
Simulation Results: Ch B
Definition
L-SIG pass: Parity pass, Rsvd bit = 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
Youhan Kim, et al.

16. Observations Option 1: VHT-SIG CRC covers only VHT-SIG
May 2010
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. May 2010
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 parity passes
Option 2
Include L-SIG length field (or the entire L-SIG) in the VHT-SIG CRC computation
Youhan Kim, et al.

18. May 2010
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.