1. VHT Packet Duration Signaling
Month Year
doc.: IEEE yy/xxxxr0
July 2010
VHT Packet Duration Signaling
Date:
Authors:
Youhan Kim, et al.
John Doe, Some Company

2. July 2010
Introduction
Bits in VHT-SIG A are very precious resource to signal important system parameters
Length field in L-SIG already contains sufficient information to convey the duration of a VHT packet [1]
More details are provided on how to signal the VHT packet duration using the L-SIG length field
Youhan Kim, et al.

3. July 2010
11ac Packet Length
A 4 ms long VHT packet using 80 MHz, 4 streams, 256-QAM 5/6, short GI can transport ~800 Kbytes
Requires 20 bits if length in bytes was indicated in preamble as in 11n
Number of bits required for length in bytes for VHT packets is actually higher since 11ac supports 160 MHz, max. 8 streams and 256-QAM 5/6 [2]
22 bits required to support 4 ms packet duration
2 symbol VHT-SIG A has 34 useful bits
48 bits – 8 CRC bits – 6 tail bits
22 bit length field would use 65% of the valuable VHT-SIG A resource
And this is not even considering DL MU-MIMO
Clearly, signaling the length of the packet in bytes in VHT-SIG A is not an efficient scheme
Youhan Kim, et al.

4. 11ac Packet Duration: Number of Symbols
July 2010
11ac Packet Duration: Number of Symbols
Alternative is to signal the number of OFDM symbols of the packet
But L-SIG already has sufficient information for RX to determine the number of OFDM symbols
See following slides
Do not need to indicate VHT packet duration again in VHT-SIG A
Very efficient usage of the valuable VHT-SIG A resources
Youhan Kim, et al.

5. L-SIG Length Conveys Number of Symbols (1)
July 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.

6. L-SIG Length Conveys Number of Symbols (2)
July 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.

7. L-SIG Length Conveys Number of Symbols (2)
July 2010
L-SIG Length Conveys Number of Symbols (2)
Short GI packet
3.6 us / VHT symbol
L-SIG can only indicate time in units of 4 us
End of frame may not be aligned to a 4 us boundary
Two 3.6 us short GI boundaries may map to the same 4 us normal GI boundary used by L-SIG
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.

8. Short GI Packet Duration Signaling
July 2010
Short GI Packet Duration Signaling
Use two bits for short GI indication in VHT-SIG A
b00 : Long GI packet
# symbols = ceil(L-SIG length / 3)
b01 : Reserved
b10 : Short GI packet
# symbols = floor(ceil(L-SIG length / 3) * 4 / 3.6 )
b11 : Short GI packet
# symbols = floor(ceil(L-SIG length / 3) * 4 / 3.6 ) – 1
L-SIG spoof with M symbols 4 4 4
Short GI packet with N+1 symbols
Short GI: b10
3.6
3.6
3.6
3.6
Short GI packet with N symbols
Short GI: b11
3.6
3.6
3.6
Youhan Kim, et al.

9. July 2010
Conclusions
Do not need to indicate VHT packet duration again in VHT-SIG A
Length field in L-SIG already has sufficient information to signal the duration of a VHT packet
Propose to use 2 bits for short GI indication in VHT-SIG A
One bit for long/short GI indication
One bit for short GI packet length ambiguity mitigation
Youhan Kim, et al.

10. July 2010
Straw Poll #1
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.
Youhan Kim, et al.

11. July 2010
Straw Poll #2
Do you support adding the following row into Table 1 in the specification framework document, 11-09/0992?
Field
Bits
Description
Short GI 2
– b00 : Long GI packet
• # symbols = ceil(L-SIG length / 3)
– b01 : Reserved
– b10 : Short GI packet
• # symbols = floor(ceil(L-SIG length / 3) * 4 / 3.6 )
– b11 : Short GI packet
• # symbols = floor(ceil(L-SIG length / 3) * 4 / 3.6 ) – 1
Youhan Kim, et al.

12. July 2010
References
[1] Kim, Y. et al., Duration in L-SIG, IEEE /0534r1, May 2010 [2] Stacey, R. et al., Specification Framework for TGac, IEEE /0992r11, May 2010 [3] Liu, Y. et al., VHT frame padding, IEEE /0064r5, May 2010
Youhan Kim, et al.