1. VHTSIG Considerations
Month Year
doc.: IEEE yy/xxxxr0
May 2010
VHTSIG Considerations
Date:
Authors:
Hongyuan Zhang, et al.
John Doe, Some Company

2. Overview
Our earlier preamble proposal [1] shows a preamble structure with 2 VHTSIG segments: VHTSIGA as common PHY info for all MU clients, and VHTSIGB containing client-specific info.
Our new proposal [2] showed that it is sufficient to use LSIG Length field in a VHT packet to signal the packet duration.
Motivations: Without duration field, a lot of free bits in VHTSIGA may allow us to squeeze all the necessary PHY information bits into VHTSIGA.
Assume up to 4 clients in each DLMU packet.
Benefits by not having VHTSIGB in 11ac preamble:
Save 4us for better efficiency.
Cleaner receiver process: the whole flow will be similar to 11n (next slide).
While VHTSIGB decoding and its corresponding PHY info processing requires extra handling compared with 11n.
Slide 2
Hongyuan Zhang, et al.

3. Illustrations VHT Preamble structure in [1]
2 symbols
1 symbol
L-STF
L-LTF
L-SIG
VHTSIGA
VHT-STF
VHT-LTFs
VHTSIGB
VHTData
VHT Preamble structure without VHTSIGB
2 symbols
L-STF
L-LTF
L-SIG
VHTSIG
VHT-STF
VHT-LTFs
VHTData
(48 Available bits)
HT MM Preamble structure
2 symbols
L-STF
L-LTF
L-SIG
HTSIG
HT-STF
HT-LTFs
HTData
Slide 3
Hongyuan Zhang, et al.

4. Example of VHTSIG Bit Allocation for DLMU
Bits:
2-4(TBD) 1 1 1
33(TBD)
4(TBD) 6 BW
SGI
STBC
Coding
GrpID Fields
CRC
Tail 6
25(TBD)
0-6(TBD)
Grp ID>0
MU MCS Field
Rsvd
Example MU MCS (25 bits): 9
3-4(TBD)
3-4(TBD)
3-4(TBD)
3-4(TBD)
Nsts LUT for 4 Clients
MCS1
MCS2
MCS3
MCS4
MCS per user represents modulation and coding for the corresponding of spatial streams.
STBC and LDPC are both common for all the clients
The 9-bit Nsts LUT compresses all 493 possible stream assignment combinations, assuming:
Up to 4 streams per user
Up to 8 total number of streams
Up to 4 users per MU packet
The actual location of bits in VHT-SIG is subject to change if necessary
Slide 4
Hongyuan Zhang, et al.

5. Example of VHTSIG Bit Allocation for SU
Bits:
2-4 (TBD) 1 1 1
33 (TBD)
4(TBD) 6 BW
SGI
STBC
Coding
GrpID Fields
CRC
Tail 6
TBD
TBD
TBD
Grp ID=0
SU MCS Field
AID
Rsvd
The actual location of bits in VHT-SIG is subject to change if necessary
Slide 5
Hongyuan Zhang, et al.

6. Feasibility of 4 bit CRC
It might be possible to reduce CRC to 4 bit in VHTSIG and still get reliable false positive protection.
Simulation of False positive prob as below:
Hongyuan Zhang, et al.
Slide 6

7. Conclusions
It is possible to squeeze all the necessary VHT PHY info bits in 2 symbols of VHTSIGA, if
VHT packet duration is signaled by LSIG Length field.
Limit to at most 4 clients in each DLMU packet.
No further PHY info bits needed in VHTSIGB.
It may worth investigating this possibility.
Slide 7
Hongyuan Zhang, et al.

8. March 2010
References
[1] IEEE /0070r5, ac preamble. [2] IEEE /0534r0, Duration in L-SIG.
Hongyuan Zhang, et al.