1. VHT Packet Length Calculation
March 2010
doc.: IEEE /0xxxr0
July 2010
VHT Packet Length Calculation
Date:
Authors:
Yuichi Morioka
Sony Corporation
Kitashinagawa
Shinagawa, Tokyo
Yuichi Morioka, Sony Corporation

2. Background and Motivation
March 2010
doc.: IEEE /0xxxr0
July 2010
Background and Motivation
ac-duration-in-l-sig
suggested that the Length Field in VHT SIG be eliminated in order to save precious bits in VTH SIG for other uses
ad-why-we-need-length-in-vht-sig identified a number of draw backs if the Length Field is not available in VHT SIG, including
PPDU size limitation
TxOP mechanism can not be used
Receiver blind decoding
Yuichi Morioka, Sony Corporation

3. Length in multiple of n1/n2/n3/n4
March 2010
doc.: IEEE /0xxxr0
July 2010
Our proposal
Keep the Length Field in L-SIG as currently defined
Include a bit Length Field in VHT SIGA to indicate a multiple of n bytes (for e.g. n=8/16/32/64 if 2 bits are used) to be added or subtracted from the Length Field in L-SIG to form the actual length
8-bit CRC in VHT SIG-A to cover the Length Fields of L-SIG and VHT SIG
Bits:
12-13
TBD 1
TBD
TBD
Reserved 8 6 BW
STBC
Group ID
Nsts
TBD
Length
CRC
Tail 1 10
1-2 1
Length in multiple of n1/n2/n3/n4
bytes
n1/n2
/n3/n4
+/-
Slide 3
Page 3
Yuichi Morioka, Sony Corporation

4. VHT SIGA with this proposal Ref: 11-10-0382-02
March 2010
doc.: IEEE /0xxxr0
July 2010
VHT SIGA with this proposal Ref:
No of bits BW
2-3
STBC 1 GI
GroupID 6
Nsts 9
Length
12-13
CRC 8
Reserved
Tail
Total bits
Slide 4
Page 4
Yuichi Morioka, Sony Corporation

5. March 2010
doc.: IEEE /0xxxr0
July 2010
Suggested VHT SIGB (user specific) with this proposal Ref:
No of bits
MCS 4
Length 8
CRC
LDPC 1
Tail 6
Total bits
Length = 5 bits (max value = 32)
size = 1-2 bits (value = 8/16/32/64 bytes)
Subtract/add (from computed VHT SIG length) = 1 bit
Slide 5
Page 5
Yuichi Morioka, Sony Corporation

6. Keeping the Length Field of L-SIG as defined
March 2010
doc.: IEEE /0xxxr0
July 2010
Keeping the Length Field of L-SIG as defined
Pros
Maintain backward compatibility with 11a/n devices
Regardless of how reliable or unreliable the auto-detection mechanism is going to be, legacy devices have the “known” Length Field to fall back on.
Cons
The length field is still protected by 1 parity bit – this is an inherent problem with 11a/n
Slide 6
Page 6
Yuichi Morioka, Sony Corporation

7. Include a 12-bit Length Field in VHT SIG
March 2010
doc.: IEEE /0xxxr0
July 2010
Include a 12-bit Length Field in VHT SIG
Pros
PPDU size is extended to 64 Kbyte (or higher if needed) to cover most applications
TxOP mechanism can still be used by 11a/n devices
Improve decoder’s efficiency since receiver has a knowledge of when the packet is terminated
Save at least 7 precious bits in VHT SIG that may help to accommodate the full definition of Group ID, Nsts and other needed fields
Cons
For packet size > 2340 bytes, if the incremental count is chosen to be very high, padding may be needed.
Slide 7
Page 7
Yuichi Morioka, Sony Corporation

8. 8-bit CRC in VHT SIG to cover the Length Fields of L-SIG and VHT SIG
March 2010
doc.: IEEE /0xxxr0
July 2010
8-bit CRC in VHT SIG to cover the Length Fields of L-SIG and VHT SIG
Pros
Eliminate false positive problems associated with L-SIG
Offer the same level of protection compared to 11n
Slide 8
Page 8
Yuichi Morioka, Sony Corporation

9. March 2010
doc.: IEEE /0xxxr0
July 2010
Conclusions
There are major drawbacks in losing the VHT-Length Field as highlighted in ad-why-we-need-length-in-vht-sig
PPDU Size limitation (3ms)
Blind decoding without knowledge of actual end of packet
TXOP Protection can not be used
This proposal avoids all of the above drawbacks and offers >40% saving in the number of bits required to specify the VHT length.
Slide 9
Slide 9
Page 9
Yuichi Morioka, Sony Corporation

10. March 2010
doc.: IEEE /0xxxr0
July 2010
Strawpoll
Do you support keeping the length field in L-SIG and add to the specification framework the method to compute the packet length of VHT packets for SU-MIMO as shown on slide #3 in this proposal?
Yes No
Abstain
Slide 10
Slide 10
Page 10
Yuichi Morioka, Sony Corporation

11. Reference July 2010 March 2010 doc.: IEEE 802.11-10/0xxxr0
Yuichi Morioka, Sony Corporation