1. 802.11ac Preamble Discussions
Month Year
doc.: IEEE yy/xxxxr0
802.11ac Preamble Discussions
Date:
Authors:
John Doe, Some Company

2. Outlines Design goals Spoofing and auto-detection
Possible preamble structures
Conclusions

3. Design Goals Backward compatibility
Robust legacy 11a deferral
Robust legacy 11n deferral
Reliable auto-detection among 11a, 11n (MM and GF), and VHT preambles
Single preamble structure in SU and MU
Signaling of VHT PHY information by VHTSIG.
Training for wider channels, and detection and deferral in each sub-channel.
Low PAPR
Minimize overall preamble length
Receiver design flow close to 11n.

4. Spoofing and Auto-detection
Use L-SIG spoofing for both 11a and 11n receivers:
As 11n spoofing for 11a/g receivers.
Rate=6Mbps, Length/Rate indicates duration.
Use 90-deg rotated BPSK (QBPSK) on VHT-SIG symbol(s) for VHT auto-detection.
11n receiver will treat the packet as 11a packet (L-SIG spoofing).

5. Preamble Option 1 T Rate=6Mbps Length determined by T 2 symbols
L-STF
L-LTF
L-SIG
VHTSIGA
VHT-STF
VHT-LTFs
VHTSIGB
VHTData T
VHT auto-detection

6. Option 1 Discussions Overall 3 OFDM symbols in VHTSIG.
Totally 72 bits (48 data tones per symbol as in 11a/n)
VHTSIGA and VHTSIGB both have CRC and BCC tail bits.
In DL-MU packet, VHTSIGA contains the same bits for all clients, VHTSIGB contains user-specific information (e.g. MCS) and is spatially multiplexed for different clients.
May need to signal number of space-time streams and STBC for each user in VHTSIGA, e.g. by different subfields.
May need prior user assignment frame exchange before DL-MU packets.
In SU packet, some bits in VHTSIGA may be redefined for “SU-Only” information.
Need an “MU Indication” bit in VHTSIGA
Example of “SU-Only” info: sounding related information

7. Preamble Option 2 T Rate=6Mbps Length determined by T 3 symbols L-STF
L-LTF
L-SIG
VHTSIG
VHT-STF
VHT-LTFs
VHTData T
VHT auto-detection

8. Option 2 Discussions
Overall 3 OFDM symbols (consecutively placed) in VHTSIG.
Totally 72 bits
More usable bits than Option 1—single CRC and BCC tail.
In DL-MU packet, VHTSIG contains the same bits for all clients.
Need to signal MCS and STBC for different clients, e.g. by different subfields.
More bits required than signaling Nsts in VHTSIGA of Option 1.
Need prior user assignment frame exchange before DL-MU packets.
In SU packet, some bits in VHTSIG may be redefined for “SU-Only” information.
Need an “MU Indication” bit in VHTSIGA

9. Example in 80MHz (Option 1)
(Four 20MHz subbands)
frequency
L-STF
L-LTF
L-SIG
VHTSIGA
VHT-STF
VHT-LTFs
VHTSIGB
VHT-Data
L-STF
L-LTF
L-SIG
VHTSIGA
VHTSIGB
L-STF
L-LTF
L-SIG
VHTSIGA
VHTSIGB
Phase rotated over 4 subbands
for low PAPR
L-STF
L-LTF
L-SIG
VHTSIGA
VHTSIGB
time

10. Conclusions Two possible structures proposed: Further Investigations:
Use L-SIG spoofing, and auto-detection with QBPSK in VHT-SIG.
Minimum preamble length for DL-MU.
Single structure for SU and MU.
Further Investigations:
Choose between Option 1 and Option 2, based on the PHY features to be supported in 11ac, which determines the required number of VHTSIG bits.
VHTSIG subfield designs (depending on PHY features)
VHTSTF, VHTLTF designs (data and sounding).
Phase rotations in 80MHz.