1. 802.11ac Preamble Date: 2010-02-10 Authors: Month Year Month Year
doc.: IEEE yy/xxxxr0
doc.: IEEE yy/xxxxr0
802.11ac Preamble
Date:
Authors:
Slide 1
Hongyuan Zhang et al.
Page 1
John Doe, Some Company
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2. I. Numerology
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3. Proposed Basic Numerology (previously presented to TGac)
Month Year
doc.: IEEE yy/xxxxr0
Proposed Basic Numerology (previously presented to TGac)
Max number of transmit (Tx) antennas sounded: 8
Reasonable complexity, cost, and preamble length trade-off
Max number of Nss (spatial streams) in the SU case: 8
Given that 8 Tx antennas are proposed to be sounded, then there is inherent support for up to 8 spatial streams
Max number of Nss per user in the MU case: 4
Given that multiple users will share spatial streams, it is natural to make this number smaller than 8
Fits VHT-SIG size limitations, reduces number of representation bits required
Maximum number of Nss summed over users in the MU case: 8
Max number of MU users: 4
Larger number significantly increases MAC/PHY complexity
Hongyuan Zhang et al.
John Doe, Some Company

4. Maximum number of transmit antennas sounded = 8
Month Year
doc.: IEEE yy/xxxxr0
Maximum number of transmit antennas sounded = 8
Meets PAR requirements
For single user case 8 antenna with Nss=8 allows for > 500Mbps throughput
For multi user case 8 antenna sounding allows for > 1 Gbps throughput
Physical limitation on AP and STAs to put more than 8 antennas
Going to 16 antenna sounding increases preamble length
Number of bits required to indicate number of antennas sounded also increases – limited number of bits available in preamble
Hongyuan Zhang et al.
John Doe, Some Company

5. Max number of Nss (spatial streams) in the SU case = 8
Month Year
doc.: IEEE yy/xxxxr0
Max number of Nss (spatial streams) in the SU case = 8
Meets PAR requirements
For single user case 8 spatial stream allows for > 500Mbps throughput
Maximum number of Nss <= Maximum number of Antennas sounded
Hongyuan Zhang et al.
John Doe, Some Company

6. Max number of Nss per user in the MU case: 4
Month Year
doc.: IEEE yy/xxxxr0
Max number of Nss per user in the MU case: 4
Meets PAR requirements
For multi user transmission two transmissions of Nss=4 allows for > 1Gbps throughput
Given that multiple users will share spatial streams, it is natural to make this number smaller than 8
Fits VHT-SIG size limitations, reduces number of representation bits required
3 bits required to define Nsts per user for MU transmission
For resolvable LTFs these bits have to be in VHT-SIGA
Hongyuan Zhang et al.
John Doe, Some Company

7. Maximum number of Nss summed over users in the MU case: 8
Month Year
doc.: IEEE yy/xxxxr0
Maximum number of Nss summed over users in the MU case: 8
Meets PAR requirements
For multi user transmission sum of Nss equal to 8 leads to throughput > 1Gbps
Given that 8 Tx antennas are proposed to be sounded, then there is inherent support for up to 8 spatial streams
Hongyuan Zhang et al.
John Doe, Some Company

8. Max number of MU users: 4 Meets PAR requirements
Month Year
doc.: IEEE yy/xxxxr0
Max number of MU users: 4
Meets PAR requirements
For multi user transmission 4 users with 2 streams per user > 1Gbps throughput
Larger number significantly increases MAC/PHY complexity
Each users stream has to be separately encrypted and modulated
Fits VHT-SIG size limitations, reduces number of representation bits required
Nss bits have to be pre-allocated for each user in VHT-SIGA.
Even with 4 MU users, most of the VHT-SIGA bits are already allocated
Hongyuan Zhang et al.
John Doe, Some Company

9. II. Preamble Comparisons
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10. TGac Preamble Proposals
Two proposals in TGac on preamble: (1) our proposal 10/070r0, and (2) 10/130r0 (Tu, et al).
Major differences:
Auto detection:
(1) 90-deg rotation on 2nd VHTSIG symbol,
(2) Manipulate constellation of 1st VHTSIG symbol, e.g. alternative 90-deg rotate, or 45-deg rotate.
Modulation of VHT-SIG:
(1) Same as 11n/a: BPSK r=1/2
(2) Allows QPSK from 2nd VHTSIG symbol
Green Field:
(1) A single preamble for SU/MU, no GF.
(2) Allow GF
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11. Preamble Structure in (1)-0070r0
Rate=6Mbps
Length determined by T
2 symbols
1 symbol
L-STF
L-LTF
L-SIG
VHTSIGA
VHT-STF
VHT-LTFs
VHTSIGB
VHTData T
VHT auto-detection
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12. Preamble Structure in (2)-0130r0
Month Year
doc.: IEEE yy/xxxxr0
Preamble Structure in (2)-0130r0
Or the “90-deg Orthogonal shift” may be replaced by +-45-deg shift.
Hongyuan Zhang et al.
John Doe, Some Company

13. Comparisons—Auto Detection
Proposal (1) is more reliable than (2):
Guarantees the most reliable spoofing of existing 11n receivers (as 11a packet), regardless of what 11n auto-detect algorithm was implemented.
Guarantees the most reliable 11ac auto detection, largest Euclidean Distance (BPSK vs QBPSK).
It is risky to manipulate modulation of the 1st VHTSIG symbol.
Given various existing implementations of 11n auto-detections.
Not fair to pre-assume any 11n auto-detect approach as in proposal (2)
More likely that an 11n device false-detects HTSIG, and goes into ED-CCA stage.
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14. Comparisons—VHTSIG Modulation
Preferable to keep using the lowest possible MCS to modulate VHTSIG fields:
MCS0 is still necessary to guarantee the longest range.
Make sure header is not worse than Data.
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15. Comparisons—Green Field
Preferable not to define the second preamble format (GF):
11n GF has seen limited usage so far.
One of the arguments in favor of GF in 11n was the existence of green space in 5 GHz due to the limited use of 11a
If there are no 5 GHz deployments of 11n, then there is no point to TGac
The assumption should be that there will be 5 GHz deployments of 11n.
Like in 11n, multiple preamble types compounds the difficulty of auto-detection for small PHY efficiency improvement.
GF protection exchanges offsets the PHY improvement.
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16. III. Re-present of Our Preamble Proposal (1)
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17. Preamble 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
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18. 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 VHTSIG symbol for VHT auto-detection.
11n receiver will treat the packet as 11a packet (L-SIG spoofing).
Hongyuan Zhang et al.

19. Preamble Structure 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
Hongyuan Zhang et al.

20. MU Packets
VHT-SIG indicates PPDU duration only (number of OFDM symbols)
No per user length indication in preamble
Use A-MPDU structure to provide length information for individual MPDUs
Require that A-MPDU always be used with VHT frame
MAC provides an A-MPDU that fills the frame up to the last byte for each per-user stream, and PHY provides 0-7 bits of padding.
Same padding scheme also defined in SU packets.
“Aggregation” bit in VHTSIG is then not needed.
Details refer to document r0 (VHT frame padding).
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21. Further Discussions on VHTSIG
In MU, VHTSIGA contains the “common” bits for all clients.
Indicates number of space-time streams (NSTS) for each user.
Need prior multiuser group and user ID assignment frame exchanges before DL-MU packets (e.g. by sounding and/or management frames).
Each user will be able to get its own NSTS information from VHTSIGA..
Details refer to document r0 (Group ID Concept for Downlink MU-MIMO Transmission).
VHTSIGB contains user-specific information (e.g. modulation and coding rate) and is spatially multiplexed for different clients.
It is placed after all the VHTLTFs to enable better receiver side interference mitigation in DL-MU before decoding VHTSIGB.
This requires each client getting as many LTFs as needed to train the total number of spatial streams across all users—named as “resolvable VHTLTF”.
“Non-resolvable VHTLTF” may be selected if all clients do not support receiver side interference mitigation, or if interference mitigation is not required .
Rx interference mitigation in DL-MU refer to document r0 (Interference Cancellation for Downlink MU-MIMO).
Hongyuan Zhang et al.

22. VHTSIG Fields Considerations (SU)
Duration
Bandwidth
Short GI
MU/SU Indication (=SU)
MCS
STBC
Coding Type
Smoothing
Sounding
CRC & Tail
For further investigations:
Number of Extension Streams
MAC ID
Hongyuan Zhang et al.

23. VHTSIG Fields Considerations (MU)
Duration
Bandwidth
Short GI
MU/SU Indication (=MU)
Group ID
Indication of Number of Space-Time Streams for each user
Mod/Coding per user
STBC
Coding Type
CRC & Tail
For further investigations:
Sounding
Number of Extension Streams
Smoothing
Resolvable/Non-resolvable LTF Indication
Hongyuan Zhang et al.