802.11ac Preamble

1. 802.11ac Preamble Date: 2010-03-15 Authors: Slide 1 Hongyuan Zhang et al.

2. Abstract • Changes from r1 (with italic font in slides) • Remove duration field in VHT-SIGA for bit allocation consideration • Remove SU/MU bit in VHT-SIGA for bit allocation consideration • Combine “Field/Bits consideration slides for SU and MU” into a single slide • Update comparison slide for AutoDetection • Update references with revision number • Minor editorial changes • Add a couple of strawpolls at the end • Changes from r2/r3 • Modify strawpolls Hongyuan Zhang et al.

3. I. Numerology Hongyuan Zhang et al.

4. 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 • Given that 8 Tx antennas are proposed to be sounded, then there is inherent support for up to 8 spatial streams • Max number of MU users: 4 • Larger number significantly increases MAC/PHY complexity • Fits VHT-SIG size limitations, reduces number of representation bits required Hongyuan Zhang et al.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. II. Preamble Comparisons Hongyuan Zhang et al.

11. TGac Preamble Proposals • Two proposals in TGac on preamble: (1) our proposal 10/070r1, 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 Hongyuan Zhang et al.

12. 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 Hongyuan Zhang et al.

13. Preamble Structure in (2)-0130r0 • Or the “90-deg Orthogonal shift” may be replaced by +-45-deg shift. Hongyuan Zhang et al.

14. 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. • On timing issue for detection • VHT-STF AGC may be deferred by approximate FFT processing time (before VHT detection). • 11ac will most likely run faster clock to support higher throughput; therefore AGC computation is faster than HT devices. • May use partial GI of VHTLTF1 for AGC computation. • There are much more complex functions (e.g. DLMU, faster decoder, etc) required for 11ac. VHT AGC enhancement is trivial. • A reliable legacy spoofing is more important than the extra complexity of AGC enhancement. Hongyuan Zhang et al.

15. 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. Hongyuan Zhang et al.

16. 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. Hongyuan Zhang et al.

17. III. Re-present of Our Preamble Proposal (1) Hongyuan Zhang et al.

18. 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 Hongyuan Zhang et al.

19. 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.

20. Preamble Structure 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 Hongyuan Zhang et al.

21. Aggregation bit in VHT-SIGs for MU Packets? Last Symbol PHY • There is no need to indicate the duration of the packet in VHT-SIG again • Length information can be obtain from L-SIG • 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 11-10-0064r1 (VHT frame padding). Service VHT A - MPDU Tail Pad VHTSIG PHY L - TFs L - SIG VHTSIGA VHT - TFs Service VHT A - MPDU Tail B Pad PHY Service VHT A - MPDU Tail Pad Less than 0 - 3 MPDU MPDU 8 - bit octets Length = 0 Length = 0 Last byte A - MPDU A - MPDU A - MPDU Null Null MAC boundary subframe 1 subframe 2 subframe n subframe subframe Pad Hongyuan Zhang et al.

22. Summary on VHT-SIGs • In MU, VHT-SIGA 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 11-10-0073r2 (Group ID Concept for Downlink MU-MIMO Transmission). • VHT-SIGB contains user-specific information (e.g. modulation and coding rate) and is spatially multiplexed for different clients. • It is placed after all the VHT-LTFs to enable better receiver side interference mitigation in DL-MU before decoding VHT-SIGB. • This requires each client getting as many LTFs as needed to train the total number of spatial streams across all users—named as “resolvable VHT-LTF”. • “Non-resolvable VHT-LTF” 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 11-09-1234r1 (Interference Cancellation for Downlink MU-MIMO). Hongyuan Zhang et al.

23. VHT-SIG Fields Considerations • Bandwidth • Short GI • Group ID Field • MCS • STBC • Sounding • Smoothing • Coding Type • CRC & Tail • For further investigation • Full/partial MAC ID • Number of Extension Streams • Resolvable/Non-resolvable LTF Indication Details of bit allocation are subject to change if necessary Hongyuan Zhang et al.

24. Straw Poll on Numerology • Do you support adding a basic guideline on the numerology for 11ac device described as in Section I of 11-10/0070r2, excluding slide 9 (max Number of users for MU remains TBD), to the spec framework document, 11-09-0992? • Yes: • No: • Abs: Hongyuan Zhang et al.

25. Straw Poll on Preamble Structure • Do you support adding the 11ac preamble structure with two SIGNAL fields (VHT-SIGA located before VHT-STF and VHT-SIGB located after VHT-LTFs) as in Section III (Slide 22) of 11-10/0070r2 to the spec framework document, 11-09-0992? • Yes: • No: • Abs: Hongyuan Zhang et al.

26. Follow-up Straw Poll on Preamble Structure • Do you support to have 2 OFDM symbols for VHT-SIGA and a single OFDM symbol for VHT-SIGB, and to edit the spec framework document, 11-09-0992, accordingly? • Yes: • No: • Abs: Hongyuan Zhang et al.

27. Straw Poll on Spoofing • Do you support to have BPSK on the 1st VHT-SIGA symbol and 90-deg rotated BPSK (QBPSK) on the 2nd VHT-SIGA symbol for VHT auto-detection as in Section III (Slide 20) of 11-10/0070r2, and to edit the spec framework document, 11-09-0992, accordingly? • Yes: • No: • Abs: Hongyuan Zhang et al.