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Single Stream Pilots for 11ah. Authors:. Date: 2012-01-16. Authors continued:. Slide 2. Abstract. This presentation proposes pilot waveform designs for 11ah. In [1][2], we proposed preamble formats for 1MHz, and >=2MHz SU/MU.

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single stream pilots for 11ah
Single Stream Pilots for 11ah

Authors:

Date: 2012-01-16

Hongyuan Zhang, et. Al.

slide2

Authors continued:

Hongyuan Zhang, et al

Slide 2

abstract
Abstract

This presentation proposes pilot waveform designs for 11ah.

Hongyuan Zhang, et. Al.

preamble overview

In [1][2], we proposed preamble formats for 1MHz, and >=2MHz SU/MU.

In multi-stream cases, LTF1 and LTF2 are placed much further way, compared with 11n/11ac—due to (1) downclocking and (2) the “green-field type” preambles.

Example of 2SS:

Preamble Overview

2MHz SU:

STF

LTF1

SIG

LTF2

DATA

120us

2MHz MU:

STF

LTF1

SIGA

MU-STF

MU-LTF1

MU-LTF2

SIGB

DATA

40us

STF

LTF1

SIG

LTF2

DATA

1MHz SU:

(Assume 6-symb SIG)

280us

Hongyuan Zhang, et. Al.

phase shift issues

11n defines pilot tones per-stream, and HTLTF does not have specific pilot tones.

    • Receiver has to equalize multiple streams to track the pilots, during data portion.
  • 11ac uses single stream pilots in VHTLTFs, SIGB and Data.
    • Due to larger phase drift with up to 8 VHTLTFs
  • The LTF phase drift issue is exaggerated in 11ah:
    • Example: residue 0.1ppm@900MHz (after the LTF1 fine CFO acquisition) still leads to a ~9o phase shift between LTF1 and LTF2 in 1MHz (assume 6 symbol SIG field).
    • Typical worst case residue ppm could be much larger than 0.1ppm (e.g. 0.5ppm).
    • Besides residue CFO, there is also phase noise that contributes to phase differences between LTFs.
    • Large phase shift between LTFs distorts the MIMO receiver—see equation below.
  • Therefore LTF phase tracking is then essential for multi-stream PPDUs in 11ah.

January 2012

Phase Shift Issues

Received two LTFs

Hongyuan Zhang, et. Al.

simulation set1

2x2, 2SS, MCS5, DNLOS.

  • 0.5ppm residue CFO, and 0.1ppm residue CFO, fc=900MHz
  • 2MHz, and 1MHz packets.
  • Legends:
    • “LTF2 and LTF1 1-symbol apart” mimics 2MHz MU.
    • “LTF2 and LTF1 3-symbol apart” mimics 2MHz SU.
    • “LTF2 and LTF1 7-symbol apart” mimics 1MHz SU.
  • By doing LTF tracking, we assume single stream pilot tones in LTFs mapped by the first column of P matrix.
Simulation Set1

Hongyuan Zhang, et. Al.

2mhz residue 0 5ppm
2MHz, Residue 0.5ppm

Hongyuan Zhang, et. Al.

2mhz residue 0 1ppm
2MHz, Residue 0.1ppm

Hongyuan Zhang, et. Al.

1mhz residue 0 1ppm
1MHz, Residue 0.1ppm

Hongyuan Zhang, et. Al.

simulation set2

1MHz SU, 2x2, 2SS, 6-symbol SIG field.

  • fc=900MHz , 40ppm raw CFO, estimate CFO by STF (coarse) and LTF1 (fine) in each packet.
    • Different from Sims set1, now we do actual CFO acquisitions at beginning of packet
  • By doing LTF tracking, we assume single stream pilot tones in LTFs mapped by the first column of P matrix.
Simulation Set2

Hongyuan Zhang, et. Al.

1mhz mcs6
1MHz, MCS6

Hongyuan Zhang, et. Al.

residue cfo histogram at snr 40db 1000 samples
Residue CFO Histogram at SNR=40dB (1000 samples)

Worst case ~0.17 ppm

Hongyuan Zhang, et. Al.

discussions

Significant performance hit with a small ppm such as 0.1ppm, especially for 1MHz.

  • Simulations did not cover some real case impairments that may further affect CFO estimations, e.g. timing error, oscillator drift, etc.
  • Average PER may not reflect the whole picture: once a device hits a bad channel (static) with large residue CFO, the MIMO traffic may not be able to work.
    • While for 11n GF, this issue was not as serious, except a 1~2dB performance hit. Because data portion still has pilots and the MIMO CE phase impact was much smaller.
  • Rather than betting on a super accurate CFO estimation on receiving each packet, introducing a single stream pilot turns out to be a trivial DSP add-on that rules out all these analog uncertainties.
    • 11ac receivers already implemented the same scheme.
Discussions

Hongyuan Zhang, et. Al.

proposed single stream pilot waveform

Data tones in LTF1 in SU open-loop formats (“greenfield”) uses the first column of P matrix to map to multi STS.

  • Suggest to also use the 1stcolumn of P matrix to map the single stream pilot across the PPDU—no phase “jump” from adjacent data tones in LTF1.
    • Facilitate channel smoothing in single stream SU open-loop packet (the most common scenario).
    • Facilitate 1M/2M classification using LTF sequences [1][2][3].
  • LTF sequence design should be based on the above proposed single stream pilot—e.g. to optimize PAPR, as well as get low cross-correlation between 1M and 2M LTF signs in the overlapping tones [3].
Proposed Single Stream Pilot Waveform

Hongyuan Zhang, et. Al.

cont d

For >=2Mhz MU packet [2], the “omni” portion is single stream as in 11a/g, or the legacy portion in11n/ac MM formats, therefore pilots are in SIGA field as in 11n/ac, LTF1 does not need specifically define any pilot.

“MU” portion applies the same single stream pilot as SU packet using the 1st column of P to map.

Cont’d

Hongyuan Zhang, et. Al.

straw poll single stream pilots

Do you agree that 11ah defines single stream pilots in the LTF, SIG and Data fields of SU packets, using the first column of P matrix for multi-stream mapping, as below?

    • For MU packets, the same single stream pilot is applied starting from MU-LTF1.
Straw Poll: Single Stream Pilots

The transmitted pilot tone signals, in the k-th tone and n-th OFDM symbol is expressed as:

Hongyuan Zhang, et. Al.

references

[1] 11-11-1482-00-00ah-preamble-format-for-1-MHz

[2] 11-11-1483-00-00ah-11ah-preamble-for-2MHz-and-beyond

[3] 11-12-0115-00-00ah-32FFT-STF-LTF-Sequences

References

Hongyuan Zhang, et. Al.