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32FFT STF/LTF Sequences

32FFT STF/LTF Sequences

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32FFT STF/LTF Sequences

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  1. 32FFT STF/LTF Sequences Authors: Date: 2012-01-16 Ron Porat, Broadcom Slide 1

  2. Authors continued: Ron Porat, Broadcom Slide 2

  3. Outline • Contribution 11/1482r2 describes the 1MHz (32FFT) preamble structure • We propose here specific STF and LTF sequences for the 32FFT preamble Ron Porat, Broadcom Slide 3

  4. STF Sequence • Recall from 1482r2 the STF signal is boosted by 3dB for MCS0 rep2 to allow robust packet detection at low SNR • Hence low PAPR sequence is of paramount importance to minimize clipping distortion when power boosted for MCS0 rep2 • We propose the following sequence • Tone index=[-12 -8 -4 4 8 12] • Values: [0.5, -1, 1, -1, -1, -0.5]x(1+j)x    where is a normalization factor   • = 2.4 for MCS0 rep2, 1.7 otherwise • The sequence uses power tapering at the band edge to reduce time domain PAPR • The resulting PAPR is 0.45dB.  Ron Porat, Broadcom Slide 4

  5. Cont. – Time Domain Plot Ron Porat, Broadcom Slide 5

  6. LTF Sequence • Recall from 1482r2 the requirement for the 32FFT LTF sequence to be orthogonal to each half of the 64FFT LTF sequence to allow 1 vs. 2MHz mode detection. • In addition, in order to aid packet detection an important feature of the LTF sequence is to maximize the speed of autocorrelation drop after the STF field • The third important attribute of the LTF sequence is lower PAPR than the PAPR of DATA symbols Ron Porat, Broadcom Slide 6

  7. Cont. • We propose the following sequence: • Tone index is [-16 -15 -14 …. -1 0 1 ….. 14 15] =[0 0 0 1 -1 1 -1 -1 1 -1 1 1 -1 1 1 1 0 -1 -1 -1 1 -1 -1 -1 1 -1 1 1 1 -1 0 0] • Sequence achieves zero cross correlation with each half of the 64FFT LTF sequence • Maximizes speed at which STF autocorrelator drops (aiding timing recovery) • PAPR=3.7dB, which is lower than that of DATA Ron Porat, Broadcom Slide 7

  8. Summary • 32FFT STF and LTF sequences that meet the design criteria are proposed Ron Porat, Broadcom Slide 8

  9. Straw Poll • Do you support the proposed STF and LTF sequences? • Y • N • A Ron Porat, Broadcom Slide 9

  10. Appendix A - LTF Orthogonality Metric • The orthogonality metric used for searching for an optimal 32FFT LTF sequence is • are the LTF sequences for 32FFT and the upper and lower portions of the 64FFT • The index k is limited to indices [-16:-1 1:15] (skipping 32FFT DC) • The metric enables a frequency domain cross correlation to be performed • Where y(k) is the final received and averaged LTF value (used for channel estimation), p(k) is the transmitted LTF sequence belonging to a 64FFT or 32FFT. Ron Porat, Broadcom Slide 10

  11. Cross correlation with the transmitted sequence finds the most likely transmitted sequence and autocorrelation of successive tones removes the effect of the channel • For the correct sequence we get a large positive number • For the incorrect sequence we get a small number – the sum of random numbers Cont. Ron Porat, Broadcom Slide 11

  12. Appendix B – Autocorrelation Drop Ron Porat, Broadcom Slide 12