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After performing a time and frequency tuning of a signal, and plotting the result in frequency domain(abs value), the following ripple structure can be witnessed.

enter image description here

The extracted bits are LFSR sequence(row wise bellow).

0 0 0 0 0 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 0 1 1 1 0 1 0 0 0 1 1 0 1 0 1 1 1 0 0 1 0 1 1 1 0 1 0 1 1 0 1 1 1 1 1 0 1 1 1 1 0 1 0 1 0 1 1 1 1 1 1 0 1 1 1 0 1 0 1 0 1 0 0 0 0 0 1 0 0 0 1 0 0 1 1 0 0 0 1 1 0 1 1 0 1 0 0 1 1 1 1 0 1 0 0 0 1 1 1 1 1 1 0 1 1 0 0 0 0 0 0 0 0 1 0 1 1 0 0 1 1 0 0 1 0 0 0 0 1 1 1 1 0 0 0 1 1 0 0 0 0 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 1 0 0 0 0 1 1 1 0 1 0 1 1 0 0 1 0 1 0 0 0 1 0 1 1 0 1 1 1 0 0 1 0 0 0 0 1 0 1 0 0 1 0 0 1 1 0 1 1 1 0 0 0 1 1 1 1 0 1 0 1 1 0 0 0 0 0 0 1 0 0 0 0 1 1 0 0 1 1 1 0 0 1 1 1 1 0 0 0 0 0 1 1 1 1 1 1 0 0 1 1 0 1 0 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0 0 0 1 0 1 0 0 0 0 0 1 1 0 1 1 1 0 0 1 1 0 1 0 0 0 0 0 1 1 1 1 0 1 1 0 0 1 1 1 1 1 1 0 1 0 0 1 0 1 0 1 0 1 1 1 0 0 0 1 0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 0 0 1 1 1 1 1 0 0 0 1 1 0 1 0 1 0 1 1 0 0 0 0 1 0 0 0 1 0 1 1 0 0 1 0 0 1 1 1 0 1 0 0 1 0 0 1 0 1 0 0 0 1

BPSK modulation is used. Total number of SCs used in Preamble - 432.

enter image description here

Seems that this structure is intended by design. Wondered what may be the reason for such preamble design.

Thanks in Advance.

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    $\begingroup$ How long is the transmitter's DFT? To me, this doesn't look like a typical OFDM signal's spectral shape at all, unless you're considering some low-subcarrier-count OFDM signal with some extremely oversampling observation DFT. $\endgroup$ May 3 '17 at 9:39
  • $\begingroup$ The extracted LFSR sequence length is 418 bins --> DFT size is 512. The up sampled symbol size is 3657. Every second bin in the relevant range is zero due to concatenation of 2 preamble waveforms. $\endgroup$
    – SenSen
    May 3 '17 at 10:45
  • $\begingroup$ Hm, the LFSR sequence isn't very long. You'd normally use that to make a pseudorandom noise sequence as white as possible. Could you give us info about the subcarrier modulation, the LFSR coefficients, and the number of OFDM subcarriers (by editing your question), please? $\endgroup$ May 3 '17 at 10:58
  • $\begingroup$ Are you sure this isn't the result of a poorly designed interpolation or decimation filter? Unless the transmitter knows the channel gains (and can therefore perform water-filling), I don't know why anyone would do this on purpose. $\endgroup$
    – hops
    May 3 '17 at 14:14
  • $\begingroup$ This is a pure signal brought to BaseBand. No filters were applied. Doing the same with the training sequence symbol, yielded the expected OFDM-like, flat response, as can be seen below: drive.google.com/file/d/0B_qN8nR8P1vZcUNaRU9ZZjF3T2M/… $\endgroup$
    – SenSen
    May 3 '17 at 17:25

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