3

The sharp rectangular shape of the spectrum, with the immensely flat top definitely says "OFDM, with suitable whitening/PAPR reduction"; what constellation is used on the individual carriers is a bit hard to tell. Can you, with a no-samples-left-out spectrogram (e.g. gr-fosphor sink, or by playing back your recorded file slower and using a suitably high ...


2

The nonlinear trick is in fact often used to recover the symbol clock of $\frac \pi {2N}$ phase-shifted signals, e.g. BPSK, QPSK: Squaring samples $x[n]$ (or even taking $x^4$) leads to a removal of phase information, and you can then optimize timing with something that is pseudo-derivate based like Müller&Muller. Now, the problem with QAM is usually ...


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