A CDMA signal is considered for this application.

I would like to have an information on the generated signal oscillator's quality using a phase noise measure. If the signal was a pure carrier, it would have been easy to make a spectral measurement using a Spectrum analyzer for example to estimate the phase noise and have an information on the oscillator.

Is it possible to estimate the phase noise using a spectral measurement on this modulated signal without demodulating the spread spectrum signal ?

  • $\begingroup$ what kind of CDMA? I.e. what kind of coding are you doing? (For example,CS-DSSS would lend itself relatively nicely to that; FHSS-CDMA with actually changing oscillators would make it a lot harder. Binary DSSS might simply boil down to actually doing the despreading as the easiest way…) $\endgroup$ – Marcus Müller Jun 25 '19 at 8:01
  • $\begingroup$ Thank you for your comment Mr Muller, Actually its a GPS signal, a CS-DSSS scheme would Apply. However, I am trying to avoid demodulating the signal since the sampling clock is way less accurate than the generator's oscillator. If a Spectrum analyzer can be used to have a spectral estimation of the oscillator's instability via phase noise, my results would be less impacted. $\endgroup$ – M.FAKHFAKH Jun 25 '19 at 13:02
  • $\begingroup$ Well, the usual method would be completely demodulating the GPS signal in software, mathematically recovering a theoretical GPS clock. You could then, through windowed cross-correlation a recreation of that with your receive signal, get something like a phase variance, and from that infer something about phase noise power. Without de-spreading, however, your SNR is so negative that you simply won't be able to say anything about the (very low) phase noise of the GPS signal compared to the (very high) additive noise. $\endgroup$ – Marcus Müller Jun 25 '19 at 14:06
  • $\begingroup$ Actually, unlike most of GNSS receivers' use cases, the considered one provides a GPS signal power level that is much higher than noise level ( high gain antenna ). However it was still not clear for me to find a way to measure its phase noise due to the spreading code presence. Performing a demodulation implies using a sampling clock which in my case has poor oscillator's performance. This is why I am trying to figure out a spectral method rather than using a gps receiver's output and compute an allan variance metric. $\endgroup$ – M.FAKHFAKH Jun 25 '19 at 14:43
  • $\begingroup$ But: almost any GPS receiver has a clock that's way way worse than the GPS clock. Your spectrum analyzer, too! No, you'll still need the processing gain to make reliable statements on oscillator performance. Yes, you'll need a lot of smarts to still be able to make statements about a clock that is way, way better than that of your spectrum analyzer or your SDR. $\endgroup$ – Marcus Müller Jun 25 '19 at 14:56

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