Hello, Thank you for these wonderfully helpful answers. I think I understand now that my signal still appears above the noise floor because it is the noise and signal power compounded in that bandwidth. That video is also incredibly helpful, especially since I am working with physical SDR's. Having said that, I think I have what I need, so thank you again.
On Wed, Jul 3, 2019, 4:09 AM Jonas Manthey <jonas.mant...@u-blox.com> wrote: > Hi, > > > > Small addition: that’s why in the GNSS world (and others I guess) we use > C/N0: https://en.wikipedia.org/wiki/Carrier-to-noise-density_ratio which > is “normed” to 1Hz. > > > > Kyeong’s video is a must watch if you are new to this field! > > > > Cheers, > > Jonas > > > > *From:* Discuss-gnuradio [mailto:discuss-gnuradio-bounces+jonas.manthey= > u-blox....@gnu.org] *On Behalf Of *Kyeong Su Shin > *Sent:* Mittwoch, 3. Juli 2019 03:43 > *To:* Vladislav Fomitchev <vladislav.fomitc...@gmail.com>; > discuss-gnuradio@gnu.org > *Subject:* Re: [Discuss-gnuradio] Help getting accurate AWGN SNR values > > > > Hello Vladislav: > > > > In my eyes, those plots look correct. > > > > "For example, when I have a signal power ratio of 0db, I can still see my > signal several db above the noise floor." -> That can totally happen; you > must not forget that the bandwidth of the noise and the bandwidth of the > signal that you are using for the SNR estimation are different (you are > using 2 samples/symbol for the PSK symbol generation). > > > > OK, here is an example: Let's say that you are transmitting a CW-like tone > (1 Hz BW) with -160 dBm power, and the noise power of the receiver is -170 > dBm/Hz. If you are observing 10 Hz of the BW, you are getting -160 dBm of > noise power (0 dB SNR). If you observe 2 Hz instead, you get 7 dB SNR. But > the spectrogram (or the PSD estimates) of the spectrum stays same - except > that you are seeing a narrower range now. > > > > Also, the power you are observing is the power of the PSK signals AND the > power of the noise, so you do get a stronger RF power (per every > FFT bin) when you increase the noise power. It is just that the power of > the PSK signals is something that you want and the power of the noise is > something that you do not want. > > > > Also see: https://www.youtube.com/watch?v=PNMOwhEHE6w > > > > [image: Image removed by sender.] > <https://www.youtube.com/watch?v=PNMOwhEHE6w> > > GRCon16 - Why Doesn't My Signal Look Like the Textbook?, Matt Ettus > <https://www.youtube.com/watch?v=PNMOwhEHE6w> > > www.youtube.com > > All GRCon16 slides available here: http://gnuradio.org/grcon-2016/talks/ > GNU Radio - the Free & Open-Source Toolkit for Software Radio > http://gnuradio.org/ > > > > > > Regards, > > Kyeong Su Shin > > > ------------------------------ > > *보낸* *사람**:* Vladislav Fomitchev <vladislav.fomitc...@gmail.com> 대신 > Discuss-gnuradio <discuss-gnuradio-bounces+ksshin=postech.ac...@gnu.org> > *보낸* *날짜**:* 2019년 7월 3일 수요일 오전 4:06:11 > *받는* *사람**:* discuss-gnuradio@gnu.org > *제목**:* [Discuss-gnuradio] Help getting accurate AWGN SNR values > > > > Hello, > > I have been trying to create a simulation with some basic PSK signals at > varying SNR values by separately generating a signal and gaussian noise and > adding them together. I am however having an issue at lower SNR’s where my > measured SNR does not match what I observe in the waterfall or FFT. For > example, when I have a signal power ratio of 0db, I can still see my signal > several db above the noise floor. I used the gnuradio noise power example > to setup my power measurements and noise generation, and it is fairly > accurate until I get close to the noise floor. The specific issue seems to > be that for some reason the amplitude of the noise also changes the peak > amplitude of my psk signal. You can observe this in the following images > where all I change is the noise power, yet the peak of my signal also > moves. Data 0: is the spectral signal power, Data 1: is the spectral noise > power, and Data 2: is the SNR or ratio of the signal and noise powers. > Top Left > > Here is the same signal with a lower noise power. Note how the peak is > ~4dB lower even though signal power is unchanged. > Top Middle > > Here is a reference shot of a 10dB SNR signal to show that my measurements > do work. > Right > > I have attached a composite of the images and my flowgraph. > > I would really appreciate any help with this. I feel I must be making a > simple mistake somewhere since this is such an elementary task. As a note I > have tried using the different channel model blocks, and different noise > distributions which all had the same issue for me. Thanks in advance. > > Vlad F. > > >
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