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


[https://www.bing.com/th?id=OVP.CTFByXJ7pplIU19UtauzlgEsDh&pid=Api]<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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