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Re: [Discuss-gnuradio] Help getting accurate AWGN SNR values

From: Jonas Manthey
Subject: Re: [Discuss-gnuradio] Help getting accurate AWGN SNR values
Date: Wed, 3 Jul 2019 08:08:58 +0000



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!





From: Discuss-gnuradio [mailto:discuss-gnuradio-bounces+jonas.manthey=address@hidden] On Behalf Of Kyeong Su Shin
Sent: Mittwoch, 3. Juli 2019 03:43
To: Vladislav Fomitchev <address@hidden>; address@hidden
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 removed by sender.


All GRCon16 slides available here: http://gnuradio.org/grcon-2016/talks/ GNU Radio - the Free & Open-Source Toolkit for Software Radio http://gnuradio.org/




Kyeong Su Shin


보낸 사람: Vladislav Fomitchev <address@hidden> 대신 Discuss-gnuradio <discuss-gnuradio-bounces+ksshin=address@hidden>
보낸 날짜: 2019 7 3 수요일 오전 4:06:11
받는 사람: address@hidden
제목: [Discuss-gnuradio] Help getting accurate AWGN SNR values



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.

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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