[RSM] Reverse Beacon - SNR - Understanding

[Gord Kosmenko]

Hello RSM Members,

This was interesting to me, hope it is interesting to you.

The link to this is: 
http://reversebeacon.blogspot.com/2014/03/understanding-signal-to-noise-ratio-snr.html

73, Gord VE6SV


      Understanding Signal-to-Noise Ratio (SNR)

One of the most frequently asked questions about the RBN has to do with 
the meaning of the SNR that is included in each spot.  I've been 
answering these questions, and, as I learned yesterday, propagating some 
incorrect information.  The following is derived verbatim from an e-mail 
exchange I had yesterday with Alex, VE3NEA, and is reproduced here with 
his permission.

"The SNR in CW Skimmer and in SkimServ is computed the same way. The 
signal is extracted from the I/Q stream using a filter with a bandwidth 
of 50 Hz. First the key-on/key-off decision is made for every sample in 
the signal, as part of CW detection. The samples corresponding to the 
key-off state and on/off transitions are discarded, the key-on samples 
are analyzed. The signal strength is computed assuming the Rayleigh 
fading model (in other words, signal strength variations due to QSB are 
expected and taken into account). The noise density is estimated from 
the flat part of the power spectrum. The noise power in 500 Hz isthen 
computed by multiplying the power density by the bandwidth. Finally, the 
ratio of the signal power to the 500-Hz noise power is computed."

One issue people have been wondering about is whether stations operating 
in the crowded lower part of the band might be "penalized" with lower 
SNR reports due to QRM, as compared with stations CQing higher up in the 
band.  Alex disagrees.  He says:

"I have performed a series of SNR accuracy tests on the simulated CW 
signals and verified that the SNR is determined correctly even at the 
station densities as high as 10 stations per kHz, and the average SNR of 
those stations of at least 60 dB."  He goes on, "The SNR is computed 
using the history of the measurements. The signal strength is estimated 
in the 50-Hz filter passband, while the noise is estimated globally in 
the whole receiver bandwidth of 48/96/192 kHz."
I had earlier understood that SNR was calculated in a relatively short 
interval just before the spot was sent to the Telnet server. Turns out 
that the calculation is based on data collected in the 50-Hz decoder 
channel for about 45 seconds*up to the moment*when the spot is validated 
and forwarded to the Telnet server.

So, I hope this helps.  The SNR numbers are better than I had thought, 
though still relative and dependent on each RBN contributor's antenna, 
receiver(s) and noise envirnment.