
From:  Tom Rondeau 
Subject:  Re: [Discussgnuradio] m2m4 SNR estimator off by 3 dB 
Date:  Thu, 28 Aug 2014 07:04:26 0700 
It appears that the MPSK m2m4 SNR estimator has an extra factor of 2.
gnuradio/grdigital/mpsk_snr_est.cc:169:175
double
mpsk_snr_est_m2m4::snr()
{
double y1_2 = d_y1*d_y1;
return 10.0*log10(2.0*sqrt(2*y1_2  d_y2) /
(d_y1  sqrt(2*y1_2  d_y2)));
}
where d_y1 and d_y2 are the running 2nd and 4th moments of the input, respectively.
The 2.0* is not present in the estimate _expression_ from the reference (Pauluzzi and Beaulieu eq. 39).
The 3 dB offset can also be seen by generating a signal (say random BPSK) with AWGN, measuring the signal and noise powers separately while estimating the SNR via the m2m4 on the sum.
It's an easy fix, I just wanted to get verification and notify other users.
Thanks,
Jared.
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