|Subject:||Re: [Discuss-gnuradio] m2m4 SNR estimator off by 3 dB|
|Date:||Thu, 28 Aug 2014 08:51:47 -0700|
On Wed, Aug 27, 2014 at 9:33 AM, Jared Dulmage <address@hidden> wrote:
It appears that the MPSK m2m4 SNR estimator has an extra factor of 2.
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.
Jared.Hi Jared,I'd have to review the paper again, but I suspect you're right. I was using this estimator just the other day and scratching my head about the results. I was on a plane, though, and was really focused on something else.Thanks!Tom
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