Forum: GNU Radio question to measure SNR

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85ffd97bae0e78056e1d06312744d7d1?d=identicon&s=25 feldmaus (Guest)
on 2009-03-25 13:24
(Received via mailing list)
Hi ALL,

i saw there is a function called <gr_probe_mpsk_snr_c>,
but i doesn't know how to get this to work.

Is there a example which is using this function ?
Can i use this to measure my SNR ?
I have a test sin signal.


I also found the possibility by measuring the SNR by
examine the bit-error-rate.

Are there any example which do this ?

Regards Markus
D0072e69d706bb3ca211d33a1b536e2c?d=identicon&s=25 Johnathan Corgan (Guest)
on 2009-03-25 20:56
(Received via mailing list)
On Wed, Mar 25, 2009 at 5:20 AM, feldmaus <feldmann_markus@gmx.de>
wrote:

> i saw there is a function called <gr_probe_mpsk_snr_c>,
> but i doesn't know how to get this to work.

This a sink you can attach to a raw MPSK constellation complex sample
stream, and it will estimate the signal-to-noise ratio using the
"squared signal over noise variance" (SNV) technique for SNR
estimation.  It's simple and fast, but not all that accurate below
about 8 or 9 dB.

> Is there a example which is using this function ?

It is used in the bit error rate tester (digital-bert) example
receiver.  The block is designed to be queried for its current SNR
estimate by an external thread (created by the user outside the
flowgraph).  In the BERT receiver, a background thread once per second
queries the flowgraph for the current SNR, bit error rate, frequency
offset, and timing error, then displays these to the screen.

> Can i use this to measure my SNR ?
> I have a test sin signal.

No, it is explicitly designed for BPSK or QPSK signals, not carriers.
Importantly, it needs to follow a frequency/phase recovery loop such
that there is no residual carrier in the signal.

> I also found the possibility by measuring the SNR by
> examine the bit-error-rate.
>
> Are there any example which do this ?

It the digital BERT example, you can see the BER and SNR once per
second, and in live testing, they do correlate.  But you can't assume
a fixed relationship between BER and SNR except in a the ideal ("not
real world") case of pure additive white gaussian noise.

Johnathan
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