Re: [Discuss-gnuradio] BER over air

[Martin Braun]

On 12/25/2014 11:06 AM, Mostafa Alizadeh wrote:
> I have a question regarding how it's possible to measure SNR @ an Rx
> with a USRP? In fact to obtain a graph of BER vs SNR I need to measure
> SNR ! 
> 
> I didn't find anything specific to this question. However, I've heard
> it's possible to estimate an initial SNR in Rx by accounting noise
> figure of the USRP, then by changing the transmitter's power, different
> SNRs can be seen at Rx. 
> 
> Any idea please!? 

Measuring exact SNR is a difficult science. What you can do quite easily
is to use the SNR estimator blocks. However, for accurate BER/SNR
curves, there's a catch-22 here because you're using the same noise to
estimate noise power that's affecting the signal (and hence, BER). So,
if you're writing a paper, don't forget to mention and explain this.

Also, when you say SNR, what *exactly* do you want? Is it Eb/N0?

If you want to figure out SNR by analog measurements (i.e. by measuring
the exact rx power and estimating the exact noise figure), that's a lot
of work. And your calibrations will be off if you open the window and
the temperature drops (depending on how accurate you need these values).

Also, when doing real measurements, you're including lots of other
effects (multipath, phase noise, quantization noise, frequency offset,
timing offset... most of these can be modeled as an additional noise
figure if they're static, but with some limited accuracy). Is that
really what you want?

If multipath is negligible you might be able to transmit a very long
known BPSK sequence and use the analytical functions for BER in AWGN to
estimate Eb/N0 (measure BER and solve for the Eb/N0). I'd need to have
another look at the math for our SNR estimators, but they probably do
something similar. This only works for *perfect time and frequency
synchronisation*!

M