[Discuss-gnuradio] Re: questions on SNR estimation from digital-bert examples

[William Cox]

Here's an image of the BER vs. SNR plot plotted along with the theoretical BPSK line.

http://www.flickr.com/photos/gallamine/5580109538/

You can see that the experimental data is 3-4 dB greater than the theoretical BPSK values.

My setup looks like:

GNURadio -> USRP -> LED -> water -> photodiode -> USRP -> GNURadio.

I'm using the "benchmark_tx.py" and "benchmark_rx.py" code with default settings. Carrier is 5e6 hz and rate is 1e6 symbol/sec. Decimation at the receiver is 32. 

Above 12 dB the BER values saturate down at 1.85e-11.

Any ideas why the experimental results are so far off theoretical?

Thanks.

-William

On Tue, Mar 29, 2011 at 7:52 PM, William Cox <address@hidden> wrote:

Newbie here. I have some questions regarding the example programs under gnuradio-examples/python/digital-bert. I just ran a link and swept my signal power. I recorded the estimated snr and ber and I'm trying to plot them and verify things are working properly. Is the SNR estimation done at baseband or passband? When I plot the theoretical BPSK snr vs. ber curve, there's a significant offset between my experimental data and theoretical plot (and yes, I'm ignoring SNR values below 7 dB). Also, a more basic question is what is the best method to find documentation for the code? The class documentation under the wiki seems rather overwhelming and I have a hard time finding what I need.

My second question is how the BER is calculated? I'm seeing values < 1e-11. But this is entirely unreasonable, as I'd have to be sending > 100 gb of data to be getting a BER close to that value. Looking at the code, its not clear to me over what packet size the BER is calculated and what the actual BER confidence value should be.

Thanks for any help that can be offered.

-William