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

From: William Cox
Subject: Re: [Discuss-gnuradio] Re: questions on SNR estimation from digital-bert examples
Date: Fri, 1 Apr 2011 15:41:40 -0400

Yes, for what I'm doing, the optical link will be AWGN. I'm doing direct detection (lots of photons) with a uncooled Si photodiode.

Thanks for the feedback. I have limited (none) experience with realtime receivers so it's hard for me to judge how much loss I'll experience from the realtime snr estimation, timing recovery, etc. For other experiments I've done, using a ML snr estimator and offline timing recovery, we can get nearly perfect agreement with theoretical curves, so I really wasn't sure what to expect from this setup. 

I was mostly checking to see if people would say, "woah somethings wrong", or "that's close enough".

PS. I've made good use of your gstreamer streaming video program. Very helpful. Thanks!

On Fri, Apr 1, 2011 at 2:52 PM, Alexandru Csete <address@hidden> wrote:
On Fri, Apr 1, 2011 at 7:07 PM, William Cox <address@hidden> wrote:
> 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?

Hi William,

What would you consider to be a reasonable deviation from the theoretical value?
In my limited experience, 3-4 dB "implementation loss" is not that bad
when using hardware and software that haven't been tuned and optimized
for the specific purpose.
Also note that you compare your results with the theoretical values
for transmission over an AWGN channel
http://en.wikipedia.org/wiki/AWGN - does water qualify as such?


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