Re: [Discuss-gnuradio] OFDM SNR

[Martin Braun]

On 03/31/2014 01:03 PM, Piotr Potocki wrote:
> 1. I want to estimate SNR using IQ samples. But when I receive IQ
> samples on the receiver (img iq_samples_eq) they are all "perfect". So
> my first question is, is there some kind of "hard decision" in OFDM
> Frame Equalizer? Without OFDM Frame Equalizer (img iq_samples_no_s)

Yes -- the current equalizer discards soft information. That's a clear
shortcoming, but I simply haven't had the time to implement a proper
equalizer so far, and if anyone else has, they haven't committed it
upstream.

You can write your own equalizers by subclassing ofdm_equalizer_base.
It's pretty simple, if you already have an algorithm in C++.

> 2. The second problem is, that I need to have a white noise not coloured
> noise.
> And to to that i need to get rid of the equalizer part of the OFDM Frame
> Equalizer. The part of correction frequency shift on the symbols need to
> stay.
> Do You have some hints how to do that? (Just commented "//" this part ?
> // Do the equalizing
>       d_eq->reset();
>       d_eq->equalize(out, frame_len, d_channel_state);
>       d_eq->get_channel_state(d_channel_state);

That should work. The cleaner way would be to add a 'dummy' equalizer
that can be dropped in.
Note that if you leave the equalizer out, you will be able to do nothing
but SNR estimation. I don't know the details of those algorithms by
heart, but some might not work either, since symbols will be phase rotated.
Maybe you can intercept the channel state PMT in a custom block, modify
that to your needs, and use the ofdm_equalizer_static.

> 3. The 3 question is about Constellation Plot in Gnuradio. My system is
> based on BPSK so on Constellation Plot I am expecting only the +1, -1
> values. The IQ samples are showing me that this is correct +1,-1 so why
> on the plot i have the values (-0.7,-0.7),(0.7,0.7) (img Tx_snr_run) ?
> Why Constellation Plot is shifting my values?

Use the scope sink in XY mode, not the constellation plot. That'll try
and actually rx.

> 4. Last question is about the MPSK SNR Estimator. I am wondering is
> there a possibility to apply this block to OFDM?

If you had a flat fading channel, you could use that after the
serialization. For more usual OFDM channels, probably not, at least not
out-of-the box. There are several papers that modify these algorithms
for OFDM, and I don't think we've implemented any of them.

M