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 _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
