Re: [Discuss-gnuradio] OFDM channel tap questions

[Jeff Long]

Hmm, that wasn't the whole answer. After reading the code a little more, 
it looks like the taps are a function of the sync symbols too, not just 
the channel.

      for (int i = loop_start; i < loop_end; i++) {
        if ((d_ref_sym[i-carr_offset] != gr_complex(0, 0))) {
          taps[i-carr_offset] = sym[i] / d_ref_sym[i-carr_offset];
        }
      }

On 01/10/2018 06:19 PM, Jeff Long wrote:
> Noise has some bias and structure when measured over a short period. You 
> can see this on a FFT display, where larger transforms give you a 
> smoother noise floor. You'll notice that the taps change randomly with 
> every packet as the estimator adjusts to the noise.
>
> On 01/10/2018 04:59 PM, Edwin Li wrote:
> > Hi all,
> >
> > I run the example found in the 
> > ~/prefix/share/gnuradio/examples/digital/ofdm/rx_ofdm.grc. The ofdm 
> > signals can be decoded correctly. But I don't understand the channel 
> > taps.
> > The channel taps are shown to be
> >   Key: ofdm_sync_chan_taps   Value: #[(0,0) (0,0) (0,0) (0,0) (0,0) 
> > (0,0) (64.2159,-3.65375) (47.2264,-42.4505) (8.44977,-62.8107) 
> > (-32.7931,-54.5247) (-59.9085,-21.3488) (-59.8031,23.3288) 
> > (-32.3382,55.4137) (9.05079,63.8184) (46.331,42.7923) 
> > (65.1778,2.83559) (51.1081,-38.6363) (15.815,-62.1237) 
> > (-27.1419,-57.8722) (-58.2753,-27.4043) (-62.3711,15.658) 
> > (-37.9986,51.1873) (3.2302,64.2116) (5.44772,5.36124) 
> > (6.75629,1.41069) (54.079,-33.4137) (2.19591,-6.12473) 
> > (-1.91544,-6.56722) (-5.87699,-3.166) (-6.83039,0.994718) 
> > (-4.49682,5.42143) (-0.363706,6.79664) (0,0) (6.653,1.39701) 
> > (6.73628,-2.57781) (2.83963,-6.40383) (-1.3027,-7.62756) 
> > (-5.2442,-4.20318) (-7.30496,-0.464149) (-46.0595,43.0102) 
> > (-1.1131,6.52395) (3.28888,5.98481) (6.67649,1.87579) 
> > (6.12278,-2.4737) (3.20924,-5.88361) (-1.73027,-7.05576) 
> > (-5.08706,-4.94171) (-6.68616,-0.0110321) (-5.45217,3.92032) 
> > (-1.28323,7.08399) (3.24904,6.21134) (6.67752,2.90531) 
> > (7.18779,-1.51598) (36.8116,-51.7724) (0.146267,-7.00651) 
> > (-4.51996,-4.89137) (-7.37838,-1.41322) (-6.20339,3.98806) 
> > (-2.49422,6.80633) (0,0) (0,0) (0,0) (0,0) (0,0)]
> >
> > This is weired. The channel is just AWGN.  Why are not the channel 
> > taps 1?
> > I found the same question posted before. 
> > https://lists.gnu.org/archive/html/discuss-gnuradio/2017-05/msg00057.html. 
> > But didn't see the answer.
> >
> > I'm running GNU Radio Companion 3.7.12 by the way. The example can be 
> > found in the attachment.
> >
> > Regards,
> > Edwin
> >
> >
> >
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