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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