Hi, I posted a question about the very same lines of code four days ago, but did not get a reply yet
http://lists.gnu.org/archive/html/discuss-gnuradio/2016-02/msg00254.html <http://lists.gnu.org/archive/html/discuss-gnuradio/2016-02/msg00254.html> I’m not sure whats going on there, but some wild guesses: The Doppler Frequency per sinusoid is distributed according to that U-shaped thing you see everywhere. Now, instead of rolling the dice once during initialisation and sticking to that Doppler frequency forever, this implementation is doing something like a random walk through the Doppler Spectrum. alpha_n is going back and forth between -pi and pi (plus some initial phase offset), so fDTs * cos(alpha_n), the normalised Doppler Frequency, follows this U-distribution. This has the advantage that you can start the block once and your results will converge to the mean. (Otherwise, you would have to do a lot of repetitions to get a lot of random initialisations.) What I don’t get (and what I asked in the other thread) is why this is multiplied with d_m. I think that, per sample, the current Doppler Frequency should be used to calculate an incremental angle to the previous value. (I guess the sincostable is just a lookup table for sin and cos values for speed optimisation.) I hope that didn’t confuse even more... Best, Bastian > On 22 Feb 2016, at 06:41, Nasi <nesaz...@mail.ru> wrote: > > Hello, > > The question is about how does the given Doppler shift progress, or how is > the Doppler induced phase shift implemented. > > I select a simple frequency selective fading block and feed in it some > gr_complex(1, 0) values. For simplicity I run one fader (num of sinusoids). > in file: > https://github.com/osh/gnuradio.old/blob/master/gr-channels/lib/flat_fader_impl.cc > > <https://github.com/osh/gnuradio.old/blob/master/gr-channels/lib/flat_fader_impl.cc> > > in the code below, > #elif FASTSINCOS == 2 > float s_i = > scale_sin*d_table.cos(2*M_PI*d_fDTs*d_m*d_table.cos(alpha_n)+d_psi[n+1]); > float s_q = > scale_sin*d_table.cos(2*M_PI*d_fDTs*d_m*d_table.sin(alpha_n)+d_phi[n+1]); > > #else > d_m shows that the Doppler shift must progress sequencially. However, the > value of "2*M_PI*d_fDTs*d_m*d_table.cos(alpha_n)" as a whole, produces > floating point numbers which results in kind of random values out of > d_table.cos() function in file > https://github.com/osh/gnuradio.old/blob/master/gr-channels/lib/sincostable.h > > Some more explanation: > the value: 2*M_PI*d_fDTs*d_m*d_table.cos(alpha_n) gets in as x below (in file > .../lib/sincostable.h) > (((int)(x*d_scale)) + d_sz) % d_sz; - this is a random integer value (may be > not, can you please help me with that?) > therefore it returns a random cos value as: return d_cos[idx]; > > The issue arises when that floating point values inside cos() function is > converted to integers as given above. > > Now, my question is, did you do that random phase shift/Doppler shift on > purpose? If yes, what is the reasoning behind that. > As far as I know, the Doppler shift should be somehow linear progressive. > > -- > NE > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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