Hi, (I was not in the mailing list for a long time) yes, you are right. d_m is incrementation or index after initialization, which is correct thing there. Yes, as you also said that is per sample. cos(alpha_n) is constant there, I checked that. cos table looks like just cosine values, which must be there somehow.
But the value are selected out of cos table kind of randomly. I think there is a "bug" in the code. Otherwise, let someone explain it better. The Doppler shift cannot jump from sample to sample randomly. If I make a mistake please correct it. P.S.: In matlab that thing is not random. After some samples that changes a little bit depending on the max Doppler shift. - Nasimi >Понедельник, 22 февраля 2016, 9:54 -08:00 от Bastian Bloessl ><bloe...@ccs-labs.org>: > >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 > >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 >> >>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 > -- NE
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