Hi Bastian,
> > 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. > Exactly. The dynamic channel in GNURadio models the Doppler spectrum accurately. 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. > > To answer this, I would consider of the correspondence (via a Fourier Transform) of a Doppler shift in the time domain; A shift in frequency corresponds to a "time" dependant phase shift in the time domain. Thus a Doppler shift of Fd in the time domain corresponds to exp(j*2*pi*Fd*t). And since Dynamic channel model(flat_fader to be precise) in GNURadio models the Doppler in the time domain, the d_m could be a way to model the time. I would also ask a supplementing question that I am having problems understanding. I would expect the Doppler shift to be modelled by a complex exponential(cos[2*pi*Fd*t*cos(alpha)] + i sin [2*pi*Fd*t*cos(alpha)]) which corroborates what we know from the Fourier correspondence of a Doppler shift. Why then, in the Dynamic channel model in GNURadio (flat_fader to be precise), the Doppler is modelled by (cos[2*pi*Fd*t*cos(alpha)] + i cos [2*pi*Fd*t*sin(alpha)]) ?? I.e why is the imaginary part a cos ? Any hints? > > > 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 > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > -- Ph.D. scholar: Kelvin Chelli OFDM based wireless systems Telecommunications Lab, Uni Saarland Campus C6 3, Room 10.07 66123 Saarbrücken, Germany. <kelvin.che...@gmail.com>
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