Re: [Discuss-gnuradio] how does Doppler shift increment in flat fading channel GNU radio

[Bastian Bloessl]

Hi,

On 22 Feb 2016, at 10:54, Nasi <address@hidden> wrote:

d_m is incrementation or index after initialization, which is correct thing there.

This is what I don’t get. d_m is the absolute sample index. Let’s say you have a slight change in Doppler Frequency, then, from the first to the second sample this results in a slight phase change d_m * delta_f.

Now assume the same delta_f after the 10e6 sample. AFAIS, this will just go crazy and maybe result in seemingly random accesses to your sincostable.

Yes, as you also said that is per sample.
cos(alpha_n) is constant there,

alpha_n is not constant. It depends on d_theta, which is updated after each sample

https://github.com/gnuradio/gnuradio/blob/master/gr-channels/lib/flat_fader_impl.cc#L67

Best,

Bastian

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 <address@hidden>:

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 <address@hidden> 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

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NE