Re: Using Fractional or Rational resampler to simulate Doppler

[Daniel Estévez]

Hi all,

Whether a resample (i.e., group delay change) is needed depends on the 
intended application. The question you need to ask yourself is: is the 
fact that in real life the Doppler at each frequency in your passband is 
slightly different because they are all different carrier frequencies 
relevant, or is it not and can we just take the Doppler for the centre 
frequency as Doppler for all frequencies?

If a resample is needed, then it is easy to do a constant resampler rate 
(corresponding to constant Doppler) by using the Polyphase Arbitrary 
Resampler (or the Channel Model block, which contains a Polyphase 
Arbitrary Resampler). Currently there isn't a simple way to make the 
resample rate change over time (corresponding to Doppler change over 
time). Perhaps a simple solution is to periodically call the callback to 
update the resample rate, according to the required Doppler curve.

Best,
Daniel.

On 27/07/2023 22:54, Tim Meehan wrote:
> Hi Jose,
>
> I would take a look at gr_sattelite.  The work described here
> https://destevez.net/2022/07/real-time-doppler-correction-with-gnu-radio/
>   was integrated.  A true doppler correction would need to also
> resample, but the time varying frequency correction may be enough for
> your use case.
>
> Tim
>
> On Thu, Jul 27, 2023 at 1:19 PM Jose Ruvalcaba <joruvalc@gmail.com> wrote:
> > Hello,
> >
> > I am trying to simulate an accurate representation of Doppler Shift in a 
> > channel and I was told that to do this I would have to multiply a complex 
> > signal with a cosine and implement a resampler block to simulate the signal 
> > samples that are coming closer or farther away from each other.
> >
> > My question is this, can I use either the rational resampler or fractional 
> > resampler block in GNU Radio to dynamically simulate samples moving closer or 
> > farther than each other?  Or would I need to make my own OOT block to do this?
> >
> > I tried using a QT GUI Range block to change the resampling ratio of a 
> > fractional resampler but my flowgraph froze.
> >
> > Any suggestions?
> >
> > Thanks,
> > Jose

OpenPGP_signature.asc
Description: OpenPGP digital signature