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I am trying to emulate a satellite link where the delay time between the satellite and ground station changes over time due to the satellite's motion. I plan do this by converting to the frequency domain via FFT, multiplying it by $e^{-2\pi \omega_{delay} }$, and converting back to the time domain via IFFT.

However, when I implement this in GNURadio, it fails to work as I expect.

enter image description here

I generate a signal, pass it through the delay, and then match filter the original signal with the delayed signal. I then change the delay with a GUI slider. When I use the frequency-domain-phase-shifting method that I implemented, the correlation peak does not move when I change the delay. However, if I substitute in gr-baz's variable delay block, the peak does move when I change the delay.

Why does my implementation of the shifting method not work? Is it because my delay block only works with a finite window of the signal?

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    $\begingroup$ At first glance you are properly implementing a delay in the frequency domain, however how do you handle the stream from one FFT block to another (or are your FFT the entire message in one block?). If mutliple blocks, how do you ensure the delay implemented is contiguous? $\endgroup$ – Dan Boschen Aug 16 '18 at 20:02
  • $\begingroup$ I am not totally sure what you mean. All the FFTs in my flowchart process 2048 samples at a time. I need to be able to handle an indefinitely long signal source in real time, so I cannot load the entire signal into the FFT. Is it not possible to implement the phase shifting method with a FIR? $\endgroup$ – rytse Aug 16 '18 at 20:17
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    $\begingroup$ You can absolutely implement a variable delay with an FIR filter (although that wasn't your question; your question was why the FFT approach was not working). To implement a variable delay consider a polyphase implementation (I think I have a post showing how that can be used nicely as a variable delay, will look up later if I can find it) or a Farrow Structure FIR filter. $\endgroup$ – Dan Boschen Aug 16 '18 at 20:31
  • $\begingroup$ This link may help: dsp.stackexchange.com/questions/28252/… and mathworks.com/help/dsp/examples/… $\endgroup$ – Dan Boschen Aug 16 '18 at 20:33
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Please see the answer to this question How to implement Polyphase filter? on how to implement a polyphase interpolator, and then note that the same structure without actually commutating the output can be used as a discrete variable delay; each output is the input within the passband of the filter at a different delay in uniform discrete steps. By stepping through the output continuously as we do in the interpolator, we achieve a higher sampled version of the input. By staying at one output we get the input at a specific delay based on that filter, by changing to another output we get another delay. The design of the filter as given in the link will properly create the filters such that each is a different delay of the other (with the same ampltitude response) in uniform steps.

polyphase resampler

Continuing with the example shown of "Resampling" further shows how a variable delay device results:

Here is shown a IS95 CDMA waveform with 2 samples per symbol (as indicated by the red dots on this eye diagram). The analog waveform that is represented by these samples is shown in blue.

CDMA before resampling

If we used the polyphase structure as an interpolator, we would get all the samples in between the 2 samples per symbol we have, evenly spaced, given by the number of filter banks in the polyphase structure.

Therefore it should be clear that we can select any filter output that is closest to the desired delay we want to achieve (in this case within one symbol duration).

CDMA after resampling

As mentioned in the comments a Farrow Filter is another approach to implementing a variable fractional delay in an FIR filter.

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