RE: Using Fractional or Rational resampler to simulate Doppler

[Dave Miller]

Jose,

During my talk at the last GNU Radio Conference (September 2022), I was asked by an audience member about how to handle Doppler for spacecraft links so I later developed Transmit/Receive Flowgraphs with Doppler that are located at:

https://github.com/DavidToddMiller/gr-HighDataRate_Modem

You can find the flowgraphs that include Doppler in the following folder on the repository site:

“examples/Doppler_And_CCSDS_TTC_Flowgraphs_LowRate”

When running one of the flowgraphs, you will see the Transmit carrier (PM modulation with a subcarrier) slowly drifting across the Frequency Sink display in the GUI (about 200-300 Hz per second to simulate typical S-band link Doppler in Low Earth Orbit). On the receive end of the flowgraph, a continuous FFT is used to acquire and track the carrier. Generating the Doppler portion of the flowgraph is done with just standard In-Tree GNU Radio blocks.

Since the GNU Radio Conference, I have added a lot more to this repository including Doppler and a lot of codes: CCSDS Reed Solomon, CCSDS Convolutional, and CCSDS DVB-S2 to test them at high data rates with CPU multicores. However, the Doppler flowgraphs to which I am pointing you is low data rate.

Support for a CCSDS rate ½ LDPC Encoder/Decoder is at my other repository, but it does not include Doppler yet in the flowgraphs (maybe later this year):

https://github.com/DavidToddMiller/gr-aff3ct_codes

Regards,

Dave Miller

On Thu, Jul 27, 2023 at 4:19 PM Jose Ruvalcaba 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