[srslte-users] C-V2X sidelink without GPSDO

[Andre Puschmann]

Hey Sebastian,

On 3/3/21 20:36, sebi at faho.rwth-aachen.de wrote:
> has anyone tried using the C-V2X sidelink without a GPSDO on a USRP B200/B210 by directly applying a PPS signal from an external GPS receiver?
> 
> I followed the application note at [1] but am unable to receive packets from a commercial C-V2X modem. The I/Q-Plots look very noisy and no constellation pattern can be identified. Do I need both a PPS signal and a 10 MHz reference signal? Unfortunately, my GPS receiver does not have a 10 MHz reference clock output. I even tried adjusting the code in [2] in order to set the USRP time to the GPS synchronized system time in the absence of a GPSDO but to no avail. Can I somehow check the srsLTE synchronization status?

I don't think what you're trying to do will work. And here is why:

When you operate Sidelink without a SyncRefUE you need GNSS for sync.
As far as I know it's the only mode that COTS modems support. In other
words, you need GPS on both nodes.

If you just feed only PPS to the USRP you can't guarantee time and
frequency alignment.

Thanks
Andre

> 
> Thanks in advance for your help!
> 
> Best regards,
> Sebastian
> 
> [1] https://docs.srslte.com/en/latest/app_notes/source/cv2x/source/index.html
> [2] https://github.com/srsLTE/srsLTE/blob/master/lib/src/phy/rf/rf_uhd_imp.cc
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