MIMO cable, frequency synchronization

[zhenjason]

Will the frequency be synchronized if I use a MIMO cable to connect two USRPs? I thought it will be but I saw small frequency offset in the received signal. Also, it will have constant phase offset by connecting two USRPs using MIMO cable, right?

[Carl_W]

Using the MIMO cable should take care of the synchronization. In the NI-USRP Help document under Programming Reference >> MIMO Synchronization talks about the ways you can do this.

[zhenjason]

Thank you fo your reply. I have tried to connect two USPRs via MIMO cable and transmit and receive packets continuously between them. There is no frequency offset for 99% of the transmission. However, still I can see that the signal has frequency offset sometimes. The set up is that I use an RF connects the two USRPs with 70dB attenuation (three attenuators two 30dB and one 10 dB). Please see the attached figures for the received normarlized signal. 

Any idea what may cause this? Thank you.

[cochenob]

If you're using an external RF source, rather than using one of the USRPs as your source....then you will most likely always have a small frequency offset.  You may be able to phase lock the external source to the USRPs (using a 10 MHz ref)...but since you're not actually sharing LO's...they won't necessarily be the same frequency.  At least this has been my experience. 

[zhenjason]

Thank you for your reply. I did not make myself clear. I meant that I used an RF cable with attenuators to connect two USPRs (It is not over-the-air transmission) and at the same time used MIMO cable to share the clock between two USRPs. Normally, there should not be frequency offset. But I saw it sometimes.