phase change Rx USRP

[feeeeeelix]

Hi!

we are using the .vi modulations examples for two USRP-2920. The problem is that when we receive, the symbol phase changes constantly. This brings that each time we execute it, we receive different symbols. For example, if we send all 1's constantly there are times that we receive all 1's, sometimes all 0's or even 1 and 0 alternately. We tested with all modulations from the examples but all is the same. How can we solve this problem?

Thanks.

I add some screenshots so you can see the problem.

This is what we are sending:

And this is what we are receiving:

Like you can see, the received bits are very different from the sended bits.

Thank you for the attention.

[cochenob]

If you're using two receivers, you'll need to remember that they are not phase locked, even if you set the LO's to the same frequency.  This could result in extra phase changes during transmission due to drifting oscillators.  You can eliminate this by locking the two USRPs together with a MIMO cable.  Still....you'll likely have some phase offset between the two each time you turn them on (at least now it will be constant during transmission).  Depending on how you're doing your recovery, you might decode all 1's and 0's and vice versa depending on what the constant phase offset between the two was.  You can get around this with either some clever differential encoding, or, a by using a known training sequence to 'cue' the Rx at the beginning into determining what phase it should assign to 1's and what phase to assign to 0's.

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Brandon