[Discuss-gnuradio] Channel Response in OFDM with MIMO USRP

[Unforgiven11 dreams]

Hello -

I've created a MIMO setup connecting 2 USRPs using an external clock reference (10MHz) and 1-PPS input. However, when I transmit packets between them, I see a phase rotation experienced by the subcarriers. I've attached an image highlighting the same.

The x-axis is the subcarrier index (80 subcarriers = 1 OFDM symbol) and the y-axis is the unwrapped channel phase.

I'm trying to understand why this would happen. As far as I know only sampling offset should be the reason for the same, but since these 2 USRPs now share the external clock, can this still be a problem? Or could there be another reason why I see this happening. The slope of this channel phase remains seemingly consistent for all the packets. I'm trying to achieve a scenario where I see a constant phase for all the subcarriers. 

Using USRP N200's rev 4.

Thanks.

phase.png
Description: PNG image