I’ve created a MIMO setup connecting 2 USRPs using an external clock
reference (10MHz) and 1-PPS input. However, when I transmit packets
them, I see a phase rotation experienced by the subcarriers. I’ve
an image highlighting the same.
The x-axis is the subcarrier index (80 subcarriers = 1 OFDM symbol) and
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
now share the external clock, can this still be a problem? Or could
be another reason why I see this happening. The slope of this channel
remains seemingly consistent for all the packets. I’m trying to achieve
scenario where I see a constant phase for all the subcarriers.
Using USRP N200’s rev 4.