[USRP-users] USRP N200 + octoclock MIMO channel sample time alignment

Juha Vierinen x at mit.edu
Fri Sep 6 10:29:26 EDT 2013


I've been testing MIMO alignment of a usrp N200 setup. I've got four usrps
hooked up to a octoclock-g, which is showing the gps lock light. I then
periodically start 8 channel recordings with external 10 MHz and PPS
locking. All channels are fed the same signal, originating from a 8 way

The relative phase differences between the channels is relatively stable,
although over tens of hours, I can detect some drift in the relative phase
differences between the channels. Also, from some reason, a few of the
USRPs seem to be consistently either 2 microseconds behind, or 2
microseconds ahead of each other, ie., the samples aren't aligned for some
reason. The timing cables aren't exactly equal length, but the  differences
in the cable lengths is the order of 1 m or less, not 300 m.

What kind of timing alignment should I expect for a four usrp N200 system?
Is it expected that in a MIMO setup there will be this much difference in
sample alignment? The rx_time tags suggest perfect channel alignment.

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