Hello Roohollah Parvizi:
-There could be many reasons, but first, you want to check
the link speed that you are getting from the switch (requires
1000Mbps connection - 100Mbps won't work).
-rx_samples_to_file is part of UHD (not GNURadio
application, although somewhat related). I believe it is not
capable of MIMO synchronization without modifications (please
correct me if I am wrong). You can download its source code
from the UHD Github project and adapt it if you want.
-It is probably easier to use GNURadio. I am actually not
sure if you have GNURadio on your system, since that
rx_samples_to_file is bundled with UHD, not with GNURadio.
Anyway, if you have it installed on your PC, you should be
able to use GNURadio Companion to write a short program out of
it. On GNURadio Compaion, you should add a "UHD: USRP Source"
block and set "Num Mboards" = 2 and match settings with your
configurations (Will have to set device address, Mb0/1
Clock/Time source, etc).
Kyeong Su Shin