[Discuss-gnuradio] Periodically varying channel gain mesurements

[Veljko Pejovic]

Hi,

I'm using Ubuntu 10.04 with a recent gnuradio-next branch and the UHD
driver on USRP2/WBX boards.
In an outdoor setup I have two receiving nodes about 40m away from
each other and from the transmitter. The transmitter is a PC with two
USRP2s connected with a MIMO cable. It is sending two different PN
sequences every 120ms on both interfaces at the same time. I correlate
the known sequences at the receivers to measure the channel gain. The
results can be seen here (x-axis is the packet number, y is the
absolute gain):

http://cs.ucsb.edu/~veljko/downloads/channel_gains_outdoor.jpg

The gain seems to be varying with around 2 Hz frequency. Since the
setup is static and the tx/rx gain is not modified during the course
of the experiment I'm almost certain that the artefact comes from the
system rather than from the environment. Does anyone have a good
explanation for this? Perhaps there is some sort of a gain control in
the USRP2 that I'm not aware of? I got similar behavior when I tried
the same experiment indoors.

Thanks,


Veljko