On Thu, Feb 24, 2011 at 16:54, Veljko Pejovic
<address@hidden> wrote:
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.
This looks like the effect of timing skew between the transmitters and receivers. Your correlation magnitude will vary in time with a period related to how long it takes for the receiver sample times to "slide" through one PN chip length. One way to test whether this is the case is to (temporarily) lock both the transmitter and receiver to the same external 10 MHz reference; your correlation magnitudes should then be the same (with some small noise variance.)
Johnathan