Re: [Discuss-gnuradio] Periodically varying channel gain mesurements

[Veljko Pejovic]

Thanks for the suggestions. I'll see if the same 10MHz clock changes
anything. However, my initial idea was to have devices far apart.
Clocking the with the same ref clock in that setup seems impossible.

Cheers,

Veljko


On Thu, Feb 24, 2011 at 5:16 PM, Johnathan Corgan
<address@hidden> wrote:
> 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
>
>
>