[Discuss-gnuradio] Transceiver Residual Carrier Offset

[dougekim]

I recently purchased two new USRP2 boards each with a XCVR2450 transceiver
board.  In testing the two boards, I began by conducting a simple tx/rx test
in which both boards share a COMMON 10MHz reference clock, as opposed to two
independent references.  This test was conducted to determine if the
measured  channel would remain relatively unchanged (time invariant) in a
static channel environemnt. 

A single 2MHz tone was used as my baseband test signal (no BPSK or QAM).  In
comparing the baseband tx signal to the baseband rx signal, I noticed that
although both signals were 2MHz in frequency, they were out of phase as
expected.  However, the phase difference was not constant over time.  Rather
the phase difference was changing over time quite a bit.   

This occurrence may limit the MIMO capabilities of these boards since the
residual carrier offsets must be corrected for individually for each
transceiver.  Take a 2x2 MIMO system fore example.  Four different carrier
offsets may need to be corrected rather than one as in the case of a fully
coherent system. 

I'm curious as to whether other users have encountered similar problems with
their systems as well, and if so, would be able to explain this occurrence.
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