Dear all,
I have developed GNU Radio Companion app in order to investigate MIMO RX channels coherent receiving issue reported here. My idea is to transmit known QPSK signal via TX 0 channel at some frequency and observe constellations of signals received via both RX channels at the very same frequency, and compare their rotation angle to get RX channels mutual phase offset. I am using USRP Sink/Source blocks, not Osmocom ones. So the driver stack is like: USRP Sink/Source -> SoapyUHD -> SoapySDR -> Limesuite. I have tested this app with USRP B200 mini and it seems to be working okay.
However, when using LimeSDR, I have observed rotation of received constellations for many frequencies, which means that TX/RX frequency offset is present. That probably means that still two PLLs are being used (FDD mode) instead of only one (TDD mode), which should be the case here.
So here is the question: how to force TDD mode in GNU Radio Companion? I have tested both master and RestructureLimeSuite repositories available on LimeSuite github page, because according to this commit the latter seems to force TDD mode when TX and RX frequency is the same. Still no success, though.
@andrewback mentioned some time ago about plans of implementing GNU Radio block which will be some kind of interface to low-level LMS7002 chip settings. Any news about that?
By the way, when using driver stack mentioned above, I cannot change TX gain after execution of flowgraph (depending on branch, it crashes or just stops receiving everytime I try to do it).