We're working on transmit beamforming with 2 Tx USRP-2943 (4 Tx antennas) and 1 Rx USRP-2943 (1 Rx antenna), where the daughterboard in our devices is CBX-120, and we expect to see different signal power at different receiving angles.
However, we encountered USRP phase offset issues while we're trying to send aligned signals with multiple USRPs, and we found that even the two antenna ports on the same USRP can't generate aligned signals (by saying "aligned" we mean the phase of two signals are the identical). The figure below shows the unaligned signals we measured with an oscilloscope, where the carrier frequency was 2.55GHz, and the baseband signal carried was a 1kHz pure sine wave. The two curves on the top were measured from the same USRP-2943, while the two curves at the bottom were from another 2943.
So we'd like to know how can we synchronize our NI 2943 devices ? Or the only way to solve this issue is switching the daughterboard from CBX-120 to other types, such as SBX-120 ? Thanks in advance.
The phase offset is the expected behavior because USRP-2943s are not absolutely identical even though two USRPs use the same daughter boards. You need to implement a phase offset compensation algorithm using the reference signal (ex: a preamble signal).