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.
I think you share PPS and Ref signals between two USRP-2943s for the transmission.
Please refer to the following link: https://kb.ettus.com/Synchronization_and_MIMO_Capability_with_USRP_Devices
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).
We've referred to the link you provided and implemented a phase offset compensation algorithm, but while we were implementing the algorithm, we found that the Tx ports of our USRP 2943 seemed to be sending signals one after another instead of simultaneously, as shown in the figure.
Do you have any idea why is this happening? And how can we fix this?
Do you use LabVIEW for implementing your application? If yes, please refer to the USRP shipping example:niUSRP EX Tx Multiple Synchronized Outputs (PPS Trig).vi. In the example, you can find out how to configure the transmission start time of multiple devices.
From your screenshot, I think you did only hardware configuration.
Thank you so much for the help. We did study the niUSRP EX Tx Multiple Synchronized Outputs (PPS Trig).vi, and after couple of days of trying, we found an interesting phenomenon. It seems like the received signals from 4 Tx antennas (2 antennas on 2 USRP 2943) can be aligned by simply giving a lower sample rate (the value in the red block). The output waveform below isn't the one we transmitted so ignore it anyway.
We set the sample rate to 15.36, 30.72, and 46.08 MS/sec, and with these sample rate values, we were able to send synchronized signals:
However, after we set the sample rate to 61.44 MS/sec, then we saw the transmitted signals become something like this:
It seems like the transmission of multiple devices can start simultaneously with lower sample rates, but they'll send data one after another when higher sample rates is configured.
Sorry we're new to LabVIEW and USRP, so we may be asking some basic questions; so far we have no idea why this is happening, do you have any thought on this phenomenon ?