For a synchronization across two USRPs or two RX ports, each channel in the system must meet the following two requirements:
- The sample clocks of two RX ports should be synchronized and aligned.
- DSP operations must be performed on samples aligned in (or from) the same sample clock edge.
These two requirements provide the aligned sampling capability of ADCs for each RF path. For this synchronization, PPS signal and the reference clock signal should be shared when you use two USRPs. You can refer to LabVIEW USRP examples: niUSRP EX RX Multiple Synchronized Inputs (MIMO Expansion).vi
for MIMO Expansion cable or niUSRP EX RX Multiple Synchronized Inputs (PPS Trig).vi
for PPS and reference clock connection.
But from the signal point of view, there are additional requirements for synchronization. Each RF path has a phase offset because the RF characteristics of each RF component constituting the RF path are not identical. In addition to the above two requirements, you should compensate the phase offset to synchronize two signal at the signal level. The phase offset can be measured using the known signal like a preamble signal.