Synchronization of multiple USRP SDRs for coherent receiver applications

This paper describes the necessary means to combine multiple Ettus Research USRP X310 software-defined radios to a multichannel coherent receiver for direction-of-arrival (DoA) and passive radar applications. The requirements to combine several software-defined radios to a multichannel coherent receiver are examined in general. In particular the requirement of phase coherence necessitates a closer look on the receiver synchronization, since the straightforward approach of synchronizing the systems with a common reference clock will in most cases lead to phase ambiguities between the channels. The mechanism inducing these phase ambiguities between several systems that are phase-locked to a common reference is discussed in detail. Results regarding the achieved phase stability and a preliminary measurement demonstrating the DoA capabilities of the system are shown.