Experimental Analysis of Phase Error in Centralized and Distributed SDR Systems

Understanding the behavior of phase errors between radio frequency (RF) chains in software-defined radios (SDRs) is crucial to the success of implementing many phase-sensitive applications, such as beamforming. Even if SDRs are provided the same clocking signal, initial local oscillator (LO) phase offsets across devices will inevitably be different. Despite its known effect on many wireless applications, there are only few works that experimentally discuss random phase errors in SDRs. To address this issue, we perform experiments and analyze the results of tens of experiments in an attempt to understand the nature of this phase offset. In particular, we target the USRP (Universal Standard Radio Peripheral) N310 platform as it provides up to 4 simultaneous transmit/receive chains that could be attractive for beamforming applications. We first model the system used in this study and demonstrate how phase errors can affect distributed beamforming gains. Then, we introduce our experimental setup, procedures and analysis of the results of the measured phase error. We do so first between two chains of the same/different transceiver boards within the same USRP, and then between chains of distributed USRPs that are geographically separated. We calculate the mean and standard deviation of this phase error, investigate its behavior over time, and demonstrate how the distribution of this error can vary based on whether it is measured in a centralized or a distributed fashion.