An empirical study of the statistics of phase drift of off-the-shelf oscillators for distributed MIMO applications

We report on the results of an empirical study of the phase drift of commercial RF oscillators, and the performance limitations implied by these drifts for distributed multi-input multi-output (DMIMO) wireless systems. The stochastic clock drifts of the oscillators at the nodes of the virtual array and the resulting synchronization errors limit the performance of DMIMO systems. Furthermore the synchronization requirements for DMIMO applications are subtly different from other synchronization problems. We examine two commercially available oscillators and present a series of measurements and calculations to characterize their short-term stability. Specifically we quantify the decrease in phase drift for a temperature controlled oscillator (TCXO) as compared to an uncompensated (VCXO) crystal oscillator. We also present the results of tracking the clock drifts of both oscillators using a simple algorithm based on an Extended Kalman Filtering framework. One key observation coming out of our study is that while the temperature compensated oscillators are an order of magnitude better than the uncompensated ones as measured by their frequency stability, the corresponding performance improvement for DMIMO applications is quite modest. This suggests the need for new figures of merit and novel oscillator designs specifically optimized for DMIMO applications.