Real-Time High-Accuracy Digital Wireless Time, Frequency, and Phase Calibration For Coherent Distributed Antenna Arrays

TL;DR

This paper introduces a real-time, high-accuracy digital calibration method for coherent distributed antenna arrays using wireless time and frequency synchronization, eliminating the need for cables or external references.

Contribution

A novel two-way time transfer approach for wireless time and frequency calibration of COTS SDRs, achieving high precision and robustness in static and dynamic environments.

Findings

Achieved ~60-70 ps time and phase precision

Median coherent gains above 99% in static scenarios

Beamforming frequency RMSE of 3.73 ppb in dynamic scenarios

Abstract

This work presents a fully-digital high-accuracy real-time calibration procedure for frequency and time alignment of open-loop wirelessly coordinated coherent distributed antenna array (CDA) modems, enabling radio frequency (RF) phase coherence of spatially separated commercial off-the-shelf (COTS) software-defined radios (SDRs) without cables or external references such as global navigation satellite system (GNSS). Building on previous work using high-accuracy spectrally-sparse time of arrival (ToA) waveforms and a multi-step ToA refinement process, a high-accuracy two-way time transfer (TWTT)-based timefrequency coordination approach is demonstrated. Due to the two-way nature of the high-accuracy TWTT approach, the time and frequency estimates are Doppler and multi-path tolerant, so long as the channel is reciprocal over the synchronization epoch. This technique is experimentally verified using COTS SDRs in a lab environment in static and dynamic scenarios and with significant multipath scatterers. Time, frequency, and phase stability were evaluated by beamforming over coaxial cables to an oscilloscope which achieved time and phase precisions of ~60 ps-70 ps, with median coherent gains above 99 % using optimized coordination parameters, and a beamforming frequency root-mean-square error (RMSE) of 3.73 ppb in a dynamic scenario. Finally, experiments were conducted to compare the performance of this technique with previous works using an analog continuous-wave two-tone (CWTT) frequency reference technique in both static and dynamic settings.