Combined Wireless Ranging and Frequency Transfer for Internode Coordination in Open-Loop Coherent Distributed Antenna Arrays

Abstract

Beamforming in distributed antenna arrays requires accurate coordination of the relative phasing between the separate wireless nodes. Principally, this coordination constitutes measuring the node separations and transferring a stable frequency reference, enabling beamsteering and ensuring that the local oscillators on the separate platforms are locked. A 4.5-GHz wireless system for simultaneous high-accuracy estimation of node separation and estimation of a wirelessly transferred 10-MHz frequency reference is presented in this article. Ranging is achieved using a spectrally sparse two-tone waveform with a tone separation of 200 MHz and pulsewidth of $2~\mu \text{s}$ ; a 10-MHz frequency reference is modulated onto one of the ranging tones. The use of spectrally sparse waveforms allows simple transceiver designs compared to wideband ranging systems since the tones can be individually generated and received. The theoretical bounds for range and frequency estimation are derived for a set of common modulation formats, and experimental measurements of ranging accuracy and frequency estimation accuracy are presented, demonstrating that the proposed waveform can achieve high accuracy in both domains, with range accuracy of 3.35 mm and phase errors less than 18°, supporting coherent distributed array operations up to 5.96 GHz. The results show that the ranging estimation is effectively independent of the modulation format of the reference frequency and the frequency estimation performance is independent of the sparse waveform design for ranging. Thus, the performance of range estimation and frequency transfer may be approached independently but in the same waveform.