Open-Loop Distributed Beamforming Using Wireless Frequency Synchronization

We present an open-loop microwave distributed beamforming system using a self-mixing circuit for wireless frequency synchronization between two transmitting nodes in relative motion. Distributed beamforming on arrays of wireless nodes requires accurate coordination of the electrical states of each node to ensure that transmitted signals arrive at the desired destination with sufficient phase alignment for high signal gain. Of the necessary coordination parameters, synchronizing the frequencies of the internal oscillators is among the most crucial to ensure distributed coherence. In this work, we demonstrate distributed beamforming between separate microwave wireless transmitters using wireless frequency synchronization in a hierarchical coordination topology that is directly scalable to larger array sizes. The primary node transmits a spectrally sparse two-tone waveform that is received by a self-mixing circuit at the secondary node. A 10-MHz frequency reference of the primary node is modulated onto the tone separation of the waveform; upon self-mixing, the demodulated 10-MHz frequency reference is passed to a phase-locked loop to discipline the oscillator of the secondary node. We demonstrate beamforming at a 1.5-GHz carrier frequency between the two nodes, performed while moving the primary node, demonstrating the ability to maintain greater than 90% ideal distributed beamforming gain.