  We present an adaptive approach for synchronizing both the phase and
frequency of radio-frequency transceivers over long-range wireless links to
support distributed antenna array applications. To enable distributed
beamforming between separate wireless nodes, the oscillators in the
transceivers must operate at the same frequency, and their phases must be
appropriately aligned to support phase-coherent beamsteering. Based on a
spectrally-sparse waveform, a self-mixing circuit, and an adaptive control
loop, we present a system capable of synchronizing the RF oscillators in
separate transceivers over distances of nearly 100 m. The approach is based on
a spectrally-sparse waveform for joint inter-node ranging and frequency
transfer. A frequency reference is modulated onto one signal of a two-tone
waveform transmitted by the primary node which is demodulated and used to lock
the oscillator of the secondary node. The secondary node retransmits the
two-tone signal which the primary node uses for a high-accuracy range
measurement. From this range, the phase of the two transceivers can be aligned
to support beamforming. We furthermore implemented an adaptive phase control
approach to support high-accuracy phase coordination in changing environmental
conditions. We demonstrate continuous high accuracy links over a 90 m distance
in an outdoor environment for durations up to seven days, demonstrating
sufficient phase coordination in changing weather conditions to support
distributed beamforming at frequencies up to 3 GHz.
