  We demonstrate a wireless, decentralized time-alignment method for
distributed antenna arrays and distributed wireless networks that achieves
picosecond-level synchronization. Distributed antenna arrays consist of
spatially separated antennas that coordinate their functionality at the
wavelength level to achieve coherent operations such as distributed
beamforming. Accurate time alignment (synchronization) of the local clocks on
each node in the array is necessary to support accurate time-delay beamforming
of modulated signals. In this work we combine a consensus averaging algorithm
and a high-accuracy wireless two-way time transfer method to achieve
decentralized time alignment, correcting for the time-varying bias of the
clocks in a method that has no central node. Internode time transfer is based
on a spectrally-sparse, two-tone signal achieving near-optimal time delay
accuracy. We experimentally demonstrate the approach in a wireless four-node
software-defined radio system, with various network connectivity graphs. We
show that within 20 iterations all the nodes achieve convergence within a bias
of less than 12 ps and a standard deviation of less than 3 ps. The performance
is evaluated versus the bandwidth of the two-tone waveform, which impacts the
synchronization error, and versus the signal-to-noise ratio.
