  In this study, we propose a general-purpose synchronization method that
allows a set of software-defined radios (SDRs) to transmit or receive any
in-phase/quadrature data with precise timings while maintaining the baseband
processing in the corresponding companion computers. The proposed method relies
on the detection of a synchronization waveform in both receive and transmit
directions and controlling the direct memory access blocks jointly with the
processing system. By implementing this synchronization method on a set of
low-cost SDRs, we demonstrate the performance of frequency-shift keying
(FSK)-based majority vote (MV), i.e., an over-the-air computation scheme for
federated edge learning, and introduce the corresponding procedures. Our
experiment shows that the test accuracy can reach more than 95% for homogeneous
and heterogeneous data distributions without using channel state information at
the edge devices.
