A simple approach to scalable wireless frequency synchronization in coherent distributed antenna arrays is presented. Coherent distributed arrays require accurate alignment of frequency, phase, and timing between antenna nodes to enable distributed beamforming. Since each node operates independently, the frequency of the oscillators on each node will inherently contain some drift relative to all others, even when designed to operate at the same frequency. Traditionally, this has been approached using two-way communications to determine the frequency offsets between nodes. In this work a simple, one-way frequency transfer approach is presented that uses a self-mixing circuit to demodulate a 10 MHz frequency reference from a modulated transmitted signal. The 10 MHz signal is input to software-defined radio nodes, synchronizing the internal oscillators to the transmitting node. Experimental results are shown for a three-node system.
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