Experimental demonstration of a distributed antenna array pre-synchronized for retrodirective transmission

We describe the key ideas behind our implementation of a distributed antenna array fully pre-synchronized for retrodirective transmission to an external receiver. In our implementation, a number of wireless transceivers in a network use a sequence of simple in-band wireless message exchanges to calibrate themselves so that these transceivers can obtain their channel gains to an external receiver using reciprocity simply by observing a single incoming transmission from that receiver without any channel feedback or other cooperation from the receiver. Some notable features of our implementation are as follows: (a) it automatically calibrates and corrects for unknown channel gains, oscillator offsets and drifts between the array nodes as well as the effect of non-reciprocal RF hardware; (b) it is fully wireless and endogenous i.e., does not use any wired backhaul connections or side channels including GPS; and (c) it uses simple signal processing on a standard and widely available software-defined radio platform based on off-the-shelf hardware and open-source software. Also, to the best of our knowledge, this is the first ever demonstration of a pre-synchronized distributed array, and thus our implementation serves as a proof-of-concept and allows for the development of more advanced distributed array techniques building on this capability.