Shameless plug: More than a decade ago, I wrote a paper [1] on how the random perturbations in the wireless channel between an ambient RF transmitter (FM radio, TV) to the two devices, allow nearby devices to authenticate locality because the perturbations are correlated only if they are nearby (where nearby is relative to the wavelength being monitored)

[1] https://dspace.mit.edu/handle/1721.1/121443