Is it possible to reuse the frequency of a commercial FM station for short-range secondary FM transmission and be able to recover the secondary FM-modulated signal despite potentially strong and time-varying interference from the primary? The answer is yes, and this paper explains how this can be done, from a review of pertinent theory and prior art to transceiver design and laboratory experiments which demonstrate the concept using off-the-shelf programmable radios. This is the first successful demonstration of analog FM underlay without any coordination between the primary and sec-ondary system. Interestingly, this only works in the ana-lytic signal domain, because the FM signal has memory. As the spectrum at premium analog broadcasting frequencies becomes more valuable, this opens the door to a number of interesting communication scenarios, including emergency response.
