Covert communications based on signal overlay

A covert communication technique based on the principle of signal overlay is presented here. A weak narrow-band signal is added to the primary signal and shares the same frequency band. Covertness of the overlay signal is ensured by careful signal design. At the receiver a stationary filtering approach is ineffective in separating the signals. However, the recently introduced raised cosine empirical mode decomposition technique is ideally suited to separate these time-varying signals with overlapping frequency components. An application to commercial frequency modulation overlay is presented with associated analysis and empirical performance results.