Bio-inspired RF steganography via linear chirp radar signals

The chirp signal is one of the first bio-inspired signals commonly used in RF applications where the term chirp is a reference to the chirping sound made by birds. It has since been recognized that birds communicate through such chirping sounds to attract other birds of the same species, to transmit an alarm for specific threats, and so on. However, birds of a different species, or sometime even birds in a different social group within a species, are unable to connect a specific meaning to certain calls - they will simply hear a bird chirping. Inspired by such, this article provides a tutorial on a novel RF steganography scheme to conceal digital communication in linear chirp radar signals. We first provide a review of the linear chirp signal and existing communication systems using chirp waveforms. Next we discuss how to implement the RF steganography and hide digitally modulated communication information inside a linear chirp radar signal to prevent an enemy from detecting the existence of such hidden information. A new modulation called reduced phase shift keying is employed to make the modulated chirp waveform almost identical to the unmodulated chirp signal. Furthermore, variable symbol durations are employed to eliminate cyclostationary features that might otherwise be exploited by an enemy to detect the existence of the hidden information.