Detection of Dolphin Whistle-Like Biomimicking Signals by Phase Analysis

The application of low probability of detection (LPD) in underwater acoustic communication is challenging due to the limited bandwidth and frequency band available that allows simple interception using energy detection. Confronting that, recent LPD schemes offer to disguise the communication signal as a vocalization of a marine mammal. This way, the signals can be transmitted at high power, while the interceptor believes these are biological sounds. In this paper, we propose a first interceptor tailored to distinguish between anthropogenic and biological sounds. Our main assumption is that, due to limitations on the dump-off factor of the acoustic projector, the phase of a real whistle would be much more diverse than that of a disguised whistle-like signal. We therefore propose as a classification measure the randomness of the signal's phase. The phase is calculated by a phase-locked-loop, while the phase's randomness is measured by entropy. Our results show that the approximate and sample entropies, which both uncover regularities in a signal, are good classification metrics. Analysis of data obtained from two sea experiments and from a large database of tagged dolphins' whistles, shows that our interception scheme can well distinguish between real and biomimicked signals.