Compressive sensing: a strategy for fluttering target discrimination employed by bats emitting broadband calls

When foraging, so-called FM-bats emit sequences of frequency modulated (FM) calls in order to detect, identify, and localize edible prey. Once a potential target has been detected, various call and call sequence parameters, such as frequency sweep, pulse duration, and inter pulse interval (IPI) vary. In this paper, the possible functions of the variation of the IPI are studied. In particular, it is conjectured that the IPI patterns are an adaptive behavior that optimizes the signal design parameters in order to improve information retrieval. Such an irregular sampling strategy would be useful whenever bats need to characterize signal modulation (e.g., the wing beat of an insect) using a call emission rate lower than the signal modulation of interest. This problem can be recast as extracting features, in this case the joint acoustic and modulation frequency representation, from signals sampled at frequencies well below the Nyquist cut-off frequency. To study the possibility of such target classification using a sub-Nyquist sampling scheme, results derived in the context of compressive sensing are used. Processing echoes collected from both rotating computer fans and fluttering locusts, it is shown that such a strategy would allow FM-bats to discriminate between targets based on their different fluttering rates.