Covert underwater communication based on combined encoding of diverse time-frequency characteristics of sperm whale clicks

Abstract

Bio-inspired camouflage covert underwater acoustic communication has been studied widely recently. In order to improve the camouflage ability and communication rate, a bio-inspired covert underwater acoustic communication method based on combined encoding of diverse time–frequency characteristics of sperm whale clicks is proposed. The characteristics of time–frequency contour shapes and frequency-band distributions are classified and counted, and the communication codes are constructed based on the two characteristics. According to the inter-click interval characteristic of sperm whale click pulse trains, the bionic communication sequences are constructed through differential time delay difference encoding between different communication codes. Based on the proposed combined encoding method of diverse time–frequency characteristics, the constructed communication sequence contains a large number of different modulated clicks. It conforms to the characteristic that there exists a large number of and a variety of clicks in the true click pulse train, which improves the camouflage ability. The true sperm whale click pulses are inserted into the communication sequences as camouflage pulses to further improve the camouflage ability. Furthermore, the communication rate is improved due to using multiple characteristics to represent different encoding information in the proposed method. Simulations are implemented to demonstrate the effectiveness, reliability and covertness of the proposed covert communication method.