Bio-inspired secure underwater acoustic communication by quasi-orthogonal keying

•Cetacean whistles can be used to camouflage the information to realize secure underwater acoustic communications.•Quasi-orthogonal splicing waveforms with the same start and end frequencies can be obtained by combining different chirps.•By transmitting messages with quasi-orthogonal waveforms, communication signals show continuous frequencies and phases.•Natural cetacean whistles can be employed as the frame head to improve communication performance and security. The bio-inspired secure underwater acoustic communication strategy camouflages the messages with cetacean calls, tricking the enemy into mistaking the communication signals for natural marine mammal sounds to obtain communication security, showing distinct advantages over the conventional low probability of detection secure strategies. Inspired by this, this paper proposes a bio-inspired secure underwater acoustic communication strategy based on quasi-orthogonal keying. This strategy camouflages the information with whale whistles, conveying the messages by different quasi-orthogonal splicing waveforms. Firstly, the construction and selection procedures of the splicing waveforms are illustrated, which is the key to realizing continuous frequency and continuous phase of the transmitted whistles and further a close similarity to the natural ones. Next, based on the autocorrelation and the wideband ambiguity function, the selection and construction procedures of the communication frame heads are demonstrated, improving the communication performance while not reducing that similarity. Then, the encoding and decoding procedures are introduced, followed by the experimental results to verify the effectiveness and security of the proposed strategy. The proposed bio-inspired secure communication strategy can serve the needs of a variety of underwater military communications, such as communications with submarines and submersibles.