###### Abstract  
In order to meet the requirements of communication security and concealment, as well as to protect marine life, bionic covert communication has become a hot research topic for underwater acoustic communication (UAC). In this paper, we propose a bionic covert UAC (BC-UAC) method based on the time-frequency contour (TFC) of the bottlenose dolphin whistle, which can overcome the safety problem of traditional low signal-noise ratio (SNR) covert communication and make the detected communication signal be excluded as marine biological noise. In the proposed BC-UAC method, the TFC of the bottlenose dolphin whistle is segmented to improve the transmission rate. Two BC-UAC schemes based on the segmented TFC of the whistle, the BC-UAC scheme using the whistle signal with time-delay (BC-UAC-TD) and the BC-UAC scheme using the whistle signal with frequency-shift (BC-UAC-FS), are addressed. The original whistle signal is used as a synchronization signal. Moreover, the virtual time reversal mirror (VTRM) technique is adopted to equalize the channel for mitigating the multipath effect. The performance of the proposed BC-UAC method, in terms of the Pearson correlation coefficient (PCC) and bit error rate (BER), is evaluated under simulated and measured underwater channels. Numerical results show that the proposed BC-UAC method performs well on covertness and reliability. Furthermore, the covertness of the bionic modulated signal in BC-UAC-TD is better than that of BC-UAC-FS, although the reliability of BC-UAC-FS is better than that of BC-UAC-TD. b 2022  
Since the detection probability is proportional to the signal-to-noise ratio (SNR), the LPD-based covert UAC method uses some technologies, such as orthogonal frequency division multiplexing (OFDM) [7], direct sequence spread spectrum (DSSS) [8], spatial diversity, frequency diversity and so on, to protect the marine ecological environment without exposing the communication equipment. However, as SNR decreases, the reliability, communication rate and communication distance of these LPD-based covert UAC schemes will obviously degrade. On the other hand, the LPI-based covert UAC method is to encode the digital symbol in a high SNR [9]. This method will not only expose the communication equipment, but also affect the marine environment. However, this method performs well on reliability, communication rate and communication distance. In the actual detection process, the passive sonar can still detect the LPD-based communication signal through a long-time integration. As the adversary approaches, the location of the communication equipment will be exposed. Moreover, since the signal transmitted by the LPD-based covert UAC method has obvious characteristics, the information is exposed, and the security is invalid once detected by the adversary. In addition, one finds that man-made noise can damage the sonar navigation system of cetaceans, which makes toothed whales unable to communicate, prey and identify directions, and even die in extreme cases [10; 11; 12]. Figure 1 illustrates the effect of human activities on toothed whales. In recent years, the LPR-based covert UAC method, which uses bionic communication technology to improve communication distance and reliability, as well to protect the marine environment and eliminate the exposure of communication equipment, has become a hot research topic. The LPR-based covert UAC method is to imitate the sounds in the marine environment, such as the sounds of marine mammals, human activities and natural sounds, or artificially generated sound signals. For bionic covert UAC (BC-UAC), the communication signals are usually identified as marine environmental noise and filtered out in signal processing [13; 14; 15]. Hence, the BC-UAC method can ensure the security of communication and the concealment of communication equipment, as well as providing environmental friendliness. Figure 1: The effect of human activities on toothed whales.