###### Abstract
Since the usage of sound to achieve covert UAC was first proposed by the Office of Naval Research in 2001 [16], many BC-UAC methods have been proposed [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. Firstly, the characteristics of marine animal sound signals are used for modulation [17; 18; 19; 20; 21; 22; 23]. In [17], the time interval between dolphin clicks is used to modulate information bits, and the dolphin whistle is used as the synchronization signal. In [18], the dual-orthogonal modulation method is proposed based on the compressed sea lion click signal, as well as using the sea lion whistle signal as the synchronization signal. In addition, the concealment of communication is improved by passive time reversal mirror technology. In [19], a new frame hopping structure is proposed. The appropriate signal is selected from the original signals, according to the ambiguity function and correlation characteristic. In [20], a bionic camouflage communication system is designed, where the killer whale sound signal is used for communication address coding. In [21], a bionic Morse coding mimicking the humpback whale song is used in covert underwater communication. In [22], a bio-inspired covert UAC based on sperm whale clicks is proposed, where frequency-hopping modulation is used on different frequency band distributions of the sperm whale tick, and the encoded sequence is inserted into the corresponding position of the original signal. Secondly, traditional signals and marine animal sound signals are used to synthesize the transmitting signals [23; 24; 25; 26]. In [23], watermarking is embedded into the bottlenogle dolphin signal, and the embedded information is extracted by an extended Prony method. In [24], a bionic communication scheme is proposed, where the minimum shift keying (MSK) modulated signal is copied to the dolphin whistle contour with continuous phase. To realize underwater target detection and covert communication, original clicks are combined with traditional sonar waveforms [25]. In [26], the framework of covert UAC is constructed, where the whistle is modulated by a special spectrogram pattern and demodulated by the correlation peak. Thirdly, some researchers try to simulate the inherent acoustical signal to realize covert communication [27; 28; 29; 30; 31]. In [27], the navigation test of imitating dolphin whistles and clicks is completed with the pulse position modulation (PPM) method, but the corresponding demodulation scheme is not given. The generalized frequency modulated (GFM) signal is used to imitate the fundamental wave of a dolphin whistle, and information bits are mapped to the parameters of GFM biomimetic signals [28]. Moreover, based on imitated high frequency clicks, the time hopping code is used to modulate each bit, and the PPM is used to modulate the frame sequence [29]. In [30], a BC-UAC method based on camouflaging sea piling sounds is proposed, where the periodic PPM is adopted. In [31], a method of simulating ship-radiated noise with chaotic signals is proposed, and a BC-UAC scheme based on mimicking ship-radiated noise is presented. Although many BC-UAC methods have been proposed in the past twenty years, the concealment and communication rate of these methods should be improved due to the complexity and variability of the underwater environment. Moreover, considering that the wide frequency band of clicks is not suitable for practical transducers, and the frequency band of the whistle signal is narrow and has good time-frequency characteristics, the whistle signal of cetaceans is selected for bionic covert communication. Furthermore, since the communication rate in the existing BC-UAC methods is generally low, the BC-UAC method with a higher communication rate is studied in this work. Hence, the problem of BC-UAC based on the time-frequency contour (TFC) of the bottlenose dolphin whistle is investigated in this paper.