## 1 Introduction  
For military underwater acoustic (UWA) communication systems, low probabilities of detection/intercept (LPD/LPI) are important parameters [1; 2; 3; 4; 5; 6; 7]. In general, since the energy of the received signals is measured for detecting the existence of an enemy, covert communications have been developed to reduce the power spectral density of the communication signal that spreads the transmission energy over a wide bandwidth to make it appear similar to background noise [1; 2; 3; 4; 5; 6; 7]. In the UWA communications, however, the available bandwidth is too small to spread the signal energy [1; 2; 3; 4; 5; 6; 7]. Even though low energy transmission methods with narrow bandwidths have been developed, these schemes suffer from a large bit error rate (BER) [1; 2; 3; 4; 5; 6; 7]. As an alternative, biomimetic covert UWA communications which mimic the biological sounds of underwater animals have been developed to overcome the problem of the conventional covert communications [8; 9; 10; 11; 12; 13; 14; 15]. The mimicked bio-sounds enable the enemy to confuse the communication signals with the underwater animal sounds. Since the mimicked sound does not need to reduce the transmission energy, a better BER performance can be achieved than that of the conventional covert communications [8; 9; 10; 11; 12; 13; 14; 15]. Thus, the biomimetic covert communication schemes for the UWA communications have been considered as one of emerging covert communications and dolphin whistles have been utilized for the covert underwater animal sounds [8; 9; 10; 11; 12; 13; 14; 15]. Many bio-mimetic communication schemes are listed in Table 1.  
Bio-mimetic communication schemes have been researched since 2013. The authors in [8] developed the Pulse Position Modulation (PPM) based dolphin whistles. This method was tested in river experiments and its BER performance was not analyzed by the computer simulations. Only selected whistles were tested to obtain better BER results. In [9], the phase shift keying (PSK) modulation with dolphin whistles was utilized, but BER at an Signal to Noise Ratio (SNR) range of 5 dB to 10 dB showed \(10^{-2}\), which was inappropriate for communication, and the scheme was not tested in ocean experiments. Chirp spread spectrum (CSS), frequency shift keying (FSK) and PSK were utilized based on the dolphin whistle contour [10; 11; 12; 13]. However, the schemes in [10; 11; 12; 13] distorted dolphin whistles due to allocating binary information to the whistles, and had low covertness. Even though the methods in [14; 15] were developed to utilize dolphin whistles without distorting the whistles, the algorithm in [14] showed a relatively large BER performance (\(10^{-2}\)) at an SNR range of 5 dB to 10 dB, and the scheme in [15] had to utilize only high auto-correlated whistles for modulation, which decreased the covertness. Thus, a low BER performance in the ocean experiments and covertness, i.e., degree of mimic (DoM), are the most important issues of the biomimetic communication scheme. In this paper, we propose a biomimetic covert communication scheme that modulates the information bits into various whistle patterns to increase the DoM with the link information among consecutive transmitted whistles, and detects the distorted whistles--via the link information--with the UWA channel using a machine learning based detector. The proposed method divides a large number of dolphin whistles into groups based on the similarity of the patterns. Each group is used as a symbol and mapped to information bits. To convey information bits and maximize the DoM, the randomly selected whistle in a chosen group is transmitted, and different whistles are sequentially transmitted. When a number of transmitted whistles pass through the UWA channel and background noise is added, the conventional machine learning based detectors suffer from detecting many distorted whistles.