# Underwater Biomimetic Covert Acoustic Communications Mimicking Multiple Dolphin Whistles
## 1 Introduction  
Underwater biomimetic communication ensures the covertness of communication signals by mimicking the communication signals with the sounds of underwater organisms. The underwater biomimetic communication method has been researched to overcome the large, low-probability detection problem of the conventional direct sequence spread spectrum method in underwater communication [1; 2; 3; 4; 5; 6; 7]. Mimicking the dolphin whistles is commonly used for underwater biomimetic covert acoustic communications [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. The dolphin whistles have chirp-like patterns varying in time and frequency with a usable frequency bandwidth of the projector, while whales and other animals generate the sounds with a low and wide bandwidth, respectively. Thus, dolphin whistles are more adequate than other ocean animal sounds for biomimetic underwater communication. Conventional biomimetic communications have been proposed using chirp spread spectrum, frequency shift keying, differential phase shift keying (DPSK), continuously varying carrier frequency modulation (CV-CFM), and time-frequency shift keying (TFSK) that transmit bits according to the time-frequency position of the whistle or the shape of the whistle pattern [8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. However, since most dolphins are social animals and live in groups, they produce multiple whistles. Mimicking the multiple dolphin group sounds is necessary to increase the covertness of underwater covert communication. Thus, the conventional whistle-mimicking method has a limitation of covertness by mimicking a single dolphin whistle, not a group of dolphins, which does not reflect the ecology of dolphins living in groups. When the group dolphin whistles are used for communication, the multiple whistles may increase the data rate but cause interference among the whistles, which decreases bit error rate (BER) performance. In this paper, we propose a method to convey information by mimicking the multiple whistles produced by a group of dolphins to increase the covertness and the data rate. Multiple whistles can be generated by simultaneously transmitting the single dolphin sounds, but these have a problem in that interference occurs between the overlapped multiple whistles, resulting in low detection performance at the receiver. The proposed method combines TFSK and CV-CFM to achieve high covertness and a high data rate by mimicking multiple dolphin whistles and mitigating the interference caused by overlapped multiple whistles. To obtain the high transmission rate, the proposed method for the multiple whistles sequentially modulates using TFSK followed by CV-CFM. When interference occurs at the overlapped multiple whistles, the CV-CFM method with phase modulation makes it difficult to demodulate the transmitted information at the interfered whistles. To solve the interference problem, we search the interference whistle locations and utilize spread orthogonal codes on the interfered whistles to mitigate the interference so that the interfered whistle can be decoded at the receiver. In addition, interleaving is applied to further reduce the effect of the remaining interference on the whistles. Since the proposed modulation method combines two modulation techniques, the complexity of the demodulation process is large and needs to be reduced. We propose a decoding algorithm in which the two modulation schemes do not interfere with each other's decoding performance. Thus, TFSK is demodulated first to estimate the time-frequency position of the whistles, and the CV-CFM of individual whistles is demodulated. To compare BER performance and the covertness of the proposed method, computational simulations, real ocean experiments, and mean opinion score (MOS) tests were conducted. Through computational simulations and ocean experiments, the proposed algorithm demonstrated lower BER and higher transmission rates compared to the conventional CV-CFM.