# The Long-Range Biomimetic Covert Communication Method Mimicking Large Whale
## 1 Introduction  
Recently, a biomimetic communication method has been studied for military purposes. Biomimetic communication methods make the enemy confuse our communication signals with dolphin or whale sounds to achieve covertness [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. Traditional covert communication systems decrease the signal strength and hide the existence of the signal using various techniques (e.g., spread spectrum, chaotic modulation, etc.) [15; 16; 17; 18]. Low-power spectral density of the transmitted signal causes a low SNR at the receiver, which results in low bit-error-rate (BER) performance. Especially for long-distance communication, low frequency and narrow bandwidth are inevitable, and it is difficult to conceal the signal using these methods [7; 8; 9; 10; 11; 12; 13; 14]. Biomimetic underwater acoustic communication has been developed to overcome the disadvantage of traditional covert communication. This method allows high transmit power without considering a low detection probability, and large SNR at the receiver can be preserved. Conventional biomimetic communication methods utilize various modulation methods such as chirp spread spectrum (CSS), frequency shift keying (FSK), phase shift keying (PSK), and time-frequency shift keying (TFSK) to modulate bits into whistles [7; 8; 9; 10; 11; 12; 13; 14]. Conventional biomimetic communication studies have demonstrated their communication performance through ocean experiments. In addition, the conventional methods have evaluated the degree of mimic (DoM) by comparing the shape of mimicking the signal with the real whistle. Especially in studies of CV-CFM (based on PSK) and TFSK, the authors performed the mean-of-score (MOS) test and showed that humans could hardly distinguish a mimicking signal from a real dolphin whistle [11; 12; 13; 14]. All of the conventional studies mimicked the whistle of small dolphins (such as white-sided dolphins utilizing tens of kilohertz sounds) and conducted ocean experiments at a short range within 5 km [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. However, considering the operation area of military-purpose underwater vehicles from tens to hundreds of km, biomimetic communication should support long ranges. For long-distance communication, using a low-frequency signal (i.e., less than several hundred hertz) is advantageous over a high-frequency signal (i.e., over tens of kilohertz) due to absorption loss, and large whales use these low-frequency whistles. Thus, it is necessary to mimic a large whale for long-range biomimicking communication, and long-range underwater acoustic communication is difficult with conventional biomimetic methods. The first problem is an increase in BER due to a large multipath delay. As the range between transmitter and receiver increases to tens of kilometers, the multipath delay also increases up to a few seconds [19; 20; 21; 22; 23; 24; 25]. However, conventional biomimetic communication methods do not consider a few seconds of multipath delay because these methods are designed for a short communication range within 5 km. Thus, when a multipath delay occurs for a few seconds, the BER of the conventional biomimetic methods increases. The second problem is a degradation of the DoM performance. Human psychoacoustic ability is the most sensitive to sound in hundreds of hertz to kilohertz [26]. On the other hand, small dolphins utilize high-frequency whistles exceeding the human auditory sense. When the conventional methods mimic small dolphins, these facts have an advantage in the DoM performance. As a result of the DoM evaluation in Ref. [14], it was determined that the DoM of killer whales (2 kHz-4 kHz) is lower than that of white-sided dolphins (15 kHz-25 kHz). Therefore, two things are required to enable long-range communication with biomimetic modulation. The first one is the robustness against large multipath delay, and the second is the performance improvement of the DoM for whistles with hundreds of hertz to kilohertz.