# Dolphin Sounds-Inspired Covert Underwater Acoustic Communication and Micro-Modem
## 1 Introduction  
With the growing demand for marine development, underwater acoustic communication (UAC) and networks have been used in many fields, such as marine environmental monitoring, natural disaster warning, etc [1; 2]. Furthermore, military applications include communication between divers, underwater unmanned vehicles (UUVs), submarines and wireless sensor networks, where the main focus is to lower the probability of detection and interception [3; 4]. Based on the different modulation methods, the modems are divided into several categories including quadrature phase shift keying (QPSK) modems, frequency shift keying (FSK) modems, quadrature amplitude modulation (QAM) modems, and so on. A compact and low power micro-modem [5] designed by WHOI has the capability to perform low-rate frequency-hopping frequency-shift keying (FH-FSK), variable rate phase-coherent keying (PSK) and long base line navigation. As a successor, the WHOI micro-modem-2 [6] is significantly more capable, allowing new, advanced capabilities such as enhanced communication rates and underwater network deployments. [PERSON] demonstrated the real-time capabilities with both a floating point and a fixed-point DSP with QPSK modulation and described its detailed implementation in [7]. [PERSON], [PERSON] presented the implementation of the physical layer of a reconfigurable underwater acoustic modem using QPSK modulation [8]. Blueprint Subsea developed the SeaTrac modem with QPSK modulation based on front-end specifications by Newcastle University in [9]. Previously, we focused on remote and robust underwater acoustic communication modems; we have developed and tested our modems, such as HEU OFDM-modem with QPSK modulation for UAC and networking [10; 11; 12]. Compared to QPSK modulation, FSK modulation is more widely used in modems. A low-power acoustic modem for dense underwater sensor networks using FSK modulation was developed by the University of Southern California in 2006 [13] and was tested by transmitting and receiving bits in the air. [PERSON] designed a modem named Aqua Node mainly used in underwater wireless sensor networks and verified its performance in a lake, river, and ocean [14]. [PERSON] presented an ultrasonic communication system for biotelemetry in extremely shallow waters by using the FSK modulation scheme in [15]. [PERSON] et al. developed a low-cost underwater acoustic modem called SeaModem with MFSK modulation which is reprogrammable in both the physical and network-layer [16]. [PERSON], [PERSON] et al. developed a QAM-based MIMO Orthogonal Frequency-Division Multiplexing (OFDM) underwater acoustic modem in [17]. Communication between divers and their commander needs to be kept highly secure and the location of activity has to be kept secret. Using simple UAC, transmitted messages are more vulnerable to being intercepted by the enemy and the location can be easily traced. This calls for covert underwater acoustic communication which keeps the communication between divers highly secure and undetected by eavesdroppers. Some papers [18; 19; 20] have proposed using or simulating marine mammal sounds as the communication carrier to realize communication. The primary purpose of this article is to design a Portable Biomimetics-based Covert (PBC) UAC micro-modem and realize covert communication by using real dolphin sounds. The communication carrier waveform in the PBC micro-modem is natural dolphin or whale sounds. In the specific communication process, the dolphin signal will be selected according to the actual situation to realize the effect of covert communication. For example, local dolphin calls should be selected for communication in different sea areas, and the seasonal dolphin calls should be used for communication in different seasons. In order to improve the communication speed and restrain the multipath interference between adjacent symbols, low correlation dolphin calls should be selected. The dolphin- and whale-call samples used in the communication algorithm are stored in the SD card in advance.