## 2 Principles of Underwater Communication
### Communication Mediums in Water  
#### 2.1.1 Underwater Acoustic Communication (UAC)  
In underwater acoustic communication (UWAC), information is transmitted via sound waves, and information is loaded by mapping parameters, such as phase, frequency, and amplitude. In underwater acoustic communication, sound waves are used to transmit information. Parameters of the sound wave, such as phase, amplitude, and frequency, are manipulated according to the information [10]. Acoustic waves are capable of traveling long distances compared to electromagnetic and optical waves [11]. However, acoustic communication encounters several challenges, such as strong multipath propagation. Multipath propagation occurs in an underwater acoustic channel when the sound wave reflects on the seawater surface, seabed, and obstacles. It leads to severe fading and long delay spread, which decreases the effectiveness of underwater communication and causes signal distortions [12]. In addition to that, the Doppler effect is another challenge in acoustic communication, which is caused by the movement of the transmitter and receiver. The combination of multipath propagation and Doppler shift creates frequency shifting and spreading. It results in frequency-dependent distortion in acoustic waves. As the frequency of the signal increases, the Doppler effect increases, making acoustic communication inefficient at high frequencies [13]. Several methods are employed to mitigate the effects of the Doppler effect and multipath propagation. Decision Feedback Equalizers (DFEs) are used to handle multipath signals. In the DFE, the received acoustic signal is processed with the feedback from past decisions. A Digital Phase-Locked Loop (DPLL) is considered an effective method to reduce the Doppler effect. DPLL continuously tracks the phase of the incoming acoustic wave since a slight motion of the transmitter and the receiver can cause a significant Doppler effect due to the low propagation velocity of acoustic waves in water. DPLL provides an output signal after adjusting the phase of the received. In addition to that, enhanced techniques use additional DPLLs along with DFE to address the phase shifts of each multipath signal [14]. A fundamental acoustic communication model is represented in Figure 1. The main components of an underwater acoustic modem consist of an acoustic transducer which is used to convert electrical signals into acoustic waves and vice versa, a communication circuitry that handles several functions such as signal processing, error correction, and data transmission, and a control unit which manages the operations of the modem [11]. Figure 1: Fundamental acoustic communication system. Unfortunately, acoustic communication systems inherit several limitations such as significantly lower data rates compared to RF communication. Sound waves travel at a much slower speed in water. Hence, when it comes to long-distance communication, there is a significant transmission delay, leading to high latency in communication [15]. Underwater acoustic signals are impacted by ambient noise which can be generated from marine life, ships, and natural phenomena such as waves and currents. These kinds of sources of noise introduce fluctuations in signal strength and quality, further decreasing the reliability of acoustic communication [15]. There are mainly four types of noise in underwater acoustic channels. Wave noise is the most significant noise and it appears 100 Hz to 100 kHz range. It is introduced due to the sea surface movements. Thermal noise is generated at frequencies above 100 kHz and is caused by the thermal agitation of molecules. Shipping noise is generated by ships and marine vehicles. It exists in the 10 to 100 Hz frequency range. Most random noise is turbulence noise and is caused by wave currents below 10 Hz frequency [11]. In order to overcome challenges in underwater acoustic communication, researchers conduct experiments involving several adaptive and predictive techniques.