# Performance Analysis of a WPCN-Based Underwater Acoustic Communication System
## 1 Introduction  
Underwater acoustic communication (UWAC) using acoustic waves as the carrier can realize long-distance information transmission in the underwater environment. UWAC is considered to be the best choice and mainstream technology for underwater communications so far and for a long time to come. As shown in Figure 1, a typical UWAC system usually consists of sensor nodes, underwater vehicles, and communication buoys. The sensor nodes are responsible for sensing the underwater environment and collecting data. The underwater vehicles, which include submarines, autonomous underwater vehicles (AUVs), and remotely operated vehicles, are used for various tasks such as surveillance, exploration, maintenance, or intermediate nodes for relaying communication signals between the sensor nodes and the communication buoys. The communication buoys serveas the gateway nodes to interconnect underwater sensor networks with other networks through wireless communication. UWAC systems comprising static sensors and underwater vehicles have been applied to many scenarios, such as undersea oil exploration, environmental monitoring, and coastal surveillance [1]. However, due to the harshness of the underwater environment, the development of UWAC systems has encountered many obstacles, including the energy limitation of the terminal nodes [2]. Underwater terminal nodes, such as sensors, are usually equipped with energy-constrained batteries. Since manually replacing batteries is very costly and even impossible in the harsh underwater environment, underwater terminal nodes can usually only work for a short period of time, which leads to an unsatisfactory lifetime and reliability of the UWAC systems, as well as significant resource waste. Developing an efficient and sustainable energy supply for the energy-limited terminals has become an urgent issue for the UWAC systems. Wireless powered communication network (WPCN) is one of the most attractable and promising energy supplement technologies that allows energy-limited terminals to harvest energy from the energy sources and ambient environment to prolong the lifetime and reliability of the terminals [3]. In WPCNs, the hybrid access point (H-AP) transfers energy to the energy-limited terminals in the downlink, while in the uplink, the terminals use the harvested energy to transfer information to the H-AP. With the tunable transmit power, waveform, frequency, etc., of the energy signals of H-AP, WPCN can provide stable, comparably controllable, and self-sustainable power supplies and eliminate the need for manual battery recharging/replacement, thus having a wide range of application perspectives in extreme environments such as underwater, body of area networks, etc. [4; 5]. Moreover, WPCN can realize the joint optimization design of downlink energy transmission/harvesting and uplink wireless information transmission to obtain optimal resource allocation and realize optimal use of wireless resources and network infrastructure [5]. WPCN is particularly suitable for the application fields of underwater acoustic sensing networks, such as offshore oil extraction monitoring and marine environmental monitoring. In these applications, the underwater acoustic sensors require extremely high reliability and a long service lifetime, while only needing to transmit a few data bits per hour; hence, WPCN can meet the power consumption requirement of the sensors [6]. For example, assuming that the transmitted acoustic power of the energy transmitter (base station/access point, buoy, etc.) is \(P_{ts}=50\) W, considering that the carrier frequency \(f=15\) kHz, transmission distance \(d=0.5\) km, spreading factor \(m=1.5\), and transducer directivity \(DI=10\) dB, the signal sound pressure level (SPL) received at the destination node can be calculated as  
Figure 1: The architecture of UWAC systems. \(SPL=170.8+10\log_{10}(P_{\text{fs}})+DI-TL=156.1\) dB re \(\upmu\)Pa, where \(TL\) is the acoustic path loss given in dB that can be calculated using Equation (3) in Section 2.3.