# Performance Analysis of a WPCN-Based Underwater Acoustic Communication System
## 1 Introduction
### Related Work  
UWAC began in World War II and has a long history. However, due to the harshness of the underwater environments and channels, it has been difficult to realize and develop underwater communications, especially before the 1990s of the last century. Since the end of the last century, researchers from various countries have successively applied various signal processing technologies in terrestrial wireless electromagnetic wave communication to UWAC systems, achieving rich results, and the theories and technologies of UWAC have been developed rapidly [8; 9; 10; 11; 12], where the research in [8; 9; 10; 11; 12] studied physical layer signal transmission and processing technology, the studies in [13; 14; 15; 16] exploited medium access control (MAC) layer and routing protocol, and the authors in [17; 18; 19; 20] investigated the experiment and application of UWAC networking. Specifically, in [8; 9], orthogonal frequency division multiplexing (OFDM) modulation was applied to the UWAC to counter inter-symbol interference. In [10; 11], the authors introduced relay nodes to the UWAC system to improve transmission reliability and reduce power consumption, while in [12], multiple-input multiple-output (MIMO) technology was applied to improve the transmission rate. In [13; 14], the authors summarized the challenges in designing underwater MAC protocols from different perspectives. The authors of [15] proposed the Integrated Secure MAC principle and Long Short-Term Memory architecture for organizing real-time neighbor monitoring tasks to protect the data. A non-orthogonal multiple access (NOMA)-based MAC protocol was proposed in [16] to achieve efficient underwater concurrent communications and high capacity. In [17], the concept of \"Seaweb\" was first proposed, and [18] briefly described the typical communication architecture of an underwater network. The development of a protocol stack for self-reconfigurable UWAC systems that autonomously adapt to changing environmental conditions and operational needs was reported by the European Defense Agency [19]. In [20], the authors designed and expounded on a set of UWAC systems and signal processing systems that effectively ensure the stability and simplicity of the data transmission process. The above works mainly focus on communication performance, including bit error rate (BER), outage probability, and data rate, whereas the energy limitation issues are not concerned. With the rapid development of UWAC network theory and technology, the problem of energy limitation and the resulting short lifetime of underwater terminal nodes has attracted growing attention [21; 22; 23; 24; 25]. In particular, the Monterey Bay Aquarium Research Institute designed a portable mooring system that can collect solar and wind energy to provide power for shore-side terminal nodes [21]. In [22], the authors studied the energy efficiency performance of a three-node underwater full-duplex relay network, where the relay can harvest energy from the ambient environment. To provide a reference for realizing efficient and flexible energy supply for autonomous underwater vehicles (AUVs), the authors in [23] analyzed several effective methods to reduce the high energy loss during underwater wireless power transfer, including coupling mechanism optimization, adaptive control, and docking device design. In [24], to solve the energy supply problem of AUVs, the authors proposed a novel \"ID\"-shaped magnetic coupler with a pendulum-type receiver, which has characteristics of compact structure, lightweight, and high receiver power density. In [25], the authors summarized the energy optimization techniques in underwater communication networks proposed in recent years, including a power-efficient routing protocol [26], an efficiency reservation MAC protocol [27], etc. In [28], the authors designed a batteryless underwater sensor node that can be wirelessly recharged through ultrasonic waves and validated that the harvested energy was sufficient to meet the requirement of power consumption through experiments.