## 1 Introduction
### Motivations and Contributions  
A physical layer for the UWAC channel is needed to ensure adequate operation in various UW environmental classes [15; 16]. The efficient operation of FD-UWAC systems using the physical layer necessitates a variable quality of service (QoS) as well as limited bandwidth systems. Many challenges confront FD physical layer-based systems, including synchronization and delays, self-loop and SI, LTI, noise, attenuation, multipath effects, and Doppler spread. To address these issues, researchers have developed various FD phys  
Figure 1: Classification of full-duplex physical layered UWAC systems and MAC layered protocol. ical layer-based systems, which are divided into three categories: OFDM-based, digital modulation-based, and spread spectrum-based, as shown in Figure 1. The most promising physical layer for FD-UWAC systems is OFDM, and various OFDM-based systems have been developed. Cooperative OFDM systems are used to combat asynchronous time issues that occur between groups of nodes/relays, which limit the efficiency of OFDM-based systems, increase relative delay and fading, and trigger ICI [17,18]. Similarly, these system are used to combat the low spectral efficiency [19]. To deal with the problem of insufficient bandwidth, IBFD-OFDM systems allow for the most efficient use of the available bandwidth [20,21]. These systems, however, suffer from a high peak-to-average-power ratio (PAPR) due to the use of OFDM as a modulation technique. Furthermore, self-interference limits IBFD-OFDM systems, and various systems (e.g., HSIC [20], TRDC [22], ML-SC-IBFD [23], and A-IBFD [24]) have been developed to resolve this issue. SI cancellation is the goal of digital modulation-based FD systems, and they are classified as such. The proposed RLS-adaptive filter digital modulation systems are based on a digital SI cancellation scheme [25, 26, 27, 28], substantially improves its efficiency. The addition of fast convergence, high numerical stability, and low computational complexity are all advantages of these schemes. However, these UWAC systems have issues with far-field interference cancellation and transmission rage. The FSK based digital modulation FD systems are energy-efficient, capable of transmitting over longer distances with long battery life, and safe data transmission [29], but they are complex and incapable of dealing with multipath interference, necessitating further research in this field. Spread spectrum-based FD systems has multiple access capabilities for different users. Further, the low probability of detection (LPD) has been addressed using spread spectrum. Hence, researchers showed interest in direct sequence spread spectrum as a result of these advantages, and it was used in various FD-UWAC systems, where it demonstrated significant efficiency by reducing the effects of fading, noise, jamming, and self-interference [30, 31, 32]. A hybrid SS approach is often used in order to establish an FD link connection between multi-user modems for the transmission of data streams and to preserve synchronization between the simultaneously received signals [33]. MAC protocols for underwater acoustic communication have been developed by researchers over the last few decades. The vast majority were based on terrestrial contact protocols [34]. The reformed protocols significantly improved the underwater network's performance in terms of service quality and energy efficiency [34,35]. However, time synchronization, low bandwidth usage, collisions, hidden and revealed terminal problems, and significant delays plague the developed underwater acoustics MAC protocols. Researchers have developed various FD MAC layered-based UWAC protocols to resolve these issues, which are divided into contention-free and contention-based protocols, as shown in Figure 1. The contention-free FD MAC protocols used various methodologies to protect the network from massive collisions, frequency selective fading, and improved transmission efficiency.