## 2 Physical Layer-Based Full-Duplex Underwater Acoustic Communication Systems
### Challenges Faced by FD Physical Layered UWAC Systems  
Some significant challenges faced by researchers in the design of FD-physical layered UWAC-based protocols are listed in this section, as well as the solutions proposed in previous works to solve each problem. Synchronization: Due to the low propagation speed of sound, the existence of large delays at the receiver end is a substantial challenge to accurately synchronize the UWA communication systems. Additionally, this causes huge delays at receiving section of the specific relay or node [19]. Maintaining precise synchronization between the nodes/relays in order to receive signal simultaneously as it passes from source to destination becomes a substantial challenge [19; 40]. To address the issues of synchronization and delays, various modulation schemes have been used in various protocols. The FD-OFDM-based modulation schemes, for example, are implemented by lengthening OFDM blocks [40]. In the FD-UWA communication system, another FD-OFDM-based scheme implemented the two-way-relay system, which significantly reduced the huge delays and comfortably allowed the synchronization between node/relays [19]. Self-interference (SI): The efficiency of FD-UWAC protocols is influenced by two types of interference: Multiple reflection interference (MRI) and SLI [22; 23; 24; 41]. The SLI has a significant impact on the performance of IBFD-UWAC systems, which usually appear after a long period of time. The IBFD-UWAC protocols struggle to suppress the SLI because of the longer delays [23; 41; 42; 43]. Various methods have been developed to suppress SLI, such as [14], which implements a method that uses both analog and digital cancellation to reduce SLI. Soft-decision feedback turbo equalization (SDFE) is used to remove ISI and residual self-interference in [22]. However, the SI problem remains a significant challenge for the protocols listed. Local transmission Interference (LTI): Since FD-UWAC protocols are implemented using modems, they experience their own transmission interference (i.e., LTI), which is caused by the use of an omni-directional transducer in the modems [33]. Acoustic baffles are used between the transmitting transducer and the receiving sensor vectors in a few methods to remove LTI. A distinction between the omni-directional transducer and receiving vectors is maintained by inserting a rod between them for more effective suppression of LTI. The methods described above also assist in improving the received signal's signal-to-noise ratio (SNR). Noises: Noise is a widespread and fundamental issue that has a significant impact on the efficiency of all types of underwater FD physical layered UWAC systems [22; 41]. Ambient noise has a major impact on FD-physical layer dependent systems at the receiver section, where an increase in noise power (dB) is directly proportional to an increase in residual analog interference. To reduce this, the IBFD-UWAC framework [41] introduced a powerful analog self-interference cancellation scheme. Attenuation: The long transmission range of the UW acoustic signals induces attenuation, converting the acoustic signal to heat [44]. Furthermore, in FD-physical layer-based UWAC systems [45], high frequencies are used to increase data rates, resulting in a serious attenuation problem [44]. The presence of sound absorbing obstructions, as well as changes in shallow sea water salinity and temperature in the FD physical layered dependent UWAC systems, are two other common causes of increased attenuation. Multipath Phenomenon: Reflections from the sea's surface and bottom induce the multipath effect, which affects the transmitted signal's phase and amplitude [46]. Due to the availability of limited FD-UWAC channel bandwidth, ISI is caused by time-varying multipath effects [46, 47]. The multipath effect occurs in shallow water interaction due to the surface reflection and refraction effect [48], causing substantial end-to-end delays and an increase in bit error rates [32, 49].