## 5 Conclusions and Future Research Directions
### Future Directions  
The FD-UWAC physical layered systems, as well as MAC layered protocols, operate in a dynamic and challenging underwater environment. When going over the FD, we discussed the advantages and disadvantages of physical layered systems and MAC protocols, and we suggested potential future directions in this section. Physical layered FD-UWAC systems are mostly based on OFDM, as we pointed out. These OFDM-based FD systems outperformed in terms of low ISI, ICI, and transmission latency reduction [19; 40]. However, further research is required to improve the power efficiency of these FD systems as well as to reduce the hardware complexity. To make maximum use of available bandwidth, OFDM-based IBFD-UWAC systems have been widely implemented [23; 24; 25; 41]. They have achieved fast convergence speed, high precision, high robustness, high numerical stability, and low computational complexity. In addition, these FD systems are typically used to inhibit SI effects such as optical SI, analog SI, SLI, multiple reflection interference, and ISI [22; 23; 41; 24]. These FD systems, on the other hand, also need to increase their overall system throughputs as well as spectral performance, and they must do so while using limited bandwidth. Furthermore, these FD- systems need improvements in ambient noise cancellation and hardware reliability. The FSK-FD-UWAC systems protocols were developed to build a long-range communication link between UWAC robots, achieving high data rates while conserving energy, and have been implemented for off-shore oil productions [29]. However, these FD-systems need improvements in robustness and multipath interference cancellation, as well as a reduction in hardware complexity [29], which could be very promising directions for future study. We discovered that the FD-MAC protocols worked well, significantly reduced collision, resolved the hidden and exposed terminal problem, and had higher throughputs and energy efficiencies. These protocols, on the other hand, face a number of problems, which are illustrated and potential directions are proposed accordingly. While implementing in an ad-hoc UWAC system [39], a self-organized HS-FD-MAC protocol successfully eliminated unwanted collisions, demonstrated better channel utilization in high network load, and exhibited high throughput benefit. However, a power efficiency study is needed to confirm the protocol's validity. The HS-FD-DA-UWAC protocols in [30], demonstrated improved performance by resolving the hidden and exposed terminal issue, as well as high overall and energy efficiency. On the other hand, it is recommended that this protocol be checked for new and drop nodes, as well as its implementation in a large UWAC network. Furthermore, this protocol demonstrated a low data rate, which is also a problem. In [37], the proposed HS-FS-MACA protocol, which showed a major improvement in collision avoidance, high throughput, and improved channel bandwidth. However, this protocol is incapable of dealing with long propagation delays, and time synchronization remains an issue. The HS-FDCP protocol proposed in [39], increased throughput by demonstrating substantial collision avoidance at the transmitter and receiver ends, as well as improved network throughputs, channel utilization, and temporal and spatial reuse. However, this protocol consumes more energy and has low robustness, and its practical implementation is also needed, leaving an open research question. The HS-IBFD-MAC UWAC protocol used twice the connection capacity, effectively reducing high node collisions and long propagation delays while consuming less power [35]. However, SI cancellation and the multipath effect are not taken into account in this protocol. This protocol may also be expanded to include cross-layered architecture. When implementing for a heavy load traffic network, the HS-FD-MAC UWAC protocol proposed in [69] significantly reduced SLI, improved throughput, and reduced power consumption. However, due to the long propagation delays, this protocol is ineffective.