## 3 Medium Access Control (MAC) Layered UWAC Protocols
### Challenges Facing by FD-UINAC MAC Protocols  
Some significant challenges faced by researchers in the design of FD MAC layered based protocols are listed in this section, as well as the solutions proposed in previous works to solve each problem.  
**Time synchronization**. In the design of an FD-MAC protocol, time synchronization has always been a crucial problem [30; 46]. The duty cycling is central to the MAC protocols [24; 25]. To perform an effective cycling between the nodes, all of the nodes must be correctly synchronized [31; 35]. If the time synchronization between the nodes isn't well designed, it can affect simultaneous transmission between the FD-nodes, causing propagation delays to increase.  
**Hidden and Exposed terminal issue**. The hidden and exposed terminal issue usually arises in the contention based collision avoidance FD-MAC layer based protocols [30; 34]. The hidden terminal problem occurs when a node cannot detect the transmission from one or more nodes during simultaneous transmission and instead sends its data to a node that is already receiving data from another node, resulting in collisions [30; 35]. The exposed terminal problem occurs when one node overhears another node's transmission in order to prevent collisions [30; 31].  
**Huge delays**. Due to large propagation delays, handshaking-based FD-MAC protocols are severely hampered, lowering channel utilization rates [30; 34]. The large propagation delays is the cause of relative propagation delays between the nodes, which is a serious challenge while designing of HS-FD-MAC protocols for underwater acoustic communication.  
**Power consumption**. The FD-MAC protocols are designed to transmit data simultaneously, and they take a lot of energy to do so accurately [35]. Unexpected collisions during the transmission of data between the nodes also increase power consumption [35; 37]. As a result, it is essential to develop FD-MAC protocols that can prevent collisions and save energy in a harsh underwater environment.  
**Noises**. For all types of underwater FD-MAC protocols, noise is a common and fundamental problem [37; 46; 50]. There are two forms of noises: (a) noise produces by the human interference, such as machinery noise, shipping movement, cavitation (b) Ambient noise, such as hydrodynamics, biologics, seismic activates and so on. [50]. In order to counteract the noise, researchers have developed a variety of techniques. For instance, signal detection error correction coding and iterative (turbo) algorithms [46; 68] have been established, resulting in improved UW channel efficiency, particularly in warm shallow water [46; 68].  
**Attenuation**. The transformation of acoustic energy into heat occurs during UWA wave transmission due to the increase in transmission range and frequency [50]. Low data rates and transmission bandwidths of a few kHz, as well as high communication frequencies, have a major effect on the acoustic signal and cause attenuation [30; 46]. The rise in ocean depth, secreting, and echo production due to the irregular water surface and seabed are other factors that cause attenuation of the FD-UWA-MAC protocols signal [35].  
**Doppler spread/ Doppler shift**. Doppler spread is caused by changes in water temperature, density, and salinity as a result of spatial-temporal differences in both deep and shallow water, reducing signal strength [46; 50]. The existence of motion at the transmitter, receiver, and medium itself results in a huge Doppler shift in the frequency which further degrades the performance of high data rates, adjacent symbol interference, and generates ISI [46; 50].