# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 3 Modeling of UWA Channel Problems and Challenges  
The complex channel of communication is an important parameter that makes the UWC different from terrestrial communication. Thus, modeling the perfect underwater channel is one of the most crucial parts of the UWC because of the inhomogeneous nature of seawater. It is impossible to capture the key propagation aspects of the sea environment with a single acoustic channel model. The absence of any fixed environment in the UWAC results in no universal UWA channel. It has been determined by the properties of the oceanic water as well as the presence of significant noise sources that the sound channel is formed by the continual change in sound speed in various layers of seawater [9]. As noise sources provide various problems at different depth levels and frequencies, distinct channels for shallow water and deep seas must be simulated. The low speed of sound in seawater creates a time-varying multipath which has a substantial influence on the system's output and must be considered while constructing the UWA tube as well as other problems, such as noise and transmission failure. Robotic vehicles attached to surface ships with expensive and heavy cables are mostly used to conduct communications underneath the ocean. The use of cables significantly limits the range of robots. Moreover, due to heavy attenuation, signals penetrate only a few meters in the conductive seawater making wireless electromagnetic communication difficult under the ocean. Thus, maximum attention intended for the UWC has concentrated on the acoustic channel, with hydrophones and acoustic transducers performing the functionsof receivers and transmitters, correspondingly. Methodologies found for the underwater channel can be used in common UWWC. The UWA channel has been considered among a few of the most exciting mediums for wireless communication. The environmental dynamics in the ocean introduce a large volume of ISI and an abrupt time-varying channel. Because of frequency-related attenuation, the bandwidth of the channel is restricted to within kilo Hertz (kHz). The delay spread of the channel is almost 50 msec owing to the highly scattering environment. There is substantial Doppler spread because of slow-speed waves and mass propagation in the oceans. Therefore, researchers aim to propose a methodology that can prevent these problems and provide definitive and rapid communication in the UWA channel. Particularly, to prevent the time differences in the UWA channels. We aspire to design a rateless communication system aiming to work at rates near the channel capacity under all channel environments. The authors in [4; 31; 148] provided a thorough explanation of the UWAC problems and current advancements in the UWA signal processing methodologies. The design of an authentic UWC system has remained an active research area. The UWAC system design is very important to know where the UWA wireless sensor nodes will be deployed. Vertical communication across the ocean has been considered the easiest regime, normally due to a slight multipath. Due to the ensured channel fidelity, the atmosphere is named the reliable acoustic path (RAP) [149]. The surface bounces effects are reduced using Baffles and/or directional hydrophones. The channel of communication in deep water is probably sparse, time-invariant, and widespread in delay due to lesser contact with the surface. Modeling practices are commonly used in deeper water systems to know the position where communication is achievable because of channel physics. The shortest path usually arrives at the end in deeper waters as the natural waveguide surrounds the sound speed minimum in deeper water [150]. It is not possible to avoid contact with the time-varying ocean surface in shallow waters. Added disturbances come from wave noises, marine life, interactions between the bottom and proximate obstacles, and shipping traffic.