# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 1 Introduction
The addition of images and videos of high-quality and autonomous usage in underwater networks has induced a constant increase in the demand for a high data rate in the UWAC. Increasing the bandwidth cannot directly improve the data rate due to the frequency-selective nature of the UWA channel. However, the frequency range of a few Hz to 1 MHz can be used for UWAC. The long distance under water restricts the higher frequencies to travel because these frequencies fade and are absorbed rapidly [7]. Propagation of sound waves in seawater at frequencies below 10 Hz typically requires deep penetration into the seabed. Conversely, frequencies above 1 MHz are rarely employed as they are rapidly absorbed by the water. The range of frequencies mentioned above mostly depend on the size of transducers and country restrictions. In the UWAC, a relatively higher data rate can be attained using efficient modulation techniques and effective modeling of the channel [7]. Some of the acoustic waves' properties bounce from the bottom and top surfaces of water. Therefore, the sound signals develop possible multiple paths to form their specific path to the receiver. Inter symbol interference (ISI) occurs in receiving symbols at the receiver's end due to the multipath that requires extensive attention in the receiver. The multipath configuration can be predicted using a propagation model which requires whole system modeling of the UWAC. The level of noise can prevent the data from being carried and mask part of the signal. There is an assumption that the Gaussian distribution has a direct relation with the ocean ambient noise according to most underwater signal processors. This assumption is mathematically addressed using the central limit theorem [8; 9]. The basic purpose of using Gaussian processes is that it exploits the second-order statistics that explain the attributes of a Gaussian signal. The reasons behind the impulsive nature of the ambient noise in the shallow water are interference, heavy rain, rainstorms, strong wind, and marine life noise which are explained by the heavy tail. Communication system engineers/designers face numerous challenges while designing a UWA channel. In addition, communication via UWA channels poses significant challenges [10; 11; 12]. The combined sensors and vehicular technology, wireless UWCs have become a potent communication technology in recent applications. This includes a collection of oceanographic data to monitor the environment, marine archaeology, and exploration and saving assignments. Effective submarine missions require advanced UWAC technologies capable of transmitting high-speed data, including video and pictures. With recent advancements in UWAC, there is a need for further development and implementation of such technologies to enhance communication capabilities for submarines operating in challenging environments. The physical properties of the UWA channel pose significant challenges to data transmission, resulting in limited data rates. As a result, current acoustic communication approaches are only capable of delivering data at restricted rates [13]. Substantial challenges occurred with the time and spatially varying oceans environment for designing a successful system for UWWC. In the UWC, the signals from electromagnetic waves, due to their significant ocean as well as seawater absorption, are not considered a practiced solution. Similarly, the optical signals experience the scattering defect due to an excessive amount of ambient light, and suspended particles in the upper and lower column of seawater, respectively. Therefore, acoustic waves have been considered one of the most essential technologies for UWWC [14]. The Ray and normal modes theory delivers the basis for propagation modeling in multiple paths [15]. Random fluctuation is added to each propagation path from the moving sea surface that generates Doppler spread, resulting in the impulse response of the channel in a variable time domain.