# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 1 Introduction  
Effective underwater and underground wireless communication are the most challenging communication. Underground wireless communication refers to the transmission of information employing radio waves or other wireless technologies in an environment where physical barriers, such as soil, rock, and concrete, restrict the propagation of signals; this has recently become an extremely active and a new targeted area for researchers. This kind of communication is frequently employed in underground mines, tunnels, and other underwater constructions. For underwater communications, the propagation media, be the sea or ocean water, is critical for various military and civilian applications including submarine communication, exploration of natural resources in the underwater environment, target localization, ocean sensor networks, and the internet of underwater things (IoUTs) [1; 2; 3]. In this paper, we will focus on the physical layer techniques and the challenges in underwater communication techniques. The deep-sea environment topographies, such as frequency-selective fading, double-side-spreading, significant delay, Doppler spreads, Doppler shift, and limited bandwidth influence the performance of underwater acoustic (UWA) channels [4]. Conversely, the most effective underwater communication (UWC) in the oceans is crucial to many kinds of civil, military, and scientific missions, such as undersea rescue and disaster response, ocean monitoring, and exploration. The understanding of the oceans and human knowledge depends on the capability to gather information from distinct places under water. Methods for designing underwater wireless information transmission have been an active research topic for more than a decade. The absorption of the electromagnetic waves in the underwater channel limit their broadcast over extremely small distances. In contrast, acoustic waves can travel over long distances in water due to their excellent sound-conducting properties. Sound waves can propagate through the water with minimal attenuation or loss of signal strength. The frequency range of acoustic waves is well-suited to UWC, as low-frequency sound waves can penetrate the water column and travel long distances with minimal attenuation. This is in contrast with higher-frequency electromagnetic waves, which are rapidly absorbed and scattered by the water. Underwater acoustic communication (UWAC) has attracted huge attention in the last two decades due to its vast applicability in different domains. The supported domains include the exploration of natural calamities including tsunamis and cyclones, changes in climate, monitoring of flora and fauna, offshore exploration, seawater heterogeneous characteristics, oil production facilities, monitoring of marine life and pollution, development of aquaculture, mapping the seabed, etc. Furthermore, it is also applied in numerous military applications communicating deep in the sea including submarines in underwater wireless sensor networks (UWWSN), harbor defense systems, and unmanned underwater vehicles (UUVs) [5]. The speed of sound in water is about four times greater than the speed of sound in air, but the propagation speed of electromagnetic waves used in terrestrial radio channels is much faster than the speed of sound in either medium. The UWCs have no resemblance with terrestrial communication. The propagation speed of acoustic waves in water is influenced by various factors that vary across different ocean environments. These elements, such as temperature, salinity, pressure, and ambient noise levels, can significantly affect the performance and reliability of UWAC systems. Several sorts of noises, including site-specific and ambient noises, affect the UWA signal propagation. The ambient noise occurs from the background noise of the deep stable quiet sea. The site-specific noise is present when ice cracks in the Polar Regions and the shrimp snap in warm waters. Breaking waves, wind, rain, distant shipping, and turbulence are the primary causes of ambient noise [6].