# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 4 UWA Modern Technologies
Thermoelectric generators can convert thermal energy into electrical energy, but their efficiency is limited due to the low-temperature gradient in underwater environments. Piezoelectric generators can harvest energy from underwater vibrations and acoustic waves, making them a promising technology for energy harvesting in underwater environments. Underwater mmWave communication is a promising technology that can provide high data rate communication for underwater platforms. MmWave communication can overcome the limitations of UWAC, such as low data rates and limited bandwidth. Underwater mmWave communication uses frequencies above 30 GHz, which are not affected by the signal attenuation caused by seawater. However, underwater mmWave communication faces several challenges, including the need for highly directional antennas and the vulnerability of mmWave signals to scattering and reflection. Recent research has explored the use of mmWave communication for underwater applications, such as underwater imaging and video streaming, demonstrating the potential of this technology to revolutionize UWC. CS is a signal processing technique that allows the reconstruction of a signal using a reduced number of samples, which can result in lower power consumption and improved efficiency in UWAC. One of the future research directions in this area is to develop new CS  
Figure 19: Underwater wireless communication modern technologies. algorithms that are optimized for the underwater environment, taking into account factors, such as multipath interference, attenuation, and noise. Researchers could also explore the potential of compressive sensing for other applications, such as underwater imaging and sensing. FBMC is a modulation technique that can provide high spectral efficiency and improved robustness to multipath interference in UWAC. Future research in this area could focus on developing new FBMC-based communication systems that are optimized for the underwater environment considering factors, such as acoustic propagation characteristics, Doppler shifts, and noise. Researchers could also explore the potential of FBMC for other applications, such as underwater positioning and navigation. WSNs are networks of wireless sensor nodes that can be used for environmental monitoring, underwater surveillance, and ocean exploration. Future research in this area could focus on developing new communication protocols and routing algorithms that are optimized for WSNs in the underwater environment, taking into account factors, such as limited energy resources, variable network topologies, and harsh environmental conditions. Researchers could also explore the potential of WSNs for other applications, such as underwater energy harvesting and underwater wireless power transfer. Cooperative communication is an approach that involves multiple nodes working together to improve the reliability and efficiency of UWAC. Future research in this area could focus on developing new cooperative communication schemes that are optimized for the underwater environment considering factors, such as node mobility, channel fading, and multipath interference. Researchers could also explore the potential of cooperative communication for other applications, such as underwater localization and underwater acoustic source tracking. Hybrid acoustic and optical communication is an approach that combines acoustic and optical communication to achieve high data rate communication in underwater environments. Future research in this area could focus on developing new techniques for integrating acoustic and optical communication, considering factors such as signal interference, transmission range, and energy consumption. Researchers could also explore the potential of hybrid communication for other applications, such as underwater sensing and imaging.