A survey on energy efficiency in underwater wireless communications

Underwater wireless communication (UWC) networks have attracted substantial attention in recent years. UWC can facilitate critical emerging services including communications for: Autonomous Underwater Vehicles (AUVs) and Remotely Operated Vehicles (ROVs), environmental monitoring, surveillance, navigation, and ex-ploration. Most UWC devices are battery-powered, where recharging/swapping batteries is not straightforward. Since the reliability of a UWC network depends on limited energy storage, energy-efficient communication is critical for its operation. Whilst communication range and data rate in UWC have been popular research topics, until recently, this energy-efficiency aspect of UWC has not received the same level of attention. In this paper, we provide a comprehensive literature review on existing contributions in the area of energy-efficient UWC. At first, we provide a detailed overview of UWC network architectures and relevant challenges for UWC networks. Next, we discuss and compare the most commonly-used UWC physical layer technologies, reviewing the existing physical layer power-saving techniques proposed in the literature. Then, we review various energy-saving techniques in the upper layers, including Medium Access Control (MAC) protocols, routing protocols, and localization techniques. Lastly, we provide a detailed discussion of alternative energy sources in UWC networks before highlighting future research directions in the field and challenges related to the widespread adoption of the Internet of Underwater Things (IoUT).