Underwater Wireless Communication Systems and Networks: A Survey, Future Directions, and Novel Proposals

In recent years, underwater wireless sensor networks (UWSNs) have attracted significant attention due to their wide range of civil and defense applications. However, underwater environments impose severe challenges, including dynamic channel conditions, lack of accurate three-dimensional localization, spatial sparsity, high propagation delays, limited bandwidth, energy-constrained sensor nodes, interference, and strong background noise. Addressing these challenges requires efficient and integrated network stack designs spanning the physical, medium access control (MAC), and routing layers. Furthermore, additional performance gains can be achieved through the integration of heterogeneous communication technologies, such as acoustic and optical systems. This paper first presents an up-to-date comprehensive survey of UWSN protocols, covering the physical, MAC, and routing layers, with a detailed analysis of their design trade-offs and performance limitations. In addition, existing network stack protocols, underwater modems, and network security threats are reviewed, with particular emphasis on the Smart Adaptive Long- and Short-range Acoustic (SALSA) stack and its operational challenges. Building on this comprehensive survey, the paper identifies key research gaps and outlines future directions to enhance the development and integration of different network layers. Building on the insights derived from this survey, two main contributions are introduced. First, novel Developed-SALSA (D-SALSA) stack protocols are proposed, designed to enhance adaptability, energy efficiency, throughput, and communication reliability in dynamic underwater environments. Second, a heterogeneous network architecture is presented that leverages the complementary advantages of acoustic and optical communication technologies, with a novel management protocol adopted within the proposed D-SALSA stack protocols to support highrate and large-scale marine data transmission. By combining research directions built on an up-to-date comprehensive survey with the design of novel stack protocols and heterogeneous networking paradigms, this work provides a unified framework for advancing UWSN performance and addressing persistent challenges in underwater communications.