# An On-Demand Scheduling-Based MAC Protocol for UW-WiFi Networks
## 1 Introduction  
In recent years, underwater acoustic networks (UANs), as a momentous extension of Internet-of-Things technology to the underwater environment, have captured growing attention in academia and industry because of their wide range of use, for example, ocean monitoring, oceanographic data collection, disaster warning, diving, and so on. In order to enable various underwater nodes with diverse missions to be networked in a flexible manner to meet those applications, a UAN with the WiFi network architecture, called UW-WiFi, is presented in [1], similarly to the traditional terrestrial WiFi network, which provides wireless access services for underwater nodes (e.g., marine engineering equipment, automatic underwater vehicles, divers, sensors, and so on). The UW-WiFi network affords distinct types of data collecting activities in uplinks, command control actions in downlinks and interacting functions between any two nodes. To accomplish the UW-WiFi network consisting of heterogeneous nodes, designing a practical medium access control (MAC) protocol is indispensable. Nevertheless, MACprotocols, mainly including carrier sense multiple access (CSMA), time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA), designed for the terrestrial WiFi network [2] are not applicable for the UW-WiFi network, due to typical characteristics of the underwater acoustic channel, such as low available bandwidth, long and variable propagation delay, high bit error rate, and time-varying. The underwater acoustic speed is approximately 1500 m/s, five orders of magnitude lower than the optical speed (\(3\times 10^{8}\) m/s), which makes CSMA based on the Request To Send/Clear To Send (RTS/CTS) exchanging costly. Because of the narrow available bandwidth, FDMA is impractical. TDMA requires precise time synchronization, which is difficult and needs extra cost to achieve. The near-far effect is usually difficult to be eradicated in the time-varying acoustic channel, leading CDMA to be not suitable for the UW-WiFi network. Several related works have been proposed for underwater data collection networks [3; 4; 5]. A customized Slotted-FAMA protocol is introduced for underwater acoustic WiFi networks [3]. This protocol only caters for applicable scenarios with low node density and traffic load, in which as the data traffic increases, the conflict of control packets will become sharply harsh. Two MAC protocols based on TDMA, namely, Transmit Delay Allocation MAC (TDA-MAC) and Accelerated TDA-MAC, are proposed [4], which have no need for clock synchronization. However, the algorithm is quite inflexible and unable to adapt to the various data volume requirements of nodes. A deep reinforcement learning-based MAC protocol is proposed [5], which is highly complex, and has high requirements for hardware computing and energy consumption, which makes it hard to be applicable in real situations. Furthermore, the hardship and high-cost of setting up real underwater network systems and executing field experiments lead to most research being based on theoretical analysis and simulations. However, dynamic underwater acoustic channel and complicated communication systems cannot be well reflected by simulation. Given the above issues, and considering the limitations of underwater acoustic communication channel, energy conservation, the different needs of heterogeneous nodes for the shared channel and the usability of the algorithm, a time sequence-based dynamic on-demand assignment (SDDA) MAC protocol is proposed and implemented in an actual system for the UW-WiFi network. By combining the main point of TDMA and reservation, this protocol improves channel utilization and packet delivery ratio, decreases end-to-end delay, and achieves good fairness among terminal nodes at different ranges. This protocol mainly includes two phases, the reservation phase and the time sequence scheduling phase.