# A Dynamic Surface Gateway Placement Scheme for Mobile Underwater Networks +
## 1 Introduction  
Over recent years, underwater sensor networks (UWSNs) have attracted considerable attention from academia and industry [1; 2; 3; 4; 5; 6] allowing a wide range of marine applications, including coastal surveillance, environmental monitoring, undersea exploration, disaster prevention, and mine reconnaissance. However, due to the high attenuation of radio waves in water, acoustic waves were chosen by UWSNs as the best solution for underwater communication. On the other hand, underwater acoustic communications and networking imply unique and challenging characteristics, such as low bandwidth, long propagation delay, high-error probability, and sensor node mobility (passive or proactive) found in mobile networks. These challenges should be considered for each layer of the network protocol stack [7; 8; 9; 10; 11; 12]. In an UWSN, sensor nodes are deployed to monitor and detect environmental events occurring in their vicinity. This information is then transferred through the underwater network to a hybrid node, typically a surface gateway node that uses radio waves to relay data received from an underwater acoustic network. By deploying multiple surface gateways, the number of receivers of acoustic transmission increases which allows sensor nodes to forward packets to nearby gateways directly [13; 14]. Thus, all the surface-level gateways and control station may be regarded as a single virtual sink. This approach reduces the total propagation and transmission time because electromagnetic wave propagation is of orders of magnitude faster than acoustic waves and the bandwidth of radio is much wider than acoustic. Since acoustic communications consume more energy than radio communications [2], energy consumption can also be reduced. As a critical hub for underwater sensor networks and shore control centers, the surface gateway deployment is particularly essential. Another challenge that needs to be addressed is that the underwater sensor nodes are typically mobile with the water current. Therefore, deployment of surface sink nodes needs to be readjusted at a set of intervals to ensure the connectivity and coverage of the whole network. However, when the number and frequency of adjustments increases, it is bound to increase the complexity of the deployment and deviate from the original intention. Thus, it requires a design for a deployment strategy to guarantees the maximum coverage and connectivity in both time and space for underwater sensor networks. Fundamentally, optimal deployment of multiple surface-level gateways must consider many factors such as the deployment surface area and the budget constraints that will limit the number of purchased gateways. A surface gateway deployment may also be designed to satisfy a certain set of desired network performance measures, such as life time, average end-to-end delay, coverage, and connectivity. The previous research [15] assumes _a priori_ knowledge of network demand and statistical models to characterize channel status, MAC protocols, routing protocols, and mobility patterns. However, these simplifying assumptions cannot account for possible changes in the environment such as evolving mission requirements or the mobility and status of underwater nodes, eliminating static gateway deployment as a viable strategy. Another work [16] adds the idea of prediction, but the optimization algorithm and initialization are relatively simple. This paper presents Prediction-Assisted Dynamic Surface Gateway Placement, named PADP, a dynamic surface gateway deployment scheme not only incorporating coverage, the end-to-end delay, and connectivity constraints, but also extending the time interval of each redeployment.