# RECRP: An Underwater Reliable Energy-Efficient Cross-Layer Routing Protocol +
in underwater environments is around 1500 m/s, which is five orders of magnitude slower than the speed of RF. As shown in Figure 2, sound velocity changes along with the factors such as water temperature, depth, density, and salinity, which means that acoustic wave propagation in underwater environments is nonlinear. The nonlinear propagation causes shadow zones where the transducers cannot receive from others, which leads to communication voids. Moreover, the transmission power required by underwater acoustic transceivers is orders of magnitude higher than that of terrestrial devices. Since sensor nodes are often battery-powered and deployed deeply, the batteries are unavailable to replace when they are exhausted. Thus, the lifetime of an underwater node is severely restricted by its local power source [6]. Since the major concern for energy cost is transmitting rather than receiving, the overall energy consumption along the path to sink nodes must be carefully controlled. In addition, neighbor nodes near sink nodes are used frequently for data forwarding, which accelerates energy consumption. This results in an imbalance of energy consumption as well as premature exhaustion of node energy. Terrestrial routing protocols have poor performance in underwater environments. Existing routing protocols proposed for terrestrial mobile and ad-hoc networks are usually divided into two categories: proactive and reactive. However, neither of these protocols is suitable for underwater environments. Proactive protocols require too much signaling overhead to establish end-to-end routing, especially when the network first establishes. Once the topology changes, the establishment and maintenance of  
Figure 1: Target scenario. Figure 2: Generation of the shadow zone. the routing table causes huge network overhead. Constant movement of underwater nodes leads to frequent network topology changes, which aggravates the situation. Reactive routing protocols are suitable for dynamic environments, but they face large delays as they require source-initiated flooding for the route discovery process. Moreover, experiments show that reactive routing protocols perform poorly in asymmetric links (UWSNs). Thus, more and more researchers are devoted to the design of new routing protocols for UWSNs [7]. Greedy routing protocols are studied extensively to improve the packet delivery rate with minimum cost [8]. Due to high mobility of underwater nodes, greedy hop-by-hop routing is the most reliable method. Since this technique relies on a simple strategy that forwards packets to the locally optimal node with a strict positive progress towards the sink node, it does not work properly when data packets reach a node that has no neighbors with positive progress towards the sink node. This phenomenon might be caused by node mobility or the existence of shadow zones. This is the problem greedy routing protocols are generally faced with, known as the communication void problem. In this paper, we propose a reliable energy-efficient cross-layer routing protocol called RECRP. RECRP is a location-free single-copy protocol, and does not rely on location and depth information. To improve two-hop packet delivery rate and balance energy among the network, we propose an optimal max-min method that dynamically controls transmission power and channel frequency. Since the Doppler scale shift can be measured from a single message exchange between two nodes, we adopt information from the physical layer such as Doppler scale shift measurement, Received Signal Strength Indicator (RSSI), etc. [9; 10] to select next hop without the use of any additional hardware (e.g., depth probes, inertial sensors). Hence, RECRP reduces the equipment cost significantly. The meaning of cross-layer routing is to use and adjust more underlying information in the routing protocol, such as using Doppler scale shift to estimate the relative speed between nodes, and adjusting the transmission power and frequency to improve the transmission efficiency.