# A Novel Cross-Layer Routing Protocol Based on Network Coding for Underwater Sensor Networks
## 1 Introduction  
Underwater wireless sensor networks (UWSNs) have attracted much attention in recent years due to the wide application of marine mining and exploitation [1; 2; 3; 4; 5; 6]. However, the adverse characteristics of UWSNs raise challenge to the design of reliable data transmission protocols. One of the outstanding prerequisites to establish underwater ad-hoc networks is to find a reliable path to forward data packets to sink nodes. So the design of underwater routing protocols is a key issue in UWSNs applications. A proper routing protocol can significantly reduce the transmission delay, decrease the energy consumption and improve the stability of system. However, there are several unfavorable factors that limit the design of underwater routing protocol. The optical waves and radio waves do not propagate well in underwater environment [7; 8]. So most UWSNs applications choose sound waves for data transmissions. The average propagation speed of sound waves in underwater environment is only 1500 m/s, which is much lower than that of optical waves and radio waves (about 3 \(\times\) 10\({}^{8}\) m/s). So the propagation delay and round-trip time (RTT) in UWSNs are much longer than that in terrestrial applications. Besides, the underwater channel is relatively complex as carrier frequencies of sound waves often share the same frequency band with noise frequencies caused by marine organisms, shipping activities, sea winds and so on. Meanwhile, sound rays are easily affected by the angle of incidence, reflections, sea water temperature, salinity and so on, which will cause severe multipath effects. Moreover, the nodes placed in sea water often move passively with water currents and some communication devices can move at a high speed (like submarine). These movements can result in severe doppler distortions. So underwater data transmission will suffer serious interference by these factors and packet error rate (PER) is extremely high especially when we need long-distance communications. What is worse, the energy consumption of nodes for underwater applications is relatively high due to the difficulties of recharging batteries and the usage of acoustic communication [9]. So we must consider these factors when designing the routing protocols for UWSNs. Current terrestrial wireless routing protocols often require the acquisition of nodes information in the entire network and this may raise the demands of instant information interaction and frequent feedbacks. However, the long propagation delay and high energy consumption in UWSNs limit the utilization of such protocols. Based on the adverse characteristics of UWSNs, many researchers make great contributions to the exploration of underwater routing protocols. Geographic routing [10; 11; 12], also called position-based routing, has been proposed recently as a proper solution for UWSNs routing protocol. It tempers the demands for global establishment of complete routes, and the nodes in network can make routing decisions locally. The abundant research of underwater positioning techniques also stimulates the development of geographic routing. To further overcome the void area problem in UWSNs, opportunistic routing [9; 13] is introduced into UWSNs routing design. In opportunistic routing, packets forwarding is enhanced by taking advantage of the multiple nodes reception and forwarding. However, the combination of geographic routing and opportunistic routing often causes the redundant packet transmission problem and decreases the channel utilization. Besides, the void area problem still exists as many current UWSNs routing protocols cannot fully use all the receiving nodes in the network and many alternative paths are suppressed to prevent duplicated packet transmissions. Furthermore, the network coding technique [14; 15; 16] has been proposed for data transmissions in underwater environment for several years and has been verified to be a promising technique for reliable data transmission in UWSNs.