# A Novel Cross-Layer Routing Protocol Based on Network Coding for Underwater Sensor Networks
## 2 Related Works
In HHVBF, each node can be treated as the source node when it needs to forward data packets and a series of pipe lines can be built to search for more alternative routes towards the destination. DBR protocol only uses the depth information for routing process. The nodes in the network just forward data packets to the nodes with lower depth. This operation decreases the difficulty of acquiring 3D geographic position and is especially suitable for applications with multiple sinks. WDFAD-DBR improves DBR by selecting the next forwarding nodes according to the weighting sum of depth difference of two hops. So the probability of meeting void areas can be reduced. Meanwhile, a mechanism of neighbor node prediction is designed to improve the transmission efficiency. Compared with sender-side candidate set selection procedures, these protocols do not need to update neighborhood information by using beacon messages, thus energy consumption is reduced and channel utilization rate is increased. However, these protocol cannot eliminate duplicated transmissions completely. This adverse effect is particularly obvious when considering high PER and node mobility in underwater environment. Besides, when there is a void area in network, these protocols cannot sense the void region in advance and have to increase redundant transmissions to improve packet delivery ratio. Meanwhile, in these protocols, all nodes need to obtain the real-time position of destination nodes or destination nodes need to be densely distributed on water surface. This means any position change of destination nodes needs to be broadcast to the whole network, or we must allocate enough destination nodes on water surface. So the use of such protocols is limited in UWSNs. The hybrid protocols are relatively complex compared with sender-side and receiver-side protocols. There are three steps in the hybrid protocols: (1) Sender nodes broadcast request packets into the network. (2) Neighborhood nodes of the sender determine if they are responsible for forwarding the packet according to the request information in the packet. Then the chosen nodes will send a reply to the sender. (3) The sender then choose appropriate nodes according to these replies and send data packets to the chosen node. Some typical underwater routing hybrid protocols in this category are focused beam routing (FBR) [20] and channel-aware routing (CARP) [21]. In FBR, nodes know their own location and the location of their final destination. When a node determine to send a packet, it sends a control packet first and waits for replies from its neighbors. The neighbors in a cone centered on the line between the sender and the sink reply with their position along with other information. The neighbor node with the smallest distance towards the sink is chosen as the next hop forwarder. So the selection of the next relay is made at each step of the path. CARP improves FBR by considering link quality for selecting the next-hop relay. The history of successful transmissions to neighbor nodes is maintained in the senders and the relay selection is based on hop count, link quality, buffer space and residual energy. Advantages and disadvantages of this kind of protocols are obvious. The beacon messages do not need to be updated periodically and the sender can obtain its neighborhood information before sending packets. However, the propagation delay of the three steps procedure can increase the communication delay to a great extent and thus this kind of protocols can only be used in low communication rate applications. To overcome the high packet error rate and unstable channel quality in UWSNs, network coding is proposed and introduced into underwater data transmissions. In [14], the authors apply network coding to UWSNs for reliable data transfer and demonstrate the efficiency of the scheme. The performance can be improved by reducing the paths as in [15], where a twin path and network coding (TPNC) protocol is proposed to achieve lower energy consumption.