# Path Diversity Improved Opportunistic Routing for Underwater Sensor Networks
## 2 Related Work  
Generally, routing protocols for UWSNs are mainly classified into three types including geographic-based, depth-based and link-state based routing protocols. Here, we consider two important factors: candidates selection procedures and candidate coordination to survey the routing protocols. Geographic-Based Routing Protocol-Vector-based forwarding (VBF) routing protocol [9] assumes every node already knows its location. It employs the geographic-based candidate selection strategy, and the nodes are allowed to forward packets when its distance to the forwarding vector is less than a pre-defined radius, where the forwarding vector is guided by the location from the source to the destination. A TDMA-based candidate coordination is applied, and the holding timer depends on the distance to the forwarding vector. An enhanced version, HH-VBF [10], is proposed for sparse network scenarios. It uses a dynamic pipeline at each hop originated from every intermediate node to the destination. VBVA is also a vector-based routing protocol, which is proposed to address the void problem in mobile network scenarios. It uses vector-shift and back-pressure mechanisms to bypass a concave void. HH-VBF and VBVA employ the same candidate selection scheme with VBF, but adopt different forwarding vector. The focused bream routing (FBR) protocol [8] assumes every node has its location information and every source node knows the final destination location. Candidate nodes are those that lie within a cone of angle emanating from the transmitter towards the final destination. The RTS-CTS based candidate coordination solution is applied. In another geographic-based routing protocol, introduced in [12], a greedy forwarding strategy, neighbors that have the smallest distance to the sonobuoy will be selected as the next-hop forwarder. The centralized and distributed topology control methods by adjusting the depth of nodes are proposed to reduce the impact of the routing void problem and improve packet forwarding efficiency. Depth-Based Routing Protocol-Depth-based routing (DBR) protocol [13] employs depth-based candidate selection strategy. It is a greedy forwarding strategy where nodes are allowed to forward packets when their depth is lower than that embedded in the packets. Otherwise, they discard the packets. A TDMA-based candidate coordination is applied, and the holding timer depends on the depth between the forwarder and itself. HydroCast [14] uses the EPA-based candidate selection metric, which is the normalized sum of advancements made by this neighboring set. The normalizedadvancement is associated with the packet delivery probability and the progress to the destination. The candidate selection algorithms proposed in [14] try to find a forwarding set of hidden-terminal free nodes that maximizes EPA. It adopts normalized advance (NADV) metric [21] as the priority metric. The linear holding timer for candidate coordination depends on the depth of the receiver. In HydroCast, a routing recovery technique is proposed to deal with the maximum voids problem. The local maximum node will search for a path to a lower depth node using 2D surface flooding. Void-aware pressure routing (VAPR) protocol [15] employs an enhanced beacon technique to build a directional trail to the closest sonobuoy. It uses the sequence number, hop count and depth information in periodic beacons to set up next-hop direction. When a beacon is received from a shallower depth node, the direction is set as up. Otherwise, it is set as down. The candidate selection algorithm tries to find the cluster with the highest expected packet advancement toward the selected direction. Opportunistic void avoidance routing (OVAR) protocol [16] takes advantage of the distributed beacon to construct the adjacency graph at each hop. The sink node initiates the beacon process, and the receiving node decides how to deal with the beacon according to hop count value of the beacon and itself. Gradually, the beacon will be cascaded down to the network.