# Ocean-Current-Motion-Model-Based Routing Protocol for Void-Avoided UASNs
## 2 Related Research  
The routing protocol has become one of the most important links in UASNs to ensure efficient delivery to the destination node [19]. A well-designed routing protocol is crucial for UASNs to achieve reliable and efficient data transmission. Therefore, scholars from different countries have carried out different studies on the routing protocols of UASNs. At the beginning of the routing protocol, many studies were based on the geographic locations of sensor nodes with the assumption of their own location information. A routing protocol called vector-based forwarding (VBF) is proposed in [20]. It improves the energy efficiency of the networks with the node location information and creates a virtual routing pipeline. The relay node receives the data transmitted from the sender and then sends them to next receiver within the virtual pipeline. However, nodes falling within this range will be used at a higher frequency than those outside of this range, resulting in uneven energy consumption and a shorter lifetime. However, VBF could not find the relay nodes when the routing void occurred within the pipeline. Hop-by-hop VBF (HH-VBF) was proposed in [21] to address this issue. The pipeline of HH-VBF is different to that of VBF; the routing pipeline is dynamically adjusted according to the location information between the forwarding node and the receiving node in the sparse network, which increases the reliability of data transmission. In [22], an LBL acoustic repeater system was presented, a method based on GNSS. Although the study proved the feasibility of this scheme for obtaining the location of the underwater node, it required four buoys and deduced the underwater node position through three parameters: the surface position, transmission time and depth information of the node. It would eventually cause a higher cumulative error once a parameter error occurred. In order to avoid using the geographic locations of sensor nodes, a depth-based routing protocol called DBR was proposed in [23]. The packets are greedily delivered from the deep nodes to the shallow nodes. The PDR is improved, and the latency is reduced. However, DBR cannot find the next node as the forwarder when a routing void happens in a sparse network. To address this issue, distance-vector-based opportunistic routing (DVOR) was proposed in [24]. The distance vector is established between the node and destination with the node query mechanism, and the packets are transmitted to the destination node via the shortest path of opportunity. Energy efficiency in energy-based routing protocols is also an important factor for UASNs because of the energy limitation of underwater sensor nodes. A power-efficient routing protocol (PER) [25] was proposed to reduce the unnecessary power consumption; the overall lifetime of the underwater network will be extended during data forwarding. In addition, a reliable energy-efficient cross-layer routing protocol (RECRP) was proposed in [26]; with a scale-measurement-based Doppler and received-signal-strength indicator, the PDR is increased, and the energy is balanced by a max-min model. However, it is hard to achieve the goal because of the particularity of the underwater environment. Localization-free energy-efficient routing (LEFER) is proposed in [27] to save energy. Although the energy is lowest with an energy-saving mechanism, all nodes will forward the data within the transmission range based on the maximum residual energy. Energy efficiency can still be considered in cluster-based routing protocols. [PERSON] et al. [28] reported a low-energy adaptive clustering hierarchy (LEACH) protocol to reduce the amount of information and minimize energy dissipation with a merging data strategy. The energy load is balanced and benefits from the fair mechanism of cluster head selection. [PERSON] et al. [29] proposed a multi-layer cluster-based energy-efficient (MLCEE) protocol to solve the problem of unbalanced transmission via the division of the layer, clustering of the sensor nodes,and forwarding of the data.