# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 3 Routing Protocols for UWSNs
### Location-Based Routing Schemes
#### EMGGR  
EMGGR [25] has three main components: (1) gateway election procedure, (2) gateway information update procedure, and (3) data packet forwarding procedure. The gateway election procedure deals with the selection of a gateway or relay node for each cell of the logical grid (the network area is partitioned into a three-dimensional logical grid). Gateway selection is based on a value \"W\" that is based on the remaining energy level of the candidate node, and its distance from the center of the cell. The node that maximizes W is selected as gateway for its cell. Selection of a node as gateway based on its distance from the center of the cluster and residual energy ensures avoidance of burdening one node with the role of gateway node, thus achieving balanced energy dissipation, and reducing the chances of void creation. After selection, every gateway periodically checks its remaining energy level. If its remaining energy level is higher than a certain threshold value, it transmits an update message, and continues to work as gateway (the update message resets the \"existence timer\" (maintained by ordinary nodes) for this gateway). However, if its residual energy level is lower than the threshold value, the gateway silently abandons its role as gateway. A new election process is carried out upon expiry of the existence timer. The third component of EMGGR is a data packet forwarding mechanism that can be divided into two parts: (1) construction of node disjoint paths, and (2) Packet transmission through node disjoint paths via gateways. Construction of node-disjoint paths increases the reachability of the network. Moreover, parallel packet transmission over multiple node-disjoint paths adds reliability and immunity against connectivity holes. The downside of this protocol is the need for geographic coordinates that adds extra overhead. Moreover, it has high end-to-end delay, due to the higher number of hops between source and sink. RBCRP  
RBCRP [26] is a cooperative routing scheme that works in two phases. During the initial setup phase, nodes discover neighbors (potential forwarders) by broadcasting \"hello\" packets. The hello packets contain the SNR value, energy level, depth, and ID of the sender. Relay nodes are selected based on these parameters with the help of a function that aims to diminish outage likelihood of relays by balancing data load. The setup phase is followed by a steady state phase, during which packets are forwarded to selected relay nodes. Retransmission is requested if BER (calculated at the destination) is higher than a predefined threshold value. In order to improve packet delivery ratios (PDR), RBCRP employs mobile sink nodes. These mobile sink nodes traverse the entire network by following horizontal and vertical traversal paths (Figure 5), thus minimizing communication distance for transmitting nodes, thereby reducing packet drop rates, and improving total throughput and network lifetime. The performance of RBCRP with regard to network lifetime and throughput depends mainly on the SNR threshold value and number of relay/forwarder nodes. The proposed scheme achieves reliability, load balancing, and end-to-end delay by the inclusion of SNR, available energy, and depth, respectively, as the next hop selection criteria. The downside of this scheme is that in order to function properly, it requires the locations of the nodes. Location estimation, which is a communication intensive process, increases the overall communication overhead. Figure 4: Node agency. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Reference** & **Main Conduction** & **Neri Hop Selection Criteria** & **Mobility** & **Location Required** & **Clustering** & **Connectivity Wald Handling** & **Sink** & **Deployment** \\ \hline HECAR-AC [24] & Void Avalanche, Network Lifetime, & Node-energy, propagation delay, hop count, & Yes & Yes & Yes & Yes & Multiple Sink & 3D \\  & EET delay: & & Combined quality &  
ECBCCP  
ECBCCP [27] is a cluster-oriented routing scheme.