## 2 Related Works
### Geographic Routing
#### 2.1.3 Opportunistic Routing Protocols  
Opportunistic protocols have been presented to avoid retransmissions in the network. In the existing literature, an EnOR: energy balancing routing protocol for UWSNs is proposed to avoid immutable selection of forwarder nodes by assigning priority to each node [8]. This routing algorithm considers three parameters for forwarder node selection; the link reliability, the energy, and packet advancement towards the destination. With the help of aforesaid parameters, the rotation of forwarder node selection is ensured. Additionally, EnOR extends the network lifespan and avoids the occurrence of void node by continuously rotating the forwarder node.  
An OR protocol for void avoidance is proposed in [24]. In this routing protocol, instead of traditional void node recovery methods, depth adjustment mechanism is used to relocate node in vertical direction. This scheme constructs adjacency graph with neighbor nodes at each to elect potential forwarder nodes. The distinctive aspect of the scheme is to elect forwarders from the transmission range of every node which resolves the issue of hidden terminal problem. This scheme also has the ability to by pass any kinds of void by utilizing omni directional routes. However, the trade-off of high energy consumption persists in [24].  
A delay sensitive OR protocol is presented [25] to achieve high goodput and minimum delay in the network. Authors consider \(EEL|_{success}(Fi)\) (expected end-to-end latency [25]) parameter to ensure at least one of the forwarder delivers data packet successfully at the destination. To predict close value, two step heuristic scheme is presented based on the per node forwarding set determination and assigning priority to advancement node. This scheme shows high goodput and efficient-energy consumption in the network.  
In [26], VAPR protocol exploits two hop depth information and hop counts to select next hop forwarder. It is easier to get depth information as compared to location coordinates. VAPR opts two fold procedure: improved beaconing and opportunistic forwarding. A node initiates a beacon containing information like its depth, direction and number of hops to initiate communication. Then, data is delivered solely based on next-hop direction, to deliver data in minimum number of hops. Due to efficient beaconing, VAPR is robust against failures and node mobility. In [27], hydrocast uses pressure information of sender and neighbor nodes along with two hop neighboring distance. During forwarding process, hydrocast selects a set of neighboring nodes based on greedy advancement towards destination, considering hidden terminal problem as well. Both VAPR and hydrocast maintain routing path to avoid void holes and the trade-off is high energy consumption.  
[PERSON] et al. proposed reliable and energy-efficient pressure-based routing (RE-PBR) [28] algorithm by taking into account; link quality, depth, and residual energy for the selection of immediate forwarder node towards the destination. The inclusion of link metric balanced the battery depletion because of continuous rotation of forwarder node. This scheme achieved balanced energy consumption due to the avoidance of immutable forwarder selection.  
Authors proposed autonomous underwater vehicles (AUVs) for data gathering from underwater sensor nodes to minimize transmission power of node [29]. In this scheme a cluster is formed and data collection from an individual cluster head node is performed for balanced energy dissipation across the network nodes. The validity of the work is confirmed through Monte Carlo simulations and deduced that energy is efficiently utilized directly effecting the network lifespan. However, the delay is high because no multi-hop mechanism is incorporated if AUV is not within the communication range. Additionally, the immutable selection of cluster head node will create void holes and degrades the network performance.