## 2 Literature Review
GFGD is a geographic multi-path routing protocol based on geo-spatial division in duty-cycled UWSNs. In this work, all sensor nodes can switch their states from active to sleep and vice versa which can save EC of the network. A node with shortest path loss and highest energy with minimum delay is selected as next forwarder in both GFGD and GGFGD. Greedy forwarding approach is used in both protocols. GFGD takes directional forwarding into account and saves energy with affordable delay as compared to GGFGD. Next selection of neighbors in GGFGD increase the delay in processing and propagation. [PERSON] et al. proposed Spherical Hole Repair Technique (SHORT) protocol in UWSNs [15]. SHORT is a spherical repair technique. This protocol avoids energy holes and coverage holes. In this protocol, if a node is about to die, it broadcasts a control message to its neighbors. Therefore, neighbors stop forwarding the packets to this node. To fill this coverage, a sensor node from the dense region is introduced into this region by ensuring no coverage hole in another location in the network. The nodes with hidden cross triangles have a higher probability to move into a new position removing the coverage hole. This mechanism increases the throughput and lifetime of the network at the cost of high end-to-end delay. In [16], Reliable Energy-Efficient Pressure-Based Routing (RE-PBR) protocol is proposed by [PERSON] et al. RE-PBR is reliable and energy efficient pressure-based routing protocol. Link eminence, depth and the enduring energy constraints are taken into account for the selection of forwarder nodes towards the destination. This scheme successfully achieved the balanced EC due to the avoidance of immutable forwarder-selection of neighbors. OR protocol is proposed for void hole region avoidances in [17]. In this routing protocol, instead of the current void node retrieval method, depth adjustment technique is used to change the position of nodes in the vertical direction. Adjacency graph of neighbor nodes is maintained at every node to select the potential-forwarder nodes towards the destination. This protocol can bypass the void nodes by using directional routing in every direction. However, the compensation of high EC persists in [17]. In this paper, [PERSON] et al. proposed Geo-Opportunistic Routing (GOR) protocol of immutable selection of forwarder node to balance the EC and void hole regions in UWSNs [18]. The total volume of the network is distributed into cubes to reduce the intrusion problem and to make well-versed decisions for proficient energy use. Moreover, data packets are recovered from the void regions by introducing mobile sinks to reduce the traffic flow. However, sparse regions effect the propagation time in proposed protocols. Three Efficient routing protocols named: Sparsity-aware Energy-Efficient Clustering (SEEC), Circular Sparsity-aware Energy-Efficient Clustering (CSEEC) and Circular Depth-based Sparsity-aware Energy-Efficient Clustering (CDSEEC) are proposed in [19] by [PERSON] et al. Proposed protocols helped to monitor the fields with square and circular geometries in UWSNs. The purpose of the proposed work is to minimize the energy in sparse regions and to stop the redundant transmissions. Therefore, the increase in the sparse region also increases the energy and communication overhead. As, energy dissipation has direct relation with distance. Therefore, as the sparse region increases, energy overhead also increases. Energy-Efficient Multi-hopping Routing (EAMR) protocol is proposed by [PERSON] et al. in [20]. Fixed clustering is performed in the proposed protocol. EAMR successfully minimized the overhead in communication with which lifetime of the network is also increased. Inter-cluster transmission increases the network scalability. However, in sparse regions cluster heads cannot bear load [20]. The authors in [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20] have achieved specific parameters, i.e., minimum EC, duplicate packets generations and void hole avoidance in UWSNs.