## 2 Related Work
Then, it requests to forwarder nodes to transmit their data packets to destination nodes. The MRC technique is applied at the destination nodes. It improves the PDR and decreases the errors probability. However, the techniques used in this protocol increase the energy consumption. In [24], a protocol is proposed, in which the network is split into three zones and nodes spread with different depth and positions. The nodes deployed in the upper zone have depth from 0 m to 300 m and can transmit the information packets to final destination node directly. The nodes present in the middle zone have the depth from 300 m to 700 m and these nodes work as the relay nodes for lower zone nodes. The optimal relay nodes have depth range from 400 m to 600 m. The nodes selected in this specified depth region are called optimal relay nodes for lower zone nodes. The middle zone nodes can directly communicate with the final destination node or through relay nodes. The lower zone nodes have the depth of 700 m to 1000 m. If no relay nodes found in the transmission area of the source nodes, then it communicates straight with the sink node with maximum energy consumption. The protocol highly improves the total energy consumption. However, by choosing only one sink node, delay of the network increases. The protocol presented in [5] extends the work of [24] by adding cooperation between the destination and forwarder nodes. The lowest depth and lowest distance values are considered to choose the destination nodes. The distance values of the nodes can be found by the current distance of the nodes from the final node. The nearest nodes to the destination are considered as the optimal relay nodes. In this way, two data packet copies are merged via MRC and checked for BER. The BER threshold is fixed at 0.5. If the BER value of the received information bags at the destination node is greater than the BER threshold value, then it requests to the forwarder node for sending its own data packet. If BER satisfies the threshold value, then destination node accepts and forwards the information packet towards the sink node. If no relay nodes are available in the source nodes range, then only destination nodes forward the information packets in the direction of sink node. This protocol increases the packets delivery ratio and decreases the total packet drop. On the other hand, by selecting only one sink node and due to cooperation in data packets forwarding, delay increases. In (DEADS) [25], [PERSON] et.al propose cooperation based protocol. The nodes are deployed in a random manner where one-hop neighbor nodes know the depth and residual energy of the neighbor nodes. The network periodically updates the depth information. The network has four different regions in which nodes are deployed. If there are more alive nodes, the depth threshold value is greater and vice versa. The depth information is needed for one neighbor hop instead of the whole network, thus data flooding is eliminated. The nodes having the lowest depth and maximum residual energy values are used as forwarders. The destination nodes can select single or multiple relay nodes from its neighbor nodes. If there are no neighbor nodes to source nodes, then the source nodes perform no function and go to sleep mode. If final destination nodes are in the field of source nodes, then source nodes directly transmit the information packets to the final destination nodes. While performing cooperation by the nodes, the destination nodes receive two copies of data packets which are combined by MRC and directed towards the next destination or sink nodes. The nodes follow linear or elliptical motion. This protocol out-performs some existing protocols in alive nodes, dead nodes, residual energy and PDR. However, packets drop increases because of not optimizing the elliptical mobility pattern. The energy issue is not only related with the UWSNs but also it is important in terrestrial networks.