## 2 Literature and Background of UA-WSNs  
[PERSON] et al. proposed a cooperative algorithm with sink mobility for UA-WSNs in [28]. Two sinks move linearly in the middle of the network and gather the information and data from the nodes. The node forwards data in a multi-hop fashion with the help of relay when the MS is not in the coverage area of the node. When the MS is present in the coverage area of the node, then it delivers the data directly to the MS. The efficiency of the network is improved such as the lifespan and minimal packets being dropped compared to its counterpart schemes. However, due to the cooperation of nodes, the latency and energy cost of the network are high. Another idea that uses two MSs is presented in [29]. The circular regions are defined in the network. In these circular regions, the MSs move in a clockwise direction. The nodes deliver data and information directly to the MSs when they come in the nodes' transmission range. This arrangement gains better lifetime and delivery to the base station. However, in this method, the packet is dropped in case of the absence of the MS in the coverage area of the node. The same idea of the data forwarding mechanism is followed in [30]. However, the network division is done in ten rectangular sectors. Two MSs pick up the data from the nodes. The algorithm gains better lifetime, throughput, and stability. The authors in [32] proposed an algorithm in which the network is split into three small horizontal sectors. Two MSs move in the horizontal orientation, whereas two MSs in the vertical orientation in the network. The MS receives data directly from the node when the MS is in the coverage area of the node, otherwise cooperative routing is performed. For further improvement and to operate the network for a greater time, energy-harvesting techniques are utilized. This approach improves the lifetime and throughput of the network. However, this approach still uses multi-hop communication, which leads to deliver data with latency. [PERSON] et al. proposed an algorithm for UA-WSNs with sink mobility in [33]. The network is split into two vertical sectors. Further, each vertical sector is partitioned into five sectors. Overall, ten sectors are formed in the whole network. Two MSs move in their own vertical region for information collection and one static sink fixed at the network top center position. The MSs do not follow a specific path, but they search for the sparse regions and move to the sparse regions for data collection from the nodes. The nodes in the dense area communicate with the cluster head. Then, it advances data to the MSs or to the base station. The throughput and network life period are maximized in this scheme. A MobiSink protocol with cooperation mechanism is presented by [PERSON] in [34]. Four horizontal sections are formed in the network, and four MSs move horizontally in each section of the network. The direct data reception is done when the node in its coverage area finds an MS. Otherwise, the data are delivered to the MS through a neighbor node. MobiSink achieves better throughput by paying a greater cost of power of the nodes. In [35], [PERSON] et al. presented a routing mechanism for a 3D network that uses the MS and cluster nodes. The MS and cluster nodes move in the defined region formed in the network for data collection from the nodes instead of multi-hop data forwarding to improve the network lifetime, throughput, and path loss. If a node has an MS in its coverage area, then it forwards data to the MS. Otherwise, in the absence of the MS, the data are received by the cluster node and then proceeded to the MS. The algorithm presented in [36] partitioned the whole network into four small squares. Instead of static sinks, two MSs are used in the network. The nodes search for an MS in proximity to deliver data. The direct exchange of the data is accomplished when the MS is present in the proximity of the node. While in absence of the MS, the data are discarded.