## 2 Related Works
### Mobile Sink Based Data Collection Methods  
With the help of one or more mobile sinks, data can be uploaded to the base station within one hop. This greatly reduces the energy consumption during transmission, and the \"energy hole problem\" can also be mitigated to some extent. Furthermore, the hotspots around the sink change as it moves, and the workload around the sink can also be distributed to more nodes, which balances the energy consumption [24]. To reduce energy consumption during data collection, [PERSON] et al. [25] have divided the sensor nodes into sub-sink nodes which are in a direct communication area or far-away nodes that are within the distance of the multi-hop communication area. Sinks move along a fixed path to gather as much  
Figure 1: A type of uneven clustering network model. data as possible. Similar to this, a type of Data Gathering method based on one mobile sink moving along fixed traverse points (DGFP) has been proposed in our previous work [26]. With the help of the sensing and coverage models, an optimal trajectory for the mobile sink was built to achieve a balanced energy consumption. In addition, a sleep scheduling strategy has also been introduced to further reduce energy consumption. Moreover, [PERSON] et al. [27] proposed a type of rendezvous-based data collection method, named MobiCluster. The mobile sink is mounted on public buses circulating within urban environments on fixed trajectories and near-periodic schedules. Nodes are often deployed in urban areas in proximity to public transportation vehicle routes. To balance energy consumption, nodes located near the sink trajectory are grouped in small-sized clusters while nodes located farther away are grouped in clusters of larger size, as shown in Figure 2. In this case, the overhead of multi-hop data relaying to the edge rendezvous nodes is minimized [27]. However, the trajectory of the mobile sink is not controllable and its route does not adapt to specific WSN deployments. In [28], the network is uniformly divided into sectors, and the nodes in each sector form a cluster. All the CHs link into a chain and the CH nearest to mobile sink is selected as the leader. It forwards data sent from other CHs to the mobile sink. This strategy mitigates the hotspots problem to some extent, and the CHs far away from the sink consume less energy for data transmission. However, the packet delay is increased due to a large number of hops between the nodes and the sink. In addition, data will be dropped if one node in the chain fails. In [29], a Mobile Sink-based adaptive Immune Energy-Efficient clustering Protocol (MSIEEP) was proposed to alleviate the energy hole problem. In this algorithm, an adaptive immune algorithm was used to find the sojourn locations of the mobile sink as well as the optimum number of CHs. Thus, the total energy dissipated in communication and the overhead control packets of all sensor nodes can be minimized. Although the lifetime of network is improved, the unbalanced load problem still exists. With the help of multiple mobile Sinks, [PERSON] et al. proposed a two-stage greedy algorithm to obtain the upper bound of an optimal cluster size interval [30]. Then, by designing the circular motion of the mobile sinks with varied velocity, the energy consumption of CHs in different coronas achieves balance. However, the optimal sojourn time for the mobile sinks to stay at different locations was not discussed in this paper. On the other hand, one should not ignore the energy consumption when frequently broadcasting the location of the mobile sink. To solve this problem, several mobile relay nodes were introduced in the network and they were regarded as the mobile sinks of the sub-networks [31]. These mobile relay nodes independently collect data in each sub-network and forward them to the static sink. However, due to their limited energy, node failure may occur frequently, which causes the disconnection of the  
Figure 2: Network structure of MobiCluster [27]. whole network. Moreover, [PERSON] et al.