## 2 Related Work
In [15], the authors proposed a new data gathering scheme called SPAT to enable reliable and efficient data gathering by mobile sinks. This scheme guaranteed the mobile sink to collect data from all sensor nodes, and ensured fairness regarding to equally collecting data from the sensor nodes. During this stage, not only single mobile sink but also multiple mobile sinks based routing algorithms were proposed together with other techniques such as clustering, data aggregation _etc._ The joint problem of multiple mobile sinks and efficient routing is a more complex and challenging research issue, which was put forwarded without good solutions then. Even though mobile sinks can improve network performance, they might also cause degraded network performance like long data collection delay, additional overhead of frequent location update _etc._ In recent years, the authors in [16] recently proposed an efficient data driven routing protocol (DDRP), which exploited the broadcast feature of wireless medium for route learning. The overhearing of transmission of data packet, which carries an additional option recording, will gratuitously provide each listener a route to a mobile sink. Random walk routing simply will be adopted for data packet forwarding, when no route to mobile sink is known. In [17], the authors first proposed a general mobility-assisted data collection (MADC) model which includes many important parameters such as number of mobile sinks, sink velocity, and traveling path. Then they developed a comprehensive theoretical approach to obtain achievable throughput capacity and lifetime. By applying the proposed approach, they investigated the behaviors of WSNs with one or more mobile sinks. In [18], the authors proposed a novel localized integrated location service and routing scheme, based on the geographic routing protocol GFG. In their scheme, sink updates location to neighboring sensor nodes after or before a link breaks. Considering both unpredictable and predictable sink mobility, the authors compared their scheme with an existing competing algorithm through simulation and validated that their scheme can generate routes close to shortest paths at dramatically lower message cost. In [19], the authors provided a simulation-based analysis of the energy efficiency of WSNs with static and mobile sinks where the focus was on two important configuration parameters: mobility path of the sink and duty cycling value of the nodes. The authors quantitatively analyzed the influence of duty cycling and the mobility radius of the sink as well as their interrelationship in terms of energy consumption for a well-defined model scenario. The analysis started from general load considerations and was refined into a geometrical model. This model was validated by simulations which are more realistic in terms of duty cycling than previous work. In [20], a survey about mobile sinks supported routing protocols for WSNs are presented, where the advantages and disadvantages of state-of-the-art routing protocols are categorized and discussed. During this stage, both the advantage and disadvantage of introducing multiple mobile sinks are more deeply studied with various constraints under different application scenarios. Joint optimization of sink moving strategy and routing algorithm becomes more challenging under more constrained conditions.