## 2 Related Work
In [16], an energy-aware clustering algorithm (EADC) was proposed using competition range to construct clusters of even sizes. The routing algorithm of EADC increases forwarding tasks of the nodes in scarcely covered areas by forcing cluster heads to choose nodes with higher energy. Recently, several applications which introduce sink mobility into the wireless sensor networks have appeared. In some applications, mobile elements have been taken forward to attach network node for data collection [32, 33, 34, 35]. It is very promising to use mobile sink to improve network lifetime without causing negative impacts to the network. This is because the role of hot spot node will rotate among most sensors, which will basically balance the traffic load throughout the whole sensor network. In [17], a mobility-based clustering (MBC) protocol for WSNs with mobile nodes is proposed. The authors consider residual energy together with the current speed of each sensor node. A threshold value is multiplied by the factors representing the residual energy and the current speed of a node. Using this threshold, the nodes with more residual energy and lower speed may have more probability to be selected as cluster heads. MBC used a heuristic mechanism in which each sensor wakes itself up one timeslot before its scheduled timeslot according to the TDMA schedule and goes back to sleep mode after its timeslot. A network infrastructure based on the use of controllably mobile elements was discussed in [36], with the essential of reducing the communication energy consumption at the energy constrained nodes and, thus, increasing useful network lifetime. The controllable mobile infrastructure can reduce energy consumption at the energy constrained nodes and, thus, increase useful network lifetime. In particular, the infrastructure focuses on network protocols and motion control strategies. The significant issue to be noticed is that the controllably mobile infrastructure tests using a practical system and do not assume idealistic radio range models or operation in unobstructed environments. A novel geographic routing for mobile sinks has been proposed by [37] to address the issue that frequent location updates of mobile sinks may lead to both rapid energy consumption of the sensor nodes and increased collisions in wireless transmissions. The proposed scheme takes advantage of the wireless broadcast transmission nature of wireless sensor nodes. When a sink moves, the new location information is propagated along the reverse geographic routing path to the source during data delivery. An Energy-Balanced Data Collection (EBDC) mechanism using a mobile data collector in WSNs was proposed in [38]. EBDC considers a circular monitoring region which has been geographically partitioned into a number of circular tracks. To cope with the energy unbalance problem, the authors determined the trajectory of a mobile data collector (or mobile sink) such that the data-relaying workloads of all sensors can be totally balanced. In [39], based on sensor transmission range \(r\) and the velocity of the mobile sink \(v\), the authors evaluated the energy performance for both static and mobile sinks. They evaluated their algorithm in C\(++\) simulator with limited energy. The authors found an balance for the mobile sink's moving speed, making sensors meet mobile sink more commonly, besides a sufficiently long session interval for the sensor and sink to successfully exchange one potentially long packet. The Backbone-based Virtual Infrastructure (BVI) approach has been proposed to avoid the routing structure construction [18]. The BVI approach supports sink mobility without global position information. However, BVI networks are always considered as single hop networks, which makes the tree organize into too many cluster heads. Thus, in [19], a novel BVI-based communication protocol to support sink mobility without global position information was proposed. The authors used multi-hop clusters and rendezvous cluster heads to reduce the number of cluster heads.