# Energy-Efficient Region Shift Scheme to Support Mobile Sink Group in Wireless Sensor Networks
## 1 Introduction  
Wireless sensor networks (WSNs) have been regarded as a prominent network technology for realizing intelligent applications with interaction between cyber and physical worlds. A set of small sensor devices is deployed to application fields and detects surrounding events. Sinks collect and process the event data as required by the applications [1]. In a considerable number of sensor networks applications, mobile sinks are utilized to distribute load due to the restricted energy resource or to perform their own missions such as fire extinguishment, battlefield surveillance, or habitat monitoring [2]. In addition, a number of sinks could compose a group and have geographically collective movement [3], e.g., a squad of fire fighters and a platoon of soldiers. They typically try to acquire the same data to execute communal missions. The mobile sink groups have different aspects compared to individual mobile sinks in terms of communications. Group movement has created novel routing opportunities. Utilizing the geographical adjacency, it could effectively deliver data to them without identification of each destination one by one. Since a group usually has a certain radius of action, group region could be considered as a circle with a center point and a radius. Data can be delivered by geocast routing methods into the region [4]. It significantly reduces control overhead to identify every location where destinations are located. In order to support mobility for such groups, it is necessary to periodically obtain positions of the group. Previous studies for mobile sink groups [5; 6] exploit flooding to figure out a region including all member sinks. However, the schemes have a significant energy consumption problem due to the periodic and extensive flooding in a group region. A representative sink who discovers a region of a group queries each location of member sinks in the whole expected area where the group could be located. Each member sink that received the request replies with its own location to the representative sink. Because of that, it does not only consume a large amount of energy, but it also causes frequent retransmission due to congestion and collision around the representative sink. Meanwhile, our observation drives new possibilities. The sink groups have an inherent feature in which members of a mobile sink group typically have similar motion patterns. They typically move toward the same point while freely swinging in a certain radius of a group region. We categorize the character of the group movement into two sub-movements: macro movement and micro movement. If the group region is discovered one time, the region could be effectively discovered hereafter by shifting the region according to the macro movement. Since every member has similar movement, we could estimate macro movement of a group by investigation of only a few members' motion information. Some errors due to the involvement of the micro movement could also be corrected by the other members' information. In this paper, we propose a new scheme that discovers a group region without periodic flooding, called Region-Shift-based Mobile Geocasting Protocol (RSMGP). Focusing on the inherent feature stated previously, the proposed scheme could discover a location of a group by utilizing movement information of partial member sinks without large energy consumption. We consider that movement of a group could be depicted as movement of center point. If a sink surpasses the region that the sink belongs to, the group region is updated. Of course, estimating exact group region is not easy unless region information is given at all times. Instead of that, our scheme integrates movement patterns of members out of the region so that the calculation could approximately imitate the entire movement pattern of the group. Unlike the previous schemes that periodically figure out a location of a group, the proposed scheme is more efficient since updating location of the group is triggered only when the group moves over a certain distance.