# Energy-Efficient Wireless Hopping Sensor Relocation Based on Prediction of Terrain Conditions
## 2 Previous Work
### Basic Assumptions about Hopping Sensors and Relocation Protocol  
In the traditional research fields of wireless sensor networks (WSNs), there are various methods for clustering and cluster header selection of sensor nodes randomly distributed in the interested areas [24]. In fact, since the purpose of this paper is to study the relocation of hopping sensor nodes, it is assumed that the clustering and selection of the cluster headers have already been completed by various existing methods. For example, hopping sensor nodes are randomly scattered firstly on the interested area in which we want to collect data. After that, the area is divided by an appropriate clustering algorithm, and a cluster header node is selected for each cluster zone. The cluster header periodically communicates with member sensor nodes in its cluster zone, and it manages representative information of each member. Considering the network connectivity problem, communication between each cluster header and all member sensor nodes could jump to send and receive messages with its maximum transmission radius. Moreover, every hopping sensor node contains a GPS unit capable of knowing its current location [25].  
The terms used in this paper are explained in Figure 2. Every hopping sensor node can be either a cluster header node or a member sensor node. Since the maximum transmission radius is defined as a cluster zone when the cluster header maximally jumps, there is a high possibility that direct communication between the cluster headers could be impossible. However, some member sensor nodes in the area intersecting with the cluster zones (that is, near the maximum transmission radius of each cluster header) may communicate with two or more cluster headers. This hopping sensor node is called as a relay node, and the role of the node could help to facilitate communication between cluster headers [18].  
Figure 1: An example of the movement of a hopping sensor considering movement-error.  
Figure 2: The defined terms for hopping sensor network.  
Unlike hopping sensor relocation protocols, which were only theoretically studied, the paper [19] proposed a relocation protocol suitable for a distributed environment so that it could be applied to a real environment. The protocol proposed in the mentioned paper is briefly summarized using an example in Figure 3. If the cluster zone C is a sensing hole, the protocol strategy to recover the problem is as follows.  
**Step 1**. The cluster header \(\mathrm{H_{C}}\) sends a REQ message requesting one hopping sensor to the relay node R2.  
**Step 2**. The relay node R2 forwards the REQ message received from \(\mathrm{H_{C}}\) to the cluster header \(\mathrm{H_{B}}\) of the cluster zone B.  
**Step 3**. The cluster header \(\mathrm{H_{B}}\) sends a MOVE message for moving to neighbor cluster zone C to the hopping sensor M3 selected as a moveable member in its zone B.  
**Step 4**. At the same time, the cluster header \(\mathrm{H_{B}}\) predicts that its zone could also be a sensing hole, and it sends a REQ message for requesting one hopping sensor to relay node R1.  
**Step 5**. The relay node R1 forwards the message REQ to its another cluster header \(\mathrm{H_{A}}\).  
**Step 6**. The cluster header \(\mathrm{H_{A}}\) chooses M2 among hopping member sensor nodes of its zone A in its zone and commands the movement. As a result, one sensor is properly allocated to each cluster zone, and the sensing zone (cluster zone C) could be recovered.