# Energy-Efficient Wireless Hopping Sensor Relocation Based on Prediction of Terrain Conditions
## 3 The Proposed Relocation Protocol for Rugged Terrains
### Descriptions of the Proposed Protocol  
So far, most previous studies on hopping sensor relocation are central-based manner which each cluster header know all network information, but this is not a very practical study. The research of [19] only conducted a distributed-based approach to consider the realistic environment, but it still failed to take into account the surrounding obstacle information. That is, it is important to consider the environment using hopping sensors, not using the wheel-based ones. The following examples of simple scenarios in Figure 5 are described to understand the difference between cases reflected the surrounding obstacle information or not. In Figure 4(a), we assume that the cluster zone B has to maintain five member sensor nodes, except for the cluster header H\({}_{\text{B}}\), for data collection. The cluster header determines that the sensing hole occurs when the number of members is smaller than 5. Here, the cluster zone B was a sensing hole due to a lack of one member (i.e., before moving M1). In order to recover the sensing hole, after the cluster header H\({}_{\text{B}}\) of zone B requests one member from zone A (the first REQ), the member sensor node M1 of zone A moves to zone B. In the first scenario, let us consider the relocation of hopping sensors without taking into account obstacles as mentioned previous literatures. In Figure 4(b), two member sensor nodes fail due to energy  
Figure 5: Examples of comparison between the previous and proposed relocations. exhausted in the cluster zone B. The cluster header H\({}_{\text{B}}\) is aware of the state of sensing hole, and it requests two members from the neighbor cluster zone A (the second REQ). Two members, which were ordered to move to the cluster zone B, are moving as shown in Figure 5c. One member M2 is successfully moved to the cluster zone B, while the other M3 is failed to move due to obstacle. The cluster header of the cluster zone B perceives the state of sensing hole again, and it requests one member from the cluster zone A (the third REQ). In Figure 5d, since the selected member M4 fails to move because of unexpected obstacle, the cluster zone B is still a sensing hole. In order to overcome the sensing hole, the cluster header H\({}_{\text{B}}\) requests one member from the cluster zone A repeatedly (the fourth REQ). The member M5 is successfully moved, and the sensing hole is finally recovered. In the second scenario, let us take into account the relocation based on probabilities of existence of obstacles. In Figure 5b, the cluster header H\({}_{\text{B}}\) should recognize the state of the sensing hole and request some members from the neighbor cluster zone A. First of all, let the initial value of the movement success rate \(p\) be 1. The success rate \(p\) is used to calculate the requested number of member sensors. According to Equation (2), the number of the requested members is 2, Ceiling[\((5-3)*(1+(1-1))\)], the cluster header H\({}_{\text{B}}\) requests two members from the cluster zone A (the second REQ). In Figure 5c, the cluster header H\({}_{\text{A}}\) commands two members to move to the cluster zone B. As described in the above first scenario, one member M2 moves successfully, but the other member M3 fails to move due to the obstacle. Thus, the cluster header H\({}_{\text{B}}\) updates the value of \(p\) as 1/2, and it is also aware of the sensing hole. The cluster H\({}_{\text{B}}\) calculates the number of requesting members using Equation (2), Ceiling[\((5-4)*(1+(1-1/2))\)], so two members are requested to the cluster H\({}_{\text{A}}\) (the third REQ). One of the two members of A, the member M3 fails to move to the zone B, as in the first previous scenario. However, the other member M2 can successfully move to the zone B to overcome the state of sensing hole. As we are looking at the mentioned two scenarios, it might be thought that the probability of successful movement represents the state of obstacles between cluster zones.