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1612 JOURNAL OF COMPUTERS, VOL. 8, NO. 6, JUNE 2013 Figure 1. Supposed that sensor i has a neighbor sensor j. S i and S j denote the circle sensing area covered by node i and j respectively. d ij is the distance between node i and j. And S denotes the sensing area that is covered by node i i∩ j and j, as shown in Figure 1. Refer to [22], we can get that: ⎧ 2 d 2 ij dij ⎪2rs arccos −dijrs 1− d ≤2 2 ij rs Si∩ j = (2) ⎨ 2rs 4rs ⎪⎩ 0 otherwise So from formula (2), we can get that when the distance between node i and j is less than or equal to 0.5r, the redundant coverage area S i∩ j is more than about 68.5% of S i . When the distance of node i and node j is more than 1.75r, the area S i∩ j is very small, about 0.052 S i. These results can be used in our nodes scheduling. If d ij ≥1.75r, the effects that node i to node j will be ignored in this paper. If θ is the percentage of the redundant area covered by all the neighbors of node i. Refer to paper [22, 23], θ can be expressed as ∪ θ = S ∩ S S = Si = 1 − m Si j (1 − ) S − S S S i∩j j i j∈N() i i N() i j= 1 i i ∏ ∩ (3) S N() i is the area that covered by sensor i but not covered by its neighbors. Then, if node i has a neighbor node k and d ik ≤0.5r. Based on formula (2) and (3), the θ of node i can be expressed as m Si j θ ≥1− 0.32 • ∏(1 − ∩ ) (4) S j= 1 j≠k Suppose node j is a neighbor node of node i. Based on the above definition, the distance between node i and j satisfy the condition: 0
JOURNAL OF COMPUTERS, VOL. 8, NO. 6, JUNE 2013 1613 the condition to sleep, it enters the pre-sleep state with a random short time T w. If the node received other sensor’s sleep-message at the pre-sleep state, it will return the active state. Otherwise, it broadcasts itself sleep-message after waiting T w time and then goes to sleep state; fall asleep for a period of time Ts. Based on the classic LEACH cluster protocol, time is divided into fixed-length time periods called rounds. Each round begins with a competition phase, in which every node determines whether it can be active or sleep. Then those active sensors enter into clustering and sensing. We detail the steps as follows. Step1: Networks initialization. We assumed that all sensors are active initially. Each sensor broadcasts messages to estimate the distance between itself and its every neighbor and then record these information. According to the QoS demand (the coverage rate θ) of network, sink broadcasts the system message including the two parameters HT and AT. Where HT is the minimum number of active neighbors with one half-hop neighbor and AT is the minimum number of neighbor nodes that have no half-hop neighbor. For example, the network coverage (θ) is required to 85%. According to tableⅡ, we can set HT = 5 and AT = 8. While the coverage rateθis more than 90%, we can set HT = 6 and AT = 9. Start cluster heads broadcast hello messages and other active nodes select the closest head to join. Step 4: Sensing. Step 5: The current round end and return step 2. IV. SIMULATION RESULTS We focus on the construction of one cover and assume that 1000 nodes are deployed randomly in a 100 meter×100 meter square. Each sensor has a sensing range of 15 meters. The transmitting, receiving (idling), and sleeping power consumption ratio is 20:4:0.01[21]. We conducted simulations with matlab simulator for comparing among two sleep scheduling methods: LDAS and our proposed scheme (ECBS). A. Coverage Effectiveness Set θ≥90%. Run LDAS and ECBS at the same condition to compare. We sampled on the No.100 round respectively as shown in Figure 4 (only active sensors are marked to see clearly). Only 58 nodes are active in our algorithm, but 150 nodes are on-duty by LDAS algorithm. And Figure 5 shows the coverage condition with the active nodes on No.100 round by different algorithm. It can be easy to see that the fewer numbers of active nodes are needed in our algorithm to meet the same coverage required and the sensors distribute more uniform in Figure 4(b). However, there is more redundancy coverage in Figure 5(a). Ni>HT N Keep active Y Ndi>0 N Y Ni>AT N The maximal residual energy of sensor in ND > Ei N Y Sensor i sleep Y Sensor j with the minimum residual energy in ND sleep 100 90 (a) LDAS 80 End 70 60 Figure 3 The scheduling process of an active sensor i Step 2: Nodes-scheduling. At the beginning of each round, each active node determines whether it is a redundancy sensor or not. The scheduling scheme is detailed in Figure 3. Where N i is the number of sensor i ’s active neighbors, and N di is the number of sensor i’s half-hop neighbors, ND is the set of sensor i’s half-hop neighbors, E i is the residual energy of sensor i. Step 3: Clustering. Active nodes randomly select nodes as cluster heads based on LEACH algorithm. Then the 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 (b) ECBS Figure 4 The distribution of active nodes on No.100 round © 2013 ACADEMY PUBLISHER
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Journal of Computers ISSN 1796-203X
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Corn Moisture Measurement using a C
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Page 261: Call for Papers and Special Issues
Page 264: (Contents Continued from Back Cover
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