網路的生命週期長短是決定無線感測器網路好壞的重要因素之一。本文提出
新的佈置節點方式來提升網路的生命週期。首先考慮一個比網路要覆蓋的目標區
域小很多的三角形，把三個或四個節點佈置在三角形的頂點與中心，調整這些節
點的感測範圍使得該三角形區域之覆蓋密度降到最低，接著再使用這些三角形區
域有效率的覆蓋整個網路，可減少感測目標所需之能源。因為佈置在三角形中心
的節點感測範圍較小，所消耗感測能源也少很多，中心的節點可作為中繼節點
（relay node）幫忙傳遞資料，減低其他三個節點的負擔。靠近匯集點（sink node）傳遞資料量多的區域就用這些中心有節點的三角形區域佈置成正六角形
（hexagonal deployment），而遠離匯集點的區域節點密度較小。既有的節點佈置策略（Distance-Based Energy-Efficient Placement Strategy）並沒有節點可調整感測範圍的機制，本篇論文既維持同樣的覆蓋範圍，又改進計算節點佈置距離的方法，使得節點縮小其感測範圍，同時平衡不同位置節點的負載。
實驗結果證實，本文提出的節點佈置比既有的節點佈置策略更能達到提升網
路生命週期的目的。

Network lifetime is one of the key characteristics for estimating wireless sensor networks (WSNs). This thesis proposed a density-based hexagonal node deployment for improving network lifetime of WSNs. First, divide the large target area into small triangular areas, if each small triangular area is fully covered by three nodes or four nodes, then the whole target area is covered. Nodes can be placed on the vertices (or centers) of those triangular areas. These nodes adjust their sensing ranges according to coverage constraint to minimize the coverage density of triangular areas. In a 4-node tile, node at center has a sensing range much smaller than sensing ranges of nodes at vertices. So node at center consumes less sensing energy. They can be assigned as a relay node to reduce the transmission energy consumption of other nodes. Thus 4-node tiles should be placed near the sink node since the transmission loading of nodes near to sink node is heavier. Because the existing Distance-Based Energy-Efficient Placement (DBEEP strategy) only reduces transmission energy consumption but sensing energy consumption was not taken into consideration, in this thesis, a new distance-based approach is also proposed. Simulation results show that proposed density-based hexagonal node deployment can achieve better lifetime than distance-based approach in low-communication loading scenario and high communication loading scenario.

[1] I. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cayirci, “Wireless Sensor Networks: A Survey,” Computer Networks 38, pp. 393-422, Dec. 2002.
[2] L. L. Peterson and B. S. Davie, Computer Networks A Systems Approach, 4th ed. Morgan Kaufmann, 2007.
[3] G. Xing, T. Wang, W. Jia, and M. Li, “Rendezvous Design Algorithms for Wireless Sensor Networks with a Mobile Base Station,” in ACM International Symposium on Mobile Ad Hoc Networking and Computing, May 2008.
[4] I. Dietrich and F. Dressler, “On the Lifetime of Wireless Sensor Networks,” in ACM Transactions on Sensor Networks, vol. 5, no. 1, article 5, Feb. 2009.
[5] K. Kweon, H. Ghim, J. Hong, and H. Yoon, “Grid-Based Energy-Efficient Routing from Multiple Sources to Multiple Mobile Sinks in Wireless Sensor Networks,” in IEEE International Symposium on Wireless Pervasive Computing, pp. 1-5, Mar. 2009.
[6] J. Eriksson, M. Faloutsos, and S. V. Krishnamurthy, “DART: Dynamic Address RouTing for Scalable Ad Hoc and Mesh Networks,” in IEEE Transactions on Networking, vol. 15, no. 1, pp. 119-132, Feb. 2007.
[7] H. Ling and T. Znati, “Energy Efficient Adaptive Sensing for Dynamic Coverage in Wireless Sensor Networks,” in IEEE Wireless Communications and Networking Conference, pp. 1-6, Apr. 2009.
[8] K. Miyao, H. Nakayama, N. Ansari, and N. Kato, “LTRT: An Efficient and Reliable Topology Control Algorithm for Ad-Hoc Networks,” in IEEE Transactions on Wireless Communications, vol. 8, no. 12, pp. 6050-6058, Dec. 2009.
[9] S. S. Dhillon and K. Chakrabarty, “Sensor Placement for Effective Coverage and Surveillance in Distributed Sensor Networks,” in IEEE Wireless Communications and Networking Conference, vol. 3, pp. 1609-1614, May 2003.
[10] M. B. Abbasy, G. Barrantes, and G. Marin, “Performance Analysis of Sensor Placement Strategies on a Wireless Sensor Network,” in IEEE International Conference on Sensor Technologies and Applications, pp. 609-617, Aug. 2010.
[11] R. Lijie, G. Zhongwen, and M. Renzhong, “Distance-based Energy Efficient Placement in Wireless Sensor Networks,” in IEEE Conference on Industrial Electronics and Applications, pp. 2031-2035, Aug. 2008.
[12] Y. Mao and M. Wu, “Coordinated Sensor Deployment for Improving Secure Communications and Sensing Coverage,” in ACM Workshop on Security of Ad Hoc and Sensor Networks, Nov. 2005.
[13] H. Zhang and J. C. Hou, “Maintaining Sensing Coverage and Connectivity in Large Sensor Networks,” in International Journal of Wireless Ad Hoc and Sensor Networks, vol. 1, no. 1-2, pp. 89-123, Jan. 2005.
[14] N. D. Nguyen, V. Zalyubovskiy, M. T. Ha, and H. Choo, “Energy-efficient Models for Coverage Problem using Sensors with Adjustable Sensing Ranges,” in IEEE Wireless Communications and Networking Conference, pp. 1-6, July 2010.
[15] J. Wu and S. Yang, “Energy-efficient Node Scheduling Models in Sensor Networks with Adjustable Ranges,” in World Scientific International Journal of Foundations of Computer Science, vol. 16, pp. 3-17, Nov. 2005.