||A Decentralized Temporal Coverage Maintenance Protocol and Obstacle-Resist Routing and Localization Algorithms in WSNs
||Master's Program in Networking and Communications, Department of Computer Science and Information Engineering
||無線感測網路已廣泛應用於環境監控、軍事及醫療監控。根據其不同的應用與場景，在研究上急待解決的問題包括覆蓋、繞徑、定位等研究議題。位置資訊的利用在過去許多國內外的研究中一直被高度重視，位置資訊的取得與否，將影響演算法發展的難易度及效率。本論文以位置資訊的獲得程度將研究主題區分為兩大部份：(1) 在精確位置資訊的條件下，探討克障繞徑以及時間覆蓋的議題。(2) 在不精確的位置資訊的環境下，開發克障且電量平衡的繞徑協定。
在第一部份的研究主題上，本論文假設每個感測器均具有精準的位置資訊，並據此提出一新的繞徑演算法，稱為WRGP。其改善了環境中障礙物對Greedy Forwarding Routing所造成的不良影響。在運作之初，WRGP會先找出包圍障礙物四周的節點，我們稱這些節點為Border Node。隨後，位於障礙物凹洞區的節點便開始進行權重值更動的工作，利用權重值的更動，這些Border Nodes便能夠建立一禁閉區以防止封包進入凹洞區。最後，WRGP選出一些具有特殊地位的Border Nodes成為Effective Border Nodes，這些Effective Border Nodes主要用於建立一條自身至Sink的最佳封包傳送路徑。本論文所提出的WRGP不儘降低了網路初始化的通訊成本，也可利用Effective Border Nodes來導引封包至最佳的傳送路徑。此外，本論文亦提出一繞徑協定，稱為M-WRGP，以克服環境中具有多障礙物並提昇繞徑效能。
||Wireless sensor networks (WSNs) have been widely applied on applications such as environment monitoring, military, and remote medical system. In the past few years, research issues including coverage, routing and localization have received much attention and required more efforts to improve the performance of existing work. The location information is a critical factor that it determines the difficulty of the algorithm design and significantly impacts their performance. Based on the different levels of obtained location information, the research issues proposed in this thesis can be categorized into two parts: (1) The issues of obstacle-resist routing protocol and temporal coverage with accurate location information; (2) The obstacle-resist and energy-balanced routing protocol with inaccurate location information.
In the first part, this thesis proposes a novel mechanism, called WRGP, which removes the impact of obstacles on the greedy forwarding routing. The proposed WRGP initially identifies the border nodes that surround the obstacle. The border nodes in the concave region of the obstacle then initiate the weight assigning process aiming at establishing a forbidden region to prevent the packets from entering the concave region. Finally the WRGP identifies some border nodes to act as the effective border nodes for constructing the optimal routes from themselves to the sink node. The proposed WRGP reduces the control overheads and guides the packets moving along the shortest path from the encountered effective border node to the sink node. In addition, the M-WRGP is further developed to cope with the multi-obstacle problem.
In addition to the routing issue, the coverage problem is one of the most important issues in WSNs and has been widely discussed in last decade. However, the spatial full-coverage only can be achieved when the surplus mobile sensors contribute a larger coverage area than the hole size. When there is no surplus mobile sensor to cover the hole, the mobile sensor moving the hole from one location to another can achieve the purpose of temporal full-coverage. However, if only some mobile sensors participate in the hole-movement task, the WSN will be energy-unbalanced. This thesis further considers a mobile WSN that contains holes but no redundant mobile sensor exists to heal the hole. To achieve the temporal full-coverage purpose, a distributed hole-movement mechanism is proposed aiming to balance the energy consumptions of mobile sensors.
The second part of this thesis focuses on the routing issue based on the network environment where each sensor has inaccurate location information. Constructing a best route is a common goal of existing researches on the routing issue. In a WSN, the best route is the shortest path that minimizes the end-to-end delay from source to destination. However, the sensor nodes that participate in the best route might consume more energy to deliver the packet than other sensor nodes. On the other hand, the inaccurate location information and the existence of obstacles can significantly drop the performances of existing routing protocols. This thesis proposes a novel algorithm, called JLR, which takes both the energy balancing and obstacle resistance issues into consideration under the network environment where all sensors have inaccurate location information. Initially, a robot localization approach based on the directional-antenna system is proposed to offer the inaccurate location information to each sensor node. Afterward, the sensor nodes that are nearby the obstacles cooperatively establish the forbidden regions to prevent the packet from entering the concave regions. Compared with the well known GPSR routing protocol, the JLR expects to improve the performance in terms of packet delivery ratio and network lifetime.
||Table of Contents
Table of Contents I
List of Figures III
List of Tables VIII
1. Introduction 1
1.1 ROUTE WITH THE ACCURATE LOCATION INFORMATION 1
1.2 COVERAGE 3
1.3 ROUTE WITH THE INACCURATE LOCATION INFORMATION 6
2. Related Work 9
2.1 ROUTE WITH THE ACCURATE LOCATION INFORMATION 9
2.2 COVERAGE 13
2.3 ROUTE WITH THE INACCURATE LOCATION INFORMATION 15
2.3.1 Obstacle Resistance 15
2.3.2 Energy Balancing 17
2.3.3 JLR 18
3. The Proposed WRGP, EBHMM and JLR 19
3.1 WRGP 19
3.1.1 Network Environment and Problem Statement 19
3.1.2 Basic Concepts of the WRGP 20
3.1.3 The WRGP Protocol 21
3.1.4 Multi-Obstacles 37
3.1.5. Analysis of the WRGP 41
3.1.6 Performance Study of the WRGP 43
3.2 EBHMM 55
3.2.1 Network Environment and Problem Statement 55
3.2.2 The Proposed Temporal Full-Coverage Mechanism 57
3.2.3 Intra-Cell and Inter-Cell Hole-Healing Algorithms 69
3.2.4 Performance Study of the EBHMM 71
3.3 JLR 76
3.3.1 Directional-Antenna Based Localization 77
3.3.2 The JLR Routing Protocol 78
18.104.22.168 Energy Balancing 79
22.214.171.124 Obstacle Resistance 80
3.3.3 Performance Study of the JLR 87
4. Conclusion 92
List of Figures
Figure 1. An example of ping-pong effect in . The number in each node represents the weight cost of that node. Node a changes its weight cost twice as shown from (a) to (d). 10
Figure 2. The WRGP constructs an efficient route R1 while PAGER  and construct an inefficient route R2. 12
Figure 3. (a) The WRGP selects border nodes in a distributed manner in Phase I. (b) The WRGP determines the effective concave region A in Phase II. (c)The WRGP constructs the Forbidden region in Phase III. (d) In Phase IV, each effective border node derives the best route from itself to the sink node. 20
Figure 4. A failure of the proposed hole-detection rule of study  since all intra-angles are smaller than 120o. 22
Figure 5. A network environment with an obstacle. 22
Figure 6. (a) The node s, ni, ni+1 should not play the border role since the small hole did not block the communication of any pair of these three nodes (b) An example that there exists a physical obstacle which blocks the communication of node ni and ni+1. Node s is located nearby the obstacle and therefore should play a border role. 25
Figure 7. (a) The physical obstacle blocks the communication between ni and ni+1 even though their distance is smaller than the communication range. (b) Node s will not serve as a border node since the voronoi neighbors of node s separate obstacle and node s. 26
Figure 8. An example of applying the Self Pruning Rule. 27
Figure 9. An example for illustrating the farthest, critical, and effective border nodes. 31
Figure 10. A special case for identification of cs and cn. 31
Figure 11. An example to illustrate the case operations for the CE packets sent from the critical border node cs. 36
Figure 12. An example to illustrate that S-WRGP results in an inefficient route due to the existence of multiple obstacles. 36
Figure 13. The new obstacle exploitation and the optimal path construction designed in M-WRGP. 38
Figure 14. The monitoring region is partitioned into n hexagonal cells. The communication circle of sensor a intersects with the hexagon H whose edge passing through k+1 hexagonal cells. 41
Figure 15. The shapes of the obstacles in our simulation environment. 44
Figure 16. Comparison of PAGER and S-WRGP in terms of the control overhead in the environment shown in Fig. 15(a). 46
Figure 17. The environments with single obstacle and multiple obstacles used to examine the effect on routing length. 47
Figure 18. Comparison of the accumulated path length between PAGER and S-WRGP under various numbers of the neighboring nodes in Fig. 17(a). 48
Figure 19. Comparison of the path length between PAGER and S-WRGP to the source located in sub-area 9 in Fig. 17(a). 49
Figure 20. Comparison of the accumulated path length between PAGER, S-WRGP and M-WRGP under various numbers of the neighboring nodes in Fig. 17(b). 49
Figure 21. Comparison of PAGER and M-WRGP in terms of accumulated path length. The source location is varied at various sub-areas as shown in Fig. 17(b). 49
Figure 22. Simulation environment that changes the distance of two obstacles for measuring the performance improvement of M-WRGP against the PAGER and S-WRGP in terms of accumulated routing length. 50
Figure 23. Performance comparison of PAGER, S-WRGP and M-WRGP in terms of accumulated path length by varying the distance of two obstacles ranging from 100m to 233m.. 51
Figure 24. Five distributions of two obstacles for measuring their impacts on routing path. 52
Figure 25. The impact of obstacle distributions on the route efficiency. The obstacle distributions are shown in Fig. 24. Five distributions of two obstacles are compared. M-WRGP significantly outperforms PAGER and S-WRGP in terms of accumulated path length when the obstacle distributions are (a), (c), and (e). 53
Figure 26. The obstacle distributions with various numbers of obstacles. 53
Figure 27. Performance study of accumulated path length by varying the number of obstacles. The proposed M-WRGP significantly outperforms PAGER and S-WRGP in all cases. 54
Figure 28. The working scenario of M-WRGP. 55
Figure 29. The state diagram of the proposed mechanism applied in each mobile sensor. 57
Figure 30. (a) An example of the partitioned network. The white cell denotes the hole cell. The arrows denote the hole trajectory. (b) Width compression for constructing a Hamiltonian Circuit path. 60
Figure 31. The relationship between coverage disks and hexagonal cell. 62
Figure 32. An example for choosing the best Inter-Cell Hole-Healer. 65
Figure 33. The algorithms of Intra-Cell and Inter-Cell Hole-Healing. 70
Figure 34. The network lifetime of compared three mechanisms in td =1500 and td =2100. 72
Figure 35. The comparison of EBHMM and ADA in terms of the event detection ratio for 10 days starting from the day that the compared temporal full-coverage mechanism is applied. 73
Figure 36. The comparison of EBHMM, ADA and TCSC in terms of average energy difference by varying the number of deployed sensor nodes. 74
Figure 37. The comparison of EBHMM, ADA and TCSC in terms of average energy difference by varying the value of . 74
Figure 38. The comparison of EBHMM, ADA and TCSC in terms of MCI by varying the number of sensor nodes. 75
Figure 39. The beacon node b with four directional antennas partitions its transmission range into four regions R1, R2, R3 and R4. The gray area in the area that outsides the transmission range but belongs to the bounding box of beacon node b. 78
Figure 40. A monitoring region with forbidden and permitted cells. 79
Figure 41. An example of the destination-location determination of the dead end. 83
Figure 42. An example of the packet delivery from permitted cell to forbidden cell. 85
Figure 43. The state diagram of the proposed JLR. 87
Figure 44. The comparison of the GPSR, LEBR and JLR in terms of EBI. 88
Figure 45. The comparison of three routing protocols in terms of the packet delivery ratio. 89
Figure 46. The comparison of three routing protocols in terms of EI. 89
Figure 47. The scenarios of three routing protocols in terms of the traffic load. 90
List of Tables
Table 1. The performances of S-WRGP examined in various shapes of the obstacle of 46
Table 2. The Abbreviations of Compared Mechanisms 71
Table 3. Simulation Parameters 71
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