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Worst and Best-Case Coverage
in Sensor Networks
Seapahn Megerian, Farinaz Koushanfar, Miodrag Potkonjak, and
Mani B. Srivastava, Senior Member, IEEE
Abstract—Wireless ad hoc sensor networks have recently emerged as a premier research topic. They have great long-term economic potential, ability to transform our lives, and pose many new system-building challenges. Sensor networks also pose a number of new conceptual and optimization problems. Here, we address one of the fundamental problems, namely, coverage. Sensor coverage, in general, answers the questions about the quality of service (surveillance) that can be provided by a particular sensor network. We briefly discuss the definition of the coverage problem from several points of view and formally define the worst and best-case coverage in a sensor network. By combining computational geometry and graph theoretic techniques, specifically the Voronoi diagram and graph search algorithms, we establish the main highlight of the paper—an optimal polynomial time worst and average case algorithm for coverage calculation for homogeneous isotropic sensors. We also present several experimental results and analyze potential applications, such as using best and worst-case coverage information as heuristics to deploy sensors to improve coverage.
Index Terms—Sensor networks, coverage, maximal breach, maximal support, best-case coverage, worst-case coverage.
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1 I NTRODUCTION
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S our personal computing era evolves into a ubiquitous computing one, there is a need for a world of fully connected devices with inexpensive wireless networks. Improvements in wireless network technology interfacing with emerging microsensors based on MEMs technology [2] is allowing sophisticated, yet inexpensive, sensing, storage, processing, and communication capabilities to be unobtru- sively embedded into our everyday physical world. More- over, embedded Web servers [1], [3] can be used to connect the physical world of sensors and actuators to the virtual world of information, utilities, and services. Consequently, a flurry of research activity has commenced in the sensor networks domain, especially in wireless ad hoc sensor networks. Although many of the sensor technologies are not new, certain physical and technological barriers of perform- ing wireless communications have limited the feasibility of such devices in the past. Some of the benefits of the newer, more capable sensor nodes are their abilities to form large- scale networks, implement sophisticated protocols, reduce the amount of communication (wireless) required to per- form tasks by distributed and local computations, and implement complex power saving modes of operation
depending on the environment, the application, and the state of the network. Due to the above-mentioned advances in sensor network technology, more and more practical applications of wireless sensor networks continue to emerge. As an example, consider the millions of acres that are lost around the world, due to forest fires every year. In all fires, early warnings are critical in preventing small harmless brush fires from becoming monstrous infernos. By deploying specialized wireless sensor nodes in strategically selected high-risk areas, the detection time for such disasters can be drastically reduced, increasing the likelihood of success in early extinguishing efforts. Also, since the nodes are self- configuring and do not need constant monitoring, the cost of such a deployment may be minimal compared to the huge losses incurred in large blazes. In addition to the new applications, wireless sensor networks provide a viable alternative to several existing technologies. For example, large buildings contain hun- dreds of environmental sensors that are wired to central air conditioning and ventilation systems. The significant wiring costs limit the complexity of current environmental controls and the reconfigurability of these systems. However, replacing the hard-wired monitoring units with ad hoc wireless sensor nodes can improve the quality and energy efficiency of the environmental system while allowing al- most unlimited reconfiguration and customization in the future. In many instances, the savings in the wiring costs alone justify the use of the wireless sensor nodes. One of the fundamental issues that arise naturally in sensor networks is coverage. Due to the large variety of sensors and their applications, sensor coverage is subject to a wide range of interpretations. In general, coverage can be considered as a measure of the quality of service of a sensor network. For instance, in the previous fire detection sensor network example, one may ask how well the network can observe a given area and what the chances are that a fire starting in a specific location will be detected in a given
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. S. Megerian is with the Electrical and Computer Engineering Department, University of Wisconsin at Madison, 3424 Engineering Hall, 1415 Engineering Drive, Madison, WI 53706. E-mail: [email protected]. . F. Koushanfar is with the EECS Department, University of California at Berkeley, 545T Cory hall (DOP Center), Berkeley, CA 94720. E-mail: [email protected]. . M. Potkonjak is with the Computer Science Department, University of California at Los Angeles, 3532G Boelter Hall, Los Angeles, CA 90095- 1596. E-mail: [email protected]. . M.B. Srivastava is with the Electrical Engineering Department, University of California at Los Angeles, MC #951594, 6731-H Boelter Hall, Los Angeles, CA 90095-1594. E-mail: [email protected]. Manuscript received 8 Aug. 2002; revised 7 Dec. 2003; accepted 7 Jan. 2004; published online 1 Dec. 2004. For information on obtaining reprints of this article, please send e-mail to: [email protected], and reference IEEECS Log Number 2-082002. 1536-1233/05/$20.00 ß 2005 IEEE Published by the IEEE CS, CASS, ComSoc, IES, & SPS
time frame. Furthermore, coverage formulations can try to find weak points in a sensor field and suggest future deployment or reconfiguration schemes for improving the overall quality of service. Here, we focus our attention to the isotropic class of sensors, deployed in a field to detect certain activities. An example of such a scenario may be seismic or acoustic sensors deployed in a battle field to detect enemy move- ments. Throughout our discussions, we use the term agent to denote the phenomenon being detected by the sensors (for example, an enemy tank moving in the field). In order to approach the coverage problem here, we formulate the worst and best-case coverage and present algorithms for their calculations. In the worst-case coverage problem, we want to find the closest distance to sensors that an agent traveling on any path in the sensor field must encounter at least once. The main idea here is that the closest distance to sensors is one metric by which sensor coverage of the field can be characterized. The scheme is “worst-case” since we determine the closest distance to sensors even if the agent tries to optimally avoid the sensors. In the analogous best-case coverage problem, we want to find the farthest distance to sensors that an agent traveling on any path in the sensor field must have from sensors, even if it tries to stay as close to sensors as possible. Clearly, at some points, the agent must move away from sensors in order to be able to traverse the field. Although the two problems seem similar (they are duals in some sense), they solve two problems which have very different physical interpretations. From the conceptual and algorithmic point of view, the main contribution is provably optimal polynomial time algorithm for best and worst-case coverage calculation in a sensor network. As we discuss in Section 5, we combine existing computational geometry techniques and constructs such as the Voronoi diagram, with graph theoretical techniques. The use of Voronoi diagram, efficiently and without loss of optimality, transforms the continuous geometric coverage problem into a discrete graph problem. Furthermore, it enables direct application of search techni- ques in the resulting graph representation.
1.1 Organization
The remainder of this article is organized as follows: In the next section, we summarize the related work. In Section 3, we survey several key technologies that are fundamental to our study of coverage. Section 4 contains a brief overview of deterministic sensor deployment and coverage. In Section 5, we present formal definitions of the worst (breach) and best-case (support) coverage and propose optimal poly- nomial-time algorithms for solving each case. Section 6 presents some empirical results followed by a brief discussion of future research directions and the conclusion.
2 RELATED W ORK
The increasing trend in research efforts in the areas referred to as smart spaces or pervasive computing are directly related to many problems in sensor networks. Although many researchers in the sensor network area have men- tioned the critical notion of coverage in the network, to our knowledge this is the first time that an algorithmic approach combined with computational geometry constructs was
adopted in the context of ad hoc sensor networks. Kang and Golay [11] describes a general systematic method for developing an advanced sensor network for monitoring complex systems such as those found in nuclear power plants, but does not present any general coverage algo- rithms. The Art Gallery Problem [12] deals with determining the number of observers necessary to cover an art gallery room such that every point is seen by at least one observer. It has found several applications in many domains such as for optimal antenna placement problems for wireless commu- nication. The Art Gallery problem was solved optimally in 2D and was shown to be NP-hard in the 3D case. Marengoni et al. [12] proposes heuristics for solving the 3D case using Delaunay triangulations. Sensor coverage for detecting global ocean color where sensors observe the distribution and abundance of oceanic phytoplankton is approached in [7] by assembling and merging data from satellites at different orbits. Perhaps the most related works are the attempts on coverage of an initially unknown environment for mobile robots [4], [6]. However, when the geometry of the environment is known in advance, coverage becomes a special case of path planning [10]. Both of these problems are solved using generalized Voronoi diagrams. Radar and sonar coverage also present several related challenges. The radar and sonar netting optimization is of great importance in networking technologies and the optimal distribution of detection and tracking in a surveil- lance area [15]. Based on the measured radar cross sections and the coverage diagrams for different radars, [16] proposes a method for optimally locating the radars to achieve a satisfactory surveillance area with limited radar resources. Coverage studies to maintain connectivity in wireless networks have also been the focus of study. For example, [13] and [14] calculate the optimal number of base stations required to achieve a system operator’s service objectives. When base stations are present, connectivity is achieved through mobile client attachments to a base station. However, the connectivity coverage is more complex in the case of ad hoc wireless networks since the connections are peer-to-peer. Haas [9] shows the improvement in network coverage due to multihop routing features of ad hoc networks and optimizes the coverage constraint subject to a limited path length. In the best and worst-case sensor coverage formulations we present here, the distance of an agent to the closest sensor is of importance while in exposure-based methods presented in [19], the detection probability (observability) in the sensor field is represented by a path dependent integral of multiple sensor intensities. It is interesting to note that in both of these schemes, the types of actions that an agent performs impact the coverage metric. For example, the sensor field may have a different coverage level if an agent is traveling west to east as opposed to north to south, or along any other arbitrary paths.
3 P RELIMINARIES
3.1 Topology of the Network and Sensor Model
Generally, wireless sensor networks are targeted to the extremes of miniaturization, availability, accuracy, reliability, and power savings. This requires a networked infrastructure
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constant describing the overhead per bit. Given this super linear relationship between energy and distance, generally using several short intermediate hops to send a bit is more energy efficient than using one longer hop. However, an incorrect conclusion would be to use an infinite number of hops over the smallest possible distances. In reality, this is impractical for two reasons:
- The number of intermediate hops is limited by the number of nodes between A and B.
- The energy is not limited to the energy radiated through the antenna; there is also energy dissipated in the radio for receiving a bit and readying a bit for retransmission. For applications such as coverage calculations, the energy of computations per node is also a component of the energy metric. It is important to note that technology scaling will gradually reduce the processing costs, with the transmis- sion cost remaining relatively constant. Using compression techniques, one can reduce the number of transmitted bits, thus reducing the cost of transmission at the expense of more computation. This communication-computation trade off is the core idea behind low energy sensor networks. From this discussion, it is apparent that a good algorithm designed for wireless sensor networks will require a minimal amount of communication. This is in sharp contrast with classical distributed systems [18] where the goal generally is maximizing the speed of execution. This renders the classical distributed algorithm irrelevant for developing wireless sensor networks algorithms. The most relevant metrics in wireless networks is power. Experimental measurements indicate that the communica- tion cost in wireless ad hoc networks can be two orders of magnitude higher than computation costs in terms of consumed power [22]. Note that the coverage problem presented in this paper is intrinsically global in the sense that lack of knowledge of location of any node may result in the problem not being solved correctly. Therefore, any algorithm which aims to provide the correct solution must inherently use all location data. Throughout our discussions, we assume a centralized model of computation. Recently, Li et al. [20] proposed a localized approach for solving a variation of the best-case coverage (maximal support) in sensor networks. In addi- tion, a variation of the localized exposure algorithm presented in [21] can be used to solve the worst-case coverage problem locally. However, a detailed treatment of this topic is beyond our scope here.
4 DETERMINISTIC C OVERAGE
In order to achieve deterministic coverage, a static network must be deployed according to a predefined shape. The predefined locations of the sensors can be uniform in different areas of the sensor field or can be weighted to compensate for the more critically monitored areas. An example of a uniform deterministic coverage is the grid- based sensor deployment where nodes are located on the intersection points of a grid. In this case, the problem of coverage of the sensor field reduces to the problem of coverage of one cell and its neighborhood due to the symmetric and periodic deployment scheme.
Examples of weighted predefined deployment are the security sensor systems used in museums. The more valuable exhibit items are equipped with more sensors to maximize the coverage of the monitoring scheme. Another deterministic deployment scheme can be found in the 3D Art Gallery Problem heuristics solutions discussed in [12]. Our proposed coverage algorithm can be used in all predefined (deterministic) deployment schemes to deter- mine the coverage in the sensor field.
5 S TOCHASTIC COVERAGE
In many situations, deterministic deployment is neither feasible nor practical. Another deployment option is to cover the sensor field with sensors randomly distributed in the environment. The stochastic random distribution model can be uniform, Gaussian, or any other distribution based on the application at hand. In the simulation studies for this paper, we have generally assumed uniform sensor distribu- tion, although our algorithm is applicable to any other deployment scheme of the sensor nodes.
5.1 Worst-Case Coverage and Maximal Breach Path
In order to introduce the worst-case coverage problem, we first formally define breach for a path in the sensor field. Given: A field A instrumented with sensors S, where for each sensor s (^) i 2 S, the location ðx (^) i ; y (^) iÞ is known; areas I and F corresponding to initial (I) and final (F ) locations of an agent. Definition: Breach. Given a path P connecting areas I and F , breach is defined as the minimum Euclidean distance from P to any sensor in S.
Thus, the worst-case, breach-based, coverage problem discussed above can formally be stated as: Problem: Maximal Breach Path. Identify a Maximal Breach Path P (^) B, in A, connecting the areas I and F.
The regions I and F are arbitrary regions determined by the task at hand and may be located anywhere inside or outside the sensor field. Theorem 1. At least one Maximal Breach Path must lie on the line segments of the bounded Voronoi diagram formed by the locations of the sensors in S. Proof. Since by construction, the line segments of the Voronoi diagram maximize distance from the closest sites, a Maximal Breach Path PB, must lie on the line segments of the Voronoi diagram corresponding to the sensors in S. If any point p on the path PB deviates from Voronoi line segments, by definition, it must be closer to at least one sensor in S. tu
The following steps outline the algorithm for finding P (^) B:
- Generate Voronoi diagram D for S.
- Apply graph theoretic abstraction by transforming D to a weighted graph.
- Find P (^) B using binary-search and breadth-first- search.
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The first part of this algorithm, detailed in Algorithm 1, generates the Voronoi diagram corresponding to the sensors in S. The weighted, undirected graph G is constructed by creating a node for each vertex and an edge corresponding to each line segment in the Voronoi diagram. Each edge in graph G is assigned a weight equal to its minimum distance from the closest sensor in S. The algorithm then performs a binary search between the smallest and largest edge weights in G. In each step, breadth-first-search (BFS) is used to check the existence of a path from I to F using only edges with weights that are larger than the search criteria called breach_weight. If a path exists, breach_weight is increased to further restrict the edges considered in the next search iteration. If a path is not found, breach_weight is lowered to relax the constraint on the search. Upon completion, the algorithm has found a Maximal Breach Path, which is a path from I to F with its smallest weighted edge being as large as possible.
As an example, consider the random sensor network instance depicted in Fig. 2 with areas I and F shown. Fig. 3 shows the weighted, Voronoi diagram corresponding to this network. Note that each edge in the Voronoi diagram is labeled with its weight in graph G, namely, its closest distance to a sensor. Fig. 4 shows an intermediate step in the search algorithm for finding PB, where the breach_weight threshold is set at 40. Consequently, all edges with weights
less than 40 are ignored in the search and are thus shown as dotted lines. As can be seen, a path can be found from I to F in this case, indicating that the search threshold should be increased. Finally, Fig. 5 shows the result at the termination of the algorithm. The optimal breach_weight has been found to be 57. The critical edge is marked in bold and the Maximal Breach Path connecting I and F can be seen as a bold dotted line.
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Fig. 2. A random sensor network instance.
Fig. 3. Weighted Voronoi diagram of the sensor network in Fig. 2.
Fig. 4. Maximal Breach Path algorithm iteration: Edges with weights less than 40 are ignored.
any path through the field A, from I to F , must encounter at least once. If additional sensors can be deployed or existing sensors moved such that support_weight is decreased, then the best-case coverage is improved.
5.3 Complexity
Given n sensors, the best known algorithms for the generation of the Voronoi diagram have Oðn log nÞ com- plexities. In 2D, Voronoi diagrams are essentially linear complexity in terms of vertices and edges. So, for n points, jV j and jEj (vertices and edges) in the Voronoi graph are both OðnÞ. So, the resulting graph used later in the search phase of the algorithm is OðnÞ in terms of the edges. Thus, the BFS and binary search phase has a complexity of Oðn log rangeÞ, where range is the difference between highest and lowest weighted edge in the Voronoi graph. In practice, the complexity of the algorithm is dominated by the Voronoi diagram generation procedure which has a large constant factor in its complexity.
6 EXPERIMENTAL RESULTS
6.1 Experimentation Platform
The coverage algorithms presented here have been im- plemented and used in several studies as stand-alone C packages. In this section, we present several results and try to provide an overview and analysis of the applications. Fig. 7 shows an instance of the coverage problem where 30 sensors are deployed at random. The Maximal Breach
Path (PB) and the corresponding edge with breach_weight depicts where the breach takes place in the field. The Maximal Support Path (PS ) and the corresponding edge with support_weight are also shown. Fig. 8 shows the underlying bounded Voronoi diagram for the same problem instance depicted in Fig. 7. Extra edges with 0 weight are used to connect the I and F regions to the structure so that all possible paths can be considered in the search algorithm. The 0-weight edges are drawn between all points where Voronoi edges intersect the boundary of the field and the corresponding point (I or F ). Fig. 9 shows the corresponding Delaunay triangulation. In this case, only two extra edges are introduced to connect I and F to the closest sensors in the structure.
6.2 Sensor Deployment Heuristics
The edges corresponding to breach_weight described in Section 5 can be used as a guide for future sensor deployments. Since breach_weight corresponds to the edge in the breach path where PB is closest to the sensors, deploying additional sensors along that edge can be used as a heuristic to improve overall coverage. Fig. 10 shows the average improvement in breach coverage when up to four additional sensors are introduced succes- sively in the network, according to the heuristic described above. Note that after each additional sensor deployment, the algorithm was repeated to find the new breach region. The results represent average improvements over 100 random
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Fig. 7. Sensor field with Maximal Breach Path (PB) and Maximal Support Path (PS).
Fig. 8. Sensor field with Voronoi Diagram and a Maximal Breach Path.
Fig. 9. Sensor field with Delaunay triangulation and a Maximal Support Path (PS).
Fig. 10. Average worst-case coverage (breach) improvement by additional sensor deployments.
deployments. It is interesting that, even after deploying 100 sensors, breach coverage can be improved by about 10 percent by deploying just one more sensor. Similarly, the support_weight and the midpoint of the corresponding edge in the Delaunay triangulation can be used as a heuristic for deploying additional sensors to improve support coverage. As shown in Fig. 11, on average, a 50 percent improvement can be achieved in support coverage by adding one additional sensor when five nodes have already been randomly deployed. After deploying 100 random sensors, on average, a 10 percent support coverage improvement can be expected by using the heuristic to deploy one more sensor.
6.3 Stochastic Deployment—Asymptotic Behavior
The graph in Fig. 12 shows how the coverage of randomly placed (uniform) sensor nodes in a field varies as the number of sensors is changed. The results shown in the graph represent the average field breach and support coverages for 1,000 random sensor placements. For each placement, two uniform random variables X and Y are used to determine the coordinates ðx (^) i ; y (^) iÞ of each sensor si in a unit square field. Fig. 12 demonstrates the asymptotic nature of these metrics from the sensor field operator’s point of view who wants to minimize breach and maximize support (minimize support_weight). Thus, for clarity, the figure shows a normalized plot of breach_weight (values closer to 0 preferred) and 1-support_weight (values closer to 1 preferred) as a function of the number of sensors. Given the unit square field and using the distance-based sensor model described earlier, on average, after deploying about 100 sensors, additional random sensors do not
improve coverage very significantly. This asymptotic nature of breach and support coverage suggests that by analyzing a given field and selecting the proper number of sensor nodes, certain levels of coverage can be expected even if sensor deployment cannot be performed according to an exact plan.
7 FUTURE R ESEARCH DIRECTIONS
Although our algorithm was developed for a wireless ad hoc sensor network, we have assumed a centralized computation model. A natural course of study would be to compare the centralized coverage algorithm to localized ones in terms of power consumption, cost, performance, and accuracy. Here, we have assumed identical sensor sensitivity models where the coverage depends only on the Euclidean distances from sensors. In practice, other factors influence coverage such as obstacles, environmental condi- tions, and noise. In addition to nonhomogeneous sensors, other possible sensor models can deal with nonisotropic sensor sensitivities, where sensors have different sensitiv- ities in different directions. The integration of multiple types of sensors such as seismic, acoustic, optical, etc., in one network platform and the study of the overall coverage of the system also presents several interesting challenges. In addition, the general problem of where to deploy additional sensors to improve the coverage remains open. Although we have introduced heuristics based on this coverage model that may perform well for single-sensor deployment, it is interesting to investigate methods of optimally deploying multiple sensors at a time.
8 CONCLUSION
We presented best and worst-case formulations for isotropic sensor coverage in wireless ad hoc sensor networks. An optimal polynomial time algorithm that uses graph theore- tic and computational geometry constructs was proposed for solving for best and worst-case coverages. Experimental results show some applications of the theoretic coverage formulations and algorithms specifically for solving for Maximal Breach (worst-case coverage), Maximal Support (best-case coverage), additional sensor deployment heur- istics to improve coverage, and stochastic field coverage.
ACKNOWLEDGMENT
The authors would like to thank John Agre at Rockwell Science Center for providing the initial insights regarding Voronoi diagrams and coverage in sensor networks. His comments were key enabling factors that lead to the development of the optimal breach and support path-based coverage algorithms. This material is based upon work partially supported by the US National Science Foundation under Grant No. NI-0085773 and the US Defense Advanced Research Projects Agency and Air Force Research Labora- tory under Contract No. F30602-99-C-0128.
REFERENCES
[1] D. Tennenhouse, “Proactive Computing,” Comm. ACM, vol. 43, pp. 43-50, May 2000.
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Fig. 12. Normalized worst-case (breach_weight) and best-case (1-support_weight) coverage as a function of the number of sensors.
Fig. 11. Average best-case coverage (support) improvement by additional sensor deployments.
