Wenping Liu

Connectivity-Based Skeleton Extraction in Wireless Sensor Networks

Many sensor network applications are tightly coupled with the geometric environment where the sensor nodes are deployed. The topological skeleton extraction for the topology has shown great impact on the performance of such services as location, routing, and path planning in wireless sensor networks. Nonetheless, current studies focus on using skeleton extraction for various applications in wireless sensor networks. How to achieve a better skeleton extraction has not been thoroughly investigated. There are studies on skeleton extraction from the computer vision community; their centralized algorithms for continuous space, however, are not immediately applicable for the discrete and distributed wireless sensor networks. In this paper, we present a novel Connectivity-bAsed Skeleton Extraction (CASE) algorithm to compute skeleton graph that is robust to noise, and accurate in preservation of the original topology. In addition, CASE is distributed as no centralized operation is required, and is scalable as both its time complexity and its message complexity are linearly proportional to the network size. The skeleton graph is extracted by partitioning the boundary of the sensor network to identify the skeleton points, then generating the skeleton arcs, connecting these arcs, and finally refining the coarse skeleton graph. We believe that CASE has broad applications and present a skeleton-assisted segmentation algorithm as an example. Our evaluation shows that CASE is able to extract a well-connected skeleton graph in the presence of significant noise and shape variations, and outperforms the state-of-the-art algorithms.

Approximate convex decomposition based localization in wireless sensor networks

Accurate localization in wireless sensor networks is the foundation for many applications, such as geographic routing and position-aware data processing. An important research direction for localization is to develop schemes using connectivity information only. These schemes primary apply hop counts to distance estimation. Not surprisingly, they work well only when the network topology has a convex shape. In this paper, we develop a new Localization protocol based on Approximate Convex Decomposition (ACDL). It can calculate the node virtual locations for a large-scale sensor network with arbitrary shapes. The basic idea is to decompose the network into convex subregions. It is not straight-forward, however. We first examine the influential factors on the localization accuracy when the network is concave such as the sharpness of concave angle and the depth of the concave valley. We show that after decomposition, the depth of the concave valley becomes irrelevant. We thus define concavity according to the angle at a concave point, which can reflect the localization error. We then propose ACDL protocol for network localization. It consists of four main steps. First, convex and concave nodes are recognized and network boundaries are segmented. As the sensor network is discrete, we show that it is acceptable to approximately identify the concave nodes to control the localization error. Second, an approximate convex decomposition is conducted. Our convex decomposition requires only local information and we show that it has low message overhead. Third, for each convex subsection of the network, an improved Multi-Dimensional Scaling (MDS) algorithm is proposed to compute a relative location map. Fourth, a fast and low complexity merging algorithm is developed to construct the global location map. Our simulation on several representative networks demonstrated that ACDL has localization error that is 60%-90% smaller as compared with the typical MDS-MAP algorithm and 20%-30% smaller as compared to a recent state-of-the-art localization algorithm CATL.

Distance Transform-Based Skeleton Extraction and Its Applications in Sensor Networks

We study the problem of skeleton extraction for large-scale sensor networks with reliance purely on connectivity information. Existing efforts in this line highly depend on the boundary detection algorithms, which are used to extract accurate boundary nodes. One challenge is that in practical this could limit the applicability of the boundary detection algorithms. For instance, in low node density networks where boundary detection algorithms do not work well, the extracted boundary nodes are often incomplete. This paper brings a new view to skeleton extraction from a distance transform perspective, bridging the distance transform of the network and the incomplete boundaries. As such, we propose a distributed and scalable algorithm for skeleton extraction, called DIST, based on DIStance Transform, while incurring low communication overhead. The proposed algorithm does not require that the boundaries are complete or accurate, which makes the proposed algorithm more practical in applications. First, we compute the distance transform of the network. Specifically, the distance (hop count) of each node to the boundaries of a sensor network is estimated. The node map consisting of the distance values is considered as the distance transform (the distance map). The distance map is then used to identify skeleton nodes. Next, skeleton arcs are generated by controlled flooding within the identified skeleton nodes, thereby connecting these skeleton arcs, to extract a coarse skeleton. Finally, we refine the coarse skeleton by building shortest path trees followed by a prune phase. The obtained skeleton is robust to boundary noise or shape variations. Besides, we present two specific applications that benefit from the extracted skeleton: identifying complete boundaries and shape segmentation. First, with the extracted skeleton using DIST, we propose to identify more boundary nodes to form a meaningful boundary curve. Second, the utilization of the derived skeleton to segment the network into approximately convex pieces has been shown to be effective.

Skeleton Extraction from Incomplete Boundaries in Sensor Networks Based on Distance Transform

We study the problem of skeleton extraction for large-scale sensor networks using only connectivity information. Existing solutions for this problem heavily depend on an algorithm that can accurately detect network boundaries. This dependence may seriously affect the effectiveness of skeleton extraction. For example, in low density networks, boundary detection algorithms normally do not work well, potentially leading to an incorrect skeleton being generated. This paper proposes a novel approach, named DIST, to skeleton extraction from incomplete boundaries using the idea of distance transform, a concept in the computer graphics area. The main contribution is a distributed and low-cost algorithm that produces accurate network skeletons without requiring that the boundaries be complete or tight. The algorithm first establishes the networks distance transform - the hop distance of each node to the networks boundaries. Based on this, some critical skeleton nodes are identified. Next, a set of skeleton arcs are generated by controlled flooding; connecting these skeleton arcs then gives us a coarse skeleton. The algorithm finally refines the coarse skeleton by building shortest path trees, followed by a prune phase. The obtained skeletons are robust to boundary noise and shape variations.

A unified framework for line-like skeleton extraction in 2D/3D sensor networks

In sensor networks, skeleton extraction has emerged as an appealing approach to support many applications such as load-balanced routing and location-free segmentation. While significant advances have been made for 2D cases, so far skeleton extraction for 3D sensor networks has not been thoroughly studied. In this paper, we conduct the first work of a unified framework providing a connectivity-based and distributed solution for line-like skeleton extraction in both 2D and 3D sensor networks. We highlight its practice as: 1) it has linear time/message complexity; 2) it provides reasonable skeleton results when the network has low node density; 3) the obtained skeletons are robust to shape variations, node densities, boundary noise and communication radio model. In addition, to confirm the effectiveness of the line-like skeleton, a 3D routing scheme is derived based on the extracted skeleton, which achieves balanced traffic load, guaranteed delivery, as well as low stretch factor.

An Approximate Convex Decomposition Protocol for Wireless Sensor Network Localization in Arbitrary-Shaped Fields

Accurate localization in wireless sensor networks is the foundation for many applications, such as geographic routing and position-aware data processing. In this paper, we develop a new localization protocol based on approximate convex decomposition (ACDL), with reliance on network connectivity information only. ACDL can calculate the node virtual locations for a large-scale sensor network with a complex shape. We first examine one representative localization algorithm and study the influential factors on the localization accuracy, including the sharpness of the angle at the concave point and the depth of the concave valley. We show that after decomposition, the depth of the concave valley becomes irrelevant. We thus define the concavity according to the angle at a concave point, which reflects the localization error. We then propose ACDL protocol for network localization. It consists of four main steps. First, convex and concave nodes are recognized and network boundaries are segmented. As the sensor network is discrete, we show that it is acceptable to approximately identify the concave nodes to control the localization error. Second, an approximate convex decomposition is conducted. Our convex decomposition requires only local information and we show that it has low message overhead. Third, for each convex section of the network, an improved MDS algorithm is proposed to compute a relative location map. Fourth, a fast and low complexity merging algorithm is developed to construct the global location map. Besides, by slight modification on the third step, we propose a variant of ACDL, denoted by ACDL-Tri, which is fully distributed and scalable while the localization accuracy is still comparable. We finally show the efficiency of ACDL by extensive simulations.

Energy-efficient compressed data aggregation in underwater acoustic sensor networks

Abstract In this paper, we propose an energy-efficient compressed data aggregation framework for three-dimensional underwater acoustic sensor networks (UASNs). The proposed framework consists of two layers, where the goal is to minimize the total energy consumption of transmitting the data sensed by nodes. The lower layer is the compressed sampling layer, where nodes are divided into clusters. Nodes are randomly selected to conduct sampling, and then send the data to the cluster heads through random access channels. The upper layer is the data aggregation layer, where full sampling is adopted. We also develop methods to determine the number of clusters and the probability that a node participates in data sampling. Simulation results show that the proposed framework can effectively reduce the amount of sampling nodes, so as to reduce the total energy consumption of the UASNs.

Surface skeleton extraction and its application for data storage in 3D sensor networks

In-network data storage and retrieval are fundamental functions of sensor networks. Among many proposals, geographical hash table (GHT) is perhaps most appealing as it is very simple yet powerful with low communication cost, where the key is to correctly define the bounding box. It is envisioned that the skeleton has the power to facilitate computing a precise bounding box. In existing works, the focus has been on skeleton extraction algorithms targeting for 2D sensor networks, which usually delivers a 1-manifold skeleton consisting of 1D curves. It faces a set of non-trivial challenges when 3D sensor networks are considered, in order to properly extract the surface skeleton composed of a set of 2-manifolds and possibly 1D curves. In this paper, we study the problem of surface skeleton extraction in 3D sensor networks. We propose a scalable and distributed connectivity-based algorithm to extract the surface skeleton of 3D sensor networks. First, we propose a novel approach to identifying surface skeleton nodes by computing the \textit{extended feature nodes} such that it is robust against boundary noise, etc. We then find the maximal independent set of the identified skeleton nodes and triangulate them to form a compact representation of the 3D sensor network. Furthermore, to react to the dynamics of the sensor networks caused by node failure, insertion, etc., we design an efficient updating scheme to reconstruct the surface skeleton. Finally, we apply the extracted surface skeleton to facilitate the data storage protocol design. Extensive simulations show the robustness of the proposed algorithm to shape variation, node density, node distribution and communication radio model, and its effectiveness for data storage application with respect to load balancing.

A Unified Framework for Line-Like Skeleton Extraction in 2D/3D Sensor Networks

In sensor networks, skeleton extraction has emerged as an appealing approach to support many applications such as load-balanced routing and location-free segmentation. While significant advances have been made for 2D cases, so far skeleton extraction for 3D sensor networks has not been thoroughly studied. In this paper, we conduct the first work of a unified framework providing a connectivity-based and distributed solution for line-like skeleton extraction in both 2D and 3D sensor networks. We highlight its practice as: 1) it has linear time/message complexity; 2) it provides reasonable skeleton results when the network has low node density; 3) the obtained skeletons are robust to shape variations, node densities, boundary noise and communication radio model. In addition, to confirm the effectiveness of the line-like skeleton, a 3D routing scheme is derived based on the extracted skeleton, which achieves balanced traffic load, guaranteed delivery, as well as low stretch factor.

The Extraction and Evaluation of Skeleton in Sensor Networks

In sensor networks community, the skeleton (or medial axis), as an important infrastructure which can correctly capture the topological and geometrical features of the underlying network, has been widely used for facilitating routing, navigation, segmentation, etc. Even though there are a handful of skeleton extraction solutions, the measurement of the goodness of the derived skeleton is often application-oriented, and there is no quantitative metric for this task. In this paper, we study the problem of skeleton extraction and conduct the first work on quantitative evaluation of skeleton in sensor networks. Different from traditional schemes which assume complete or incomplete boundaries, the proposed skeleton extraction algorithm is based on mere connectivity information, without reliance on any boundary information. More specifically, for each node we compute its variability factor based on the neighborhood sizes of the node and its neighbors, which can reflect how central a sensor node is to the network, and a sensor node identifies itself as a skeleton node if its variability factor is locally maximal. Next, we present a light-weight scheme to connect these skeleton nodes. Finally, we proposed a metric, named visibility coefficient, to quantitatively evaluate the derived skeleton.