Waleed Alsalih

Placement of multiple mobile data collectors in wireless sensor networks

A major challenge affecting the lifetime of Wireless Sensor Networks (WSNs) comes from the unbalanced energy consumption over different parts of the network. This unbalanced energy consumption is a direct result of having a stationary sink: nodes near the sink are intensively used to relay data for other nodes to the sink. A natural solution to such a problem is to have multiple mobile sink nodes (which we call data collectors), and to change their locations periodically so that the load is distributed evenly among all sensor nodes. In this paper we propose a mobile data collector placement scheme for extending the lifetime of the network. In our scheme the lifetime of the network is divided into rounds and data collectors are moved to new locations at the beginning of each round. While previous work has focused on placing data collectors at predefined spots (e.g., the work in Gandham et al. (2003) [1]) or at the boundary of the network (e.g., the work in Azad and Chockalingam (2006) [2]), we define and solve two problems which are more general: the on-track placement where data collectors can be placed only along predefined tracks (roads) spanning the sensing field, and the general placement where data collectors may be placed at any point in the sensing field. We formulate the problems as Mixed Integer Linear Programs (MILPs) and use a MILP solver (with a constant time limit) to find near-optimal placements of the data collectors and to find routing paths to deliver data to data collectors. Our experiments show that our schemes make significant extension to the lifetime of the network.

Distributed voronoi diagram computation in wireless sensor networks

We present a distributed algorithm to compute the Voronoi Diagram (VD) of a wireless network in the plane. Our contribution is twofold: the complete VD is computed, as opposed to the approximate or the bounded VD, and the number of transmissions used is relatively small.

Energy-aware task scheduling: towards enabling mobile computing over MANETs

In this paper, we look into the problem of distributing computational tasks amongst a set of mobile computing devices in a mobile wireless ad hoc network (MANET) in such a way that conserves energy and improves performance. In such a distributed environment, the assignment of computational tasks to different devices and the order of their execution play a vital role in energy conservation and performance improvement. The main contributions of this paper are formulating a novel energy-aware scheduling problem and proposing a heuristic algorithm to solve it. Our scheduling algorithm schedules a set of computational tasks, which may have dependencies and communication, into a set of heterogeneous processors in such a way that minimizes both the total consumed energy and the makespan (i.e., the time by which all tasks complete their execution). Experiments show that significant improvement can be achieved by using our scheduler.

Using neighbor and tag estimations for redundant reader eliminations in RFID networks

Deployments of Radio Frequency Identification (RFID) networks are anticipated to be dense and ad-hoc. These deployments usually involve redundant readers having overlapping interrogation zones and, hence, causing immense reader collisions. Elimination of redundant readers, from the network, is of utmost importance as otherwise they affect the lifetime and the operational capacity of the overall RFID network. In this paper, we propose a light-weight greedy algorithm that detects and eliminates redundant readers from the network. Our algorithm uses the ratio of tag counts to the number of neighboring readers of each reader to estimate the likelihood for that reader to be redundant. The proposed algorithm is highly scalable and poses a minimal communication overhead as compared with existing schemes in the literature.

Optimized relay placement for wireless sensor networks federation in environmental applications

Advances in sensing and wireless communication technologies have enabled a wide spectrum of Outdoor Environment Monitoring applications. In such applications, several wireless sensor network sectors tend to collaborate to achieve more sophisticated missions that require the existence of a communication backbone connecting (federating) different sectors. Federating these sectors is an intricate task because of the huge distances between them and because of the harsh operational conditions. A natural choice in defeating these challenges is to have multiple relay nodes (RNs) that provide vast coverage and sustain the network connectivity in harsh environments. However, these RNs are expensive; thus, the least possible number of such devices should be deployed. Furthermore, because of the harsh operational conditions in Outdoor Environment Monitoring applications, fault tolerance becomes crucial, which imposes further challenges; RNs should be deployed in such a way that tolerates failures in some links or nodes. In this paper, we propose two optimized relay placement strategies with the objective of federating disjoint wireless sensor network sectors with the maximum connectivity under a cost constraint on the total number of RNs to be deployed. The performance of the proposed approach is validated and assessed through extensive simulations and comparisons assuming practical considerations in outdoor environments. Copyright

Optimal distance-based clustering for tag anti-collision in RFID systems

Tag collisions can impose a major delay in radio frequency identification (RFID) systems. Such collisions are hard to overcome with passive tags due to their limited capabilities. In this paper, we look into the problem of minimizing the time required to read a set of passive tags. We propose a novel approach, the distance-based clustering, in which the interrogation zone of an RFID reader is divided into equal sized clusters (discs), and tags of different clusters are read separately. The novel contributions of this paper are the following. First, we provide a mathematical analysis to the problem and derive a closed-form formula relating delay to the number of tags and clusters. Second, we devise a method to efficiently find the optimal number of clusters. The proposed scheme can be augmented with any tree-based anti-collision scheme, and substantially improve its performance. Simulation results show that our approach makes significant improvements in reducing collisions and delay.

Routing to a Mobile Data Collector on a Predefined Trajectory

In this paper, we propose a distributed scheme for data gathering using a mobile data collector in Wireless Sensor Networks (WSNs). In our scheme, a mobile data collector moves along a predefined track over the sensing field and data are forwarded to nodes whose transmission disks overlap with the trajectory of the data collector; these nodes are called relaying nodes. Data are classified into two categories: delay-sensitive data and delay-tolerant data. While delay-sensitive data are sent to the data collector directly, delay-tolerant data may be sent to a nearby relaying node, where they wait for the data collector to come and pick them up. We give a theoretical analysis to quantify the impact of data collector mobility on the lifetime of the network as compared to a WSN with a stationary data collector. Moreover, we use simulations to evaluate our scheme in practice. Simulation results show that our scheme has the potential to prolong the lifetime of the network significantly.

Set-Cover Approximation Algorithms for Load-Aware Readers Placement in RFID Networks

Radio Frequency Identification (RFID) is an emerging wireless network technology that poses new fundamental challenges. One such challenge is coverage in RFID networks which is the ability to accurately read a set of RFID tags. Accurate coverage is of utmost importance in RFID networks as missing some tags may result in missing important events and, for some RFID applications, losing asset and revenue. In this paper, we address an optimization problem related to the deployment of RFID readers to cover a set of RFID tags with the objectives of minimizing the number of readers, reducing overlapping among readers coverage and balancing the load. We propose a set-cover based approximation algorithm for RFID coverage with the minimum number of readers. We extend this algorithm to consider the objective of reducing overlap and interference among readers interrogation ranges. And finally, we devise a load balancing algorithm that evenly distributes the load among different readers. Comprehensive experiments that study the performance of our algorithms are presented.

A novel wireless sensor and actor network framework for autonomous monitoring and maintenance of lifeline infrastructures

This position paper introduces a novel wireless sensor and actor network (WSAN) framework for autonomous monitoring and maintenance of pipe and power line (oil, gas, water, electricity) infrastructures in an efficient and cost-effective manner. The main focus is on boosting the availability of lifeline infrastructures through advancements in the WSAN technology. First, we categorize and classify the existing lifeline monitoring systems. Second, we identify the requirements for effective and efficient monitoring and maintenance of lifeline infrastructures. Third, we propose a novel WSAN architecture that combines sensing with distributed decision-making and acting capabilities through advanced robotics. Two operational models for the proposed architecture are also presented. The first is a push-up model that employs low-cost, multi-functional sensors along the lifeline to observe certain phenomena of interest, e.g., leakage, ruptures, clogs, etc., in real time and reports to actors over wireless links. The actors process the received data, coordinate with each other in order to identify the most appropriate response. The second is a pull-down model that capitalizes the resources of elite nodes (i.e. actors) in the network.

RFID Localization Using Angle of Arrival Cluster Forming

Radio Frequency IDentification (RFID) has been increasingly used to identify and track objects automatically. RFID has also been used to localize tagged objects. Several RFID localization schemes have been proposed in the literature; some of these schemes estimate the distance between the tag and the reader using the Received Signal Strength Index (RSSI). From a theoretical point of view, RSSI is an excellent approach to estimate the distance between a sender and a receiver. However, our experiments show that there are many factors that influence the RSSI value substantially and that, in turn, has a negative effect on the accuracy of the estimated distance. Another approach that has been recently proposed is utilizing transmission power control from the reader side. Our experiments show that power control results are more stable and accurate than RSSI results. In this paper, we present a test-bed comparison between the power control and the RSSI distance estimation approaches for active RFID tags. We also present the Angle of arrival Cluster Forming (ACF) localization scheme that uses both the angle of arrival of the tags signal and the readers transmission power control to localize active tags. Our experiments show that ACF is very accurate in estimating the location of active RFID tags.