Dimitrios Zorbas

Solving coverage problems in wireless sensor networks using cover sets

To achieve power efficient monitoring of targets by sensor networks, various coverage algorithms have been proposed. These algorithms divide the sensor nodes into cover sets, where each cover set is capable of monitoring all targets. Generating the maximum number of cover sets has been proven to be an NP-complete problem and, thus, algorithms producing sub-optimal solutions have been proposed. In this paper we present a novel and efficient coverage algorithm, that can produce both disjoint cover sets, i.e. cover sets with no common sensor nodes, as well as non-disjoint cover sets. While searching for the best sensor to include in a cover set, our algorithm uses a cost function that takes into account the monitoring capabilities of a sensor, its association with poorly monitored targets, but also the sensors remaining battery life. Through simulations, we show that the proposed algorithm outperforms similar heuristic algorithms found in the literature, producing collections of cover sets of optimal (or near-optimal) size. The increased availability offered by these cover sets along with the short execution time of the proposed algorithm make it desirable for a wide range of node deployment environments.

Connected coverage in WSNs based on critical targets

Abstract One of the recent challenges in wireless sensor networks is the design of efficient algorithms to monitor a set of discrete targets lying at a field. A set of active nodes must cover all the available targets and at the same time retain connectivity with the sink. Such a set can remain active until one active node depletes its battery. In this paper, we analyze the problem of finding the proper sensor scheduling in order to maximize the total network lifetime. We present OCCH (optimized connected coverage heuristic) an efficient algorithm that is based on a general connected coverage methodology. This methodology takes into account the association of the sensors with the poorly covered targets that set an upper bound on the overall computed lifetime. Two solutions are presented to efficiently manage the battery life of these sensors followed by other minor improvements that prolong the network lifetime. Extensive simulation results are presented that show that our solution outperforms other known algorithms found in the literature in terms of achievable network lifetime.

Connected partial target coverage and network lifetime in wireless sensor networks

Improving the lifetime of sensor networks that are used to monitor a number of targets lying on a field has lately received considerable attention. The most common approach to tackle this problem divides the sensor nodes into cover sets, where the sensors in each set are capable of monitoring all targets in the field. By activating one cover set at a time, the sensor network lifetime can be extended. In this paper, we analyze the problem of connected partial target coverage where cover sets are allowed to monitor a subset of the targets at any point in time, while connectivity with the base station is retained. We propose an algorithm that computes the desired cover sets and we compare its performance, in terms of network lifetime, to that of a full coverage algorithm. Our solution takes into account several characteristics of the problem, such as the specific topology parameters, the coverage status of the sensors as well as their remaining battery life. Extensive experimental results are presented for different sensor and target placement topologies. Results show that monitoring 90% of the targets may yield twice the network lifetime provided by a full coverage approach.

Satisfying coverage and connectivity in bandwidth constrained sensor networks

One of the main problems in wireless sensor networks is the successful coverage of a number of targets lying on a field and at the same time the maximization of the lifetime of the network. Many algorithms have been proposed in the literature in order to find the maximum number of disjoint or non-disjoint sets of sensors (cover sets), where one set can be active at any one time. The complexity of the problem increases if we consider that the channel bandwidth of the base station, where the sensor nodes transmit their monitoring data, is limited. The limited channel bandwidth leads to cover sets that cannot monitor all the available targets resulting in a coverage breach. In this paper, we propose an algorithm that computes cover sets that minimize the overall coverage breach. This algorithm takes into account several problem parameters, such as the connectivity requirement between the sensors and the base station, the coverage status of the nodes and their remaining battery life. Our experimental results show that our algorithm outperforms existing approaches in various performance metrics and that the connectivity constraint sets an upper bound on the algorithms output.

The charger positioning problem in clustered RF-power harvesting wireless sensor networks

Abstract Wireless charging brings forward several new challenges in designing energy efficient wireless sensor networks. In a wireless charging scenario, a number of chargers with high energy resources are placed in the network to recharge power constrained nodes. Nevertheless, due to the fading effect of the signals, a few only nodes can remarkably benefit from the charger power emission. For this reason we examine whether the organization of the nodes in clusters may extend the network lifetime. In this paper, we compute the maximum size of the cluster and we propose an efficient localized algorithm as well as a centralized one to compute the charger position so as to maximize the cluster lifetime. We compare to other solutions in the literature and we present both theoretical and simulation results to show the effectiveness of our approach. The results show an up to 360% increase in lifetime in comparison with the traditional 1-hop communication method between the nodes and the sink.

On the optimal number of chargers in battery-less wirelessly powered sensor networks

A major advantage of wirelessly powered devices is the use of wire-free and sometimes battery-free nodes that can operate for extremely long times. However, to achieve infinite network lifetime a set of chargers is needed to periodically transmit energy to the nodes through the emission of RF signals. In this paper, we study the problem of finding the optimal number of chargers so that a group of nodes can operate without using power sources other than wireless charging. We show that this problem is equivalent to the set-cover problem which is NP-Complete. We propose an efficient heuristic that bypasses the high computational cost of finding the overlapping segments of the harvesting sensor disks and we compare to optimal and non-optimal set-cover solutions. The results show significant performance gains in terms of execution time while keeping the number of chargers close to the optimal.

Target location based sink positioning in wireless sensor networks

One of the main challenges in wireless sensor networks is to prolong the network lifetime by efficiently handling the limited battery life of the nodes. This problem becomes harder in applications where the nodes are randomly dropped in the field. In this paper we deal with the problem of the sink placement and of the network longevity, assuming a number of points in the field with known positions which must be covered by the sensors. Unlike other approaches, we consider the more realistic scenario where the coordinates of the sensors are not assumed to be known in advance and, thus, they cannot be used for the computation of the positions of the sinks. We present two solutions for the above problem; one based on the distance between the points and the second on the probability that a sensor may cover many points. We evaluate our approaches and compare them to algorithms that use the knowledge of the positions of the sensors in order to compute likely sink locations. It is shown that both proposed approaches present similar or better performance concerning network lifetime, while at the same time they significantly decrease the algorithm complexity.

Efficient target coverage in WSNs with bidirectional communication

Several applications in wireless sensor networks require a bidirectional communication between the sensing nodes and the sink. A number of relay nodes is used to achieve this bidirectional communication. The relay nodes adjust their communication range in order to consume less energy, constructing an asymmetric network with unidirectional links. In this paper, we examine the case where a bidirectional path must be established between the sensing nodes and the sink, focusing on the extension of the network lifetime. We propose an efficient heuristic that establishes two unidirectional paths in order to distribute the communication cost among the nodes. Our evaluation shows an over 20% improvement in some cases compared to the commonly used approach where a single bidirectional path is used.

Collision-Free Broadcast Methods for IEEE 802.15.4-TSCH Networks Formation

One of the most recent and reliable MAC protocols for low-rate wireless personal area networks is the IEEE802.15.4-TSCH. The formation of an IEEE802.15.4-TSCH network depends on the periodic transmission of Enhanced Beacons (EBs), and, by extension, on the scheduling of EB transmissions. In this paper, we present and analyze a negative phenomenon that can occur in most of the autonomous EB scheduling methods proposed in the literature. This phenomenon, which we call full collision, takes place when all the neighboring EB transmissions of a joining node collide. As a consequence, a node may not be able to join the network fast, consuming a considerable amount of energy as well. In order to eliminate collisions during EB transmissions, and, thus, to avoid the occurrence of this phenomenon, we propose a novel autonomous collision-free EB scheduling policy. The results of our simulations demonstrate the superiority of our policy compared to two other recently proposed policies.

Assessing the cost of deploying and maintaining indoor wireless sensor networks with RF-power harvesting properties

Abstract Since Wireless Sensor Networks (WSNs) consist of nodes with limited power resources, methods that extend their energy lifespan are always in the spotlight. A potential method is the use of RF-power harvesting antennas which can absorb energy from radio frequency (RF) signals and transform a part of it into electricity. Dedicated energy transmitters (ETs) are used to emit power to the nodes. In this paper, we model the amount of harvesting energy as a function of several parameters such as the received power, the efficiency of the harvesting module and the transmission time. We consider a simple communication model that separates the ETs’ transmissions with the node data transmissions to avoid interference whilst we allow multi-hop energy transfer between the nodes when it is achievable. However, the ultimate purpose of this paper is to examine whether the cost of the investment of using energy harvesting nodes can be covered by achieving a lower operation cost; that is longer and cheaper operation times and, thus, less frequent maintenance. We consider several scenarios with different node densities and transmitter populations. Simulation results show that the use of RF-energy harvesting nodes can save a significant amount of energy, while the cost of the investment can be (theoretically) covered in less than 7 years for dense networks.