Elsevier

Computer Networks

Volume 97, 14 March 2016, Pages 98-112
Computer Networks

Hierarchical, collaborative wireless energy transfer in sensor networks with multiple Mobile Chargers

https://doi.org/10.1016/j.comnet.2016.01.007Get rights and content

Abstract

Wireless energy transfer is used to fundamentally address energy management problems in Wireless Rechargeable Sensor Networks (WRSNs). In such networks mobile entities traverse the network and wirelessly replenish the energy of sensor nodes. In recent research on collaborative wireless charging, the mobile entities are also allowed to charge each other.

In this work, we enhance the collaborative feature by forming a hierarchical charging structure. We distinguish the Chargers in two groups, the hierarchically lower Mobile Chargers which charge sensor nodes and the hierarchically higher Special Chargers which charge Mobile Chargers. We define the Coordination Decision Problem and prove that it is NP-complete. Also, we propose a new protocol for 1-D networks which we compare with a state of the art protocol. Motivated by the improvement in 1-D networks, we propose and implement four new collaborative charging protocols for 2-D networks, in order to achieve efficient charging and improve important network properties. Our protocols are either centralized or distributed, and assume different levels of network knowledge.

Extensive simulation findings demonstrate significant performance gains, with respect to non-collaborative state of the art charging methods. In particular, our protocols improve several network properties and metrics, such as the network lifetime, routing robustness, coverage and connectivity. A useful feature of our methods is that they can be suitably added on top of non-collaborative protocols to further enhance their performance.

Section snippets

Introduction and contribution

In Wireless Sensors Networks (WSNs) the sensor nodes are equipped with small batteries and thus, the lifetime of the network is limited. Although there are several approaches that try to address this fundamental problem, the proposed solutions are still limited since the energy that is replenished is either uncontrollable (such as environmental harvesting approaches) or require the nodes to be accessible by people or robots in a very accurate way (such as battery replacement approaches).

Related work and comparison

Wireless energy transfer technology inspired a lot of researchers to investigate how to exploit it in WSNs efficiently. In [5], the authors used a realistic scenario where the sensor nodes are mobile and the Chargers are stationary. They proposed two protocols to address the problem of how to schedule the Chargers activity so as to maximize either the charging efficiency or the energy balance. Also, they conducted real experiments to evaluate the protocols’ performance. In [6], the objective

The model

Our model features four types of devices: N stationary sensor nodes, M Mobile Chargers which charge sensor nodes, S Special Chargers which charge Mobile Chargers and a single stationary Sink. The sensor nodes of wireless communication range r are uniformly distributed at random in a circular area of radius R. The Mobile Chargers and the Special Chargers are initially deployed at the center of circular area. The Sink serves only as data collector.

In our model, we assume that neither the Mobile

The coordination decision problem

Definition 1

Consider a set S of S Special Chargers. For each SCk (1 ≤ kS), we denote by Ek the percentage of its current energy level to the total amount of energy of all Special Chargers i.e., Ek=ESCki=1SESCi(1kS).

Also, consider a set M of M Mobile Chargers. For each MCj (1 ≤ jM), we define EMCjlack=EMCmaxEMCj the amount of energy that Mobile Charger j can receive until it is fully recharged and denote by εj the percentage of its energy lack to the total energy lack of all Mobile Chargers, i.e.,

The charging protocols

We present a new protocol operating in 1-D networks and four new protocols operating in 2-D networks. Our protocols use hierarchical collaborative charging. Since there is plenty of research on how multiple Mobile Chargers can charge sensor nodes we focus on how we can efficiently use the available Special Chargers. In all protocols we investigate the following three design aspects:

Coordination: a Special Charger consumes its energy according to the energy depletion on its area, i.e. the energy

Performance evaluation

The simulation environment for conducting the experiments is Matlab 7.12. The Sink is placed at the center of the circular area. The number of sensor nodes is set to 2000, the number of Mobile Chargers to 15 and the number of Special Chargers to 3. In the simulations, the number of the Mobile Charges in non-collaborative protocols equals to the sum of the Mobile Chargers and the Special Chargers in the hierarchical protocols, so, in protocols that do not use Special Chargers, the number of

Conclusion and future work

In this work we study the problem of efficient collaborative wireless charging in Wireless Sensor Networks. We propose a new design approach, according to which, the set of chargers is partitioned into two groups, one hierarchically higher, called Special Chargers and one hierarchically lower, called Mobile Chargers. The Mobile Chargers are responsible for charging the sensor nodes whereas the Special Chargers charge Mobile Chargers. This hierarchical structure provides a more controllable and

Acknowledgments

This research was partially supported by the EU/FIRE IoT Lab project – STREP ICT-610477 and the European Social Fund (ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) – Research Funding Program: Thalis-DISFER, investing in knowledge society through the European Social Fund.

Adelina Madhja is an M.Sc. student at the Computer Engineering and Informatics Department, University of Patras, Greece and a Researcher at the Computer Technology Institute & Press “Diophantus”. Her research interests focus on the design of energy efficient algorithms for Wireless Sensor Networks, Distributed Systems, and Internet of Things. She has co-authored two publications in international refereed conferences and a journal.

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Adelina Madhja is an M.Sc. student at the Computer Engineering and Informatics Department, University of Patras, Greece and a Researcher at the Computer Technology Institute & Press “Diophantus”. Her research interests focus on the design of energy efficient algorithms for Wireless Sensor Networks, Distributed Systems, and Internet of Things. She has co-authored two publications in international refereed conferences and a journal.

Sotiris Nikoletseas is a Professor at the Computer Engineering and Informatics Department of Patras University, Greece and Director of the SensorsLab at CTI. His research interests include Algorithmic Techniques in Distributed Computing (focus on sensor and mobile networks), Probabilistic Techniques and Random Graphs, and Algorithmic Engineering. He has coauthored over 200 publications in Journals and refereed Conferences, several Book Chapters and two Books (one on the Probabilistic Method and another on sensor networks), while he has delivered several invited talks and tutorials. He has served as the Program Committee Chair of many Conferences, and as Editorial Board Member of major Journals. He has co-initiated international conferences on sensor networking. He has coordinated several externally funded European Union R&D Projects related to fundamental aspects of modern networks.

Theofanis P. Raptis is a Research Engineer at Computer Technology Institute and Press “Diophantus”, Greece and a Ph.D. candidate at the Computer Engineering and Informatics Department, University of Patras, Greece. His current research interests include wireless power transfer algorithms in sensor networks, mobile crowdsensing systems and future internet platforms and testbeds. He has co-authored more than 20 publications in acclaimed international refereed journals, conferences and books and has participated in several relevant European Union funded R&D projects.

A preliminary version of this paper appeared in [1].

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