1. On Renewable Sensor Networks with Wireless Energy Transfer IEEE INFOCOM 2011 Yi Shi, Liguang Xie, Y. Thomas Hou, Hanif D. Sherali

2. OUTLINE Introduction Goal Network Model Overview Problem Description Algorithm Simulation Conclusions

3. Introduction Wireless sensor networks (WSNs) today are mainly powered by batteries. limited battery energy WSN can only remain operational for a limited amount of time. To prolong the network lifetime, there have been a flourish of research efforts in the last decade. energy-harvesting techniques

4. Introduction Wireless energy transfer technology exploiting a novel technique called magnetic resonance [11] A. Kurs, A. Karalis, R. Moffatt, J.D. Joannopoulos, P. Fisher, and M. Soljacic, “Wireless power transfer via trongly coupled magnetic resonances,” Science, vol. 317, no. 5834, pp. 83–86, 2007. ◎ wireless energy transfer : the ability to transfer electric energy from one storage device to another without any plugs or wires

5. Goal Employing a mobile vehicle carrying a battery charging station periodically visit each sensor node and charge it wirelessly can remove the lifetime performance bottleneck from a battery- powered WSN

6. Network Model sensor node distributed over a two-dimensional area has a battery capacity of E max and is fully charged initially minimum energy E min mobile wireless charging vehicle(WCM) base station(B) service station multi-hop data routing

7. Overview Service station Base station Sensor node Mobile WCM

8. Overview Service station Base station Sensor node Mobile WCM resting period vacation time

9. Problem Description multi-hop data routing flow balance constraint at each sensor node i f ij : the flow rate from sensor node i to sensor node j f iB : the flow rate from sensor node i to the base station B Each sensor node i generates sensing data with a rate R i of (in b/s)

10. Problem Description Each sensor node consumes energy for data transmission and reception. the rate of energy consumption for receiving a unit of data rate C ij (or C iB ) is the rate of energy consumption for transmitting a unit of data rate from node i to node j (or the base station B).

11. Problem Formulation maximize the percentage of time in a cycle that the WCV can take vacation. max s.t.

12. RENEWABLE CYCLE CONSTRUCTION

15. CONSTRUCTION OF INITIAL TRANSIENT CYCLE Initial transient cycle must meet the following criterion :

16. CONSTRUCTION OF INITIAL TRANSIENT CYCLE ˆa i is the arrival time of the WCV at node i in the initial transient cycle.

17. Simulation Number of nodes : 50 Data rate from each node : randomly generated within [1,10]kbps Base station location : (500, 500) (m) V=5 (m/s) E max =10.8KJ E min =540J U=5W

18. Simulation

19. An optimal traveling path for the 50-node sensor network. Only counter clockwise traveling direction is shown. [18] Concorde TSP Solver, http://www.tsp.gatech.edu/concorde/.

20. Simulation

21. Property 1: In an optimal solution, there exists at least one node in the network with its battery energy dropping to E min when the WCV arrives at this node.

22. Conclusions This paper exploits recent breakthrough in wireless energy transfer technology for a WSN. Exploiting a mobile charging vehicle periodically travels inside the network and charges each sensor node wirelessly without any plugs or wires. we showed that a sensor network operating under our solution can indeed remain operational with unlimited lifetime.