| Over the past decade, sensor networks have consistently been a focus of the computer research community, and a large number of prototypes have been built for military and civilian applications. One of the fundamental research issues of sensor networks is the energy scarcity problem. Due to the nature of sensor networks, e.g., small-size sensor nodes and large-scale deployment, sensor nodes cannot carry a large amount of energy or be conveniently recharged. As a result, the amount of energy that can be carried by a sensor node fundamentally limits the use of sensor networks. A large number of schemes have been proposed to address this issue. These schemes, however, have one or more the following drawbacks: (i) energy cannot be replenished to the network, and thus the network lifetime is bounded by the amount of energy preloaded to sensor nodes; (ii) the ways of replenishing energy to the network may not be practical and reliable, and thus may not support normal operations of the network; and (iii) sensor nodes drained of energy are left in the deployment field, and thus may cause pollution to the environment.;Fundamentally addressing this problem requires energy to be continually replenished to sensor nodes. This can be achieved in two approaches (i) The Node Reclamation and Replacement Approach: Sensor nodes with low or no energy are reclaimed periodically, and are replaced with fully charged ones. (ii) The Wireless Recharging Approach: Sensor nodes are periodically recharged with energy transmitted from wireless chargers over radio. Both the approaches exploit mobility in accomplishing energy replenishment. Specifically, one or more mobile agents, which could be human technicians or robots, travel around the network, and perform sensor reclamation and replacement or wireless recharging task.;In this dissertation, we propose an array of new mobility-assisted energy replenishment schemes. Firstly, for the node reclamation and replacement approach, we propose a node replacement and reclamation (NRR) strategy, with which a mobile robot or human labor periodically traverses the sensor network, reclaims nodes with low or no power supply, replaces them with fully-charged ones, and brings the reclaimed nodes back to an energy station for recharging. To effectively and efficiently realize the NRR strategy for different application scenarios, we present several implementing schemes for NRR under point coverage and area coverage models, respectively. We also present schemes to improve reliability in implementation of NRR .;Secondly, the wireless recharging approach takes advantage of emerging wireless recharging technology to continually transfer energy into the network. To support long network lifetime with the wireless recharging approach, the recharging agents' activities and sensors' activities can be scheduled in a similar way as with the node reclamation and replacement approach. Therefore, in this line of research, we focus on a unique problem with the wireless recharging technology, that is, how wireless recharging affects sensor network deployment and routing arrangement. We prove the problem is NP-complete, and propose heuristic algorithms to solve it.;Extensive analysis and simulations have been conducted to verify the effectiveness and efficiency of the proposed schemes. As the battery technology lags far behind that of MEMS, we believe energy replenishment is necessary to long-lived surveillance sensor networks. To the best of our knowledge, our works of sensor node reclamation and replacement and wireless recharging are among the first efforts on studying how to re-design sensor networks to fully leverage different energy replenishment techniques. |
