Analysis And Optimization Of Wireless Rechargeable Sensor Networks

Wireless Rechargeable Sensor Networks (WRSN) is an emerging technology which integrates sensing, communication and computation capabilities. Different from the traditional sensor nodes powered by batteries, wireless rechargeable sensor nodes gather their energy from the transmission of energy sources such as RFID readers. Given its small form factors and universal sensing capa-bilities, it is expected that wireless rechargeable sensor will be a promising platform for different applications such as warehouse inventory management, supply chain and environmental monitor-ing, authentication and so on. Based on the state-of-the-art study, this dissertation researches on analysis and optimization in WRSN. The main work and contributions are summarized as follows:1. A brief review of the background, overview, technical challenges and related works on WRSN is provided.2. Research on node localization in WRSN. For many WRSN applications, the locations of sensor nodes are required for them to function properly. For example, for warehouse inven-tory management or environmental monitoring, it is usually necessary to identify the location where sensor readings are originated from, which further benefits the robot-assisted search and delivery. Localization and tracking of RFID-based nodes can help supermarkets bet-ter understand the consumers’ shopping habits, and provide significant benefits in hospital care environment and rescue services. For example, it facilitates medical equipment tracking under emergency situations and enables the detection of firemen in the building on fire, or victims under the wall. In addition, some geographic routing protocols and network manage-ment optimization can only be implemented with the location of each sensor already known. In this project, we exploit the unique wireless charging properties of the wireless rechargeable sensor nodes and propose the concept of Time of Charge (TOC), the time for a sensor node to be charged above its working threshold, to localize individual nodes in a WRSN. we consider the scenario that a mobile charger moves and stops at different locations to wirelessly charge nodes and obtain the time of charge (TOC) for nodes in its surrounding area. The novel idea of TOC is to estimate each sensor’s location by utilizing Time of Charge sequences from both temporal and spatial domain. Furthermore, TOC also optimizes the charger stop positions based on the estimated sensor locations.3. Research on velocity control of mobile charger in WRSN. Mobile chargers are good can-didates for charging WRSN nodes. Compared to deploying multiple static wireless charg-ers, using one mobile charger is cost-efficient and flexible in dealing with network topology changes. Moreover, the mobile charger can be combined with the mobile base station to help alleviate network congestion and avoid energy hot spots during data collection. However, mobile charging creates several challenges in energy provision in WRSNs. One fundamental challenge is how to control the speed of the mobile charger. In the case of omnidirectional wireless charging, the amount of energy charged in nodes is dictated by 1) the distance be-tween sensor nodes and the charger, and 2) the duration of charging each node. Specifically, the charging power at nodes decreases as the distance to the charger increases. Given a fixed charging distance, the charged energy at nodes increases with the charging duration. It is thus desirable to charge nodes as long as possible and at minimum distances from the charger. However, due to location diversity, the charged energy at different nodes cannot be maximized simultaneously. If we choose to fully charge part of the network, there may not be enough time for the charger to move and charge other parts of the network, especially under the time-bounded charging constraint. Thus, given constrained moving trajectories, the velocity of the charger plays an important role in energy provision in WRSNs. Due to the non-uniform distribution of sensor nodes and different shapes of the moving trajectory, it is non-trivial to continuously determine if the charger should move faster or slower along the trajectory, in order to maximize the charged energy at nodes. To tackle this problem, we consider a common scenario where the charger travels along a pre-planned trajectory and determine the optimal velocity of the charger subject to a given traveling time constraint, such that the network lifetime is maximized. Specifically, we aim to maximize the minimum charged energy among all nodes in the network. This way, we can mitigate the uneven energy replenishment of sensor nodes which has a significant effect on sensing quality, data delivery reliability, network throughput, and so on.4. Research on joint energy replenishment and sensor scheduling in WRSN. Operation schedul- ing of sensor nodes in a Wireless Sensor Network (WSN) while considering the sensing cov-erage has been studied extensively. Most of existing work considers a fixed amount of energy aiming to minimize the total energy consumption. However, in a WRSN, sensor operation scheduling becomes more sophisticated since each node’s energy balance does not monoton-ically decrease due to the energy replenishment by charger(s). In traditional sensor networks, network lifetime with a coverage guarantee is subject to critical nodes located in the essential sensing area with few nearby neighbors, thus keeping themselves alive for a longer period of time and draining their energy faster than others. However in a WRSN, the energy of these nodes can be replenished by a charger, thus making them no longer the "bottleneck" nodes. A key problem is, therefore, to design the active/sleep schedule for nodes, and charge them in an energy-balanced manner. We address the above problem by developing a joint energy re-plenishment and operation scheduling mechanism that maximizes the network lifetime while providing strict sensing guarantees. Specifically, given the charging capacity of a charger, we want to find the best strategy of operation scheduling and energy partitioning. This way, we mitigate the gap between the heterogeneous energy consumption among nodes and the unbalanced initial energy via energy replenishment, and therefore increase the total energy utility and extend the network lifetime.5. Research on communication delay minimizing in WRSN. For a typical warehouse inventory monitoring application, wireless rechargeable sensor nodes are attached to various containers or individual products for sensing and monitoring vital product statuses such as temperature, humidity and vibration. To obtain the sensory data measured by wireless rechargeable sensor nodes, an RFID reader is normally deployed and can move around to communicate with all wireless rechargeable sensor nodes. So in order to minimize the total time duration for the RFID reader to obtain the sensory information from all wireless rechargeable sensor nodes, intuitively, we should have the RFID reader cover as many sensor nodes as possible. On the other hand, as the number of sensor nodes within the communication range of an RFID reader increases, the collisions among sensor nodes also increase, which lead to a larger communication delay. In this project, instead of designing solutions for reducing collisions among sensor nodes, we study an orthogonal approach and focus on how to optimally plan the movement of the RFID reader, so as to minimize the communication delay of obtaining sensory data from all RFID-based wireless rechargeable sensor nodes. Rather than designing new protocols from scratch, our solutions exploit the fundamental relationship between the total communication delay and the number of wireless rechargeable sensor nodes within the communication range of an RFID reader. Based on this observation, we introduce novel solutions for determining the optimal movement pattern of the RFID reader along the fixed linear trajectory and generic two-dimensional space.6. Research on application design in WRSN. Access control is a mechanism which enables an authority to control access to restricted areas and resources at a given physical facility or computer-based information system. In general, authentication methods in access control systems can be divided into two broad categories. The first category is based on mechan-ical matching, such as keys and combination locks. The other category of authentication for access control systems is electronic authentication including barcode, magnetic stripe, biometrics and etc. Compared with mechanical matching authentications, the electronic au-thentications such as RFID-based smart card offer much more convenience and flexibility for both administrators and users of access control systems. However, it still suffers from similar problem of key loss since authentication is only based on the encoded identification data on the card. Anyone who carries the card will be granted the access and the security of the system still can be compromised. To bridge the gap between insufficiency of exist-ing electronic authentication solutions and the increasing demand of high security guarantee for access control systems, we design a novel electronic proximity authentication framework that enhances the security level of existing RFID-based access control systems with back-ward compatibility. Specifically, we add dynamic data into the traditional authentication information by using sensors such as accelerometer, gyroscope and etc. This authentication framework is adaptive to the change of encryption complexity of the access control systems and could be adopted with minor modification of existing infrastructure.The conclusions are drawn with future work at the end of the dissertation.