Compact, Low Power Wireless Sensor Network System for Line Crossing Recognition Chung-Ching Shen, Roni Kupershtok, Bo Yang, Felice Maria Vanin, Xi Shao, Datta Sheth, Neil Goldsman, Quirino Balzano, and Shuvra S. Bhattacharyya Department of Electrical and Computer Engineering University of Maryland, College Park 20742 {ccshen, akuper, boyang, felvanin, xshcn, sheth, neil, qbalzano, ssb}@umd.edu Abstract— Many application-specific wireless sensor network (WSN) systems require small size and low power features due to their limited resources, and their use in distributed, wireless environments. In this paper, we present a light-weight distributed algorithm for line-crossing recognition, together with its analysis, implementation, and experimental evaluation within a prototype wireless sensor network platform. The algorithm is developed in conjunction with a TDMA-based communi- cation protocol such that the proposed system provides for low duty cycle and energy efficient operation. An accurate lifetime model is proposed with consideration of detailed energy usage to analyze and estimate the system lifetime. Our experimental results demonstrate the accuracy of this lifetime model, and its utility in optimizing network implementation. The design and experimental evaluation of our prototype network demonstrates the compactness and functionality of the proposed distributed WSN system for line-crossing recognition. I. I NTRODUCTION AND RELATED WORK While considering the design of a distributed WSN system applied in an indoor environment, we propose a light-weight, distributed algorithm for line-crossing recognition. We implement this algorithm as part of an operational wireless sensor network, and develop a highly customized and streamlined printed circuit board (PCB) design to support the individual sensor nodes. Our complete system imple- mentation involves application-specific optimizations at the algorithm level, network level, and node level, and for this implementation, we demonstrate the resulting features of small size and high energy efficiency. The purpose of this distributed WSN system is to periodically reach consensus in deciding whether or not an object (“intruder”) has crossed a specific boundary (“line”) in a noisy environment that is being monitored. Furthermore, upon detecting an intrusion, the system determines where the line was crossed (i.e., between which nodes in the line). For example, in Figure 1, the sensor nodes are placed in a circle inside a room. In this practical configuration, the WSN system recognizes when and where a subject has crossed the circle. All sensor nodes communicate with each other through an efficient, wireless time division multiple access (TDMA) protocol so that each node can transmit and receive at designated time slots, and can “sleep” during other times for energy savings. To demonstrate the practical realization of the targeted application, we develop a customized system prototype using an emerging family of off-the-shelf system-on-chip (SoCs), and a custom-designed minia- ture antenna that we have developed. Each SoC component employed provides an embedded microprocessor and wireless communications transceiver together within a single integrated circuit. Our design is different from related wireless sensor network implementations (e.g., see [1], [2]) in that a single SoC is used to replace separate integrated circuits for microcontroller and transceiver components, and furthermore, a smaller antenna is designed and installed. Both design features contribute to significantly reducing PCB size and the size of each network node. customized sensory node approaching sound subject sound source detected μC +T P S μC +T P S token passing path μC+T P S μC+T P S μC +T P S μC +T P S Fig. 1. An indoor environment scenario with the use of our distributed system for line-crossing recognition. All N nodes within the system run the proposed distributed algorithm, and reach a consensus based on local decisions of C nodes while a subject is being detected (C ≤ N ). The number of data bits for the data that is communicated by each node is O (log (C )) (i.e., is logarithmically bounded with respect to C). In [3], Hirschberg and Sinclair proved an upper bound of O (log (N )) on the number of bits that are sent by every node during a consensus task of N nodes arranged in a bidirectional ring topology. Every node that executes their algorithm has an initial input, and has no additional inputs during its execution. In [4], Dinitz et al. proved an upper bound of O (N ) on the number of bits that is sent by all nodes during a consensus task of N nodes arranged in a tree topology (a chain topology is a special case). Their proof is based on the collection of information with feedback (CIF) algorithm. Every node that runs the CIF algorithm may have an initial input without additional inputs during its execution. After one round that includes two phases, the ‘collect” phase and “feedback” phase, all nodes reach consensus. In our proposed distributed algorithm — in contrast to the ap- proaches described above — each node can obtain many inputs (i.e., either from the received data or from the sensed data) during its execution, and based on these inputs, all of the nodes decide whether or not a subject is approaching and crossing the given line. Also, our algorithm requires that either O (log (C )) or O (log (N )) data bits are needed depending on the protocol stage (i.e., synchronization stage or communication stage) instead of N . Furthermore, during most of the system lifetime, our implemented system communicates with only O (log (C )) data bits. Here, C — a design parameter — is the minimum number of nodes that must sense the subject in order to reach consensus that an intruder is approaching and crossing the line. Higher values of C provide higher system accuracy at the expense of higher communication requirements and higher recognition latency. Since energy consumption during transmission is high, our approach reduces energy consumption significantly by reducing the number bits that need to be transmitted for overall system operation. 2506 1-4244-0921-7/07 $25.00 © 2007 IEEE. In Proceedings of the International Symposium on Circuits and Systems, pages 2506-2509, New Orleans, Louisiana, May 2007