Hyuntae Cho

Implementation of a precision time protocol over low rate wireless personal area networks

Time synchronization is essential for a number of network applications. As the era of ubiquitous computing is ushered in, high precision time synchronization of nodes in wireless networks is required. High precision time synchronization can enable a variety of extensions of applications. This paper includes the design and implementation of the precision time protocol over low rate wireless personal area networks (LR-WPANpsilas). To achieve high precision in LR-WPANpsilas, we analyze the factors of latency and jitter in wireless environments, and we aim to minimize these factors. In addition, this paper presents experiments and the performance evaluation of the precision time protocol in LR-WPANpsilas. The result is that we established for nodes in a network to maintain their clocks to within a 50 nanosecond offset from the reference clock.

Precision Time Synchronization Using IEEE 1588 for Wireless Sensor Networks

Wireless sensor networks are evolving from relatively undemanding applications to applications which have stronger requirements. The coordination of distributed entities and events requires time synchronization. Although a number of methods have been studied for WSNs, some applications require high precision time synchronization. Precision time synchronization enables a variety of extensions of applications. The IEEE 1588 precision time protocol (PTP) provides a standard method to synchronize devices in a network with sub-microsecond precision. This paper deals with precision time synchronization using IEEE 1588 over wireless sensor networks. Precision time synchronization using IEEE 1588 provides compatibility between heterogeneous systems in WSNs. This paper also presents experiments and performance evaluation of precision time synchronization in WSNs. Our result established a method for nodes in a network to maintain their clocks to within a 200 nanosecond offset from the reference clock of a master node.

Design and implementation of an active RFID system platform

An active RFID system has the active tag that incorporates an internal battery providing a robust, long-range transmission signal. Typical application using the active RFID is the identification and the position tracking of warehouse inventory-item and freight container. This paper presents the design and implementation of an active RFID system platform which complies with the ISO/TEC 18000-7 standard. Our system design focus on three parts: 1) the standard compliance with the ISO/TEC 18000-7, 2) the energy saving mechanism to maintain the longevity of tags as long as possible, and 3) the high identification rate in the presence of multiple tags. We describe the detailed design of the system and the commercial off-the-shelf (COTS) implementation. The performance evaluation of the system is also included.

Large scale active RFID system utilizing ZigBee networks

As the era of the ubiquitous computing is ushered in, consumers have been primed for a variety of automation applications. Active RFIDs can be used for a number of automation applications because it may also have other sensors to extend its applications. An active RFID instinctively focuses on a long communication range. Nevertheless, active RFIDs cannot be applied to a large-scale area due to their limited radio communication and obstacles. This paper introduces a large-scale active RFID system based on multi-hop deployment utilizing dual radio frequency to overcome radio shadow areas that do not reach signals from the RFID reader. The proposed system consists of multiple RFID readers deployed in an ad hoc pattern. It uses a multihop collection scheme utilizing ZigBee networks to extend the coverage of the RFID reader and collect RFID tags outside its communication range. This paper also includes an experiment evaluating performance of the proposed method, using an implemented RFID system that complies with the standard for 433 MHz active RFIDs based on ISO/IEC 18000-7.

Practical localization system for consumer devices using zigbee networks

As the era of ubiquitous computing dawns, there is a growing need for a reliable, efficient positioning and tracking system. A localization system involves ongoing tracking of the location of assets and personnel. This paper presents a practical localization system for consumer devices on Zigbee networks. The proposed system is based on time-difference-of-arrival (TDOA). Localization based on TDOA involves estimating the location of the device by calculating the time-difference-of-arrival of the signal received from a device. In order to calculate the time difference of the signal, TDOA-based methods require precision time measurements of the signal, and reader time synchronization accurate to within a few nanoseconds. We also propose a precise time stamping unit, which enables the system to determine the arrival time of the radio signal, and a precision time synchronization protocol, which enables readers to have a reference clock. In addition, this paper includes an experimental performance evaluation of the localization system. The performance shows that the localization system has a DRMS of approximately two meters in a harsh environment.

LITeTag: Design and Implementation of an RFID System for IT-based Port Logistics

Logistics has grown dramatically, especially where identifying, locating, or tracking objects in ports are important, because ports are gateways to extended markets. In this paper, we analyze the new requirements of port logistics after a brief introduction and present a system prototype. Then we describe the design and implementation of an RFID system for IT-based port logistics. Our solutionnamed Logistics Information Technology electronic Tag (LITeTag) consists of three parts: a smart tag, a smart electronic container seal, and an RTLS system. These offer a complete range of port logistics services that cover the entire port environment. Also, our system design focuses on fourparts: 1) the standard compliance with the ISO, 2) conservation of energy to extend the live of the tags for as long as possible, 3) a high identification rate in the presence of multiple tags, and 4) the fastest possible tag response time. A performance evaluation of the system is also included inthe paper.

LDFSA: a learning-based dynamic framed slotted ALOHA for collision arbitration in active RFID systems

In recent large scale deployment of active RFID systems has been introduced by many applications, but a variety of critical issues remain unresolved. Especially, the impact of collision is the most essential problem. In this paper, we propose a Learning-based Dynamic Framed Slotted ALOHA algorithm (LDFSA) which mitigates collision from the active RFID tags and complies with international standard, ISO/IEC 18000-7. In addition, this paper includes the performance evaluations of the proposed LDFSA algorithm with the conventional algorithms. According to the result, the proposed LDFSA algorithm shows better performance than other conventional algorithms.

Enhanced Precision Time Synchronization for Wireless Sensor Networks

Time synchronization in wireless sensor networks (WSNs) is a fundamental issue for the coordination of distributed entities and events. Nondeterministic latency, which may decrease the accuracy and precision of time synchronization can occur at any point in the network layers. Specially, random back-off by channel contention leads to a large uncertainty. In order to reduce the large nondeterministic uncertainty from channel contention, we propose an enhanced precision time synchronization protocol in this paper. The proposed method reduces the traffic needed for the synchronization procedure by selectively forwarding the packet. Furthermore, the time difference between sensor nodes increases as time advances because of the use of a clock source with a cheap crystal oscillator. In addition, we provide a means to maintain accurate time by adopting hardware-assisted time stamp and drift correction. Experiments are conducted to evaluate the performance of the proposed method, for which sensor nodes are designed and implemented. According to the evaluation results, the performance of the proposed method is better than that of a traditional time synchronization protocol.

Real Time Locating System for Wireless Networks using IEEE 802.15.4 Radio

Real time locating systems (RTLS) determine and track the location of assets and person using active tags. Two or more readers can estimate the tags range from each reader and determine its location. In order to determine tags location, there are several methods. This paper presents a real time locating system which is based on time difference of arrival(TDOA) of a signal, for wireless networks using IEEE 802.15.4 radio. In order to measure the time that a signal arrives at, exact time measurement is crucial. This paper proposes a multi-phase radio method to provide exact time measurement. In addition, to calculate the time difference, readers in the network should be synchronized with themselves. We also present a precision time synchronization protocol. This paper includes the performance evaluation. The performance shows that our RTLS has accuracy of within 3 meters.

Precise location tracking system based on time difference of arrival over LR-WPAN

As the era of ubiquitous computing approaches, there is a growing need for a reliable, efficient positioning and tracking system. Localization involves continuous determination and tracking of the location of assets and personnel. Localization using time difference of arrival (TDOA) involves determining the location of a tag by calculating the time difference of arrival of the signal received from the tag. In order to calculate the time difference of the signal, TDOA-based methods require precision time synchronization of within a few nanoseconds. This paper presents a location tracking system(LTS), which consists of readers, tags, and an engine, based on TDOA in IEEE 802.15.4. In addition, we propose a precise time measurement method and a precision time synchronization protocol which provides a common clock with an accuracy of within a few nanoseconds. The proposed precision time synchronization protocol results in accurate locating services. The performance shows that readers achieve precision time synchronization of within 5 nanoseconds from a reference clock, and the location tracking system has a location error of within 2 meters.