Hyunsung Jang

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.

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.

Robustness of Distributed RTLS for Dense Large-Scale Environments

RTLS (Real Time Locating Systems) are used to track the location of an object or person in real time. However, if there are many tags and readers, the conventional centralized RTLS server with a single location engine may fail to estimate the location of the tags. Furthermore, if the server does not receive a tag’s signal due to pass loss or none line-of-sight (NLOS) from more than three readers, the server fails to estimate the location of the tag. In this paper, we propose a special reader that incorporates an RTLS location engine for distributed RTLS. Furthermore, we use a multidirectional antenna array, alleviating the multipath effect and allowing estimation of a tag’s location using only two readers. To assess the performance of the system, we implement an entire RTLS system based on IEEE 802.15.4a. The experiment results show that the distributed RTLS enhances the location response time and reduces network confusion. Moreover, we can obtain the tag’s location with only two readers using directional information.

Time synchronization via clock skew correction on ZigBee networks

Time synchronization in distributed networks is one of the most crucial and fundamental issues. Inaccurate factors in synchronizing clocks between nodes in the network can occur at any point in the network layers. This proposes synchronization via clock skew correction to minimize a time representation error at the radio transceiver and to improve synchronization accuracy.

Multi-phase correlator-based realistic clock synchronization for IEEE 802.15.4 networks

Clock synchronization is one of the most crucial and fundamental issues in distributed networks. Inaccurate factors in synchronizing clocks between nodes in the network can occur at any point in the network layers. Most uncertainties caused at the upper layers can be eliminated by hardware-assisted time stamping. However, eliminating the uncertainty of a physical layer is difficult.This paper proposes a multi-phase correlator-based clock synchronization method to mitigate the time representation error at the physical layer and improve synchronization accuracy by introducing a time representation error, which is one of physical uncertainties. Further, to apply the proposed method to a realistic environment, we implement and evaluate the proposed multi-phase correlator. Our experimental results show that the accuracy of the proposed method is better than that of a conventional approach in terms of minimizing the time representation error.

Multi-Phase Correlator-based Realistic Clock Synchronization for Wireless Networks on IEEE 802.15.4

Clock synchronization in distributed networks is one of the most crucial and fundamental issues. Inaccurate factors in synchronizing clocks between nodes in the network can occur at any point in the network layers. The most uncertainties caused at upper layers can be eliminated by hardware assisted time stamping. However, eliminating the uncertainty of a physical layer is difficult. This paper introduces an uncertainty called a time representation error and caused the physical layer, and proposes a multi-phase correlatorbased clock synchronization method. The proposed method minimizes the time representation error at the physical layer to improve synchronization accuracy. Further, to apply the proposed method to realistic environment, we implement and evaluate the proposed multi-phase correlator. Experimental results show that the proposed method has superior accuracy than a conventional approach in terms of the time representation error.