wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

30-4 Industrial Communication Systems
30.3 Localization
Healthcare environments encompass human resources, such as physicians, nurses, medical staff, and
equipment, that must be correctly managed to efficiently satisfy a given set of objectives. Resource localization
is an important factor when evaluating management actions, since it provides information that allows
the improvement of medical response times, which is of the utmost significance in life-critical scenarios.
In addition, localization can potentially have a strong impact concerning security and safety in healthcare
environments. Given that many hospital equipments are expensive and can be handheld, the possibility
of theft can be avoided by having a real-time localization system tracking their localization and triggering
an alarm when carried beyond a predefined physical area. Furthermore, in scenarios where patients must
remain in a restricted area (the psychiatric ward or the neonatal ward of a hospital, for example), localization
information is highly relevant as it hinders occurrences in which hospitals may be held accountable (violence,
child kidnap, etc.). Regarding safety, in healthcare environments having patients with a high risk of
brain injury and stroke (e.g., hospitals with elderly inpatients), localization can be of paramount importance,
as it can shorten the medical response time and reduce the chances of irreversible physical impairments.
Currently, four types of localization are available: direct identification, time acquisition, angle detection,
and receiver signal strength (RSS). Direct identification simply detects if a tag is present/absent in
the neighborhood, i.e., the result of a localization round is either true (present) or false (absent). Radiofrequency
identification (RFID) [6] is the most common technology employed in this type of localization.
Time acquisition localization, as the name suggests, is based on time measurements and includes
techniques such as time of arrival (TOA) [7,10,11], time difference of arrival (TDOA) [8,11,12], and differential
time difference of arrival (DTDOA) [9]. Angle detection includes the angle-of-arrival (AOA)
localization technique [10–12] employing stations capable of determining the direction of propagation
of a packet by using multiple receivers. RSS localization [10–12] is supported on the ability to measure
the energy of a received packet, which provides a rough estimate of the traveled distance (if the transmission
power is known). Several localization systems using these techniques have been proposed in the
literature. However, only a few have become commercially available and can currently be used in real
deployments. The following paragraphs provide an overview of such systems.
The Exavera eShepherd• [13] system is supported on direct identification and was specifically
designed targeting healthcare environments allowing the tracking of patients (Vera-T Bracelets), medical
staff (Staff Badges), and objects (Asset Tags). Besides the RFID-enabled mobile devices (bracelets,
badges, and tags), this system includes Vera-Fi devices (RFID reader plus Wi-Fi Access Point) and relay
RFID transceivers (RRT), responsible for relaying data packets. Mobile devices periodically transmit
RFID packets, which are received by neighbor Vera-Fi devices and transmitted (using Wi-Fi) to a hospital
information system identifying the area where the “tag” is localized. The Exavera eShepherd localization
system specifies an accuracy of 3.m (95th percentile, herein assumed as default).
Ubisense [14] is a real-time localization solution with subsecond response and proven robustness in
challenging manufacturing environments that employs a combination of the AOA and TDOA techniques.
Although originally designed targeting industrial applications, it can also be applied in healthcare
environments. The Ubisense system is composed of a network of sensors (Ubisensors)—a cell,
mobile tags (Ubitags), and a Ubisense software platform (USP) running in a PC. The Ubisensors are
equipped with UWB and RFID transceivers installed in known positions, connected by an Ethernet
network. A Ubisensor simultaneously transmits a UWB impulse sequence and an RF message with its
identification (ID). These messages will be received by the nearby fixed Ubisensors. Each cell has a single
Ubisensor playing the role of the master, i.e., responsible for controlling the medium access using time
division multiple access (TDMA). In addition to the received RF identification, Ubisensors determine
the UWB impulse angle of arrival (and instant of reception) through the use of an array of antennas. The
master Ubisensor calculates the TDOA for each Ubisensor relative to a given Ubitag and sends all data
(ID, AOA, and TDOA) to the USP, which computes the Ubitag localization. This solution is particularly
suited for scenarios requiring high accuracy (30.cm).
© 2011 by Taylor and Francis Group, LLC