Aug 22, 2011RFID localization can be accomplished either by triangulation or by zoning. Triangulation involves using sophisticated algorithms to localize an RFID tag, based on compared readings collected by three or more interrogators. Zoning, on the other hand, consists of allocating a reader to each zone (room), and tuning its gain in order to detect only tags within said zone. Alternately, with second-generation zoning systems, a single reader can be installed and tuned to cover a large area, with a beacon device deployed within each individual room, in order to delineate a zone.
Since the early days of active RFID, manufacturers have announced that they would be capable of accurately localizing objects with only three readers, using RF triangulation algorithms. More than 10 years after these great promises were first made, however, it has become clear that for indoor real-time location systems (RTLS), a zoning-based approach provides far more reliable results than the respective triangulation-based methods, while at the same time being more cost-effective.
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Triangulation Methods
Triangulation methods were initially based on using RF expansion equations, and comparing the relative signal intensity received by three different readers from a single tag. Each reader's location (coordinates) is recorded at the site of its initial installation. Solving these equations should theoretically lead to knowing the tag's coordinates. This method is known as relative signal strength intensity (RSSI) triangulation. The initial results rapidly showed that this method was not entirely accurate outdoors, and that it was clearly unreliable in indoor environments, especially with partitions. Several causes for these inaccuracies were identified.
The accuracy of the results obtained outdoors with RSSI triangulation was typically plus or minus 10 percent to 15 percent of the distances between the readers. If the interrogators are situated 50 meters (164 feet) apart, for example, the location accuracy will be +/-7 meters (23 feet) or more.
The main reasons for these inaccuracies are:
1. The sensitivity of commercial readers varies from one reader to the other.
2. Reflections in the passage of the RF beams toward each of the readers (from the floor or other elements) are not identical, and can not be ignored.
3. Readers at certain distances can be affected by destructive interferences between the direct beam and a reflected beam from the floor or any other possible reflection.
4. The tag is not uniformly emitting to a range of 360 degrees.
The triangulation-based application in indoor environments with partitions suffers from all of the abovementioned shortcomings. What's more, the results of RSSI-based triangulation indoors within an environment with partitions are especially erratic and unstable. The received signal intensity depends on the types of walls, furniture, persons and other partitions encountered by the beam on its way from the tag to the reader. A path going through open doors, windows and so forth often arrives at the receiver with an intensity that is much higher than the direct path, despite the fact that the distance traveled by said indirect beam was much greater that the direct line.
Wi-Fi-Based Systems
Some companies utilize Wi-Fi tags and make triangulation calculations according to the RSSI received by different Wi-Fi access points. Such systems employ RSSI triangulation methods, using network access points to read and measure the signal strength.
The main argument in favor of these systems is the supposed use of the existing Wi-Fi network. In real terms, however, the number of additional network points and their respective costs that are necessary in order to reach an accuracy of +/-3 meters (9.8 feet) in indoor environments, with partition, seriously challenge these sales arguments.
Furthermore, numerous articles written by experts note the challenges inherent to deploying Wi-Fi based technology (for examples, see The Impact of Next Gen Wi-Fi Technology on Healthcare and Wi-Fi Location-Based Services—Design and Deployment Considerations).
Time Difference of Arrival (TDOA)
The second approach to localization by triangulation is to compare each signal's time of arrival, rather than their intensities. The underlying principle is simple: If a tag's distance varies between the different readers, the closest reader will receive the signal before the second closest, and so forth. The difference in the time of receiving the signals divided by the speed of light should be an indication of the difference in distances between the tag and the three interrogators, thereby allowing us to calculate the tag's position.
For indoor applications, the expected time differences will be in the order of magnitude of a few nanoseconds. Consequently, the readers must utilize expensive internal crystal clocks, and the local-area network (LAN) must be fully synchronized at the nanosecond level. These requirements inevitably make the system expensive. Furthermore, for indoor applications, more than three readers are necessary in order to obtain reasonably accurate results, even in an area measuring only 20 meters by 20 meters (66 feet by 66 feet)—typically, 10 to 12 rooms, including passages.
TDOA-based systems are definitely more accurate than those based on RSSI. Their accuracy varies between 5 percent and 10 percent of the distance between the readers, depending on the respective systems and environments involved. In practical terms, in an indoor environment with partitions (such as offices or hospitals), four synchronized readers are required to cover approximately 15 rooms. The location accuracy with such systems, for indoor applications with partitions, varies between +/-2 meters (6.6 feet) and +/-3 meters (9.8 feet).
Since the triangulation results are expressed in x,y coordinates, ignoring the partitions, a localization accuracy of +/-2 meters or +/-3 meters can easily lead to localizing a person or object in the incorrect room.
Zoning Methods
Zoning methods can be used when the localization requirement is for a zone (room-level accuracy). Initially, zoning methods utilized a reader within each zone (room). Each reader's gain is tuned to receive only signals from tags within said zone. When used in open space, however, this method is not very accurate, as it is impossible to delimit the RF zones by virtual lines. Reflection and tag orientation can easily lead to locating the tag in an adjacent zone. Such errors, however, can be reduced by comparing the respective RSSI levels, though they can not be completely eliminated.
Dynamic filtering is also used to further increase localization reliability. Such algorithms take into account the localization of the previous tag signals, in case of complex situations. Practically speaking, readers deployed using the zoning method, along with RSSI and sophisticated software, can warrant approximately 99 percent accuracy in tag localization, whether situated on a person or on an asset.
The communication protocols used by active tags for first-generation zoning systems are normally unidirectional—that is, they talk to the reader, but the reader can not call the tag ("tag talks first"). Such tags have the advantage of being rather inexpensive compared with other types of active tags. A zoning system's installation is simple and stable, which is also a cost-savings factor. The big disadvantage for this type of system is that it requires a single reader per room, which implies the device's cost, plus the expense of a network point and an electricity point.
Second-Generation Zoning
The active tags employed in second-generation zoning systems are bidirectional—that is, they can both emit and receive signals. A single interrogator is installed and tuned to cover a large area—typically, a circle with a radius of 15 meters to 20 meters (49 feet to 66 feet), passing through several walls.
Zone indicators typically cost only 20 to 30 percent of the readers' price. They are wireless, and some are even battery-operated, so their installation is very easy and requires no infrastructure. In order to conserve the tag battery, some manufactures put the tag in sleep mode. The tag is then awoken by the zone indicator.
While some companies keep confusing the issue for commercial reasons, it is clear, beyond any doubt, that for indoor applications, where room-level accuracy localization is required, the second generation of zoning-based systems is the most suited approach:
• Their localization accuracy is superior.
• The stability of their results is much greater.
• Their cost is significantly lower
• Their installation is very simple and requires almost no infrastructure, other than a network connection and electricity for each reader, covering up to 20 rooms.
Abraham Blonder is the president of Vizbee RFID Systems, a provider of real-time tracking software designed to support all leading RFID technologies, including different types within the same project, for security, inventory and workflow applications. For more information, read How to Implement RFID Projects Without Risk in Days.
