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Why Zone-Based Real-time Location Systems Are Superior
For indoor applications, zone-based approaches provide far more reliable results than triangulation-based methods, and at a cheaper cost.
Aug 22, 2011—RFID 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.
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.
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