Why Zoning is preferable than Triangulation for Indoor RTLS
Performance and cost effectiveness of Zoning Real-time location in comparison with Triangulation methods for indoor applications
The purpose of this article is to show why for indoor RTLS, a zoning-based approach gives far more reliable results than the respective triangulation-based methods, while at the same time being more cost effective.
RFID localization can be done either by triangulation, i.e. using sophisticated algorithms to localize the RFID tag based on compared readings by three or more readers, or by zoning, i.e. allocating a reader to each zone (room), tuning its gain in order to detect only tags in said zone. This article will compare the cost effectiveness and performances of these two approaches in outdoor and indoor environments.
Since the early days of active RFID, manufacturers announced that they would be capable of accurately localizing objects with only three readers, using RF triangulation algorithms. More than ten years after these great promises, let’s check what today's situation in real conditions is.
- Relative Signal Strength Intensity (RSSI)
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. The location (coordinates) of the readers is recorded at their initial installation. Solving these equations should theoretically lead to knowing the coordinates of the tag. This method is known as RSSI triangulation. The initial results rapidly showed that this method was not totally accurate outdoors, and clearly unreliable in indoor environments, especially with partitions. Several reasons were identified as the cause of these inaccuracies:
Outdoor environments with line of sight :
The accuracy of the results obtained outdoors with RSSI triangulation were typically +/- 10-15% of the distances between the readers. If the readers are 50 meters apart, the location accuracy will be +/- 7 meters or more.
The main reasons for these inaccuracies are:
The sensitivity of commercial readers varies from one reader to the other.
Reflections in the passage of the beams toward each one of readers (from the floor or other elements) are not identical and cannot be ignored.
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.
The tag in not uniformly emitting to a range of 360˚.
Indoor environments with partitions
The triangulation-based application in indoor environments with partitions suffers from all the shortcomings mentioned above, in addition to the following problems:
The results of RSSI-based triangulation indoors in an environment with partitions are erratic and unstable. The received signal intensity depends on the type of walls, furniture, persons etc. encountered by the beam on its way between the tag and the reader. Often a path going through open doors, windows and so forth, arrives at the receiver with an intensity that is much higher than the direct path, despite the fact that the length of the pattern of said indirect beam was much greater that the direct line.
Although manufacturers of such systems are trying to average their results over several readings and calibrate the readers upon installation using special installation software, the results remain inaccurate and unstable. Opening a window or a person passing by can move the result by several meters, despite all these attempts to overcome problems that are inherent to this method.
- Wi-Fi based systems
Some companies use Wi-Fi tags and make triangulation calculations according the RSSI received by different access points. Despite being called Wi-Fi, these technologies are in fact a type of RSSI triangulation methods, using special network access points (reading and transmitting the measurement of the signal strength) instead of readers.
The main argument in favor of these systems is the supposed use of the existing Wi-Fi network. However, in real terms the number of additional network points and their respective costs that are necessary in order to reach the accuracy of +/- 3 meters in indoor environments with partition seriously challenge this sales arguments.
Furthermore numerous articles by experts are pointing out significant negative impacts of Wi-Fi based systems; see as one example: http://www.integrasystems.org/main/images/stories/Whitepapers/healthcare-wp-020310.pdf or Why not Wi-Fi by David Hoglund.
These systems are very complex to deploy and maintain. See another example: http://docstore.mik.ua/univercd/cc/td/doc/solution/wifidesi.pdf
- Time Difference on Arrival (TDOA)
The second approach to localization by triangulation is to compare the time of arrival of the signals rather than their intensities. The underlying principle is simple: If the distance of a tag varies between the different readers, the closest reader will receive the signal before the second closest reader and so on. 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 readers, thereby allowing us to calculate the tag’s position.
In indoor applications the expected time differences will be in the order of magnitude of few nanoseconds (10-9 second). Consequently:
The readers must use expensive internal crystal clocks.
The LAN must be fully synchronized at the nanosecond level.
These requirements inevitably make the system expensive. Furthermore, in indoor applications more than three readers are necessary in order to get reasonably accurate results even on an area of only 20 x 20 meters, i.e. 10-12 rooms including passages.
In order to compensate for the difference between the electronics of one reader and another some companies use complex tuning procedures as part of the project implementation, while others use patented double-pass measurements that make the system even more expensive.
Some systems use Ultra-wide- band (UWB) in order to improve the range of detection by using short pulses over a broader spectral range, generally using higher frequencies (3.1 to 10 Ghz) .
All these technologies suffer from multi-pass beams that reduce their localization accuracy.
Outdoor environments with line of sight
TDOA based system are definitely more accurate than RSSI based systems. Their accuracy varies between 5-10% of the distance between the readers, depending on the respective systems and environments.
In practical terms, in an indoor environment with partitions (offices, hospital etc.), four synchronized readers are required to cover approximately 15 rooms. The location accuracy with these systems in indoor applications with partitions varies between +/- 2 to +/-3 meters.
Since the results of the triangulation are expressed in x,y coordinates, ignoring the partitions, a localization accuracy of +/- 2m or +/- 3 meters easily leads to localize a person or an object in the incorrect room.
Zoning methods can be used when the localization requirement is for a zone (room level accuracy). The zoning methods initially used a reader in each zone (room). The gain of each reader is tuned to receive only the signal from tags in said zone.
The results of this method when used in open space are not very accurate, as it is impossible to delimit the RF zones by virtual lines. Reflection and tag orientation can easily lead to locate the tag in the adjacent zone. These errors, however, can be reduced by comparing the respective RSSI levels, but cannot be completely eliminated.
The zoning method is mostly for passages (doors) or indoor environments like office spaces, hospitals and so forth. Tuning the reader for each room at the installation allows precise and stable room level accuracy. Even if a tag close to a wall could be detected by the reader covering the adjacent room, the intensity of the signal - after going through the wall - would be significantly lower than the intensity of the signal received by the reader in the same zone. The software application will therefore consider that the tag is in the correct area.
Dynamic filtering is also used to further increase the localization reliability. Such algorithms take into account the localization of the previous tag signals in the decision process in case of complex situations.
Practically speaking, readers including RSSI tag transmission combined with sophisticated software can warrant approximately 99% accuracy in tag localization, whether on a person or an asset.
- Zoning method first generation
The active tags used by such systems are normally uni-directional, i.e. they “talk” to the reader, but the reader cannot “call” the tag (“Tag talk first”). Such tags have the advantage of being rather inexpensive compared to other system tags.
The installation of a zoning system is simple and stable, which is also a cost-saving factor.
The big disadvantage of this type of system is that it requires one reader per room, which implies the cost of the reader plus the cost of a network point and an electricity point.
- Zoning method second generation
The active tags in this method are bi-directional, i.e. they can emit as well as receive. A single reader is installed and tuned to cover a large area (typically a circle radius of 15 to 20 meters passing through several walls) .
A device (zone indicator) is installed in each individual room. This device periodically emits signals indicating to the tags the ID of the zone in which they are located. The zone indicators can be tuned to reach only the tags that are in the room.
Upon receiving the signal from the zone indicator, periodically, or in response to a call by the reader, the tags emit a signal that includes their own ID as well as the ID of the zone in which they are situated.
Zone indicators typically cost only 20-30% of the price of the reader. They are wireless and some of them are even battery operated, so that their installation is very easy and doesn’t require any infrastructure.
In order to save the tag battery, some manufactures put the tag in sleep mode. The tag is woken up by the zone indicator.
Zone indicators are normally radio emitters. Some companies use IR emitters as zone indicators, in which case the tags must include an IR sensor.
While for commercial reasons some companies keep confusing the issue, 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 definitely the most suited approach from any point of view in comparison to all the methods that we have reviewed :
Their localization accuracy is superior
The stability of their results is much better
Their cost is significantly lower
Their installation is very simple and requires almost no infrastructure, other than one network point and electricity per reader, covering up to 20 rooms.