The RSSI value is an indicator of signal strength, in praxis used for estimating distance. By applying RSSI filters one can optimize RFID reading and writing conditions for specific applications, so that only tags within a certain distance are registered.
Authors: Hanna Östman & Toni Heijari
WHAT DOES THE RSSI VALUE TELL US AND WHAT IS
IT GOOD FOR?
It's fairly simple. RSSI stands for "Received Signal Strength
Indicator" and it tells us the strength of the signal sent from an
RFID tag to an RFID reader. But for what do we use that information
- For estimating the distance between an RFID tag and an RFID
- For identifying the movement direction of an RFID tag
- For setting optimal reading and writing conditions
- For analysing and optimizing the environment
- For locating lost tags
The RSSI value gives us a clue to what the distance between an
RFID reader and an RFID tag might be. The environment has a huge
effect on the results, but as a rule of thumb we can say that the
higher the RSSI value, the closer the RFID tag is to the reader.
The RSSI value can give us an indication of the distance to the
tag, but only after we analyse the environment and take it into
The RSSI value can also be used to determine in what direction a
tag is moving. As the RFID tag moves closer to the RFID reader, the
RSSI value increases. When the RFID tag moves away from the RFID
reader, the signal gets weaker. The RSSI value only tells us the
distance to the tag, not where it is going. Several RFID readers
are usually required to achieve reliable information about the
direction. It is also usually recommended to use supporting
techniques, such as "phase difference", for more reliable
Measurements of the RSSI value can be used to analyse the tag
reading and writing conditions. The optimal condition is to be able
to use as little power as possible for achieving a strong enough
signal to perform the RFID actions in question. Different actions
require different power outputs and RSSI settings.
Many retail and supply chain operations require a "locate tag"
application. The RSSI value can be used for guiding the user closer
and closer to the target, much like a Geiger meter.
ENVIRONMENTAL EFFECTS ON RSSI VALUES
We are constantly surrounded by obstructions and environmental
challenges, especially indoors. This can create problems when
trying to estimate the location and movements of a tag.
Factors that can cause RSSI variability:
- Metals and other reflecting materials causes the signals to
- Liquid elements that absorb the signal
- Some types of materials that RFID tags are attached to affect
- Blocking objects and constructions between the RFID tag and the
- Height difference between the RFID tag and the RFID reader
(causes the distance to appear longer)
- Relative orientation of the RFID tag and the RFID reader
APPLYING RSSI FILTERS
During RFID scanning we want the right tags, and only the right
tags, to be registered. This can be done by applying a filter that
rules out tags with too strong or too weak RSSI values. In plain
English this means that altering the RSSI filter will allow the
reader to only see tags that are within the set distance range,
e.g. 30-60 cm (12-24 inches), depending on the needs of that
When writing tags it is important that the RSSI value is high,
so that the tag has sufficient power. RSSI filtering is used for
ensuring that the RFID reader is close enough to the RFID tag to
complete the writing operation.
In store environments and during the supply chain we often run
into situations where it is important that a reader only registers
tags that are in a certain distance from the reader. By finding the
right filter settings for the particular environment and operation,
we can assure that only the right tags are read avoiding
RSSI FILTER TEST BY TONI HEIJARI
Toni Heijari, RF Design Engineer at Nordic ID, performed RSSI
tests to demonstrate the effects that RSSI filters have on reading
Used settings and equipment:
- Used power level: 100mW
- Tag: UPM RAFLATAC Belt
- Environment: Office environment, room temperature 20±2°C.
- RFID Reader: Nordic ID Sampo S1 UHF RFID Reader
- External antenna (gain: 0dBic)
MIN RSSI setting defines the minimum tag
reply power level
MAX RSSI setting defines the maximum tag
reply power level
Picture 1. Reading distances
with minimum and maximum RSSI value settings.
The blue line demonstrates the effect of using the MIN RSSI
setting with a maximum read distance. The orange line describes the
minimum read distance by using the MAX RSSI setting.
Picture 2. Distance range with
same RSSI value
The window that is left open between the two graphs, shown in
picture 2, is the area where the tag reply RSSI value is the same
for a certain distance range. This means that for example the RSSI
level -60dBm can be found between 25-45 cm (10-18 inches) from the
Table 1 indicates the MIN and MAX RSSI values that the reader can
Table 1. MIN and MAX RSSI
The test results of this experiment only apply for this specific
scenario. If any of the used settings and equipments change, the
experiment should be redone.
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