UNDERSTAND RSSI VALUE TO GAIN MORE CONTROL OVER YOUR RFID MANAGEMENT SYSTEM

The RSSI value is an indicator of signal strength. In practice, it’s used for estimating distance and works a bit like wi-fi, reading the tags in a given area.

By applying RSSI filters you can optimize RFID reading and writing conditions for specific applications, so that only tags within a certain distance are registered.

Ultimately, understanding how RSSI value works will give you more control over your RFID management system. 

WHAT DOES THE RSSI VALUE TELL US AND WHAT IS IT GOOD FOR?

It’s fairly simple. RSSI means Received Signal Strength Indicator and it represents the strength of the signal sent from an RFID tag as detected by an RFID reader.

But what can you use that information for in practice?

  • Measuring the distance between an RFID tag and an RFID reader
  • Identifying the movement direction of an RFID tag
  • Setting optimal reading and writing conditions
  • Analyzing and optimizing the environment
  • Locating lost tags

The RSSI value provides a clue to the distance between an RFID reader and an RFID tag. Generally, the higher the RSSI level, the closer the RFID tag is to the reader. Since the environment has a huge effect on the results, you must also analyze and account for the environment.  

Changes in the RSSI value can be used to determine the direction that a tag is moving in. 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. Since 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 movement direction. It is also usually recommended to use supporting techniques, such as “phase difference”, for more reliable results.

RSSI value measurements can also be used to analyze the tag reading and writing conditions. Optimally, minimize the power needed to obtain the signal strength required for the RFID actions you want. The required power outputs and RSSI settings depend on the action.

Many retail and supply chain operations require a “locate tag” application. The RSSI value can be used to lead the user toward 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 levels to vary:

  • Metals and other reflecting materials can cause signals to bounce uncontrollably
  • Liquid elements can absorb signals
  • Some materials affect the signals from RFID tags that are attached to it
  • Blocking objects and constructions between the RFID tag and the RFID reader 
  • Height difference between the RFID tag and the RFID reader,  which causes distances to appear greater than they are)
  • Relative orientation of the RFID tag and the RFID reader

APPLYING RSSI FILTERS

During RFID scanning we want only the right tags, and no others, to be registered. This can be done by applying a filter that rules out tags with too strong or too weak RSSI values. Simply put, the filter will allow the reader to see only the tags that are within a defined distance range, e.g. 30-60 cm (12-24 inches), depending on the needs of that particular operation. 

When writing tags, it is important to have a high RSSI value, so that the tag has sufficient power. You can use RSSI filtering to ensure that the RFID reader is close enough to the RFID tag to complete the writing operation.

In-store environments and in supply chain contexts there are often situations where it is important that a reader only registers tags that are at a certain distance from the reader. By using appropriate filter settings for the particular environment and operation, you can avoid unwanted cross-reading and ensure that only the desired tags are read.

RSSI FILTER TEST BY NORDIC ID

One of the RF Design Engineers at Nordic ID performed RSSI tests to demonstrate the effects that RSSI filters have on reading distance. 

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

Figure 1: Reading distances with minimum and maximum RSSI value settings

In figure 1, the blue line demonstrates the effect of using the MIN RSSI setting with a maximum read distance, while the orange line describes the minimum read distance by using the MAX RSSI setting.

Figure 2: Distance range for the same RSSI value


When the 2 graphs are combined, as in figure 2, the area between the lines represents the distance range within which the tag reply RSSI value is the same. This means that, for example, between 25 to 45 cm (10 to 18 inches) from the RFID reader, the RSSI level -60dBm  is read.


Table1 : MIN and MAX RSSI values that can be received 

The results of this experiment are specific to the environment and scenario. If any of the settings or equipment change, these results are no longer valid.