RFID Systems & Testing: Transmit Power vs. Receive Sensitivity

Introduction

When implementing an RFID system, sometimes a good amount of RFID testing is required in order to ensure that the optimal read area/distance is achieved for the application. While some RFID applications are not particular about the distance or size of the  read zone, conversely, quite a few applications are focused on reading tags in a specific zone, or coverage area.

An example of this is a vehicle manufacturing facility outfitted with multiple read zones for work-in-process. During the manufacturing process, the vehicle is constantly moving throughout the facility, entering and exiting different stations, each station being unique to install a different component. At each station, an RFID system exists to read the vehicle’s unique tag once it enters the specific station so that each vehicle’s progress is tracked throughout the facility. If one station’s RFID system is reading over 20 feet and is able to pick up vehicles in different stations, the system is flawed. The vehicle in station #20 is now being read in stations #20 and #21 - meaning that the data is now inaccurate. This can be remedied by defining each read zone using RFID reader settings such as Reader Transmit Power, and Reader Sensitivity.

RFID Transmit Power

RFID transmit power is the amount of RF energy emitted by an RFID reader in order to communicate with RFID tags in the area. RFID transmit power is measured and specified in decibels per milliwatt, or dBm. A reader’s transmit power is configurable from 0 dBm up to its max power level, which is usually between 27 dBm and 31.5 dBm. An RFID reader’s transmit power can easily be converted from dBm to milliWatts or even to Watts via a table like the one  available here.

During RFID testing, a reader’s transmit power can easily be changed and configured in order to control the read distance at which an RFID system will read RFID tags. Reducing or increasing a reader’s transmit power is not a simple change because transmit power is measured on the logarithmic scale. For example, if you reduce a reader’s transmit power from 30 dBm to 27 dBm, you are effectively reducing your reader’s power by 50%. When experimenting with reader transmit power, it’s important to remember that changing your transmit power by 3 dBm will half or double the power at which your RFID system currently resides. To learn more about RFID reader’s transmit power, take a look at our article “ 10 Facts about Reader Transmit Power”

Below is a quick video that shows how to change a reader’s transmit power via Impinj’s ItemTest Software and the effect the change has on reading an RFID tag.

When you raise or lower the transmit power, you are basically just changing the amount of power being pushed out through the RFID antenna to read tags at a further or shorter distance. A lower transmit power can be important for defining a read area and avoiding stray tag reads. Be sure to experiment with different transmit powers during RFID testing in order to understand your system's needs.

RFID Receive Sensitivity

RFID Receive Sensitivity (Rx Sensitivity) is basically a measure of how sensitive the RFID reader is when detecting RFID tags. Before we dive into receive sensitivity, it’s important to first understand the concept of  RSSI or Received Signal Strength Indicator.

In a UHF RFID system, readers send an interrogation signal to a nearby tag, which responds in turn via backscatter. The reader analyzes the response and reports the tag’s data along with the signal’s RSSI. RSSI, or Received Signal Strength Indicator, is a measurement of the power received from the returned signal from an RFID tag when interrogated by a reader.

When a reader reports a tag’s RSSI value, it is effectively reporting the power level of the tag’s backscattered response signal as it relates to the power level of the reader’s initial transmission signal.

An RFID tag’s RSSI value can range anywhere from - 30 to -92 dBm and the closer the RSSI value is to 0, the better the signal. The more sensitive the RFID reader is, the more likely the reader is to read tags with weak signals. Most fixed RFID readers have a max receive sensitivity between -84 dBm and -92 dBm and these values can be configured just like reader transmit power.

Users can actually control receive sensitivity by inputting an RSSI value range that they would like to see. For instance, a user can tell the  Impinj R700 to only report RFID tags with an RSSI value between -30 and -55. With this set parameter, the RFID reader would still read all the RFID tags in the area, but it would only report tags within the set RSSI value. Configuring a reader’s receive sensitivity is comparable to placing a filter on RFID tag reads.

Below is a quick video that shows how to change a reader’s receive sensitivity via Impinj’s ItemTest Software and the effect the change has on reading an RFID tag.

When to Configure Reader Transmit Power vs. Receive Sensitivity

In most RFID systems, it is recommended to test different settings for both transmit power and receive sensitivity in order to create the ideal read zone. But below I will provide two examples of applications that might lean toward using one over the other.

In  smart shelving applications, tagged items are typically very close together, which means that configuring receive sensitivity might be slightly more difficult. For example, the RSSI values on one shelf antenna might be around - 30 dBm, and the tagged items on either side could be as close as around -35 dBm. With RSSI values that close together, applying a filter on receive sensitivity could lead to missing important tag reads. In addition to tagged item proximity, shelf antennas already have a very defined read area, created to avoid reading stray RFID tags. In this type of application, configuring reader transmit power could be the most impactful.

In manufacturing facilities with multiple RFID systems and read zones, if the read zones are spaced out within the environment, configuring receive sensitivity might be the most impactful for defining these zones. This is because the RFID-tagged vehicles will either be directly in the read area, or a few feet or meters away in a different read area - which would mean the tag RSSI values would be clearly different.

Those two examples are not any type of guarantee of which setting you should use for your application, because every application and application environment is different. Typically, during RFID testing, when defining a read zone, both transmit power and receive sensitivity will have to be configured. Adjusting both of these settings can ensure that the correct tagged items are read within a zone and any stray tag reads are eliminated.

Conclusion

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