Maintaining RFID Performance while Reducing RF Emissions [VIDEO]

The Impinj Speedway Revolution RFID reader offers a variety of options to professional RFID installers to reduce interference with other devices operating in the unlicensed 900MHz Industrial, Scientific and Medical (ISM) band. This post will explain those features and how to enable them.

The first feature of the Impinj Speedway Revolution reader which allows a user to optimize performance where it's needed while reducing RF emissions is the ability to set the transmit power independently for each of the four antenna ports. This means that only those read zones which require higher power can be set accordingly while other zones can have the conducted power reduced to as low as +10dBm. The key thing here is not to violate the allowed maximum radiated transmit power in the region where the reader is deployed. This maximum radiated power is expressed as either Effective Isotropic Radiated Power (EIRP) or Effective Radiated Power (ERP) depending on the region and is a combination of the conducted power (the power out of the reader antenna port) and the gain of the connected antenna minus any cable loss.

In addition to allowing each antenna port transmit power to be independently set, the Speedway Revolution reader also allows for up to 16 channels to be selected at which to operate at a reduced power of 0dBm (available for Rev 5 Speedway readers in FCC region only).

While not nearly as congested as the 2.4GHz Industrial, Medical and Scientific (ISM) band, there are a number of devices which operate in the unlicensed 900 MHz ISM band. Let's look at an example of deploying an Impinj RFID system in a hospital which uses 900MHz Medical Telemetry System (most MTS devices use other frequencies but we'll use this device as an extreme example).
These devices operate at a fixed frequency of 916.5 MHz in accordance with FCC CFR 47part 15.249.

The maximum permitted field strength of this example device for compliance with FCC Part 15.249 is 50mV/m or 93.98 dBmV/m. Since the field strength limits are specified at a distance of 3m from the radiating source, this equates to a conducted power of approximately -1 dBm when measured at the antenna port.

The frequency of 916.5 MHz falls between the FCC-allocated channels 28 (916.25MHz) and 29 (916.75 MHz). So by reducing the transmit power on these two channels a user can significantly decrease the possibility that power will 'leak' into the MTS channel (without significantly impacting the RFID system performance as the other 48 channels continue to transmit at higher powers).

This feature can be implemented from the ItemTest GUI in the Reader Configuration, under the "Frequency Settings" section, or it can be implemented pragmatically using the Impinj SDK command:ReducedPowerTxFreqeunciesInMhz.Add(910.250)

RFID readers operating in this Ultra-High Frequency (UHF) band in the United States must conform to FCC part 15.247 and utilize Frequency Hoping Spread Spectrum (FHSS) across fifty channels; the 20dB bandwidth of each channel is limited to 500 kHz. The dwell time on each channel is not allowed to exceed 400ms in a 20 second interval.

By simply selecting AutoSet Multiple Reader (when # of readers deployed is greater than half of the available channels in a given region) or AutoSet Single Reader (where # of readers < 1/2 available channels) the Speedway Revolution will automatically adjust and optimize settings so that throughput is maximized while unnecessary RF emissions are minimized. Another Autopilot feature implemented in the Speedway Revolution Reader is to only spend time reading on antenna ports where tags are present; this will happen without any setup required or an installer may choose to implement Low Duty Cycle.

It is possible to set up some logic which tells the Speedway Revolution reader how long to wait with no tags being detected before going into a reduced mode of operation as well as define how often to ping for tags in the read field. In the video below, the Revolution reader is set up to go into Low Duty Cycle (LDC) mode after 5 seconds of not seeing any tags and to scan every 3 seconds for any tags in the read field - this will have the effect of not only reducing RF emissions but also power consumption and dissipated heat on the part of the reader.


Rather than have the Revolution reader constantly transmitting, you can set up for a trigger based on a light beam or pressure mat. In this way, only when the trigger event occurs will the reader begin transmitting to look for tags in the read field.

- Sensitivity -
When considering reduction of RF interference, a system solution approach is recommended that includes not only the RFID reader but also the tags. The reason is that selecting an RFID tag that is EPC GEN 2 (ISO 18000-6C) compliant and offers a lower sensitivity (amount of power required from the reader to activate the tag) will allow the user to reduce the power from a reader and still get the required performance.
For example, tags which use the Monza 4 IC from Impinj offer over 6dB of read sensitivity advantage over some other tag ICs on the market; this means a user should be able to reduce the reader power by 6dB (a 75% reduction in radiated power) and, when combined with the -80dBm read sensitivity of the Revolution reader, still be able to have similar performance.

- Filtering -
There are two methods which can be used to filter tag reads, one is to implement a filter in the application layer by enabling this software filter in middle ware. In this approach all the tags are still transmitting their data in the physical (RF) layer, they are just not passed on to the user.

The other approach which is supported by both Impinj readers and Monza-based tags is GEN 2 filtering. This approach actually instructs those tags not matching certain filter data set by the user to remain silent (do not backscatter/transmit their data), thereby reducing RF emissions and actually improving system performance as only the tags of interest will respond.


One nice feature of UHF RFID is that it inherently is less intrusive on other devices, especially on implantable pacemakers and cardioverter-defibrillators (ICDs) as shown in this report which summerizes in part...

"For both pacemakers and ICDs, no reactions were observed during exposure to UHF RFID. Pacemakers and ICDs were most susceptible to modulated LF RFID readers."

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