Backscattering is the heart of passive UHF RFID systems. Understanding backscattering will help you optimize the read range in your RFID systems.

In this previous post we talked about read range in UHF RFID systems. Now that you understand better the implications of the values given as ‘read range’, we will take you to the heart of where these numbers come from.

UHF RFID is based on a physical phenomenon called backscattering. This phenomenon allows you to build up systems to power up and communicate with battery-free devices – UHF RFID tags in this case.

RFID tags do not transmit information but backscatter the signal created by the reader. Transmitting data requires too much energy for a battery-free device so passive RFID use the signal sent by the reader to communicate data back.

You can think about backscattering as the tag reflecting the electromagnetic waves back to the reader. RFID tags have the ability to reflect these waves or let them pass, which means the ability to generate communication: the reader will receive this wave back or not, which is interpreted as a basic digital ‘message’ – zeros and ones.

RFID backscattering

RFID backscattering

Reader to tag communication

The RFID reader – together with the reader antenna, do not forget this part – generates an RF field. This field’s ‘size’ and ‘pattern’ will depend on the reader and antenna selected. Note that environmental characteristics will also have an impact on this field – different materials impact the propagation of the electromagnetic fields in a different way.

This RF field carries both energy and signal. The tag antenna harvests the energy from the RF field and delivers it to the RFID chip. We now have an active chip that can decode the signal sent by the reader within the RF field.

The RFID tag’s sensitivity will be the limitation to the read range in this part (reader to tag power transfer and communication). The lower the energy required for the tag to power up and receive the communication from the reader, the longer the read range.

Note that sensitivity in the RFID tag is linked to two activities: energy harvesting and correct signal demodulation. Reader to tag link may be limited by one or another, so the sensitivity is the value of the worst of them.

Tag to reader communication

Once the tag is powered up, it will demodulate the signal, do some internal processing – simple when using just RFID tags and a little bit more complex if they also include sensors or actuators – and communicate the data back to the RFID reader.

As discussed earlier, the tag does not transmit this information but backscatters it. No signal is generated by the tag but the field generated by the reader is used to convey the data back to the reader by reflecting or letting the waves pass by.

The key here is to generate reflections good enough so that these are sensed correctly at the reader. This implies two things:

  • The RFID tag needs to generate correct modulations – basically being able to reflect and let waves pass in a good timing.
  • The reader needs to be able to sense the reflected waves. This is a specification in RFID readers that you will find as ‘sensitivity’. Note that the reflected waves have very low power so it is complex to sense them. Luckily, there is power at the reader side so more complex electronics are allowed.

Read range will be limited by the worst of these two: reader to tag communication and tag to reader communication. Understanding this will help you identify the limitation and working on finding the correct component for your application – reader with better sensitivity, just higher output power, tag with a higher antenna gain, etc.

To learn more about the topic, download the free eBook ‘The secrets of read range in UHF RFID sensor systems‘.

Read range in RFID systems eBook

Read range in RFID systems eBook