RFID Journal Staff asked 13 years ago What exactly is done at the modulation site of a tag's IC to put the information into the side band carrier "far" away from the base band of a reader? In the article, there is described a sort of "listen-before-talk" (LBT) function—if a reader "hears" another interrogator using that channel, it will go to another channel to avoid interfering with the reader on that channel.
—Werner (Germany)
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Werner,
I'm not sure to which article you refer, but I reached out to Impinj, a provider of ultrahigh-frequency (UHF) RFID chips, to answer your question regarding what happens on a transponder's chip to enable tags to avoid interference. Here is the response from the company's engineers:
"There are two types of modulation for tags: FM0 and Miller-modulated subcarrier. A Miller-modulated subcarrier is one that moves energy away from a carrier. Section 6.3.1.3.2.3 of the Class 1, Gen 2 specification (available here) provides a function description and diagrams."
As for the "listen-before-talk" protocol, a reader "listens" for RF signals from other tags or interrogators. If the device receives signals on a specified channel, it will not emit energy on that channel to power up tags. Instead, it will try another channel until it finds one on which no signals are being received. It will then broadcast energy to power up the tags, which will reflect back a signal on that channel.
I hope that answers your question.
—Mark Roberti, Founder and Editor, RFID Journal
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