From what I've found so far, when scanning multiple passive RFID tags at once, collisions will occur so some collision avoidance algorithms like pure aloha and slotted aloha are probably used.What I don't understand is how do passive RFID tags even support these algorithms?Does this mean that they have a CPU?Another thing I don't understand is how does a passive RFID tag know which information bit (serial number for example) to send when it receives a signal from a reader?
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2 Answers
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Passive RFID tags just react to the right RF field. If you don't want collisions, it's up to you to make sure only one at a time is within the near field of the antenna.
Fortunately for you, between the extent of the near field, and the distance at which enough power is transmitted to run the tag, the useful volume around a RFID reader is rather small. For most normal use cases, it's not too hard to isolate individual tags to read them.
answered Apr 2, 2018 at 11:13
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\$\begingroup\$ If only one at the time should be read, how are passive RFID tags supposed to be used in retail industry for quick shopping (just take things and walk out of the store)?How do they know when to send the next bit from their ROM?To me it looks like they should have some RAM memory but how would they even store electric charge (for both RAM and ROM)? \$\endgroup\$ Apr 2, 2018 at 11:29
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\$\begingroup\$ @user3711671 There is anti-collision mechanisms built into the protocol. For example, ISO14443, which specifies the low level protocol used in many popular NFC / RFID tags, has anticollision, so 2 tags can be in the field of view of a reader and both tags can be read. \$\endgroup\$– CHendrixApr 2, 2018 at 12:14
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Passive RFID tags do have a full featured CPU with RAM and ROM. This fact should answer your questions. You could build your own tag with a device like Arduino. The tag is powered by inductive coupling of two coils, similar to a transformer with some distance between the coils.
answered Apr 2, 2018 at 12:37
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\$\begingroup\$ How do they store electrical charge then?How many clock cycles can they get out of one radio signal? \$\endgroup\$ Apr 3, 2018 at 7:33