Avoiding Tag & Reader Collision

While scanning for RFID tags does your reader appear confused, unable to process the information you expect it to receive? Are there a large number of tags within the read field? If so, your RFID scanner is likely unable to decipher the information due to tag collision, a problem that arises when multiple tags are energized by an RFID reader simultaneously, leaving the reader unable to differentiate each tags signal.

There are many different methods of solving the problem. For example, readers can be programmed to recognize tag collision and upon doing so can send a special signal called a “gap pulse.” Once the tags receive the signal, each one consults a random number counter to determine the interval to wait before sending its data. Since every tag gets its own unique number interval, each one will send its data at a different time.

Another solution is to use a tree-walking algorithm, in which a reader will essentially send sequential messages to each tag prompting them to respond if their binary digits follow a certain pattern. For example, a reader might say to a tag, “If the first binary digit in your serial number is 0 respond.” Twenty tags might respond. The reader can then say, “If your first digit is a 0 and your second digit is a 1, respond.” 10 tags might respond. The reader can then ask about the third digit and so forth, until only a single tag responds. If neither of these methods strike your fancy, you can try using an ALOHA protocol, a method in which a transmitting station listens to the two-way communication between readers and tags to tell whether the two groups had a successful exchange. If communication was unsuccessful, it’s likely because a tag collision took place, at this point the sender station transmits again after a random wait period. Tags will then periodically send data after the waiting period.

Reader collision can also happen when two or more readers are being used near one another. The problem occurs when the coverage area of one RFID reader overlaps with that of another reader, causing two problems: signal interference and multiple reads of the same tag. This can sometimes be fixed by simply changing the frequencies of the readers. For example, if multiple readers are using high frequencies, they are much more likely to have their signals overlap. Another option is to use “frequency hopping,” in which a reader randomly jumps between channels. If one channel is in use, the reader can then move to an alternate channel and broadcast a signal to the tags.

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