Dense Reader Environments

A dense reader environment is an area where multiple readers are operating in proximity of one another. Imagine five readers, each with eight antennas, operating in a fifty square foot area. In such a situation, you can experience something called “contention.” Contention is when several readers talk at the same time. The noisy environment makes it harder to hear tag responses and typically results in degraded performance. It’s like trying to carry on a conversation at a crowded restaurant. You cannot always hear what the other person is saying and you’re never sure you are being heard above the din.

In these situations the range and rate at which interrogators read tags can be improved by preventing interference from other interrogators. Again, going back to the language analogy, when multiple people speak at the same time, it is more difficult to hear a specific voice. One of the most subtle but important points is Gen 2 does not require interrogators to be designed for dense-interrogator operation.
Additionally, the method which the interrogators use to coexist with other interrogators is allowed to vary from manufacturer to manufacturer. That means that an interrogator from brand A may not work well with one from brand B, though they are both Gen 2 compatible.

Solutions and approaches
First, remember that interoperability between readers should be confirmed in advance of purchase. There are several other methods of preventing the contention caused by dense environments: Listen-before-talk, hardware synchronization, reader network synchronization and frequency-division multiplexing.

Listen-before-talk is a method that tells one reader to listen for another reader’s transmissions before it starts it own. It is similar to being polite in that conversation example above. I don’t start talking until you have finished your sentence.

Hardware synchronization requires the readers have a second wire running between them, so readers can signal each other when one is about to start talking. This method not only creates additional wiring worries, but it is probably manufacturer specific. UHF Gen 2 compatible equipment uses a feature called Dense Interrogator Mode.

Network synchronization is a similar method, only the coordination occurs via a network conversation. Network synchronization typically provides better performance than listen-before-talk because a scheduler guarantees a time for every reader to talk and keeps neighbor readers from talking. The problem with synchronization is when readers from multiple manufacturers try to coexist. Brand A’s method of synchronization is most likely different than brand B’s, so they are going to talk all over one another. Some middleware applications now support this capability with readers from different manufacturers.

Gen 2 has a Dense Interrogator Mode which uses frequency-division multiplexing, which allows all readers to talk simultaneously without synchronization. This is the most commonly supported method of avoiding contention.

If you’re operating in Europe or Japan, readers are required to use listen-before-talk due to local governing regulations. In order to improve performance, some reader manufacturers combine listen-before-talk and network synchronization – this combination offers the best possible performance in dense reader mode environments.