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What Is the Ideal Ratio of Tags to Readers for Active RFID-based Real-time Location Systems?

Posted By RFID Journal, 12.05.2011
Is there a point at which there might be too many tagged objects for a particular number of interrogators to read?

—Name withheld

I don't believe there is an ideal ratio of tags to readers. In most deployments, the focus is on using the proper number of readers to cover the area in which a user wishes to locate objects. The number of interrogators may vary for the same number of square feet or meters, depending on the layout and the location accuracy desired. If a facility has a lot of corridors at angles to one another, a higher number of readers might be required than within a rectangular facility measuring the same number of square feet. With many systems, adding additional interrogators allows a company to locate an object more precisely—say, to within five feet instead of 12 feet.

The number of tagged objects is not usually a problem with conventional active systems or real-time location system (RTLS) solutions, because active tags are frequently utilized on larger assets, equipment, tools or vehicles, and these are not typically found in high densities within a small area. You might have 300 shirts tagged with passive tags in a shipping carton, for example, but you would rarely find 1,000 oxygen tagged pumps within a single hospital room.

Active RFID systems have communications protocols allowing a reader to receive signals from multiple tags in rapid succession, just as cell towers have protocols enabling them to receive calls from many cell phones within the same area. I have not heard of an instance in which an active RFID-based RTLS was hampered by a high number of tagged objects.

—Mark Roberti, Founder and Editor,
RFID Journal

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