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What Factors Affect the Reading Performance of RFID Systems Within a Warehouse?

Posted By RFID Journal, 10.13.2011
Thanks in advance for your cooperation.

—Mayadah Hassan
(Ph.D. student at Brunel University, in West London)

Dear Mayadah,

The answer to your question depends on the type of RFID system that you are using within a warehouse. A company might employ high-frequency (HF) or ultrahigh-frequency (UHF) passive tags, or an active RFID system. Each type performs differently, and different factors would affect the performance of each technology.

Active tags broadcast a signal, just as a cell phone does. For the most part, active tags do not have many performance issues. Because an active tag has a battery and sends out a signal, that signal can usually reach a reader. If there are metal storage units that could shield the tag from interrogators, however, this could cause problems. Other than that, active tags should work consistently.

Passive HF tags communicate with readers through inductive coupling. Loop antennas on the tag and reader form an electromagnetic field. The tag modulates and demodulates its antenna, causing subtle changes within the read field, which are then picked up by the reader antenna. (Imagine two people with their eyes closed, each with one hand on a balloon; these individuals could use Morse code to communicate by squeezing that balloon.)

Once you've set up HF readers on shelves to interrogate the tags, they can usually be read consistently. The read range is short—only about 3 feet—so there is typically little interference.

Most likely, you are considering utilizing UHF tags, since their read range is longer than for HF, and they cost less than active tags. Reading UHF tags can be challenging, however, due to how the tag and reader communicate. Instead of forming an electromagnetic field, the interrogator emits electromagnetic waves, and the tag reflects back a signal from the reader using a method known as backscatter.

The tag signal is very weak, compared to the energy emitted by the reader, and is thus prone to interference. (Imagine someone on a boat holding up a flashlight—the reader—and another person on another boat holding up a mirror to reflect back light. By moving the mirror up or down, these individuals could communicate in Morse code via the flashlight, though clouds or a third boat passing between them might prevent the two from communicating.)

So what might affect a UHF system's performance within a warehouse? Florescent lights, electric motors and other RF devices operating in the UHF band all create noise that can make it difficult for a reader antenna to detect relatively faint tag signals. And metal shelving, diamond plating and rebar in a concrete floor can reflect radio waves, causing a multipath situation—radio waves can travel along more than a single path, thereby canceling one another out and preventing a tag from being read.

Additionally, water absorbs RF energy in the UHF spectrum—which is why food heats up in a microwave oven—so water, or products containing high liquid levels, can absorb energy and make it difficult for tags to be read.

When deploying a passive UHF system within a warehouse, you will first need to conduct a site survey. This will require that you bring in an expert with a spectrometer who can determine the types of RF noise existing within your particular environment, as well as any other factors that might impact your ability to read tags consistently.

—Mark Roberti, Founder and Editor,
RFID Journal

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