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How Important Is RSSI Value to RFID?

Posted By RFID Journal, 11.17.2017

Also, what exactly does the received signal strength indicator measure?




Received signal strength indicator (RSSI) is a measure of the strength of a signal received from a tag. I assume you are talking about passive UHF RFID systems, which use RSSI to determine how close a tag is to a reader antenna. The answer to your question depends on what you are trying to achieve.

If you were to install a fixed reader with antennas around a dock door, you might be concerned only that you read a tag on a pallet moving through the dock door. In that case, RSSI might not be very important. If, however, you were using a handheld reader to locate a tagged object within a store, then RSSI would be critical since it would tell you whether you were moving closer to the tagged item or further away. Handhelds typically beep louder or more frequently as the signal strength from a tag increases, indicating that you are nearing the tag.

—Mark Roberti, Founder and Editor, RFID Journal


Neil Mitchell 2017-11-18 12:08:22 PM
It is also important to understand what factors impact RSSI to understand how relevant it is to your application. RSSI is a value the RFID READER interprets from the strength of the signal coming back from the tag selected. The RSSI varies dependent on the distance of the tag from the readers antenna but that is not the only factor. There are several other factors, including: 1) the orientation of the tag with respect to the Reader antenna. This is very important to understand. RFID tags are a dipole. This means it’s a in essence a single plain antenna. The implication is that dipole antenna’s work very well in one orientation to the antenna (usually the Tags longest dimension is parallel to the plain of the Reader antenna) and very poorly in other (short dimension of most Tags facing the Reader antenna). So it is common to have a tag close to the reader’s antenna, but the tags short end facing the Reader antenna and a tag further away with its longest width facing the Reader antenna and the tag furthest away may have a stronger RSSI signal. 2) The material the tag is applied to. If the tags you are comparing are on different materials (ones that varying in their RF reflective or obsorbing properties (like metallic or water based content) this also would provide different values. This is why RSSI based location solutions are imperfect and open to significant errors. That said, Mark is correct that using a handheld to find a single tag based on RSSI values is vary useful. It’s compating two tags RSSI values that is so difficult.

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