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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?

—Monica

———

Monica,

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

USER COMMENTS

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.
Mike Maurer 2018-06-11 10:57:17 AM
Active RFID for RTLS (campus-wide coverage strategy): I agree with Neil with regard to error using RSSI but applications can be built around the known error. For example: If one requires an average accuracy of between 3-5 meters (15 feet), higher frequencies at 933MHz and above may not suffice due to antenna orientation drift coupled with inability to penetrate various materials of construction consistently enough to reliably track tags in real-time. Dropping down to 433MHz penetrates materials of construction found in most facilities well enough to locate within the above margin of error regardless of antenna orientation to the reader. Collating tag RSSI from multiple readers (e.g. on a 50-60 foot grid) and accounting for the margin of error (making RSSI readings linear based on multiple samplings) can provide location accuracy consistently within the 3-5 meter range to accommodate even life-safety applications. Using 433MHz RF to blanket a campus provides long-range confidence without fear of losing tags (attached to people or things). To isolate to the correct floor, simply adding 125KHz Exciters before/after portals provides floor logic and tag direction. For room level accuracy, adding IR to the same infrastructure significantly reduces cost over installing IR in every single space (fallback to 433MHz for campus-wide coverage to the above accuracy). Need accuracy to a cabinet or simply at the portals? There are many passive solutions that play well with the above triple technology approach. The ultimate in patient safety and process optimization. Finally, the active tags will need to have all three technologies (plus passive sticker tag), low battery, tamper, temperature notifications in a small footprint. Combining the above active RFID triple technology approach with Passive RFID provides accuracy that meets most if not all RTLS healthcare use cases.

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