The venerable RFID tag traces its ancestry to the “friend or foe” transponder systems developed for military aircraft beginning in WWII. Since then, RFID has earned its place as a reliable asset identification system. Recently, it has been marketed as a solution for real-time indoor positioning. In this post, I want to provide a brief overview of how RFID works, how it might be used for indoor asset tracking, and how it compares to alternatives.
An RFID system—RFID stands for "radio-frequency-identification"—comprises two components: a transponder (or tag) containing data that can be read over RF and an interrogator (or reader) that can read the transponder's data.
The specific way in which these components communicate (known as their “coupling mechanism”) determines the range, complexity, and cost of the system. (“Coupling” in this context refers to an energy transfer between tag and reader.) Currently, three types of coupling mechanisms compete in the market: inductive, capacitive, and backscatter.
The RFID market delineates systems according to how RFID tags are powered. Whether a tag has onboard power available affects its size, price, read range, and whether it can support additional sensors.
Before assessing RFID’s merits as an asset tracking technology, we need to clarify what we mean by “tracking". RFID has, since its inception, been used to track assets in a sort of spreadsheet sense. It makes it simple to identify and log which tracked items are nearby. If your goal is to make sure all of the train cars that went through gate A also made it through gate B, or whether an employee swiped into a building, then RFID is a well-tested, proven solution.
In such use cases, RFID competes most directly with barcodes or QR codes. It offers the obvious advantage of being readable at a distance. Active or semi-active RFID tags can provide valuable sensor information. On the other hand, passive readers are very expensive, and powered tags are costly and have a limited lifespan.
A more challenging type of tracking is knowing the (nearly) real-time location of a tracked asset. Although this is a relatively recent use case for RFID, there are already quite a few commercial solutions on the market.
The way these systems work varies. Some systems use RFID purely for object identification while leveraging another technology for ranging. Those that rely purely on RFID almost exclusively use active RFID tags. There is some exciting research that uses passive RFID tags, but the cost of passive readers and the low range of these systems makes them commercially prohibitive.
Real-time location systems (RTLS) that use active RFID tags behave similarly to competing technologies—Bluetooth, Bluetooth Low-Energy (BLE), WiFi, Ultrasonic, and Ultra-Wideband (UWB). The RFID versions are largely based on the LANDMARC system, which determines location by comparing the Received Signal Strength (RSS) of an active tag’s signals with the RSS of reference tags with a known location.
Active RFID has a much greater range than BLE. It's capable of spanning a kilometer in open air compared to BLE’s ~70m. This is less important in indoor environments with obstructions (e.g. walls or floors), but in warehouses or barns active RFID's range might allow businesses to make do with fewer readers, cutting costs and reducing potential failure points.
RFID has a few drawbacks as a tracking solution. Like all RF / RSSI based solutions, it suffers from bleed-through. Since RF signals can penetrate walls, it becomes difficult to determine from which room a tag is transmitting. The high bandwidths used by active trackers—especially long range trackers—is highly subject to interference. And, compared to BLE, both the tags and the readers are very expensive.
RFID finds its greatest success as part of a hybrid system. It provides reliable identification, which can supplement systems that rely on ultrasonic, infrared, or ultra-wideband for location information.
Currently, RFID technology is not ready to provide a standalone solution for indoor positioning. It is not alone in that respect. As part of a multi-technology system, however, RFID brings to indoor positioning its decades-long history of reliable identification.
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