Behind the curtain: how fixed RFID infrastructure readers work

Fixed infrastructure RFID doesn’t work like you think it works

People who are not familiar with a technology on a deep level, often create a ‘mental model’ of the technology. A mental model is a simplified explanation of how the technology works, that gives ‘good enough’ guidance to use the technology. For example, if the battery of your car has 50% left, you can assume that you can still drive 50% of the maximum range. However, the reality is a lot more complicated: it depends on the terrain you are driving on, the outside temperature, driving style and the exact battery depletion curve. However, the 50% is a good approximation of reality.

The same holds with RFID. If you walk around a store with an RFID handheld, the mental model is relatively simple: the handheld picks up items in a radius of a few meters, and somewhere between two hundred up to a thousand items per second. It is completely logical that it takes anywhere between tens of minutes to an hour to count a whole store, because you need to walk along all the shelves.

Why fixed infrastructure reading is different

For overhead reading, this mental model is different: you have readers in the ceiling that are continuously reading, so you instantly know everything in the store and see all changes in real-time. Right? … right?

Unfortunately, the reality is a bit more complicated. Let me take you through this step-by-step.

Why “instant” reading isn’t instant

The first challenge is that we still have the limitations of the RFID protocol: readers can only read approximately up to a thousand RFID labels per second - in ideal circumstances. With a typical store containing tens of thousands of items - that’s the first limitation. It will literally take tens of seconds to just read all the products once.

When multiple readers interfere with each other

When two (or even more readers) try to read the same tag, it might yield to no reader being able to read the tag. There are multiple physical effects playing a role, but the main take-away is that you cannot simply assume that when you have two readers, you read twice as fast: in most fixed infrastructure systems readers read sequentially - causing even more delay.

Additional factors that slow down reading

And then there are some more complicating efforts:

• Some of the hardware vendors require multiple sightings (observations) of the same item for locating purposes - this further increases the time to get to a good read.
• Some of the hardware vendors use multiple antennas and/or readers at the same time to read a tag, which also slows down reading.
• There are also other readers in the same store (or in nearby stores) that need some ‘space’ to operate in - and as RFID operates in a shared channel, systems should facilitate co-existence.

Overall, it takes at least a few minutes to read a whole store in what we call a ‘cycle’ - and while the system is trying to read the store, items move around, get sold or might arrive to the store.