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Several UHF RFID readers in the DC, part 1

A staff member is performing a stock inventory with a mobile UHF RFID reader. Everything flows smoothly until the mobile UHF RFID reader notes that there is another UHF RFID reader in the DC – a fixed one. What happens – do they both freak out?

Authors: Toni Heijari & Suvi Dalén

The Tech Geek will tell you.

Shortly, what happens is that the readers detect each other. This is called the RFID reader collision problem. By using the term 'reader collision' we define it as a phenomenon where an interrogation signal from a specific reader interferences the operation of a tag or another reader. This sort of a problem is augmented particularly in a dense reading environment, for example in a warehouse where multiple readers are operating in the same environment.

Several UHF RFID Readers

But where does the problem arise?

The channels used earlier in the EU region were defined by EN 302 208 or EN 300 220, and were allocated in a way that there were 10 high power channels. Also, the LBT system (Listen Before Talk) was in use. The idea was that a reader would listen if the channel is free before it transmits signal - also the tag would respond in the same channel. This method works quite nicely if the number of readers is limited to only a few readers. The threshold for specific channel was low in such a way that in order to receive permission to use that channel it was required that there were no other readers operating in a certain channel for about 1 km radius (when operating at maximum output power). In other words, you could say that there could be only 10 readers operating in the area of 1 km radius without any reduction in performance.

Another problem is that when Reader A is reading a specific tag population and Reader B is operating in close proximity, the tags in question can interfere from the Reader B signal. This is due to the reason that the tags have no channel selectivity. This in turn means that they interoperate only the power they are receiving. As the stronger interrogation signal will run over the weaker one, only one reader can read the specific tag population at a time as a consequence.

Introducing the DRM

When the reader collision problem with multiple readers using LBT was acknowledged, the ETSI (European Telecommunications Standards Institute) introduced the new channel plan and method for multiple readers to operate in a dense reading environment. This method was called DRM (Dense Reader Mode).The number of high power channels in which readers operate was limited to 4. The free channels, which were dedicated to tag replays, were left between those channels. The Miller sub-carrier encoding and link frequencies of 200 - 400 kHz were adopted into use in order to get the tag replay away from the channel the reader was using. Generally, link frequency defines the spacing between the reader transmission frequency and the tag replay frequency. The order of the Miller encoding (M2/M4/M8) impacts on the spectral density of the tag replay concentrating around the link frequency offset. The ETSI channel plan is introduced in Picture 1.

 Several UHF RFID Readers _graph

Picture 1. ETSI channel plan (EN 302 208 v1.4.1).

How to avoid the collision

As the technology constantly develops, certain types of problems are already been avoided. When adapting the new channel plan and the DRM into use, the reader is no longer required to use the LBT technique. This will increase the reading speed since the reader doesn't have to listen every time before it transmits. Moreover, now a single reader will no longer reserve the channel for itself in a large surrounding area. Also the tag replays, which are 40 - 120 dB lower than the readers' signals, are not blocked because of the dedicated tag replay channels.

But there are still some limitations. The performance of the DRM filters inside the reader and the overall robustness of the receiver design will now limit the performance when readers are operating in a close proximity. Also the used channels and the directivity of the reader antenna will affect the performance because they greatly determine the isolation between the two readers.

The used channels and the antenna radiation pattern issues will be further discussed when the Tech Geek returns with Several UHF RFID readers in the DC, part 2. In fixed installations a good practice is not to put two readers equipped with directive antennas facing toward each other, or in that case the reading should be sequential.

To be continued…

Sources:

ETSI 302 208 v1.4.1 specification

2 comments on “Several UHF RFID readers in the DC, part 1”

  1. Gravatar of Yi An ChenYi An Chen
    Posted 22 December 2015 at 04:41:56

    When the continued topics will be available?

  2. Posted 12 May 2017 at 20:15:19

    Radio frequency identification or RFID collision avoidance where a RFID reader is needed to avoid reader collision is an extreme problem. It's a good thing you've enumerated here few of the tricks one has to do in order to get out of the maze. But you kept us in the loop waiting for the next series of events.

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