Authors: Toni Heijari & Suvi Dalén

As introduced in Several UHF RFID readers in the DC, part 1, the reader collision occurs when two UHF RFID readers equipped with high gain antennas are operating in a close proximity in the same DC. An advice for how to avoid the possible collision problem was not to put two readers equipped with directive antennas facing towards each other. This time the Tech Geek takes a closer look.

The roles of the reader and the receiver

When the readers operate in a dense reader mode, the performance is mainly affected by the channel or the DRM filters, which are in the receiver. The overall design of the receiver, which includes mixers and LNA (low noise amplifier) gain stages, has an effect on the interference tolerance. Some of those parameters can be adjusted via registers of the UHF RFID chipset - in that case, those parameters and the settings will have an impact on the performance in the environments where readers are densely operated. A rule of thumb is that by lowering the gain in pre-amplifiers or in a mixer stage more tolerance to other readers is received. But, in other hand some of the sensitivity of the receiver will simultaneously be lost. 

Today most of the UHF RFID readers use direct conversion technique inside the receiver meaning that the received signal is down-converted directly from the original frequency (840 - 960 MHz) to a baseband. For this reason, there is no intermediate frequency used between. After the down-conversion the tag replay signal is located around the used link frequency, which is usually 256 - 320 kHz (ETSI regions). Now all the other signals outside the wanted frequency band have to be filtered out, and the response curve of this filter will determine how much interference signals are attenuated. This is explained in Picture 1.

The meaning of the antenna radiation pattern

As stated, the filters, the mixers and the settings have a direct impact on how much Reader A is withstanding the interference from the Reader B. Let's say that the readers have an output power of 1 W (30 dBm) and Reader A can tolerate an interference signal of -15dBm (worst case scenario) or lower without any decrease in performance. Note that this does not mean that if the interference signal goes higher the reading would stop completely - it might slightly slow down. Hence, the transmission of the Reader B has to be attenuated 45 dB in order to not to have any effect to Reader A. The correspondent distance is about 5 meters. This example assumes that both readers have an isotropic antenna radiation pattern, meaning that the antennas would radiate to all directions with a gain of 0 dBi. Thus, the radiation patterns would be a circle. In real life the antennas used in the UHF RFID readers are usually directive antennas meaning that the maximum gain has a certain direction.

Picture 2 describes the radiation pattern of the Nordic ID Merlin UHF RFID Cross Dipole. As can be seen from the picture, the antenna radiates to a 90-degree angle from the maximum direction with a gain of -5 dBi. Now if the same gain of -5 dBi is used in the calculation, the 45dB isolation requirement means about 1.5 m distance. This way, the antenna radiation patterns have a large effect on when discussing multiple readers operating in small area, for instance in a DC.  

Taking one step further

The calculations presented assume the 45 dB isolation requirement with 30 dBm transmitter power. This might be a case when two neighboring transmitter channels are used and they have a spectral separation of 600 kHz. When the next channel is taken into use, the separation grows to 1.2 MHz. Now the DRM filters have a larger attenuation (10 dB) to interfering signals (Reader B), and for this reason the attenuation requirement is reduced to 35 dB. This way, the overall throughput could be improved by for example using high power channel 1 with a fixed reader and portable readers could use high power channels 3 and 4 or other way round. When using the fixed channels, the ETSI 302 208 channel time requirements must be applied. Another way to reduce the interference between fixed readers and portable readers is to use duty cycles if this is acceptable by the use case.

In following pictures, there are some radiation characteristics of the different high gain portal antenna arrays, first with 2 x 2 elements and the second with 1 x 4 elements. As can be seen, there is a significant difference in patterns meaning that the right one for the right use should be chosen. By doing this way it is possible to limit the reading field to certain narrow areas. 

Lesson learned

What did we learn from this? The Tech Geek advises to pay attention in the reading process as an entirety, also the DC design. The readers should not be located too close to each other in order to maximize the reading efficiency - also the radiation patterns of antennas should be taken into account when installing the devices.