The reader collision problem was earlier introduced in part 1. Now the Tech Geek returns with the second part. This time we discuss further how to improve the reading efficiency by for instance taking a closer look at the reader and the antennas.
Authors: Toni Heijari
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.
