Installation guide

88
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
RFID Tag Considerations
Figure 58 Passive Tag Load Modulation
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Backscatter modulation and electromagnetic coupling—In this approach (shown in Figure 59), the
RFID reader provides a medium-range electromagnetic field that the passive RFID tag uses for both
power and as a communication medium. Via electromagnetic coupling, the passive RFID tag also
draws energy from the electromagnetic field of the RFID reader to power the tag. However, the
energy contained in the incoming electromagnetic field is partially reflected back to the RFID reader
by the passive tag antenna. The precise characteristics of this reflection depend on the load
(resistance) connected to the antenna. The tag varies the size of the load that is placed in parallel
with the antenna to apply amplitude modulation to the reflected electromagnetic waves, thereby
enabling it to communicate information payloads back to the RFID reader via backscatter
modulation. Backscatter modulation and electromagnetic coupling typically provides longer range
than inductively coupled tags and can be found most commonly among passive RFID tags operating
at 868 MHz and higher frequencies.
Figure 59 Passive Tag Backscatter Modulation
Note that neither of these two techniques allow passive RFID tags to communicate directly with 802.11
infrastructure access points. All communication from the passive RFID tag occurs via the RFID reader.
Passive RFID tags are less costly to manufacture than active RFID tags and require almost zero
maintenance. These traits of long life and low-cost disposability make passive RFID tags attractive to
retailers and manufacturers for unit, case, and pallet-level tagging in open-loop supply chains (where
there is little control over whether an RFID tag leaves the control of the tag owner or originator). Because
of their dependence on external reader energy fields and their low reflected power output, passive RFID
tags have a much shorter read range (from a few inches for tags using load modulation up to a few meters
for those using backscatter modulation) as well as lower read reliability when compared to active RFID
tags.
190591
Tag
Reader
Tag modulates
inductive coupling
Reader detects
load modulation
190592
Tag
Reader
Tag Reflects
electromagnetic
waves
Reader detects changes
in reflected power